Synthesis of a-C coatings by HPPMS using Ar, Ne and He as process gases

Abstract

Diamond-like carbon (DLC) coatings are used in numerous tribological applications, for example components of the automotive powertrain. The universal hardness HU of these coatings correlates with the sp3/sp2 bond ratio between the carbon atoms. Research on the synthesis of these coatings is upon physical vapor deposition (PVD) processes, especially the high power pulsed/impulse magnetron sputtering (HPPMS/HiPIMS). Synthesizing DLC coatings with a high sp3/sp2 ratio requires high energetic carbon ions, whose generation strongly depends on the HPPMS process parameters. Plasma diagnostics have shown that the energy of the carbon ions in the plasma strongly depends on the process gas mixture, the process gas pressure and the bias voltage. An increased fraction of high energetic carbon ions in the HPPMS plasma has been reported for process gases with a higher ionization energy as well as for higher bias voltage and lower process gas pressure. Therefore, the aim of this work is to investigate the correlation of the plasma parameters with the properties of DLC coatings. Ar, Ne and He were used as process gases. Process gas pressure and bias voltage were varied between p=0.5–2.5Pa and UB=−300–0V, respectively. The coatings were deposited in a high volume semi-industrial coating unit. It was observed that the coating properties strongly correlate with the results from the plasma diagnostics. Dependencies of the sp3/sp2 ratios and hardness on the ionization energy of the process gas, the process gas pressure and bias voltages could be found. Hardness values up to HU=45GPa have been achieved. In conclusion it can be claimed that synthesis of DLC coatings with high hardness is possible by HPPMS processes using process gas mixtures of Ar, Ne and He.