<P> Mechanical preparation is the most common preparation method . Successively finer abrasive particles are used to remove material from the sample surface until the desired surface quality is achieved . Many different machines are available for doing this grinding and polishing, which are able to meet different demands for quality, capacity, and reproducibility . </P> <P> A systematic preparation method is the easiest way to achieve the true structure . Sample preparation must therefore pursue rules which are suitable for most materials . Different materials with similar properties (hardness and ductility) will respond alike and thus require the same consumables during preparation . </P> <P> Metallographic specimens are typically "mounted" using a hot compression thermosetting resin . In the past, phenolic thermosetting resins have been used, but modern epoxy is becoming more popular because reduced shrinkage during curing results in a better mount with superior edge retention . A typical mounting cycle will compress the specimen and mounting media to 4,000 psi (28 MPa) and heat to a temperature of 350 ° F (177 ° C). When specimens are very sensitive to temperature, "cold mounts" may be made with a two - part epoxy resin . Mounting a specimen provides a safe, standardized, and ergonomic way by which to hold a sample during the grinding and polishing operations . </P> <P> After mounting, the specimen is wet ground to reveal the surface of the metal . The specimen is successively ground with finer and finer abrasive media . Silicon carbide abrasive paper was the first method of grinding and is still used today . Many metallographers, however, prefer to use a diamond grit suspension which is dosed onto a reusable fabric pad throughout the polishing process . Diamond grit in suspension might start at 9 micrometres and finish at one micrometre . Generally, polishing with diamond suspension gives finer results than using silicon carbide papers (SiC papers), especially with revealing porosity, which silicon carbide paper sometimes "smear" over . After grinding the specimen, polishing is performed . Typically, a specimen is polished with a slurry of alumina, silica, or diamond on a napless cloth to produce a scratch - free mirror finish, free from smear, drag, or pull - outs and with minimal deformation remaining from the preparation process . </P>

Why some of metallographic samples have to be mounted
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