Modem grinding abrasives mainly fall into one of two groups, namely,
• Conventional abrasives based either on silicon carbide (SiC) or aluminum oxide (Alox), and
• Superabrasives based either on diamond or cubic boron nitride (CBN).
The division into two groups is based on a dramatic difference in hardness of the grains leading to very different wheel wear characteristics and grinding strategies. The division is also based on cost; wheels made using superabrasives are typically 10 to 100 times more expensive.
3.2.1 Development of SiC
SiC was first synthesized in 1891 by Dr. E. G. Acheson, who gave it the trade name “Carborundum.” It was initially produced in only small quantities and sold for $0.40/ct or $880/lb as a substitute for diamond powder for lapping precious stones. In its time, it might well have been described as the first synthetic “superabrasive,” certainly compared to the natural emery and corundum minerals then otherwise available. However, once a commercially viable process of manufacturing was determined, its price fell precipitously, and by 1938 it sold for $0.10/lb [Heywood 1938]. Today the material costs about $0.80/lb.
3.2.2 Manufacture of SiC
SiC is manufactured in an Acheson resistance heating furnace through the reaction of silica sand and coke at a temperature of around 2,400°C. The overall reaction is described by the equation
SiO2 + 3C ^ SiC + 2CO
A large carbon resistor rod is placed on a bed of raw materials to which a heavy current is applied. The raw material also includes sawdust to add porosity to help release the CO, and salt to remove iron impurities. The whole process takes about 36 hours and yields 10 to 50 tons of product. From the time it is formed, the SiC remains a solid as no melting occurs (SiC sublimates at 2,700°C). After cooling, the SiC is sorted by color; from green SiC, which is 99% pure, to black SiC, which is 97% pure. It is then crushed and sized as described for alumina below.
3.2.3 Hardness of SiC
SiC has a Knoop hardness of about 2,500 to 2,800 and is very friable. The impurities within the black grade increase the toughness somewhat but the resulting grain is still significantly more friable than alumina. Above 750°C, SiC shows a chemical reactivity toward metals with an affinity for carbon, such as iron and nickel. This limits its use to grinding hard, nonferrous metals. SiC also reacts with boron oxide and sodium silicate, common constituents of vitrified wheel bonds [Viernekes 1987].