Unraveling the effects of anionic vacancies and temperature on mechanical properties of NbC and NbN: Insights from Quantum Mechanical Study
Transition metal carbides and nitrides (TMCNs), such as niobium carbide (NbC) and niobium nitride (NbN), are of great technological interest due to their exceptional hardness, high melting points, and thermal stability. While previous studies have focused on their groundstate properties (at 0 K), limited information exists on their mechanical behavior under realistic operational conditions involving elevated temperatures and the presence of defects. In this study, we employ ab initio molecular dynamics (AIMD) simulations to investigate the effects of temperature (300 to 1500 K) and anionic vacancies on the mechanical properties of NbC and NbN in rocksalt (RS), zincblende (ZB), and wurtzite (WZ) structures. The results reveal a nonlinear decrease in elastic constants, bulk, shear, and Youngs moduli with both increasing temperature and defect concentration. Hardness and toughness analyses, based on Pughs ratio and Poisson ratio, show ductility and brittleness transitions that are sensitive to structure, defect level, and thermal effects. Furthermore, vacancy migration energies computed using the nudged elastic band (NEB) method demonstrate strong structural dependence, with RS exhibiting the highest barriers and WZ the lowest. These findings provide new insights into the defect and temperature interplay in NbC and NbN, offering guidelines for their optimization in high-temperature and wear-resistant applications.