Superhard nanocomposite nc-TiC/a-C:H coatings: The effect of HiPIMS on coating microstructure and mechanical properties

Nanocomposite coatings consisting of titanium carbide crystallites embedded in amorphous carbon matrix were deposited utilizing direct current magnetron sputtering (DCMS) and high power impulse magnetron sputtering (HiPIMS) of titanium target in argon/acetylene atmosphere. DC driven depositions of coatings with high amount of carbon were accompanied by significant arcing during the deposition. HiPIMS utilization made reliable arc-free deposition of coatings with high carbon content possible. Analyses of the coating structure revealed that utilization of HiPIMS favours carbon incorporation into the TiCx grains rather than to the matrix. HiPIMS deposited coatings exhibited stoichiometry of the TiCx grains closer to unity with grains exhibiting lower lattice parameter and a lower fraction of the a-C matrix. This led to a smaller mean TiCx grain separation by the a-C:H matrix. Also the surface structure of the HiPIMS deposited coatings was much finer. The structure refinement resulted in overall higher hardness of HiPIMS deposited coatings with the hardest coatings reaching the superhard region of > 40 GPa and in 40% increase of H/E ratio and nearly threefold increase of H3/E2 ratio.