The influence of the surface texture of hydrogen-free tetrahedral amorphous carbon films on their wear performance
Hydrogen-free and predominantly tetrahedrally bonded amorphous carbon thin films (ta-C) are excellent coatings to protect surfaces from wear due to their low coefficient of friction and high hardness. Since these coatings may be several times harder than common engineering materials counterpart wear can be significant. Therefore the surface texture of the ta-C coating is critical to wear applications. While the surface roughness is an important factor, the paper shows that other surface texture parameters have to be considered as well to predict the wear performance of the coating. Wear data are compared of as deposited, polished and brushed ta-C coatings. The results show that typically referenced average values for the surface roughness such as R-a and R-z may prove insufficient to reliably predict the wear behavior of the coating. Additional parameters describing the surface texture such as the "Skewness" (R-sk) and "Kurtosis" (R-ku) can provide relevant information. For example, a brushed ta-C surface with an average roughness of R-a = 31 nm showed a tenfold improved wear performance over a polished ta-C surface with an average roughness of R-a = 10 nm. This phenomenon is explained by analyzing the R-sk and R-ku data, which prove to more closely capture the post-treatment specific changes to the surface texture of the coatings.