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

The 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. The DLC films can be divided into two major groups based on their hydrogen content, namely to hydrogenated and hydrogen-free carbon coatings. This presentation describes the research work on the amorphous hydrogenated carbon films (a-C:H) and hydrogen-free tetrahedral amorphous carbon films (ta-C). These coating types have low friction performance and good wear resistance, but they also have some dissimilarities in friction and wear properties due to different hydrogen content and microstructure.

The a-C:H films deposited for this study were produced by r.f. plasma deposition technique and the ta-C films by pulsed vacuum are technique. The tribological performance of a-C:H and ta-C coatings was evaluated by pin-on-disc tests carried out in normal atmosphere. Selected coatings were evaluated also in dry atmosphere. The wear resistance of ta-C was higher compared to a-G:H films, but the ta-C films caused higher wear of the counter part. The increase in normal load and sliding velocity decreased the friction coefficient of a-e:H coating against steel and alumina counter face, whereas the ta-C 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 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 friction performance of DLC films.