Structural properties, internal stress and thermal stability of nc-TiN/a-Si3N4, nc-TiN/TiSix and nc-(Ti1-yAlySix)N superhard nanocomposite coatings reaching the hardness of diamond

The average crystallite size, d, in the range of about 3-8 nm 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 less than or equal to 0.4 GPa for less than or equal to 10 mu m thick films is fairly low. The random stress determined from the Warren-Averbach analysis increases with decreasing crystallite size from about 1 GPa for d greater than or equal to 10 nm to almost 10 GPa for d approximate to 3 nm. A strong increase is observed for the stability of the nanostructure and of the hardness upon annealing: the recrystallization temperature increases from about 850 degrees C for d greater than or equal to 5 nm to greater than or equal to 1150 degrees C for d less than or equal to 3 nm. This is explained by thermodynamical stabilization of the grain boundaries due to segregation. Superhardness remains constant up to recrystallization. For superhardness of about 100 GPa, the elastic modulus of 70-500 GPa and the universal hardness of about 17-22 GPa (loads between 30 and 100 mN) 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%. (C) 1999 Elsevier Science S.A. All rights reserved.