arXiv subjects

P. W. Muchiri

Publications and source records attributed to P. W. Muchiri.

2 recordsLinked to original sources

arXiv2026

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.

cond-mat.mtrl-sci↗

arXiv2020

The Impact of Anionic Vacancies on the Mechanical Properties of NbC and NbN: An ab initio Study

The development of super-hard materials has recently focused on systems containing a heavy transition metal and light main group elements. Niobium carbides and nitrides have previously been identified as potential candidates, however, the volatility of carbon and nitrogen during synthesis makes them prone to the formation of anionic vacancies, which have the ability to change the electronic structure, dynamical stability and adversely affecting the mechanical properties. Here, we present ab initio Density Functional Theory calculations that probe the occurrence of anionic vacancies as a function of concentration, thereafter, pertinent mechanical properties are investigated. Our results showed that the presence of anionic vacancies in NbC and NbN tends to deteriorate the mechanical properties and ultimately the mechanical hardness due to vacancy softening that can be attributed to defect induced covalent to metallic bond transition. Further, it was observed that anionic vacancies in NbC tend to modify its toughness, in particular, NbC in ZB becomes brittle while NbC in WZ becomes ductile in the presences of C vacancies of up to 6%. On the other hand, the toughness of NbN was found to be insensitive to defect concentration of even up to 8%. Consequently, stringent control of anionic defects during the synthesis of NbC and NbN is critical for the realization of the desired mechanical response that can make these materials ideal for super-hard and related applications.

cond-mat.mtrl-sci↗