Recent Attempts to Design New Super- and Ultrahard Solids Leads to Nano-Sized and Nano-Structured Materials and Coatings

A brief overview of recent attempts to design new super- and ultrahard materials, which are based on the assumption that materials with high elastic moduli should be super- or ultrahard, is presented in order to show that meeting this condition is not sufficient. Instead, electronic and structural stability upon a finite, relatively large shear strain at atomic level is necessary to avoid structural transformations to softer phases or even a collapse of the structure. We discuss several examples where very high hardness of >70 GPa has been obtained due to the nano-sized and nano-structured effects. Superhard nano-sized and/or nano-structured (nanocomposites) materials can be prepared either by limited diffusion or by spinodal phase segregation during their synthesis. The advantages of the latter mentioned approach is the formation of a stable nanostructure with strong interfaces, that avoids grain boundary shear and concomitant softening when the crystallite size decreases below about 10-20 nm. In such a way, hardness enhancement by a factor of 4 to 5 has been achieved. We shall show that nc-TiN/a-Si3N4 nanocomposites can achieve hardness in excess of 100 GPa when properly designed, and prepared with low density of flaws and impurities. The paper finishes with a short overview of industrial applications of the nanocomposites as wear protection coatings on tools for machining.