Mechanisms of friction in diamondlike nanocomposite coatings

Diamondlike nanocomposite (DLN) coatings (C:H:Si:O) processed from siloxane precursors by plasma enhanced chemical vapor deposition are well known for their low friction and wear behaviors. In the current study, we have investigated the fundamental mechanisms of friction and interfacial shear strength in DLN coatings and the roles of contact stress and environment on their tribological behavior. Friction and wear measurements were performed from 0.25 to 0.6 GPa contact pressures in three environments: dry (< 1% RH) nitrogen, dry (< 1% RH) air, and humid (50% RH) air, with precise control of dew point and oxygen content. At 0.3 GPa contact stress, the coefficient of friction (COF) in dry nitrogen was extremely low, similar to 0.02, whereas in humid air it increased to similar to 0.2, with minimal amount of wear in both environments. The coatings also exhibited non-Amontonian friction behavior, with COF decreasing with an increase in Hertzian contact stress. The main mechanism responsible for low friction and wear under varying contact stresses and environments is governed by the interfacial sliding between the DLN coating and the friction-induced transfer film adhered to the ball counterface. This interfacial shear strength, computed from COF-inverse Hertzian contact stress plots, was found to be 9 MPa in dry nitrogen and 78 MPa in humid air. Time-of-flight secondary ion mass spectroscopy analysis of the interfaces (wear tracks and transfer films) was used to explain the tribochemical effects in both environments. The transfer films generated in humid air were found to be enriched with SiO2 containing fragments, whereas those formed in dry nitrogen had hydrogenated and long range ordered carbons with practically no SiO2 fragments, ultimately resulting in much lower interfacial shear strength and COF. (c) 2007 American Institute of Physics.