Studies on vacuum arc evaporation behavior of modified graphite cathodes and influence on particle-induced defects in deposited (t)a-C films with and without dopant

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Among the thin film PVD processes, the vacuum arc process plays a prominent role in the deposition of hard and wear-protective coatings for tool and component applications. The main advantage, in addition to the presence of an almost completely ionized, high-energy coating plasma, is a robust evaporation technology that allows stable and efficient process control during industrial deposition of the coatings. Superhard, tetrahedral amorphous carbon (ta-C) coatings produced by arc processes play a special role in this context. Thanks to their almost universally favorable tribological properties, they are being used in an increasing number of applications. For all the mentioned advantages of arc processes, they also have an inherent disadvantage, namely droplet or particle emission, which is intrinsically linked to the discharge process at the cathode spot of the arc from the vaporized cathode material. This phenomenon is particularly pronounced in ta-C coatings and leads to defects and hard roughness peaks in the deposited coatings, which make the coatings unusable for many applications or require a great effort in post-smoothing the coatings. A fundamentally new aspect in this context has been demonstrated in recent work: In investigations with low element additions (5 at%) to graphite cathodes, doped ta-C films were produced which exhibited a particle-induced defect density that was in part considerable, up to a factor of 10 lower. This circumstance is surprising in that the evaporated cathodes still consisted of 95% graphite. There were large element-dependent differences in particle reduction. Thus, B and Mo caused a very strong, while Si caused almost no change in the particle-induced defect density. The phenomenon of particle reduction in ta-C(:X) films by dopants is a previously unexplored, scientifically interesting phenomenon that should be thoroughly investigated and understood. The overall objective of this research project is to develop an understanding of the key factors influencing the formation of particles during arc evaporation of graphite and composite graphite cathodes and their incorporation into the growing (t)a-C:X films. The processes and phenomena at the cathode, in the plasma and in the film growth will be analyzed and considered in context in order to understand the decisive influences on the particle reduction in doped ta-C layers and to derive suitable models from them.