  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.
