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

Extensive research on diamond and diamondlike carbon (DLC) films in our lab oratory 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 hydro gen-to-carbon (H/C) ratios of source gases and the friction and wear coeffi cients 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), w hereas that of hydrogen-free DLC films (with essentially zero H/C ratio) wa s very high (0.65). The friction coefficients of films grown in source gase s with intermediate H/C ratios were between 0.003 and 0.65. Experiments als o revealed that the frictional properties of these films were very sensitiv e to test environments. Specifically, when tested in open air, the friction coefficient of hydrogen-free DLC dropped to 0.25, whereas that of highly-h ydrogenated DLC increased to 0.06. Fundamental knowledge combined with surf ace analytical and tribological studies have led to the conclusion that the type and extent of chemical interactions between carbon-carbon, carbon-hyd rogen, and carbon-adsorbate atoms at the sliding-contact interfaces determi ne the friction and wear properties of DLC films. (C) 2001 Elsevier Science B.V. All rights reserved.