Tribochemistry between hydrogen and diamond-like carbon films

The objective of the present work is to propose a model related to the role of hydrogen on the friction mechanism of DLC films. An up-to-date review o f the effect of hydrogen on the tribology of DLC films is presented first. Selected experiments performed on two model hydrogenated DLC films are then presented to demonstrate how hydrogen, both as a constituent of the carbon aceous film or as a gaseous species introduced in the surrounding environme nt during the friction process can influence the intermediate and steady-st ate friction regimes, in the absence of any oxidating species. For the film with the highest hydrogen content, superlow friction (10(-3) range) is rea ched rapidly in an ultrahigh vacuum. For the film containing the lowest hyd rogen content, the combination of a controlled temperature during friction (150 degreesC) with hydrogen diffusion from the bulk of the film towards th e sliding activated surfaces of the hydrogen carbon-to-carbon is responsibl e for an intermediate period with friction in the 10(-3) to 10(-2) range. T hen the steady-state friction coefficient rises up to 0.6, typical for low hydrogenated a-C:H films in vacuum or inert atmospheres. A superlow frictio n steady-state regime may be controlled over longer periods by introducing a significant pressure of pure hydrogen surrounding the contact during the friction process. Argon at the same pressure does not have any similar lubr icating effects. Tribochemistry between hydrogen and the carbonaceous netwo rk is thus responsible for the control of the superlow friction regime obse rved with a-C:H coatings in selected conditions of film composition and atm osphere. (C) 2001 Elsevier Science B.V. All rights reserved.