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Diamond is the hardest material known in the world, with an experimental Vickers hardness of about 100 GPa. However, last year the domestic research team published an article in Nature, reporting the new synthesis of nanostructured materials that are harder than diamond. The experimental Vickers hardness of this new material reached 108 GPa. The material is a nano twin crystal cubic boron nitrogen (nt-cBN) formed by introducing a twin crystal surface separated by a few nanometers in a common cubic boron nitride material, and the experimental Vickers hardness is experimentally compared with the single crystal cubic boron nitrogen. The hardness (about 60GPa) is nearly doubled. It has long been believed that the twin faces in the material are similar to the general crystal interface and can hinder the movement of defects such as dislocations, thereby enhancing the strength or hardness of the material (Hall-Petch effect). However, when the spacing between the crystal interfaces is less than a certain critical value (ten to tens of nanometers), the movement of the crystal interface itself causes a decrease in the strength or hardness of the material (abnormal Hall-Petch effect). The nt-cBN's twin interface spacing is much smaller than the critical distance. Not only does the anomalous Hall-Petch effect occur, but the hardness of nt-cBN is multiplied. This result is very confused and has a great interest for domestic and foreign counterparts. . The physical mechanism of the influence of very small twins on the strength of materials is being studied.
The material has a variety of different crystal faces and directions, and has different resistance to deformation along different directions, and the hardness of the material follows the principle of the barrel, that is, the direction with the lowest resistance to deformation, which often determines the hardness of the material. Sun Hong's research team has long been engaged in the study of first-principles calculations of super-hard materials, using a first-principles calculation method to establish a set of calculation software for accurately predicting the Vickers strength of materials. The software explains the experimental Vickers hardness of diamond, cubic boron nitride, FeB4, CrB4, and BC2N. According to long-term research, they found that the single-crystal cubic boron nitrogen determines the hardness of the material in the weakest direction of 62Ga, while the opposite direction is the strongest direction of the material, and its intensity is 130GPa. On this basis, their in-depth research shows that since the energy of the twin plane is slightly higher than that of the single crystal structure, when the twin plane is introduced, the atomic bond deformation tends to accumulate near the twin plane, as the deformation increases. Eventually, the atomic bonds on the twin faces are recombined into new atomic bonds, so that the original weak bond direction in the material is converted into a strong bond direction, and finally the intensity of the original weak bond direction in the nt-cBN material reaches 130 GPa. Significantly increased the strength or hardness of the material. The effect of this strength enhancement is very similar to the stretching process of the rubber band. The rubber band is soft at the beginning of stretching, and the rubber band becomes stronger as the tensile deformation increases (strain stiffening effect).
This mechanism, the calculation results of Sun Hong's research group, explains the physical nature of nt-cBN material hardness higher than diamond from the microscopic scale of atomic molecules, and solves the confusion of abnormal Hall-Petch effect. The study was funded by the National Natural Science Foundation of China.
Shanghai Jiaotong University's research results provide new ideas for artificial synthesis of nano-materials with super diamond hardness
Abstract Recently, the research team of Professor Sun Hong from the Department of Physics and Astronomy of Shanghai Jiaotong University published a book entitled "Largeindentationstrainstiffenin..." in the international journal "Nature".
Recently, the research team of Professor Sun Hong from the Department of Physics and Astronomy of Shanghai Jiaotong University published a paper entitled "Large indentation strain stiffening in nanotwinned cubic boron nitride" in the international journal "Nature". [Nature Communications 5, 4965 (2014)], provides a new idea for the synthesis of nanomaterials with super diamond hardness.