Microscopic theory of hardness and design of novel superhard crystals

Hardness can be defined microscopically as the combined resistance of chemical bonds in a material to indentation. The current review presents three most popular microscopic models based on distinct scaling schemes of this resistance, namely the bond resistance, bond strength, and electronegativity models, with key points during employing these microscopic models addressed. These models can be used to estimate the hardness of known crystals. More importantly, hardness prediction based on the designed crystal structures becomes feasible with these models. Consequently, a straightforward and powerful criterion for novel superhard materials is provided. The current focuses of research on potential superhard materials are also discussed.

► Three most popular microscopic hardness models are presented.
► A microscopic definition of hardness is suggested.
► Key points for employing these models are addressed.
► The current research focuses of novel superhard materials are discussed.