| In this work we used Ab Initio Method based on DFT. Using the VASP package we calculate the structure and mechanical properties of Ti-Si-N like composite materials. This study has two objectives:⑴We studied the lowest energies configuration of such composite materials;⑵The mechanical properties of such composites had been analyzed by calculating the elastic modulus and elastic constants to analyze.The first part of the task is that the method of studing the lowest energies conformation is by calculating the total energies and cohesive energies. In order to calculate the cohesive energies, we firstly calculated the monatomic energies and the total energies of the lowest energies configuration. The calculating cohesive energies have the group of titanium and vanadium nitrides and the corresponding enter silicon atoms. The results show that:⑴Ti-Si-N composite materials were inspected the location of adding silicon atoms in the grain, they have hollow spaces, plane hollow spaces, occupies a N atom and occupies a Ti atom, and the cohesive energies were obtained to 451.73eV, 433.05 eV, 443.18eV and, 446.91eV, they are lower than the cohesive energies of TiN crystal 457.69eV. That saw silicon atoms can not exist within the TiN grains, which can only exist in the boundary. When the boundary is 2.75? the cohesive energies is 425.86eV. But it enhances 426.47eV when adding silicon atom on the boundary, that is easy to silicon atoms can stay in the boundary.⑵The composite materials of Zr-Si-N, Hf-Si-N, Nb-Si-N and Ta-Si-N for the cohesive energies 478.4eV, 496.1eV, 457.9eV and 485.2eV are lower than the corresponding cohesive energies of nitrides 482.6eV, 502.6eV, 462.6eV and 485.9eV, and only the V-Si-N cohesive energies of 408.0eV cohesive energies higher than the VN of 407.8eV. It means that the the silicon atoms can enter the VN grains, and other only stay on the boundary.The second part of the task is that analyzed mechanical properties through the calculation of elastic modulus and elastic constants, and under the conditions of small strain elastic modulus and the elastic are the quadratic linearity constants of total energy change and strain. In this paper, we calculated elastic modulus and elastic constants calculated the group of titanium and vanadium nitrides and the corresponding enter silicon atoms.⑴.The bulk modulus of TiN, ZrN, HfN, VN, NbN and TaN were 280.9GPa, 258.5GPa, 277.9GPa, 316.6GPa, 312.45GPa and 293.61GPa and the corresponding experimental value were 260GPa, 264GPa, 278GPa, 333GPa, 317GPa and 338GPa, the relative error is in the range of 0.03% -13%, it means that the model and methods are accurate.⑵.Studing mechanical properties, the model of Ti-Si-N composite materials has two situations. It is that a silicon atom is in the hollow or the boundary TiN. The bulk modulus is 285GPa in the hollow higher than the conditions which is 280.9GPa has no silicon atom, the silicon atom enter the boundary which width is 2.75?, we calculated the elastic constants C44 is 159.7Gpa, it is lower than the value of TiN elastic constants C44 is 170.61GPa. The results are still not enough explain that silicon atoms into the boundary of TiN influenced the superhardness of Ti-Si-N. It requires further study. |
