Elastic property and intrinsic hardness of novel superhard ternary nitrides (CSi2N4 and SiC2N4)

In this work, the new ternary nitrides (CSi₂N₄ and SiC₂N₄) are designed by the substitution method. The structures, elastic properties, intrinsic hardness and Debye temperature of the new ternary nitrides (CSi₂N₄ and SiC₂N₄) are studied by first-principles calculations based on the density-functional theory. The elastic constants C_ij$C_{ij}$ of these new ternary nitrides are obtained using the stress–strain method. Derived elastic constants, such as bulk modulus, shear modulus, Young's modulus, Poisson coefficient and brittle/ductile behavior are estimated using Voigt–Reuss–Hill theories. The results indicate that γ-CSi₂N₄, p-CSi₂N₄ and p-SiC₂N₄ are mechanically stable. Calculated B/G$B/G$ values and Poisson's ratio for γ-CSi₂N₄, p-CSi₂N₄ and p-SiC₂N4 indicate that these materials are brittle. The calculated anisotropy parameters indicate that γ-CSi₂N₄ shows weak anisotropy and p-SiC₂N₄ and p-CSi₂N₄ have larger anisotropy. Based on the microscopic hardness model, p-CSi₂N₄, p-SiC₂N₄ and γ-CSi₂N₄ should be viewed as superhard materials with some peculiar mechanical properties.