Friction Characteristics Between CVD Diamond and High-Speed Tool Steel in Air and an Argon

Abstract

To elucidate the mechanisms of the low friction coefficient in CVD diamond film, a series of pin-on-plate type friction tests was performed in ambient air and in argon. As the test piece, a combination of high-speed tool steel JIS-SKH51 pin and CVD diamond plate was used. Results obtained in air showed that the coefficient of friction decreased concomitantly with the increase of the applied load. Furthermore, the existence of the graphite-like material was confirmed from the Raman spectrum of the wear debris. These results confirmed that, for friction between SKH51 and diamond in air, frictional heat promoted the graphitization of diamond. However, in tests conducted in argon, the load dependence was the same as that in air in the low load region but the coefficient of friction increased at the highest load. In tests conducted in argon, the possibility was considered that a low coefficient of friction might develop because of adsorbed oxygen and oxide film formed on the SKH51 pin. To clarify their effects, a friction test was conducted using a non-heat-treated pin and a pin that was heat-treated at 550 °C for 1 h in air. Test results show that the heat-treated pin maintained a low coefficient of friction for a long time. Results suggest that the adsorbed oxygen and the oxygen in oxide film promote graphitization of diamond in argon in the low-load region. However, at the highest load, the coefficient of friction increased because adsorbed oxygen and oxygen in oxide film alone were unable to produce sufficient graphite-like material to accommodate the increased friction area. These results indicate that when oxygen in ambient air or oxide film is present on the friction surface, diamond transformed to the soft graphite-like material by frictional heat shows a low coefficient of friction.