Natural Diamond

Diamond is chemically pure carbon that crystallises as a natural product under ex­treme pressure and heat over a long period of time. About 80 % of natural dia­monds are rejected for use as decorative objects because of their small size and de­ficient purity, making them available to industry. Their properties are nearly identical to synthetic diamonds [LEIC75]. Large diamonds of high purity are in­corporated into drawing dies, hardness testers, dressing tools and diamond lathes. Other natural diamonds of the purer variety are used in drill bits for the natural gas industry and tunnel-boring machines. Impure, irregular diamonds, so-called “borts”, are used in bonded form for grinding in diverse diamond tools and in loose form for polishing. It is processed by the abrasive grit industry in grit sizes of 1 to 1100 |im grit diameter. Natural diamond is primarily used for grinding and slicing glass, ceramic products, and carbides, plastics as well as concrete, natural
and synthetic stones and in stationary and rotating dressing tools. However, the diamond is not suited for grinding steel for two reasons. Firstly, in grinding, high shear loads arise, as well as high temperatures under low pressures compared with the conditions for the synthesis of diamonds. These conditions favour the diffusion of the carbon from the diamond into a material in which carbon is easily dis­solved, e. g. steel workpieces. The second mechanism also arises in the grinding conditions described above. Thereby, diamond graphitises only in the layers near the surface, which is accelerated by the presence of a catalyst such as oxygen. The carbon in graphite dissolves well in iron [COLL80, KOMA76].