STONE GRINDING WHEELS FOR SAMPLE PREPARATION
Hardness: 2500 HV
Although Al2O3, alumina, has certain advantages, especially for grinding of steel, it is not used much for metallographic/materialographic grinding except in ceramic grinding stones and in connection with zirconia in wet grinding paper for plane grinding. Very fine alumina is used for polishing. Aluminum oxide was the first grinding medium found in nature, typically emery about 50 % Al2O3 with other oxides, principally iron oxide_ and corundum. Around 1900 a process was found to turn bauxite into Al2O3 and today it is the most used abrasive. Al2O3 is available in different crystals with the _ particles mostly used for grinding. Al2O3 easily forms substitutional solid solutions like the combination Al2O3 and ZrO2 mentioned above. Being made in many different types, Al2O3 also has very different surface structure. The types with relatively smooth surfaces are used for rough grinding operations. Types with a surface with sharp facets are used for finer grinding.
Hardness: 2700 HV
Silicon Carbide plays an important role in metallographic/materialographic grinding with Silicon Carbide grinding paper being used for both plane grinding and fine grinding. Silicon Carbide is the first synthetic abrasive, made in the 1890s. There are two grades of Silicon Carbide. The green type is relatively friable and has fewer impurities; the black type has the same hardness as the green type but is less friable. Most of the Silicon Carbide used for grinding is of the black type. Silicon carbide is the second most widely used type of abrasive. The surface structure of an Silicon Carbide grain is rather irregular, resembling fractured glass. This is an advantage when coa ted on an Silicon Carbide grinding paper because the grain will be fixed firmly in the coating cement but the irregular surface combined with an irregular shape will give an increased deformation of the specimen surface