Role of Interfaces and Effect of Impurities in Nitride-based Superhard Nanocomposites

Recently, a hardness similar to that of diamond has been reported for the ternary nitride-based nanocomposite, $nc$-TiN/ $a$-Si$_{3}$N$_{4}$/$a$- and $nc$-TiSi$_2$ [1]. The reproducibility, however, has proved difficult, as has the superhardness of the related, prototypical, binary nanocomposite $nc$-TiN/$a$-Si$_{3}$N$_{4}$. Extensive density- functional theory calculations indicate that the hardness enhancement in the latter system is due to the preferential formation of TiN(111) polar interfaces with a thin Si-layer which is N-coordinated and tetrahedrally bonded [2]. The tensile strength of TiN in the [111] direction is very similar to the weakest bonding direction in diamond. Oxygen impurities cause a significant reduction of the interface strength which could partly explain the conflicting results, and signals the importance of avoiding such contaminants for achieving super- and ultra-hard nanocomposites.\\ $[1]$ S. Veprek et al. Surf. Coat. Technol. 133-134, 152 (2000).\\ $[2]$ S. Hao, B. Delley, and C. Stampfl, to be published.