Environment and Tribochemistry: Two Key-Factors for the Tribology of Diamond-Like Carbon Coatings

Amorphous carbon coatings have been widely recognized as being good solid lubricants. A thin layer of DLC (diamond-like carbon) coating can protect a substrate from the degradation induced by a sliding contact. It is also well known that the tribological behavior is very sensitive to the exact nature of the DLC layer, and strongly depends on the environmental conditions. Actually, the friction coefficient value can vary by more than one order of magnitude, from atmospheric pressure to vacuum, however the cause for this strong dependence has not yet been elucidated, taking into account the diversity of DLC coatings, depending on the elaboration process. This paper presents tribological properties of DLC coatings, tested in different environments: ambient air at atmospheric pressure (AA, RH<60%), dry air at atmospheric pressure (DA, RH<3%), dry nitrogen (DN, RH<3%) and ultra high vacuum (UHV< pressure less than 10(sup-7) Pa). The test were conducted on two DLC films, about 50 nm thick, deposited from different precursors: pure cyclohexane or a mixture of cyclohexane and argon. The tribological behavior was evaluated in terms of lifetime, values of fiction coefficients (f) or mean shear stress, and observation of wear scars and debris.