Tribological behavior of superhard amorphous carbon films

Due to their unique combination of low adhesion, high abrasive resistance and chemical inertness, the amorphous carbon films are increasingly becoming established as optimum tribological coatings for many applications. The breakthrough for protective carbon coatings on components was achieved by their application in Diesel injection systems. Application on tools is limited at higher temperatures, especially in the case of steel machining, because of the high solubility of carbon in steel. Hence, carbon coated tools are preferably to be used for machining of non-iron materials and for forming processes. Up to now only hydrogenated carbon films (a-C:H and related systems), are commercially available. A great potential for future applications is offered by the ta-C films with their markedly higher hardness and their specific interaction with lubricants. Fraunhofer IWS has developed coating technology for deposition of ta-C films in the industrial scale. The tribological behavior of amorphous carbon films is determined by monomolecular covering layers strongly attached to the surface. They cause the very low friction in normal humid air, their absence in dry air or vacuums leads to high friction. Lubricants usually do not improve the tribological behavior in comparison to air. However with non-hydrogenated ta-C films a marked reduction of friction is possible by attaching specially adapted lubricants.