Use of Thermal Spray as an Aerospace Chrome Plating

Transcription

LINKING GLOBAL TECHNOLOGIES WITH MARKETS
Use of Thermal Spray
as an Aerospace Chrome
Plating Alternative
Courtesy U.S. Navy. Photo by Ensign John Gay
Report to:
William Green
Geo-Centers
Rowan Project #: 3105JSF3
Contract Number: N00173-98-D-2006, D.O. 0002
Subcontract Number: GC-3363-99-004
P.O. Number: 28578MK
Report Number: Final
Date: October 27, 2000
Authors:
Keith Legg (mailto:klegg@rowantechnology.com)
(Rowan Technology Group, Principal Investigator)
John Sauer
(Sauer Engineering)
UNCLASSIFIED NON-PROPRIETARY - Distribution Statement A
ROWAN TECHNOLOGY GROUP
1590 S. Milwaukee Ave., Suite 205, Libertyville, IL 60048, U.S.A 847-680-9420
Email: rowan@rowantechnology.com
www.rowantechnology.com
Fax: 847-680-9682
EXECUTIVE SUMMARY
Thermal spray coatings have been used for many years in aircraft turbine
engines as wear and erosion resistant coatings, thermal barriers, and
clearance control coatings. As increasing environmental and safety
issues have driven a search for chrome plating alternatives, engineers
have found that thermal spray coating, long used for gas turbine engines,
can be a very cost-effective alternative to hard chrome plating. Although
the initial driver for the substitution of thermal spray coatings for chrome
was environmental, the alternatives are now being widely adopted
because of their better performance, higher reliability, and lower life-cycle
cost.
This document summarizes the current state-of-the-art, property and
performance data, and usage of thermal spray coatings as replacements
for hard chrome plating on aerospace components. The information covers
the use of hard chrome for both original equipment and for overhaul and
repair. Its purpose is to provide in one place a summary of information on
thermal spray coatings that will be useful for engineers engaged in the design
and maintenance of aircraft components.
This document is designed as an electronic book, with links to guide the
user directly to information of interest. The document itself contains data
summaries and examples, with a large number of underlying full-text
references (available at the click of a mouse) to provide as much detail as
possible. The information is current as of August 2000, but the document
is intended to be readily revised and updated as more information is
generated. After a brief introduction, the document is broken into four
parts:
Part 1. Aerospace Usage of Chrome – An overview of the types of
components and applications in which hard chrome is
currently used in the aircraft industry, and the requirements
for chrome replacement.
Part 2 Overview of Thermal Spray – Types and principles of
thermal spray, especially High Velocity Oxy-Fuel (HVOF) and
Plasma Spray – the two primary chrome replacement
technologies. This Part includes thermal spray equipment
and powders, thermal spray producibility and quality control,
stripping, and finishing.
Part 3. Thermal Spray Data – Summary of current data on structure,
properties, and performance of thermal spray coatings –
hardness, adhesion, embrittlement, corrosion, fatigue, wear,
hydraulic rig testing, landing gear rig testing, and flight testing.
The text contains data summaries and graphs, with the
underlying data accessible via full-text documents.
Part 4. Specifications and Qualified Components – Summary of
thermal spray specifications, and of thermal spray-qualified
applications and components.
Rowan Technology Group
Project #: 3105JSF3 ; Report #: Final
Page i
In summary, the data shows that in all critical respects HVOF coatings
perform as well as (and in most cases better than) hard chrome. This is
certainly true in critical areas, including hardness, wear, fatigue,
corrosion, hydraulic testing, and extended flight testing. HVOF can be
applied to almost any material without causing hydrogen embrittlement,
and in many cases the fatigue debit can be completely eliminated. As a
result HVOF coatings (primarily tungsten carbide cermets) are now
specified on more than a hundred components on Boeing aircraft,
and are used extensively for overhaul and repair of landing gear
cylinders and axles, and flap and slat tracks. The new Boeing 767400 is specified for HVOF-coated or chrome plated landing gear,
whichever customers request. Parker-Hannifin is eliminating chrome
plate, and using thermal spray coatings on all new aerospace hydraulic
actuator designs. Airlines such as Delta, Lufthansa, and United are all
qualifying HVOF for landing gear overhaul. There are several standard
and widely used aerospace specifications for thermal spray processes
and for the powder materials they employ. However, thermal spray is not
a simple drop-in replacement for chrome plate. As a dry spraying
process rather than an electroplate it fits differently into the OEM
production and overhaul sequence. Although it can be done in-house,
and is in fact available at most repair shops and DoD depots, OEMs
frequently contract it out. Furthermore, HVOF coatings, the most
common chrome alternative, cannot be used on internal diameters,
although plasma spray can be used on diameters down to about 2”.
Thermal spray cannot be used to replace thin dense or flash chrome,
since it cannot be made thin enough.
The process lends itself to a large number of different coating materials
and a wide range of deposition conditions. This makes it highly flexible
but more complex to use. Therefore the specifications for a thermal spray
coating must be properly defined, and the process optimized to fit both
the material being processed and the coating material being applied. For
example:
•
Since it utilizes a torch or plasma gun, it is possible to overheat
heat-sensitive components, making proper temperature
measurement and control an essential part of the process
specification.
•
The coating material must fit the substrate material. The most
common coating material is tungsten carbide, but thermal sprayed
hard alloys, such as Tribaloy, give better fatigue performance on
aluminum alloys.
•
The thermal spray coating must be optimized properly for the
application. Some thermal spray coatings have performed poorly
because they used the wrong coating material or used a
deposition process that was optimized for the wrong application.
For example, thermal spray coatings optimized for wear
resistance may have as large a fatigue debit as chrome (or even
larger). Re-optimizing the coating for fatigue has reduced, or even
eliminated, the fatigue debit while still retaining superior resistance
to wear.
Page ii
•
The finishing specifications for thermal spray coatings are not
necessarily the same as for chrome. Thermal spray coatings
must in general be finish-ground or superfinished to a much finer
surface than is typical for chrome plate. For example, a 16 µinch
finish is typically specified for chrome plated hydraulics. Using
HVOF coatings with this finish leads to very rapid seal failure.
With a 4 µinch or better finish, however, both seal life and rod life
are greatly extended.
In summary, the thermal spray process is highly recommended and
growing as a replacement for hard chrome plate, but it must be used
properly, with accurate specifications, a qualified sprayer, and proper
account taken for the materials and applications in which it is used.
Page iii
ACKNOWLEDGEMENTS
This compilation would not have been possible without the assistance of a great
many people in the aerospace industry. We would therefore like to acknowledge
the many people and organizations that have contributed to this document.
Funding for the work was provided by the Joint Strike Fighter Program Office,
NADEP Jacksonville, poc Jean Hawkins. HCAT information has been provided
by the members of the HCAT team, courtesy of Bruce Sartwell, NRL, the team leader.
Many documents and other information have been provided by companies such as
Boeing, Messier-Dowty, Praxair, Sulzer Metco, Southwest Aeroservice, Metcut
Research, and the National Research Council of Canada, among others. Many of
the documents have been provided by courtesy of ASM International, Materials
Park, OH 44073-0002, other documents by courtesy of Gorham Advanced Materials,
Gorham, Maine, and by numerous individual authors as indicated in the text.
Page iv
HOW TO USE THIS REPORT
This document is designed to be used electronically, so that it can be a
living document that can be updated with the latest information as it
becomes available. It is extensively hyperlinked to permit the reader to
jump immediately to items of interest in the text.
Many items in the text – figures, tables, references, section headings, etc.
– are hyperlinked, and can be identified by their blue text. Clicking on the
text takes you to the item.
The report is designed to contain the most important information within
the text. Details and backup information are provided in the form of
attached documents, which can be recognized by the yellow boxes, like
the one below. Clicking on the icon within the box brings up the
document, making all the details readily available.
Later cross-references to these documents are shown in blue, and the
Document can be opened by clicking on the blue text.
"HVOF Applications
Listing SWA.PDF"
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Page v
TABLE OF CONTENTS
Executive Summary........................................................i
Acknowledgements ......................................................iv
How to use this report ...................................................v
Table of Contents .........................................................vi
Index of Tables.............................................................xv
Index of Figures ..........................................................xix
Table of Documents..................................................xxiii
Table of Acronyms ...................................................xxvi
1.
Introduction.............................................................................. 1
1.1.
Documents ........................................................................... 2
1.2.
Recent data on health effects of Cr6+ .................................... 2
1.3.
Progress in chrome replacement .......................................... 3
Part 1. Aerospace usage of chrome ............................6
2.
Typical Chrome Plated Components ....................................... 6
2.1.
New equipment usage.......................................................... 6
2.2.
Overhaul and repair usage ................................................... 7
2.3.
Landing gear components .................................................... 7
2.4.
Hydraulic actuators............................................................... 9
3.
Chrome replacement options and requirements .................... 10
3.1.
Hard chrome replacement criteria....................................... 11
3.2.
Thermal spray for hard chrome replacement ...................... 13
Page vi
Part 2. Overview of thermal spray .............................14
4.
Principles of thermal spray..................................................... 14
4.1.
Summary............................................................................ 14
4.2.
Documents ......................................................................... 14
4.3.
General .............................................................................. 15
4.4.
Thermal spray processes ................................................... 16
4.5.
Factors determining coating properties............................... 18
4.6.
Applications of common thermal spray coatings ................. 19
4.7.
Limitations of thermal spray................................................ 20
4.7.1.
Line of sight issues.......................................................... 21
4.7.2.
Heating issues................................................................. 21
4.7.3.
Coating thickness............................................................ 21
5.
Thermal spray coatings ......................................................... 23
5.1.
Summary............................................................................ 23
5.2.
Thermal spray materials ..................................................... 23
5.2.1.
General ........................................................................... 23
5.2.2.
Powders frequently used for chrome replacement 26
5.3.
Typical structural properties of thermal spray coatings ....... 27
5.4.
Typical applications of thermal spray coatings.................... 29
6.
Types of thermal spray processes ......................................... 31
6.1.
Flame spray........................................................................ 31
6.2.
Arc spray ............................................................................ 31
6.3.
Plasma spray...................................................................... 32
6.4.
High velocity oxy-fuel (HVOF) spray and detonation gun.... 33
7.
Thermal spray producibility .................................................... 34
7.1.
Summary............................................................................ 34
7.2.
Documents ......................................................................... 34
7.3.
Quality Control Of the Thermal Spray Process ................... 35
7.3.1.
Choice of powder ............................................................ 35
7.3.2.
General ........................................................................... 35
7.3.3.
Metallography.................................................................. 37
Page vii
7.3.4.
Hardness......................................................................... 39
7.3.5.
Tensile/Adhesion............................................................. 40
7.3.6.
Temperature monitoring .................................................. 40
7.3.7.
Monitoring residual stress................................................ 43
7.4.
Process optimization and control ........................................ 44
7.4.1.
General ........................................................................... 44
7.4.2.
Example 1 – Optimization of WC-Co ............................... 46
7.4.3.
Example 2 – Optimization of WC-CoCr ........................... 48
7.5.
Stripping ............................................................................. 52
7.5.1.
Documents...................................................................... 52
7.5.2.
Stripping of WC-Co ......................................................... 53
7.5.2.1.
Southwest Aeroservice.............................................. 53
7.5.2.2.
Sulzer-Metco ............................................................. 54
7.5.2.3.
Lufthansa .................................................................. 55
7.5.2.4.
Other specifications................................................... 55
7.5.3.
Stripping of WC-CoCr...................................................... 56
7.5.4.
Stripping of Tribaloy 400 ................................................. 56
7.5.5.
Water-jet stripping ........................................................... 57
7.6.
Finishing............................................................................. 57
7.6.1.
Documents...................................................................... 57
7.6.2.
General requirements...................................................... 57
7.6.3.
Specifying the surface finish............................................ 58
7.6.4.
Superfinishing ................................................................. 61
7.6.5.
Rig test experience.......................................................... 63
7.6.6.
Flight experience............................................................. 64
7.7.
8.
9.
Inspection........................................................................... 65
Thermal spray equipment ...................................................... 66
Thermal spray services.......................................................... 67
Part 3. Thermal Spray Data ........................................69
10.
Coating structure ................................................................ 70
10.1.
Summary............................................................................ 70
Page viii
10.2.
Documents ......................................................................... 70
10.3.
General .............................................................................. 71
10.4.
Microstructure..................................................................... 71
10.4.1.
General Description and Test Methods ........................ 71
10.4.2.
Microstructural Features .............................................. 73
10.4.2.1. Porosity/Voids ......................................................... 73
10.4.2.1.1.
Comparison of Porosity vs. Particle Velocity ..... 73
10.4.2.2. Matrix/Carbide Phases/Composition........................ 74
10.4.2.3. Transverse Cracks .................................................. 76
10.4.3.
10.5.
General trend of microstructural features ..................... 76
Phase Determination and Effect ......................................... 76
10.5.1.
10.5.2.
Phase Determination and Effect Results ...................... 77
10.5.2.1. Carbide Phase Comparison vs. Process Type......... 77
10.5.2.2. Carbide Degradation Indexing ................................. 79
10.5.3.
General Trend of Carbide Phase Distribution............... 79
11.
Coating properties .............................................................. 81
11.1.
Summary............................................................................ 81
11.2.
General Background........................................................... 81
11.3.
Hardness............................................................................ 82
11.3.1.
Documents................................................................... 82
11.3.2.
11.3.3.
Hardness Results......................................................... 82
11.3.4.
General Trend of Hardness Results ............................. 85
11.4.
Adhesion ............................................................................ 85
11.4.1.
Documents................................................................... 85
11.4.2.
11.4.3.
Tensile/Adhesion Results............................................. 86
11.4.4.
General Trend of Tensile Results................................. 86
11.5.
Residual Stress .................................................................. 86
11.5.1.
Documents................................................................... 86
11.5.2.
11.5.3.
Residual Stress Results ............................................... 89
11.5.3.1. Almen strip .............................................................. 89
Page ix
11.5.3.2. Almen/Residual Stress Comparison ........................ 90
11.5.3.3. Modified Layer Removal Technique ........................ 93
11.5.3.4. Residual Stress by X-ray Diffraction ........................ 95
11.5.3.5. Residual Stress by Neutron Diffraction .................... 96
11.5.4.
General Trend of Residual Stress Results ................... 98
12.
Coating performance .......................................................... 99
12.1.
Summary............................................................................ 99
12.2.
General Background......................................................... 100
12.3.
Documents ....................................................................... 100
12.4.
Test Protocol Summaries ................................................. 101
12.4.1.
Start-up test Protocol ................................................. 101
12.4.2.
JTP for Landing Gear................................................. 102
12.4.3.
Other Protocols .......................................................... 102
12.5.
Corrosion.......................................................................... 103
12.5.1.
Documents................................................................. 103
12.5.2.
Corrosion Test Methods............................................. 104
12.5.2.1. Atmospheric Methodology ..................................... 104
12.5.2.2. Simulated Cabinet Testing..................................... 104
12.5.3.
Corrosion Data........................................................... 108
12.5.3.1. Simulated Cabinet Results from Lufthansa............ 108
12.5.3.2. Cabinet and Atmospheric Testing - HCAT ............. 111
12.5.3.2.1.
ASTM B117 Salt Fog Testing .......................... 111
12.5.3.2.2.
GM 9540P/B Testing....................................... 114
12.5.3.2.3.
Atmospheric Testing ....................................... 114
12.5.3.2.4.
Interpretation of results.................................... 115
12.5.3.3. Electrochemical Testing of Carbide Coatings ........ 115
12.5.3.3.1.
12.5.3.4. Corrosion Work Planned in JTP Landing Gear ...... 117
12.5.4.
12.6.
General Trend of Corrosion Results........................... 118
Fatigue ............................................................................. 118
12.6.1.
Documents................................................................. 118
12.6.2.
General Description and Test Method........................ 119
12.6.3.
Fatigue Results .......................................................... 123
12.6.3.1. Comparison of Hard Chrome vs. HVOF WC-Co and
Page x
T400...................................................................... 123
12.6.3.2. Comparison of Hard Chrome vs. HVOF WC-Co for
Landing Gear......................................................... 127
12.6.3.3. Comparison of Hard Chrome vs HVOF WC-CoCr . 132
12.6.3.4. Other ..................................................................... 134
12.6.3.5. Comparative Study of Compressive Stress Effects on
Fatigue for HVOF .................................................. 134
12.6.3.5.1.
12.6.4.
12.7.
General Trend of Fatigue Results .............................. 137
Wear – Erosion, Abrasion, Sliding, Fretting ...................... 138
12.7.1.
Documents................................................................. 138
12.7.2.
General Description and Test Methods ...................... 139
12.7.3.
Test Methods ............................................................. 141
12.7.3.1. Erosion Testing per ASTM G 76 ............................ 141
12.7.3.2. Abrasion Testing ................................................... 142
12.7.3.3. Sliding/Fretting Wear Methods .............................. 142
12.7.4.
Wear Results ............................................................. 143
12.7.4.1. ASTM G 65 Erosion Testing .................................. 143
12.7.4.1.1.
12.7.4.2. ASTM G 76 Abrasion Testing ................................ 145
12.7.4.2.1.
12.7.4.3. Other Abrasion Tests............................................. 147
12.7.4.4. Sliding and Fretting Wear Results ......................... 148
12.7.4.4.1.
DARPA program – GEAE/NU.......................... 148
12.7.4.4.2.
JTP for Landing Gear ...................................... 150
12.7.5.
12.8.
General Trend of Wear Results.................................. 151
Impact .............................................................................. 152
12.8.1.
12.8.2.
Impact Test Results ................................................... 152
12.9.
Hydrogen Embrittlement ................................................... 153
12.9.1.
12.9.1.1. Embrittlement Testing:........................................... 153
12.9.2.
Lufthansa embrittlement tests .................................... 153
12.9.3.
Hydrogen Embrittlement Tests Planned - HCAT ........ 154
12.9.4.
General Trend of Hydrogen Embrittlement Results ... 154
Page xi
12.10.
Creep ............................................................................ 155
12.10.1.
General Description and Test Methods...................... 155
12.10.2.
Documents ................................................................ 155
12.10.3.
Creep Testing Results ............................................... 156
12.10.3.1. Results for HVOF WC-Co and T400...................... 156
12.10.3.1.1.
Test conditions .............................................. 156
12.10.3.1.2.
Results.......................................................... 157
12.10.3.1.3.
Interpretation of results.................................. 157
12.10.4.
General Trend of Creep Results ................................ 157
13.
System performance ........................................................ 158
13.1.
Summary.......................................................................... 158
13.2.
Rig tests ........................................................................... 159
13.2.1.
Hydraulic Seals – Green, Tweed Phase 2 hydraulic rig
test ........................................................................... 159
13.2.1.1. Documents ............................................................ 159
13.2.1.2. Test Description .................................................... 159
13.2.1.3. Test Conditions ..................................................... 159
13.2.1.4. Results .................................................................. 160
13.2.1.5. Interpretation of Results ........................................ 161
13.2.1.6. Comments............................................................. 162
13.2.2.
Landing Gear Pins – Boeing landing gear rig test ...... 163
13.2.2.1. Documents ............................................................ 163
13.2.2.2. Test Conditions ..................................................... 163
13.2.2.3. Results .................................................................. 164
13.2.2.4. Interpretation of results.......................................... 164
13.2.2.5. Comments............................................................. 164
13.2.3.
13.3.
Rig tests under development – Messier-Dowty .......... 164
Flight tests........................................................................ 165
13.3.1.
Lufthansa ................................................................... 165
13.3.1.1. Documents ............................................................ 165
13.3.1.2. Test Conditions ..................................................... 165
13.3.1.3. Results .................................................................. 166
13.3.1.5. Comments............................................................. 166
Page xii
13.3.2.
Delta .......................................................................... 166
13.3.2.1. Test Conditions ..................................................... 167
13.3.2.2. Results .................................................................. 169
13.3.2.4. Comments............................................................. 170
13.3.3.
F-18 landing gear repair............................................. 170
13.3.4.
Flight tests under way or under development ............. 171
Part 4. Specifications and qualified components...172
14.
Specifications and standards for thermal spray ................ 172
14.1.
Documents ....................................................................... 172
14.2.
Boeing thermal spray specs – method, powder, grinding.. 172
14.2.1.
Boeing Thermal Spray Spec – BAC 5851 .................. 172
14.2.2.
Boeing Powder Spec – BMS 10-67 ............................ 174
14.2.3.
Boeing Grinding Spec – BAC 5855 ............................ 174
14.3.
Hamilton-Sundstrand – HS 4412 ...................................... 174
14.4.
Society of Automotive Engineers - AMS 2447 .................. 174
14.5.
American Welding Society – AWS C.2-19-XX .................. 175
14.6.
AMS standards under development.................................. 176
15.
Qualified Thermal Sprayed Airframe Components............ 177
15.1.
Documents ....................................................................... 177
15.2.
Usage of thermal spray in Gas Turbine Engines............... 177
15.3.
Summary of thermal spray coatings on non-engine
components ................................................................. 179
15.4.
Boeing – qualified thermal sprayed components .............. 180
15.5.
Landing gear .................................................................... 181
15.5.1.
OEM Production - Boeing 767-400 landing gear ........ 181
15.5.2.
Flight tested landing gear repair - Canadian F-18 MLG
axle........................................................................... 183
15.5.3.
Other qualified landing gear applications.................... 184
15.5.4.
Boeing overhaul manual revision ............................... 184
15.5.5.
Delta Airlines qualified landing gear repair – Boeing 737,
757, 767 ................................................................... 185
15.5.6.
Qualified landing gear repair ...................................... 186
Page xiii
15.6.
Hydraulics ........................................................................ 186
15.6.1.
P&W F-119 engine convergent nozzle
actuator .................................................................... 187
15.6.2.
Flight test – Sikorsky CH-53 blade damper ................ 187
15.7.
Production - Flap and slat tracks ...................................... 188
15.7.1.
OEM tracks - Boeing .................................................. 188
15.7.2.
OEM tracks - Bombardier........................................... 188
15.7.3.
Flap track repair – Bombardier Dash 8....................... 189
15.7.4.
O&R of tracks – Boeing and other aircraft .................. 190
15.8.
Other components............................................................ 191
References .................................................................193
Page xiv
INDEX OF TABLES
Table 1. Hard Chrome Alternatives Team members (full list available on
HCAT web site). ............................................................................... 4
Table 2. Some typical OEM chrome plated components......................... 6
Table 3. Some differences between OEM and O&R chrome
replacement.................................................................................... 10
Table 4. Hard chrome replacement criteria. .......................................... 12
Table 5. Typical characteristics of thermal spray coating processes. .... 17
Table 6. Some fundamental terms that define the quality of thermal spray
coatings. ......................................................................................... 19
Table 7. Some common thermal spray coatings, their structure,
performance, and applications........................................................ 20
Table 8. Some major thermal spray powder classifications................... 24
Table 9. Important parameters defining thermal spray powders and
electric arc wire............................................................................... 25
Table 10. Examples of thermal spray powder used in chrome
replacement operations. ................................................................. 26
Table 11 Comparison of thermal spray coating processes – general
properties. ...................................................................................... 28
Table 12. Comparison of thermal spray coating processes - permeability,
thickness. ....................................................................................... 29
Table 13. Some applications of thermal spray coatings.. ...................... 30
Table 14. Producibility summary and links. ........................................... 34
Table 15 Commonly Used Quality Control Tests................................... 36
Table 16 Common Characteristics Evaluated in Metallographic
Specimens...................................................................................... 38
Table 17. Thermal spray process parameters....................................... 45
Table 18. Design of Experiment analysis tool. ...................................... 47
Table 19 Response vs. Coating Property............................................. 48
Table 20. Comparison of hydrogen versus propylene DOE. ................. 48
Table 21. Electrolytic stripping method for HVOF WC-Co (Courtesy
Southwest Aeroservice).................................................................. 53
Sulzer Metco). ................................................................................ 54
Table 23. Electrolytic stripping method for “aged” HVOF WC-Co
(Courtesy Lufthansa). ..................................................................... 55
Table 24. Electrolytic stripping method for "new" HVOF WC-Co (Courtesy
Page xv
Lufthansa). ..................................................................................... 55
Table 25. Electrolytic stripping method for HVOF WC-Co (NTS/NADEP
Cherry Point). ................................................................................. 56
Table 26. Seal life in HVOF-WC-Co sprayed landing gear.18, ................ 64
Table 27. Qualified providers for Boeing 5851 thermal spray coatings, as
of June 2000 (Source, Boeing Aircraft Corporation)........................ 67
Table 28. Common microstructural Characteristics Observed in Tungsten
Carbide Materials. .......................................................................... 72
Table 29. Features seen in Figure 35. .................................................. 75
Table 30. Physical properties of coatings produced by different guns. .. 78
Table 31. Effect of Gas Flows and Cooling Gases on Retained Carbon.
....................................................................................................... 78
Table 32. Retained C and XRD phases. ............................................... 78
Table 33. Microhardness for various HVOF coatings and equipment
(Courtesy Praxair Surface Technology). ......................................... 83
Table 34. Comparison of Microhardness Values and Resultant Variation
(Courtesy Sulzer Metco and SUNY Stony Brook). .......................... 84
Table 35. Qualitative techniques for measuring residual stress............. 88
Table 36. Common quantitative residual stress measurement techniques.
....................................................................................................... 89
Table 37. Zone analysis of thermal spray coatings. .............................. 92
Table 38. WC coating system designations for Document 24. .............. 93
Table 39. Comparison of Residual Stress by Varied Techniques.......... 96
Table 40. Experimental set-up for neutron diffraction............................ 96
Table 41. Summary of performance tests. ............................................ 99
Table 42. Chemistry of Tribaloys. ....................................................... 101
Table 43. Materials and heat treats for HCAT Landing Gear JTP. ...... 102
Table 44 Common Corrosion Testing Methods................................... 104
Table 45 GM9540 Protocol for Corrosion testing. .............................. 106
Table 46. Visual ranking criteria (ASTM B537-70). ............................. 107
Table 47. Coatings tested (Lufthansa). Note: 25µm+0.001”................ 109
Table 48. Summary of corrosion ratings for coatings tested by Lufthansa.
..................................................................................................... 110
Table 49. Coatings and substrates - HCAT corrosion testing. ............. 111
Table 50. Corrosion of 4340 steel with HVOF and Cr coatings appearance and protection rankings............................................. 113
Table 51. GM9540P/B corrosion of 4340 steel with HVOF and Cr
coatings - appearance and protection rankings............................. 113
Page xvi
Table 52. HVOF coatings used for Comparison of Electrochemical
Corrosion Potential. ...................................................................... 116
Table 53. HCAT/C-HCAT corrosion test matrix for landing gear steels
and coatings. ................................................................................ 117
Table 54. Fatigue testing variables. .................................................... 122
Table 55. Materials and Substrates in Study....................................... 123
Table 56 Fatigue Test Parameters...................................................... 123
Table 57. Fatigue Matrix for Initial Validation ...................................... 124
Table 58. Substrate and coating materials - landing gear JTP. ........... 128
Table 59. Test conditions for landing gear JTP. .................................. 128
Table 60. Tungsten Carbide Coating System Designations (Volvo) .... 135
Table 61. Fatigue Test Parameters for Volvo Evaluation .................... 136
Table 62. Four Primary Wear Mechanisms ......................................... 140
Table 63. Erosion Results as Conducted By Praxair........................... 144
Table 64. ASTM G76 data from Praxair. ............................................. 146
Table 65. ASTM G76 Data from NRC. ................................................ 146
Table 66. ASTM G76 Data from NRL and Sulzer Metco. .................... 147
Table 67. Average wear coefficients, K, expressed in units of 10-4 mm3/Nm, for the various coating/substrate combinations. ....................... 148
Table 68. Fretting test parameters. ..................................................... 149
Table 69. Wear test variables for DOE factors. ................................... 151
Table 70. Creep test parameters ........................................................ 156
Table 71. Summary of rig and flight testing data. ................................ 158
Table 72. Hydraulic test conditions. .................................................... 159
Table 73. Stroke and frequency profile for hydraulic tests................... 160
Table 74. Military flight tests of HVOF-coated components................. 171
Table 75. Boeing thermal spray coating types. ................................... 173
Table 76. AMS 2447 HVOF Coating specifications. ............................ 175
Table 77. Summary of thermal spray-qualified non-engine components.
(Click on links to access data directly.) ......................................... 179
Table 78. Summary of Boeing components specified for thermal spray.
..................................................................................................... 180
Table 79. Other landing gear components qualified for OEM HVOF WCCo (Courtesy Southwest Aeroservice). ......................................... 184
Table 80. Landing gear components commonly repaired with HVOF WCCo (Courtesy Southwest Aeroservice). ......................................... 186
Table 81. Flap and slat tracks specified for thermal spray coating with
Page xvii
Super D-gun WC-18Co (Courtesy Boeing). .................................. 189
Table 82. Bombardier Dash 8-100, -200, -300 flap tracks qualified for
HVOF repair (Courtesy Vac Aero). ............................................... 190
Table 83. Common flap/slat track repairs using HVOF WC-Co (Courtesy
Southwest Aeroservice)................................................................ 191
Table 84. Other OEM HVOF WC-Co applications (Courtesy Southwest
Aeroservice). ................................................................................ 192
Table 85. United Airlines O&R components qualified for HVOF in place of
chrome plate................................................................................. 193
Page xviii
INDEX OF FIGURES
Figure 1. Boeing 737 nose landing gear inner cylinder. Shiny areas are
chromed - piston and four axle journals (Courtesy Sulzer Metco). .... 8
Figure 2. Boeing 767 main landing gear (Courtesy Sulzer Metco). ......... 8
Figure 3. F-18 main landing gear Oleo Attach Pin (Courtesy Boeing). .... 9
Figure 4. F/A-18 E/F aileron servocylinder, manufactured by HR Textron
(Courtesy Boeing)............................................................................. 9
Figure 5. Thermal spray process schematic (left); close-up view of
surface (right). ................................................................................ 15
Figure 6. Types of thermal spray processes. Types covered in this report
shown in green.8 ............................................................................. 16
Figure 7. Structure of Thermal Spray Deposit at 100-500X.8................. 18
Figure 8. Example of powder definition for sintered irregularly shaped
88/12 Tungsten Carbide Cobalt powder.......................................... 27
Figure 9. Components of a typical plasma spray system. ..................... 32
Figure 10. Typical HVOF coating cross sections; Ni-Al left (200x), WC-Co
right (500x). (Courtesy Praxair-TAFA)............................................ 38
Figure 11 Tensile Assembly from ASTM C-633 .................................... 40
Figure 12. Typical non-contact temperature arrangement for HVOF. .... 41
Figure 13 Typical Temperature Plot From a Spray Cycle (J. Schell,
GEAE, Courtesy HCAT). ................................................................ 42
Figure 14. Almen “N” Test Strip. ........................................................... 43
Figure 15. Almen holding fixtures (Electronics Inc.). ............................. 44
Figure 16. Almen measuring instrument (Electronics Inc). .................... 44
Figure 17. Kinetic versus thermal energy for the main thermal spray
technologies. .................................................................................. 46
Figure 18. Best propylene results. Degradation index = 4.25. .............. 49
Figure 19. Best hydrogen results. Degradation index = 3.46................ 49
Figure 20. Graph showing the temperature/velocity profile with varied fuel
types............................................................................................... 50
Figure 21. Microstructure/morphology of selected powders. ................. 51
Figure 22. Particle size distribution for the three best powders. ............ 51
Figure 23. Typical surface profile. ......................................................... 59
Figure 24. Three different surfaces with the same Ra. .......................... 60
Figure 25. Other surface roughness parameters................................... 60
Figure 26. Definition of bearing ratio. .................................................... 61
Page xix
Figure 27. Effect of various finishing methods on an HVOF coating at
175x (Courtesy Supfina). ................................................................ 62
Figure 28. Surface finishes obtained on Chrome and HVOF WC-CoCr by
various finishing methods.18 ............................................................ 63
Figure 29. Sulzer Metco F210 ID plasma spray gun (Courtesy Sulzer
Metco). ........................................................................................... 66
Figure 30. HVOF spraying of WC-CoCr on landing gear with TAFA gun
(Courtesy Praxair-TAFA). ............................................................... 66
Figure 31. Northwest Mettech Axial III tri-electrode plasma system
(Courtesy Northwest Mettech).. ...................................................... 66
Figure 32. Stellite Jet-Kote HVOF gun (Courtesy Deloro Stellite).......... 66
Figure 33. Comparison of porosity at 200x and 1000x magnification. ... 73
Figure 34. Relationship between velocity and porosity.......................... 74
Figure 35. Comparison of Carbide Distributions in 88-12 WC-Co (left) vs.
83-17 WC-Co (right) at 500X (Courtesy Praxair/TAFA)................... 74
Figure 36. Microstructure of WC-CoCr 1000X....................................... 75
Figure 37. Transverse cracking in plasma sprayed carbide coatings.25 . 76
Figure 38. X-ray diffraction plot of powder(lower curve) and coating
(upper curve). ................................................................................. 77
Figure 39. Comparison of carbide content in as-sprayed A-12 (12%
cobalt ). Hybrid 2600 gun (left), air-cooled DiamondJet (right). ...... 79
Figure 40. Tensile assembly from ASTM C-633.................................... 85
Figure 41. Stress as a function of coating thickness for HVOF WC-CoCr.
....................................................................................................... 90
Figure 42. Almen strip stress measurement.......................................... 91
Figure 43. Average residual stress as a function of spray distance. ...... 91
Figure 44. Average residual stress as a function of powder feed rate. . 92
Figure 45. Bend test technique, evaluation criteria, results. .................. 94
Figure 46. Typical stress profile for modified layer removal technique. . 95
Figure 47. Air Plasma Spray residual stress pattern. ............................ 97
Figure 48. Wire Arc Spray residual stress pattern. ................................ 98
Figure 49. HVOF Spray residual stress pattern..................................... 98
Figure 50. B117 Appearance Rankings for coatings on 4340 high
strength steel, PH13-8Mo stainless steel, and 7075 Al. ................ 112
Figure 51. GM9540P/B Appearance Rankings for coatings on 4340 high
strength steel, PH13-8Mo stainless steel, and 7075 Al. ................ 112
Figure 52. 4340 steel 18-month beach exposure tests, with and without
scribing. ........................................................................................ 114
Page xx
Figure 53. 7075 Al 18-month beach exposure samples with and without
scribing. ........................................................................................ 115
Figure 54. Corrosion Current for an Aerated 0.1 N HCl Solution.40 ..... 117
Figure 55. Typical hourglass-shaped fatigue bar................................. 121
Figure 56. Typical smooth fatigue bar. ................................................ 121
Figure 57. Flat Kb fatigue bar.............................................................. 122
Figure 58. Comparison of Fatigue Data on Smooth Bars for 4340 ...... 125
Figure 59. Comparison of fatigue data on Kb bars for 4340 ................ 125
Figure 60. Fatigue of coated 4340 steel - hourglass samples. ............ 126
Figure 61. Fatigue Results for HVOF and Chrome on 7075 Aluminum 127
Figure 62. Fatigue Curve for 300M with .24”dia. hourglass tested in air –
coating thickness 0 .003”. ............................................................. 129
Figure 63. Fatigue Curve for 300M with .24”dia. hourglass comparing air
results with samples tested in NaCl and .003” Coating Thickness. 130
Figure 64. Fatigue Curve Comparing Thickness Effects 0.003” (.250”
dia.) vs. 0.010” (.500” dia.) on 4340 using hourglass configuration
tested in air. .................................................................................. 131
Figure 65. Fatigue of HVOF-coated and chrome plated high strength
steels, Kt=1.5, Boeing qualification testing. (Courtesy Engelhard
Surface Technology)..................................................................... 133
Figure 66. Comparison of fatigue for chrome and HVOF WC-CoCr
deposited with Jet Kote and Diamond Jet guns. (Courtesy
Southwest Aeroservice.)............................................................... 133
Figure 67. Comparison of Residual Stress and Resistance of Coating to
Crack Initiation.............................................................................. 136
Figure 68. Comparison of Final Fatigue Life with Residual Stress ...... 137
Figure 69. Typical Set-up of ASTM G76 erosion test. ........................ 141
Figure 70. ASTM G 76 set-up. ............................................................ 142
Figure 71. Schematic of Sliding Wear Apparatus for hydrualics ......... 143
Figure 72. Side view of fretting apparatus. .......................................... 143
Figure 73. Erosion Results As Conducted By Stony Brook/Sulzer Metco
..................................................................................................... 144
Figure 74. Comparison of HVOF Processes and WC-Co Powders ..... 148
Figure 75. Fretting wear of hard chrome, HVOF WC-17Co, and HVOF
T400. (Note – the zero wear measurement resulted from material
transfer from the uncoated block to the coated shoe, protecting it
from wear.) ................................................................................... 150
Figure 76. Average creep measured by direct micrometer readings. .. 157
Figure 77. Cumulative hydraulic fluid leakage in rig tests.................... 161
Page xxi
Figure 78. Seal wear during hydraulic rig tests..................................... 162
Figure 79. F/A-18E/F main landing gear, showing locations of HVOFcoated pins....................................................................................... 164
Figure 80. Landing gear components HVOF sprayed for flight testing by
Delta Airlines (sprayed areas numbered). (Courtesy Delta Airlines.)
......................................................................................................... 168
Figure 81. Boeing 737 nose landing gear inner cylinder undergoing flight
test inspection at Delta Airlines (Courtesy Delta Airlines). ............. 168
Figure 82. Boeing 757 axle sleeves HVOF-sprayed with WC-CoCr
(Courtesy Delta Airlines).................................................................. 169
Figure 83. Canadian F-18 main landing gear polygon repair (Courtesy
Messier-Dowty)................................................................................ 171
Figure 84. Thermal spray coatings used in a typical gas turbine engine.
(Courtesy GE Aircraft Engines)50 .................................................... 178
Figure 85. Boeing 767-400 with HVOF coated landing gear. ............... 181
Figure 86. Boeing 767-400 main landing gear (Courtesy Sulzer Metco).
......................................................................................................... 181
Figure 87. Boeing 767 main landing gear axle (part # 2207-85-10),
showing HVOF areas (engineering note 3). (Courtesy Sulzer Metco.)
......................................................................................................... 182
Figure 88. Boeing 767-400 main landing gear inner cylinder (Part # 22074-10) with asterisks showing locations of HVOF coatings (Courtesy
Sulzer Metco.).................................................................................. 183
Figure 89. Canadian F-18 main landing gear axle (Courtesy MessierDowty).............................................................................................. 183
Figure 90. Repair area of F-18 main landing gear polygon (Courtesty
Messier-Dowty)................................................................................ 184
Figure 91. HVOF WC-Co repair of Boeing 737 nose landing gear inner
cylinder (Courtesty Southwest Aeroservice). .................................. 186
Figure 92. Thermal spray actuator coating system developed by
Praxair.............................................................................................. 187
Figure 93. CH-53 helicopter (Sikorsky). ................................................ 188
Figure 94. Bombardier Q-400 (Courtesy Bombardier.)......................... 189
Figure 95. Typical flap track - Bombardier Dash 8 (Courtesy Vac Aero,
Canada). .......................................................................................... 190
Figure 96. HVOF-sprayed Dash 8 flap track. Coated areas are dark.
(Courtesy, Vac Aero, Canada.) ....................................................... 191
Figure 97. Flap and slat track repair by HVOF (Southwest Aeroservice).
......................................................................................................... 191
Figure 98. Boeing 737 nose landing gear lower bearing shock strut, Part
# 69-76508. HVOF WC-Co coated and super finished. (Courtesy
Sulzer Metco.).................................................................................. 192
Page xxii
TABLE OF DOCUMENTS
Document 1. Hard Chrome Coatings - Advanced Technology for Waste
Elimination, DARPA Grant MDA972-93-1-0006, Keith O. Legg, Jerry
Schell, George Nichols, Robert Altkorn............................................. 2
Document 2. Lung Cancer Among Workers in Chromium Chemical
Production, Herman J. Gibb et. al., American Journal of Industrial
Medicine, 38, 115-126 (2000). (Courtesy of the authors and
American Journal of Industrial Medicine.) ......................................... 2
Document 3. Clinical Findings of Irritation Among Chromium Chemical
Production Workers, Herman J. Gibb et. al., American Journal of
Industrial Medicine, 38, 127-131 (2000). (Courtesy of the authors
and American Journal of Industrial Medicine.) .................................. 2
Document 4. JSF Phase 1 Report: Chrome Replacements for Internals
and Small Parts. (Rowan Technology Group). ................................ 14
Document 5. JSF Phase 2 Report: Optimal Chrome Replacement
Technologies for Internal Diameters and Heat-Sensitive Parts.
(Rowan Technology Group)............................................................ 15
Document 6. Common thermal spray powder types.............................. 25
Document 7. Test standardization: a Key Tool in Coating System
Implementation (Courtesy Sauer Engineering, Gorham Advanced
Materials)........................................................................................ 34
Document 8. Training in Coating Evaluation Techniques: a Unique
Approach for Discussion (Courtesy Sauer Engineering). ................ 34
Document 9. The Use of Metallographic Standards in Calibration of the
Polishing Process (Courtesy Sauer Engineering). .......................... 34
Document 10. Metallographic Preparation of Thermal Spray Coatings:
Coating Sensitivity and the Effect of Polishing Intangibles (Courtesy
Sauer Engineering)......................................................................... 35
Document 11. Tensile Bond Variance of Thermally Sprayed Coatings
with Respect to Adhesive Type. ..................................................... 35
Document 12. Almen Strips and Temperature Measurement During
HVOF Processing (Courtesy, Sauer Engineering). ......................... 35
Document 13. Design of Experiment for HVOF WC-Co process
(Courtesy HCAT, www.hcat.org)..................................................... 35
Document 14. Summary of DOE results for optimization of HVOF WCCoCr (Courtesy NRC Montreal and C-HCAT). ................................ 35
Document 15. NTS Stripping Report,PDF. ........................................... 52
Document 16. NDCEE Evaluation of Stripping Methods. ..................... 52
Document 17. Stripping of WC Coatings from Aermet 100, Southwest
Aeroservice, Menasco, Carpenter Technology (Courtesy Southwest
Aeroservice). ................................................................................. 52
Page xxiii
Document 18. Surface Metrology Guide (Courtesy Precision Devices,
Inc.). .............................................................................................. 57
Document 19. Superfinishing of Hard Chrome and HVOF Coated
Workpieces (Courtesy Supfina and Gorham Advanced Materials). 57
Document 20. Surface Finishing of Tungsten Carbide Cobalt Coatings, J.
Nuse, J. Falkowski. ........................................................................ 57
Document 21. Barkhausen Noise as a Quality Control Tool (Courtesy
Stresstech Inc., Finland). ............................................................... 57
Document 22 Evaluation of Four High Velocity Thermal Spray Guns
Using WC-10Co-4Cr Cermets (Courtesy of J.G. Legoux,
B.Arsenault, C. Moreau, V. Bouyer, L. Leblanc). ............................ 70
Document 23. Fracture Toughness of HVOF Sprayed WC-Co Coatings
(Courtesy of S. De Palo, et al). ...................................................... 70
Document 24. Tungsten Carbide-Cobalt Coatings for Industrial
Applications (Courtesy of M. Dorfman, J Defalco, J. Karthikeyan). 70
Document 25. A Critical Evaluation of the Employment of Microhardness
Techniques for Characterizing and Optimizing Thermal Spray
Coatings 2000 (Courtesy of M. Factor and I. Roman, Hebrew
University). .................................................................................... 82
Document 26 Behaviour of Tungsten Carbide Thermal Spray Coatings
1995, J. Wigren et al....................................................................... 86
Document 27 An ASM Recommended Practice for Modified Layer
Removal Method (MLRM) to Evaluate Residual Stress in thermal
Spray Coatings 2000, Ed Rybicki and ASM TSS Committee. ........ 86
Document 28 Properties of WC-Co Components Produced Using the
HVOF Thermal Spray Process 2000, J. Stokes and L. Looney. ..... 86
Document 29 X-ray diffraction residual stress techniques, P.S. Prevey.
....................................................................................................... 86
Document 30 Processing Effects on Residual Stress in Ni+5%Al
Coatings-Comparison of Different Spraying Methods 2000,
J.Matejicek et al. ............................................................................ 87
Document 31 Residual Stress Measurement in Plasma Sprayed
Coatings by X-Ray Diffraction (Courtesy of J. Matejicek et al) 1997.
....................................................................................................... 95
Document 32 HCAT Test Protocol for Initial Work 1996 (Courtesy of
HCAT Team) ................................................................................ 100
Document 33 Joint Test Protocol (JTP) for Landing Gear 1998
(Courtesy of HCAT and CHCAT Teams) ...................................... 100
Document 34 Joint Test Protocol (JTP) for Propeller Hub Components
2000 (Courtesy of HCAT, JG-PP, and C-HCAT Teams) .............. 100
Document 35 Joint Test Protocol (JTP) for Gas Turbine Engines 2000
(Courtesy of HCAT and PEWG Teams ......................................... 101
Page xxiv
Document 36. Report of Replacement of Chromium Electroplating Using
HVOF Thermal Spray Coatings AESF Plating Forum 1998 (Courtesy
of Bruce Sartwell and HCAT Team). ............................................ 103
Document 37. Replacement of Chrome Plating by Thermal Spray –
Results of Corrosion Testing of HVOF Coatings 1996 (Courtesy of
Lufthansa). .................................................................................. 103
Document 38. Replacement of chrome plating by thermal spray coatings
– Summary of tests (Courtesy of Lufthansa). ............................... 103
Document 39 Performance of HVOF Sprayed Carbide Coatings in
Aqueous Corrosive Environments 2000 (Courtesy of S. Simard
(NRC) et al). ................................................................................ 103
Document 40 Summary of 4340 Data from Initial HCAT Protocol
(Courtesy of Phil Bretz Metcut) ..................................................... 118
Document 41 Summary of 7075 Al Data from Initial HCAT Protocol
Document 42 Summary of 13-8 Stainless Data from Initial HCAT Protocol
Document 43. HCAT landing gear JTP fatigue data - HVOF WC-Co on
4340, 300M, AerMet 100 in air and NaCl solution......................... 119
Document 44 Advanced Thermal Spray Coatings for Fatigue Sensitive
Applications (Courtesy of John Quets Praxair).............................. 119
Document 45 Compressive Creep Tests of Hard Chrome and HVOF
coatings 1998, J. Schell, GE Aircraft Engines. .............................. 155
Document 46. Evaluation of Chrome Rod Alternative Coatings, Tony
Degennaro, Green Tweed, 1999................................................... 159
Document 47. F/A-18E/F Main Landing Gear HVOF-coated Pin Testing
and Evaluation.............................................................................. 163
Document 48. Table of contents of BAC 5851 Thermal Spray
Specification, 2000 (Courtesy Boeing Aircraft Corp.). .................. 172
Document 49. Standards for the Thermal Spray Industry, Bhusari and
Sulit. ............................................................................................ 172
Document 50. HVOF WC aerospace applications for OEM and rebuild
(Courtesy Southwest Aeroservice). .............................................. 177
Document 51. Thermal Spray Applications at GE Aircraft Engines
(Dorothy Comassar, Courtesy GE Aircraft Engines). ................... 177
Document 52. OEM Approval for HVOF Wear Resistant and MCrAlY
Coatings (Gary Naisbitt and Gorham Advanced Materials). .......... 177
Document 53. Replacement of Chromium Electroplating on Gas Turbine
Engines. ....................................................................................... 177
Document 54. List of Boeing thermal sprayed parts (Courtesy, Boeing
Aircraft Corp). ............................................................................... 180
Page xxv
TABLE OF ACRONYMS
AFRL
Air Force Materials Lab (Dayton, OH)
ALC
Air Logistics Center (Air Force maintenance depot)
AMS
Aircraft Materials Specification (a specification of the
Society of Automotive Engineers)
APS, VPS
Air Plasma Spray, Vacuum Plasma Spray
BFG
B.F. Goodrich
C-HCAT
Canadian Hard Chrome Alternatives Team
Cr
6+
Hexavalent chrome
DARPA
Defense Advanced Research Projects Agency
D-Gun
Detonation gun (also Super D-Gun) – high velocity
thermal spray method based on fuel detonation
(proprietary to Praxair)
DND
Department of National Defence (Canada)
DoD
Department of Defense (US)
DOE
Design of Experiment (statistically designed matrix of
experiments used for process optimization)
EPA
Environmental Protection Agency
ESTCP
Environmental Security Technology Certification
Program (funding HCAT)
GEAE
General Electric Aircraft Engines
GTE
Gas turbine engine
HCAT
Hard Chrome Alternatives Team
HVOF
High Velocity Oxy-Fuel thermal spray
ID
Inside diameter
JG-PP
Joint Group – Pollution Prevention (DoD environmental
group assisting with qualifying clean processes)
JSF
Joint Strike Fighter
JSF IPT
Joint Strike Fighter Integrated Product Team
JTP
Joint Test Protocol
NADEP
Naval Aviation Depot (Navy Maintenance depot)
NAWC
Naval Air Warfare Center
NDCEE
National Defense Center for Environmental Excellence
NRC
National Research Council of Canada
Page xxvi
NTS
National Technical Systems, Inc.
O&R
Overhaul and Repair
OD, ID
Outside diameter, inside diameter
OEM
Original Equipment Manufacturer
OSHA
Occupational Health and Safety Administration
PC
Personal computer
PEWG
Propulsion Environmental Working Group (turbine
engine environmental issues)
PVD
Physical Vapor Deposition (vacuum coating deposition
process)
P&W
Pratt and Whitney
QPL
Qualified Provider List
R&O
Repair and Overhaul
SERDP
Strategic Environmental Research and Development
Program (funding ID chrome replacements)
TPC
Technology Partnerships Canada (funding C-HCAT)
WC
Tungsten Carbide
WC-Co, WC- Cobalt cemented WC (usually WC-17Co or WC-12Co)
CoCr
and cobalt-chrome alloy cemented WC (usually WC10Co4Cr). (Percentages by weight.)
Page xxvii
1.
Introduction
There is currently intense activity in the area of replacing chrome plating
in aircraft, both for original equipment (OEM) use and for overhaul and
repair (O&R) use. The most commonly-used alternative to chrome
plating is thermal spray, which has now replaced chrome plating in many
aircraft OEM and repair applications. While there is a great deal of
information on the performance of thermal spray coatings, it tends to be
scattered across a large number of disparate documents, few of which
are publicly available.
This report brings this data into one place for ready access. Its aim is to
provide the underlying technical data, as well as information on
specifications and qualified components needed by engineers in charge
of component design, coating specification, or process and material
qualification. In its electronic format, it is intended to be a living
document, providing a source of information that can be constantly
updated as new data become available.
The report is split into four parts:
Part 1. Aerospace Usage of Chrome – Types of components and
applications in which hard chrome is currently used in the
aircraft industry.
Part 2 Overview of Thermal Spray – Types and principles of
thermal spray, with emphasis on the primary method used for
chrome replacement, HVOF and APS. This Part includes
information on thermal spray producibility and quality control,
stripping, and finishing.
Part 3. Thermal Spray Data – Compilation of data on structure,
properties, and performance of thermal spray coatings. This
includes hardness, adhesion, corrosion, fatigue, wear,
hydraulic and landing gear rig testing and flight testing.
Part 4. Specifications and Qualified Components – This Part
summarizes the primary specifications used for thermal
spray, as well as the aircraft and components on which
thermal spray coatings are presently qualified.
The report is extensively hyperlinked so that the reader can jump directly
to sections, tables, figures, references, etc. Data and reference materials
are summarized in this document, while the underlying documents, where
they can be made available, may be accessed directly by double-clicking
on the yellow boxes adjacent to their Document Captions.
Page 1
1.1.
"DARPA chrome Final
Report.PDF"
"Lung Cancer Chromium Chemical W
Documents
Document 1. Hard Chrome Coatings - Advanced Technology for
Waste Elimination, DARPA Grant MDA972-93-1-0006, Keith O. Legg,
Jerry Schell, George Nichols, Robert Altkorn. 1
This document is the final report for an initial DARPA-funded program
which showed that HVOF is the most reasonable dry alternative to
chrome plating. HVOF, PVD, and laser clad coatings were tested.
Document 2. Lung Cancer Among Workers in Chromium Chemical
Production, Herman J. Gibb et. al., American Journal of Industrial
Medicine, 38, 115-126 (2000). (Courtesy of the authors and American
Journal of Industrial Medicine.)2
This is the most recent report at time of writing that documents in detail
the lung cancer risks of hexavalent chrome for workers in industries
where they are exposed.
"Clinical findings Chromium among wo
Document 3. Clinical Findings of Irritation Among Chromium
Chemical Production Workers, Herman J. Gibb et. al., American
Journal of Industrial Medicine, 38, 127-131 (2000). (Courtesy of the
authors and American Journal of Industrial Medicine.)3
This study is the most recent report at time of writing that documents
other health effects of hexavalent chrome for workers in industries where
they are exposed.
1.2. Recent data on health effects of
Cr6+
Recently, new data on the health effects of hexavalent chrome have been
developed by the EPA and John Hopkins University. Document 2 and
Document 3 detail the health effects of hexavalent chrome exposure on
worker health in general and incidence of lung cancer in particular.
Document 2 concludes that even the lowest suggested level (0.5 µg
Cr6+/m3) produces a measurable increase in lung cancer rate.
This study is expected to be used by OSHA in lowering the permissible
exposure limit (PEL) from its current 100µg/m3, and may well lead to its
being lowered very significantly – almost certainly into the range 0.5-5
µg/m3, and quite possibly into the lower part of that range. The effect of
the lowest limit would be a large increase in cost associated with
providing adequate worker protection, while the increased liability risks
would be likely to drive many vendors out of the chrome plating business.
No matter what the details of the final outcome, there is every reason to
believe that the environmental and health pressures on chrome plating
will increase in the coming years, and that the move toward chrome
alternatives will accelerate.
Page 2
1.3. Progress in chrome replacement
The initial reason for considering alternatives to chrome plate was the
increasing regulation of chrome (and especially hexavalent chrome)
processes. This regulation was designed to combat the environmental
and worker health and safety problems inherent in the use of Cr6+ in
plating baths, as well as chrome generation in stripping and grinding
operations. However, as our experience with alternatives has grown and
more data has become available, users are increasingly adopting chrome
alternatives because they nearly always have better performance, and
frequently have lower cost.
Thermal spray coating is the principal technology that has long been used
for coating high performance gas turbine engine components (See
Document 49, for example). As a result the engine industry has many
years of experience with thermal spray processes, such as plasma spray
and High Velocity Oxy-Fuel (HVOF), which have been qualified on
hundreds, if not thousands, of turbine engine components. Over the
past 5 years or so, thermal spray coatings have been qualified for
numerous airframe components. In this case, the primary replacement
for chrome plating is HVOF, which is now being used in the manufacture
and repair of aircraft landing gear. Some examples of the use of thermal
spray on airframe components include:
•
Thermal spray coatings (primarily HVOF) are now qualified and
used on over 100 airframe components made by Boeing (see
Section 15.4).
•
The landing gear on the new Boeing 767-400 is now specified for
HVOF or chrome, depending on customer wishes. Several
airlines now require the HVOF version. (See Section 15.5.1.)
•
Boeing has specified HVOF tungsten carbide coatings as a
qualified replacement for chrome plating for overhaul and repair of
landing gear (see Section 15.5.4).
•
HVOF tungsten carbide coatings are used for new flap and slat
tracks, and are now qualified and widely used for repair of older
tracks (see Section 15.7).
Qualification of thermal spray coatings to replace chrome plating is now
the subject of extensive laboratory, rig, and flight testing in the defense
and commercial sectors.
Hard Chrome Alternatives Team (HCAT) – This binational integrated
team is the primary program for chrome replacement in the Department of
Defense (HCAT in the US) and the landing gear industry (C-HCAT in
Canada). The team comprises members from the aircraft industry in the
US and Canada, military depots, DoD (US) and DND (Canada) offices,
Industry Canada, various and laboratories. This program is run by Bruce
Sartwell of the Naval Research Laboratory and is funded by the
Environmental Security Technology Certification Program (ESTCP) and
other DoD organizations in the US, and by Technology Partnerships
Canada (TPC), the Canadian Department of National Defence (DND),
Page 3
and the landing gear makers (BF Goodrich, Messier-Dowty, and Heroux)
in Canada.
The program is validating thermal spray coatings (primarily HVOF WC-Co
and WC-CoCr) for chrome replacement on landing gear, propeller hubs,
and helicopter head components. A PEWG/HCAT program is qualifying
thermal spray chrome replacements for gas turbine engine overhaul,
while an HCAT/JG-PP program is qualifying HVOF for aircraft hydraulics.
The HCAT team includes major aerospace manufacturers, overhaul and
repair companies, thermal spray companies, and DoD repair depots.
Team members are shown in Table 1.
Table 1. Hard Chrome Alternatives Team members (full list
available on HCAT web site).
B.F. Goodrich
Messier-Dowty
PEWG
Boeing Aircraft Corp
Metcut Research
Pratt and Whitney
Corpus Christi Army
Depot
NADEP Cherry Point
Praxair Surface
Technologies
Delta Airlines
NADEP Jacksonville
QuesTek Innovations
Engelhard Surface
Technologies
NADEP North Island
Rolls Royce
GE Aircraft Engines
NTS (McClellan)
Rowan Technology
Group
Green-Tweed and Co.
National Research
Council (Canada)
Southwest Aeroservice
Hamilton-Sundstrand
Naval Research Lab
Sulzer Metco
Heroux
NAWC PAX
Technology
Partnerships Canada
Industry Canada
OC_ALC
Vac Aero
JG-PP
OO-ALC
Westaim Corp
Lockheed Martin
Orenda Aerospace
Boeing – Boeing has been introducing HVOF and D-gun coatings on
airframe components for several years and now has over 100 parts
specified for thermal spray coatings (see Section 15.4). These
components include slat tracks, landing gear, and pins. The new Boeing
767-400 aircraft now has HVOF WC-CoCr specified for its landing gear
axles and inner cylinders (Section 15.5.1).
Boeing has now approved HVOF coatings in place of chrome as a repair
procedure for landing gear (Section 15.5.4).
Other manufacturers – Thermal spray coatings have been approved by
other manufacturers, including Sikorsky (helicopter landing gear,
Page 4
actuators), Hamilton Sundstrand (actuators), Bombardier (flap tracks),
Messier-Dowty (landing gear), and Parker-Hannifin (actuators) (See
Section 15).
Airlines – Delta Airlines has been flight testing HVOF coatings on landing
gear of Boeing 737, 757, and 767 aircraft, and has now qualified HVOF
WC-CoCr coatings for overhaul and repair in place of chrome on a
number of parts. Lufthansa has also flight tested and qualified HVOF
WC-CoCr coatings for use on landing gear. (See Section 13.3.2 and
13.3.1.) United Airlines has been using HVOF coatings in place of
chrome for some years (Section 15.8).
The use of thermal spray in place of chrome for OEM parts and for repair
is therefore spreading from its initial use in gas turbine engines to
components throughout the aircraft.
Page 5
PART 1. AEROSPACE USAGE OF CHROME
2.
Typical Chrome Plated Components
Hard chrome plating is used in many areas of new, Original Equipment
Manufacturer (OEM) components, as well as for many rebuild
applications.
2.1. New equipment usage
Table 2. Some typical OEM chrome plated components.
System
Component
Notes
Landing gear
Inner cylinder OD
Dynamic seal
Outer cylinder ID
Thin dense Cr or flash Cr often used
for IDs
Uplock and downlock
hydraulics
Axles
Pins
High-load rotation
Lug faces
Hydraulic actuators
Rods
Dynamic seal
Outer cylinder ID
Turbine engines
Pins
Mostly OD, some ID
Power shafts
Wear and press-fits
Bearing holders
Press-fits
Seals
Actuators
Gears
Propeller/rotor
Not gear teeth
Propeller hubs
Rotor head components
Gears
Not active profile of gear teeth
Shafts
Dampers
Hydraulic rods and outer cylinder IDs
Page 6
Part 1 – Aerospace Usage of Chrome
Hard chrome is used wherever wear is known or expected to be a
problem. Table 2 shows some of the many aircraft components that are
typically chrome plated.
The primary OEM usages of chrome plating are landing gear components
and hydraulic actuators. These account for the largest chrome plated
areas. Most OEM applications are for areas that are subject to wear.
However, some applications, especially on shafts and bearing holders,
use chrome to provide a press-fit interface that will minimize galling on
assembly. Other applications are for locations where high precision is
required and a ground chrome surface is used to provide a better surface
finish or more accurate dimensions. OEMs also use chrome plating for
restoring dimensions on mismachined parts.
OEM usage is typically quite thin – 0.003” is common. Where corrosion
resistance is needed and wear is not a serious issue, thin dense chrome
(typically 0.0003” thick) is commonly used. Thin dense chrome is
frequently used, for example, on hydraulic outer cylinder IDs.
2.2. Overhaul and repair usage
During overhaul and repair chrome is frequently replaced on originallychromed areas that have been worn, pitted, or are otherwise out-ofspecification. Hard chrome is also used for general rebuild of many
components that may be worn or damaged, but were never originally
chromed. Most, if not all, DoD maintenance depots and aircraft O&R
shops are equipped with hard chrome plating tanks.
Rebuild usage is typically thicker than OEM usage – 0.010” – 0.020”
being quite common. Both externals and internals may be plated,
although ID plating is a more specialized process that is often contracted
out.
2.3. Landing gear components
Landing gear are primarily made of 300M high strength steel, Aermet 100
steel, and in some cases aluminum or titanium alloys.
Page 7
Figure 1. Boeing 737 nose landing gear inner cylinder.
Shiny areas are chromed - piston and four axle
journals (Courtesy Sulzer Metco).
Landing gear inner cylinders are the largest aircraft components
commonly chrome plated. The Boeing 737 landing gear inner cylinder of
Figure 1 is relatively small – about 24” high and wide.
The Boeing 767 main landing gear, on the other hand, is far larger, with
Figure 2. Boeing 767 main landing gear (Courtesy
Sulzer Metco).
four sets of wheels on two removable axles, each almost 60” long (Figure
2). As with other landing gear, the axle journals and the piston are
chrome plated.
Page 8
Figure 3. F-18 main landing gear Oleo Attach
Pin (Courtesy Boeing).
Items such as pins are much smaller – generally 1-2” in diameter, and a
few inches long. These pins link hydraulic actuators to landing gear and
airframe attachment points. Most are coated on the outside, while some
are ID coated to reduce corrosion and wear from end caps.
2.4.
Hydraulic actuators
Since hydraulic actuators are used throughout the aircraft, they constitute
the second most important application for aerospace chrome plating.
Almost all actuator rods are chrome plated, while on the actuator shown
in Figure 4 the bore of the outer cylinder is also chromed.
2.5”
5.7”
Figure 4. F/A-18 E/F aileron servocylinder, manufactured by HR
Textron (Courtesy Boeing).
Chrome plating prevents wear of the metal by the seals and also serves
to hold hydraulic fluid to lubricate the seals and reduce seal wear.
Page 9
3.
Chrome replacement options and
requirements
The requirements for OEM and O&R chrome replacement are somewhat
different. One of the primary differences, although difficult to quantify, is
that at the OEM the processing environment is quite well defined and
there is a limited number of different components to be plated, whereas in
O&R operations many different components must be processed, each
with its own unique problems resulting from its field history. Some of the
major differences are summarized in Table 3.
Table 3. Some differences between OEM and O&R chrome replacement.
OEM
O&R
Limited numbers of different components,
processed in significant quantities on a
regular basis. Standard production lines
Many different components, sporadic work
loads
New substrate material; only new coatings
need be stripped in case of processing
errors
Old, dirty substrates; old coating must be
stripped prior to recoat. Coating must
withstand component cleaning and
servicing
Coating thickness typically 0.0003” (thin
dense chrome) to 0.004”
Coating thickness up to 0.020” as-coated,
0.010” finished, for rebuild
Approvals from OEM engineers
Approvals from OEMs, NAVAIR, ATCOM,
Single Item Managers, Program Managers
One issue to consider very carefully is whether a process for OEM can
also be used for O&R. Although in principle there is no reason that O&R
processes cannot be different from OEM processes, it makes validation
and acceptance doubly expensive and time-consuming if two different
processes must be validated, rather than a single one accepted by
OEMs, DoD stakeholders, and depots alike. This is an especially
important issue in view of the wide range of coating requirements for
chrome replacement, from 0.0003” to 0.015” thick. Some processes are
good for OEM use but cannot be used for rebuild, while others cannot
reliably deposit a coating thinner than about 0.001”.
Since most hard chrome applications are 0.003” or more in thickness, the
requirements for thin dense chrome and flash chrome are not critical for
the vast majority of cases. Inability to replace these specialized thin
chrome coatings is not a critical issue in a general hard chrome
replacement since they are not strictly hard chrome applications.
Page 10
3.1. Hard chrome replacement criteria
The following is drawn from the Brooks AFB Statement of Need
describing the basic requirements for a chrome replacement4:
“A coating process (or processes) is needed that will meet the
requirements of chromium without the environmental and health hazards
associated with chromic acid. Ideally, the process would not use any EPA
17 chemicals. The process must not cause hydrogen embrittlement.
Fatigue loss should be no worse than electro-deposited chromium. The
deposit should exhibit adhesion to steel equivalent to electro-deposited
chromium. The deposit must be machinable or grindable to produce
surface finishes of approximately 8 rms. The deposit must be easily
strippable. Good corrosion protection would be a plus. The finished
surface must have low friction characteristics and must not gall. The
process should be relatively easy to control. It should not require large
amounts of capital to install and should fit into existing space.”
Replacing hard chrome involves a great deal more than meeting the
technical requirements for wear, corrosion, fatigue, etc. To be viable in
the aerospace community the replacement must fit into the way the
industry works at both the OEM and O&R level (commercial shops and
military depots). Technical performance cannot be considered in isolation
from other technical issues such as stripping and finishing, from
environmental and safety issues, from issues of complexity and cost, or
from the more “political” issues of acceptance and validation. The most
important criteria that an alternative must meet to replace chrome
successfully on IDs are summarized in Table 4. There are many more
detailed issues, but these will in general be different for each different
application.
The Hard Chrome Alternatives (HCAT) team started out with the
approach that, to be viable, a hard chrome alternative must meet the
same performance standards as hard chrome in all critical areas (as we
have indicated in Table 4). As a practical matter, however, the team has
found that the alternative must exceed the performance of hard chrome.
If it does not, there is no strong driver to specify the replacement, since all
changes of this type involve both cost and risk. In general, environmental
drivers are very weak, especially in an industry as complex as aerospace,
where responsibility for change is diffused and decision makers are often
not directly affected by their decisions. However an alternative will be
strongly and rapidly embraced by engineers and other stakeholders if the
replacement provides a clear technical and cost benefit, and especially if
it provides some critical capability that chrome lacks.
Page 11
Table 4. Hard chrome replacement criteria.
Issue
Criteria
Notes
ID coating requirements
Coating thickness
OEM: 0.003”
OEM thin dense Cr: 0.0003”
Note: Widely different OEM and O&R
needs
O&R: 0.003-0.015”
Smoothness
16µ” Ra typical, some
replacements may need to be 4µ”
Ra
Note: Thermal spray coatings generally
need to be smoother than Cr for the same
application.
High strength steels: <250C
Aluminum alloys: <150C
Critical issue is time-at-temperature.
Overheating may cause fatigue reduction
due to changed surface microstructure.
Wear resistance and
hardness
Match performance of chrome on
actual components. (Cr is 8001000HV)
Critical issue is wear life (wear rate x
thickness) in service, and avoidance of seal
wear in hydraulics
Hydrogen
embrittlement
None (Note: Cr requires bakeout
for embrittlement relief)
This is a critical flight safety issue
Corrosion resistance
Must match chrome - primarily
B117 salt fog
Microcracks make chrome a poor corrosion
inhibitor – good corrosion resistance
requires sealer or Ni underlay.
Fatigue
Fatigue debit must not exceed
chrome
Navy particularly concerned with NaCl and
SO2 atmospheres for corrosion and fatigue
- critical flight safety issue.
Process must be stable
Both OEM and O&R environments
Within day-to-day operating
parameters
Simple, reasonable QC needed
Total production cost comparable
to chrome.
Production cost needs to include cleaning,
masking, finishing, heat treating, waste
disposal, etc.
Deposition temperature
Technical issues
Producibility
Reproducibility
Process window
Cost
Life-cycle cost < chrome
Reasonable capital cost
OEM and O&R fit
Stripping
Must be able to be stripped - safe
chemicals, water jet, etc
Strippability is crucial to O&R.
Must withstand O&R cleaning,
chemicals, hydraulic fluid, etc.
Must not deteriorate when put through O&R
process, including plating
Must be environmentally benign
and safe for workers
Note that O&R operations are more diverse
and less easily controlled
Specifications
AMS and/or aircraft company
specifications needed
Cannot be specified and put on drawings
without specs.
Proprietary technology
Cannot be proprietary to one
company
If possible, should be able to be done at
general O&R site to avoid outsourcing.
Field and O&R
chemical stability
Environment/safety
Acceptance issues
Page 12
3.2. Thermal spray for hard chrome
replacement
Thermal spray in general, and HVOF in particular, is being accepted in
the aerospace industry as the most widely-used chrome alternative for a
number of reasons:
•
Thermal spray coatings are already widely used in the industry for
turbine engines – as a result aerospace engineers are familiar
with the technology, and most O&R shops and DoD repair depots
are equipped with some kind of thermal spray.
•
Thermal spray is a relatively simple technique that avoids the
complications of vacuum coatings.
•
Thermal spray processes are environmentally benign. They do,
however generate particulates in the overspray (material that does
not melt properly and bounces off the substrate), which are caught
in a standard bag-house dust collector.
•
The HVOF method is not exceptionally capital-intensive,
especially for a shop already set up for other types of thermal
spray.
•
HVOF is a high-quality coating that can be made with very low
porosity and can be machined, ground, and superfinished as
necessary without chipping or delaminating.
•
The main process limitation is that HVOF cannot be used on deep
IDs (such as hydraulic or landing gear outer cylinders), except the
very largest in normal use (about 11” ID). Plasma spray can be
used down to 3” ID, and perhaps smaller, but the material quality
is not as good. These methods are under development.5
HVOF thermal spray is now being used (both on OEM equipment and for
O&R), on landing gear, flap and slat tracks, hydraulic actuators, and an
increasing number of components. Users include airframers, landing
gear manufacturers, airlines, and repair shop
Page 13
PART 2. OVERVIEW OF THERMAL SPRAY
Thermal spray is a coating process that is widely used in the aerospace
industry. Its advantages are that it is clean, very flexible, can be done in
air, and can be used both for OEM and for O&R, since it can produce
coatings from 0.003” typically for original equipment, and 0.020” or more
for rebuilding worn parts.
4.
Principles of thermal spray
4.1. Summary
This section is intended to be introductory, providing a quick overview of
thermal spray, with details provided in subsequent sections. We describe
•
the basic principles of thermal spray,
•
the types of thermal spray equipment,
•
what controls the properties and performance of thermal spray
coatings, and
•
the limitations of the process.
The thermal spray process applies coatings to the substrate via a
combination of thermal and kinetic energy, which makes the particles
soften and “splat” onto the surface. Many thermal spray processes exist
to produce a dense structure that will meet the needs of a wide variety of
applications including wear, corrosion, erosion, electrical conductivity, etc.
Since thermal spray coatings can vary in thickness from 0.003” to
>0.020”, the method can be used both for new OEM components and for
rebuilding worn or damaged items.
Emphasis is placed upon the carbide and metallic coatings since
these materials are the primary thermal spray alternatives for many
of the current chrome applications.
4.2.
Documents
Document 4. JSF Phase 1 Report: Chrome Replacements for
Internals and Small Parts. (Rowan Technology Group).6
"JSF Final ID Report
Phase 1.pdf"
This document is a technology analysis carried out for the Joint Striker
Fighter Program office evaluating the various options for chrome plating
in internal replacement diameters and heat-sensitive items.
Page 14
Part 2 – Overview of Thermal Spray
Technologies for Internal Diameters and Heat-Sensitive Parts.
(Rowan Technology Group).7
"JSF Final ID Report
Phase 2.pdf"
This report evaluates the critical issues and costs for bringing the best
technologies found in Phase 1 to production in aircraft.
4.3. General
Figure 5. Thermal spray process schematic (left); close-up view of
surface (right).8
Thermal spray is a widely-used coating process in which materials
ranging from metals and ceramics to plastics are melted into individual
droplets and fired onto the substrate being sprayed to form a final coating
structure. The thermal spray coating technique is a “line of sight”
procedure, meaning that the coating material is deposited only on the
surfaces directly open to the spray stream. The coating “overlays” the
substrate and is “mechanically “ bonded to the part. To ensure a good
bond the surface is usually roughened by grit blasting before spraying.
There are three major components to the thermal spray process:
1. Coating material in either powder or wire form.
2. Kinetic energy – used to accelerate the particles to high speed,
usually by injecting them into a high-velocity gas stream.
3. Thermal energy – usually an electric arc plasma or a high
temperature flame, to soften or melt the coating material.
Page 15
As illustrated in Figure 5, the molten particles individually “splat” against
the substrate via this combination of thermal/kinetic energy, resulting in a
solidified surface deposit.
4.4.
Thermal spray processes
As with other manufacturing processes, there are many techniques to
achieve the final coating structure. Many people commonly interchange
the use of plasma spray and thermal spray; however, plasma spray is
only one type of thermal spray. Broadly, thermal spray technologies are
categorized by the heat source:
•
Combustion (flame or detonation)
•
Plasma (intense electric discharge)
•
Electric Wire Arc (sparking between wires)
Figure 6 illustrates the typical commercial processes, broken down under
these broad categories.
Figure 6. Types of thermal spray processes. Types covered in this
report shown in green.8
Page 16
Table 5. Typical characteristics of thermal spray coating processes.
Flame Spray
HVOF*
D-Gun**
Wire Arc
Air Plasma
(Combustion)
(Combustion)
(Combustion)
Jet Temp. (K)
3500
5500
5500
>25000
15000
Jet Velocities
50-100
500-1200
>1000
50-100
300-1000
Gas Flow (slm)
100-0200
400-1100
N/A
500-3000
100-200
Gas Types
02, Acetylene
CH4 ,C3 H6
,H2, O2
02,
Acetylene
Air, N2, Ar
He, N2, Ar,
H2
Power Input
20
150-300
N/A
2-5
40-200
Particle temp.
max, (deg C)
2500
3300
N/A
>3800
>3800
Particle
velocities,
50-100
200-1000
N/A
50-100
200-800
30-50
15-50
N/A
50-100
200-800
Density Range
85-90
95-98
95-98
80-95
90-95
Bond Strength
7-18
82
82
10-40
40-68
high
small
small
Moderate
to high
Moderate
to coarse
Attribute
Gas Jet
(m/s)
(kW equiv)
Particle Feed
(m/s)
Material feed
rate,
(g/min)
Deposit/Coating
(MPa)
Oxides
* High Velocity Oxy-Fuel
** Detonation Gun-A proprietary process from Praxair
Table 5 summarizes the typical characteristics of the main types of
thermal spray coatings.
Page 17
4.5. Factors determining coating
properties
When evaluating a coating, the characteristics can primarily be divided
into two major categories:
a) Properties and structure of the material, and
b) How the coating affects the performance of the component
Figure 7 illustrates a typical structure of a plasma spray coating. All
coatings will possess theses characteristics to varying degrees, which
affect final coating performance.
The specification for a thermal spray coating usually defines, among other
Figure 7. Structure of Thermal Spray Deposit at 100-500X.8
things, the coating method to be used, and the allowable number of
imperfections such as porosity, oxide particles, and unmelts (see Table
6).
The quality control of coating structure is monitored by tests such as
metallography, macro/microhardness, and tensile testing, to name a few.
However, care must be taken with these evaluations since the evaluator
is analyzing a composite structure (coating/substrate) in lieu of just a
metallic coupon, and response to testing can change dramatically with
different combinations of substrate and coating material. Careful control
of QC procedures is necessary for consistent and repeatable results.
Page 18
Table 6. Some fundamental terms that define the quality of thermal
spray coatings.
Coating Characteristic
Cause and Background
Splat
Molten particles that have hit the surface and
solidified as elongated shapes parallel to the
substrate (the ideal form of deposition).
Porosity (Voids)
With individual particles “splatting’ as irregular
shapes in the deposit, porosity or voids may
be formed on solidification if the thermal and
kinetic energy are not sufficient to minimize
this effect.
Oxide
With many thermal spray processes
conducted in air, oxides will form as the molten
particles travel to the substrate.
Unmelt
Dependent upon the process used, not all
particles will see the same heat input and
therefore some will not have enough energy to
splat into an elongated shape and will instead
retain the shape of the starting stock.
Layer lines
Dependent upon the coating material, lines
between splats will be evident.
4.6. Applications of common thermal
spray coatings
Performance measures how the coated system (component, with its
particular substrate material, plus coating) performs in various
environments. Performance in turn determines what applications the
system is suited to. With the wide variety of materials that can be
sprayed by thermal spray techniques, the applications for these coatings
are widespread. Some typical applications and performance
characteristics of some coating materials are shown in Table 7.
Page 19
Materials can range from pure metals/alloys to cermets, carbides and
plastics. Because the starting material is usually in powder or wire form,
it can usually be formulated to meet the needs of general applications or
of special requirements if so desired.
Table 7. Some common thermal spray coatings, their structure,
performance, and applications.
Coating
Structure and
Performance
Characteristics
Nickel Graphite
CompositePowder
Structure
Metallography
Hardness
Tensile
Abradable Coating
(NiG)
Structure
Metallography
Hardness
Wear coating
Performance
Erosion
Corrosion
Tungsten Carbide
Cobalt Powder
Wear Coating
Structure
Metallography
Hardness
(WC-Co)
Performance
Erosion
Corrosion
Zinc Wire
Structure
Metallography
Performance
Corrosion
4.7.
Sprayed on cowling parts to
allow blades to “cut into”
coating and form seal,
preventing air bypass.
Performance
Erosion
Abradability
Chrome Carbide
Nickel Chrome
Powder
(Cr3C2 Ni Cr)
Application
Sprayed in areas on pump
housings where high velocity
flow causes erosion and
corrosion.
Sprayed on blade tips to resist
erosion of material being
ingested as plane goes down
runway.
Used for protection of iron and
steel against corrosion in
fresh/salt water.
Limitations of thermal spray
As with any process, limitations exist for application and usage. The
major limitations of thermal spray are:
•
Thermal spray is a line-of-sight process
•
Substrate heating by the thermal spray process
•
Thickness (maximum and minimum).
Page 20
4.7.1.
Line of sight issues
The issues of line-of sight processing are covered at length in two prior
reports for the Joint Strike Fighter Program Office.
Document 4. JSF Phase 1 Report: Chrome Replacements for Internals
and Small Parts. (Rowan Technology Group)
Technologies for Internal Diameters and Heat-Sensitive Parts. (Rowan
Technology Group
The area to be sprayed must be accessible to the thermal spray gun,
which can limit ID applications or use in tight areas. This leaves two
options:
1. Use an ID gun, able to reach into the ID and deposit directly onto
the wall
•
HVOF guns cannot be used inside an ID < 11” diameter, which
is the diameter of the largest landing gear outer cylinders
•
Plasma spray guns can be used down to 3” ID, although the
coating quality is generally inferior to HVOF
•
Some miniature plasma guns can be used down to 1.5 – 2” ID,
but these are less well characterized.
2. Spray from outside the hole
•
4.7.2.
The general rule of thumb is that a hole can be sprayed from
outside if the ratio depth/diameter < 1, which corresponds to
an impact angle of no more than 45° from the vertical. It is
possible, in some cases, to obtain good coatings at 60° from
the vertical, while in other cases the properties of the coatings
diminish at 20° from the vertical.
Heating issues
Since the thermal spray process incorporates both thermal and kinetic
energy, the part being sprayed will experience some temperature rise,
especially at the surface of the part. With the temperatures of the
particles shown in Table 5, temperatures can be reached that will
degrade the properties of the substrate – especially fatigue properties.
The solution to overheating is generally to apply sufficient cooling air (by
air jets surrounding the component, and proper matching of the rotation of
the component and movement of the gun to prevent the gun spending too
long on any area. This issue is discussed at some depth in Section 7.3.6.
4.7.3.
Coating thickness
There is a practical lower limit to the coating thickness, which is set by the
size of the particles and the requirement for a continuous coating. The
minimum thickness possible as a practical matter is about 10 splat
Page 21
thicknesses, or 0.001” (25µm). This is because the size of most carbide
particles is about 2.5µm (0.0001”), while typical 25 - 60µm spray particles
tend to splat into a ratio of about 10:1 when they hit the substrate. For
coatings that are to be ground, it is found that a minimum thickness to
avoid spalling during grinding is about 0.003” (see Section 13.3.2).
In principle there is no upper thickness, provided the coating is not
deposited under conditions that that build up a high level of stress with
thickness. Boeing has specified an upper thickness limit for landing gear
repair by HVOF tungsten carbide as 0.010” (see Section 15.5.4).
However, much thicker repairs have been developed, either with a single
coating, or a combination of metallic build-up with WC cermet hard
overcoating (Section 15.5).
Page 22
5.
Thermal spray coatings
5.1. Summary
This section provides an overview of the general materials, properties,
performance, and usage of thermal spray coatings. It is intended to
provide an overview of the general applications of thermal spray coatings,
and the materials used to produce them. The most common coating
materials for chrome replacement are carbides (such as cobalt-cemented
tungsten carbide, WC-Co, which is the same material as that commonly
used in carbide cutters and inserts) and alloys, such as Tribaloy.
A critical requirement for high quality coatings is the proper definition of
the starting material (i.e. the powder in plasma, HVOF, and D-gun
processes). Examples are given of typical powders.
5.2.
Thermal spray materials
5.2.1.
General
A wide variety of materials is used in the application of thermal spray
powders. The general classifications are shown in Table 8. The starting
stock is a critical part of the thermal spray process. Some important
characteristics of the starting stock are summarized in Table 9.
Page 23
Table 8. Some major thermal spray powder classifications.
Description
Abradables and Plastics
•
Aluminum Base
Abradables
•
Cobalt Base Abradables
•
Copper Base
Abradables
•
Nickel Base Abradables
•
Plastics
Pure Metals, Alloys, Cermets,
Composites, and Blends
• Pure Metals
• Aluminum Base
Powders
• Cobalt Base Powders
• Copper Base Powders
• Iron Base Powders
• Molydenum Base
Powders
• Nickel Base Powders
• Cermet Powders
Carbides
•
Chromium Carbides
•
Tungsten Carbide
Ceramics
•
Aluminum Oxide Base
Powders
•
Chromium Oxide Base
Powders
•
Titanium Oxide Base
Powders
•
Zirconium Oxide Base
Powders
HVOF
(High Velocity OxyFuel)
Thermal Spray Wires(Arc and
Combustion)
Uses
Application Methods
Abradables provide clearance control
in high-speed applications where nearzero clearances between moving parts
are required. Typically used by jet
engine manufacturers to improve
engine performance by reducing the
clearance between the blades of the
compressor and the surrounding
casing.
For surface enhancement, corrosion,
oxidation and abrasion resistance,
bestowing electrical
conductivity/shielding characteristics,
superalloy repair and everything else in
between.
Selected primarily for their wear
resistance, abrasion resistance and
erosion resistance, coatings of these
materials are especially suitable for use
in many harsh environments.
Ceramic coatings exhibit properties
such as wear resistance, high dielectric
strength, hot corrosion resistance,
chemical resistance and even
bioactivity
Air plasma and flame
spray
Air plasma, flame
spray, and HVOF
Plasma spray and
HVOF
Plasma spray
These materials are properly sized for
High Velocity Oxy-Fuel spray. For
applying dense, strongly bonded
coatings with reduced oxides, porosity
and excellent wear resistance
properties
HVOF
Used for a broad range of corrosion
control and machine element repair
applications, these wires are specially
manufactured to tight tolerances for
thermal spraying. Used particularly for
on site applications.
Electric Arc
These characteristics must be considered when choosing the appropriate
application technique to maximize production and ease of producibility.
Specific examples of how this information is supplied for varied material
Page 24
types is shown in the next sections on carbides and pure metals
Table 9. Important parameters defining thermal spray powders and
electric arc wire.
Characteristic
Description
Form
Wire/Powder
Size
Wire-diameter
Powder-size distribution of particles
from fine to coarse sizes
Shape
Manufacturing Method
Surface Finish
Physical properties
Spherical, nears spherical, and
regularly shaped powders with
smooth surfaces (preferred) vs.
non-spherical, irregular and “cuspy”
particles where surface
irregularities exist
Water atomized, gas atomized,
sintered and crushed,
agglomerated, etc.
Characteristics for feeding powder
or melting wire
Melting temperature, density,
composition, thermal properties,
flow characteristics, etc.
The following are links to the web sites of some of the major
manufacturers of thermal spray materials:
PRAXAIR-Thermal Spray: Powders
TAFA-Thermal Spray Powders
Sulzer Metco Wire and Powder Product Portfolio
Powder Alloy Corporation-Powders
Stellite Powders
Two material classes have been chosen to illustrate materials,
applications, and general properties: Tungsten Carbides and Pure
Metals.
"Common powder
types.pdf"
Document 6. Common thermal spray powder types.9
Examples of different types of carbide and metal powders are given in
this document.
Page 25
5.2.2.
Powders frequently used for chrome replacement
Carbides are a major material type as listed in Table 8 and can be further
subdivided into categories defining the particular carbide phase such as
tungsten or chrome carbides. The most common application of these
powders for wear resistance, which is why tungsten carbides are
commonly used to replace chrome. Examples of the most common
powders used in chrome replacement are summarized in
Table 10. Examples of thermal spray powder used in chrome replacement
operations.
Powder type
Manufacturer,
product
Powder size
Comments
WC-17Co
Sulzer Metco
Diamalloy 2005
-53 +11µm
Specified for BMS 10-67
Type 1
WC-12Co
Praxair WC 106
-45 +5µm
BMS 10-67
WC-10Co4Cr
Sulzer Metco 5847
-53 +11µm
Chosen by C-HCAT for
chrome replacement
(Section 7.4.3)
Tribaloy 400,
800
Stellite
Softer than Cr, but better
wear in hydraulics (Section
13.2.1), useful on Al alloys
(Section 12)
Ni5Al
Metco 450NS
Used for build-up
Note: This is a very limited listing. See links in Section 5.2.1 for more details.
An example of how data is supplied for such a powder is shown in Figure
8. Note that these powders are generally referred to by the type of
carbide powder (tungsten carbide, WC), the type of binder metal holding
the carbide grains together (cobalt), and the percentage by weight of the
constituents. Hence, with 12% by weight of Co, this material is referred to
as WC-12Co or 88/12 WC-Co. Powder size is given in mesh size (lines
per inch) or microns. Hall Flow is a measure of how well the powder
flows, measured by a Hall flowmeter funnel according to ASTM B855-94.
Page 26
TYPICAL POWDER PROPERTIES
Chemical Composition
Cobalt (Co) …………………………11.0 to 13.0%
Tungsten Carbide (WC) ………….…..Remainder
Sieve Analysis
-325 Mesh + 10 µm ………………(-45µm+10µm)
Melting Temperatures
Cobalt (Co) ……………………..5031°F (2777°C)
Tungsten Carbide (WC) ………2723°F (1495°C)
Hall Flow
16 grams per minute
Apparent Density
0.177 lbs/cu in ………………….……….(4.9g/cc)
Starting powder at 500X
Figure 8. Example of powder definition for sintered irregularly shaped 88/12
Tungsten Carbide Cobalt powder.
Even for the same chemical composition, powder can come in a variety of
different types (depending on how it was made) and sizes. This makes it
necessary either to define the allowable powder characteristics to be sure
of reliable coatings, or to require that a consistent powder be used to
ensure reproducibility between test items and production items.
5.3. Typical structural properties of
thermal spray coatings
With the wide variety of materials available, the applications for thermal
spray coatings run across many industries and applications.
Two material classes (carbides and pure metals) have been highlighted
thus far, and applications will be summarized detailing the critical
properties of each material. Table 11 and Table 12 give a general
summary of properties and process expectations with varied spray
Page 27
techniques and coating materials.
Table 11 Comparison of thermal spray coating processes –
general properties.
Wire
Flame
Spray
Coating Type
Powder
Flame
Spray
Electric
Wire Arc
Spray
(1)
HVOF
Spray
3
Plasma
Spray
3
Gas Temperature 10 ºC (10 ºF)
3
(5.4)
All Coatings
3
(5.4)
2-3
(3.6 - 5.4)
N/A
12 - 16
(21.6 - 28.8)
3
Bond Strength MPa (10 psi)
Ferrous metals
14 - 28
(2 - 4)
14 - 21
(2 - 3)
48 - 62
(7 - 9)
28 - 41
(4 - 6)
22 - 34
(3 - 5)
Non-ferrous metals
7 - 34
(1 - 5)
7 - 34
(1 - 5)
48 - 62
(7 - 9)
14 - 28
(2 - 7)
14 - 28
(2 - 7)
Self-fluxing alloys
---
83+ (12+)
62 (9)**
---
---
Ceramics
---
14 - 32
(2 - 5)
---
---
21 - 41
(3 - 6)
Carbides
---
34 - 48
(5 - 7)
83+
(12+)
---
55 - 69
(8 - 10)
Density, % of equivalent wrought material
Ferrous metals
85 - 90
85 - 90
95 - 98+
85 - 95
90 - 95
Non-ferrous metals
85 - 90
85 - 90
95 - 98+
85 - 95
90 - 95
Self-fluxing alloys
---
100*
98+**
---
---
Ceramics
---
90 - 95
---
---
90 - 95+
Carbides
---
85 - 90
95 - 98+
---
90 - 95+
Hardness
Ferrous metals
84Rb-35Rc
80Rb-35Rc
90Rb-45Rc
85Rb-40Rc
80Rb-40Rc
Non-ferrous metals
95Rh-40Rc
30Rh-20Rc
100Rh-55Rc
40Rh-35Rc
40Rh-50Rc
Self-fluxing alloys
---
30 - 60Rc
50 - 60Rc
---
---
Ceramics
---
40 - 65Rc
---
---
45 - 65Rc
Carbides
---
45 - 55Rc
55 - 72Rc
---
50 - 65Rc
Page 28
Table 12. Comparison of thermal spray coating processes permeability, thickness.
Wire
Flame
Spray
Coating Type
Powder
Flame
Spray
(1)
HVOF
Spray
Electric
Wire Arc
Spray
Plasma
Spray
Permeability
Ferrous metals
High
Medium
Negligible
Medium
Medium
Non-ferrous metals
High
Medium
Negligible
Medium
Medium
Self-fluxing alloys
---
None*
Negligible*
---
---
Ceramics
---
Medium
---
---
Low-Medium
Carbides
---
Medium
Negligible
---
Low-Medium
Coating Thickness Limitation mm (in.)
Ferrous metals
0.5 - 2.0
0.5 - 2.0
0.6 - 2.5
(0.02 - 0.08) (0.02 - 0.08) (0.025 - 0.1)
0.5 - 2.5
0.4 - 2.5
(0.02 - 0.1) (0.015 - 0.1)
Non-ferrous metals
0.5 - 2.0
0.5 - 2.0
0.6 - 2.5
(0.02 - 0.08) (0.02 - 0.08) (0.025 - 0.1)
0.5 - 2.5
0.4 - 2.5
(0.02 - 0.1) (0.015 - 0.1)
Self-fluxing alloys
---
0.4 - 2.5
0.4 - 3.8
(0.02 - 0.2) (0.015-0.15)
---
---
Ceramics
---
0.4 - 0.8
(0.015 - 0.1)
---
(0.4 - 5.0
(0.015 - 0.2)
Carbides
---
0.4 - 0.8
0.4 - 5.0+
(0.015 - 0.1) (0.015-0.2+)
---
(0.4 - 5.0
(0.015 - 0.2)
* Fused Coating
---
(1)
High Velocity Oxygen Fuel
** Unfused Coating
5.4. Typical applications of thermal
spray coatings
Thermal spray coatings are used for thousands of applications in aircraft,
machinery, infrastructure, and elsewhere. Table 13 summarizes some a
few of these applications.
Page 29
Table 13. Some applications of thermal spray coatings..
Coating type
Application
Notes
Carbides (WC-Co,
Cr3C2-NiCr, etc.)
Wear
Easy to spray. Most common
chrome replacement
Zn, Zn-Al, Al
Corrosion, conductivity
(aluminum)
Used on landing gear,
bridges
Zirconium oxide
Thermal barrier
Turbine blades
Ni5Al, In 718, stainless
steels
Build-up
Repair
Aluminides
High temperature
oxidation
Gas turbines
Chrome oxide
Anilox print rolls
Laser engraved for very high
resolution flexographic
printing
Babbit and other soft
alloys
Bearing journals
Ships, machinery
Hasteloys, Inconels,
Stellites, MCrAlYs
High temperature
oxidation and wear
Turbine engines
Mo
Wear, lubricity
Piston rings for trucks, cars
Abradables
Clearance coatings
Turbine engines
Page 30
6.
Types of thermal spray processes
There are four general types of thermal spray in widespread commercial
use, in order of increasing coating quality – flame spray, arc spray,
plasma spray, and high velocity oxy-fuel (HVOF) spray.
There are other technologies in development or specialized use, including
•
Cold spray (or gas dynamic spray) – in which there is no
combustion, but the particles are accelerated to high speed by an
ultra-high velocity gas stream. When they strike the surface they
soften and melt by a combination of conversion of kinetic to
thermal energy, and cold forging of each coating layer by the high
velocity incoming particles that form the subsequent layers. At
this point the method is in the early development stage and is only
suitable for depositing low melting point metals such as Cu and Al.
•
High velocity air-fuel spray (HVAF) – similar to HVOF, but with
air as the oxidizer. These guns are made for use in areas of the
world where oxygen is not easily obtained.
•
Spray forming (the Osprey process) – in which an alloy is melted
and atomized into a gas jet so as to spray the molten particles. It
is not generally used for coating, but for spray-casting threedimensional objects.
This report is concerned only with plasma spray and HVOF, as they
are used for aerospace components. We describe these technologies in
the following sections.
6.1.
Flame spray
This is the simplest thermal spray method, which is used for lower-quality
alloy coatings. Powder is entrained in a gas jet and fed through a flame.
The coating is generally of quite poor quality (porous and low adhesion),
but the method is used for some aircraft components. It is not generally
suitable as an alternative to hard chrome.
6.2.
Arc spray
In this method an electric arc serves both as the source of heat and as
the source of molten metal droplets. The arc is struck between two feed
wires (or a feed wire and an electrode), and the molten droplets are
driven to the component surface by a gas jet. The method is used to
spray metal and alloy coatings. Commercial arc guns are used to spray
interiors of automobile engine cylinders, and they are gaining increased
currency for aircraft applications. At this point, they are not generally
suitable as an alternative to hard chrome. However, arc spray technology
is constantly improving and the applications for arc spray coatings are
growing.
Page 31
6.3.
Plasma spray
Plasma Spray is perhaps the most flexible of all of the thermal spray
processes as it can develop sufficient energy to melt any material.
Plasma spray is usually the best choice for facilities where many different
surfaces must be applied, and it is the only technique able to spray most
high-quality ceramic coatings. The components of a typical system are
shown in Figure 9.
Figure 9. Components of a typical plasma spray system.
Since it uses powder as the coating feedstock, the number of coating
materials that can be used in the plasma spray process is almost
unlimited. The plasma gun incorporates a cathode (electrode) and an
anode (nozzle) separated by a small gap forming a chamber between the
two. DC power is applied to the cathode and arcs across to the anode. At
the same time, gases are passed through the chamber. The arc
temperature is sufficient to strip the gases of their electrons, and the state
of matter known as plasma is formed. As the unstable plasma
recombines back to the gaseous state thermal energy is released.
Because of the inherent instability of plasma, the ions in the plasma
plume rapidly recombine to the gaseous state and cool. At the point of
recombination, temperatures can be 6,600 ºC to 16,600 ºC (12,000 ºF to
30,000 ºF),which exceeds surface temperatures of the sun. By injecting
the coating material into the gas plume, it is melted and propelled towards
the target component.
Typical plasma gases are Hydrogen, Nitrogen, Argon and Helium.
Various mixtures of these gases (usually 2 of the 4) are used in
combination with the applied current to the electrode to control the
amount of energy produced by a plasma system. Since the flow of each
Page 32
of the gases and the applied current can be accurately regulated,
repeatable and predictable coating results can be obtained. In addition,
the point and angle that the material is injected into the plume as well as
the distance of the gun to the target component can also be controlled.
This provides a high degree of flexibility to develop appropriate spray
parameters for materials with melting temperatures across a very large
range.
The distance of the plasma gun from the target components, gun and
component speeds relative to each other and part cooling (usually with
the help of air jets focused on the target component) keep the part at a
comfortable temperature that is usually in the range of 38 ºC to 260 ºC
(100 ºF to 500 ºF). Several commercial plasma spray guns can coat IDs
down to 3”, while a few can coat to IDs as small as 1.5”.
6.4. High velocity oxy-fuel (HVOF) spray
and detonation gun
We include under the definition of HVOF, the Detonation gun (D-gun)
and Super D-gun, which are proprietary to Praxair. HVOF uses a fuel
and oxygen, which burn in a combustion chamber and exit the gun
nozzle at supersonic speed. The powder is injected into the combustion
chamber and is accelerated to high (but not supersonic) speed by the
gas. In the detonation guns the fuel and oxygen are burned in distinct
detonations (similar to a machine gun with blank cartridges), while in
other HVOF guns the combustion is continuous. The high speed of the
particles makes HVOF coatings the highest quality thermal sprays. The
most common fuels are hydrogen, propylene, acetylene, and kerosene.
Natural gas is beginning to be used because of its lower cost, but
hydrogen, propylene and kerosene remain the primary fuels used for
aircraft applications.
Sulzer Metco High Velocity Oxy-Fuel Thermal Spray Process
This link contains data on the Diamond Jet HVOF System
Praxair HV-2000.pdf
This link contains data on the HV 2000 HVOF gun from Praxair
Page 33
7.
Thermal spray producibility
7.1. Summary
This section is concerned with issues involved in production thermal
spray processing.
Table 14. Producibility summary and links.
Item
Item
Quality control
Stripping
Metallography
WC-Co
Hardness
WC-CoCr
Adhesion
Tribaloy
Temperature monitoring Finishing
Residual stress monitoring
Process optimization
Inspection
7.2.
"Gorham
Conf.Pres.pdf"
Superfinishing
Hydraulic rig test
experience
Flight test experience
Documents
Document 7. Test standardization: a Key Tool in Coating System
Implementation (Courtesy Sauer Engineering, Gorham Advanced
Materials).10
Document 8. Training in Coating Evaluation Techniques: a Unique
Approach for Discussion (Courtesy Sauer Engineering).
"Training in coating
evaluation.pdf"
Document 9. The Use of Metallographic Standards in Calibration of
the Polishing Process (Courtesy Sauer Engineering).
"Metallographic
standards in polishing
Page 34
"Metallographic
standards in polishing
Document 10. Metallographic Preparation of Thermal Spray
Coatings: Coating Sensitivity and the Effect of Polishing Intangibles
(Courtesy Sauer Engineering).
Document 11. Tensile Bond Variance of Thermally Sprayed
Coatings with Respect to Adhesive Type. 11
"Comparison of
tensile adhesives.pdf"
This article by K. A. Evans summarizes a comparison of epoxy types, and
the strength level/degree of variation experienced between liquid and film
epoxies. Actual tensile results may be found in Section 11.4.3.
Document 12. Almen Strips and Temperature Measurement During
HVOF Processing (Courtesy, Sauer Engineering).
Almen-Temp.pdf
This document, by John Sauer, is a discussion of practical problems of
temperature measurement and almen strip set-up for an actual case
history problem.
Document 13. Design of Experiment for HVOF WC-Co process
(Courtesy HCAT, www.hcat.org).
HitemcoDOE.PDF
This document summarizes the design of experiment used to transfer an
HVOF process developed at one location to another spray shop. This
was an 11-run DOE (including 3 duplicate runs at the center points).
Document 14. Summary of DOE results for optimization of HVOF
WC-CoCr (Courtesy NRC Montreal and C-HCAT).12
Jean-Gabriel.pdf
This document describes an extensive DOE that includes the effects of
different gases and powders.
7.3. Quality Control Of the Thermal
Spray Process
7.3.1.
Choice of powder
This is a very important issue, which has been covered in Section 5.
7.3.2.
General
An important first step in thermal spray producibility is understanding the
tools used to control the process. Many of the tests used to evaluate the
thermal spray process are conventional methods specified for the
analysis of metallic products such as metallography, tensile, and
hardness testing. However, in the case of coatings, the evaluation is now
being performed on a composite structure consisting of the coating on top
of the substrate. The response to testing variation can therefore become
very significant given this “dual” structure and the category of coating
Page 35
material being investigated. A thorough understanding of the testing
process is therefore critical to properly provide results that lead to
optimization and control of the thermal spray process.
Many methods are used to evaluate coating properties and performance.
This section will present the varied tests used primarily in daily quality
control of the thermal spray process. Methods such as fatigue,
corrosion testing, wear testing, etc. are sometimes referred to as
“capability” or “performance” tests that validate the process but are not
performed on a daily basis. Sections 11and 12 present more detailed
Table 15 Commonly Used Quality Control Tests
Method
General Description
Metallography
Test coupons are sprayed and then sectioned,
mounted, and polished for evaluation.
Hardness
Coupons are coated and then macrohardness
performed. Some microhardness evaluation on
mounted cross sections from above.
Tensile
Buttons or tensile adaptors are coated, bonded
together with adhesive, and then pulled in
tensile machine.
Residual stress
This method is primarily used for HVOF
coatings where Almen Strips (same as shot
peening) are coated on one side and the stress
from coating application measured as a
deflection.
Temperature monitoring
This method is primarily used for HVOF
coatings where monitoring temperatures effects
is critical to minimize affect on fatigue
properties.
Surface roughness
For many applications such as wear or thermal
transfer, surface roughness is monitored via
use of profilometers.
information on these methods and summarize actual results for the varied
analysis techniques. The commonly used methods are shown in Table
15.
The remainder of this section will provide greater detail on the application
of these test methods. Currently, no general specification test methods
with the exception of ASTM C-633 on Tensile Testing exist for usage. All
other test methods are covered under general metal testing.
Efforts are now in progress to standardize the methodology used in
evaluation of coatings. Document 7 summarizes some of the groups and
actions involved in process standardization .
Page 36
7.3.3.
Metallography
Metallography is a destructive test which provides information about the
structure of the coating. It is commonly performed on coupons that are
sprayed at the same time as the part in question. Common
metallographic techniques can be applied but consideration must be
given to the composite structure of coating and substrate. Some
commons issues to consider and carefully monitor in coating
metallographic preparation are:
•
Hardness of substrate vs. coating
•
Format being used to grind-polish: disc vs. papers
•
Cutting method
•
Hardness of polishing abrasive vs. coating
•
Type of polishing cloth: nap vs. no nap
•
Mounting material hardness vs. coating
•
Impregnation of coating with mounting material if porous
•
Concentration and type of polishing abrasive
After the microstructure is prepared and ready for evaluation, the test lab
technician will commonly review the structure for the characteristics in
Table 16 (Section 4.5 for explanation of how characteristics evolve as
part of the thermal spray process).
Page 37
Table 16 Common Characteristics Evaluated in Metallographic
Specimens.
Characteristic
Description
Interface contamination
Grit that is embedded at the interface form
the grit blasting operation-usually compare
to a photostandard to assess degree.
Porosity
Assess distribution of voids/holes in
coating against photostandard from
incomplete splat bonding
Oxides
May be in form of stringers or clusters from
powder traveling through the air-again
compare to photos
Unmelted particles
All particles do not obtain sufficient thermal
energy in flame to deform when
accelerated toward the substrate-usually
count particles and a shape is defined for
characterization as unmelt
Phase content
Distribution and content of phases is
critical for some coatings such as carbides
in a wear coating-compare to photos
Delaminations and cracks
Can be located at interface and within
coatings-a definition on length and
frequency critical to evaluator
Figure 10. Typical HVOF coating cross sections; Ni-Al left
(200x), WC-Co right (500x). (Courtesy Praxair-TAFA)
Figure 10 represents typical photomicrographs of thermal spray coatings,
cross-sectioned, metallographically mounted, and polished. The
photograph on the left shows a NiAl coating with varied features such as
oxides, porosity, and unmelts. The photograph on the right shows a WCCo coating with a distribution of dark carbide phase in a matrix of “white”
or cobalt material. Note the manner in which the material forms layers
Page 38
parallel to the surface (top of pictures). This layering is typical of thermal
spray coatings.
As mentioned earlier, a mounted cross section such as this is commonly
characterized against photostandards for acceptance rejection criteria.
There is work currently in progress to use a rapid growing technique
called image analysis to quantify the phases in coatings. Due to the
heterogeneous nature of some coating types, it has been difficult to apply
this method but the tool has been used successfully on material that are
more homogeneous in nature. A more detailed description of coating
microstructure analysis is found in Section 10.4.
Further information on metallography can be found at the websites shown
below or in the referenced PDF files Document 8 to Document 10, and in
the web sites below.
Buehler web site: http://www.buehler.com/
Struers website: http://www.struers.com/
7.3.4.
Hardness
Hardness testing on coatings is very similar in methodology to that
performed on metallic materials. Macro and microhardness are covered
by specifications ASTM E-19 and ASTM E-384 respectively. Coupons
used for metallography are sometimes tested for hardness first and then
cut and polished for microstructure analysis.
Macro or Rockwell hardness for coatings is most often performed using
the R15N superficial scale which applies a small load of 15 kg. With the
usual thickness range of .002-.008”, the use of conventional Rc or Ra
scales with loads of 150 and 60 kg respectively would result in
penetration of the indentor thru the coating thickness. The hardness
reading would then be a composite of the substrate/coating which is not
representative of true hardness. The surface is usually lightly sanded to
remove irregularities and a series of readings distributed randomly across
the face of the coupon are taken to determine hardness. The readings
are averaged to obtain a composite value since values may vary
dependent upon the phases present under the indenter due to the
heterogeneous nature of some coatings.
Microhardness can be performed using either a Knoop (elongated
impression) or Vickers (diamond impression). For most coating
applications, the Vickers method is used. Since phase distribution is
critical for many coatings, microhardness can identify any segregation or
absence of important constituents in the microstructure. The pattern for
impressions may be random as described for macrohardness but some
specifications require a stair step pattern thru the thickness. This can
check for variations in the spray process as passes are applied to the
coupon. If something changes in spraying, a hardness change should be
identified in the progression of readings.
For more information on hardness, see Section 11.3 or click the link
below for the ASTM website and pertinent specifications.
Page 39
ASTM website: http://www.astm.org/
7.3.5.
Tensile/Adhesion
The tensile strength of coatings is monitored by use of either buttons or
tensile adaptors/loading fixture sprayed with the material in question. The
typical tensile assembly used for determining strength is shown in Figure
11 from the ASTM C-633 specification on tensile testing.
Figure 11 Tensile Assembly from ASTM C-633
Figure 11 shows a button as part of the assembly; if one fixture is sprayed
the adaptors are bonded together with a single adhesive application.
Currently, both liquid and film adhesive are being used for this purpose.
The assemblies are cured at temperatures between 300-450 degrees F
for 1-3 hours and then cooled before pulling. A sample with no coating is
usually placed in a furnace run to verify proper epoxy curing. Epoxy only
values should exceed 10,000 psi and normally pull in the 12,000 psi
range. When the test is required, the material is sprayed with the part in
question and a set of three samples is usually processed. Test values
can range from 100 psi for very soft abradable coatings to epoxy-only
failures at over 12,000 psi for HVOF materials (see Document 11). This
is due to the limited strength value of the epoxy and is not a true test of
the coating strength. Research is in progress to develop stronger epoxies
or alternative testing methods for coating strength. See, for example
Document 11.
7.3.6.
Temperature monitoring
As described in Section 4.7.2, heat can be transferred to the substrate as
the material is coated. Although the part does not see the 1,000°+ F of
the thermal plume, sufficient heat can be transferred to degrade the
Page 40
surface material properties. This can be controlled by either choice of
coating process or process modification to allow cooling time during the
spray cycles.
Temperature monitoring is therefore very critical if surface properties such
as fatigue are important. An obvious mechanism to monitor temperature
would be a contact pyrometer after the part has been sprayed. However,
this is not practical for the thermal spray process due to booth and safety
constraints. Most spraying is performed in a booth with robotics for part
manipulation and air circulation to insure a proper environment for worker
health and safety. With cooling air also placed on the part for
temperature reduction, temperature monitoring must be instantaneous to
monitor the true effects of the spray process.
Infared (often called
non-contact)
temperature
measurement
equipment is
currently being used
to achieve this
performance.
Infrared instruments
can be set-up inside
the booth and data
transmitted remotely
to a PC for later
analysis. A peak
hold capability allows
the operator to view
maximum
Figure 12. Typical non-contact temperature
temperature and
arrangement for HVOF.
modify the spray
process if data
indicates values above the upper material limit. A typical arrangement for
non-contact temperature measurement is shown in Figure 12. Note that
the substrate (a landing gear cylinder or hydraulic rod, for example)
rotates while the gun traverses up and down on a robot arm. The
pyrometer is set near the center of the part (or at the most critical area, or
the area most likely to be overheated), aimed such that the deposition
spot never passes directly into its field of view (so that it is not affected by
the high radiation from the flame itself). Cooling air jets are usually
arranged around the part in strategic locations to give thorough cooling.
Page 41
Temperature [F]
Figure 13 Typical Temperature Plot From a Spray Cycle (J. Schell,
GEAE, Courtesy HCAT).
350
300
250
200
150
100
50
0
11: 11: 11: 11: 12: 12: 12: 12: 12: 12: 12: 12: 12: 12: 12: 12:
56 57 58 59 00 01 02 03 04 05 06 07 08 09 10 11
Time
A typical plot of temperature vs time is shown in Figure 13. This plot was
taken as the HVOF gun was moved up and down a rotating sample.
Each peak corresponds to the gun crossing the level of the pyrometer
measurement spot. Note how rapidly the temperature rises and falls.
Clearly, by the time the deposition was stopped, the booth opened, and
the temperature measured with a simple contact probe, the surface
temperature would have fallen about 100°F, making the measurement
almost meaningless.
Two potential problem issues with infared pyrometry are emissivity and
spot size.
Emissivity is the measurement of how the radiation emitted form the
target in question compares to that of a blackbody. It is therefore a ratio
and will be different between say a coated and coated piece of steel. The
instrument must therefore be set to the proper emissivity for the specific
material. There are some published values but a value for a specific
material can be obtained by comparing the infared values from the
instrument to contact pyrometer on the same samples temperature values
and then dialing in the proper emissivity value on the calibration for the
unit.
Spot size is related to how focused the infared beam in relation to the
part being measured. If a small part is being measured and the beam is
bigger thab the piece, compensations must be made for the background
also being measured by the unit.
Page 42
More detailed information on temperature measurement is given in
Document 12. Web sites for a number of instrument manufactures are
also listed.
Some infrared measurement system manufacturers:
Raytek:
http://www.raytek.com/
Ircon:
http://www.ircon.com/
Omega:
http://www.omega.com/
7.3.7.
Monitoring residual stress
Residual stress can be an important factor in the performance of coatings
and the substrate being used. If a compressive residual stress can be
introduced at the substrate surface, this will greatly enhance the fatigue
properties of the material. There are many techniques to measure
residual stress that are mentioned in Section 11.5, such as hole drilling,
X-ray diffraction, and possibly neutron diffraction. These methods are not
quality control tools and cannot be applied on a daily basis to qualify
spray runs.
(1) SAE 1070 Cold Rolled Spring Steel
(2) Edge Number One (on 3 inch edge)
(3) Blue Temper (or Bright) Finish
(4) Uniformly hardened and tempered to Rockwell hardness C44 to C50
(5) Flatness tolerance is ± 0.0015 inch arc height as measured on an Almen gauge
(6) Dimensions in inches
Figure 14. Almen “N” Test Strip.
At the present time, the technique of using Almen strip deflection is the
method being used as a quality control tool. The Almen strip concept has
been used in shot peening for many years to measure the residual stress
Page 43
effect of varied peening media by blasting only one side and inducing a
deflection in the coupon as shown in Figure 14. For HVOF quality control
Almen “N” strips are commonly
used.
The Almen strip is attached to the
fixture shown in Figure 15 and
then sprayed with the production
lot. After removal from the
holding fixture, the deflection is
measured in a fixture similar to
Figure 16.
Figure 15. Almen holding fixtures
(Electronics Inc.).
Almen strip measurements and
temperature monitoring are critical
items for process control especially in
HVOF coatings. The link below
highlights current processing and
techniques for use of Almen strips in
HVOF spraying. Work is currently in
progress to refine this methodology
as more spraying on components is
performed (see Document 12. Almen
Strips and Temperature
Measurement During HVOF
Processing (Courtesy, Sauer
Engineering).)
Figure 16. Almen measuring
instrument (Electronics Inc).
7.4. Process optimization and control
7.4.1.
General
Many coating providers simply deposit thermal spray coatings with
general parameters supplied by the equipment or powder manufacturer,
or use a process optimized for some other use. Past experience shows
that such coatings are seldom optimized for fatigue-critical applications.
The deposition conditions for a particular thermal spray powder with a
particular gun should be optimized with respect to the most critical enduse parameters (fatigue, corrosion, wear, etc.).
Optimization of the process involves controlling the many parameters
which are part of the thermal spray process. Table 17 details the large
number of parameters/variables that are possible in the thermal spray
process.
However, the process can really be broken down into two major
components:
Page 44
1. How the material is fed into the system
2. Jet temperature/particle velocity distributions
Table 17. Thermal spray process parameters.
Jet
Formation
Process type (combustion, arc, plasma), gas composition, gas
flow rate, power/current, power/voltage, power/gas heat
content, nozzle diameter, nozzle length, nozzle cooling,
chamber/nozzle ambient pressures, nozzle internal pressure
Materials
Composition, melting range, thermal properties, size, shape,
morphology/manufacturing method, form (wire, rod powder),
coefficient of thermal expansion
Substrate
Size, geometry, surface preparation, surface texture,
temperature, thermal properties, coefficient of thermal
expansion
Material feed
Feed rate, location of feed relative to jet, angle of particle
injection, diameter of feed port, carńer gas flow, mass-toparticle ratio, particle shape, particle specific gravity
Deposition
Angle of deposit/particle impact angle, particle velocities,
distance to substrate, pressure, environment composition,
speed of traverse, pattern of traverse, relative motion of pattern
Control of these items affects deposit quality because our final structure
depends upon how much thermal energy is absorbed by the particles
while in the flame and the kinetic energy imparted as the material impacts
on the substrate.
Page 45
Figure 17. Kinetic versus thermal energy for the main thermal spray
technologies.
Figure 17 illustrates the relative contributions from both the
thermal/kinetic energy.
With the use of statistical process control, a process can be optimized
cost-effectively using Design of Experiment or DOE methods. This
methodology combines knowledge and experience with the process in
conjunction with statistics to determine what parameters are significant.
When the important parameters for the technique in question are
identified, experiments are run that then zeroes in on the best settings to
optimize the spray results.
7.4.2.
Example 1 – Optimization of WC-Co
This is a basic DOE optimization where the process is fairly well known
and the powder defined. It was used by Jerry Schell of GE Aircraft
Engines to transfer a known HCAT WC-Co HVOF process to a new
vendor. The illustration below, Table 18, shows the eight-run experiment
in optimizing the HVOF process with 4 variables identified as significant
issues. Prior work was already performed to arrive at these parameters
as most important to the process.
The factors identified as most important were:
1. Surface feet per minute How fast was the thermal spray gun
traversing across the substrate surface
2. Combustion gas How much combustion gas was flowing in the
process
3. Stoichiometry Ratio The ratio of combustion gas to oxygen during
Page 46
the combustion process. This ratio will is based upon a complete
chemical reaction and combustion between the gases which in
production never occurs. This variable is critical to jet temperature
and particle melting.
4. Spray Distance How far away is the nozzle from the substrate.
Other factors of the process are fixed, either from prior DOE work or
previous experience in the process. When this work is performed, the
response to these variables must be monitored to determine how a
change affects coating quality. The responses monitored in this case are
shown in Table 19. Note that, since Almen number relates directly to
coating residual stress, which in turn is directly related to fatigue, Almen
number was especially important in this DOE.
A full DOE review of this particular analysis can be reviewed in Document
13.
Table 18. Design of Experiment analysis tool.
Design 1: Use L8 design plus Center Points, 11 runs total
FIXED:
Levels
FACTORS:
-1
+1
54 grit alumina grit blast at 40 psi, 6 inches
C Pt
Substrate is 4340 steel, 260-280 ksi
A
Surf Speed,Feed Rate
1335, 5.1
1835, 3.5
1585 ipm, 4.3
Powder size/type is WC-17Co, Diamalloy 2005, Lot 54480
B
Combustion Gas
1525 scfh
1825 scfh
1675 scfh
Powder Feed Rate**
C
Stoic Ratio
0.405
0.485
0.445
Spray angle is 90 degrees
D
Spray Distance
10 inch
13 inch
11.5 inch
100 psi cooling air, 4 AJs @ 6 inch spaced over coupon area
8.5 lbs/hr
Carrier gas N2 at 148 psi, 55 flow, air vib @ 20 psi
Turntable
Robot Spd
Robot % @
A Factor:
RPM
ipm
mm/sec
750 mm/sec
Spots/Rev
(-1)
212
25
10.6
1.41%
5.1
C Pt
252
35
14.8
1.98%
4.3
(+1)
292
50
21.2
2.82%
3.5
(B,C) Factor Combinations:
Comb Gas Stoic Ratio
Hyd SCFH
Oxy SCFH
1675
0.445
1159
332
Air SCFH Point (CG,SR)
920
1525
0.405
1085
258
1525
0.485
1027
314
1825
0.405
1299
342
1825
0.485
1229
412
Spray pattern length
Approximately 13 inch
Fixture diameter
2 inch
RESPONSES:
RELATED CTG FUNCTION:
1) Part temperature
Fatigue
2) Almen strip
Fatigue, ctg residual stress
( 0, 0)
3) Hardness, HV300
Wear
920
(-1,-1)
4) Coating dep/pass
Cost
920
(-1,+1)
5) Porosity
Ctg quality, corrosion
920
(+1,-1)
6) Oxides
Ctg quality
920
(+1,+1)
7) Carbides
Ctg quality, wear
8) Tensile bond
Adhesion/cohesion
Page 47
Table 19 Response vs. Coating Property
Response
Coating property
Part temperature
Fatigue
Almen strip
Fatigue; coating
residual stress
Hardness (HV300)
Wear, abrasion
Deposition/pass
Cost
Porosity
Coating quality,
corrosion
Oxides
Coating quality
Carbides
Coating quality,
hardness, wear
7.4.3.
Example 2 – Optimization of WC-CoCr
Another example of optimization was conducted by NRC-IMI of Canada12.
This was a highly detailed DOE that included not only the best
parameters for spraying with hydrogen but also the use of propylene and
powder sprayability involving 7 different powder morphologies. Table 20
compares the parameters between hydrogen and propylene.
Table 20. Comparison of hydrogen versus propylene DOE.
DOE Using Propylene …
!
!
Parameters that were varied.
DOE using hydrogen
Parameters that were varied.
–
Powder
– Powder
–
Flow rates
– Total flow rates
–
Propylene
– Stoichiometry
–
Air
– Stand off distance
–
Carrier
– Powder feed rate
–
Coating build up rate
Constant parameters
!
Constant parameters
– Oxygen flow rate
– Powder feed rate
– Stand off distance
– Surface temperature
– Step size
– Step size
!
Parameters adjusted
– Gun displacement speed
!
Parameters adjusted
– Cooling
– Cooling
The response characteristics evaluated for these DOE’s were:
•
Residual stress
•
Porosity
Page 48
•
Carbide degradation – degradation index measurement is
discussed in section 10.5.2.2
The optimum results to the response characteristics are shown in Figure
18 (propylene) and Figure 19 (hydrogen).
Figure 18. Best propylene results. Degradation index = 4.25.
Figure 19. Best hydrogen results. Degradation index = 3.46.
Note that even the optimized materials are imperfect. Optimization
minimizes the imperfections, and specifications for the process should
therefore define the maximum permitted unmelted grains, oxide particles,
porosity, etc.
Page 49
2400
1200
Temperature prop
Temperature Hydrogen 1100
Velocity prop
1000
Velocity Hydrogen
Temperature (°C)
2200
2000
900
1800
800
1600
700
1400
600
1200
Velocity (m/s)
Temperature and velocity were also optimized as part of the fuel
comparison. Figure 20 shows that use of propylene results in both
higher temperature and velocity at the optimum stand-off.
500
1000
400
0
10
20
30
40
50
Position from gun (cm)
Figure 20. Graph showing the temperature/velocity profile with varied
fuel types.
This optimization concluded that hydrogen was the optimum fuel to
produce the best microstructure, with special emphasis on carbide
degradation. Note that this DOE primarily optimized structure,
whereas Example 1 optimized stress, which is directly related to
fatigue performance.
The other portion of this DOE analyzed seven different powders with
respect to a number of different characteristics to select the best
materials.
The characteristics evaluated as described earlier in Section 4 were:
•
Particle size
•
Carbide grain size
•
Morphology/microstructure of the coating
•
Porosity/phase distribution of the coating
Page 50
Three powders (SM 5847, 1350VF, and WC-636-3) out of the 7 tested
were selected with characteristics as shown in Figure 21 and Figure 22.
Note that each of these powders looks somewhat different, sprays
Figure 21. Microstructure/morphology of selected powders.
differently, and would be expected to perform somewhat differently as a
coating.
SM-5847 is composed largely of hollow spherical particles that
would be
40.0
5847
expected to
35.0
636-3
accelerate
1350-VF
faster and
30.0
heat
25.0
evenly.
•
WC-636-3
15.0
has a very
narrow
10.0
grain size
5.0
distribution,
which
0.0
should
0
10
20
30
40
50
60
70
80
90
100
ensure that
Microns
each grain
is heated
and
accelerated Figure 22. Particle size distribution for the three
in much the best powders.
same way.
•
1350 VF has a much broader size distribution, which means that
particles will strike the substrate with a broad distribution of
% Frequency
•
20.0
Page 51
velocities and temperatures.
From the results obtained in this optimization, the SM-5847 powder was
selected based on:
– Higher compressive residual stresses (related to fatigue)
– Lower porosity level
– Lower carbide degradation
However intrinsically this material produced an heterogeneous coating,
containing large Cr rich areas.
7.5.
Stripping
Stripping of thermal spray coatings is critical for O&R, since new coatings
cannot generally be sprayed over old, just as new chrome cannot be
reliably plated over old chrome. Plasma spray coatings can often be
removed by water jet stripping. However, HVOF coatings adhere more
strongly than plasma sprayed coatings, and most attempts to remove
them with water jets have not succeeded, although there has been more
success with ultra-high pressure water jets. There are, however,
standard electrochemical stripping solutions used by the industry and
approved for aerospace use.
The following is a summary of information from Sulzer Metco, Southwest
Aeroservice, and Lufthansa, as well as reports from NTS (McClellan AFB)
and National Defense Center for Environmental Excellence (NDCEE).
7.5.1.
Documents
Document 15. NTS Stripping Report,PDF. 13
"NTS stripping
report.pdf"
This report is authored by Elwin Jang of NTS (National Technical Systems,
formerly Sacramento ALC) and Robert Kestler of NADEP Cherry Point, 1998.
It describes the results of a project to evaluate the electrolytic stripping of HVOF
HVOF WC-Co and WC-CoCr from steels by use of the Rochelle Salt solution.
Document 16. NDCEE Evaluation of Stripping Methods. 14
"NDCEE evaluation
of stripping methods.pdf"
This is a report of a study funded by NRL and run by Concurrent
Technologies Corp (NDCEE), evaluating various stripping methods, and
includes data on water jet stripping experiments run at NDCEE.
Document 17. Stripping of WC Coatings from Aermet 100,
Southwest Aeroservice, Menasco, Carpenter Technology (Courtesy
Southwest Aeroservice). 15
"Stripping of Aermet
100 Southwest Aeroservice.pdf"
This
document is authored by Southwest Aeroservice, Menasco (now
B.F. Goodrich, BFG), and Carpenter Technology. It reports stripping of
HVOF WC-17Co and WC-CoCr using the standard Southwest Aeroservice
Rochelle salt bath and an alkaline non-electrolytic bath.
Page 52
7.5.2.
Stripping of WC-Co
The most common stripping solutions are electrochemical solutions
based on Rochelle Salt (sodium potassium tartrate), and are fairly innocuous.
7.5.2.1.
Southwest Aeroservice
Specification # SASP.025A
Southwest Aeroservice).
Component
Value
Notes
Anhydrous sodium
carbonate
20 - 30 oz/gal water
Sodium potassium tartrate
(Rochelle Salt)
8 - 12 oz/gal water
Temperature
104 - 150°F
150 - 225 gm/l
60 - 90 gm/l
130 -150 °F optimal
40 - 66 C
pH
11 - 12
Voltage
4 - 6 V DC
Current density
4 - 8 A/sq in
62 - 124 A dm
Parts are anodic (positive)
2
Dissolution rate
Approx 0.006”/hr
Applicable substrates
High strength
steels
A similar method used for
Ti and Al alloys by BFG
Summary of method:
1. Solvent clean
2. Strip, checking every 10 min to ensure no dissolution of base
metal.
3. Rinse
4. Dry
5. Inspect
6. Embrittle relieve
7. Inspect.
Page 53
7.5.2.2.
Sulzer-Metco
Process specification #E-2. This process is for stripping WC-Co from
steel (dated 1979).
Sulzer Metco).
Component
Value
Sodium carbonate
20%
Tartaric acid
5%
Water
75%
Temperature
160 - 180°F
Notes
70 - 80 C
Voltage
6 V DC
Current density
4 - 8 A/sq in
62 - 124 A dm
Dissolution rate
2
Approx 0.006”/hr
0.18 mm/hr
Steels
Note: The same solution dissolves tungsten (W) at a rate of about 3
mils/hr.
Page 54
7.5.2.3.
Lufthansa
Summary of method:
Table 23. Electrolytic stripping method for “aged” HVOF WCCo (Courtesy Lufthansa).
Component
Value
Citric acid
Various
Sodium hydroxide
Various
Sodium carbonate
Various
Temperature
104 - 140°F
Notes
40 - 60 C
Voltage
6 +- 0.5 V DC
Current density
1 - 3 A/dm2
Parts are anodic
(positive)
Time
3 - 10 hrs
10 hrs max for HSS
HSS, IN 718,
Ti6Al4V
BFG uses a similar
method for steels.
Note: This method is used for “old” WC-Co – i.e. material to be stripped
from parts during O&R.
For new WC-Co (i.e. material just deposited, but that must be stripped to
correct misapplication), Lufthansa uses a simple chemical immersion
method that does not work for “aged” (i.e. oxidized) WC-Co, as shown in
Table 24.
Table 24. Electrolytic stripping method for "new" HVOF WC-Co
(Courtesy Lufthansa).
Component
Value
Notes
Citric acid
Hydrogen peroxide
Temperature
30 C
Time
3 - 10 hrs
HSS, IN 718, Ti6Al4V
7.5.2.4.
10 hrs max for HSS
Other specifications
GE Aircraft Engines, Pratt and Whitney, and Praxair specify a rather
Page 55
similar electrochemical stripper for removing WC-Co from titanium alloys,
comprising a solution of Rochelle Salt, sodium hydroxide, and sodium
carbonate.
7.5.3.
Stripping of WC-CoCr
NTS and NADEP Cherry Point have tested the use of the standard
Rochelle salt stripping method for HVOF WC-Co and WC-CoCr. The
results were similar with both materials. This is reported in Document 15.
These tests were done to evaluate the standard Rochelle salt stripping
method for stripping both WC-Co and WC-CoCr. Both stripped at about
the same rate.
Table 25. Electrolytic stripping method for HVOF WC-Co
(NTS/NADEP Cherry Point).
Component
Value
Notes
Anhydrous sodium
carbonate
20 - 30 oz/gal water
Sodium potassium tartrate
(Rochelle Salt)
8 - 12 oz/gal water
Temperature
130 - 150°F
150 - 225 gm/l
60 - 90 gm/l
54 - 66 C
pH
11 - 12
Voltage
4 - 9 V DC
Current density
40 - 80 A/sq ft
Note low current density
Dissolution rate
Approx 0.001
0.002”/hr
Note low rate
4340, PH13-8 Mo,
1010
Southwest Aeroservice has tested their standard Rochelle salt WC-Co
stripper (Table 21) for WC-CoCr, and it appears to work in the same way
as for WC-Co. They report similar results for Aermet 100 substrates
coated with WC-Co and WC-CoCr in Document 17.
7.5.4.
Stripping of Tribaloy 400
Tribaloy is far more difficult to strip than WC-Co. The simple Rochelle
salt method is not effective. Southwest Aeroservice has tested 50% nitric
acid for T-400 on “Custom 450” steel. A 0.010” coating of T-400 breaks
down in 4 - 5 hours sufficiently to remove by glass bead blasting. This
method is approved and used by GE Aircraft Engines for T-400.
Page 56
7.5.5.
Water-jet stripping
Although standard water jet stripping works quite well for most plasma
sprayed coatings, it seldom works well for most HVOF coatings, since
their adhesion strength is too high.
CTC has reported some success with ultra-high pressure water jet
stripping of tribaloy from mild steel, but the method is not well defined
(see Document 16).
7.6. Finishing
7.6.1.
Documents
Document 18. Surface Metrology Guide (Courtesy Precision
Devices, Inc.). 16
"Surface Metrology
Guide - Profile Param
This document summarizes the various surface profile parameters, their
definitions, and their uses.
(See also http://www.predev.com/smg/parameters.htm)
Document 19. Superfinishing of Hard Chrome and HVOF Coated
Workpieces (Courtesy Supfina and Gorham Advanced Materials). 17
"Klotz - Supfina
superfinishing.pdf"
This paper by Norbert Klotz of Supfina gives a good overview of the
effects of Superfinishing on the surface of a component.
Document 20. Surface Finishing of Tungsten Carbide Cobalt
Coatings, J. Nuse, J. Falkowski. 18
"Nuse Falkowski AESF 2000 Paper.pdf
This paper is based on work done by Jim Nuse of Southwest Aeroservice,
and John Falkowski of Boeing. It describes flight test experience with
surface finish and a laboratory evaluation of different finishing methods.
Document 21. Barkhausen Noise as a Quality Control Tool
(Courtesy Stresstech Inc., Finland). 19
Barkhausen.pdf
This document is a brief review of the origin of Barkhausen noise and the
way in which it is used as a QC method.
(See also http://www.stresstech.fi/)
7.6.2.
General requirements
Chrome plating is usually finished by grinding with a standard carbide
wheel. Specifications for chrome plate usually define the finish in terms
of Ra (the arithmetical mean deviation of the surface profile from the
average). Typical Ra values for chrome plate are
Page 57
Axle journals
32 µ” Ra (0.8µm)
Hydraulic seal surfaces
16 µ” Ra (0.4µm)
Metallic HVOF coatings, such as Tribaloy, can be ground in a similar
manner. However, HVOF WC-Co and WC-CoCr coatings can only be
ground with a diamond wheel.
As with chrome, the specifications for surface finish depend on the
application. Although finishes are specified for aerospace-qualified
thermal sprayed components, a definitive specification for HVOF coating
finish is not yet available. However, in general it found that WC-Co
coatings must be finished to a significantly smoother surface than one
would use for chrome. The reason for this appears to be that, since WCCo is so much harder than chrome and contains many small particles of
hard carbide, rough HVOF WC-Co acts almost like a file against soft
materials such as seals and bushing alloys, causing rapid seal failure or
excessive transfer of bushing material. Furthermore, in actuators and
landing gear, typical 5µm WC particles come off rough surfaces and
become suspended in the hydraulic fluid, turning it into an abrasive
cutting fluid.
For this reason HVOF coatings are usually specified as <8 µ” Ra, and
preferably as <4 µ” Ra. However, some users specify much finer
superfinishes (down to 1 µin Ra), and others specify the surface finish
more thoroughly than with just an Ra number. Although this emphasis on
very smooth finishes runs counter to the generally accepted belief that
some roughness is necessary for oil retention, testing clearly shows that
smoother HVOF surfaces usually perform better.
7.6.3.
Specifying the surface finish
Methods of specifying surface finish are discussed in Document 18 and at
the following web sites:
http://intranet.siu.edu/~cafs/surface/file10.html
and
http://www.predev.com/smg/parameters.htm
A review of standards for surface finish measurement can be found at
http://www.predev.com/smg/standards.htm
A typical surface after grinding is covered in grinding marks and loose
debris. The profile of the surface (usually measured with a stylus that
traverses the surface in a line) shows sharp “hills” and “valleys” about a
mean. In most cases the mean is also wavy on a larger distance scale.
Page 58
Figure 23. Typical surface profile.
The surface roughness, Ra (see Figure 23), is simply the arithmetic
average of the absolute deviations from the mean line (which can be
defined from the area under the curve between the profile and the mean).
This is the most common surface finish parameter, specified in most
drawings. However, it is becoming clear that Ra is a very unsatisfactory
measure of surface finish as it relates to performance, since many
different surface profiles can have the same Ra, but will perform very
differently. For example, the surfaces in Figure 24 all have the same Ra,
but the top one will tend to cut into seals, while the middle one will tend to
hold lubricant at the surface.
For this reason some users of thermal spray coatings are resorting to the
use of additional parameters to describe the surface more completely.
For example, Figure 25 shows several other surface parameters that
better show the shape of the surface.
•
Rt is a measure of the total peak-to-valley height
•
Rp shows how high the peaks rise above the centerline
•
Rv shows how deep the valleys fall below the centerline
Page 59
Figure 24. Three different surfaces with the same Ra.
In principle, a smooth surface would have a very small Rt. However, one
might actually prefer a surface with a small Rp and large Rv, since this
would have few damaging peaks that would gouge into seals and mating
surfaces, while having fissures that would hold lubricant.
Figure 25. Other surface roughness parameters.
Some users are adopting the Surface Bearing Ratio, tp, as a measure of
surface finish. This parameter is usually defined as “the cutting depth for
an X% bearing ratio”. This parameter answers the question “How deeply
do I have to cut into the surface to get an area of flat surface X% of the
entire surface?” The surface profile is integrated from the topmost peak
(Figure 26). As the graph at the right of the figure shows, as you move
inward from the peak you intersect a larger and larger area of the surface.
This ratio provides a good idea of how “mountainous” the surface is, and
how rapidly it will wear (or can be superfinished) to a flat surface.
Page 60
Figure 26. Definition of bearing ratio.
Delta has adopted superfinishing (see below) for landing gear inner
cylinders, and a set of surface finish specifications that include Ra, Rt,
and tp
7.6.4.
Superfinishing
Superfinishing is being increasingly recognized as critical to ensuring the
optimum performance of HVOF-coated hydraulic cylinders of all kinds.
Document 19 is an excellent overview of superfinishing. There are two
basic types of Superfinishing
•
Stone finishing, in which a fine honing stone is held against the
surface and vibrates axially while the cylinder rotates.
•
Belt finishing, in which an abrasive belt is vibrated against the
workpiece instead of a stone.
A simple review of superfinishing may be found at:
http://www.supfina.com/english/html/press115.html
Superfinishing is usually done after the surface has been ground to a 4 or
8 Ra finish. This ensures that the surface is true and avoids the need to
remove large amounts of material. The superfinishing tool removes
material uniformly from the surface, leaving a surface that is not only
smooth (often <1 µin Ra), but it removes any surface lay (grinding marks),
and leaves the surface free of embedded grinding materials and loose
debris. With WC-Co thermal sprays, small WC particles are often found
loosely held on the ground surface. Superfinishing removes these
particles.
Page 61
Figure 27. Effect of various finishing methods on an HVOF coating
at 175x (Courtesy Supfina).
It is generally thought that a very smooth surface will be a poor surface
for hydraulics since it cannot retain fluid to lubricate the seal. For HVOF
coatings, however, superfinishing can be used to remove the surface
“hills”, leaving the “valleys” and a small amount of porosity to hold fluid.
Page 62
Boeing, Delta, and Southwest Aeroservice have evaluated superfinishing
for landing gear inner cylinders. The results, shown in Figure 28, clearly
illustrate that one obtains different finishes on chrome and HVOF coatings
by the same finishing techniques. Grinding leaves significant debris and
striations on the HVOF surface that polishing does not remove. However,
superfinishing eliminates the debris and the surface lay, leaving a clean,
smooth surface finish with some porosity to hold fluids.
Figure 28. Surface finishes obtained on Chrome and HVOF WC-CoCr
by various finishing methods.18
7.6.5.
Rig test experience
The issue of surface finish has been evaluated by Green-Tweed for its
effect on seal life in hydraulic rig tests designed to simulate flight surface
Page 63
actuators. With a surface finish of 4-8µ” Ra, the HVOF coatings
performed much better than hard chrome (i.e. less seal leakage) when
run against PTFE seals, but much worse (seal failure) when run against
elastomeric seals. This work is described in Section 13.2.1 and
Document 46.
7.6.6.
Flight experience
There is now a great deal of flight experience with different finishes on
HVOF coatings on landing gear, and on slat and flap tracks. In particular,
Boeing and Delta Airlines have evaluated surface finish quite closely (see
Document 20.)
In their flight tests, Lufthansa’s HVOF WC-CoCr-coated landing gear
inner cylinders were 4-6µ” Ra. However, in early field testing by Boeing
Table 26. Seal life in HVOF-WC-Co sprayed landing gear.18,20
Aircraft/HVOF
coating
Finish
Seal type
Life (cycles)
Boeing 757/WC-CoCr
13µ” Ra
Elastomer 936
Boeing 737/ WC-CoCr
9-12µ” Ra
Teflon
Boeing 757/ WC-CoCr
2µ” Ra
Elastomer No failures in 1 year
(approx 2,000
cycles)
Boeing 737/WC-Co
(Lufthansa)
2µ” Ra
-
1910
Boeing 737/WC-Co
(Lufthansa)
16µin Ra
-
1100 average
855
and Delta Airlines, HVOF-coated landing gear shock struts had a 16µ”
finish – the same as that called out for chrome. Their elastomeric seals
failed rapidly. Damaged seals showed the pock-mark degradation typical
of seals run against a rough surface, and analysis of the hydraulic fluid
showed the presence of 5µm WC particles, evidently released by wear or
polishing of the HVOF-coated surface.
These seal failures were eliminated by refinishing the HVOF surface to
2µ” Ra, and then performed better than standard chrome plated gear.
Boeing, Delta, and Lufthansa flight experience with seal life is
summarized in Table 26.
The surface finish of stationary components is less critical than sealing
surfaces. Delta Airlines now specifies a finish for HVOF coatings of
4µ
µin Ra maximum on hydraulic seal surfaces, and 8µ
µin Ra maximum
on axles.
Page 64
7.7. Inspection
Detection of cracks in HVOF surfaces is not as easy as it is with chrome
plate. It is made more difficult since HVOF coatings are generally in
compression (which closes cracks), rather than in tension (which opens
them). In most chrome plated surfaces, cracks in the underlying steel are
evident at the surface, and tend to be opened by the chrome’s tensile
stress (hence its fatigue debit). Substrate cracks tend to be closed by the
compressive stress of HVOF coatings and are often not evident at the
surface.
Since most HVOF coatings are non-magnetic, magnetic particle
inspection (MPI) is not useful. Lufthansa has found that simple dye
penetrant inspection is also not useful. The only simple inspection
method for detecting cracks in the coating is fluorescent penetrant
inspection (FPI). This method has been adopted by Delta Air Lines.
Eddy current methods must be more sensitive than normal to detect
cracks under HVOF coatings. For this reason, Bombardier increased the
inspection frequency when adopting HVOF for flap tracks in place of
electroless nickel.
Boeing successfully uses Barkhausen Noise methods of evaluating
conditions beneath the coating. However the method is not widely
available, the results are difficult to interpret, and there is a lack of
recognized standards. Information on the Barkhausen method can be
found at http://www.stresstech.fi/BarkNoisAnal.html. The method is
reviewed in Document 21.
To provide better inspection methods, various techniques including
ultrasonic and eddy current methods are currently being tested and
improved by the HCAT and by Heroux.
Page 65
8.
Thermal spray equipment
Most thermal spray equipment (flame, wire, plasma, and HVOF) is
supplied by a few major international vendors, who supply guns, parts,
and complete turnkey deposition systems. The following are links to their
web sites:
•
Sulzer Metco
o
•
Praxair (including the
TAFA division)
o
•
Plasma, HVOF, ID
plasma systems,
powder
Plasma, HVOF, ID
plasma systems,
powder
Deloro Stellite
o
Figure 29. Sulzer Metco F210 ID plasma
spray gun (Courtesy Sulzer Metco).
HVOF Jet-Kote systems,
powder
Other vendors include
•
Northwest Mettech
o
3-electrode high power
plasma gun
This is not an exhaustive summary of
equipment vendors, but it contains most of
those companies selling equipment
commonly used for aerospace
applications.
Figure 30. HVOF spraying of WCCoCr on landing gear with TAFA
gun (Courtesy Praxair-TAFA).
Figure 31. Northwest Mettech
Axial III tri-electrode plasma
system (Courtesy Northwest
Mettech)..
Figure 32. Stellite Jet-Kote
HVOF gun (Courtesy Deloro
Stellite).
Page 66
9.
Thermal spray services
There are numerous aerospace-qualified companies offering thermal
spray services. Table 27 shows companies qualified by Boeing for
deposition of thermal spray coatings on airframe parts. Information can
be accessed by clicking on the Processor #. These are not the only
aerospace-qualified vendors, however, since other aerospace companies
have their own qualified vendor lists.
Table 27. Qualified providers for Boeing 5851 thermal spray
coatings, as of June 2000 (Source, Boeing Aircraft Corporation21).
Country
State
Processor
Processor Name
CANADA
MANITOBA
646547
NATIONAL COATING TECHNOLOGIES
CANADA
QUEBEC
561323
VAC AERO INTERNATIONAL
641824
PRAXAIR SURFACE TECHNOLOGIES
LTD
ENGLAND
JAPAN
SAITAMA
PREF
627739
USA
CA
560994
FLAME SPRAY INCORPORATED
USA
CA
561023
PLASMA COATING CORPORATION
USA
CA
537751
PLASMA TECHNOLOGY INC
USA
CA
305611
USA
CA
031185
SERMATECH TECHNICAL SERVICES
USA
CT
S01081
ENGELHARD SURFACE
TECHNOLOGIES
USA
CT
627738
PRAXAIR INCORPORATED
USA
IN
621420
USA
KS
632859
THERM-O-COAT INCORPORATED
USA
NY
409600
HITEMCO
USA
OK
597139
SOUTHWEST AEROSERVICE INC
Other aerospace-qualified vendors include
•
Cincinnati Thermal Spray
•
Ellison Surface Technology
•
Colonial Coatings
Page 67
•
Aerospace Welding
•
GKN Aerospace
•
Turbine Metal Technology
•
Vac Aero International
•
Sermatech International
•
Thermal Spray Technologies
•
Plasma-Tec
This is by no means an exhaustive list.
Praxair is the only major supplier of detonation gun (D-gun) coatings,
since this is a proprietary Praxair technology.
In addition several airlines and turbine engine manufacturers have their
own in-house thermal spray facilities.
Page 68
PART 3. THERMAL SPRAY DATA
There is a great deal of data available on thermal spray coatings.
However, much of it is not well-defined in terms of substrates, coatings, or
test methods. We have included in this section a limited amount of data
that is well-defined and illustrative of typical coating properties and
performance.
Page 69
Part 3 Thermal Spray Data
10. Coating structure
10.1. Summary
The structure of a coated material can be defined by both microstructure
and phase. It is not always possible to determine the presence of varied
phases by visual examination of the microstructure even with Scanning
Electron Microscopy (SEM). A combined approach is sometimes
necessary due to the effect on final coating performance caused by the
presence of varied phases.
Microstructural work can currently identify both porosity content and
primary WC carbide distribution. X-Ray diffraction can identify the phases
present but the relationship between quantifiable content and
performance properties is still under investigation.
10.2. Documents
"Evaluation of 4
HVOF guns, Legoux.p
Document 22 Evaluation of Four High Velocity Thermal Spray
Guns Using WC-10Co-4Cr Cermets (Courtesy of J.G. Legoux,
B.Arsenault, C. Moreau, V. Bouyer, L. Leblanc). 22
This summary document details the effect of gun type on the
microstructural porosity and abrasive wear characteristics of the coating.
Document 14. Summary of DOE results for optimization of HVOF
WC-CoCr (Courtesy NRC Montreal and C-HCAT).
This document summarizes the optimization work performed with varied
powders and fuel gases using the HVOF system on WC-CoCr material.
Document 23. Fracture Toughness of HVOF Sprayed WC-Co
Coatings (Courtesy of S. De Palo, et al). 23
"Frac troughness of
HVOF WC-Co DePalo
Summary of testing concerning fracture toughness and erosion
resistance on WC-Co coatings.
Applications (Courtesy of M. Dorfman, J Defalco, J. Karthikeyan). 24
"Tung carb coatings
for ind apps.pdf"
Summary of varied spray processes for deposition of coatings and study
on abrasive wear/microstructure/phase content interactions
Page 70
10.3. General
The coating structure is obviously the ultimate factor for determination of
coating properties and the performance of the HVOF alternatives in
service. Microstructure (seen by optical or scanning electron microscopy)
describes the visual appearance or structure of the coating for
characteristics like porosity, oxides, and distinguishable phases such as
carbides. The presence and distribution of features determines the
coating performance in most applications.
Phase determination supplements the identification via light microscopy
and strives to quantify all phases that are present in the coating structure.
This analysis measures the dissolution of the primary WC, for example,
and what phases form in the structure. Both methods are critical and
necessary to adequately characterize the coating morphology.
This section will summarize the test methods for both microstructure and
phase determination and explain the significance of the results for each
structural property.
10.4. Microstructure
10.4.1.
General Description and Test Methods
HVOF Tungsten Carbide Cobalt (WC-Co) and Tungsten Carbide Cobalt
Chrome (WC-CoCr) are the primary thermal spray materials used for
replacement of hard chrome. As referenced earlier in Section 7.3.3, the
major characteristics evaluated in the microstructure are listed in Table
28, with a description of those commonly found with carbides.
The most common methods used to characterize the microstructure are
metallography and microstructural interpretation via light microscopy.
These techniques are described in Section 6.
Page 71
Table 28. Common microstructural Characteristics Observed in Tungsten Carbide
Materials.
Characteristic Description
Carbide Coatings
Interface
contamination
Grit that is embedded at the
interface from the grit blasting
operation – usually compare to a
photostandard to assess degree.
This characteristic is present for all
coatings. The degree of
contamination will be closely
related to the hardness of the
substrate rather than any coating
material characteristics.
Porosity
Assess distribution against
photostandard of voids/holes in
coating resulting from incomplete
splat bonding
Present with carbide coatings but
minimized in the HVOF process
due to high particle velocities.
Oxides
May be in form of stringers or
clusters from powder traveling
through the air – again compare
to photos
Usually not observed in cobaltbased coatings (minimized
oxidation tendency) but can be
present if temperature is extremely
high.
Unmelted
particles
All particles do not obtain
sufficient thermal energy in flame
to deform when accelerated
toward the substrate – usually
count particles, defining a shape
for characterization as unmelt
Powder morphology and size
usually results in very complete
melting and thus lack of unmelts in
cobalt based carbide coatings.
Phase content
Distribution and content of phases
is critical for some coatings such
as carbides in a wear coating –
compare to photos
Distribution and size/morphology
of carbide phase are very
important in cobalt based
materials. Carbides can dissolve
in matrix
Delaminations
and cracks
Can be located at interface and
within coatings – crack length and
number of cracks is critical
Tendency in carbide coatings for
transverse (parallel to surface)
cracking, especially if dissolution
of carbide phase occurs.
Delaminations at interface
possible with poor grit blasting.
Page 72
10.4.2.
Microstructural Features
10.4.2.1.
Porosity/Voids
Porosity levels in
properly sprayed HVOF
materials are typically
very low – in the 0.5%
to 5% range. Since
cobalt is a soft phase
as compared to
tungsten carbide, there
can be a tendency in
metallographic
Figure 33. Comparison of porosity at 200x
preparation to smear
and 1000x magnification.
the cobalt into the
existing porosity of the structure, which presents what appears to be a
fully dense structure. This is illustrated to a degree in Figure 33, which
shows the cross section at two different magnifications. Although
appearing essentially porosity-free at the lower magnification, a degree of
porosity does exist as illustrated by the higher-power picture.
True porosity and void content is a controversial subject in the thermal
spray industry. Evaluations have been performed with mounting
materials containing fluorescent dyes to determine the true level of
porosity. This technique works on the basis of first using either vacuum
or pressure impregnation to force the mounting media into the pores of
the coating. The polished mount is excited using a Xenon light source
and porous areas filled with “glowing “ mounting media are characterized
as “true” porosity. With HVOF coatings, no porosity could be identified
with this technique. This is due to the lack of “interconnected” porosity in
HVOF materials as compared to their plasma sprayed counterparts. The
connected porosity of plasma sprayed materials allows the mounting
media to flow through the coating and into all void areas. It is therefore
easier to ascertain the “true” porosity level.
Work is continuing with such techniques as “cryogenic fracturing” and the
application of NDT methods such as “CAT” scans to determine the exact
porosity levels. Material will be made available on the HCAT Home Page
as research data is available.
10.4.2.1.1.
Comparison of Porosity vs. Particle Velocity
Using WC-10Co-4Cr Cermets (Courtesy of J.G. Legoux, B.Arsenault,
C. Moreau, V. Bouyer, L. Leblanc)
This documents relates void formation to process velocity. As shown in
Figure 34, the porosity is reduced with increasing particle velocity. This is
related to the high kinetic energy in the HVOF process that results in
better bonding of the particles and reduced void formation. The Mettech
Axial III is a plasma spray gun, and shows little obvious dependence of
porosity on velocity.
Page 73
Figure 34. Relationship between velocity and porosity.
10.4.2.2.
Matrix/Carbide Phases/Composition
WC-Co and other cermet coatings are essentially two phases: a
substantial amount of hard carbide particles held together by a matrix of
softer material such as a cobalt or cobalt-chrome. The distribution and
size of the carbide particles can obviously affect many properties as
described below. Figure 35 shows a comparison of carbide content
between two different compositions. The white phase is the background
matrix of cobalt while the darker particles are the carbides. Note the
obvious change a 5% difference in cobalt content makes in the apparent
carbide distribution.
Figure 35. Comparison of Carbide Distributions in 88-12 WC-Co
(left) vs. 83-17 WC-Co (right) at 500X (Courtesy Praxair/TAFA).
Figure 18 is a higher magnification illustration of a WC-CoCr coating
showing the varied distribution in carbide size in the microstructure. The
various features of Figure 18 are summarized in Table 29.
These phases will be present in all carbide/cermet coatings to some
degree. Optimization obviously involves providing a uniform distribution
Page 74
of the desired carbide particles and minimization of both carbide
dissolution (thus less secondary phases) and segregated matrix phase.
(Details may be found in Document 22.)
Figure 36. Microstructure of WC-CoCr 1000X25
Table 29. Features seen in Figure 35.
Feature
Description
Small carbides and large
carbides
Results from initial distribution of carbide particles in the
starting powder. Must be controlled for varied guns to
optimize properties and prevent areas of segregation
Chrome/cobalt rich areas
Softer matrix phase can become segregated during
powder manufacture or improper mixing during the spray
processing
Carbide/amorphous phase
Results form dissolution of primary WC carbide into
secondary phases such as W 2C, amorphous phase, and
other W ?C? compositions. Desire is to minimize these
secondary phases (see Section 11.4.3.2)
Page 75
10.4.2.3.
Transverse Cracks
Carbide coatings can be
prone to transverse
cracking, especially if the
coatings are sprayed at
too high a temperature
so that substantial
carbide dissolution
(carbide in solution) is
prevalent. Cracks can
also appear in the
microstructure from
excessive force in
clamping during cutting
Figure 37. Transverse cracking in plasma
of the test sample and
sprayed carbide coatings.25
overheating during the
sectioning process.
Properly sprayed materials will not show cracking indications in the 200500X magnification range. Cermet materials at 1000x will show some
degree of transverse separation but this can be observed with all coated
materials and does not adversely affect performance. Figure 37 shows
cracking in a plasma sprayed coating.
10.4.3.
General trend of microstructural features
Characterization of microstructural features is used to measure porosity
and carbide phase content and distribution. Levels of porosity below 2%
are easily obtainable. However, the best method to determine this
content is still the subject of debate.
Carbide distribution is relatively easy to determine on a correctly polished
sample. This content is dependent upon the primary carbide content and
composition.
These characteristics will ultimately be reflected in coating performance,
such as corrosion and wear, due to the dependence of these
characteristics on structural morphology.
10.5. Phase Determination and Effect
10.5.1.
Carbide coatings primarily consist of a distribution of carbides within the
soft cobalt or cobalt-chrome matrix. During the spray process, some
breakdown or decomposition of the WC particles can occur forming a
variety of secondary phases. X-ray diffraction (XRD) is commonly used
to quantify and identify these phases.
Page 76
A typical plot comparing the varied phases found in powder vs. final
sprayed product is shown in Figure 38.
Figure 38. X-ray diffraction plot of powder(lower curve) and coating
(upper curve).26
This change in crystalline phases or “carbide degradation/dissolution” is
critical to coating performance in properties such as wear and abrasion.
It is therefore an important aspect of coating evaluation and is highlighted
in the results section.
10.5.2.
10.5.2.1.
Phase Determination and Effect Results
Carbide Phase Comparison vs. Process Type
Applications (Courtesy of M. Dorfman, J Defalco, J. Karthikeyan)
This document shows the relationship of retained carbon content and
carbide phase when comparing data from two different spray guns. An
air cooled DiamondJet system vs. a Hybrid 2600 were compared using
A12 (12% cobalt) and A17 (17% cobalt) materials. Table 30 summarizes
the physical properties of the coatings.
Page 77
Table 31 to Table 32 summarize the phase data. The amount of carbide
and therefore carbon retained in the process is higher with the air cooled
Table 30. Physical properties of coatings produced by different guns. 27
Set 1 Propylene
Set 2 Hydrogen
Set 1 Propylene
Set 2 Hydrogen
A-12 (Air-cooled)
A-12 (Hybrid)
A-17 (Air-cooled)
A-17 (Hybrid)
(aa, micro inch)
120-170
90-140
150-250
120-160
(Ra, micro meter)
3.1-4.3
2.3-3.6
3.8-6.4
3.1-4.1
60-65
65-70
60-65
65-70
950±65
1030±46
900±76
1100±74
40-45
23-27
47-55
30-35
Carbide Size (um)
1-2
1-2
4-6
4-6
Porosity (Vol%)
2-4
<2
2-4
<2
55-60
40-45
55-60
40-45
Surface Texture
Macrohardness Rc
Microhardness HV0.3
Carbide Content Vol%
Deposit Efficiency %
system, as would be expected due to the lower operating temperatures of
the system and lower degradation rate. Although not quantitatively
shown, the authors also indicated the A-12 material showed more
degradation, possibly due to the finer carbide size and increased surface
Table 31. Effect of Gas Flows and Cooling Gases on Retained Carbon.
Spray
Set
Material
Fuel Gas
Type
Oxygen/Fuel
Ratio
Cooling Gas
Retained
Carbon
1
5
3
4
2
2
1
A-17
A-17
A-17
A-17
A-17
A-17
A-17
Propylene
Propylene
Hydrogen
Hydrogen
Hydrogen
Hydrogen
Propylene
4.7:1
3.7:1
.34:1
.27:1
.45:1
.45:1
4.7:1
Air
Nitrogen
Nitrogen
Nitrogen
Air
Air
Air
3.3
3.5
2.7
3.3
2.4
2.4
3.0
% Carbide
Out of
Solution
47-55
60-65
45-50
52-57
30-35
23-27
40-45
area for reaction. This can also be seen in comparing the carbide content
of the A-12 material. The microstructure of the hydbrid-gun material
(Figure 39) shows significantly fewer carbides, as Table 30 would lead us
to expect.
Table 32. Retained C and XRD phases.
A-12
Powder
Diamond
Jet
A-17
Hybrid
Powder
Diamond
Hybrid
Jet
Carbon Content
5.6
3.0
2.4
5.1
3.3
2.4
X-ray Diffraction
Phases
WC,Co
WC,W2C,W
WC,W2C,W
WC,Co
WC,W2C,W
WC,W2C,W
Page 78
Carbide dissolution is obviously dependent upon gun type, process
temperature, and starting carbide morphology.
Figure 39. Comparison of carbide content in as-sprayed A-12 (12% cobalt ).
Hybrid 2600 gun (left), air-cooled DiamondJet (right).
10.5.2.2.
Carbide Degradation Indexing
The most important phase in the carbide/cermet coatings is the primary
WC which should be retained through the spraying process. However,
due to carbide dissolution in the Co at high temperature in the flame, a
combination of varied secondary phases from W 2C to amorphous phase
may be present in an as-sprayed structure. It can be difficult, costly, and
time consuming to identify the discrete crystallographic morphologies that
exist in a typical spray deposit.
A semi-quantitative method of indicating the degree of dissolution of the
carbide is being used by NRC, who term it the carbide degradation index
(see Document 14). This method involves the use of XRD to determine
crystallographic phases and then calculates a ratio of primary WC to all
other phases present. The method, developed by Jean-Gabriel Legoux
at NRC, is not intended to be quantitative, but rather to be a simple
qualitative method of gaining insight into the degree of degradation or
dissolution of the carbides, which is useful in optimizing carbide coatings.
The method consists of taking the ratio of the sum of the degradation
products (W 2C, W-Co alloys, etc., which can be seen in the region
between the (100) and (101) WC peaks in Figure 38) to a well-separated
WC peak (such as the WC (001) peak in the same figure). The ratio is an
arbitrary number, but is directly dependent on the extent to which the WC
has broken down.
10.5.3.
General Trend of Carbide Phase
Distribution
In general, an optimized carbide thermal spray should retain the carbide.
In practice, however, some of the carbide is likely to dissolve in the cobalt
matrix or degrade into sub-carbides, lowering the hardness of the coating.
Page 79
Work is currently in progress to understand the relationship between
carbide dissolution and the phases present after spraying. This is critical
to understand the effects of phases present on performance properties
such as fatigue, wear, and impact. A more time efficient, precise, cost
effective method must be developed to address these needs.
Page 80
11. Coating properties
11.1. Summary
This section contains data on the physical characteristics of thermal spray
coatings. The properties covered are:
•
Hardness (Macro/micro)
•
Adhesion/Tensile strength
•
Residual stress (both coating/substrate)
With hardness, direct comparison of data with hard chrome is easily
done. Hardness values can be obtained with HVOF alternatives that
provide superior wear resistance as described in Section 12.6. For
properties such as tensile and residual stress, no direct comparison to
hard chrome exists or data is difficult to obtain.
Tensile and residual stress information are not critical for chrome plating
since the process does not allow control of these properties. The HVOF
coatings exhibit excellent repeatability in tensile and tests are being
developed to better understand this property. A controlled level of
residual stress has been identified as a need in the coatings to improve
resistance to cracking of the coating and minimize (and in some cases
eliminate) fatigue debit in the substrate.
11.2. General Background
Coating properties involve the physical and quantifiable characteristics of
the materials other than the structural and phase data covered in Section
10. These properties include, but are not limited to, hardness,
adhesion/tensile, and residual stress. Hardness is critical for the wear
performance of coatings. Adhesion/tensile is required to insure both
adequate cohesion of the deposit and adhesion of the coating to the
substrate. Residual stress is critical to the ability of the coating to resist
cracking. The magnitude of the stress in the coating must also be
balanced by the values which are present in the substrate; this
degree/magnitude/sign of substrate residual stress affects fatigue life and
the tensile magnitude must be minimized.
This section will describe test methods and results for these properties.
Page 81
11.3. Hardness
11.3.1.
"Factor - Crit Eval of
Microhardness.pdf"
Documents
Document 25. A Critical Evaluation of the Employment of
Microhardness Techniques for Characterizing and Optimizing
Thermal Spray Coatings 2000 (Courtesy of M. Factor and I. Roman,
Hebrew University). 28
11.3.2.
Hardness testing on coatings is very similar in methodology to that
performed on metallic materials. Macro and microhardness are covered
by specifications ASTM E-19 and ASTM E-384 respectively. Coupons
used for metallography are sometimes tested for hardness first and then
cut and polished for microstructure analysis.
Macro or Rockwell hardness for coatings is most often performed using
the R15N superficial scale which applies a relatively small load of 15 kg.
With the usual thickness range of .002-.008”, the use of conventional RC
or RA scales with loads of 150 and 60 kg respectively would result in
penetration of the indenter through the coating thickness. The hardness
reading would then be a composite of the substrate/coating, which is not
representative of true hardness. The surface is usually lightly sanded to
remove irregularities and a series of readings distributed randomly across
the face of the coupon are taken to determine hardness. The readings
are averaged to obtain a composite value since values may vary
dependent upon the phases present under the indenter due to the
heterogeneous nature of some coatings.
Microhardness can be performed using either a Knoop (elongated
impression) or Vickers (diamond impression). For most coating
applications, the Vickers method is used. Since phase distribution is
critical for many coatings, microhardness can identify any segregation or
absence of important constituents in the microstructure. The pattern for
impressions may be random as described for macrohardness, but some
specifications require a stair step pattern through the thickness. This can
check for variations in the spray process as the coating builds up on the
coupon. If something changes in spraying, a hardness change should be
identified in the progression of readings.
11.3.3.
Hardness Results
The goal with HVOF materials is generally to meet or exceed the value
for hard chrome. (Note, however, that hardness does not always relate
directly to wear rate, and softer materials can perform better than harder
ones depending on the surface finish, wear mechanism, etc.) The
generally recognized value for hardness of hard chrome is 800 – 1000, or
an average of about 900 HV(300g).
With HVOF materials, the process parameters can be varied to obtain a
Page 82
wide variety of hardness values normally ranging from 1000 to 1450 HV
for the carbides. Expected values for macrohardness can range from
R15N 88-92. The hardness values are also obviously dependent upon
coating composition and the percentage of carbide vs. matrix present in
coatings such as WC-Co. The ultimate hardness target in HVOF coatings
is dependent upon the final coating/substrate properties desired with
regard to wear, fatigue, etc. When a process parameter set is developed,
the hardness values can be consistently obtained on a repeatable basis.
Since the carbides are so much harder than chrome, hardness can
sometimes be traded for other desired properties, such as fracture
toughness or ease of deposition. (In fact, WC-17Co is often used instead
of the harder WC-12Co for its greater ease of deposition.)
The presentation of HVOF hardness results is primarily a summary of
quality control results obtained during spraying. Hardness is used with
HVOF sprayed materials for the following purposes:
•
A quality control tool to provide process control (both macro/micro)
•
A means of determining phase distribution in the coating structure
(primarily micro)
•
A measure of the carbide degradation that may occur (primarily
micro).
Microhardness
Table 33 lists the microhardness
values obtained in a comparison
of several coating processes and
compositions. The values
obtained for different materials
can be similar as process
parameters are varied for each
material.
Coating
Chemistry
Coating
Process
Density
g/cc
Average
Hardness
VH.3
Chrome
Cr Plate
6.9
900
WC-Co
HV 2000
13.2
1200
WC-Co
Jet Kote
13.2
1100
WC-Co
JP 5000
13.2
1125
WC-Co
D-Gun
13.2
1075
As compared to hard chrome,
WC-Co
SDG
13.2
1100
there are issues that must be
WC-Co-Cr
JP 5000
12.4
1100
recognized when performing
WC-Co-Cr
SDG
12.4
1270
hardness testing on HVOF
materials. HVOF materials are
WC-Cr-Ni
SDG
10.5
1100
heterogeneous in structure. It
should therefore be expected that Table 33. Microhardness for
various HVOF coatings and
the carbide HVOF materials will
show a larger degree of variability equipment (Courtesy Praxair
in the results when 10 readings
Surface Technology).
are taken across the sample,
since the indenter may hit various concentrations of carbide particles or
soft binder. A variation from 1000 to 1400HV is not unusual.
Document 25 is a study of this variation. Porosity in the coating can also
affect results, but this should be mitigated by the relatively high density of
HVOF materials.
Page 83
Table 34 lists results from a comparative study of varied materials all
sprayed with the same parameter set. This data shows:
•
Variability in microhardness data (1032 to 1435 for Coating D )
•
The effect of composition (Coatings C and D with more cobalt are
softer)
•
Different materials with approximately the same soft matrix
percentage will produce similar results (Coatings A/B and E/F)
Table 34. Comparison of Microhardness Values and Resultant Variation (Courtesy
Sulzer Metco and SUNY Stony Brook).
Sample
Code
Nominal
Composition
(wt%)
Manufacturing
technique
DPH300g
St. Dev.
Min.
Max.
A
WC-12Co
-45,+5
Sintered/
Crushed
1334
110
1139
1448
B
WC-12Co
-53, +11
Agglomerated/
Sintered
1261
66
1176
1373
C
WC-17Co
-53, +11
Spray Dried
1068
37
1020
1124
D
WC-17Co
-53, +11
Spray Dried
1187
116
1032
1435
E
WC-10Co-4Cr
-45, +11
Sintered/
Crushed
1389
80
1292
1581
F
WC-10Co-4Cr
-53, +11
Agglomerated
/Sintered
1209
75
1124
1340
Starting
Powder
Size (µm)
Dependent upon the specification, absolute minimum/maximum single
values are sometimes specified and/or minimum/maximum average
values are listed.
Current work shows that values of approximately 1100-1200 HV produce
optimum fatigue results for WC cermets Section 12; wear results (Section
12.6) favor values towards the 1400 range. The variability of carbides is
related to the dissolution of carbides discussed in Section 10.5.
As with WC-Co, the hardness of Tribaloy 400 coatings varies with
deposition conditions, and values of 580-670 HV are typical. This is
significantly less than the hardness of chrome plating. However, T400 is
better in fatigue on aluminum alloys than WC-Co (see Section 12.6.3),
and, although it does not have the wear performance of WC-Co, it is still
better than chrome in hydraulic rig tests (Section 13.2.1).
More actual data for the varied processes is available and usually
incorporated in the documents which are referenced in both Sections 12
and 13 on coating properties and performance respectively.
Macrohardness
The values generally obtained for R15N readings on carbide coatings can
vary from 88-92 dependent upon the spray process parameters. The
requirement for macrohardness varies from supplier to supplier, and it is
not always required. It is normally used as a quick indication of process
Page 84
control and followed by microhardness measurement as a more accurate
gauge of coating properties.
11.3.4.
General Trend of Hardness Results
The general HVOF hardness results vs. hard chrome indicate that higher
values can be obtained with the HVOF alternatives. For each application
the optimum hardness values should be balanced against the other
properties of fatigue, resistance to cracking, and wear performance.
11.4. Adhesion
11.4.1.
Documents
Document 11. Tensile Bond Variance of Thermally Sprayed
Coatings with Respect to Adhesive Type
This article compares the results obtained for tensile bond testing using
both liquid and film adhesives.
11.4.2.
The tensile strength of coatings is generally monitored by use of either
buttons or tensile adaptors/loading fixture sprayed with the material in
question. The typical tensile assembly used for determining strength is
shown in Figure 40 from the
ASTM C-633 specification on
tensile testing.
Figure 40 shows a button as
part of the assembly; if one
fixture is sprayed the
adaptors are bonded
together with a single
adhesive application.
Currently, both liquid and film
adhesive are being used for
this purpose. The
assemblies are cured at
temperatures between 300450 degrees F for 1-3 hours Figure 40. Tensile assembly from ASTM Cand then cooled before
633.
pulling. A sample with no
coating is usually placed in a furnace run to verify proper epoxy curing.
Epoxy only values should exceed 10,000 psi and normally pull in the
12,000 psi range. When the test is required, the material is sprayed with
the part in question and a set of three samples is usually processed.
Document 11 summarizes the advantages and disadvantages of both
epoxy types.
Page 85
Test values can range from 100 psi for very soft abradable coatings to
epoxy only failures at over 12,000 psi for HVOF materials. This is due to
the limited strength value of the epoxy and is not a true test of the coating
strength. Research is in progress to develop stronger epoxies or
alternate testing methods for coating strength.
11.4.3.
Tensile/Adhesion Results
Tensile results for HVOF carbide/cermet coatings are rated as epoxy
pulls because the strength of the HVOF materials exceeds the 12,500 psi
limit of the epoxy. The true strength of the coatings is therefore not truly
known. Research is underway by 3M to develop a higher strength epoxy
but his work in the developmental stages.
11.4.4.
General Trend of Tensile Results
For properly-bonded HVOF coatings, the tensile results should exceed
12,500 psi, which is the upper limit of the testing epoxy. All failures
should be in the epoxy rather than in the coating or at the substratecoating interface. This indicates both good cohesion within the coating
and excellent adhesion to the substrate material.
11.5. Residual Stress
11.5.1.
Documents
Document 26 Behaviour of Tungsten Carbide Thermal Spray
Coatings 1995, J. Wigren et al.29
"Volvo WCCo
Damper Application.p
Study involving application of WC-Co to mid-span dampers on aircraft
blades with fatigue, bend testing, and residual stress analysis.
Removal Method (MLRM) to Evaluate Residual Stress in thermal
Spray Coatings 2000, Ed Rybicki and ASM TSS Committee. 30
"MLRM paper Ed
Rybicki.pdf"
Method for determining residual stress where subsequent layers are
removed and change in stress measured by strain gages.
HVOF Thermal Spray Process 2000, J. Stokes and L. Looney. 31
"Stokes - properties
of HVOF WCCo Spray
This paper reviews residual stress intensity as a function of spray
distance and powder feed rate.
Document 29 X-ray diffraction residual stress techniques, P.S.
Prevey. 32
"Prevey - XRD stress
measurement.pdf"
Describes principles and theory of XRD methods for residual stress
measurement. Includes a number of examples.
Page 86
Coatings-Comparison of Different Spraying Methods 2000,
J.Matejicek et al. 33
"Matejicek - Proc effs
on resid stress Ni5Al.
The neutron diffraction method is used to calculate through-thickness
residual stress values in a substrate thickness of .100” and a coating
thickness of .080”.
11.5.2.
Residual stress in coatings is important for two reasons:
1. The stress state of the coating must be high enough to prevent
spalling or delamination from occurring in service, due to thermal
or mechanical changes to the system.
2. The stress state in the coating can have a direct effect on the
residual stress state of the substrate, and thus on the final
material properties, particularly fatigue.
Section 7.3.7 discusses the Almen Strip as the current quality control tool
for rapid verification of the degree of compressive residual stress present
in HVOF coatings. Mechanical property data indicates that compression
in the coating is beneficial for fatigue. Techniques for residual stress
measurement fall into two different categories: qualitative and
quantitative.
Qualitative techniques give a relative value of a specimen at a fixed set
of parameters. Examples of these techniques are listed in Table 35. This
helps to control the process but the values are relative to other
evaluations that have established acceptance or rejection criteria via
quantitative residual stress measurements. Industry procedures for
qualitative techniques are currently being formulated. The measurements
can be very technique sensitive. It is currently difficult to compare values
from location to location for consistency although in-shop consistency is
excellent when written guidelines for spraying and evaluation are
established.
Page 87
Table 35. Qualitative techniques for measuring residual stress.
Name
Description
Almen
Thin strip measured for deflection before spraying, held in fixture
during spraying, and then deflection/bowing of strip measured after
spraying. Difference in deflection is indication of residual stress.
Method most commonly used for HVOF QC.
In-process
Almen Strip
Same principle as Almen above except deflections are measured
during spraying by means of very sensitive deflectometers
Bend Testing
Coupons are sprayed with a coating and then bent to a specified
dimension. Number of cracks and lack of spalling correlated to
quantitative residual stress values and accept/reject limits
established
Quantitative techniques are the same as substrate techniques with
some allowance for the presence of an interface between coating and
substrate. Some commons methods are listed in Table 36. The
quantitative techniques provide absolute values of the residual stress in
the material. These techniques are usually more time consuming and
costly, but provide the background data that allows acceptance or
rejection criteria to be established for the qualitative methods used in QC.
As with qualitative techniques, industry procedures for quantitative
techniques are currently being formulated. The measurements can be
very technique sensitive and coating material properties are required for
the calculations. It is currently difficult to compare values from laboratory
to laboratory for consistency although in-lab consistency is excellent
when written guidelines for evaluation are established. Work is in
progress to establish consistent usage of established material properties.
The need for compressive stresses in HVOF carbide coatings has been
established but values will vary with coating material and thickness.
Page 88
Table 36. Common quantitative residual stress measurement techniques.
Name
Description
Modified Layer
Samples are polished to remove a given thickness of material and
Removal Technique change in stress is measured by strain gages attached to
specimens.
Strain in the crystal lattice is measured, and the residual stress
producing the strain is calculated. Applicable to all crystalline
materials (metallic or ceramic). Surface residual stress
measurement is non-destructive. Subsurface measurements are
obtained by removing material via electropolishing methods.
Penetration of x-rays relatively shallow compared to neutron.
X Ray
Diffraction
Neutron Diffraction
Based on the same general principals as x-ray technique. The
main difference is the size and location of the diffracting volume in
the sample as a result of the lower absorption of neutrons.
Subsurface measurements can be obtained non-destructively.
Knowledge of the unstressed lattice spacing is required in order to
calculate the residual stress from the measured strains.
11.5.3.
11.5.3.1.
Residual Stress Results
Almen strip
The simplest method of stress measurement is the Almen strip, which is
now frequently specified as a QC parameter, using Almen strips coated at
the same time as the component. The Almen data gives a good
indication of whether the stress is compressive or tensile, within the
limitations we have already described. Almen measurements have been
found to be a good predictor of fatigue life,34 since as a general rule
fatigue is enhanced by compressively stressed coatings and reduced by
coatings with tensile stress (such as chrome).
Page 89
Stress tends to build up with coating thickness. This becomes very
important when using thermal spray coatings for rebuild, because the
stress can become so high in very thick coatings that it causes
delamination, especially under high stress.
In other situations, however, it appears that the stress is self-limiting.
Figure 41 shows data for a series of HVOF WC-CoCr coatings deposited
8
7
6
Almen
5
Total Almen
Normalized Almen
4
3
2
1
0
2.3
4.5
5.9
7.1
9.5
Total thickness
Figure 41. Stress as a function of coating
thickness for HVOF WC-CoCr.35
under the same conditions but at varying thicknesses. Clearly, the total
Almen number rises (the coating becomes more compressive) with
thickness, but is limited and reaches a maximum by about a 0.007”
thickness. When normalized to a 0.005” coating (i.e. divided by
thickness/0.005”), the normalized number drops above 0.005” showing
that the stress does not build up linearly with thickness.
The reasons for these differences in stress behavior are unknown, but are
doubtless related to process conditions and powder material. For this
reason, coating specifications generally require the Almen stress to be
within a specified range for a specific thickness, and one cannot simply
make the assumption that doubling the thickness will double the stress.
11.5.3.2.
Almen/Residual Stress Comparison
HVOF Thermal Spray Process 2000, J. Stokes and L. Looney
This paper presents a study of the residual stress and microstructural
properties of thick spray formed components produced using HVOF
thermal spray processing. It is generally an analysis of the dependence
of residual stress and resultant material characteristics on spraying
distance and powder feed rate conditions.
This investigation takes the quality control tool of Almen strips, and using
the equation below calculates actual residual stress values.
Residual stress, σ = ECY/R
Page 90
where EC is coating modulus of elasticity, Y is Neutral axis of composite
strip, R is the bending radius.
This is a method to estimate residual
stress magnitude. It tends to
underestimate the actual stress in a
component due to factors such as
heat sink, thickness, etc.
Residual stress is closely related to
spraying conditions – especially spray
distance and powder feed rate. Figure
43 and Figure 44 show the
relationships between residual stress,
spray distance, and powder feed
rates. The zoning of the plots is an
excellent representation of how the
HVOF process produces varying
results as the parameters are changed
within a range. The available thermal Figure 42. Almen strip stress
energy must be used efficiently to melt measurement.
the particles but a portion of that
energy is also transferred to residual stresses in the final deposit.
Figure 43. Average residual stress as a function of spray
distance.
Page 91
Figure 44. Average residual stress as a function of powder
feed rate.
Table 37. Zone analysis of thermal spray coatings.
Spray distance analysis
Powder feed rate analysis
Zone 1
The coating is too hot because
there is little time for cooling in
such a small spray distance and
you would expect substantial
residual stress which is not
acceptable. Structure is
acceptable
Zone A
The feed rate in this range in too
low to produce particle cohesion
and the resulting microstructure is
unacceptable. With a porous
deposit, residual stress is also low.
Zone 2
An acceptable combination of
coating microstructure and
reasonable residual stress.
Zone B
An acceptable combination of
coating microstucture and
reasonable residual stress.
Zone 3
The residual stress drops due to
the reduced thermal energy from
the longer spray distance and the
microstructure degrades the
particles lack sufficient energy for
spalt formation.
Zone C
The feed rate in this range cannot
melt all the particles but a
substantial amount is till deposited
resulting in high residual stress.
The following information can be ascertained from the data:
•
Almen Strips can be used to obtain a quantifiable indication of the
numerical magnitude of residual stress
•
For spray distance, there is a target range where the particle
temperatures result in both good coating microstructure and
acceptable residual stress.
•
For powder feed rate, there is a target range where the amount of
material being deposited results in both good coating
Page 92
material being deposited results in both good coating
microstructure and acceptable residual stress.
•
The input of thermal energy and powder material must be
balanced to provide enough heat for melting the particles and still
result in acceptable residual stress upon cooling.
11.5.3.3.
Modified Layer Removal Technique
1995, J. Wigren et al.
Removal Method (MLRM) to Evaluate Residual Stress in thermal Spray
Coatings 2000, Ed Rybicki and ASM TSS Committee
Document 26 evaluates WC-Co, WC-Ni, and WC-CoCr (Table 38)
sprayed with the HVOF process at a variety of different spray parameter
settings. A wear problem was identified on engine fan blades and the
goal of the evaluation was to balance compressive residual stress in the
coating to prevent spalling in service with a minimal degradation of
substrate fatigue life. The following tests were made:
•
•
•
Three point
bend testing
for crack
resistance
Residual
stress
measured
by Modified
Layer
Removal
Technique
Table 38. WC coating system designations for
Document 24.
Coating
composition
Application process
HVOF
Plasma
A
B
C
D
E
F
G
H
1. (WC-Co)
●
●
●
●
●
●
●
●
2. (WC-Ni)
●
3. (WC-Co/Cr)
●
●
Low cycle
fatigue testing
Page 93
Residual stress and bend test results will be discussed here. Fatigue
results from this evaluation are summarized in Section 12.6.3.5 in Coating
Performance.
Figure 45 shows the
bend test information.
This must be
compared with the
residual stress results
shown in Figure 46
and obtained with the
Modified Layer
Removal Technique
summarized in
(Document 27).The
average residual
stress in the substrate
represents locations
close to the
coating/substrate
interface. As
expected, the ranking
in the bend test shows
F-1, A-1, and G-1 as
most resistant to
Coating
Ranking
Coating
Ranking
system
system
cracking due to the
F-1
1
D-1
7
compressive stresses
as measured in the
A-1
2
D-2
8
coating. This follows
G-1
3
H-1
9
the trend in all work
C-1
4
E-3
10
thus far towards
B-1
5
E-1
11
compressive stresses.
A-2
6
Coating A-1 was
chosen for
Figure 45. Bend test technique, evaluation
implementation based
criteria, results.
upon these results
and fatigue data.
Coating F-1 showed spallation in engine testing due to the higher
compressive stresses as compared to A-1.
Page 94
The residual stress and its distribution as a function of depth through the
coating and into the substrate are shown in Figure 46. Compressive
stresses are negative, and tensile stresses are positive. Note that if
the average stress in the coating is compressive, the average stress in
Figure 46. Typical stress profile for modified layer removal technique.
the substrate is negative to balance out the overall stress.
Note that this stress measurement technique is quite accurate, and yields
the stress variations with depth. As a QC method it would be far too timeconsuming and expensive. For this reason, QC relies on the average
stress, as measured by the almen strip method.
11.5.3.4.
"Matejicek - Residual
stress in plasma spray.pdf"
Residual Stress by X-ray Diffraction
Document 31 Residual Stress Measurement in Plasma
Sprayed Coatings by X-Ray Diffraction (Courtesy of J.
Matejicek et al) 1997. 36
Document 29 X-ray diffraction residual stress techniques, P.S.
Prevey.
X Ray diffraction (XRD) is one of the most common quantitative methods
used to determine residual stresses in coatings and many other metallic
materials. Features of the method are:
•
Ability to measure residual stress in a very thin surface layer (i.e. a
few splat thicknesses), thus allowing assumptions of plane stress
•
Phase distinctive – can determine stress in specific phases, rather
than only in the material as a whole
•
A through-thickness stress distribution can be measured by
electropolishing a small area together with continuous XRD
measurements
o
This method can be somewhat non-destructive if the
Page 95
electropolishing can be performed in a non-critical area.
The referenced documents contain data on XRD methodology and
analysis. To date, the majority of XRD information on coatings has been
obtained using the surface only method (Document 31) in lieu of through
thickness analysis. Work is in progress to better understand XRD
analysis and will be available on the HCAT Home Page by December
2000.
Document 31 is a summary of the XRD method for near surface residual
stress in Ni, NiCrAlY, and YSZ coatings. The effects of surface finish and
material anisotropy are discussed. A very critical detail of this summary is
the comparison of data from both neutron diffraction and blind hole
techniques to XRD for Ni
coatings as shown in Table 39.
Table 39. Comparison of Residual
Stress by Varied Techniques.
Although the comparison of
methods shows qualitative
Specimen
Ni-APS
Ni-VPS
agreement, the magnitude of
Method
the values is substantially
σxx
σxx
different. Unfortunately, this is
(MPa)
(MPa)
a result of the many
X-ray
62
-116
assumptions required in
diffraction
determination for each method
and the lack of standardization
Hole drilling
241
-55
and established material
Neutron
186
-37
constituents for coating
diffraction
materials. This is an important
area of quality assurance and is
being addressed by industry
research groups.
11.5.3.5.
Residual Stress by Neutron Diffraction
Coatings-Comparison of Different Spraying Methods 2000, J.Matejicek
This study uses neutron diffraction to determine the residual stress
patterns on thin specimens sprayed with Ni+5Al% coatings. Three
different spray methods with significantly different particle temperatures
and velocities were used to
apply the coatings.
Table 40. Experimental set-up for
The experimental set-up is
neutron diffraction.
listed in Table 40. The neutron
Spray equipment Sample
diffraction method, like X-ray
diffraction, is based upon the
HVOF
0.1”X1”X2”
accurate measurement of
Air plasma spray
Ni + 5wt% Al alloy
changes in the spacing of
different crystal planes (i.e.
Wire arc spray
.080” thick
strain) based on shifts in
positions of diffraction peaks.
From the strain, the stress can
Page 96
be calculated with the use of appropriate elastic constants (which can
sometimes be difficult to determine).
Figure 47, Figure 48, and Figure 49 summarize the data for the three
spray processes, and show the following:
•
In air plasma spray, the overall stress is slightly tensile with a
gradient towards more tension near the deposit surface as a result
of successive buildup of deposit layers with tensile quenching
stresses. The substrate stress is therefore slightly compressive.
•
In wire arc spray, the stress gradient is somewhat higher, while in
the substrate, it is negligible. This difference between air plasma
and wire arc may be caused by the difference in deposit density
which will obviously contribute to final coating residual stress.
Higher porosity will allow for stress relaxation which may be
occurring in the wire arc deposit.
•
In HVOF, the stress is mainly compressive as a result of the
peening action of the high velocity particles, whose impact
plastically deforms the previously-deposited layers. This
compressive coating stress causes a tensile residual stress in the
substrate to balance stresses. These stress gradients and levels
can be varied with different spray parameters.
The important concept is the differentiation between the cause of the
residual stress: quenching stresses in air plasma vs. a peening action in
HVOF. This supports the use of HVOF processes with the compressive
residual stresses needed to enhance coating performance characteristics.
Figure 47. Air Plasma Spray residual stress pattern.
Page 97
Figure 48. Wire Arc Spray residual stress pattern.
Figure 49. HVOF Spray residual stress pattern.
11.5.4.
General Trend of Residual Stress Results
The general trend for HVOF coatings is to have compressive stress in the
coating, which even though it results in tensile stress in the substrate, has
the overall effect of minimizing or eliminating the fatigue debit. However,
simply maximizing the coating stress is not a good approach, as this
reduces coating fracture strength and can even lead to coating
delamination if compressive stresses become too high. Compressive
stress measured by Almen strips is a simple quality control that gives an
indication of the residual stress pattern, but the actual stress in the
coating on the part will be different due to size, heat sink, etc. Work is in
progress to develop a better correlation between Almen strips and actual
residual stress coating/substrate values.
Work to date has shown that Almen strip stress measurements are highly
dependent on the temperature reached by the Almen strip, which in turn
depends on the way the strip is held, and the speed and direction in
which it is sprayed. When using Almen strips these factors must be built
in to the test procedure to ensure consistency.
Page 98
12. Coating performance
12.1. Summary
Table 41. Summary of performance tests.
Measurement
Typical test
Notes
Corrosion
Landing gear steels B117
4340, PH13-8Mo B117
4340, PH-13-8Mo, 7075Al GM
9540P/B
Lufthansa
HCAT
HCAT
Atmospheric 4340, PH-13-8Mo, Atmospheric HCAT
7075Al beach
exposure
HVOF WC cermets vs stainless Electrosteel chemical
NRC
Fatigue
Air, corrosive, various
temperatures
4340 high strength steel, PH13- ASTM
8Mo stainless, 7075 Al E466-96
HCAT general
300M, 4340, Aermet 100 ASTM
E466-96
WC-Co, WC-CoCr, WC-Ni
HCAT landing gear
Fan blade midspan evaluation,
Volvo
Abrasion, erosion, sliding,
fretting
Wear
Erosion
Various guns, coatings
Abrasion
Sliding, fretting
Impact
Hydrogen embrittlement
ASTM F519
Lufthansa results and HCAT
planned tests
GE Aircraft Engines data
Creep
Coating performance measures how the coated substrate performs in
laboratory tests. Performance depends on coating properties, substrate
Page 99
properties, and how the coating and substrate interact. Much, but not all,
of this data is derived from a number of test protocols carried out by the
HCAT/C-HCAT cooperative effort. The performance data covered in this
section are listed in Table 41. Go directly to the relevant section by
clicking on the blue links.
The majority of the data is a direct comparison between hard chrome and
the HVOF alternatives. Most, if not all, performance data to date
show that the HVOF carbides have performance equal or superior to
hard chrome plating. Some of the testing listed is currently in progress
and should be available on the HCAT Home Page in the next 6-12
months.
12.2. General Background
Chrome has been used for many years in a variety of environments with
success, and coating performance must either meet or exceed current
design allowables. A substantial amount of work is currently being
performed in the industrial sector to evaluate coating performance, but
the most extensive and detailed efforts are being conducted by the Hard
Chrome Alternatives Team (HCAT) and Canadian Hard Chrome
Alternative Team (C-HCAT). The HCAT effort has been in place since
1995 to demonstrate and validate the replacement of hard chrome plating
with HVOF coatings. The Canadian counterpart began work in 1999. As
mentioned in Section 1.2, the HVOF carbides (WC-Co and WC-CoCr)
have been identified as primary replacements for chrome plating, and
extensive test protocols have either been carried out or are in process, as
will be highlighted in this section.
12.3. Documents
Document 32 HCAT Test Protocol for Initial Work 1996 (Courtesy
of HCAT Team)
"HCAT Test Plan original.pdf"
Summarizes the testing protocol initially developed to validate HVOF as a
viable chrome alternative.
(Courtesy of HCAT and CHCAT Teams)
"HCAT JTPPart I
Landing Gear.pdf"
Protocol developed for validation of WC-Co and WC-CoCr as chrome
replacements for landing gear applications.
Document 34 Joint Test Protocol (JTP) for Propeller Hub
Components 2000 (Courtesy of HCAT, JG-PP, and C-HCAT Teams)
"HCAT JTP Propeller
hubs.pdf"
Protocol developed for validation of HVOF coatings as chrome
replacements for propeller hub applications
Page 100
Document 35 Joint Test Protocol (JTP) for Gas Turbine Engines
2000 (Courtesy of HCAT and PEWG Teams)
"PEWG GTE JTP.pdf"
Protocol developed for validation of HVOF coatings as chrome
replacements for gas turbine applications – this protocol is under
development at time of writing.
12.4. Test Protocol Summaries
12.4.1.
Document 32
HCAT Team)
Start-up test Protocol
HCAT Test Protocol for Initial Work 1996 (Courtesy of
Initial work was planned in 1996 to provide a generic evaluation of
possible HVOF thermal spray alternatives. The emphasis was placed
upon implementation of the chrome alternative on components
undergoing repair at military depots. As a generic plan, the purpose was
to evaluate a broad range of materials to assess the potential of HVOF
coatings. This work centered around two coating types:
1. WC-17Co. The most commonly used HVOF coating is WC-Co
(either WC-17%Co or WC-12%Co. Of these two materials the
easiest to spray is the WC-17Co. The WC-12Co is somewhat
harder, but is more brittle and more difficult to spray. Given that
both materials are superior to chrome in wear resistance, WC17Co was chosen.
2. Tribaloy1. Tribaloy 400 and 800 are
wear-resistant Co-based alloys
frequently used in engine and other
wear applications. Their
compositions are given in Table 42.
Three alloys were chosen:
Table 42. Chemistry of
Tribaloys.
Co Mo Cr
T400 62
28
8
Si
2
T800 52 28 17 3
1. 4340 steel. 4340 is the most
common steel used for hydraulics. It
is very similar to 4340(M) and 300M,
the most common landing gear steels. For landing gear the 4340
would be heat treated to 260-280 ksi.
2. PH13-8-Mo steel. This is a common precipitation-hardened alloy
frequently used for helicopter components.
3. 7075-T73 aluminum. This alloy is commonly used for landing
gear and other large components
Performance of the coating on component alloys was evaluated in the
following tests:
•
Friction and wear
1 Tribaloy is a trade name of Stellite Corp.
Page 101
•
Fretting
•
Fatigue
•
Corrosion
Data from this work is summarized below under the appropriate heading
of this section.
Status – This work is complete.
12.4.2.
JTP for Landing Gear
After the initial protocol work validated the potential of HVOF coatings, a
plan was formulated to address specific components for full scale
validation. This joint effort between HCAT and C-HCAT is currently in
progress. Two coatings have been chosen for evaluation:
U.S. HCAT: WC/Co (83%/17%) HVOF coatings
Canadian HCAT: WC/CoCr (86%/10%4%) HVOF coatings
Three substrate materials are being evaluated to cover the wide range of
landing gear applications, as
Table 43. Materials and heat treats
shown in Table 43.
for HCAT Landing Gear JTP.
Performance tests being
Material
Heat Treat (tensile
conducted are:
strength)
• Fatigue
4340
260-280 ksi
• Corrosion
300M
280-300 ksi
• Wear
Aermet 100
280-290 ksi
• Impact
•
Hydrogen Embrittlement
Data currently available from this work are summarized under the
appropriate heading of this section (below).
Status – Close to completion. This work should be complete by the end
of 2000.
12.4.3.
Other Protocols
There are other types of components for which coating performance
protocols are currently being formulated.
•
Propeller Hubs: Document 34 Joint Test Protocol (JTP) for
Propeller Hub Components 2000 (Courtesy of HCAT, JG-PP, and
C-HCAT Teams)
•
Gas Turbine Engines: Document 35 Joint Test Protocol (JTP)
for Gas Turbine Engines 2000 (Courtesy of HCAT and PEWG
Teams).
Page 102
•
Hydraulic actuators – in process of formulation.
•
Rotary wing (helicopter rotor head) components – in process
of formulation..
Progress and current copies can be obtained in the Joint Test Protocols
section of the HCAT Home Page.
12.5. Corrosion
12.5.1.
"AESF Plating Forum
Mar1998.pdf"
Documents
Document 36. Report of Replacement of Chromium Electroplating
Using HVOF Thermal Spray Coatings AESF Plating Forum 1998
(Courtesy of Bruce Sartwell and HCAT Team). 37
Report details the results of property and corrosion testing from protocol
referenced in Document 30.
Lufthansa). 38
"Lufthansa Corrosion
testing.pdf"
A summary is given of the ASTM B117 results which show superior
performance of HVOF coatings vs. current hard chrome plating solutions.
(Note: this is a very large document.)
Document 38. Replacement of chrome plating by thermal spray
coatings – Summary of tests (Courtesy of Lufthansa). 39
"Lufthansa
testing.pdf"
"Simard Performance of HVOF
This document summarizes the results of the testing used to qualify
HVOF WC-CoCr for flight testing, as well as the results of initial flight
tests.
Aqueous Corrosive Environments 2000 (Courtesy of S. Simard
(NRC) et al). 40
This paper summarizes corrosion studies performed using anodic
polarization (Tafel plot) methodology to assess corrosion resistance of
carbide coatings
Page 103
12.5.2.
Corrosion Test Methods
Corrosion testing can be performed by many different techniques
dependent upon the corrosion environment of the application being
evaluated. Table 44 highlights some of the most common test methods.
Table 44 Common Corrosion Testing Methods
Type
Description
Atmospheric
Expose to atmosphere in environments such as
marine air and salt spray with recording per ASTM G
33-88
Simulated Chamber Testing
Continual Salt Spray Exposure to ASTM B-117
Intermittent and Cyclic Salt Spray Exposure to GM
9540P/B
Continual Salt Spray/SO2 Exposure to
ASTM B-117
Electrochemical Potential
Determination
12.5.2.1.
Varied Methods such as Tafel Plot
Atmospheric Methodology
The atmospheric corrosion tests generally follow ASTM G33 entitled,
“Standard Practice for Recording Data from Atmospheric Corrosion Tests
of Metallic-Coated Steel Specimens.” The size of the test panels is
typically 6 inches by 4 inches by 3/16-inches in thickness and reasonable
statistics require at least five panels of each base alloy with each coating.
In order to evaluate through-thickness corrosion, which would occur if the
coating is scratched through, the coating is scribed with an “X”, which
penetrates through the coating to reveal bare surface. The atmospheric
tests are typically run for at least 4000 hours, during which the panels are
checked and data recorded at appropriate intervals.
Two types of atmospheric corrosion tests may be performed.
1. The panels are mounted on racks which are exposed to sunlight
and the marine air, but which are not subject to splashing with salt
water.
2. Similar mounting and exposure except that the panels are
subjected to periodic splashing with salt water (usually done
manually for consistency).
12.5.2.2.
Simulated Cabinet Testing
Cabinet testing of this type is normally conducted in a salt spray chamber
such as Q-Fog Model CCT600 or equivalent at ambient temperature.
Page 104
Salt Fog Corrosion Test
The standard salt fog test follows ASTM B117:
(1) Position of the panels during exposure – The specimens are
placed into a salt spray/fog chamber so that they are supported or
suspended between 15 and 30° from vertical. The specimens are
exposed to salt spray/fog continuously for 1,000 hours.
(2) Preparation of the salt solution – A typical test involves a 5
percent sodium chloride salt solution (5 ± 1 parts by weight of
NaCl in 95 parts of water). The pH of this solution should be
between 6.5 and 7.2.
(3) Specimen instpection – Visual inspection of the specimens for
surface corrosion can occur at any interval, but common periods
are after 48 hours, 96 hours, and every 100 hours between 200
and 1000 hours. It is customary to remove the samples at 500
and 1000 hours for photographing.
GM Cyclic Corrosion Test
This test is conducted in accordance with the General Motors
GM9540P/B protocol, which is indicated in Table 45. This test is intended
to be less severe than B117, but to be similar to the types of exposures
characteristic of automotive components.
Page 105
Table 45 GM9540 Protocol for Corrosion testing.
GM9540P/B Cyclic Corrosion Tests
Solution:
0.9% NaCl, 0.1% CaCl2 and 0.25%
NaHCO3
pH:
6.0 - 8.0
Test protocol:
Step 1
Subcycle step 2-3 repeat 4
times
Step 2
Salt mist
25 C
15 min
Step 3
Dry-off
25 C
75 min
Step 4
Dry-off
25 C
120 min
Step 5
RH 95100%
49 C
8 hours
Step 6
Dry-off
60 C
7 hours
Step 7
Dry-off
25 C
1 hour
Step 8
Final step, go to step 1
Note: RH = relative humidity
Test duration:
Examined every
2000 hrs
125 hrs
At each 500 hours interval, the specimens can be removed and
photographed.
SO2 Salt Fog Test
This test is conducted in accordance with ASTM G85-85. It is very similar
to ASTM B-117 as summarizes earlier with the following exceptions:
SO2 gas is introduced for one hour every 6 hours
All other parameters with respect to inspection intervals, salt spray, etc.
are identical.
This test is frequently used to simulate exposure on aircraft carrier decks,
where aircraft are exposed to NaCl spray and sulfur from jet exhaust.
Page 106
Specimens
For cabinet testing, either flat or cylindrical specimens can be used.
Groups of five are usually run and one specimen in each group will have
an “X” scribed in the coated surface. The “X” should penetrate through
the coating to evaluate through-thickness corrosion. The smaller angle of
the “X” should be 30 to 45 degrees. Each line of the “X” should be
approximately 2 inches long.
On the uncoated edges and back of each specimen, an inert wax or
epoxy is applied to ensure no galvanic couple between uncoated and
coated areas.
Based on visual inspection, a ranking is
applied to each specimen at each
interval of inspection and these rankings
are tabulated and displayed graphically.
The rankings are assigned in
accordance with ASTM B537-70 as
outlined in Table 46.
Table 46. Visual ranking
criteria (ASTM B537-70).
Defect area (%)
Rank #
0
10
0 - 0.1
9
Electrochemical Potential
0.1 - 0.25
8
Electrochemical testing usually involves
performing anodic polarization tests in
electrochemical cells. Anodic
polarization curves or Tafel plots
provide information on
0.2 - 0.5
7
0.3 - 1.0
6
1.0 - 2.5
5
2.5 - 5.0
4
5 - 10
3
10 - 25
2
25 - 50
1
>50
0
1. the relative rates of corrosion,
2. the type of corrosion (localized
versus general) occurring, and
3. the presence of connected
porosity in coatings (pores that
provide a path from the
electrolyte to the underlying
substrate).
Anodic polarization curves can be obtained as described in ASTM G5.
The electrolyte can be varied dependent upon the environment being
encountered. For work on replacing hard chrome, an electrolyte of
sodium chloride solution is a good choice because this represents many
environments encountered by DoD weapons systems. A typical test
might be a standard one compartment corrosion cell. The samples are
typically immersed in a quiescent (open to the atmosphere without air
sparging) chloride solution and allowed to stabilize in the solution for 1
hour prior to polarization. Ramping and sweep rate can vary but an
example might be a potentiodynamic ramp from the open circuit potential
to +1.6 volts at a sweep rate of 0.6 volts per hour. A saturated calomel
electrode is usually used as the reference electrode.
Page 107
Anodic polarization curves should be obtained for the uncoated and
coated samples for all coating/substrate combinations, with hard chrome
coated samples as a control. Duplicate experiments for each sample
type are a good experimental practice. The information obtained from the
polarization curves can then be used to compare the protective nature of
the coatings to that of hard chrome.
12.5.3.
12.5.3.1.
Corrosion Data
Simulated Cabinet Results from Lufthansa
Lufthansa)
This report is a very extensive evaluation of the corrosion performance of
thermal spray coatings compared with chrome plate. It summarizes in
detail comparative salt spray corrosion tests according to ASTM B117
which were performed on different HVOF coatings and different chrome
plates as shown in Table 47. The results of these tests were used by
Lufthansa as the basis for their choice of HVOF WC-CoCr as a
replacement for chrome plating on landing gear.
Page 108
Table 47. Coatings tested (Lufthansa). Note: 25µm+0.001”
Chrome
Platings
Tested
Plating
Thickness [µm]
After grinding
Chrome
“
HVOFCoatings
Tested
Specification
Coating
QQ-C-320B
100 & 200
HEEF 25
100
Spraying Equipment
Thickness [µm]
After grinding
WC-Co
DJ 2600
100 & 200
“
JP-5000
100 & 200
“
Jet Kote II
100
WC-Ni
JP-5000
200
WC-Co-Cr
JP-5000
100 & 200
Jet-Kote II
100
JP-5000
100 & 200
Jet-Kote II
100
“
Cr3C2-NiCr
“
Cr-Ni-B-Si
JP-5000
200
“Proprietary
WC-Co
Super-D-Gun
100
Coatings”
WC-Co-Cr
Super-D-Gun
100
Tested
WC-Cr-Ni
Super-D-Gun
100
The results of the testing are shown in Table 48. The investigation
showed that several HVOF sprayed WC-CoCr coatings provide much
higher corrosion resistance than chrome plate, which failed after three
days of corrosion test exposure. In fact, some WC-Co and all WC-CoCr
HVOF coatings survived 1000 hours of salt spray testing with no sign of
corrosion either of the coating or the base metal. In all cases the HVOF
WC-CoCr coatings performed the best. Cr-Ni-B-Si also performed very
well, and were easy to spray, making them attractive alternatives.
However, the lack of a chemical strip for these coatings led Lufthansa to
reject them.
WC-Co and WC-Ni showed some corrosion of the metal matrix, with
surface discoloration and roughening. WC-CoCr of WC-CoNi showed no
matrix corrosion. The use of an epoxy sealer did not change the
corrosion of any of the HVOF coatings.
Since HVOF WC-CoCr showed the best corrosion results, Lufhansa
adopted this material for flight tests.
Page 109
Interpretation of results
The analysis showed the WC-Co or WC-Ni based coatings exhibited
corrosion of the Co or Ni matrix materials. In contrast, the WC-NiCr and
WC-CoCr had excellent matrix corrosion resistance. WC-CoCr showed
no attack on the substrate metal after extended exposures of up to 1,000
hours. WC-CoCr was taken forward to flight testing since it provided the
highest level of protection on ASTM B117 salt spray tests, with far higher
performance than chrome plate.
Table 48. Summary of corrosion ratings for coatings tested by Lufthansa.
All Chrome Platings
HVOF WC-Co
HVOF WC-Co-Cr
HVOF Cr 3C2-NiCr
SDG 2040G (WC-Co)
SDG 2020
(WC-Co-Cr)
SDG 2005
(WC-Cr-Ni)
All Chrome Platings
HVOF WC-Co (DJ2600[2])
HVOF WC-Co (JP-5000[3])
HVOF WC-Ni
HVOF WC-Co-Cr
HVOF Cr-Ni-B-Si
HVOF Cr 3C2-NiCr[4]
[1]
Please note that this graphic shows an “average” of days to occurrence of corrosion with regard to the coating type. Of each coating type, at least
four specimens had been tested which generally did not fail at the same testing time. For exact data, please refer to Figure 17 (100ìm thick coatings)
and figure 18 (200ìm thick coatings).
[2] This WC-Co coating (Diamalloy 2005, DJ2600 sprayed at LHT) passed the test, whereas all other WC-CO coatings failed.
[3] In contrast to [2], the WC-Co coating applied by the JP-5000 using powder AWN 3073 failed after approx. one week due to base material
corrosion.
[4]
This coating was incorrectly sprayed, i.e. too much powder was fed to the gun, resulting in a poor microstructure.
Please note that “thinner” coatings of the same chemistry revealed a distinct better corrosion performance.
Page 110
12.5.3.2.
Cabinet and Atmospheric Testing - HCAT
Document 36. Report of Replacement
of Chromium Electroplating Using HVOF
Thermal Spray Coatings AESF Plating
Forum 1998 (Courtesy of Bruce Sartwell
and HCAT Team)
Table 49. Coatings and
substrates - HCAT corrosion
testing.
Coatings
Substrates
This report summarizes simulated
Chrome
4340
cabinet work which included ASTM
7075 Al
WC-Co
B117 salt spray and GM 9450P/B cyclic
work plus atmospheric exposure studies T-400
PH 13-8 Stainless
carried out under the initial testing
protocol (Document 32). Substrates
and coatings are listed in Table 49. All
coatings were nominally 0.004” (100µm) thick. Samples for the cabinet
work were evaluated at 125 hour intervals.
12.5.3.2.1.
ASTM B117 Salt Fog Testing
Data from the 4340 substrate in ASTM B117 Salt Fog Testing is shown in
Figure 50 and Table 51.
The ASTM standard requires that the surface be measured and ranked
without cleaning. The Appearance Ranking was therefore determined by
observation of the surface on taking the samples from the chamber.
However, cleaning of the surfaces permitted better evaluation of the
degree of actual damage to the coating – including blistering and
undercutting. The Protection Rating was therefore measured after the
sample surface had been cleaned and the blisters and undercut portions
of the coating removed. The Protection Rating was measured for the
coating surface and edges separately. The data indicates that WC-Co
actually performs better than hard chrome in a side by side comparison.
Although the T-400 showed a better appearance rating, the surface was
found to be severely blistered with over 50% of the surface affected.
Clearly, using the ASTM standard test method, there was significant
deterioration of the surface in all cases, with the WC-Co performing
somewhat better than the hard chrome, and the T400 performing the
best. However, the Protection Rating showed that the T400 provided less
overall protection, and was significantly worse than chrome. The WC-Co,
on the other hand, was comparable to, or a little better than, chrome.
For stainless steel corrosion was significantly less, as one would expect.
The WC-Co performed at the same level as the chrome, but the T400
was measurably worse.
The situation for 7075 Al was significantly different, because chrome
plating on aluminum requires a double-zincate process and copper and
nickel strikes for adhesion, which appeared to protect the surface from
corrosion better than chrome alone.
Page 111
4340 Steel Substrate
ASTM B117
4340 Steel Substrate
GM9540P/B
T400 4340
Cr 4340
8
10
8
WC-Co
6
T400
10
WC-Co 4340
Appearance Ranking
Ranking
10
6
T400
4
4
Chrome
2
2
0
0
0
250
500
750
1000
WC-Co
8
Cr
6
4
T400
Cr
2
WC-Co
0
1250
0
500
1000
1500
12
WC-Co
10
Cr
Appearance Ranking
12
10
Chrome
8
8
T400
6
6
4
4
2
2
0
0
10
T400
8
WC-Co
6
T400
WC-Co
Cr
4
2
0
0
0
250
500
2500
13-8 Stainless Steel Substrate
GM9540P/B
13-8 SS Substrate
ASTM B117
Ranking
2000
Hours
Hours
750
1000
500
1000
1500
2000
2500
Hours
1250
Hours
Aluminum Substrate
GM9540P/B
Aluminum Substrate
ASTM B117
Cr
Chrome
10
10
Ranking
8
8
T400
6
6
4
4
WC-Co
T400-Al
WC-Co-Al
Cr-Al
2
2
250
WC-Co
8
T400
6
4
T400
WC-Co
2
Cr
0
0
0
0
0
Appearance Ranking
10
500
750
1000
500
1000
1500
2000
2500
Hours
1250
Hours
Figure 50. B117 Appearance Rankings
for coatings on 4340 high strength
steel, PH13-8Mo stainless steel, and
7075 Al.
Figure 51. GM9540P/B Appearance
Rankings for coatings on 4340 high
strength steel, PH13-8Mo stainless
steel, and 7075 Al.
Page 112
Table 50. Corrosion of 4340 steel with HVOF
and Cr coatings - appearance and protection
rankings.
Coating
Appearance
Ranking
Protection Protection
Rating
Rating
Face
Edge
4340 steel
T400
5.0
1.6
1.0
WC/Co
4.0
3.4
3.2
Hard Cr
1.6
3.2
2.0
7575 Aluminum
T400
5.8
9.0
3.0
WC/Co
4.8
10
10
Hard Cr
9.8
10
10
Table 51. GM9540P/B corrosion of 4340 steel
with HVOF and Cr coatings - appearance and
protection rankings.
Coating
Appearance
Ranking
Protection Protection
Rating
Rating
Face
Edge
4340 steel
T400
8.0
9.6
2.4
WC/Co
8.0
10
8.8
Hard Cr
6.8
9.8
1.0
T400
9.0
9.6
9.8
WC/Co
8.0
10
10
Hard Cr
10
10
10
PH13-8Mo
7575 Aluminum
T400
7.5
9.2
1.8
WC/Co
7.6
10
1.6
Hard Cr
10
10
10
Page 113
12.5.3.2.2.
GM 9540P/B Testing
Data from the 4340 substrate in GM 9540P/B Testing is shown below in
Figure 51 and Table 51. As for the B117 test, the samples were
evaluated both for their appearance ranking and for the protection rating
on coating surfaces and edges.
12.5.3.2.3.
Atmospheric Testing
Atmospheric work is currently still in progress at the writing of this report.
In a first set of tests, the performance of the HVOF coatings was
significantly better than chrome plating, as illustrated in Figure 52 and
Figure 53. Additional testing is ongoing that incorporates WC-Co and
WC-CoCr samples, and a final report will be posted on the HCAT Home
Page as the corrosion cycles are completed.
Figure 52. 4340 steel 18-month beach exposure tests, with and
without scribing.
Page 114
Figure 53. 7075 Al 18-month beach exposure samples with and
without scribing.
12.5.3.2.4.
In most corrosion evaluations, the HVOF coatings show equal, and
in most cases superior, performance to hard chrome. Clearly, there
is a difference in relative results between cabinet testing and beach
exposure testing. In cabinet testing, the HVOF coatings appear to be in
some cases a little worse and other cases a little better than chrome. The
beach exposure tests show the largest variation between thermal spray
and chrome plating, with the thermal spray coatings showing significantly
less corrosion and no undercutting or blistering of the coating. The
excellent performance of chrome plated aluminum apparently stems from
the highly corrosion-resistant Ni strike used for chrome adhesion, rather
than from the chrome plate.
The fact that there is any substrate corrosion at all for the thermal sprays
shows that there is some through-porosity, which permits penetration of
liquid to the metal surface.
12.5.3.3.
Electrochemical Testing of Carbide Coatings
Aqueous Corrosive Environments 2000 (Courtesy of S. Simard (NRC) et
al.
Page 115
This paper addresses the performance of various carbide coatings HVOF
coatings (Table 52) in terms of corrosion rate and degradation mode in
two corrosion environments – HCl and HNO3. Electrochemical tests
allow a more precise evaluation of the corrosion resistance of a material
in a variety of environments and concentrations as compared to the
simulated cabinet tests. Corrosion potential can be measured by taking
electrical current measurements when an amount of acid or electrolyte is
in contact with the material being evaluated for corrosion. In this
situation, 316 stainless steel, considered a very corrosion resistant
material, was used as the comparative baseline.
An example of the results for HCl exposure is shown in Figure 54. The
graph has been
corrected for metallic
Table 52. HVOF coatings used for
fraction (normalize %
Comparison of Electrochemical Corrosion
composition) so a more
Potential.
relevant comparison
Coating HVOF coating material
can be made between
the materials. As
A
Wrought Stainless Steel (bulk)
expected, the materials
B
Stainless Steel Coating
containing the corrosion
resistant materials like
C
WC-12Co
Cr, Ni, and varied alloy
D
WC-10-Co-4 Cr
mixes show excellent
corrosion results. The
E
WC-12Co + 25% Ni Superalloy
results of alloy H were
F
WC-12Co + 35% (Cr3C2/NiCr)
surprising as compared
to the overall ratings
G
WC-17Ni
and further
H
WC-20Cr3C2-7Ni
investigation revealed a
higher porosity level in
this coating as
compared to the other
materials. This allowed material to penetrate in to the coating and
accelerate corrosion. Control of porosity is therefore critical to optimum
corrosion resistance.
Page 116
Figure 54. Corrosion Current for an Aerated 0.1 N HCl
Solution.40
12.5.3.3.1.
Electrochemical rankings of the materials the presence of Cr or a
combination of Ni/Cr greatly enhances the performance of HVOF carbide
/cermets in HCl/ HNO 3 environments. Porosity of the coatings must be
controlled to prevent penetration of the corrosive media into the coatings
which can accelerate corrosive attack.
12.5.3.4.
Corrosion Work Planned in JTP Landing Gear
Table 53. HCAT/C-HCAT corrosion test matrix for landing gear steels and coatings.
Tests
Alloys
B117
4340
GM9540P/B 300M
B117+ SO2
Coatings
Thickness
(inch)
Notes
Uncoated
0.003
Cr with and without Ni strike
Hard chrome
0.010
HVOF with and without
sealant
Aermet 100 HVOF WC-Co
HVOF WC-CoCr
With and without shot peen
The corrosion work in the Landing Gear Joint Test Protocol is currently in
progress with estimated completion by December 2000. The matrix
includes a variety of simulated cabinet testing with both cylindrical and flat
Page 117
specimens evaluating WC-Co (HCAT) and WC-CoCr (C-HCAT) on a
number of substrates. Detailed test matrices are incorporated in the test
plan, and include the materials and conditions summarized in Table 53.
This work is being performed to validate the excellent performance of the
carbides/cermets in the earlier HCAT protocol and further evaluate the
role of coating thickness and sealants on the HVOF coatings, and the
effect of hard chrome plating with and without Ni strike layers.
The landing gear JTP beach exposure, B117, GM 9540, and SO2 tests
are being evaluated at the time of writing this report. The data appear to
be showing significant differences in relative performance of chrome and
HVOF coatings, with the HVOF coatings performing much better than
chrome in beach exposure and C-HCAT cabinet tests, but worse in HCAT
cabinet tests. Tests and evaluations are ongoing, and it is too early at the
time of writing to assess either the data itself or its meaning. Updated
information will be available on the HCAT web site.
12.5.4.
General Trend of Corrosion Results
Testing in simulated cabinet (ASTM B 117, GM 9450P/B, Modified SO2),
atmospheric, and electrochemical potential measurements show HVOF
coatings to be usually equal in corrosion performance to hard chrome.
Carbides with a matrix of CoCr, CoNi, or CrNi generally have better
corrosion resistance than Co alone. Corrosion data also clearly
demonstrate the need to minimize through-porosity. However, the reason
for the differences between cabinet test and atmospheric test results is a
matter of concern which is under investigation.
12.6. Fatigue
12.6.1.
Documents
(Courtesy of Phil Bretz Metcut)
"4340 fatigue
data.PDF"
This document contains the HCAT data on fatigue of 4340 high strength
steel (initial HCAT protocol).
(Courtesy of Phil Bretz Metcut)
"7075 fatigue.pdf"
This document contains the HCAT data on fatigue of 7075 Al (initial
HCAT protocol).
Page 118
Document 42 Summary of 13-8 Stainless Data from Initial HCAT
Protocol (Courtesy of Phil Bretz Metcut)
"13-8 stainless
fatigue.pdf"
This document contains the HCAT data on fatigue of PH 13-8Mo
stainless steel (initial HCAT protocol).
Document 43. HCAT landing gear JTP fatigue data - HVOF WC-Co
on 4340, 300M, AerMet 100 in air and NaCl solution.
"Fatigue data landing gear JTP.pdf"
This document contains the HCAT data on fatigue of 4340, 300M, and
Aermet 100 high strength steels (HCAT landing gear protocol).
Document 44 Advanced Thermal Spray Coatings for Fatigue
Sensitive Applications (Courtesy of John Quets Praxair)
"Quets - Adv Th
Spray for fatigue.pdf"
Document 26 Behaviour of Tungsten Carbide Thermal Spray
Coatings 1995, J. Wigren et al.
12.6.2.
General Description and Test Method
Fatigue is a very critical property in the aerospace industry, because of
the repeated cyclic loading for landing gear, actuators, airframe parts, and
gas turbine engine components. Since fatigue performance is driven by
material strength and is especially related to near-surface effects, fatiguecritical applications require careful definition and control of the thermal
spray process to
1. minimize surface heating so as to prevent loss of mechanical
properties due to overheating, and
2. deposit thermal spray coatings with compressive stress to
minimize of eliminate any fatigue debit.
Although plasma spray processes have been widespread in the
aerospace industry for many years, they have tended to be limited to nonfatigue critical applications, largely due to the heat input of the process.
The recent emphasis and work to understand the HVOF process, which
relies more on kinetic than thermal energy for final coating properties, has
started to move the design community towards thermal spray in fatiguedriven components.
The evaluation of fatigue in a coated part is really the analysis of how the
coating affects the known values of an uncoated component. Baseline
data for the alloys used in most applications have already been
established. There are established methods and specifications for
determining fatigue properties. However, with coatings now applied,
some of the guidelines used for bulk materials are somewhat different.
For most chrome-replacement testing, axial fatigue testing (ASTM E46696) provides the most useful data for evaluation (rather than bend
Page 119
testing). In designing a fatigue testing protocol, some areas requiring
definition are:
1. What is the load carrying capability of the coating and should this
value be used in determining the applied stress? How will
thickness of the coating affect this situation?
2. What is the best bar design for fatigue evaluation of coatings?
•
Hourglass (smoothly varying cross section, thinnest at the
center)
•
Smooth section (constant cross section from some distance in
center)
•
Rectangular cross section (flat patch in center of bar for
evaluation)
3. Will the testing be load (stress) or strain control?
4. Can grinding of the coating be repeatable on the fatigue bars to
produce a consistent thickness and surface for testing?
As with all fatigue evaluations, considerations must be given to:
•
Frequency or speed of testing
o
•
Type of control (load or strain)
o
•
will determine time for testing, but to high a frequency can
cause overheating or a shift in results for sensitive
materials
will application of force to bar be controlled purely by load
or by the deformation induced in the part?
R ratio or A ratio
o
R ratio is defined as the ratio of minimum cyclic load to
maximum cyclic load. For example, an R ratio of –1.0
means the maximum and minimum loads are the same
and the loading is fully reversed from positive to negative.
o
A ratio is defined as:
(maximum stress - minimum stress)/(maximum stress +
minimum stress)
Typical fatigue bar shapes are shown in Figure 55, Figure 56, and Figure
57. The most common designs are the hourglass (which can only be
used for load-control fatigue), and the smooth bar.
Page 120
Figure 55. Typical hourglass-shaped fatigue bar.
Figure 56. Typical smooth fatigue bar.
Page 121
Figure 57. Flat Kb fatigue bar.
Fatigue testing for coating comparison must be defined by the important
parameters of the spray process and the critical controls for consistent
property evaluation as shown in Table 54.
Table 54. Fatigue testing variables.
Coating/Substrate Information
Testing Information
Peening of substrate
Frequency of testing
machine
Thickness of coating
Bar geometry
Surface finish (ground, unground,
or superfinished)
Load vs. strain control
Location of coating (patch/full
length)
R ratio, or A ratio
Almen (intrinsic stress) intensity
Corrosion fatigue must also be a consideration, given the corrosion
environments for many of the hard chrome applications. This usually
involves the same considerations as testing in air but the test area in
question is exposed to the corrosive media for the entire test or for
specific periods of time. There may also be pre-exposures for the
purpose of initiating corrosion followed by constant exposure to the
environment in question. When defining the protocol for testing, the
frequency of exposure, and the degree of replenishment must be
stipulated to best approximate the actual service conditions.
Page 122
Work in varied protocols and industrial applications will be discussed and
the general trend for comparing hard chrome to HVOF coatings in fatigue
established.
12.6.3.
Fatigue Results
12.6.3.1.
Comparison of Hard Chrome vs. HVOF WC-Co and
T400
(Courtesy of Phil Bretz Metcut).
Document 42 Summary of 13-8 Stainless Data from Initial HCAT Protocol
This review summarizes the fatigue
testing performed in the initial HCAT
protocol of Document 32 for
validation of HVOF materials as a
viable hard chrome alternative. The
materials and substrates are shown
in Table 55.
The required fatigue test parameters
as defined in the test methodology
section can be found in Table 56. A
variety of test bar configurations
were chosen to investigate the
Table 55. Materials and Substrates
in Study
Coating
materials
Substrates
Hard chrome
4340 Alloy steel
WC-Co
7075-T73
Aluminum
T-400
!3-8 PH Stainless
Table 56 Fatigue Test Parameters
Testing Information
Baseline polished
Coated bars peened
Nominal .005’ thickness
Surface finish (ground/unground)
As coated surface
Location of coating (patch/full length)
.5” patch in center of test section
Frequency of testing machine
Load control 50 Hz
Strain control 2 Hz
Bar type (hourglass vs. smooth)
Combination of all types
Combination of both types
R ratio
Load control .025
Strain control 1.0
Almen intensity
8-11 compressive
Page 123
sensitivity of each configuration to changes in fatigue as caused by
coating application. This step also addressed the concerns of many
groups that had already chosen a bar type of their past verification work.
Table 57 summarizes the number of bars tested for each condition on the
three substrates selected.
Table 57. Fatigue Matrix for Initial Validation
Low Cycle Fatigue (LCF)
Baselines
HVOF Coatings
High Cycle Fatigue (HCF)
Baseline
HVOF Coatings
(per coating)
(per coating)
10 smooth bar tests
uncoated
8 smooth bar tests
HVOF coatings
10 smooth bar tests
uncoated baseline
8 smooth bar tests
HVOF coatings
10 smooth bar tests
hard chrome
6 hourglass bar tests
HVOF coatings
10 smooth bar tests
hard chrome baseline
HVOF coatings
hard chrome
6 Kb bar tests HVOF
coatings
hard chrome
6 Kb bar tests HVOF
coatings
6 Kb bar tests hard
chrome
6 Kb bar tests hard
chrome
Totals:
Baselines:
1.
2.
20 smooth bar tests per substrate per test condition (uncoated)
20 smooth, 12 hourglass, 12 Kb bar tests per substrate per condition (hard chrome plated)
Coatings:
16 smooth, 12 hourglass, 12 Kb bar tests per coating, per test condition
Due to the substantial amount of data, representative results for only the
4340 and 7075 are presented. Data is currently available in referenced
documents (Document 40, Document 41, Document 42) and a final report
should be published and available on the HCAT Home Page by
December 2000.
All data for the HVOF materials on 4340 substrates show satisfactory
results as compared to hard chrome in Figure 58, Figure 59, and Figure
60, regardless of the test bar type which was used. An interesting
observation was the majority of failures for the T-400 coating were
outside the coated patch (suggestive of, but not proving, a fatigue
enhancement). The majority of failures for the WC-Co and hard chrome
were underneath the coating patch as would be expected. Data for
PH13-8Mo stainless steel were very similar to those for 4340.
Regardless of sample geometry, the HVOF coatings performed better on
4340 and PH13-8Mo than hard chrome, showing little or no fatigue debit.
For reasons that are not known the hourglass samples are particularly
sensitive to the coating, and the chrome plated samples show a very
marked fatigue debit.
Page 124
240
230
220
Engineering Stress Max (ksi)
210
200
190
Uncoated smooth
180
Cr plated smooth
170
T400 smooth
160
WC-17Co smooth
150
140
130
120
110
100
1.00E+03
1.00E+04
1.00E+05
1.00E+06
1.00E+07
1.00E+08
1.00E+09
Cycles to failure (Nf)
Figure 58. Comparison of Fatigue Data on Smooth Bars for 4340
220
210
Engineering Stress Max (ksi)
200
190
180
170
Cr plated Kb
160
WC-Co Kb Bar
150
T400 Kb Bar
140
130
120
110
100
1.00E+03
1.00E+04
1.00E+05
1.00E+06
1.00E+07
1.00E+08
Cycles to failure (Nf)
Figure 59. Comparison of fatigue data on Kb bars for 4340
For 7075 aluminum, the situation was found to be more complex (Figure
61). While T400 gave a fatigue debit, it was much smaller than that
caused by chrome. WC-Co, on the other hand, produced a worse fatigue
debit than chrome. It was first assumed that the heat input from the
process affected the substrate. However, extensive testing showed that
heat input had no effect, and it is currently believed that the high debit
derives from the extreme mismatch in mechanical properties (primarily
elastic modulus) between the aluminum and the HVOF WC-Co. This
mismatch is much less for T400.
Page 125
190
Max. stress (ksi)
170
150
130
WC-Co
T-400
EHC
110
90
70
50
1.00E+04
1.00E+05
1.00E+06
1.00E+07
1.00E+08
Cycles to failure
Figure 60. Fatigue of coated 4340 steel - hourglass samples.
In review of all the data, we can make the following conclusions:
•
Data for the 4340 alloy steel and 13-8 stainless steel shows fatigue
performance very close to baseline data and far superior to hard chrome.
•
Test bar configurations do not affect the ranking of the coating materials.
However, the hourglass configuration shows the most drastic variation
between chrome and HVOF. The reason for this is not known.
•
For the T-400, the majority of failures were outside the coating patch
which has not yet been explained. All chrome failures were under the
patch.
•
The 7075 material showed a degradation in fatigue properties as
compared to the hard chrome baseline.
Page 126
Low Cycle Fatigue Data
7075-T73 Smooth Gage
1.2
Baseline
S280/WCCo/Jet-Kote
S110/T-400/Jet-Kote
1
S110/Chrome
Strain Max(%)
0.8
0.6
0.4
0.2
0
1.0E+3
1.0E+4
1.0E+5
1.0E+6
Cycles (Nf)
Figure 61. Fatigue Results for HVOF and Chrome on 7075
Aluminum
12.6.3.2.
Comparison of Hard Chrome vs. HVOF WC-Co for
Landing Gear
This data is close to completion at time of writing, but is not yet quite
completed.
These data were taken to address specific requirements for fatigue in
landing gear applications, as defined by NAVAIR, the US Air Force,
Boeing, and the landing gear manufacturers. The data from the initial
HCAT work (Document 40 to Document 42) was analyzed, and areas
identified where further information and clarification was needed. The
resulting Landing Gear Joint Test Protocol (JTP) is a joint effort between
HCAT (WC-Co coating) and C-HCAT (WC-CoCr coating). The fatigue
data presented here is only for the WC-Co material; the C-HCAT testing
is currently in progress and data will be made available on the HCAT
Home Page. Since the C-HCAT data is for WC-CoCr, it is discussed in
the following section.
Page 127
Table 58 and
Table 59
summarize the
materials and test
conditions for the
landing gear JTP
data presented in
this section.
Table 58. Substrate and coating materials landing gear JTP.
Coating materials
Substrate materials
Hard chrome
4340 high strength steel
WC-Co
300 M landing gear steel
AerMet 100 landing gear steel
Table 59. Test conditions for landing gear JTP.
Testing Information
Peening of substrate:
Frequency of testing machine:
Baseline and coated bars peened
Load control 50 Hz
Thickness of ground coating:
Bar type:
Nominal thickness .003” and 0.010”
Primarily hourglass, some smooth
Surface finish:
Load vs. strain control:
Ground surface, 16µ” Ra
Load control
Location of coating:
R ratio:
0.5” patch in center of test section
Load control -1.0
Almen intensity (stress):
8-11 compressive
This work was very similar to the initial HCAT evaluation, with key
differences as listed below:
•
The coatings were ground to a 16µ” Ra finish before testing (prior
tests used as-sprayed specimens)
•
The majority of the tests were run with the hourglass configuration
(smooth bars and flat bars were used in the prior work)
•
The testing was performed with fully reversed loading R = -1.0
•
A .010” thick coating was also sprayed to evaluate thickness
dependence, in order to evaluate O&R usage
o
•
(NOTE: Test bar size for 0.010” coatings was increased to
0.50” dia. to keep the percentage of total coating area
similar to the .25”dia. bar and 0.003” coating, i.e. coating
thickness << specimen diameter)
The full data set as of the time of writing is summarized in
Document 43. Corrosion fatigue tests were performed to evaluate
the effect of salt exposure on cyclic life.
Page 128
As an example of the data, curves for 300M (a very damage intolerant
material) are shown in Figure 62 (.003” thickness-air), and Figure 63
(.003” thickness-NaCl). Thermal spray coatings on peened materials
have equivalent performance to their chrome plated counterparts. The
data correlates with the initial HCAT work on 4340 steel, which showed
superior or equal performance of the HVOF alternatives in comparison
with hard chrome. In NaCl, the performance of the HVOF coating is
marginally better than the chrome. (Note that failure to peen the material
before chrome plating leads to a very large fatigue debit, which is not the
case for the HVOF coatings.)
300M, SMALL HOURGLASS SPECIMEN
(0.003" COATING) R = -1, AIR
200
Bare/UNPeened
EHC/UNPeened
EHC/UNPeened/Average
EHC/Peened
EHC/Peened/Average
WCCo/UNPeened
WCCo/UNPeened/Average
WCCo/Peened
WCCO/Peened/Average
190
180
170
160
150
140
130
120
110
100
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
1.E+08
CYCLES TO FAILURE, Nf
Figure 62. Fatigue Curve for 300M with .24”dia. hourglass tested in air –
coating thickness 0 .003”.
Page 129
300M, R = -1, SMALL HOURGLASS SPECIMEN
AIR VS. NaCl ENVIRONMENT
200
EHC/Air
190
EHC/Air/Average
EHC/NaCl
180
EHC/NaCl/Average
WCCo/Air
170
WCCo/Air/Average
WCCo/NaCl
160
WCCo/NaCl/Average
150
140
130
120
110
100
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
1.E+08
Figure 63. Fatigue Curve for 300M with .24”dia. hourglass comparing air results with
samples tested in NaCl and .003” coating thickness.
Figure 64 shows the effect of coating thickness on 4340 samples,
showing increased fatigue debit as thickness increases. This effect is
quite commonly seen with coated materials. Experience shows that a
thicker coating or a larger bar will show a higher fatigue debit, and it
cannot yet be determined what percentage of the reduction may be
attributed to the larger bar diameter and what to the coating thickness.
Again, the HVOF material is significantly better than the same thickness
hard chrome.
Page 130
4340, R = -1, AIR
LARGE (0.010"CTNG) VS. SMALL (0.003"CTNG) HOURGLASS
200.0
LgHG/EHC/Peened
190.0
LgHG/EHC/Average
LgHG/WCCo/Peened
LgHG/WCCo/Average
180.0
SmHG/EHC/Peened
SmHG/EHC/Average
SmHG/WCCo/Peened
SmHG/WCCo/Average
170.0
160.0
150.0
140.0
130.0
120.0
110.0
100.0
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
1.E+08
Figure 64. Fatigue curve comparing thickness effects 0.003” (.250” dia.) vs. 0.010”
(.500” dia.) on 4340 using hourglass configuration tested in air.
The following observations were made during the testing:
HVOF Coating
•
Dependent upon the magnitude of loading (165-180 ksi), cracks
could sometimes be observed in the coating after only a small
number of cycles (10-1000). However, fatigue remained better
than chrome.
•
For the larger .50” dia. bars and some of the smaller diameter
bars, spalling/delamination of the coating has been observed at
specimen failure.
•
The specimen failure initiation sites were primarily subsurface,
especially at low loads, with limited surface sites, primarily at high
loads.
Hard Chrome
•
“Chicken wire” cracking can be observed on the failed bars which
is typical of hard chrome plating. No spalling/delamination has
been observed.
•
The failure initiation sites were primarily surface with limited
subsurface sites.
Page 131
These observations with regard to initiation sites are consistent with the
previously reported data that cracks in chrome plating are directly
transferred in to the substrate. In contrast, cracks in the HVOF
alternatives do not correlate with substrate fatigue initiation sites; instead
they are subsurface and not process related. This again confirms that
substrate materials properties drive fatigue performance in HVOF
coatings, whereas hard chrome coatings are detrimental to fatigue.
The data concerning cracking of the coatings indicates more surface
discontinuities with the HVOF alternatives. Clearly, the cracks in the
coating do not affect fatigue, since the HVOF coatings have longer fatigue
lives than chrome. The other possible effect of cracks might be increased
corrosion and corrosion-fatigue, since one might argue that cracks would
permit penetration of liquids to the underlying steel. The data from the
NaCl fatigue testing clearly shows that this is not so; the HVOF samples
show improved corrosion fatigue as well as dry air fatigue – in fact the
differential performance between HVOF and chrome is generally larger
under corrosive conditions.
Conclusions
The following conclusions can be drawn:
•
In air, the performance of HVOF coatings is equal or superior to
hard chrome.
•
In NaCl, the performance of HVOF coatings is superior to hard
chrome.
•
The cracking observed in the HVOF alternatives during cyclic
loading does not substantially reduce the ability of the coating to
provide protection against corrosion as evidenced by the corrosion
fatigue data.
•
The thicker coatings show a higher fatigue debit for chrome and
HVOF, which appears to be a function of coating thickness and
bar diameter.
12.6.3.3.
Comparison of Hard Chrome vs HVOF WC-CoCr
WC-10Co4Cr is now used commercially on landing gear for the Boeing
767-400 as well as for Boeing-approved O&R of landing gear (see
Section 15.5). All vendors on the Boeing Qualified Provider List (QPL)
are qualified on the basis of fatigue performance (among other criteria).
All fatigue testing is carried out on Boeing standard axial 4340 steel
fatigue test specimens, which are hourglass-shaped. Testing is done
using an R-ratio of 0.1 and loads up to 170 ksi. The HVOF coating must
perform better than chrome.
An example of this type of qualification testing is shown in Figure 65. (As
defined by Boeing in BMS 10-67 (Section 14.2.2), Type I coatings are
WC-18Co, while Type XVII coatings are WC-10Co4Cr.) Note that this
figure plots the range of values obtained for the fatigue life with each
coating material at a given stress level. Since runout was defined as 1
million cycles, most of the WC-CoCr samples went to runout.
Page 132
Cr Plate
GPX2700 (Type I)
GPX2800 (Type XVII)
Uncoated
GPX2700 (Type I)
GPX2800 (Type XVII)
Uncoated
1.00E+04
1.00E+05
1.00E+06
Cycles to failure
Figure 65. Fatigue of HVOF-coated and chrome plated high strength steels,
Kt=1.5, Boeing qualification testing. (Courtesy Engelhard Surface Technology).
Figure 66. Comparison of fatigue for chrome and HVOF WC-CoCr deposited
with Jet Kote and Diamond Jet guns. (Courtesy Southwest Aeroservice.)
Page 133
Figure 66 shows the fatigue life of HVOF WC-CoCr compared with hard
chrome using 0.25” diameter 4340 smooth fatigue bars. Two different
HVOF guns and powders gave equivalent performance coatings that
were somewhat better than chrome. (Note that the hourglass specimen
shape tends to separate the performance of different coatings, moving
the S/N curves apart, while the smooth bar shape tends to move the S/N
curves together.)
Although all prior work had shown excellent performance for WC-CoCr,
on the basis of which it is now a production process for commercial
aircraft, work by the C-HCAT team under way at the time of writing is
showing that for their specimens the WC-CoCr coating tends to rumple
and delaminate at high loads. These specimens use the same specimen
geometry as that of Figure 66. This data is under evaluation at the time
of writing to determine whether this is a materials or a testing issue, and
whether it is relevant to military usage of WC-CoCr coatings, where
higher service loads might be expected.
12.6.3.4.
Other
Other fatigue data is available in the document as listed below.
Document 44 Advanced Thermal Spray Coatings for Fatigue Sensitive
Applications (Courtesy of John Quets Praxair).
12.6.3.5.
Comparative Study of Compressive Stress Effects
on Fatigue for HVOF
1995, J. Wigren et al.
VolvoAero identified a wear problem on engine fan blades and the goal of
this study was to balance compressive residual stress in the coating to
prevent spalling in service, with a minimal degradation on substrate
fatigue life. This study primarily evaluates WC-Co (Table 60) sprayed
with the HVOF process at a variety of different spray parameter settings.
The following tests were performed:
•
Three point bend testing for crack resistance
•
Residual stress measurement by Modified Layer Removal
Technique
•
Low cycle fatigue testing
Page 134
Table 60. Tungsten Carbide Coating System
Designations (Volvo)
Coating
composition
Application process
HVOF
Plasma
A
B
C
D
E
F
G
H
1. (WC-Co)
●
●
●
●
●
●
●
●
2. (WC-Ni)
●
3. (WC-Co/Cr)
●
●
Fatigue results will be discussed here. Residual stress work from this
document was discussed earlier in Section 11.5.3.2.
The fatigue evaluation involved both work to identify crack initiation and
full fatigue performance. The test parameters are as shown in Table 61.
The specimens were loaded initially by applying incremental load steps,
each corresponding to an increase of 0.1% in strain, with 1000 cycle
cycles between each step until a crack was initiated in the coating.
Fluorescent penetrant inspection was used prior to load application and
after every 1000 cycle load increment to check for crack initiation. Since
the purpose of cracking the coating was to determine the fatigue life of
the substrate subsequent to this event, the load was increased directly
after crack initiation to a stress of 700 Mpa for 100, 000 cycles or failure
of the substrate.
Page 135
Table 61. Fatigue Test Parameters for Volvo Evaluation
Testing Information
Frequency of testing machine
Coated bars peened
Load control 50 Hz
Strain control 2 Hz
Nominal .008’ thickness
Bar type (hourglass vs.
smooth)
Smooth
Surface finish (ground/unground)
As coated surface
Stain control till crack initiationswitch to load to runout
Location of coating (patch/full
length)
R ratio
Strain control 0
Full length of test section
Residual stress intensity
Varied
The results for crack initiation are shown in Figure 67. As expected, the
F-1, A-1, and G-1 are most resistant to cracking due to the compressive
stresses as measured in the coating. This follows the trend in all work
thus far towards compressive stresses.
Figure 67. Comparison of Residual Stress and Resistance of Coating to Crack
Initiation
Figure 68 summarizes the final fatigue performance of the substrate. As
expected, the H-1, E-1, and E-3 samples show the longest lives based
upon the highest levels of compressive residual stress in the substrate.
Page 136
Figure 68. Comparison of Final Fatigue Life with Residual Stress
The goal of the Volvo study was to optimize the coating with respect to
both fatigue and residual stress. Engine tests were run to validate the
laboratory test results.
The following observations were made:
•
Although coating H-1 showed superior substrate fatigue life due to
substrate compressive residual stress, the tensile residual stress
in the coating caused spallation during engine evaluations.
•
Coating F-1 also showed spallation in engine testing. Although
the compressive residual stress would be expected to preclude
this occurrence, it is hypothesized that the high compressive
residual stress may be close to that of the WC-Co material, thus
resulting in failure.
Based upon these observations, A-1 was chosen for implementation,
balancing both residual stress and fatigue data.
12.6.3.5.1.
A balance must be obtained between residual stress and substrate
fatigue to balance the effect of the coating on substrate performance and
the ability of the coating to remain intact during application in the
environment. Properties of the coating such as ability to resist
compressive stresses are also critical and must be understood.
12.6.4.
General Trend of Fatigue Results
Hard chrome always causes a significant fatigue debit because the
coating is under tensile stress, which tends to open up cracks in the
substrate. There is also the possibility that some of the cracks in the
chrome may propagate into the substrate. The HVOF carbide materials
can exhibit cracking early in the fatigue testing process, but current
evidence, from the extended fatigue life of HVOF-coated alloys and
cross-sectional microscopy, shows those cracks do not propagate into
the substrate.
Page 137
In general, therefore, HVOF coatings can largely avoid the severe fatigue
debit that chrome plate creates, and often have no measurable fatigue
debit. Hourglass-shaped fatigue bars are particularly sensitive for fatigue
evaluation of coated materials, although the reason for this is unknown.
HVOF WC-Co shows superior performance to hard chrome on both the
4340 alloy steel and 13-8 stainless steels. On the 7075 Al, however, the
WC-Co is significantly worse than chrome, while T400 is significantly
better than chrome.
There are several important issues to take into account when replacing
chrome plating with HVOF coatings:
•
Deposition temperature – It is important to ensure that the coating
process does not overheat the substrate.
•
Intrinsic stress in the coating – The coating should be under
compressive stress, but not under very high compressive stress,
which can delaminate the coating. Coating stress can be
measured by Almen strip – a satisfactory Almen number is 0.003
– 0.012” Almen “N” for a 0.005” thick tungsten carbide coating41.
•
Coating mechanical properties – Large mismatches between the
elastic moduli of the substrate and the coating should be avoided,
as this appears to lead to reduced fatigue life. While WC-Co and
WC-CoCr work well on steels, softer coatings with lower elastic
modulus (such as Tribaloy) work better on aluminum alloys.
Corrosion fatigue is being measured in the Landing Gear JTP. Current
data shows no significant effect on material performance, even with initial
cracking in the HVOF materials.
There is an outstanding issue, brought up by initial C-HCAT data, that
WC-CoCr may be more brittle than WC-Co and therefore more prone to
crack or delaminate at high load or coating thickness. This issue is in
process of being resolved at the time of writing.
12.7. Wear – Erosion, Abrasion, Sliding,
Fretting
12.7.1.
Documents
Document 23. Fracture Toughness of HVOF Sprayed WC-Co
Coatings (Courtesy of S. De Palo, et al)
Summary of testing concerning fracture toughness and erosion resistance
on the referenced materials.
Summary of varied spray process for deposition of coatings and study on
abrasive wear/microstructure/phase content interactions.
Using WC-10Co-4Cr Cermets (Courtesy of J.G. Legoux, B.Arsenault,
C. Moreau, V. Bouyer, L. Leblanc)
Page 138
Study of spray process parameters with regard to velocity, temperature,
efficiency, etc. and relation to microstructure, wear resistance and carbide
degradation.
12.7.2.
Wear resistance has been one of the prime characteristics that warrants
the use of hard chrome in a many applications. Real-life applications
frequently involve sliding of two mating surfaces (sometimes in
combination with abrasive particles). Wear tests are usually simpler, and
there are several philosophies of wear testing:
1. Carry out simple tests to measure how materials perform
compared to each other. Typical tests are Taber abrasion, pin-ondisk, ring-on-block, etc. These tests are simple, but often fail to
rank even relative wear correctly as it is measured in service.
2. Carry out tests designed to evoke the wear mechanisms expected
in real applications. This can be quite difficult since it requires a
very good understanding of the actual wear situation.
3. Carry out tests that simulate service conditions as accurately as
possible. For aircraft this includes landing gear and hydraulic rig
tests. Because these tests are accelerated they permit
investigation of complex interactions and widely varying
conditions.
4. Evaluate in service (i.e. field testing, or for aircraft, flight testing).
This method is obviously the most true-to-life, but of course it
measures only the wear resulting from the conditions experienced
by that particular test aircraft. It is uncontrolled, takes a long time,
and is expensive. Because of the cost and risks of failure it is
usually preceded by rig testing.
Because no one method is satisfactory, most engineers depend on a
variety of test methods, but ultimately rely on rig and flight testing. This
section covers only coupon wear testing. Rig and flight testing are found
in Sections 13.1 and 13.3.
Wear can be defined by a variety of mechanisms primarily involving loss
of material and substrate integrity. For the purposes of this section, there
are four primary wear mechanisms as detailed below in Table 62.
Page 139
Table 62. Four Primary Wear Mechanisms
Wear
mechanism
Description
Primary Test
Method
Erosion
Loss of material due to impact of abrasive
material carried in a liquid or gas stream.
ASTM G 76
Abrasion
Displacement of material from a surface in
contact with hard projections on a mating surface
or with hard particles that are moving relative to
the wearing surface.
ASTM G 65
Sliding/Adhesive Two surfaces rubbing together over a defined
distance where adhesion can occur in some
areas and wear debris generated can cause
further degradation.
Varied
methods
Fretting
Varied
methods
Occurs between two mating surfaces; it is
adhesive in nature and vibration is its essential
causative factor. Relative motion in design is
usually not expected.
To define a wear environment, a number of factors must be considered
such as:
•
Lubrication – amount (if any), frequency, contamination
•
Type/size of abrasive – if involved
•
Wear stroke – relative motion, length, contact surface
•
Type of loading during cycle – side, axial, combination
•
Removal of wear debris – frequency, mechanism
•
Similarity to service geometry and conditions.
These factors are very important to assess and define, but sometimes
impossible to reproduce with reasonable accuracy in a laboratory
evaluation.
This therefore can make defining wear methodology very difficult. Even if
a “standard” procedure has been formulated such as ASTM or SAE
specifications, each laboratory may have a different interpretation of the
requirements. Comparative ranking of results from varied sources can
then sometimes present incorrect conclusions because of these
differences. With this in mind, many companies and laboratories develop
wear procedures unique to the application/environment in which the wear
problem exists.
However, problems can arise if the formulator of the test methodology
designs a test that is too severe and fails most if not all of the potential
candidates. It is easier to implement new materials when a current
baseline material such as hard chrome is being replaced by possible
alternatives. The current product provides not only a benchmark for
Page 140
comparison, but also serves to validate the test methodology if it is unique
for the application in question. Actual usage in the environment is the
final test but wear testing can hopefully rank varied materials for
consideration.
With hard chrome as the baseline, this section will define both
specification (ASTM) and industry-unique procedures designed for
comparison of hard chrome vs. alternatives in specific applications.
Results, examples, and general trends will be discussed.
12.7.3.
Test Methods
12.7.3.1.
Erosion Testing per ASTM G 76
This test involves firing of abrasive particles at a test surface using a gas
stream. Parameters to control in this test are:
•
Length and diameter of gun nozzle
•
Carrier gas type
•
Velocity
•
Size and type of abrasive (the erodent)
•
Distance from nozzle to gun
•
Angle of impingement
•
Area of impingement on sample
•
Ability to catch used erodent
•
Duration of testing
Sol id Pa rtic le E ro sio n Te st Ap pa ra tus
( mod i fi ed A S TM G 76- 89)
10 mm s tan d o f f
R ot a te
( 3 0 °- 9 0° )
C o ate d
S a mp le
N o zz le
( 1/ 16 ” ID , 2 ” lon g )
Fil ter
Fl o wm e te r
C o m pr es s ed
D ry Air
( 5 0 p s i)
( 2 0 s lp m)
P o w de r F e ede r
( 20 g / min )
6 0 m /s
A 2l O 3
( 50 u m, an gula r)
Figure 69. Typical Set-up of ASTM G76 erosion test.
Results are usually expressed in a weight loss per unit area format.
Problems can arise if the flow and mixing of gas/erodent is not uniform.
The test is therefore sometimes normalized for the amount of erodent
used in a given time frame of testing if more or less material is used for a
particular test. It is also critical to control the size/shape and distribution
of abrasion particles as shifts in these characteristics can substantially
Page 141
effect the final result and ranking. A typical set-up is shown in Figure 69.
12.7.3.2.
Abrasion Testing
ASTM G 65
This technique involves the use
of dry sand abrading against the
testing surface with pressure
applied by means of a rotating
rubber wheel. Rounded Ottawa
silica sand is normally used and
is introduced between the coated
block and the chlorobutyl rubber
wheel/rim. Rotation of the rim is
in the direction of abrasive flow.
The test specimen is pressed
against the rotating wheel by
means of a lever arm. The
Figure 70. ASTM G 76 set-up.
sample is measured before and
after the test. Weight loss is converted to volume loss by dividing by the
coating density. Results are expressed as volume loss per 1,000
revolutions.
Other
There can be many variations for abrasion testing involving use of some
abrasive media in the wear process. These specific test setups will be
highlighted in the results section for that particular test.
12.7.3.3.
Sliding/Fretting Wear Methods
For many applications in landing gear, hydraulic, and actuator designs,
wear involves either sliding of a seal against the coating over a certain
stroke length (e.g. landing gear uplock or downlock hydraulics), or
situations where the piston dithers or vibrates at a single position for
extended time frames (e.g. fly-by-wire actuators). These situations
describe perfect circumstances for occurrence of sliding and fretting wear
respectively. Actual hydraulic tests as reviewed in Section 13.2.1 are the
best option but are expensive and time consuming. Application-driven
test methods provide an option for more economical and rapid
evaluations, provided they simulate service conditions properly.
Methods for each type of test have been devised under the
HCAT/CHACT protocols. Document 33 Joint Test Protocol (JTP) for
Landing Gear 1998 (Courtesy of HCAT and CHCAT Teams). Other
methods could be acceptable but these tests are currently being used. A
short summary of each methodology is shown below.
Page 142
Sliding Wear
An oscillating piston test can simulate piston actuation which is typically
encountered.
Test
Guide
Guide
bushing
This test can be used to reflect
typical conditions of use under
a side load. The design is
Oscillating piston
shown schematically in Figure
71. Standard ASTM tests are
not applicable since they do
not reflect conditions of use in
hydraulics.
The piston consists of 4340
high strength steel 1" in
Load
diameter and 9” in length,
typical of hydraulic rods. The
Figure 71. Schematic of Sliding Wear
rod coating and finish, the
Apparatus for hydrualics
bushing material or seal, and
the wear test conditions are as detailed in the test protocol.
The rod and bushing wear specimens are dimensioned and finished as
required per protocol specifications.
Fretting Wear
The fretting wear test is
LOAD
O s c il la ti n g
used to reflect typical
M o tio n
actuator piston dithering
or vibration movement.
S h oe
This equipment is shown
schematically in Figure
B lo c k
72. It is a test system
commonly used by GE
Figure 72. Side view of fretting apparatus.
Aircraft Engines for
measuring fretting wear
of engine components,
and comprises a flat
block and a shaped shoe which move rapidly with respect to each other.
Standard ASTM tests are not used since they do not reflect conditions of
use in hydraulics.
12.7.4.
12.7.4.1.
Wear Results
ASTM G 65 Erosion Testing
Document 44. Advanced Thermal Spray Coatings for Fatigue Sensitive
Applications (Courtesy of John Quets Praxair)
Page 143
Table 63. Erosion Results as Conducted By Praxair.
Coating
Chemistry
Coating Process
Erosion
Rate
ìm/g
30°
90°
> 130
50
Chrome
HCP
WC-Co
Jet Kote
10
70
WC-Co
JP 5000
10
65
WC-Co
D-Gun
17
100
WC-Co
SDG
20
85
WC-Co-Cr
JP-5000
10
60
WC-Co-Cr
SDG
17
85
WC-Cr-Ni
SDG
20
105
Testing was performed to ASTM G65 standards according to the
description summarized in 12.7.1.
There is currently very little data available on carbides/cermets
concerning erosion. The data from Praxair is a direct comparison of hard
chrome with WC-Co produced by a number of different spray processes.
The Stony Brook/Sulzer Metco analysis is a side-by-side review of varied
WC-Co compositions/powder morphologies sprayed with the same gun.
Although the data is reported in different units, review shows the following
information:
•
Erosion is higher for all WC-Co materials in the 90° mode for both
studies.
o
This occurs because the harder thermal spray coatings
fracture and are eroded at this high angle of incidence.
(HV 900 for hard chrome vs. HV 1150 for WCCo.) In
general ceramics are avoided in these situations.
Sample
Code
Nominal
Composition
(wt%)
Starting
Powder
Size(ìm)
Manufacturing
Technique
A
WC-12Co
-45, +5
Sintered/
B
WC-12Co
-53, +11
Agglomerated/
Crushed
Sintered
C
WC-17Co
-53, +11
Spray Dried
D
WC-17Co
-53, +11
Spray Dried
E
WC-10Co-4Cr
-45, +11
Sintered/
F
WC-10Co-4Cr
-53, +11
Agglomerated/
Crushed
Figure 73. Erosion Results As Conducted By Stony Brook/Sulzer Metco
Page 144
•
At the lower 30° angle, the carbide materials substantially
outperform the hard chrome materials – again, as one would
expect.
•
As expected, the WC-17Co material showed higher erosion loss
due to the higher content of the softer matrix Co.
12.7.4.1.1.
The HVOF coatings show good performance in erosion testing at low
incidence angles.
Reduction in hardness closer to hard chrome values may be required for
thermal spray materials to show parallel performance at angles up to and
equal to 90 degrees.
12.7.4.2.
ASTM G 76 Abrasion Testing
Document 44. Advanced Thermal Spray Coatings for Fatigue Sensitive
Applications (Courtesy of John Quets Praxair)
Document 22. Evaluation of Four High Velocity Thermal Spray Guns
Using WC-10Co-4Cr Cermets (Courtesy of J.G. Legoux, B.Arsenault, C.
Moreau, V. Bouyer, L. Leblanc)
These documents summarize the testing of coatings using the ASTM G
76 rubber wheel (Taber) abrasion test as described in 12.7.1. There is
currently very little data available on carbides/cermets concerning
abrasion. The data from Praxair is a direct comparison of hard chrome
with WC-Co based materials produced by a number of different spray
processes. The NRC analysis is a side by side review of varied spray
HVOF guns with the same WCCoCr compositions/powder morphology.
A hybrid plasma gun is also analyzed. Analysis of the data shows the
following:
•
WC-Co and WC-CoCr based materials show better abrasion
resistance as compared to hard chrome sometimes by a factor of
8:1.
•
Although the NRC study is not normalized to revolution data, the
trend corresponds to Praxair data for the JP 5000 study,
supporting lower abrasive wear for higher hardness HVOF
coatings.
Page 145
Table 64. ASTM G76 data from Praxair.
Coating
Chemistry
Coating
Process
Density
Average
Hardness
g/cc
Abrasion
Wear
VH.3
3
mm /1krev
8
Chrome
Cr Plate
6.9
900
WC-Co
HV 2000
13.2
1200
3.0
WC-Co
Jet Kote
13.2
1100
1.6
WC-Co
JP 5000
13.2
1125
0.6
WC-Co
D-Gun
13.2
1075
2.9
WC-Co
SDG
13.2
1100
1.4
WC-Co-Cr
JP 5000
12.4
1100
0.6
WC-Co-Cr
SDG
12.4
1270
1.3
WC-Cr-Ni
SDG
10.5
1100
1.1
Table 65. ASTM G76 Data from NRC.
Sample Dep efficiency Substrate temp. Particle velocity Particle temp.
Porosity
Hardness
Volume loss
%
°C
m/s
°C
%
sdev
VHN
sdev
mm3
sdev
MT1*
86
205
340
2300
6.2
0.6
1082
149
6.2
0.45
MT2
86
170
340
2300
8.6
0.9
1094
184
6.7
0.26
MT3
86
165
320
2350
8.8
1.5
1086
172
7.2
0.68
MT4
84
175
340
2400
5.3
0.5
1066
133
8.7
0.42
MT5
66
140
360
2150
16.1
1.5
949
172
5.6
0.44
MT6
81.5
190
300
2600
6.5
0.5
1059
158
13.7
0.89
MT7
87
155
320
2250
12.6
1.5
1025
149
6.5
0.38
JP1
--
240
--
--
2.1
0.4
1237
134
3.6
0.32
JP2
39
200
665
1805
3.1
0.8
1124
218
2.9
0.14
JP3
31
200
620
1742
2.8
0.6
1213
142
3.2
0.1
JP4
40
200
661
1850
2.7
0.5
1114
95
3
0.32
JP5**
7.8
--
672
1465
0.6
0.1
1206
100
4.2
-
DJ1
--
--
575
1975
5.3
1
1167
178
3.6
0.24
DJ2
--
--
575
1975
3.7
0.8
1201
160
3.3
0.25
DJ3
57
--
562
1840
8.4
1.64
1067
106
3.6
0.03
DJ4
--
--
570
1980
5
0.39
1149
140
3.6
0.13
DJ5
63
--
570
1980
4.6
0.93
1239
139
3.7
0.05
DJ6
62
--
570
2005
4.5
0.39
1156
272
3.9
0.14
DJ7
62
--
570
1975
4.7
0.73
1141
157
3.4
0.12
DJ8
65
--
530
1915
7.7
1.2
1053
172
3.4
0.07
DJ9
67
--
590
2025
5.6
0.91
1214
118
3.9
0.24
*presence of air jet
** used the ST-gun
Page 146
12.7.4.2.1.
The HVOF coatings show superior performance to hard chrome in
abrasion testing. This can allow use of HVOF carbide materials in
conditions where abrasive particle contamination has eliminated past
consideration of hard chrome.
12.7.4.3.
Other Abrasion Tests
Document 36. Report of Replacement of Chromium Electroplating Using
HVOF Thermal Spray Coatings AESF Plating Forum 1998 (Courtesy of
Bruce Sartwell and HCAT Team).
Table 66. ASTM G76 Data from NRL and Sulzer Metco.
Parameter
Set-up #1-HCAT Calowear
Tester
Set-up #2 Sulzer Metco
Abrasive media
4 micrometer diameter
silicon carbide particles
-53+15 micron alumina
Carrier
Distilled water
Distilled water
How media
applied
Drip feed
-
Concentration of
media
-
150 grams of alumina/500 milligrams
of water
Mating wear
surface
2.5 cm dia. hardened steel
ball
Ground cast iron plate
Load-how
applied
Sliding normal force of
200 g/cm2
How wear is
measured
Measure volume of wear
crater
Thickness and weight loss
How expressed
Coefficient K expressed as
volume removed per unit
load and unit sliding distance
-
Wear sample
size
-
25 mm diameter
Test duration
50, 000 revolutions
5 minute intervals with total test time
of 15 minutes
.27 N
This summary describes abrasion testing other than the standard ASTM
G-65. As can be seen from comparing the test parameters from Table
66, the methodologies are significantly different, which prevents direct
correlation of the results.
Page 147
A-12 = WC-12% Co
Standard =Air cooled gun
A-17 = WC-17% Co
Hybrid = Hybrid gun
Figure 74. Comparison of HVOF Processes and WC-Co Powders
The HCAT-directed work contains a direct comparison to hard chrome
baseline materials with both WC-Co and T-400 HVOF coatings. The
Sulzer Metco work is a comparison of WC-Co powders sprayed with two
different spray systems.
Table 67. Average wear
The data for both tests are shown
coefficients, K, expressed in units
in Table 67 and Figure 74. Review
of 10-4 mm 3/N-m, for the various
of the work indicates the following:
coating/substrate combinations.
• The HVOF coatings show
Sample
# of tests
K
some improvement in
Calowear Abrasion testing
Cr-plate on 7075 Al
4
9.3
when run in side-by-side
Cr-plate on 4340
5
9.9
comparison with hard
Cr-plate on PH13-8
4
9.7
chrome, provided they are
harder than chrome (WCWC/Co on 7075 Al
5
6.7
Co), but higher wear when
WC/Co on 4340
5
5.7
they are softer (T400).
•
WC/Co on PH13-8
5
6.4
Comparison of varied
T400 on 7075 Al
5
13.3
powders/processes for WCT400 on 4340
5
15.6
Co indicates that, as
expected, the higher
T400 on PH13-8
5
18.1
hardness materials perform
better in abrasion wear testing, but that there is little difference in
wear rate between several guns and powders.
12.7.4.4.
Sliding and Fretting Wear Results
12.7.4.4.1.
DARPA program – GEAE/NU
Document 1. Hard Chrome Coatings - Advanced Technology for Waste
Elimination, DARPA Grant MDA972-93-1-0006, Keith O. Legg, Jerry
Schell, George Nichols, Robert Altkorn.
Page 148
12.7.4.4.1.1. Test conditions
Fretting wear measurements were made as described in 12.7.3.3, above.
Test conditions were as described in Table 68.
12.7.4.4.1.2. Results
In these tests the shoe was coated and the block uncoated. The results
are shown in Figure 75. The zero-values for wear of chrome plated
materials against 4340 are due to the fact that the 4340 material wore off
the bare block surface and formed a protective layer on the coated
material.
Table 68. Fretting test
parameters.
Surface finish
Ground – 32 µ”
Ra nominal
Block
Uncoated
Shoe
Coated
Coating
thickness
0.010”
Stroke
0.040”
Cycles
786,000
Frequency
10 Hz
Temperature
400°F
Load
2,000 psi
12.7.4.4.1.3. Interpretation of results
These results are in general agreement with the data obtained in
hydraulic rig tests (see Section 13.2.1) and flight tests (see Section
13.3.2). It is important to note that the surface finish was 32µ”, which is
standard for chrome plate, but very rough for a thermal spray. As a result
the thermal spray coatings caused higher wear on the adjacent materials.
The T400 coatings showed higher wear (and less wear of the adjacent
material) than the WC-Co coatings. Again this is in agreement with rig
and flight tests, which have shown similar results.
Lessons learned in rig and flight testing are that HVOF coatings cause
more wear than chrome if the surface Ra value is high, but less if the Ra
value is less than about 6 µ”.
Page 149
Wear Coeff.
1.00E-09
1.00E-10
1.00E-11
Block Wear Coeff
1.00E-12
IN718
4340
EHC
IN718
IN718
EHC
4340
4340
EHC
4340
IN718
EHC
IN718
4340
IN718
IN718
IN718
IN718
WC-Co WC-Co
WC-Co
Shoe Wear Coeff
IN718
IN718
T400
IN718
4340
T400
Shoe - coated
Block - uncoated
Coating
Figure 75. Fretting wear of hard chrome, HVOF WC-17Co, and HVOF T400.
(Note – the zero wear measurement resulted from material transfer from the
uncoated block to the coated shoe, protecting it from wear.)
12.7.4.4.2.
JTP for Landing Gear
Document 43. HCAT landing gear JTP fatigue data - HVOF WC-Co on
4340, 300M, AerMet 100 in air and NaCl solution.
Wear testing in this JTP involves both sliding and fretting wear. The test
set-up designs are shown in Section 12.7.1, which compares varied
testing methodologies. The sliding wear test involves a bushing being
oscillated along a rod with a downward side load. The fretting test
involves a shoe being pressed against a block over a short dithering
stroke.
The execution of this testing is currently in progress and should be
completed by June 2001.
Initial work has centered around the identification of the important
variables in the process using a design of experiment (DOE)
methodology. Key variables were identified as shown in Table 69.
Page 150
Table 69. Wear test variables for DOE factors.
Variable
Default value
Alternatives or Ranges
Test Type
Rod, Shoe
Materials
Bushing wear
4340
+
Hard
Chrome
Fretting wear
4340 + HVOF WC17% Co
Bushing, Block
Matls
Finish, Ra
microinches
4340
AMS 4640
8-12, (target 8)
2-6, (target 4)
Lubrication
none
MIL-H-83282
hydraulic fluid
oad, lbs.
72 lbs.
30 lbs. minimum
Stroke, inches
0.010 Fretting
0.5 Bushing
0.005 to 0.1 inch
range for fretting
Frequency (Speed)
10 Hz, Fretting
90 cpm, Bushing
20 hrs, (cycles =
720,000 fretting &
108,000 bushing)
1 - 70 Hz Fretting test
Temperature
Room temp.
200oF seals
350 oF all other
Operating
Environment
Ambient air
Duration, hrs
(cycles)
12.7.5.
0 - 48 hrs
4340
+
HVOF
WC-10Co4Cr
Anodized Al
4340 with nitrile or PTFE seals; 24 hr
hydraulic fluid pre-soak
Pressurized
to 3000 psi
for seal tests
240
lbs. capable of up to 1000 lbs
Maximum
0 to 3” range
for Bushing
test
1 - 90 cpm
Bushing test
Note: Longer times can be run if required to get
sufficient amounts of wear to distinguish between
tests.
No active cooling
(Capable of higher temps)
General Trend of Wear Results
Wear results show that the HVOF materials generally show superior
performance to hard chrome in all types of wear evaluations from erosion
to sliding/fretting mechanisms. There are exceptions, however:
•
As we would expect, abrasive wear resistance is controlled by
coating hardness, and so softer materials (such as T400) do not
perform as well in abrasion tests as chrome or carbides.
•
In erosion testing at 90 degree angles, the harder WC-Co does
not perform as well as chrome, again, as would be expected.
•
In fretting wear, HVOF coatings with rough surfaces (32µ” Ra)
tend to cause increased wear on adjacent uncoated materials. In
some cases chrome performs better in testing because of material
transfer. However, rig and flight tests (using 1 – 6 µ” Ra surface
finishes) show better wear performance for the HVOF carbides.
Page 151
12.8. Impact
12.8.1.
The purpose of this type of test is to assess the resistance to chipping or
damage, primarily from runway debris thrown up on landing or takeoff.
This debris will most commonly be in the form of small stones and similar
objects, but may occasionally be larger, heavier objects.
Two types of tests have been used to assess different damage
mechanisms that are expected for landing gear:
1. Gravelometer testing (ASTM D 3170-87) - which impinges gravel
in a high-velocity air stream onto the surface (high hardness
projectiles, high speed, low mass, small impact area).
2. Ball impact testing - in which a 1lb hardened steel ball is dropped
from varying heights (up to about 4 ft) onto the surface (relatively
soft projectile, low speed, low mass, large impact area).
The gravelometer test is most commonly used to assess the chip
resistance of paints, although it is also used to assess the brittleness and
chip resistance of hard materials such as glass. The air velocity is
approximately 100 m/s (200 mph), but the gravel velocity is considerably
less, depending on its size and shape.
Gravelometer test - ASTM D 3170-87, SAE J400
The test standard is a 5 - 10 second feed of 550ml of road gravel (size
9.5 - 16 mm) into an air stream passing at a rate of 100 cfm (47 l/s) and a
pressure of 70 or 80 psi (480 or 550 kPa). The surface is then examined
for chips. Since chrome and HVOF WC-Co materials will be more difficult
to chip than paints, or even glass, the test must be increased in severity
by increasing the air pressure or the exposure time.
After each test the surface is examined visually and with a binocular
microscope for evidence of chips or cracking of the coating. The number
and size of chips and cracks are recorded photographically and
compared between the coating and the control (chrome in this case).
Ball drop test
A 1lb ball of hardened steel is dropped down a tube onto the surface of
the coated specimen. The extent of coating damage (cracked area and
delaminated area) is recorded photographically as a function of drop
height. Since this is only a relative test, the extent of damage (cracked or
damaged area, or area of delamination) is recorded for the test coating
and control.
12.8.2.
Impact Test Results
Limited testing ball drop and gravelometry testing has been performed (by
NADEP Jacksonville and Boeing respectively) comparing hard chrome
with alternative HVOF coatings, and has been reported at HCAT
meetings. In each case the HVOF coating was reported to have
Page 152
performed better than chrome, but data is not presently available. Both
ball drop and gravelometer testing is to be done by the HCAT under the
Landing Gear Joint Test Protocol and results are expected to be available
on the HCAT Home Page by December 2000.
12.9. Hydrogen Embrittlement
Hydrogen embrittlement is a concern with the use of hard chrome
electroplating on high strength steels as a result of hydrogen produced at
the surface of the substrate during electrodeposition processes. A bake
out for approximately 1-2 hours at 350 - 400oF is required for all hard
chrome plating to remove any hydrogen generated in the plating process
that has migrated into the substrate.
HVOF does (or can) use hydrogen as the fuel gas. However, there is no
evidence that the hydrogen can become incorporated into the steel.
Partly this is because it is mostly burned in the flame, partly that it is in
molecular form (not the highly active atomic, or evanescent, form found in
plating solutions), and partly that the heat of the process itself would be
expected to liberate any dissolved hydrogen, as does a standard
hydrogen bake.
Environmental embrittlement (or re-embrittlement) occurs when corrosion
of the steel generates hydrogen, which diffuses into the bulk and causes
failure. This type of failure could in principle occur because of corrosion
through a damaged coating.
12.9.1.
12.9.1.1.
Embrittlement Testing:
To address these concerns, a test methodology has been developed in
the Landing Gear JTP (Document 33) which addresses the concerns with
HVOF materials.
The standard embrittlement test is ASTM F 519, in which a notched bar is
held under load – typically 75% UTS for 200 hours. Specimen and notch
geometries are described in detail in the standard. Environmental
embrittlement is tested by submerging the specimen in a corroding
solution, such as salt water during the test.
12.9.2.
Lufthansa embrittlement tests
– Summary of tests (Courtesy of Lufthansa). 42
This document summarizes Lufthansa’s testing of thermal spray coatings
for landing gear, which formed the basis for their flight testing.
Lufthansa has examined the possibility of hydrogen embrittlement during
processing, by beginning ASTM F519 testing of 4340 steel less than 2
hours after coating with WC-17Co and WC-CoCr. All specimens
Page 153
passed the test.
As part of their work in qualifying HVOF WC-CoCr for landing gear,
Lufthansa also intends to test for possible embrittlement during stripping
(which is electrochemical).
12.9.3.
Hydrogen Embrittlement Tests Planned HCAT
In manufacturing and maintenance the same component may be coated
with both electroplates (e.g. Cd) and chrome coatings. Since hydrogen
can easily diffuse through the cracked chrome, cadmium plating can be
done subsequent to chrome plating provided the hydrogen can diffuse out
through unplated or chrome plated areas. Whether or not hydrogen
introduced during plating can diffuse through an HVOF coating could be
important in properly sequencing coating operations.
For this reason, the HCAT is testing embrittlement of high strength steels
with HVOF coatings, addressing the following issues:
1. Demonstrating that the HVOF coating process does not contribute
to hydrogen embrittlement.
2. Evaluating whether hydrogen that might be generated in a
subsequent plate can pass through an HVOF WC-Co layer either
during the HVOF process itself (which heats the surface) or on
subsequent heat treating.
3. Evaluating whether HVOF-coated steels have any significant
difference in re-embrittlement behavior than chrome plated steels.
No testing has been performed to date comparing hard chrome with
alternative HVOF coatings. Results of testing are expected by December
2000 and will be made available on the HCAT Home Page.
12.9.4.
General Trend of Hydrogen Embrittlement
Results
All experience with thermal spray coatings to date is that they do not
cause embrittlement, and the Lufthansa results confirm this.
Issues that remain are (all are under testing by HCAT):
•
Diffusion of hydrogen through HVOF coatings – However, the fact
that these coatings have some permeability to liquids (as
evidenced in the corrosion data) suggests that they should also be
porous to hydrogen, permitting out-diffusion of hydrogen from
subsequent plating operations.
•
The possibility of embrittlement resulting from electrolytic stripping
of thermal spray coatings.
•
Possible differences in environmental embrittlement between
chrome plated and thermal sprayed steels.
Page 154
12.10.Creep
One function of hard chrome is to ensure proper interference fits between
parts, such as a bearing journal surface and the bearing itself. Shafts are
typical examples of this situation. An important aspect of this type
application has been the ability of the hard chrome to maintain a tight fit
without relaxation of the compressive interference loads due to creep. If
the chrome creeps (especially at high temperature, as can happen in an
engine), the holder will become loose, spin, and gall the shaft.
12.10.1.
There are no ASTM standard methods for creep evaluations for this
particular situation and application. A test methodology has therefore
been developed by GE Aircraft Engines as follows:
•
The coatings should be deposited to an approximate thickness of
.030” on flat substrates.
•
The coated side can be ground flat with minimal coating removal.
The substrate material is machined from the reverse side, leaving
only coated material.
•
Specimens are cut 0.25 inches square for testing.
•
Testing is done using stacks of 3 specimens to allow for greater
accuracy in measuring the height changes due to creep.
•
Alumina platens are utilized for loading the coating specimens to
assure the creep deformation is in the coating specimens and not
the platens.
•
Measurements can be obtained by two methods; 1) direct flat anvil
micrometer measurements of the individual test specimen stacks
before and after testing, and 2) dial gage extensometer readings
during the test.
•
Suggested test conditions are 800oF under varied loads and hold
times. The 800oF temperature is the upper temperature limit
where hard chrome might be used, so that these high load, high
temperature tests represented a worst case set of conditions for
compressive creep of a chrome replacement.
12.10.2.
Documents
Document 45 Compressive Creep Tests of Hard Chrome and HVOF
coatings 1998, J. Schell, GE Aircraft Engines.43
"Compressive creep
of Cr and HVOF coati
This document was produced as part of an initial evaluation of HVOF
coatings for chrome replacement.
Page 155
12.10.3.
Creep Testing Results
12.10.3.1.
Results for HVOF WC-Co and T400
Document 45. Compressive Creep Tests of Hard Chrome and HVOF
coatings 1998
This document summarizes creep testing for HVOF alternatives. Testing
was done as described in 12.10.1 General Description and Test Methods,
with the parameters shown in Table 70.
12.10.3.1.1.
Test conditions
Table 70. Creep test parameters
Description
Parameter
Hard chrome thickness
.030”
HVOF thickness
WC-17Co
.025”
T-400
.025”
Machining
Acid etching used to remove final substrate
from chrome due to fracturing problems
Test temperature
800oF
Test loads
50 ksi
100 ksi
Test Duration
300 hrs
1000 hrs
Page 156
12.10.3.1.2. Results
Test results are shown in Figure 76. WC-Co showed no measurable
creep at 50 ksi and gave the lowest compressive creep strain rate at 100
ksi, 0.000057 (10% that of chrome). The WC-Co showed such low creep
Avg. strain rate
(in/in/hr)
0.0006
0.0005
0.0004
0.0003
0.0002
0.0001
0
100ksi
50ksi
WC17Co
T400
Chrome
Figure 76. Average creep measured by direct micrometer
readings.
after the planned 300 hours at 100 ksi that the time was extended to 1000
hours and the low load test abandoned. T400 showed higher creep than
WC-Co, but much less than hard chrome.
12.10.3.1.3. Interpretation of results
Clearly, WC-Co is far less subject to creep than hard chrome, and
therefore more suitable for most engine applications that involve high load
at elevated temperature (such as shaft areas with press-fitted bearing
holders, which are quite common on main power shafts).
It is interesting to note that regardless of which set of measurement data
was examined, the hard chrome strain rates were approximately doubled
when the load was doubled while the Tribaloy 400 strain rates increased
by about four times. The hard chrome is a homogeneous single phase
material and gave linear behavior with load. However, the Tribaloy 400
material contains a dispersed hard phase (Laves phase) in an fcc alloy
matrix. The Tribaloy 400 behavior probably occurred due to a creep
mechanism change with load for the two phase structure not seen for the
single phase structure of hard chrome.
12.10.4.
General Trend of Creep Results
Results show that creep behavior of HVOF alternative materials is
superior to the current hard chrome materials. This is true for both WCCo and T-400, although the harder carbide is far more creep-resistant
than the Tribaloy.
Page 157
13. System performance
13.1. Summary
This section summarizes the data obtained in various rig and flight tests.
In most, if not all, rig and flight tests thus far HVOF coatings have proved
superior to chrome. However, this level of performance in general
requires that the surface finish of the HVOF coating is finer than that of
chrome, achieved either by grinding to 4µ” Ra or less, or by
superfinishing. Actuator tests have shown that the HVOF coatings have
little or no wear or leakage when used with PTFE seals, but rapidly
damage nitrile seals. In landing gear, however, either seal type can be
used.
As a result of rig and flight testing, HVOF coatings have been qualified as
chrome replacements by a number of airlines and manufacturers.
Table 71. Summary of rig and flight testing data.
Test
Organization
Notes
Hydraulic
actuator rig
Green, Tweed
and Co
WC-Co
F-18 landing
gear pin rig
Boeing
Limited qualification test. HVOF-coated
pins performed better than Cr.
Boeing 737,
Airbus 320
Lufthansa
Landing gear coated with WC-Co.
Performance better than Cr
Boeing 737,
757, 767
Delta Airlines
Landing gear coated with WC-CoCr.
Performance better than Cr
F-18 landing
gear repair
Messier-Dowty
Canadian F-18 main landing gear polygon
repair. Successfully flight tested. In
approval cycle.
Page 158
13.2. Rig tests
13.2.1.
Hydraulic Seals – Green, Tweed Phase 2
hydraulic rig test
13.2.1.1.
Documents
Document 46. Evaluation of Chrome Rod Alternative Coatings, Tony
Degennaro, Green Tweed, 1999. 44
"Evaluation of
Chrome Rod Alternat
This document describes the results of a 50 million cycle hydraulic
actuator test that evaluated HVOF WC-CoCr and T400 versus hard
chrome. The data from this test is also summarized on Green, Tweed’s
web site at
http://www.gtweed.com/Aerospace/ASTN/astnv11n7.htm.
13.2.1.2.
Test Description
The test is a hydraulic rig test using a fully-assembled hydraulic actuator
in which the rods were coated with various chrome alternatives and the
hydraulic cycled for a total of 50 million cycles. This test was designed to
simulate typical actuator service conditions. Measurements were made
of hydraulic fluid leakage rate as a function of time. Seal and rod wear
and condition were measured on disassembly.
13.2.1.3.
Test Conditions
Table 72. Hydraulic test conditions.
Substrate material
AISI-SAE 1566 steel, case hardened to 60-65 Rc
Coating material
EHC
HVOF T 400
HVOF WC-10Co4Cr
Coating thickness
0.002”
Surface finish
EHC 4µ” Ra
T 400 9 µ” Ra
WC-Co 4 µ” Ra
WC-CoCr 6.5 µ” Ra
Seals
ACT (elastomeric seal)
Enercap (capped-type PTFE seal)
Sample coatings and surface conditions are shown in Table 72. The
stroke and frequency profile is shown in Table 73. The surface finish for
Page 159
EHC was chosen to be comparable with the standard specified finish for
hydraulic rods. Samples were ground only - no superfinishing was used.
Table 73. Stroke and frequency profile for hydraulic tests.
Pressure (psi)
Stroke (“)
Frequency (Hz)
Cycles
3,000
3
0.5
1
2,200
1.5
1
2
1,800
.75
2
4
1,500
.3
4
10
1,000
0.09
5
33
800
0.06
5.5
50
500
0.03
6
100
200 total
Sequence repeated to 50,000,000 cycles
13.2.1.4.
Results
Results for the testing are shown in Figure 77 and Figure 78. Figure 77
shows cumulative oil leakage as a function of time. Therefore a steadily
rising curve represents an unchanging leak rate, while an increasing
slope is representative of leakage that becomes worse with time. Where
there is a peak, this means that the seal failed and had to be replaced,
which restarted the leakage measurement. After the first failure with the
WC-Co ACT seal and the second with the Tribaloy ACT seal, the
elastomeric ACT seals were replaced with PTFE Enercap seals, which
performed similarly to the other Enercap seals with the same rod
coatings.
Page 160
2500
Leakage
(grams)
2000
Chrome, Enercap
Tribaloy, Enercap
1500
WC-Co, Enercap
WC-Co-Cr, Enercap
Chrome, ACT
1000
500
5
15
25
35
45
0
Tribaloy, ACT
WC-Co, ACT
Cycles (millions)
Figure 77. Cumulative hydraulic fluid leakage in rig tests.
Figure 78 shows the actual measured seal wear and its standard
deviation.
Failures were seen in all the coatings with the ACT seals, and none with
the Enercap seals. After the failures in the ACT seals with WC-Co and
Tribaloy, the ACT seals were replaced with Enercaps, and the leakage
rate dropped to a low, steady value.
13.2.1.5.
Interpretation of Results
When HVOF coated rods are used in conjunction with PTFE seals, the
performance of the rod and seal is markedly superior to EHC, with the
WC coatings performing the best and providing seal life about an order of
magnitude longer than EHC (presumably because of their hardness).
Page 161
Both T-400 and WC-Co tend to damage elastomer seals and cause
premature failure and leakage (about one third the seal life). For this
reason, Green, Tweed does not recommend the use of elastomer seals
on flight surface actuator rods with these coatings until more work has
been done to determine whether superfinishing overcomes the problem.
Wear & Standard Deviation (inch)
0.035
0.03
Avg
Standard Deviation
0.025
0.02
0.015
0.01
0.005
0
WC-CoCr,
Enercap
WC-Co,
Enercap
Tribaloy,
Enercap
Chrome,
Enercap
Tribaloy,
ACT
Chrome,
ACT
WC-Co,
ACT
Rod, Seal Combinations
Figure 78. Seal wear during hydraulic rig tests.
13.2.1.6.
Comments
Note that this test was designed to represent control actuators, and does
not represent landing gear or intermittent use actuators such as uplock
and downlock hydraulics. Regarding elastomer seals, the Green, Tweed
report states, “The leakage rate on WC-Co is similar to chrome but the
seal life is roughly one-third that of chrome. Please note that this test
simulated a flight control duty cycle. The WC-Co/elastomeric seal
combination may be completely acceptable for landing gear and hydraulic
system utility actuators but can not be substantiated under these tested
conditions.“ See Section 13.3 for information on landing gear seal wear,
which appears to be acceptable with this combination of HVOF coating
and elastomer seals.
Page 162
13.2.2.
13.2.2.1.
Landing Gear Pins – Boeing landing gear
rig test
Documents
Document 47. F/A-18E/F Main Landing Gear HVOF-coated Pin
Testing and Evaluation.45
"Gaydos F-18 MLG
rig test summary.pdf"
This document summarizes rig testing on HVOF-coated landing gear run
at Boeing St, Louis as part of a main landing gear fatigue test.
13.2.2.2.
Test Conditions
The following pins were stripped of chrome plate and recoated with HVOF
WC-17Co per BAC 5851 specification:
•
74A400527 - Upper Sidebrace Attach Pin
•
74A400748 - Sidebrace to Universal Pin
•
74A400645 - Upper Oleo Pin
The test was an FT66 fatigue test conducted on a full scale left side main
landing gear. Loads were applied through hydraulics and strain was
measured with strain gauges. The test involved subjecting the gear to a
spectrum of loads and frequencies designed to simulate operating
conditions over a period of 14,000 standard flight hours. Load
requirements were designed to meet MIL-A-8867, and a pass required
that each component complete two lifetimes without failure. The fatigue
test was followed by a Constant Amplitude Cycling (CAC) test. Figure 79
shows the landing gear and the locations of the pins.
Page 163
Figure 79. F/A-18E/F main landing gear, showing locations of HVOFcoated pins.
13.2.2.3.
Results
The HVOF-coated pins showed no loss in dimensions, no wear damage,
and passed magnetic particle inspection. There was some material
transfer from the Cu-Be bushings onto the HVOF surfaces during the
cycling test.
13.2.2.4.
The HVOF coated pins passed the FT66 fatigue test, with performance
equal to or better than chrome. However, Boeing will require additional
testing of HVOF-coated pins before the coating can be qualified on other
components in the F-18 landing gear.
13.2.2.5.
Comments
There was some transfer of material from a Cu-Be bushing. This tends to
occur quite frequently with HVOF-coated components running against
soft bushings. Generally it appears to occur early in the wear process,
and then stabilize with little or no further transfer. Delta Airlines has
successfully eliminated material transfer by coating some bushings with
Tribaloy 400 (see Section 13.3.2).
13.2.3.
Rig tests under development – MessierDowty
Messier-Dowty, the manufacturer of the F/A-18E/F nose landing gear,
plans to carry out the following Qualification Test Procedures in the period
Page 164
2000-2001:
1. QTP 1568: Fatigue testing of high velocity oxygen fuel (HVOF)
coated F/A-18/EF nose landing gear. A complete NLG assembly,
will be tested in the ground load fatigue mode (excluding catapult
and holdback loads).
2. QTP-1569: Endurance and fatigue testing of high velocity oxygen
fuel (HVOF) coated F/A-18/EF nose landing gear drag brace
assembly. A drag brace assembly will be tested for both ground
load fatigue (including catapult and holdback loads) and
retraction-extension endurance.
The point of contact for this work is Roger Eybel:
mailto:Roger.Eybel@messier-dowty.on.ca.
13.3. Flight tests
13.3.1.
Lufthansa
Lufthansa was the first airline to place an HVOF WC-17Co-coated B737300 nose landing gear inner cylinder into service in January 1996 for a
two year flight test. The reason for moving toward thermal spray was
both environmental and economic. The normal chrome plating process
for this landing gear part is six days because of the need for special
masking and anodes, and embrittlement-relief heat treating. The HVOF
process can be done in less than 4 hours.
Subsequently, Lufthansa has adopted HVOF WC-CoCr for its improved
corrosion resistance.
13.3.1.1.
Documents
Lufthansa
This document details the corrosion test data underlying Lufthansa’s final
choice of WC-CoCr for landing gear. (Note: this is a very large
document.)
– Summary of tests (Courtesy of Lufthansa).
This document summarizes the Lufthansa data, including flight testing as
of 1997.
13.3.1.2.
Test Conditions
Both the inner cylinder seal surface and the axle journals were coated.
The aircraft was flown on standard commercial flights in Europe. The
landing gear was inspected at 6-month intervals under full extension, and
the wheels were removed to check the axles. The surfaces were
Page 165
inspected visually for chipping and flaking, and dye penetrant inspection
was used to look for cracks.
13.3.1.3.
Results
The test, which began in January 1996, ended in April 1998 after
completing 4701 flight cycles. No defects were found in the coating,
although the surface roughness had risen to 10µin due to Teflon pick-up
from the seal. Seal life was improved by about a factor of two, from a
typical 900-1100 flight cycles for chrome to 1910 flight cycles for HVOF.
A fluorescent penetrant inspection (FPI) showed crack-like indications on
the aft of the cylinder that were not visible with dye penetrant. The
cylinder was checked at Boeing with Barkhausen noise measurements,
which measures stress under an oscillating magnetic field, and would
indicate penetration of the cracks into the underlying steel. The test
showed the cracks to be confined to the coating, and the part was
returned to service. As of September 1999, no defects have been found.
13.3.1.4.
HVOF coatings perform better than chrome in flight testing. They do not
show significant wear. Provided the surface is smooth (in this case 6µin
Ra, but 2-6µin Ra is typical), seal life is improved.
13.3.1.5.
Comments
•
It is necessary to use FPI rather than simple red dye penetrant
inspection.
•
Some cracks may be seen in FPI tests, but thus far there is no
evidence of any propagation of cracks into the substrate. (This
agrees with extensive tests made on fatigue specimens – see
Section 12.6.)
•
Note that Lufthansa has now moved primarily to HVOF WC-CoCr
for its better corrosion resistance.
13.3.2.
Delta
Delta Airlines has flight tested a number of landing gear inner cylinders,
axles, and axle sleeves over the past three years.
Page 166
Aircraft
Component
Flight test status
2 each Boeing 737
Nose landing gear
piston; lower bearing
Completed successfully
4 each Boeing 757
Main landing gear
axles and axle sleeves
2 completed successfully;
2 still in test
4 each Boeing 767
Main landing gear
axles and axle sleeves
1 completed successfully;
3 still in test
13.3.2.1.
Test Conditions
All components were coated per Specification BAC5851, Class 2 or 4
(HVOF or D-gun) using BMS 10-67 Type XVII powder (WC-10Co-4Cr).
The pistons were ground to 8-16µin Ra (as specified for chrome) initially,
and later superfinished to less than 2µin Ra to prevent seal damage.
Axles were ground to 3-6µin Ra. The components and the areas coated
are shown in Figure 80.
In the case of the Boeing 757 and 767 main landing gear axles, the
HVOF-coated axles were mounted on one side of the aircraft, with
standard chrome plated landing gear on the other to permit direct
comparison.
The landing gear were inspected on the aircraft at 6-monthly intervals,
visually inspecting for damage and delamination, measuring diameters for
wear, measuring roughness, and using FPI for cracks. They were also
tracked for seal leakage on a daily basis.
Page 167
B 757 landing gear axle
B 737 landing gear
B 767 landing gear axle
Figure 80. Landing gear components HVOF sprayed for flight testing
by Delta Airlines (sprayed areas numbered). (Courtesy Delta Airlines.)
Figure 81. Boeing 737 nose landing gear inner cylinder undergoing
flight test inspection at Delta Airlines (Courtesy Delta Airlines).
Figure 81 shows a Boeing 737 undergoing inspection at Delta.
Page 168
13.3.2.2.
Results
Pistons:
It was found very quickly that the nominal 16µin Ra finish was too rough
for the pistons and caused seal failure twice in only 900 cycles. After the
piston was superfinished to <2µin Ra, the seals lasted 1910 cycles –
twice as long as a typical seal running on chrome (see Section 7.6.6).
The pistons exhibited essentially no wear. FPI showed no indication
of coating or cracking.
Axles:
As with the pistons, the axles showed neither wear nor cracking of
the coating. The chrome plated axles, on the other hand, showed
significant roughening due to wear.
The only problem found was the result of an improper hardness test,
which resulted in coating delamination at 18 months in several nearby
locations on one of the B767 axles. It was determined that these
delaminations occurred at the indents where the Rockwell hardness of
the axle had been measured (improperly) after coating. The coating
specification was revised to preclude hardness testing in the working area
and on coated areas.
(Note that coating
hardness can be
measured by Diamond
Pyramid Hardness, but
this should obviously not
be done on working
areas – see Section
7.3.4.)
It was found that the 155 PH sleeves in contact
with the HVOF-coated
axles exhibited higher
fretting wear than
against chrome (which
has been confirmed by
laboratory
measurements at
Boeing18). For this
Figure 82. Boeing 757 axle sleeves HVOFreason the sleeves were sprayed with WC-CoCr (Courtesy Delta
HVOF-sprayed inside
Airlines).
and out with WC-CoCr
and with T400. The effect of this coating on sleeve wear is still under
evaluation.
As a result of these successful flight tests Delta has converted overhaul
procedures for a majority of existing landing gear components from
chrome plating to HVOF WC-CoCr, including ODs, shallow IDs, and lugs.
Page 169
13.3.2.3.
The Delta flight tests represent a total of 20 aircraft years of flight
experience (2 years of experience with 10 aircraft). The HVOF coatings
outperformed chrome plate in coating wear and seal life. However, it was
found to be important to take steps to prevent wear of adjacent surfaces.
Seal wear was greatly reduced by superfinishing, while sleeve wear was
reduced with a similar hard coating (although this is still under
evaluation).
13.3.2.4.
Comments
Jay Randolph of Delta Airlines notes a number of other issues to be
borne in mind when using HVOF coatings on landing gear46:
•
Identification – Since it is not easy to identify HVOF-coated parts
versus chromed parts, an easily-visible marking system should be
used.
•
Finishing – Diamond grinding and (for pistons) superfinishing is
important. Ra alone is insufficient as a roughness measure, and
one should use peak-valley and bearing ratio as well. In general
static surfaces (such as axles) may have an 8µin Ra finish, but
hydraulic seal surfaces should have ≤ 4µin Ra. Most grinding
fluids are not a problem, but some can leach the CoCr binder.
•
Coating thickness – The coating thickness after grinding should
be at least 0.003” to avoid the possibility of flaking during grinding.
•
Inspection – Fluorescent penetrant inspection is necessary to
detect cracks in HVOF coatings. Magnetic particle inspection will
not pick up cracks since the coatings are non-magnetic.
•
Subsequent coating and stripping – HVOF-coated components
can be cadmium plated or nital etched (for surface examination)
after HVOF coating. Chrome and nickel strips will etch the WCCoCr, so it must be masked if these solutions are to be used to
remove Cr or Ni from an HVOF-coated item.
13.3.3.
F-18 landing gear repair
A repair for the Canadian F-18 main landing gear axle polygon has been
successfully flight-tested and is currently in the approval cycle with the
Department of National Defence (DND), Canada. This application is
described in Section 15.5.2.
Page 170
Since the polygon was
showing excessive wear
on its internal surface, a
repair procedure was
developed by MessierDowty, the landing gear
manufacturer. Since the
depth of the polygon is
quite small (see Figure
83), an HVOF WC-CoCr
coating is sprayed on
the inside surface, by
spraying at an angle so
as to reach down into
the hole. Masking is
used to prevent
overcoating of other
areas.
Figure 83. Canadian F-18 main landing
gear polygon repair (Courtesy MessierDowty).
13.3.4.
This repair has
eliminated the wear
problem and has been
successfully flight tested.
It is now in the approval
cycle with DnD.
Flight tests under way or under
development
Several flight tests are planned or in progress on HVOF-coated
components of military aircraft, as summarized in Table 74.
Table 74. Military flight tests of HVOF-coated components.
Aircraft
Component
Agency
Comments
P-3
Main landing gear
piston
NADEP
Jacksonville
In progress,
WC-Co
E-6A
Main landing gear
uplock hook shaft
NADEP
Jacksonville
In progress,
WC-Co
C-130
Nose and main
landing gear pistons
and axles
Canadian DND
Planned, WCCoCr
In addition flight tests are still in progress on commercial landing gear with
Lufthansa and Delta Airlines.
Page 171
PART 4. SPECIFICATIONS AND QUALIFIED
COMPONENTS
14. Specifications and standards for
thermal spray
14.1. Documents
Document 48. Table of contents of BAC 5851 Thermal Spray
Specification, 2000 (Courtesy Boeing Aircraft Corp.). 47
"Contents of Year
2000 Boeing 5851 Th
This is the latest version of the Boeing 5851 specification, which is used
as the industry standard. The document is still in the comment stage as
of the time of writing, and has not been issued.
Document 49. Standards for the Thermal Spray Industry, Bhusari
and Sulit. 48
"Bhusari TS
Standards.pdf"
This document gives a good overview of general industry standards for
thermal spray, mostly for applications outside the aerospace industry.
14.2. Boeing thermal spray specs –
method, powder, grinding
14.2.1.
Boeing Thermal Spray Spec – BAC 5851
Title: “Application of Thermal Spray Coatings”
Issued: October 1993; updated (greatly expanded) version to be issued
in 2000.
Availability: Available to Boeing contractors
Scope: This specification covers Plasma, HVOF, D-gun, and Super Dgun coatings.
Notes: This is the primary specification used in the aircraft industry for
thermal spray coatings. Many thermal spray coatings use this
specification, even when they are not done for Boeing.
Thermal spray coatings are specified by Class, Grade, and Type. The
Classes control the technology to be used, viz:
•
Class 1 Plasma,
Page 172
Part 4 Specs, Qualified Components
•
Class 2 High Velocity Oxygen Fuel (HVOF),
•
Class 3 Detonation Gun (D-Gun), and
•
Class 4 Super D-Gun™ (SDG)
There are two Grades:
•
Grade A General use coatings
•
Grade B Coatings for fatigue applications
There are 18 classes, covering various materials for applications such as
wear, corrosion, thermal barriers, bearings and bushings. The classes
are summarized in Table 75. Types XVI-XVIII were added to the original
table of coating types defined in the 1993 version of BAC 5851.
Table 75. Boeing thermal spray coating types.
Name
Formula
Use
Type I
Tungsten Carbide-Cobalt – High Cobalt WC-18Co
Wear
Type II
Aluminum Bronze
Cu-10Al
Soft bearing
Type III
Aluminum Oxide
Al2O3-3TiO2
Type IV
Chromium Oxide
Cr2O3
Type V
Zirconium Oxide
ZrO2-5CaO
Type VI
Nickel-Chrome
Ni-20Cr
Type VII
C. P. Aluminum
Thermal barrier
Type VIIII AISI 316 CRES
Fe-17Cr-12Ni-3Mo
Type IX
Cobalt Alloy 31
Co-25Cr-10Ni-8W
Type X
7XXX Aluminum
Al-5Zn-2Mg-2Cu
Type XI
AISI 46XX Steel
Fe-2Ni-0.3Mo
Type XII
Nickel-Aluminum
Ni-5Al
Rebuild
Type XIII
Nickel-Aluminum, prealloyed
Ni-5Al
Rebuild
Type XIV
Copper-Nickel-Indium Alloy
Cu-37Ni-5In
Type XV
Cobalt Alloy T-400
Co-28Mo-8Cr-3Si
Wear, corrosion high temperature
Type XVI
Chromium Carbide 80
Cr3C2-80Ni20Cr
Wear
Type XVII Tungsten Carbide-Cobalt-Chrome
WC-10Co-4Cr
Wear, corrosion
Type XVIII Tungsten Carbide-Cobalt – Low Cobalt
WC-12Co
Wear
The Year 2000 version of this specification covers the thermal spray
process, testing, and evaluation in great detail. The contents of the
document are summarized in Document 48.
Page 173
14.2.2.
Boeing Powder Spec – BMS 10-67
Title: “Thermal Spray Powders”
Issued: August 1993.
Scope: This specification covers the thermal spray powders to be used
in thermal spray according to BAC 5851 (see Table 75).
Notes: This specification is used in conjunction with BAC 5851. It
defines the powder chemistry and particle size distribution.
14.2.3.
Boeing Grinding Spec – BAC 5855
Title: “Grinding and Machining of Thermal Sprayed Coatings”
Issued: April 1997.
Scope: This specification covers the grinding and finishing of thermal
spray coatings made according to BAC 5851, using the materials defined
in BMS 10-67.
Notes: This specification is used in conjunction with BAC 5851 and BMS
10-67. It covers grinding and honing feeds and speeds.
14.3. Hamilton-Sundstrand – HS 4412
Title: “Coating, Plasma Spray Deposition, Process Specification for”
Issued: August 1992.
Availability: Available to Hamilton Sundstrand contractors.
Scope: This specification covers plasma spray, D-gun, and HVOF
coatings.
Notes: This specification defines two classes of coatings:
1. Coatings for wear and corrosion resistance
2. Coatings for buildup.
It defines 15 different coating chemistries, their hardnesses and bond
strengths.
14.4. Society of Automotive Engineers AMS 2447
Title: “Coating, Thermal Spray High Velocity/Fuel Process”
Issued: May 1998, revised October 1998.
Availability: Society of Automotive Engineers (SAE),
Page 174
http://www.sae.org/, Phone (724) 776-4970.
Scope: Covers engineering requirements for applying thermal spray
HVOF coatings, and the properties of HVOF coatings. Includes powders,
substrate preparation, coating methods, temperatures, test methods and
Quality assurance.
Notes: The specifications for HVOF coatings deposited under AMS 2447
are summarized in Table 76.
Table 76. AMS 2447 HVOF Coating specifications.
Coating
designation
Name/
AMS 2447-1
Stellite 31
Chemistry
Hardness,
min (HV)
Bond
strength,
min (psi)
Oxides,
max %
Voids,
max
%
Unmelts,
max (per
mm2)
400
8,000
5
1
5
500
9,000
2
1
2
800
10,000
2
1
-
275
8,000
2
1
3
350
8,000
2
1
3
375
9,000
5
1
3
1050
10,000
1
1
-
1000
10,000
1
1
-
1050
10,000
1
1
-
1000
10,000
1
1
-
57Co-25Cr-10Ni-7W
AMS 2447-2
Tribaloy 400
60Co-29Mo-8Cr-3Si
AMS 2447-3
75Cr3C2-25NiCr
69Cr-20Ni-11C
AMS 2447-4
Ni-Al Alloy
95Ni-5Al
AMS 2447-5
Ni-Cr-Al Alloy
76Ni-18Cr-6Al
AMS 2447-6
Inconel 718
60Ni-19Cr-18Fe-3Mo
AMS 2447-7
WC-17Co
78W-16Co-5.1C
AMS 2447-8
WC-12Co
88WC-11Co-4C
AMS 2447-9
WC-CoCr
82W-10Co-4Cr-3.5C
AMS 2447-10
WC-Ni
86W-10Ni-3.5C
14.5. American Welding Society – AWS
C.2-19-XX
Title: “Specifications for Thermal-Spray Coatings for Machine-Element
OEM and repair Applications”
Issued: This specification is in the latter stages of development by the
AWS C2 Subcommittee on Machine-Element Coatings. The standard is
expected to be issued in 2000.
Page 175
Availability: Not yet available. Will be available from AWS, probably in
2000.
Scope: This specification covers thermal spray coatings of coating
materials such as stainless steel, Ni-Al, Aluminum –bronze, Babbitt, NiCu, and alumina-titania for repair of machinery. It includes equipment,
spray methods, and sealer use. It incorporates the following spray
methods:
•
Arc wire
•
Flame wire
•
Flame powder
•
Plasma powder
Notes: The specification is still in draft form.
14.6. AMS standards under development
Several specifications are under development by the Society of
Automotive Engineers (SAE) Aerospace Materials Committee. These
Aerospace Materials Specifications (AMS) will cover the use of HVOF
coatings on high strength steels. They will cover the following:
•
Deposition methods
•
Powders – two specifications, AMEC 99B and 99C will cover WCCo and WC-CoCr powders
•
Grinding methods
The standards are being championed by Don Parker, NASA
mailto:Donald.Parker-1@kmail.ksc.nasa.gov (coating standards), and
Bruce Bodger, Sulzer Metco bbodger@ttown.com (powder standards),
Jon L. Devereaux, NADEP Jacksonville devereauxjl@navair.navy.mil
(grinding standards).
Page 176
15. Qualified Thermal Sprayed Airframe
Components
15.1. Documents
Document 50. HVOF WC aerospace applications for OEM and
rebuild (Courtesy Southwest Aeroservice). 49
"HVOF Applications
Listing SWA.PDF"
This document, authored by Jim Nuse, lists many of the components
currently HVOF sprayed with WC-Co for both OEM and rebuild. The list
is dominated by landing gear components and flap and slat tracks.
15.2. Usage of thermal spray in Gas
Turbine Engines
Several documents give a good overview of usage of thermal spray in
turbine engines, as well as what it takes to qualify thermal spray coatings
for engine applications.
Document 51. Thermal Spray Applications at GE Aircraft Engines
(Dorothy Comassar, Courtesy GE Aircraft Engines). 50
"Comassar, GE
Briefing.pdf"
This paper is an excellent review of the general applications, specific
materials, and specific types of components
Document 52. OEM Approval for HVOF Wear Resistant and MCrAlY
Coatings (Gary Naisbitt and Gorham Advanced Materials). 51
"Naisbitt Gorham
March 1999.pdf"
This paper covers requirements for and progress toward approving HVOF
coatings on various engine components.
Document 53. Replacement of Chromium Electroplating on Gas
Turbine Engines.52
"AESF March 2000,
GTEs, Schell.pdf"
This paper by Jerry Schell and Mark Reichtsteiner of GE Aircraft Engines
reviews a project begun in 2000 to replace chrome on aircraft engines
that is being spearheaded by PEWG (the Propulsion Environmental
Working Group).
Page 177
Part 4, Specs, Qualified Components
Thermal spray coatings have been used for many years on hundreds, if
not thousands, of components in gas turbine engines (GTEs), both at the
OEM and O&R levels. GTE coatings are most commonly plasma spray,
HVOF, or D-gun. Some of these are indicated in Figure 84. Examples
include
•
McrAlY corrosion-resistant bond coats for thermal barrier coatings
•
Zirconia thermal barrier coatings on hot section blades,
combustion and exhaust components
•
Al-Si Graphite abradable coatings for blade clearance control
•
Co-based alloys such as Tribaloy to prevent fretting of hot section
components
•
Cr3C2-NiCr for wear
•
WC-Co for shafts
Figure 84. Thermal spray coatings used in a typical gas turbine engine.
(Courtesy GE Aircraft Engines)50
The thermal spray process is therefore well-established for engines, and
is essential for proper engine function. It is only over the past 5 years or
so that thermal spray coatings have become more widely used in
airframes. This section covers the use of thermal spray for coating
airframe components.
Page 178
15.3. Summary of thermal spray coatings
on non-engine components
Table 77. Summary of thermal spray-qualified non-engine components.
(Click on links to access data directly.)
Component
Coating
Specification
Notes
Steering collars
HVOF, plasma, D-gun
WC-Co
BAC 5851
Boeing OEM coating
Slat tracks
HVOF, plasma, D-gun
WC-18Co
BAC 5851
Boeing OEM coating
D-gun WC-14Co
BAC 5851
Boeing OEM coating
Plasma Cr 2O3
BAC 5851
Boeing OEM coating
HVOF, D-gun, plasma
Tribaloy 400
BAC 5851
Boeing OEM coating
Landing gear sleeves, pins
D-gun, plasma Cu37Ni-5In, WC-18Co
BAC 5851
Boeing OEM coating
Boeing 747-400 landing gear
HVOF WC-CoCr
BAC 5851
Boeing OEM coating
Boeing 737 landing gear
HVOF WC-CoCr
BAC 5851
Overhaul and repair
Airbus 320 landing gear
HVOF WC-CoCr
F-18 Main landing gear axle
HVOF WC-CoCr
Sikorsky CH-53 blade
damper
Plasma T400
P&W F-119 Engine
Convergent Nozzle Actuator
Super D-gun WC-Co
and Plasma T400
HS 4412
Hamilton Sundstrand spec.
Engine likely to be used on JSF
Slat tracks, Boeing
Super D-gun WC18Co
BAC 5851
OEM usage
Bombardier Q-400 flap
tracks
HVOF WC-Co
Bombardier
PPS 24.04
OEM usage
Bombardier Q-400 and
Global Express door stop
pins
HVOF WC-Co
Bombardier
PPS 24.04
OEM usage
Slat and flap tracks
HVOF WC-Co
BAC 5851
Overhaul and Repair
Flap track repair –
Bombardier Dash 8
HVOF WC-Co
RD 8-571164 to 1169
Overhaul and Repair
L1011 flap tracks
HVOF WC-Co
Boeing 737, 757, 767 landing
gear
HVOF WC-Co
Parker-Hannifin actuators
Various
Pan castings
Gimbals
Bolts
Wear rings
Plungers
Fittings
Mufflers
Fairings
Row an Technology Group
Overhaul and repair
AMS 2447
Canadian Air Force – flight tested,
in final approval
In final approval for Navy CH-53
fleet
Overhaul and Repair – Titanium
alloy
BAC 5851
Main and nose landing gear O&R
at Delta Airlines
OEM usage – all new -design
hydraulics
Page 179
15.4. Boeing – qualified thermal sprayed
components
Boeing has specified thermal spray coatings on over a hundred airframe
part numbers. The majority of these components (86) are specified for
WC-Co, primarily WC-14Co or WC-18Co.
Document 54. List of Boeing thermal sprayed parts (Courtesy,
Boeing Aircraft Corp).
"Boeing Thermal
Sprayed Parts List.pd
Document 54 is Boeing’s list of components on which they have qualified
thermal spray as of June 2000. A general summary of this listing is given
in Table 78.
Table 78. Summary of Boeing components specified for thermal
spray.
Component types
Coating method
Coating material
Steering collars
HVOF, plasma,
D-gun
WC-Co
Plasma, HVOF,
D-gun, Super Dgun
WC-18Co
D-gun
WC-14Co
Plasma
Cr2O3
HVOF, D-gun,
plasma
Tribaloy 400
D-gun, plasma
Cu-37Ni-5In, WC18Co
Pins
Slat tracks
Pins
Pan castings
Gimbals
Sleeves
Bolts
Pins
Wear rings
Plungers
Fittings
Mufflers
Fairings
Landing gear sleeves, pins
Page 180
15.5. Landing gear
15.5.1.
OEM Production - Boeing 767-400 landing
gear
The landing gear of the new
Boeing 767-400 are specified for
HVOF WC-CoCr in place of hard
chrome on axles, inner cylinders,
and other components. These
items are also specified for
chrome plate, so that the two
different coatings are
interchangeable for different
Figure 85. Boeing 767-400 with HVOF
vendors. Some customers now
coated landing gear.
specify HVOF explicitly for
landing gear on their new aircraft.
This is the first full-scale OEM production use of HVOF-coated landing
gear on a commercial airliner in which the HVOF was designed on the
component at the outset, rather than being used to solve a problem after
the item had gone into production. The drawings permit either chrome of
HVOF WC-CoCr to be used. The landing gear is among the largest used
in commercial airliners, and is shown in Figure 86.
Figure 86. Boeing 767-400 main landing gear (Courtesy Sulzer
Metco).
HVOF WC-CoCr is applied to the following 767-400 landing gear
components:
•
Main Inner Cylinder
•
Main Axles
•
Bogey Beam Pin
•
Brake Rod Pins
Page 181
•
Spindles
•
Various Bushings
The major items in this list are the inner cylinder and axle. This landing
gear has four sets of wheels, with eight journals, all of which are specified
for chrome or HVOF WC-CoCr. In this landing gear the inner cylinders
are separate from the axles. Figure 87 shows the main landing gear axle,
on which the HVOF coating (or chrome plate) is indicated as engineering
callout #3. The coating is used on the center section and journal areas.
Figure 87. Boeing 767 main landing gear axle (part # 2207-85-10),
showing HVOF areas (engineering note 3). (Courtesy Sulzer Metco.)
Figure 88 shows the main landing gear inner cylinder, with asterisks
showing the locations of HVOF-sprayed areas. Note that the OD of the
cylinder is coated. The ID is also coated over a two-foot section, which is
possible only because the ID is very large (10”). (This is about the
smallest ID on which HVOF can be used, since it is limited by the gun
size and standoff.)
Page 182
Figure 88. Boeing 767-400 main landing gear inner cylinder (Part # 22074-10) with asterisks showing locations of HVOF coatings (Courtesy Sulzer
Metco.)
The specification for all Boeing landing gear repairs is BAC 5851.
15.5.2.
Flight tested landing gear repair Canadian F-18 MLG axle
Figure 89. Canadian F-18 main landing gear axle (Courtesy
Messier-Dowty).
A repair for the Canadian F-18 main landing gear axle polygon has been
successfully flight-tested and is currently in the approval cycle with the
Department of National Defence (DND), Canada. The axle is shown in
Figure 89. The axle is approximately 18” in total length, while the polygon
has a 4” equivalent diameter.
Page 183
The repair was developed
by Messier-Dowty, the
landing gear manufacturer,
for the polygon ID section of
the axle, which was showing
excessive wear (see detail
in Figure 90). The figure
shows the HVOF-repaired
area, which is the white
polygon. The coating
material is HVOF WC-CoCr,
which is sprayed into the
repair area at an angle from
the outside.
The coating process
specification for this repair is
Figure 90. Repair area of F-18 main landing gear
AMS 2447.
polygon (Courtesty Messier-Dowty).
15.5.3.
Other qualified landing gear applications
Various other landing gear components are also qualified for HVOF, as
indicated in Table 79.
Table 79. Other landing gear components qualified for
OEM HVOF WC-Co (Courtesy Southwest Aeroservice).
Item
Coating
F-18 Steering Covers
T-400
A-340 Bushings
T-400
F-18 Shock Absorber Piston Heads
T-400
Boeing 737 Nose Torsion Link Pin
WC-Co
Beech Premiere nose outer cylinder bearing lugs WC-Co
Various rotor drive keys for brake drums
WC-Co
Note that not all of these coatings are WC-Co. Tribaloy 400 is often used,
e.g. where WC-Co is too hard and causes excessive wear on adjacent
components.
15.5.4.
Boeing overhaul manual revision
In early 1999 Boeing issued a revision to its landing gear overhaul
manuals to allow an alternate repair of HVOF, D-Gun, or SDG, in lieu of
chrome plate for finished coating thicknesses of 0.010 inches or less.
This makes it possible for airlines and landing gear O&R shops to
Page 184
substitute HVOF for chrome in the overhaul of most landing gear
components. However, any such use of HVOF can only be done by a
Boeing-approved facility. Approval involves validating the repair
procedure, which includes spraying of metallographic and fatigue test
samples. As of this point, there are a number of qualified thermal spray
shops able to carry out HVOF for overhaul.
As a result of this revision, HVOF WC-Co or WC-CoCr is being used on a
number of aircraft landing gear, such as the Boeing 737 and the Airbus
32053.
15.5.5.
Delta Airlines qualified landing gear repair
– Boeing 737, 757, 767
Delta Airlines flight testing is summarized Section 13.3.2.
After 24 months of successful flight testing involving 6-monthly
inspections of several aircraft, Delta Airlines has converted the nose and
main landing gear cylinders and axle journal repairs from chrome to
HVOF WC-CoCr on the following aircraft:
•
Boeing 737-200/300
•
Boeing 757-200
•
Boeing 767-200/300
Delta have also converted other line-of sight applications, such as shallow
IDs and lug faces to HVOF. The airline has required HVOF WC-CoCr
coated landing gear on its new Boeing 767-400 aircraft, and intends to
convert most of its landing gear component repairs to HVOF WC-CoCr in
the near future.
During the course of flight testing, it was found that a smooth surface
finish of the inner cylinder was essential to prevent damage to the seals
and premature seal failure (see Section 13.3.2).
Page 185
15.5.6.
Qualified landing gear repair
With Boeing’s specification of HVOF
as an alternative to chrome in
landing gear repair, HVOF WC-Co
and WC-CoCr are now being used
extensively for repair of landing gear
inner cylinders and axles (for
example by Southwest Aeroservice).
The specification for these coatings
is BAC 5851.
Table 80. Landing gear
components commonly
repaired with HVOF WC-Co
(Courtesy Southwest
Aeroservice).
737 Nose Inner Cylinder
Main Inner Cylinder
Nose Lower Bearing
757 Main Axles
Main Brake Sleeves
767 Main Axles
Main Brake Sleeves
Numerous Pins, Bolts and
Hardware
P3 Main Inner Cylinder*
Figure 91. HVOF WC-Co repair
of Boeing 737 nose landing gear
inner cylinder (Courtesty
Figure 91 shows a Boeing 737 nose
landing gear inner cylinder being HVOFcoated. The HVOF gun on the right
traverses up and down the cylinder while the cylinder rotates about its
axis. The combination of rotation speed and cooling air jets (not shown)
is used to maintain a proper temperature.
* In flight test
Table 80 summarizes some of the landing gear components frequently
coated with HVOF WC-Co. All of the commercial components are
qualified repairs. The P3 component is currently in flight test.
15.6. Hydraulics
Thermal spray coatings are not yet as widely used on hydraulics as they
are on landing gear.
Praxair has developed a combination of coatings to replace chrome
plating on hydraulic actuator rods, piston heads, and cylinder IDs. They
supply the following combination of coatings as a general approach for
hydraulic actuators:
Page 186
•
D-gun/HVOF WC-Co, WC-CoCr and WC-CrNi on rods
•
Plasma (or low pressure plasma) sprayed Tribaloy 400 on cylinder
IDs (>2.5" diameter)
•
D-gun/HVOF WC-Co or T400 on piston heads.
Variations of this multiple-coating approach (see figure 92) have been
qualified on several actuator systems by different manufacturers.
Parker-Hannifin has made the decision that the company will not use
chrome plate on new designs 54, and instead is moving to other materials
and coatings, including thermal sprays. Some Parker flight control
actuator pistons are already being HVOF-coated. Given the ubiquity of
hydraulics in aircraft, and Parker’s position as the market leader in this
area, this decision is likely ultimately to have a major effect on chrome
usage in the aircraft industry.
D-gun/HVOF WC-Co, WC-CoCr, WC-CrNi
Plasma T400
D-gun/HVOF WC-Co, T400
Figure 92. Thermal spray actuator coating system developed by
Praxair.
15.6.1.
P&W F-119 engine convergent nozzle
actuator
The Pratt & Whitney F-119 engine is being produced for the F-22, and is
expected to be used on the JSF. The convergent nozzles are
hydraulically actuated using actuators produced by Hamilton Sundstrand.
Both the actuator rod and cylinder ID are thermal sprayed. The
specification for both of these coatings is HS 4412.
Page 187
15.6.2.
Flight test – Sikorsky CH-53 blade damper
The CH-53 is a heavy-lift
helicopter manufactured by
Sikorsky. Currently the IDs of
the blade dampers are coated
with hard chrome. HVOF
Tribaloy 400-coated outer
cylinders are in the final stages of
flight testing for eventual
replacement of all dampers on
the Navy’s CH-53 fleet. The
damper has an ID of about 3.5”
and a length of about 6”.
Figure 93. CH-53 helicopter
This application is expected to be (Sikorsky).
qualified shortly.
Page 188
15.7. Production - Flap and slat tracks
15.7.1.
OEM tracks - Boeing
Boeing specifies Super D-gun WC-18Co on a number of flap and slat
tracks, as summarized in Table 81.
Table 81. Flap and slat tracks specified for thermal spray
coating with Super D-gun WC-18Co (Courtesy Boeing).
Part #
Description
113A3941 TRACK ASSY - #2, OUTBD AFT FLAP
114A7511 MAIN TRACK ASSY - SLAT 1 AND 8 OUTBD, SS494.85
114A7512 MAIN TRACK ASSY - SLAT 1 AND 8 INBD, SS409.09
114A7521 MAIN TRACK ASSY - SLAT 2 AND 7 OUTBD, SS362.51
114A7522 MAIN TRACK ASSY - SLAT 2 AND 7 INBD, SS282.91
114A7531 MAIN TRACK ASSY - SLATS 3 AND 6 OUTBD, SS 240.95
114A7532 MAIN TRACK ASSY - SLATS 3 AND 6 INBD, SS 163.95
114A7542 MAIN TRACK ASSY - SLATS 4 AND 5 INBD, SS 47.69
Boeing 737 flap drive gimbals are also coated with HVOF WC-Co.
15.7.2.
OEM tracks - Bombardier
The flap tracks for the new
Bombardier Q-400 are now in
production with HVOF WC-Co. In
this case the thermal spray
replaces electroless Ni-B, which
was found to have inadequate life.
The aircraft has no slat tracks, but
has 10 flap tracks, which are all
coated to the same Bombardier
Production Process Specification
PPS 24.04.
Figure 94. Bombardier Q-400
(Courtesy Bombardier.)
Page 189
The basic shape of a flap track is shown in Figure 95. The track
comprises two mirror-image components fastened together. The flap
rides along the track on bearings attached to the sides of the flap. These
bearings ride on the
surfaces of the track,
and in the fully extended
position the bearings
tend to cause severe
fretting wear at their
stopping points on the
track because of the
Figure 95. Typical flap track - Bombardier
vibration of the flaps. In
Dash 8 (Courtesy Vac Aero, Canada).
the past, electroless
nickel has been used,
but it has always been less than satisfactory because of its low thickness
and limited wear life, hence the move to HVOF coatings. HVOF coatings
may cause higher wear of flap bearings, but these are inexpensive and
easily replaced on the aircraft, whereas the track is valued in excess of
$100,000 and cannot be repaired in-place.
15.7.3.
Flap track repair – Bombardier Dash 8
The Bombardier Dash 8-100, -200, and –300 aircraft all have qualified
flap track repairs using HVOF WC-Co (see Table 82).
Table 82. Bombardier Dash 8-100, -200, -300 flap tracks
qualified for HVOF repair (Courtesy Vac Aero).
Flap track #
Part #
1
85780044 – 101/102-105-106-109-110
85780185-101/102/103/104
2
85780044 – 103/104/107/108/111/112/113/114
85780185 – 103/104/107/108/111/112/113/114
3
85780035 – 101/102/103/104/105/106
85780189 – 101/102/103/104/105/106
4
85780013 – 101/102/103/104/105/106
85780183 – 101/102/103/105/107/111/113/115
5
85780014 – 101/102/103/104
85780184 – 101/102/103/105/107
Repair involves building up the damaged track areas and then coating the
entire track with HVOF WC-Co (Figure 96). Both sides of both tracks are
coated. Because of the shape of the flap tracks, the coating of the inner
surfaces must be made with the HVOF gun off normal incidence. The
complexity of the shape also precludes a post-coating grind, so that the
Page 190
coating must be smooth as-sprayed.
Repair specifications are governed by Bombardier RD 8-57-1164 to 1169.
Figure 96. HVOF-sprayed Dash 8 flap track. Coated areas are dark.
(Courtesy, Vac Aero, Canada.)
15.7.4.
O&R of tracks – Boeing and other aircraft
HVOF WC-Co is now qualified and used
for O&R on slat and/or flap tracks on a
number of aircraft. The HVOF coating
replaces other hard coatings such as
electroless nickel, providing improved
performance and longer life (see Table
83).
Table 83. Common
flap/slat track repairs using
HVOF WC-Co (Courtesy
Boeing 707
Boeing 727
Boeing 737
Boeing 767
Douglas DC-9
McDonnel Douglas MD-80
McDonnel Douglas DC-10
McDonnel Douglas MD-11
Embraer EMB120
Lockheed L-1011
Figure 97. Flap and slat
track repair by HVOF
(Southwest Aeroservice).
In the UK, TWI (The Welding Institute) of
Cambridge, England has developed a
qualified repair procedure for the titanium
alloy flap tracks of the L1011. The repair
procedure involves TIG welding to rebuild worn areas followed by
spraying with HVOF WC-Co.
Bombardier Dash 8-200/300
Page 191
15.8. Other components
Both the Bombardier Q-400 and
the Bombardier Global Express
specify HVOF WC-Co on the stop
pins for the cabin and baggage
doors.
Figure 98 shows a Boeing 737
nose landing gear bearing made
of Al-Ni-bronze that is now
repaired on the ID using WC-Co.
Because the part is 3.25” ID by
2.5” deep, and open at both ends,
it can readily be sprayed from
outside. The repair is
superfinished to prevent damage
to the mating surface.
Boeing 747 Wear Plates are also
repaired with HVOF WC-Co.
A number of other assorted OEM
applications are also approved, as
outlined in Table 84.
Since 1996 United Airlines has been
replacing chrome plating with HVOF
coatings, as well as replacing
proprietary coatings (D-gun, etc.)
with in-house HVOF. A number of
components on which chrome
plating has been replaced by HVOF
for O&R are summarized in Table
85.
Figure 98. Boeing 737 nose
landing gear lower bearing shock
strut, Part # 69-76508. HVOF WCCo coated and super finished.
(Courtesy Sulzer Metco.)
Table 84. Other OEM HVOF
WC-Co applications (Courtesy
Helicopter Hardware
Swashplate supports
Swashplate balls
Bearing sleeves
Radius rings
Miscellaneous Hardware
Boeing 767 Wear strips
Boeing 777 Fireseal Depressor
Boeing 777 Various Frames
Page 192
Table 85. United Airlines O&R components qualified for HVOF in place of chrome
plate.55
Part
Type of component
Aircraft
Butter Shaft
Accessory Component Part
Boeing 747
Disc Hi Stage Valve
Boeing 747
HPSOV Actuator Cap
Boeing 767
Shaft Disc Hi Stage Valve
Boeing 767
Valve Body
Boeing 767
Body Butterfly
Boeing 737
Shaft IOG Worm
Boeing 747, 757, 767
Hub Compressor Front Disc
Engine Component Part
JT8D engine
Page 193
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st
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26
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st
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Page 195
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35
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36
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rd
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Page 196
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54
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rd
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Global Symposium on HVOF Coatings, San Francisco, 1997.
Page 197

Use of Thermal Spray as an Aerospace Chrome Plating

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