Different approaches to superhard coatings and nanocomposites

Different approaches to the preparation of superhard coatings such as intrinsically superhard materials, coatings whose hardness is enhanced by energetic ion bombardment during deposition, and nanostructured superhard materials are discussed with the emphasis on the question of how to distinguish between the different mechanisms of hardness enhancement in thin coatings. We compare the thermal and long-term stability in air and some further properties of such coatings. The lack of success of some workers to reproduce the high value of hardness reported by Veprek et al. is explained in terms of inappropriate choice of the deposition conditions and/or impurities.