Design of ultrahard materials: Go nano!

Intrinsically super- (H 40-70 GPa) and ultrahard (H epsilon 70 GPa) materials attain high hardness through their large intrinsic strength, whereas extrinsically super- and ultrahard materials reach such hardness through their nanostructure. The recent search for intrinsically super- and ultrahard materials has concentrated on those with high elastic moduli. Several examples are presented of materials with high zero-pressure elastic moduli but relatively low hardness, because, upon imposing a large lattice distortion or substantial plastic shear strain, they undergo an instability of their electronic structure. Recent progress is then elucidated in the understanding of the origin of ultrahardness of nc-TmN/a-Si3N4 nanocomposites (Tm = transition metal that forms hard and stable nitrides), in which 3-4 nm size TmN nanocrystals are joined together by about a monolayer thick SiNx interface material layer. A combined ab initio DFT calculation of the ideal shear strength of the interfaces between the plastically non-deformable randomly oriented TmN grains, followed by Sachs-type averaging of these shear resistances in space to obtain a uniaxial yield strength, Y, employing a proper pressure enhancement of Y and considering the Tabor relation between the hardness H and yield strength Y, shows that these materials can reach hardness significantly larger than diamond, when correctly prepared and essentially free of defects. Such superhard nanocomposites are already available on an industrial scale as protective coatings on tools for machining, such as for drilling, milling, turning, forming, stamping and the like.