Conditions required for achieving superhardness of ≥45 GPa in nc-TiN/a-Si3N4 nanocomposites

Based on our generic concept for the design of superhard (HVickers ≥ 40 GPa) nanocomposites with a high thermal stability and oxidation resistance we discuss the role of nitrogen pressure and substrate temperature during the deposition and the detrimental effect of impurities on the formation of superhard nanocomposites with a high thermal stability. It is shown that inappropriate choice of the deposition parameters or impurities in the coatings are the possible reason of the poor reproducibility of our results by some authors. In order to differentiate between the superhard nanocomposites in which the superhardness originates from a stable nanostructure and ordinary coatings in which the hardness enhancement is due to energetic ion bombardment during their deposition we discuss the different behavior of such coatings and our nc-TiN/a-Si3N4 superhard nanocomposites upon annealing. It is further shown that hydrogen and oxygen impurities degrade the hardness. If the oxygen content in the coatings amounts to about 0.5 at.% or more, the hardness remains limited to 35 GPa or less.