Cutting a specimen
When cutting a
specimen from a larger piece of material, care must
be taken to ensure that it is representative of the
features found in the larger sample, or that it
contains all the information required to investigate
a feature of interest.
One problem is that
preparation of the specimen may change the
microstructure of the material, for example through
heating, chemical attack, or mechanical damage. The
amount of damage depends on the method by which the
specimen is cut and the material itself.
Cutting with
abrasives may cause a high amount of damage, while
the use of a low-speed diamond saw can lessen the
problems. There are many different cutting methods,
although some are used only for specific specimen
types.
Mounting
Mounting of
specimens is usually necessary to allow them to be
handled easily. It also minimises the amount of
damage likely to be caused to the specimen itself.
The mounting
material used should not influence the specimen as a
result of chemical reaction or mechanical stresses.
It should adhere well to the specimen, and if the
specimen is to be electropolished later in the
preparation then the mounting material should also
be electrically conducting.
Specimens can be
hot mounted (about 150 °C) using a mounting press
either in a thermosetting plastic, e.g.
phenolic resin, or a thermosoftening plastic
e.g. acrylic resin. If hot mounting will alter
the structure of the specimen a cold-setting resin
can be used, e.g. epoxy, acrylic or
polyester resin. Porous materials must be
impregnated by resin before mounting or polishing,
to prevent grit, polishing media or etchant being
trapped in the pores, and to preserve the open
structure of the material.
A mounted specimen
usually has a thickness of about half its diameter,
to prevent rocking during grinding and polishing.
The edges of the mounted specimen should also be
rounded to minimise the damage to grinding and
polishing discs.
A
mounted specimen (shows typical dimensions)
Grinding
Surface layers
damaged by cutting must be removed by grinding.
Mounted specimens are ground with rotating discs of
abrasive paper, for example wet silicon carbide
paper. The coarseness of the paper is indicated by a
number: the number of grains of silicon carbide per
square inch. So, for example, 180 grit paper is
coarser than 1200.
The grinding
procedure involves several stages, using a finer
paper (higher number) each time. Each grinding stage
removes the scratches from the previous coarser
paper. This can be easily achieved by orienting the
specimen perpendicular to the previous scratches.
Between each grade the specimen is washed thoroughly
with soapy water to prevent contamination from
coarser grit present on the specimen surface.
Typically, the finest grade of paper used is the
1200, and once the only scratches left on the
specimen are from this grade, the specimen is
thoroughly washed with water, followed by alcohol
and then allowed to dry. The drying can be made
quicker using a hot air drier.
Cleaning specimens
in an ultrasonic bath can also be helpful, but is
not essential.
The series of
photos below shows the progression of the specimen
when ground with progressively finer paper.
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Copper
specimen ground with 180 grit paper
(Click on image to view larger version) |
Copper
specimen ground with 400 grit paper
(Click on image to view larger version) |
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Copper
specimen ground with 800 grit paper
(Click on image to view larger version) |
Copper
specimen ground with 1200 grit paper
(Click on image to view larger version) |
Lapping
The lapping process
is an alternative to grinding, in which the abrasive
particles are not firmly fixed to paper. Instead a
paste and lubricant is applied to the surface of a
disc. Surface roughness from coarser preparation
steps is removed by the micro-impact of rolling
abrasive particles.
Polishing
Polishing discs are
covered with soft cloth impregnated with abrasive
diamond particles and an oily lubricant. Particles
of two different grades are used : a coarser polish
- typically with diamond particles 6 microns in
diameter which should remove the scratches produced
from the finest grinding stage, and a finer polish –
typically with diamond particles 1 micron in
diameter, to produce a smooth surface. Before using
a finer polishing wheel the specimen should be
washed thoroughly with warm soapy water followed by
alcohol to prevent contamination of the disc.
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Copper
specimen polished to 6 micron level
(Click on image to view larger version) |
Copper
specimen polished to 1 micron level. Ideally
there should be no scatches after polishing,
but it is often hard to completely remove
them all.
(Click on image to view larger version) |
Mechanical
polishing will always leave a layer of disturbed
material on the surface of the specimen.
Electropolishing or chemical polishing can be used
to remove this, leaving an undisturbed surface.
Etching
Etching is used to
reveal the microstructure of the metal through
selective chemical attack. In alloys with more than
one phase etching creates contrast between different
regions through differences in topography or the
reflectivity of the different phases. The rate of
etching is affected by crystallographic orientation,
so contrast is formed between grains, for example in
pure metals. The reagent will also preferentially
etch high energy sites such as grain boundaries.
This results in a surface relief that enables
different crystal orientations, grain boundaries,
phases and precipitates to be easily distinguished.
The specimen is
etched using a reagent. For example, for etching
stainless steel or copper and its alloys, a
saturated aqueous solution of ferric chloride,
containing a few drops of hydrochloric acid is used.
This is applied using a cotton bud wiped over the
surface a few of times (Care should be taken not to
over-etch - this is a difficult point to determine,
however, the photo below may be of some help). The
specimen should then immediately be washed in
alcohol and dried.
Following the
etching process there may be numerous small pits
present on the surface. These are etch pits caused
by localised chemical attack, and in most cases they
do not represent features of the microstructure.
They may occur preferentially in regions of high
local disorder, for example where there is a high
concentration of dislocations.
If the specimen is
over etched, ie. etched for too long, these pits
tend to grow, and obscure the main features to be
observed - as seen in the images below:
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Etched copper
specimen
(Click on image to view larger version) |
Over etched
copper specimen
(Click on image to view larger version) |
Ideally the surface
to be examined optically should be perfectly flat
and level. If not, then as the viewing area is moved
across the surface it will pass in and out of focus.
In addition, it will make it difficult to have the
whole of the field of view in focus - while the
centre is focused, the sides will be out of focus.
By using a specimen levelling press (shown below)
this problem can be avoided, as it presses the
mounted specimen into plasticene on a microscope
slide, making it level. A small piece of paper or
cloth covers the surface of the specimen to avoid
scratching.
Specimen levelling press