INFLUENCE OF HYDROGEN CONTAINED IN HARD CARBON COATINGS ON THEIR TRIBOLOGICAL BEHAVIOR

Synthesized diamond-like coatings obtained by hydrocarbons/hydrogen pl asma processes contain hydrogen. The ultra-low friction state of diamo nd coating (friction coefficient mu = 0.02) observed in an inert envir onment is not an intrinsic property of the material. It is induced by the hydrogen contained in the film. The study, under vacuum, of the fr iction coefficient evolution measured with a pin-on-disc tribometer sh ows that the transition from a low-friction state to a severe one appe ars after a critical number of revolutions. This phenomenon shows that abrasive wear during sliding motion slowly leads to the consumption o f the hydrogen, and the friction coefficient increases. The aim of thi s paper is to present the tribological behaviour of diamond-like coati ngs vs. the amount of hydrogen contained in the coating, and to propos e an explanation of the lubricating mechanism of hydrogen contained in the coating on the friction coefficient and its evolution with time a nd with the environment. A comparison of the tribological behaviours o f various hard carbon coatings synthesized with different ratios of ca rbon/hydrogen/oxygen as was described by Bachmann's equilibrium diagra m [1] is presented.