New insight into the effect of C concentration on the structural stability, elastic modulus, hardness and thermodynamic properties of Nb[sbnd]C carbides

Abstract

Although some NbC binary carbides (NbC and Nb6C5) have been reported, the structural feature and mechanical properties of the other NbC binary carbides are not unclear. In particular, the influence of C concentration on the structural stability, mechanical and thermodynamic properties of NbC is also unknown. To solve this problem, here, the influence of C concentration on the structural stability, mechanical and thermodynamic properties of NbC binary carbides is studied by the first-principles calculations. The calculated results show that two novel Nb3C2 (Cmcm and Pnma) and one Nb8C7 are predicted. The convex hull shows that the Nb6C5 has better thermodynamic stability in comparison to the other six NbC cardies. Importantly, it is found that the calculated shear modulus, Young's modulus, hardness and Debye temperature of NbC carbides increase with increasing C concentration. Essentially, the increasing mechanical properties and Debye temperature are that the high concentration of C enhances the localized hybridization between the Nb atom and C atom, which forms the strong NbC bond. However, the high concentration of C weakens the ductility of NbC carbides. Here, the results show that the Nb23C6, Nb3C2 and Nb4C3 show ductility. On the contrary, the Nb6C5, Nb8C7 and NbC exhibit brittleness. Therefore, I believe that the adjustment of C concentration can improve the mechanical and thermodynamic properties of NbC high-temperature carbides, which are potentially used in various high-temperature industries.