Superhard and Ultrahard Nanostructured Materials and Coatings

The recent search for new super- and ultrahard materials is briefly summarized. It is shown that many materials with high elastic moduli cannot be superhard because, upon finite shear, electronic instabilities and transformations to softer phases occur, particularly in materials which contain metals with d-electrons or non-binding electron pairs. Hardness enhancement can be achieved in nanostructured materials, but it is limited by grain-boundary shear when the crystallite size decreases to 10–15 nm called the “strongest size.” When however, low-energy grain boundaries, such as stacking faults and twins, or one-monolayer thin sharp grain boundaries of silicon nitride strengthened by valence charge transfer are introduced, the grain-boundary shear can be reduced and the “strongest size” shifted to a few nanometers. This results in a significantly increased hardness enhancement. In such a way, extrinsically ultrahard materials, such as nanotwinned nt-c-BN and nt-diamond, and nanocomposites consisting of 3–4 nm small transition metal nitride with about one monolayer silicon nitride interfacial layer with hardness exceeding 100 GPa have been prepared. We discuss the conditions which have to be met for such nanocomposites to be super- and ultrahard and show that not all such systems can be superhard. Impurities, mainly oxygen content of more than a few hundred ppm, are critical limitation for achieving the high hardness. The superhard nanocomposites, such as nc-(Ti1−xAl x )N/Si3N4 and nc-(Cr1−xAl x )N/Si3N4, find important applications as wear-protection coatings on tools for machining, stamping, injection molding, and the like.