Introduction

Superhard nanocomposites in the form of coatings are a novel class of materials, which have a great potential for a variety of applications. The high hardness of the nanocomposites distinguishes them from microcrystalline and amorphous materials of the same composition. For example, the nanocrystalline coatings of nc-TiN/a-Si3N4/nc-TiSi2/a-TiSi2 (nc, nanocrystalline state; a, amorphous state) display a very high hardness (up to 105 GPa [1]) although the hardness is only up to 25 GPa [2, 3] if every component is taken separately as a coarse-grained material. Consequently, it is possible to create purposely artificial nanocomposites having a total hardness that significantly exceeds the hardness of the individual components.

Recently, the search for superhard materials has been pointed in a few directions:

• Novel superhard phases, such as predicted superhard carbon nitride (^-C3N4) [4-11], hypothetical super-hard carbon with a rigid three-dimensional network of sp2 hybridization [12-14], hard graphitic-like structures [15, 16], carbon nanotubes [17], super diamond [18, 19], insufficiently explored carbynes [20-23], new possible phases of three-dimensional (3D) C60-based fullerites [24-26], transition-metal dioxides containing heavy elements [27, 28], superhard boron suboxides [29], superhard cubic BC2N [30-34], ultrahard MgAlB14 phase based on boron icosahedra [35], nonmolecular polymeric CO2 solid [36, 37], new spinel-type phase of Ti3N4 and SiTi2N4 [38], C60 compounds (one- and two-dimensional polymers) [39-44], novel W3C phase [21, 45], and so on

• Nanocrystallization of hard and superhard materials (e.g., nanodiamond [46-118], diamond-like carbon [119-127], carbon nitride [128-133], c-BN [134-144], B4C [145-149], silicon nitride [150-155], silicon carbide [156, 157], chromium nitride [158], titanium nitride [159]), trying to obtain their nanostructured state

• Development of artificial superhard nanocomposites, the superhardness of which is solely defined by their optimal nanostructure and composition

In this review, attention is focused on artificial superhard nanocomposites in the form of coatings with hardness higher than 35-40 GPa. The aim of the review is the state of the art in artificial superhard nanocomposites that demonstrate a significant enhancement of their hardness owing to a nano-structured state. A few comprehensive reviews and general papers about the artificial superhard nanocomposites have been published [160-175].

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