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 Cij 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 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.