Search for Ultrahard Materials and Recent Progress in the Understanding of Hardness Enhancement and Properties of Nanocomposites

The recent attempts to design new super- and ultrahard materials concentrate predominantly on those with high elastic moduli. This approach neglects the fact that elastic moduli describe the reversible, elastic response to small strain near equilibrium, whereas hardness is related to plastic deformation, the measurement of which involves substantial plastic strain, where the electronic structure becomes strongly distorted and can often result in structural transformations to softer phases. In the superhard nanocomposites consisting of 3-4 nm size randomly oriented nanocrystals of hard transition metal nitrides joined together by about one monolayer of silicon nitride variant, which is strengthened by negative charge transfer, the nanocrystals are free of defects and therefore reach ideal strength. Because of the strengthening of the interface and of the random orientation of the nanocrystals, these nanocomposites reach hardness of more than 100 GPa as shown experimentally. We provide a simple theoretical explanation why these materials can exceed the hardness of diamond, and outline a possible way how to design new nanocomposites with even higher hardness when reduction of Friedel oscillations of the valence charge density, which weaken the strength of the transition metal nitride, can be accomplished.