The coefficient of static friction of silicon containing diamond-like carbon films

Abstract

Diamond-like carbon (DLC) films are well known for their outstanding tribological properties, which has been shown in many studies. Because of their extreme hardness, low coefficient of sliding friction (often after some running-in time) and especially their ability to form a transfer layer on the counter-body of the frictional pair, these coatings have found different areas of application. There are many different types of these coatings on the market depending on the deposition process and on the deposition parameters used. The dependence of the film properties on the deposition conditions allows the tailoring of certain film properties to specific applications. Additionally, alloying with different elements allows further adaptations. A special tribological problem is found in applications where almost no or only little motion takes place and therefore no transfer layer can build up. In this case 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 the transformation of the topmost surface layer into a more graphitic-like state takes place. In this study, silicon containing DLC films are produced in a RF plasma-activated chemical vapor deposition system. A mixture of acetylene and an organosilicon gas is used to deposit silicon containing DLC films onto hardened steel sample plates. On top of a supporting coating, a 30-nm thick DLC layer with varying silicon content is deposited using different selected self-bias voltages. The coefficient of static friction against a hardened stainless steel surface is then measured in dependence of the silicon content in the film and of the self-bias used during deposition. The films display a constant coefficient of static friction even after many single measurements. The results are compared to results obtained for pure DLC films.