Aluminium oxide and silicon carbide belong to the group of so-called conventional abrasives, while CBN and diamond are referred to as super hard cutting materials. The hardness of the various grain types can be classified as follows:
Aluminium oxide < Silicon carbide < CBN < Diamond
Production of corundum (Aluminium oxide Al2O3)
The corundum used is crystalline aluminium oxide (Al2O3). Corundum is produced from bauxite, the starting material of aluminium. Bauxite is converted into clay/alumina using the Bayer process. Aluminium oxide (Al2O3) is then produced by melting the clay at approx. 2000 °C in an electric furnace. When it cools, it combines itself in form of crystals (crystal lattice), giving the corundum/aluminium oxide its excellent properties as an abrasive. The large lumps are then crushed in crushing plants and the various grain sizes are sieved out. The different types of aluminium oxide such as regular alumina, mixed fused alumina or fused alumina have different concentrations of aluminium oxide (Al2O3).
Production of silicon carbide (SiC)
Silicon carbide (SiC) is produced by the Acheson process also in an electric furnace. Silicon carbide (SiC) is produced from quartz sand and high carbon pet coke at temperatures of approx. 2000 °C to 2300 °C by electrochemical reaction. Silicon carbide is harder but also more brittle than aluminium oxide.