Abstract Yanshan University, State Key Laboratory of Metastable Materials Science and Technology, Professor Tian Yongjun research group led by continued funding under the National Natural Science Fund for Creative Research Groups, the focus of the project, the project surface and 973 projects, in collaboration with scientists at home and abroad In polycrystalline...
The research team led by Professor Tian Yongjun of the State Key Laboratory of Metastable Materials Preparation Technology and Science of Yanshan University cooperated with scientists at home and abroad under the continuous support of the National Natural Science Foundation of China, the Key Research Project, the Key Project, and the 973 Project of the Ministry of Science and Technology. Breakthroughs have been made in the research of polycrystalline superhard material synthesis technology and hardening mechanism of superhard materials. High-temperature and high-pressure technology was successfully used to synthesize ultra-high hardness nano-crystalline cubic boron nitride bulk materials, and a new mechanism of material hardening was proposed. The research results were published in the latest issue of Nature on January 17, 2013. Cubic boron nitride is an important superhard material and has been widely used in the iron-based material processing industry. Unfortunately, the hardness of synthetic cubic boron nitride single crystals is less than half the hardness of diamond single crystals. According to the well-known Hall-Petch relationship, the hardness of polycrystalline materials increases as the grain size decreases. Therefore, the synthesis of nanostructured cubic boron nitride has become an effective means to improve its hardness. Using the martensitic transformation of graphite-like boron nitride precursors at high temperatures and pressures, scientists have synthesized nanocrystalline cubic boron nitrides with a minimum grain size of 14 nm. Tian Yongjun and his collaborators used a special structure of onion boron nitride as a precursor to successfully synthesize a transparent nano-twinned cubic boron nitride under high pressure. The average thickness of twins is only 3.8 nm. This material exhibits excellent overall performance: Vickers hardness 108GPa reaches or exceeds synthetic diamond single crystal, fracture toughness 12.7MPa?m0.5 is higher than commercial cemented carbide, and oxidation resistance temperature is higher than cubic boron nitride single crystal itself.
A large number of experimental results and molecular dynamics simulations have shown that above the critical size (10-15nm), the hardness and strength of metal and alloy materials increase with the decrease of grain size (Hall-Petch effect); Below, the strength and hardness decrease as the grain size decreases (anti-Hall-Petch effect). Regarding the hardness of polycrystalline polar covalent materials, a recent theoretical model was proposed by the Tian Yongjun group (Int. J. Refract. Met. Hard. Mater. 33 (2012), 93-106), which predicts: at the nanoscale, In addition to the contribution of the Hall-Petch effect, the hardening mechanism should have an additional contribution to the quantum confinement effect. The experimental results show that the nano-twisted cubic boron nitride can continue to harden to 3.8 nm without softening, which confirms the existence of quantum confinement effect in the hardening mechanism of polycrystalline covalent materials. The research results have broken through the traditional understanding of the material hardening mechanism, and a new way to synthesize high performance superhard materials.
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