Ideal tensile and shear strength of β-C3N4 from first-principles calculations

Covalent solid beta-C(3)N(4) has received considerable attention in the last two decades since it was predicted to be a superhard material based on its high bulk modulus that is comparable to that of diamond. Here, we report first-principles calculations that examine its deformation modes and its ideal tensile and shear strength at large strains. The calculated results show that the ideal tensile and shear strength of beta-C(3)N(4) are both much lower than those of diamond; more significant, they are even below those of cubic boron nitride (c-BN) despite that c-BN is elastically more compliant near equilibrium. This illustrates the disparity between the mechanical properties of beta-C(3)N(4) at equilibrium and at large strains. The different bonding structures of beta-C(3)N(4) from those of diamond and c-BN introduce more complex structural deformation modes. We discuss the microscopic mechanism of the structural deformation in beta-C(3)N(4) under different strains and, in particular, examine the role of the angular deformation of the C-N bonds and its effects on the overall stress-strain relations. The results provide insights for understanding its mechanical properties at large strains at the atomic level. The knowledge may also prove useful for the design of new strong covalent solids.