Unraveling the effects of anionic vacancies and temperature on mechanical properties of NbC and NbN: Insights from Quantum Mechanical Study

TL;DR

The paper investigates how elevated temperature (300–1500 K) and anionic vacancies affect the mechanical properties of NbC and NbN in RS, ZB, and WZ structures using ab initio molecular dynamics and NEB calculations. It demonstrates a nonlinear decline of elastic constants and moduli with temperature and vacancy content, and shows that ductility generally increases with temperature while higher vacancy concentrations promote brittleness, with toughness influenced by structure. Vacancy migration barriers are strongly structure-dependent, highest in RS and lowest in WZ, and decrease with increasing defect concentration, indicating enhanced defect mobility at higher vacancy levels. The findings provide a framework for optimizing NbC/NbN-based materials for high-temperature and wear-resistant applications by accounting for temperature and defect interactions in mechanical performance.

Abstract

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.

Figures (11)

• Figure 1: Calculated elastic constants of (a) NbC and (b) NbN in RS crystal structure at different temperatures and defect concentrations.
• Figure 2: Calculated elastic constants of (a) NbC and (b) NbN in ZB crystal structure at different temperatures and defect concentrations.
• Figure 3: Calculated elastic constants of (a) NbC and (b) NbN in WZ crystal structure at different temperatures and defect concentrations.
• Figure 4: Calculated bulk modulus (a) NbC and (b) NbN for RS, ZB and WZ for temperature range 0-1500 K.
• Figure 5: Calculated Young's modulus (a) NbC and (b) NbN for RS, ZB and WZ for temperature range 0-1500 K.