The effect of microstructure on the tribological properties of a-C:H films

The tribological properties of a-C:H films were investigated from a microstructural point of view. The microstructures of a-C:H films (i.e., diamond-like, polymer-like, and graphite-like structure) were analyzed by Raman spectroscopy. Friction tests were conducted under controlled relative humidity. At low humidity condition, diamond-like-structured film having a high hardness shows a high friction coefficient due to the surface chemistry dominated by iron oxides from the counterface, and the friction coefficient decreases as the structure of a-C:H film becomes more polymer-like or graphite-like. At a higher humidity condition, the friction behavior of a-C:H film against steel ball is a trade-off of the effects of iron oxide formation and water lubrication. The friction coefficient increases by the formation of abundant iron oxides and simultaneously decreases by the water lubrication, resulting in an almost constant friction coefficient except for the highly polymer-like carbon film, whose friction coefficient increased with increasing relative humidity. [Display omitted] •Friction properties of a-C:H films with different microstructure were investigated.•Microstructure of a-C:H films were evaluated with Raman spectroscopy.•Friction properties of a-C:H films were studied in the humidity range of 10 to 75%.•In 10% RH, correlation between hardness and friction coefficient was found.•Friction properties are determined by iron oxides and adsorbed water at a high RH.