Evaluation of the tribological properties of DLC for engine applications

Diamond-like carbon (DLC) coatings are used in automotive engines for decreasing friction and increasing durability. There are many variants of DLC films which provide a wide range of mechanical, physical and tribological properties. The films can be extremely hard (>90 GPa), give low coefficients of friction against a number of counterfaces and exhibit low wear coefficients. The films are often considered to be chemically inert. The properties of DLC films depend to a large degree on the relative proportions of graphitically- (sp2) and diamond-like (sp3)-bonded carbon but the inclusion of elements such as hydrogen, nitrogen, silicon, tungsten, titanium, fluorine and sulphur can dramatically change their tribological response. Two different types of DLC, a WC/C amorphous hydrogenated DLC (WC/C a-C : H) coating and an amorphous hydrogenated DLC (a-C : H) have been investigated. The mechanical and tribological properties have been evaluated by nanoindentation, scratch and wear testing and friction testing in an instrumented cam–tappet testing rig. The deformation mechanisms and wear processes have been evaluated by scanning electron and atomic force microscopy. The results show that the harder a-C : H film was more wear resistant than the softer WC/C a-C : H film and performed better in the cam–tappet testing rig