|Metallographic grinding is one of the process steps in mechanical sample preparation for subsequent microscopic examination. The principle is based on the fact that abrasive particles in successively finer grain sizes are used to remove material from the surface until the desired result is obtained. These particles are present in bonded form during grinding and in unbonded form during polishing.
The quality to be achieved depends on the application purpose. It is not always necessary for the true structure to be available in the highest degree of perfection – often a satisfactory result is sufficient to enable subsequent testing (e. g. a hardness test).
In any case, the metallographic preparation must be carried out systematically and reproducibly. This is the only way to achieve optimum results at the lowest possible cost.
In metallographic grinding, a distinction is made between macro grinding and micro grinding.
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A macro-section is understood to be the preparation of a sample for observation and evaluation, either without any microscopic aid or under a stereoscopic microscope with a magnification range between 6x and 60x, e. g. for welded and soldered joints.
A micro-section is understood to be the preparation of a sample for observation and evaluation under a light microscope with Köhler's illumination in the magnification range from 50:1 to 1000:1, i. e. for classical structural analysis.
This distinction is useful because a finely ground surface is usually sufficient for evaluation of a macro-section. For a micro-section, a deformation-free and polished surface is essential.
Metallographic grinding is divided into three separate operations:
METALLOGRAPHIC PLANAR GRINDING
The aim of metallographic planar grinding is to remove roughness from the cutting process and to achieve flatness of the ground surfaces.
The abrasive itself depends on the material to be ground. For soft materials, this is essentially silicon carbide (SiC) as paper, foil, grinding disc or grinding stone. For harder materials ( 300 HV, e. g. hardened steel or ceramics), diamond grinding discs are however used. These are usually fixed on a magnetic carrier disc. One advantage is that the achieved flatness is maintained from the beginning to the final polishing. Furthermore, several grinding steps are not necessary. For larger samples and sample quantities, an aluminium oxide-based (Al2O3) grinding stone can also be used. It must be noted that a special stone grinding machine is required.
METALLOGRAPHIC FINE AND ULTRAFINE GRINDING
The surfaces produced with finer-grained silicon carbide paper already have only small residual deformations that can be removed directly by polishing.
Alternatively, so-called fine grinding discs can be used for metallographic sample preparation. These work by adding diamond suspension and lubricant. This is especially helpful for thin coatings, nitrided layers and decarburized edges to ensure an optimal evaluation after polishing.
With special diamond grinding discs, it is now also possible to grind down to an area that was previously reserved for metallographic polishing. Grinding discs with down to 3 µm diamond particles are available. With suitable materials, this is a possibility that should be considered from an ecological and economic point of view.
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