The role of hydrogen in tribological properties of diamond-like carbon films

Extensive research on diamond and diamondlike carbon (DLC) films in our laboratory has further confirmed that hydrogen plays an important role in the tribological properties of these films. Specifically, model experiments in inert gas environments revealed a very close relationship between the hydrogen-to-carbon (H/C) ratios of source gases and the friction and wear coefficients of the resultant DLC films. The friction coefficient of films grown in source gases with very high H/C ratios (e.g. 10) was superlow (0.003), whereas that of hydrogen-free DLC films (with essentially zero H/C ratio) was very high (0.65). The friction coefficients of films grown in source gases with intermediate H/C ratios were between 0.003 and 0.65. Experiments also revealed that the frictional properties of these films were very sensitive to test environments. Specifically, when tested in open air, the friction coefficient of hydrogen-free DLC dropped to 0.25, whereas that of highly-hydrogenated DLC increased to 0.06. Fundamental knowledge combined with surface analytical and tribological studies have led to the conclusion that the type and extent of chemical interactions between carbon–carbon, carbon–hydrogen, and carbon–adsorbate atoms at the sliding-contact interfaces determine the friction and wear properties of DLC films.