Shape may not be suitable for observation on a flat

shape may not be suitable for observation on a flat surface. They are therefore mounted inside a
polymer block This can be done cold using two components which are liquid to start with but which set
solid shortly after mixing. Cold mounting requires very simple equipment consisting of a cylindrical ring
which serves as a mould and a flat piece of glass which serves as the base of the mould. The sample is
placed on the glass within the cylinder and the mixture poured in and allowed to set Cold mounting
takes about 40 minutes to complete.
In hot-mounting the sample is surrounded by an organic polymeric powder which melts under
the influence of heat (about 200 degree Celsius). Pressure is also applied by a piston, ensuring a high
quality mould free of porosity and with intimate contact between the sample and the polymer. This is
not the case with cold mounting where the lack of proper contact and the presence of porosity can
cause problems such as the entrapment and seepage of etchant during the final stages of preparation.
Consequently, hot-mounting should be the preferred way of encapsulating specimens assuming that
time and resources permit, and assuming that the heat involved in the process does not influence the
sample.
ii. Grinding and Polishing:
Grinding is done using rotating discs covered with silicon carbide paper and water. There are a
number of grades of paper, with 180, 240, 400, 1200, grains of silicon carbide per square inch. 180 grade
therefore represents the coarsest particles and this is the grade to begin the grinding operation. Always
use light pressure applied at the center of the sample. Continue grinding until all the blemishes have
been removed, the sample surface is flat, and all the scratches are in a single orientation. Wash the
sample in water and move to the next grade, orienting the scratches from the previous grade normal to
the rotation direction. This makes it easy to see when the coarser scratches have all been removed. After
37

the final grinding operation on 1200 paper, wash the sample in water followed by alcohol and dry it
before moving to the polishers.
The polishers consist of rotating discs covered with soft cloth impregnated with diamond
particles (6 and 1 micron size) and an oily lubricant. Begin with the 6 micron grade and continue
polishing until the grinding scratches have been removed. It is of vital importance that the sample is
thoroughly cleaned using soapy water, followed by alcohol, and dried before moving onto the final 1
micron stage. Any contamination of the 1 micron polishing disc will make it impossible to achieve a
satisfactory polish.
METALLOGRAPHY-GRINDING-POLISHING-MACHINE
iii. Etching:
The purpose of etching is two-fold. Grinding and polishing operations produce a highly deformed,
thin layer on the surface which is removed chemically during etching. Secondly, the etchant attacks the
surface with preference for those sites with the highest energy, leading to surface relief which allows
different crystal orientations, grain boundaries, precipitates, phases and defects to be distinguished in
reflected light microscopy. A polished sample is etched using a cotton tip dipped in the etchant. Etching