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The most time consuming process of the sample preparation stage is the grinding stage. Grinding is a progressive process whereby scratches and imperfections are gradually removed from the surface of the sample, through the use of progressively finer abrasive surfaces. This grit is silicon carbide powder (SiC) and should remove any damage caused during the previous preparation stages.

 

For the process to be implemented an automated grinding machine which could simultaneously prepare 6 sample was used. This is achieved by applying a small vertical force to the non-critical side of the mold, putting the surface of the sample in contact with the surface of the grinding wheel. The surface of the grinding wheel is coated in a grit paper. This wheel rotates in the opposite direction to that of the samples and is constantly lubricated via a stream of cold water. The rotations of both wheels were 150RPM respectively. The machine ran for 120 – 180 seconds and the samples were then inspected by eye to ensure that all were of an even standard. If a sample appeared to be more imperfect than others then it was subject to an extra cycle on the grinding wheel.

 

This process was repeated using a grit papers of 320 grit followed by 500 grit, with a force of 20N pressing down on each sample. To attach the grit paper to the grinding wheel, special adhesive papers had to be used. For the coarsest paper of grit 320, Gekko paper was used and for grit paper of greater than or equal to 500 grit Fuga paper was used. Note that the higher the grit value, the finer the abrasive surface.

 

Between each cycle, the samples were cleaned using ethanol (C2H6O) then dried using a compressed air gun to remove any excess grit and to remove water stains.