Tribochemical Reaction of Hydrogenated Diamond-like Carbon Films: A Clue to Understand the Environmental Dependence

Tribological behavior of diamond-like carbon (DLC) films and Si incorporated DLC (Si-DLC) films deposited on Si (100) wafer by r.f.-plasma assisted chemical vapor deposition were investigated by ball-on-disk type tribometer in various environments. The friction tests were performed in an ambient air of relative humidity ranging from 0 to 90 % or dry oxygen environment. We focused on the tribochemical reactions by analyzing chemical composition, chemical bond structure and agglomerated shape of the debris. High and unstable friction behavior was observed in both humid air and dry oxygen environment. In these environments, Auger spectrum analysis showed that the debris contained large amount of Fe. Significant incorporation of Fe in the debris resulted from the wear of the steel ball, which might be enhanced by the surface oxidation of the ball. These results show that the humidity dependence of friction coefficient is not an inherent tribological property of DLC film but results from the surface reaction of the steel ball with humid environment. Two possible reasons for the Fe rich debris to affect the friction behavior were suggested. Reduced dependence of the frictional coefficient on the relative humidity observed in Si-DLC film was discussed in terms of the two possible reasons.