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Revista Latinoamericana de Metalurgia y Materiales, Vol. 20, N°2, 2000, 42-46
CHARACTERIZATION OF VANADIUM CARBIDES COATINGS
PRODUCED BY THERMOCHEMICAL DIFUSIVE TREATMENT IN
MOLTEN SALTS: COMPOSITION AND RESIDUAL STRESSES.
1
,,1
,
2
2
V. Herrera ,Lo M. Fernandez .B. Aragon e lo Zamora o
1. Centro de Estudios Aplicados al Desarrollo Nuclear (CEADEN).
Departamento de Análisis y Ensayos. Calle 30 #502 el 5ta y a, Miramar, Playa. C.Habana, Cuba.
CP 6122; E-Mail: victoria@ceaden.edu.cu
2. Centro de Investigaciones Metalúrgicas. (CIME).Ave. 51 No. 23611 el 236 y 240, San Agustín,
La Lisa. C. Habana, Cuba.
r
Resumen.
Las tensiones residuales están presentes en casi toda estructura ensamblada.
La Difracción de Rayos X, entre otros usos, es una técnica no destructiva de medición de tensiones residuales en un
campo superficial del material.
Los recubrimientos de carburos de metales de transición han ganado gran importancia en la fabricación de herramientas
gracias a su alta resistencia al desgaste.
El tratamiento termoquímico combina la acción de la temperatura con condiciones de saturación por difusión de un
metal o aleación dado con metales o no metales, de manera que propicien la realización de reacciones químicas en una capa
superficial.
En el presente trabajo se efectúa la caracterización fásica de recubrimiento s de carburo de vanadio producidos sobre
aceros de herramienta X12M y 09XBG (norma rusa) y se evalúa la tensión residual generada en el recubrimiento mediante
la técnica de Difracción de rayos X. En la caracterización de las capas además se empleó la Microscopía Electrónica de
Barrido.
Los recubrimientos obtenidos tienen espesores entre 6 y 12 um con una microestructura globular y se componen de
carburo de vanadio de estructura NaCl. Los recubrimiento s presentan macrotensiones de compresión relativamente bajas,
entre 0.3 y 0,5 GN. m-2. Se observa además ciertas microtensiones.
Palabras Clave: Recubrimientos
herramientas, MEB.
de Carburo de Vanadio,
Esfuerzos
residuales,
difracción de rayos X,
acero para
Abstract.
Residual Stresses are present in almost a11assembled structures.
X Rays Diffraction, 1S a versatile non - destructive technique, which finds a high valuable use for residual stresses
measurement on the material surface.
Coatings of transition metals carbides have acquired a remarkable importance in the tool production, due to their high
wear resistance.
The coupled action of temperature and diffusion controlled saturation of a given metal or alloy with other metals or nonmetal s is the key element of thenno - chemical treatment. As result, surface chemical reactions are induced.
In this paper X Ray phase characterization of vanadium carbide coatings produced on tool steels X12M and 09XBG
(Russian norm) is carried out. X Ray Diffraction Residual Stresses analysis is achieved on coating surface. For
characterization Scanning Electron Microscopy and Electron Probe microanalysis are also used.
Studied coatings are formed by vanadium carbide (VC) with a NaCl structure. Low compressive macrostresses (0.3 y
0,5 GN. m-2.) are measured. Some microstresses are also observed. A globular microstructure is characteristic for these
coatings.
Keywords: Vanadium Carbide Coatings; Residual Stresses; X-Ray Diffraction; Tool Steels; Sem
residual
are presenr in almost all
led srrucmre.
.dnal stresses are produced in technological
¡;:n:ceSS:5 su h as welding, heat treatment, galvanic and
diffusive coating, or as result of procedures
plastic deformation
is involved. Generally,
compressive stresses have been considered
cial for parts performance, due to the reduction of
ice tensile stresses. Oppositely, tensile residual
~~::s
can lead to unforeseen fractures.
Therefore, the information
on residual stresses
;;- nces to a large extent the concepts on technological
¡;:nJcedure,s.
Coatings of transition metals carbide ha ve acquired a
importance in the manufacture of tools and parts
ID their high wear resistance.
Therrno - chemical treatment causes surface chemical
.ons where key elements are the coupled action of
rature and diffusion controlled saturation of a given
or alloy with metal s or non-metals. As result,
sition, rnicrostructure and the stress level of
gs can be controlled.
As a way to raise the hardness, wear, cavitation and
ion resistance, the thermo- chemical treatment also
vides favorable residual stresses. Therefore, it finds
rtant applications to increase the reliability and
bility of parts.
The quality of diffusive coatings is characterized by:
its structure and phase composition,
its total or effective depth,
the concentration of diffusing element;
•
the fragile fracture capability under the action of a
localload;
the homogeneity, continuity and uniformity of the
distribution of coating along the configuration of the
metallic piece (configuration effect);
•
the magnitude and gradients of residual stresses .
Several coatings types are known, especially PVD
hysical Vapor Deposition) and the CVD (Chernical
apor Deposition) of titanium carbonitride and nitride
_-5], that have been those ones of more industrial
ímportance. Coatings of tungsten carbide, iron silicide,
hrornium carbide have been also studied.
Several references on procedures to produce carbide
layers, with MC (M = W, V, Nb) type structures [6-10]
are reported. They are applied on low alloyed steels to
raise their wear resistance.
In this paper, phase characterization of vanadium
carbide coatings onto tool steels X12M and 09XBG
(Russian standard) was carried out. Residual stresses on
coatings are also measured.
Cylíndrical amples ofX12M and 09XBG steels with
vanadium carbide coatings were obtained in molten salt
mixtures with different sort and concentration
of
reducing agent. Nominal chemical composition of cited
steels is shown on Table 1.
Depth profile for carbon, vanadium and iron in the
coating was measured by Electron Probe Microanalysis
in a JEOL Scanning Electron Microscope.
Phase composition was determined by X Ray Diffraction
(XRD). Residual macrostress on produced coatings is
also evaluated by the sin2 t¡rmethod [12,13].
XRD allows to measure the lattice strain E ~ '" directly in
the direction (\jf, <») of a rectangular coordinate system
xyz, where x- and y- axes líe on the sample surface plane
and z axe is normal.
The angle \jf is formed between the normal to the
sample surface Ns and the normal to diffracting
crystallographic planes N(hkl) in a plane normal to sample
surface.
The angle <p is forrned between the direction of the
measured stress cr~ on the sample surface and one of the
x- or y-axes.
Ns• N(hkl)
and cr~ lie in a plane, normal to the sample
surface.
The strain € in the (rp, \jf) direction is expressed by the
equation:
En = (dn-do)/do
(1)
where,
d ~ 'l'
". interplanar spacing for (hkl)
crystallographic planes measured in the (rp ,\jf) direction
on the stressed material.
do- interplanar spacing for (hkl) planes measured in the
non stressed material.
In an isotropic body the strain € ~ 'l' is given by the
equation:
2
= V2 S2 [ ( cr~ " cr33) sin \jf + o 33 + (crl3 coso + cr23
símp ) sin 2\jf] + SI [crn + cr22 + cr33] (2)
E~
IJI
where, SI and Y2 S2 - X-ray elastic constants. They are
functions of (hkl) planes
o i i - normal components of stress ten sor i = 1,2,3
cr ij - shear components.
where,
cr~=cr llCOS2 <p + cr22sin2<p + 2cr12sin<p coso (3)
For comparison purposes, the theoretical powder
diffraction pattem of vanadium carbide was ca1culated.
Revista Latinoamericana de Metalurgia y Materiales, Vol. 20, N°2,2000
44
Table I. Nominal chemical composition of tool steels X12M and 09XBG [11]
C%
Cr%
Mo%
Mn%
Si%
W%
X12M
1,45 - 1,70
11,0 - 12,5
0,5 - 0,8
-
-
-
09XBG
0,85 - 0,9
0,9 - 1,2
-
0,8 -1,0
0,15 - 0,35
1,2 - 1,6
STEEL
3.
ResuIts.
•
Chemical composition.
•
Phase composition and microstructure.
The rnicrostructure of coating controls in a great extent
its mechanical and tribological properties
Coatings thickness with values ranging from 6 to 12 um
was measured. These layers show a globular structure
with some pares (fig.2). Cracks at the interface
coating/bulk metal are not observed. (fig.3).
X Ray Diffraction diagrams show a NaCI structure
vanadium carbide VC (Table.II). In fig. 4 the calculated
diffraction pattem is given. Influence of rnicrostresses
was considered. Actually, experimental pattem exhibits
this effect. The lattice parameter appears smaller than
the calculated one. This could be related to differences in
the vanadium /carbon ratio.
The assessment of the chemical composinon of
coatings is meaningful due to the influence of impurities
on
coating
perfection.
Furthermore,
chernical,
mechanical, physical and tribological properties of
coatings are conditioned by their phase stoichiometry.
In this study observed elements in coatings are
carbon, vanadium and iron. (fig.l and 2). They are
distributed homogeneously. However, in some cases a
certain partial substitution of vanadium by iron was
observed.
400
350
300
• ~
-,"
en
e::s
o
.
~'r~
I~ ~~
250
I
--w
200
o
•
--
--
•
150
100
50
O
O
2
3
4
5
6
7
8
9
Depth,l'm
e _c~
-
v~
-----=-
FeKa-
Fig.l Depth profiles for carbon, vanadium and iron in a ve coating. SEM Electron Microprobe
v. Herrera y col./Revista Latinoamericana
Fig.2 VC Coating
de Metalurgia y Materiales
45
Fig 3b. Vanadium distribution in the coating. SEM Electron
Probe Microanalysis
SEM
•
Residual Stresses.
The residual stresses can be established on coatings,
due to deformations induced by growth defects, which
affect locally the lattice pararneter; or by anisotropy of
the elastic properties of grains during the growth of the
layer. [14].
In some coatings residual stresses can be present, that
are able to produce plastic deformation
or even
rnicroscopic cracking.
In residual stress measurements vanadium carbide
(333) peak (d = 0.0800 nm) was used. Line position for
different \jf orientations
(O < sin2\jf
<0.5) was
deterrnined by a para bola profile fit around the
maximum.
Fig.3a. Coating cross section.
Table II:Calculated and experimental XRD line intensities and positions for Vanadium Carbide VC (NaCl structure).
28teor.
Iteor.
Iexp.
Dteor.
Dexo.
(hkl)
F2
P
Iabs.
37.237
43.267
62.849
75.376
79.366
95.0lí
106.927
111.048
129.119
148.593
92.7
100.0
55.8
30.6
17.3
8.6
15.1
30.8
36.4
47.,7
100
31.8
8.5
8.0
2.4146
2.0910
1.4786
1.2609
1.2073
1.0455
.9594
.9351
.8537
.8049
2.429
2.089
1.475
1.254
1.202
1.041
.956
.930
.851
.800
111
2 OO
220
3 1 1
222
4 OO
331
420
422
333
2836.
5716
3748
1504
2796
2249
969
1904
1672
731
8
6
12
24
8
6
24
24
24
8
71.13
76.77
42.81
23.52
13.31
6.61
11.62
23.68
27.95
6.52
17
5.7
5.7
5.7
9.1
11.4
Revista Latinoamericana
46
I-
(a)
de Metalurgia y Materiales, Vol. 20, N°2,2000
I
27M
1SIl
n'
~ IIlIl
o
Sil
L
io..,
Il
31l
Sil
•
9IJ
11l
.1
,11.
11[]
13[]
...l..
1S[]
2 theta
~0r-----------------------------------'
-
(b)
ve (calculado)
4.
I
l:l
§ 100
8
50
110
70
1S0
theta
Fig.4 Experimental (a) and calculated (b) diffraction pattem of
vanadium carbide coatings
2
The obtained strain distribution is cuasilinear (fig.5),
pointing out the lack of shear components. It shows a
weak texture effect, as indicated by the small oscillation
around the linearity.
According to the slope of this
dependence, compression stresses are present.
(333) line ve
149.1
-.-
149.0
linearfit
148.3
/
0.0
2.
3.
4.
6.
7.
8.
9.
10.
11.
~ 148.7
148.4
1.
5.
148.8
148.5
5.
experimental
148.9
148.6
-~
12.
13.
0.1
0.2
0.3
0.4
0.5
Conclusions.
Residual
macrostresses
yielded during thermochemical treatment in molten salts were measured for
vanadium carbide coatings (NaCl structure) on tool steel
X12M
and
09XBG.Measured
macrostresses
are
compressive with values between 0.5 and 0,7 GN. m",
The studied coatings show some microstresses.
Produced coatings have a thickness between 6 and 12
mm with a globular microstructure. Cracking at the
coatinglbulk metal interface was not observed.
150
O~----.-----.-----.-----.-----.---~
:xl
According to [15] for vanadium carbide E =276
GN.m -2 compression stresses between 0,70 ± 0,08 and
0,49 ± 0,07 GN. m ,2 are calculated fram the measured
strains. These stresses are lower to those ones reported in
[14], where values from 2 to 3 GN. m'2 are obtained for
titanium nitride coatings (TiN) and values between 17
and 20 GN. m,2 for titanium carbonitride (Ti(C,N).
The observed halbwidth of diffraction lines gives a
measure of the microstresses in these systems. The e
may be related to a high defect density or local
composition gradients.
0.6
14.
.2
SIIl\jf
. Fig.5 Strain distribution for vanadium carbide coating onto steeJ
09XBG,
Reported values of Y oung Module E for titanium
nitride and carbide are between 200 and 600 GN. m ,2
[14]. The constant Y2 S 2 is re1ated with Poisson
coefficient E in the following way
15.
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