Fundamentals of the tribological behaviour of diamond, diamond-like carbon and cubic boron nitride coatings

First the tribologically relevant properties of cubic boron nitride, diamond and diamond-like carbon are discussed: chemical composition, lattice structure, type of bonding, hardness, heat conductivity, temperature stability, etc. Then the results of laboratory friction and wear tests are presented. They reveal that, above all, sliding couples with diamond-like carbon coatings exhibit low friction and wear coefficients, if the metal content of the coating and the counter-body material is optimized. While diamond-like carbon coatings display the best tribological behaviour under vacuum conditions or in chemically inert atmospheres, diamond coatings need adsorbed molecules to produce a low friction coefficient. The friction and wear behaviour of cubic boron nitride cannot be judged in detail, as first the adhesion to the base material has to be improved. With regard to industrial applications, diamond-like carbon seems to be suited for unlubricated sliding couples and tools. Diamond coatings are a good choice for tools in contact with non-iron materials. Cubic boron nitride is expected to reduce the wear of tools working at high temperatures. Initial results of sliding tests indicate that it may be applicable for sliding couples used under vacuum and high-temperature conditions.