Predicting potential hard materials in NbB ternary boride: First-principles calculations

To identify potential superhard materials, we conducted a comprehensive theoretical investigation of the thermodynamic and kinetic stability, mechanical properties, electronic structure, Debye temperatures and melting point of sixteen ternary transition metal borides NbTMB_x (x = 1, 2, 4 and TM = Ti, V, Fe, Co, Ni, Zr, Ru, Hf, W, Os) using first-principles methods. Our findings indicate that, with the exception of NbFeB, NbRuB, and NbWB, all other borides exhibit both thermodynamic and kinetic stability. Notably, NbTiB₄, NbVB₄, NbZrB₄ and NbHfB₄ demonstrate superior hardness and enhanced resistance to deformation, with NbTiB₄ showing an impressive hardness value of 40.84 GPa, positioning it as a promising candidate for superhard materials. Both NbVB₄ and NbTiB₄ have very high Debye temperatures and melting points and can be used in high temperature environments. We further explored the mechanical properties of NbTiB₄ at elevated temperatures by employing a combination of first-principles and quasi-static methods. Our analysis reveals that the elastic constants and moduli of NbTiB₄ decrease with increasing temperature. Additionally, bonding analysis indicates that all NbB ternary borides exhibit hybridization involving metallic, ionic, and covalent interactions, resulting in the formation of exceptionally strong covalent bonds between boron atoms.