Recent advances on understanding the origin of superhardness in nanocomposite coatings: A critical review

The remarkable mechanical properties of nanocomposite coatings, such as superhardness, high elastic modulus and recovery, excellent resistance against cracking, low wear rate, and high thermal stability, are due to their unique structures and deformation mechanisms at the nanometer scale. In this paper, recent advances are reviewed mainly with respect to the understanding of the origin of superhardness in nanocomposite coatings. A few controversial issues relevant to the identification of superhard coatings are mentioned. Also discussed are several models, based on analyses and simulations at different levels from continuum to atomistic scales, to elucidate likely superhardening mechanisms. Finally, some open problems and continuing challenges are highlighted. C 2006 Springer Science + Business Media, Inc