Unbalanced magnetron sputtered carbon composite coatings

Abstract

Carbon composite coatings combine high hardness and toughness, good adhesion, low friction coefficient, and low wear rate. These properties make it a good candidate for tribological applications. Previous studies commonly used hydrocarbon gases to produce metal (Ti, W, Cr) doped amorphous carbon coating (M–a:C–H) and sputtered Ti–TiN or TiCN as supporting layers. Relatively high hardness and good adhesion could be obtained by optimizing the composition and structure of the supporting layers and by increasing the doping level. However, the friction coefficient increased with the doping level, which resulted in high wear rates. This article reports on the deposition of carbon composite coatings on high speed steel substrates using simultaneous unbalanced magnetron sputtering of metallic and graphite targets. The coating consists of a graded Cr–Ti–N alloyed interlayer and a Cr-containing carbon composite surface layer. Raman spectroscopy, scanning electron microscopy, nanoindentation, scratch tests and pin-on-disk wear tests against alumina were used to characterize the properties of the coatings. Low friction coefficients (below 0.1 at an air humidity of 40±10 RH%), high hardness (18±3 GPa) and good adhesion (55 N in lower critical load) were found, leading to a low normalized wear rate of about 5×10−8 mm3 N−1 m−1, which is substantially lower than those of the metallic nitride coatings such as TiN and CrN. The superior wear resistance coupled with good adhesion implies that the carbon composite coatings may be ideal for sliding wear applications.