EBSD Technical Bulletin Sample preparation for EBSD Analysis Introduction

EBSD
EBSD
Sample preparation for EBSD Analysis
Introduction
Technical Bulletin
Specimen preparation for EBSD is critical, because the diffracted electrons escape from within only a few tens
of nanometres of the specimen surface. However, sample preparation requirements can typically be achieved
following some simple recipes. Some broad guidelines are given here:
•Metals and insulators: mounting in conductive resin, mechanical grinding, diamond polishing and final polishing
with colloidal silica.
•Metals: mounting in conductive resin, mechanical grinding, diamond polishing and electropolishing.
•Brittle materials (such as ceramics and geological materials): can often be fractured to reveal surfaces
immediately suitable for EBSD.
•Ion milling for materials which are not amenable to conventional metallography such as zirconium and zircalloy.
•Dual focussed ion beam: electron beam microscopes fitted with EBSD can perform in-situ specimen preparation
for EBSD.
•Plasma etching for microelectronic devices.
Additional details of the techniques mentioned are given below.
The need for sample preparation
EBSD is a surface-sensitive technique, with the
diffraction signal coming from the top few nanometers
of the crystal lattice. Therefore it is essential that this
top layer remains free from damage and also free from
contamination or oxidation and this makes sample
preparation absolutely critical for collecting good EBSD
data. The fact that the sample must be tilted up to high
angles (typically 70°), means that surface topography
must also be kept to a minimum.
For most materials, standard preparation methods
are sufficient to the final polishing stage. Then, an
additional polishing stage using colloidal silica is all that
is required to achieve a finish suitable for EBSD.
However, different materials respond differently
to common preparation methods. Therefore the
material under investigation should be considered
on an individual basis and prepared appropriately.
The manufacturers of preparation equipment can be
consulted for the applicability of a given approach for a
given material.
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EBSD
EBSD
Sample preparation for EBSD Analysis
Preparation techniques
There are a number of techniques for preparing samples
for EBSD, and choosing the correct technique is very
dependent on the composition and structure of your
sample. Here are some common examples:
Mechanical Polishing
Mechanical polishing is necessary on almost all
samples for EBSD analysis. It is the preferred method
when preparing multiphase materials, which are the
most challenging to prepare well. This is also the
best solution when preparing ceramic or geological
materials. It involves four simple steps:
1. Mounting
There is no particular mounting process that is ideal for
EBSD, but it can be useful to use a conductive medium
as this will reduce any drift or charging when working
with insulators. However, hot mounting processes
can cause some materials (such as many geological
minerals) to expand and possibly fracture.
2. Grinding
This is the first mechanical stage of preparation, and
primarily removes the deformation layer produced in
sectioning and produces a flat surface. Silicon Carbide
(SiC) paper is normally used. However, different
materials have different abrasion characteristics so
the selection of grinding material and conditions can
therefore be specific to a given sample.
After every grinding stage it is advisable to inspect
the sample surface using a light microscope in order
to ensure that all damage from the previous stage is
completely removed. Continue preparing the sample in
this manner to the finest abrasive size required, ready
for polishing. Care at this stage will greatly reduce the
amount of polishing required to achieve a good surface.
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Technical Bulletin
EBSD
EBSD
Sample preparation for EBSD Analysis
Technical Bulletin
3. Polishing
Removes most of the damage caused by grinding and can be performed with many types of abrasive and
suspension mediums. Diamond polishing compounds or slurries are good for preliminary stages for most materials.
Usually polishing is started on a hard cloth with a coarser abrasive and finished on a softer cloth with a finer
abrasive.
Vibratory Polishing removes minor deformation remaining after mechanical preparation. It is designed to prepare
high quality polished surfaces on a wide variety of materials and applications. A horizontal vibratory motion
of typically 7200 cycles per minute produces a very effective polishing action with superior quality results and
exceptional flatness. The unique vibratory action produces less deformation, flatter surfaces and reduces edge
rounding. It also yields a stress-free surface.
4. Colloidal Silica
For EBSD, it is generally necessary to include a final polishing stage using colloidal silica. Final polishing should not
be prolonged, but just sufficient to achieve the desired surface finish without causing excessive relief.
Electropolishing
Electopolishing is an ideal sample preparation solution for many metals. It involves the removal of material from the
surface of the sample by electrolytic action can work very well. This technique removes any deformation layer on
the surface, as well as most surface irregularities.
There is no single electrolyte that will work with all materials – it is necessary to use the correct solution for the
given material. Additional information can be obtained from www.metallography.com.
Ion Milling
Ion milling can produce surfaces suitable for EBSD with minimal prior preparation. This technique is useful because
it can be applied to all types of samples. It is a process applied to a sample under vacuum whereby a selected area
of the surface can be bombarded by an energetic beam of ions. The bombardment erodes the surface, but can also
cause damage by ion implantation which can lead to an amorphous layer being formed. Thus rotation and angle of
attack is important.
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Mechanical Polish
Technique
1
2
3
Electropolish
Ion mill
Standard
Colloidal Silica
Vibromet
(post mechanical
(ideally post
polish)
mechanical
Metallographic
Polish
polish)*
Metals
Single Phase
Y
Y
optional
Y
Y
Multi Phase
Y
Y
optional
No - may cause
Y
preferential
etching
Soft metals
Y
Y
very useful
very useful
Y
Y
Y
optional
Y
Y
Single Phase
Y
Y
very useful
not required
Y
Multi Phase
Y
Y
very useful
not required
Y
Y
Y
very useful
not required
Y
e.g. Al, Cu
Steels
Ceramics
Geological
Multi Phase
Table 1. Well documented and established methods for the preparation of most materials.
*Ion milling should ideally be post mechanical polishing. It can be performed
on an unprepared sample, but the milling time will be significantly longer
Summary
Sample preparation is critical for the EBSD technique. However, there are well documented and established
methods for the preparation of most materials; a summary is given in Table 1 above.
For further information visit the ebsd.com website, or www.oxinst.com/ebsd.
www.oxford-instruments.com/ebsd
The materials presented here are summary in nature, subject to change, and intended for general information only.
Performances are configuration dependent. Additional details are available. Oxford Instruments NanoAnalysis
is certified to ISO9001, ISO14001 and OHSAS 18001. AZtec and Tru-I are Registered Trademarks of Oxford Instruments plc,
all other trademarks acknowledged. © Oxford Instruments plc, 2013. All rights reserved.
Document reference: OINA/AN/SamplePreparation/1113