This review has summarized the experimental results regarding the relationship between superhardness and nanostruc-ture of relevant nanocomposites. These novel superhard nanocomposites have demonstrated the strong dependence of superhardness on optimum nanostructure. This review has confirmed that the hardness of nanocomposite coatings depends on the bilayer repeat period and on the grain size. In fact, the ability to control hardness by changing the nanograin/nanolayer size and composition suggests that nanocomposites can be considered as new types of nanoma-terials with tailored high hardness.

It was demonstrated that nanocomposites exhibit a super-hardness effect when the sizes of their functional elements (i.e., nanograins, nanolayers) achieve appropriate nanoscale dimensions. The nanostructural architecture of such materials and the fine amorphous or quasicrystalline matrix supplement each other.

There is no sole viewpoint on the extreme increase in hardness of the nanocomposites. Different possible mechanisms have been offered to explain the extreme hardness of optimal sizes of nanolayers and of nanolayered composites, as well as of nanograins for nanocrystalline composites, namely:

• Diffusional creep at room temperature [851]

• Changing grain sizes using thermal treatment [347, 852]

Crystallographic texture formation with reducing grain size [853]

Crystalline-amorphous transition [854] Domination of triple junctions in nanocrystalline materials [198, 853]

Consolidation of micropores, roughness, and stringlike surface features in nanocomposite materials [852] Dislocation-related explanations [335, 343, 360, 362, 415, 497, 855, 856]

Coherency and concomitant interfacial misfit dislocations [857]

Interfacial compositional gradients [161] Layer modulus difference [397, 399] Effect of the shape of the Fermi surface on the elastic anomaly [402, 403]

Strain caused by incoherent interfacial stresses [858] Possible elastic anomalies of nanolayered composite coatings [453, 566, 567]

Such possible explanations provide enough evidence that this domain of research and development is under progress, and further exciting results are not far off.

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