Differentiating the tribological performance of hydrogenated and hydrogen-free DLC coatings

Abstract

Diamond-like carbon (DLC) coatings cover a wide range of different types of carbon-based coatings, which generally have properties such as low friction and high wear resistance. DLC films can be divided into two major groups based on their hydrogen content, namely hydrogenated and hydrogen-free carbon coatings. This presentation describes the research work on amorphous hydrogenated carbon films (a-C:H) and hydrogen-free tetrahedral amorphous carbon films (ta-C). These coating types offer low friction performance and good wear resistance, but they also have some dissimilarities in the friction and wear properties due to their different hydrogen content and microstructure. The a-C:H films deposited for this study were produced by the rf plasma deposition technique and the ta-C films by the pulsed vacuum arc technique. The tribological performance of the a-C:H and ta-C coatings was evaluated by pin-on-disc tests carried out in normal atmosphere. Selected coatings were also evaluated in dry atmosphere. The wear resistance of the ta-C films was higher compared to the a-C:H films, but the ta-C films caused higher wear of the counterpart. The increase in normal load and sliding velocity decreased the friction coefficient of the a-C:H coating against the steel and alumina counter-face, whereas the ta-C coating showed more stable friction performance. The micro-Raman studies showed clear graphite formation for the a-C:H film, whereas the graphite formation on the ta-C film was not so evident. In dry conditions the ta-C had a high friction coefficient, which could be reduced by doping the film with hydrogen. The results show that the hydrogen content together with graphitisation plays an important role in the friction performance of DLC films.