Ultra-thin tetrahedral amorphous carbon films with strong adhesion, as measured by nanoscratch testing

Abstract

Amorphous carbon can form hard and dense films, which find applications for tooling components, magnetic recording, etc. The challenge is to prepare hard layers that have good adhesion to the substrate. Moreover, mechanical characterisation of sub-20 nm layer is still difficult and semi-quantitative. The paper is addressing these issues. We prepared 10-nm and 50-nm thick tetrahedral amorphous carbon (ta-C) and hydrogenated amorphous carbon (a-C:H) films, respectively, by filtered cathodic arc deposition (FCVA) and plasma enhanced chemical vapour deposition (PECVD). The films were characterised by nanoindentation and nano-scratching. With such hard films, blunting of the diamond tip can affect the measurements and we used a hardness slope ratio protocol, which gave more accurate results. The ta-C films were the hardest and more wear resistant, i.e. for the 50-nm thick ta-C film on Al 2O 3–TiC; H (25 nm)=51 GPa and residual depth=4 nm for a 5 mN scratch load. This is typical of the FCVA conditions, i.e. a relatively high energy, mono-energetic (approx. 25 eV) non-hydrogenated ion flux, resulting in formation of a dense sp 3 network. Ramping the scratch load to observe delamination events, we measured the highest critical loads (approx. 6 mN) again for the ta-C films. We used SEM/EDX thickness measurements to confirm these delaminations. Again, the ta-C films show the best adhesion. We believe that, the FCVA beam is sufficiently energetic to provide a dense and rigid network structure and a good ion beam mixing in the substrate but, appropriately, not enough to produce the large internal stress observed at −100 V substrate bias.