Tribochemical studies of hard carbon films as a function of load and environment

Hydrogen-free, hard carbon thin films are exciting material coatings candidates as solid lubricants. Two examples, ultrananocrystalline diamond (UNCD) and tetrahedral amorphous carbon (ta-C), are particularly promising, because their exceptional mechanical and tribological properties are combined with extremely smooth surfaces. However, their tribological performance can be seriously affected by variations in humidity. These materials do not perform well in vacuum or inert environments. The mechanisms controlling the friction and wear of UNCD and ta-C are not well understood because of a fundamental lack of physical understanding of the surface interactions. The aim of this thesis is to elucidate the fundamental mechanisms of friction and wear in UNCD and ta-C films. An experimental protocol is defined to examine the relationship between the sliding environment, tribological performance, and mechanical and chemical changes to the films. Self-mated reciprocating tribometry in controlled environments measure UNCD and ta-C friction as a function of load and relative humidity (RH). Scanning white light interferometry measures the post-mortem height profile. Finally, chemical changes inside the wear track are characterized by x-ray photoelectron emission microscopy combined with near-edge x-ray absorption fine structure (X-PEEM-NEXAFS) spectromicroscopy. Results for ta-C and UNCD show that both films, like single crystal diamond, perform better at lower loads or with higher amounts of RH in the environment. Previous hypotheses for this suggested that lubrication for these films either comes in the form of graphitization (converting carbon from diamond-type bonding to graphite-like bonding) or by passivation (the termination of broken carbon bonds by species in the environment, such as water). All spectroscopic evidence shows no evidence of graphitization, but support the passivation hypothesis. Furthermore, the spectroscopy shows that the passivation is in the form of hydroxyl groups, most likely from water. This affects the run-in (period at the start of sliding of high friction as asperities are being smoothed) behavior of these films. The level of passivation also controls whether the films have high or low friction