Superhard materials: recent research progress and prospects

Over the past several decades, great endeavors have been devoted to superhard materials research, among which two topics are of central focus. One is to understand hardness microscopically and reveal the controlling factors for superhardness, which can be used to guide the design of novel superhard crystals; the other is to synthesize superhard materials with enhanced comprehensive performance (i.e., hardness, fracture toughness, and thermal stability), with the ambition to synthesize materials harder than natural diamond. We proposed a microscopic understanding of the indentation hardness as the combined resistance of chemical bonds in a material to indentation, and established a microscopic hardness model for covalent and polar covalent crystals, which was further generalized to polycrystalline materials. Guided by the polycrystalline hardness model, we successfully synthesized nanotwinned cubic boron nitride and diamond bulks under high pressure and high temperature. These materials exhibit simultaneous improvement of hardness, fracture toughness, and thermal stability, designating a new direction for superhard materials research.