Nanostructured superhard materials

Abstract

Superhard materials play irreplaceable roles in the industry areas including mechanical processing, oil exploration, geological exploration, etc., as well as in the scientific fields such as earth science and high pressure science. Developing high-performance superhard materials with extraordinary hardness, fracture toughness, and thermal stability has been a long-lasting goal for both scientific and industrial communities. Natural diamond has been thought of the hardest material in nature since it was discovered in ancient India more than 6000 years ago. Ever since the successful synthesis of diamond in the laboratory in 1955, finding artificial materials with hardness exceeding that of the natural diamond has been a pursued dream of human beings. However, this is truly a challenge to materials science, and many researchers have argued that it is impossible to fulfill this dream. In 2003, 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 single crystals. This model reveals several key factors, such as short and strong chemical bonds, high valence electron density or high bond density, and strongly directional bonds, which are beneficial to enhance the hardness. On the base of the hardness model for covalent single crystals, we systematically investigated the hardening mechanisms in polycrystalline covalent materials, and established a hardness model for polycrystalline covalent materials. Two main hardening effects in polycrystalline covalent materials are identified, namely the Hall-Petch effect and the quantum confinement effect, both of which contribute increasingly to hardness with decreasing microstructural characteristic size. As a result, polycrystalline covalent materials can be continually hardened with the microstructural characteristic size down to the deep nanoscale, which is significantly different from the nanostructured metals and designates a brand-new direction to greatly enhance the hardness of covalent materials. Nanograining and nanotwinning are two popular strategies to minimize the microstructures. Twin boundaries possess excess energy typically one order of magnitude lower than that of grain boundaries. Nanowinning thus provides a more effective mechanism to achieve smaller microstructural characteristic size compared with nanograining. We proposed a novel idea to significantly enhance the performance of diamond and cubic boron nitride (cBN) through forming ultrathin nanotwinned microstructures, and synthesized nanotwinned diamond and cBN bulks from onion-structured carbon and boron nitride precursors, respectively, via martensitic phase transformations under high pressure and high temperature. Nanotwinned diamond and cBN both shows greatly enhanced hardness, fracture toughness and thermal stability compared with the corresponding single crystals due to the formation of ultrathin nanotwins. The hardness of nanotwinned diamond reaches 200 GPa, twice as high as that of natural diamond and turning the dream of synthesizing a material harder than natural diamond into reality. In addition, there is no hardness anisotropy or cleavage features typical for single crystals due to the randomly oriented nanograins and nanotwins. The successful synthesis of nanotwinned ultrahard materials is a great promotion to the high-performance superhard materials research. With these advantages in performance, nanotwinned ultrahard materials can produce technological innovations in industry and high pressure science.