Friction and wear behaviour of PVD chromium nitride supported carbon coatings

Amorphous hydrogenated carbon (a-C:H) films were deposited on case-hardened steel by combining sputtering from graphite targets and reactive deposition using acetylene (C2H2). To improve adhesion, the substrate was coated with a gradient multilayer consisting of chromium nitride (CrN) and carbon prior to deposition of the a-C:H film. Depending on the partial pressure of acetylene during the deposition of a-C:H, a change of the tribological properties, assessed by pin-on-disc and scratch adhesion tests, was observed. Although a change of structure could not be verified by Raman spectroscopy measurements, the acetylene affects the hardness of the a-C:H films and changes the dry friction behaviour against ball-bearing steel. Furthermore, the importance of the transition layer could be confirmed as the gradient multilayer significantly improves the adhesion resulting in critical scratch loads above 100 N.