High pressure synthesis of nanotwinned ultrahard materials

In this review, we present our recent research progress in superhard materials, with specially focusing on two topics. One topic is to understand hardness microscopically and establish the quantitative relationship between hardness and atomic parameters of crystal, which can be used to guide the design of novel superhard crystals. The other topic is to identify the fundamental principle and technological method to enhance the comprehensive performances (i.e., hardness, fracture toughness, and thermal stability) of superhard materials, and to synthesize high-performance superhard materials. Starting from the chemical bonds associated with crystal hardness and electronic structure, we propose a microscopic understanding of the indentation hardness as the combined resistance of chemical bonds in a material to indentation. Under this assumption, we establish the microscopic hardness model of covalent single crystals and further generalize it to polycrystalline materials. According to the polycrystalline hardness model, we successfully synthesize nanotwinned cubic boron nitride and diamond bulks under high pressure and high temperature. These materials exhibit simultaneous improvements in hardness, fracture toughness, and thermal stability. We also clarify a long-standing controversy about the criterion for performing a reliable indentation hardness measurement. Our research points out a new direction for developing the high-performance superhard materials, and promises innovations in both machinery processing industry and high pressure science.