*Volker Weihnacht, Frank Kaulfuss, Stefan Makowski, Falko Hofmann, Martin Zawischa, Tim Kruelle, Andreas Leson, Fraunhofer Institute for Material and Beam Technology IWS, Dresden, Germany
Diamond like amorphous carbon coatings are attractive due to their high abrasive wear resistance combined with low-friction behavior. Within this material class hydrogen-free, tetrahedral amorphous carbon films obtain even superhardness and exhibit superior wear resistance. However, this property is inevitably associated with the risk of brittleness and coating delamination because of the high Young’s modulus and intrinsic compressive stress in the films. Therefore, the application of superhard ta-C coatings is mostly limited to a low film thickness. Here, we present ways to optimize mechanical-tribological properties of ta-C by doping with minor element concentrations of Cu, Mo, Fe, B or Si. A detailed study of the properties depending on the doping element is described. It is shown how the crack behavior and the tendency to spalling are strongly dependent on the residual stresses and the Young's modulus of the doped ta-C layer. Surprisingly, it can be shown that the mechanical properties of an undoped ta-C layer can also be changed without the layer losing its super hardness. This new carbon coating has a strong mechanical anisotropy, which has been shown by measuring nanohardness and elasticity perpendicular and on cross sections of the coating. The reason of this mechanical anisotropy seems to result from a structural anisotropy, which has been observed by cross-section TEM-investigation. We will describe how the doped and the anisotropic ta-C coatings can be easily synthesized using a laser-arc deposition process. The coatings are compared to conventional ta-C coatings with respect to their friction and wear behavior.