Structural properties, internal stress and thermal stability of nc-TiN/a-Si₃N₄, nc-TiN/TiSi_x and nc-(Ti_1-yAl_ySi_x)N superhard nanocomposite coatings reaching the hardness of diamond

The average crystallite size, d, in the range of about 3-8nm determined from XRD by means of the Scherrer formula and integral width of the Bragg peaks compares well with that determined from the Warren-Averbach analysis. TiN films show (200) texture which changes to random orientation of the crystallites when the silicon content reaches about 10 at.%. The biaxial stress of ≤0.4GPa for ≤10μm thick films is fairly low. The random stress determined from the Warren-Averbach analysis increases with decreasing crystallite size from about 1GPa for d ≥10nm to almost 10GPa for d≈3nm. A strong increase is observed for the stability of the nanostructure and of the hardness upon annealing: the recrystallization temperature increases from about 850°C for d≥5nm to ≥1150°C for d≤3nm. This is explained by thermodynamical stabilization of the grain boundaries due to segregation. Superhardness remains constant up to recrystallization. For superhardness of about 100GPa, the elastic modulus of 70-500GPa and the universal hardness of about 17-22GPa (loads between 30 and 100mN) compare well with the hardness of a single-phase nanocrystalline diamond. Besides this extremely high hardness, the coatings also have a very high toughness and elastic recovery of 80-90%.