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 of the effect of hydrogen on
the tribology of DLC films is firstly presented. Selected experiments performed on two model
hydrogenated DLC films are then presented to demonstrate how hydrogen, both as a
constituent of the carbonaceous film or as a gaseous species introduced in the surrounding
environment during the friction process can influence the intermediate and steady-state
friction regimes, in the absence of any oxidating species. For the film with the highest
hydrogen content, superlow friction (10^-3 range) is reached rapidly in ultrahigh vacuum. For
the film containing the lowest hydrogen content, the combination of a controlled temperature
during friction (150°C) with exodiffusion from the bulk of the film towards the sliding
activated surfaces of the hydrogen carbon-to-carbon is responsible for an intermediate period
with friction in the 10^-3 - 10^-2 range. Then 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 friction
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 lubricative effects. Tribochemistry between
hydrogen and the carbonaceous network is thus responsible for the control of the superlow
friction regime observed with a-C:H coatings in selected conditions of film composition and
atmosphere.