Superhard nanocomposites: Origin of hardness enhancement, properties and applications

The original finding of Veprek et al. that in nc-TiN/a-Si₃N₄ and in nc-TiN/a-Si₃N₄/TiSi₂ nanocomposites, deposited under conditions which allow complete phase segregation by spinodal mechanism, the maximum hardness of ≥ 45 and > 100 GPa, respectively, is achieved when the thickness of the interfacial Si₃N₄ is about 1 monolayer, has been recently confirmed by both experiments and theory. First principle calculations explain why the decohesion and shear strength of a TiN-SiN_x-TiN sandwich is higher than that of bulk SiN_x. Combined ab initio DFT calculations of shear resistance of the interfaces, their averaging according to Sachs for randomly oriented polycrystalline material to obtain tensile yield strength, Tabor's criterion, Hertzian analysis and pressure-enhanced flow stress explain in a simple way the experimentally achieved high values of hardness of > 100 GPa, in excess of diamond. Friedel oscillations of the valence charge density, originating from negative charge transfer to the strengthened SiN_x interface, cause decohesion and ideal shear to occur between Ti-N bonds near that interface. The extraordinary mechanical properties of these and related quasi-binary superhard nanocomposites can be understood in terms of nearly flaw-free strong materials with no need to invoke any new mechanism of strengthening. We shall present selected examples of industrial applications of the superhard nanocomposite coatings.