Investigations of the coefficient of static friction diamond-like carbon films

Abstract

Hydrogenated amorphous carbon films or diamond-like carbon films have shown their outstanding tribological properties in many studies and in many applications. This is due to their extreme hardness, low coefficient of sliding friction (after some running-in time) and especially, their property to form a transfer layer on the counter-body of the frictional pair. Hydrogenated amorphous carbon coatings describe a large class of different coatings with properties ranging from relatively soft to extremely hard, from highly amorphous to more graphitic-like and from high hydrogen contents to hydrogen free, as well as any combination of these. A special problem is found in applications, where almost no or only little motion takes place, because either the static friction cannot be overcome or the running-in cannot be completed, because the transfer layer cannot form. In all these cases, a special coating is needed, which has a low coefficient of friction, already at the very beginning of any motion. A similar situation is found in low load applications, where neither a transfer layer is created nor a transformation of the topmost surface layer into a more graphitic-like state takes place. Diamond-like carbon films in this study are produced in a r.f. plasma-activated chemical vapour deposition system from acetylene and deposited onto hardened steel plates. The coefficient of static friction against a hardened stainless steel surface is then measured in dependence of the self-bias, used for the topmost coating layer. Different measurement methods have been used to prove the reproducibility. It was furthermore shown that these films displayed a constant coefficient of static friction even after 200 single measurements. The overall average coefficient of static friction in air was 0.158±0.002, and in a dry nitrogen atmosphere 0.126±0.003 as compared to 0.210±0.011 for uncoated surfaces in air.