The impact of anionic vacancies on the mechanical properties of NbC and NbN: An ab initio study

Abstract

The development of super-hard materials has recently focused on systems containing heavy transition metal and light non-metallic elements. Niobium carbides and nitrides have previously been identified as potential candidates, however, the volatility of carbon and nitrogen during synthesis make them prone to formation of anionic vacancies, which have the ability of changing the electronic structure, dynamical stability and adversely affecting the mechanical properties. This study presents 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 tend 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 structure becomes brittle while NbC in WZ phase becomes ductile in presences of C vacancies of up to 6%. The dynamical stability of NbC in RS, ZB, and WZ phases and NbN in WZ phase were found to be insensitive to the presence C and N vacancies, while NbN in RS and ZB becomes dynamically stable with the introduction of N vacancies. In addition, the toughness of NbN was found to be insensitive to defect concentration of even up to 8%. Consequently, stringent control of anionic defects during synthesis of NbC and NbN is critical for realization of the desired mechanical response that can make these materials ideal for super-hard and related applications.