Hardness, which is a practical parameter for the evaluation of wear resistance of materials, is commonly used for the characterization of the mechanical properties of materials. The hardness of materials depends on both their elastic and their plastic properties.

The hardness of materials, that is, the ability to resist deformation, depends on the type of interatomic bond as well the type of crystal lattice. Superhard nanocomposites demonstrate the kind of results that can be achieved by forming an optimal nanostructure. As will be demonstrated, the hardness of such nanocomposites can be a few times higher as compared with their microcrystalline components or their coarse-grained mixtures.

Conventionally, superhard materials are defined [156, 167] as materials having microhardness between 40 and 80 GPa. In the range of 35-40 GPa, there are a lot of nanostructured coatings that have various functional applications. In this review, the selection of superhard coatings was lowered to 35 GPa in order to consider certain nanostructured coatings that can demonstrate novel developments and potential applications of such superhard nanocomposites. Materials with hardness higher than 80 GPa are classified as ultrahard [168, 324-327].

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