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

Diamond-like carbon (DLC) coatings cover a wide range of different types of carbon-based coatings, which generally have properties such as low frictio n and high wear resistance. DLC films can be divided into two major groups based on their hydrogen content, namely hydrogenated and hydrogen-free carb on coatings. This presentation describes the research work on amorphous hyd rogenated carbon films (a-C:H) and hydrogen-free tetrahedral amorphous carb on films (ta-C). These coating types offer low friction performance and goo d wear resistance, but they also have some dissimilarities in the friction and wear properties due to their different hydrogen content and microstruct ure. The a-C:H films deposited for this study were produced by the rf plasma dep osition technique and the ta-C films by the pulsed vacuum are technique. Th e 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 decrea sed the friction coefficient of the a-C:H coating against the steel and alu mina counter-face, whereas the ta-C coating showed more stable friction per formance. 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 evide nt. 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 hyd rogen content together with graphitisation plays an important role in the f riction performance of DLC films. (C) 2001 Elsevier Science B.V. All rights reserved.