Relationship between electronic structure, phase stability and mechanical properties in the transition metal carbides, nitrides and carbo-nitrides

Abstract

The transition-metal carbides (TMCs), nitrides (TMNs) and carbo-nitrides (TMCNs) make upa class of hard materials whose melting temperatures generally exceed 3000 K. These refractory ceramics, generally referred to as ultra-high-temperature ceramics (UHTCs), are naturally of inter- est for a range of extreme environment applications, from nuclear power generation to hypersonic flight. The carbides of the Group IV and Group V transition metals—Ti, Zr, Hf, V, Nb and Ta— are of particular interest, since some of these materials are claimed to possess the highest melting temperatures among all known compounds; some are found to exceed 4000 K. The Group IV and Group V TMCs generally share the same rocksalt (NaCl) crystal structure and covalent-metallic bonding character; furthermore, they all exhibit large rocksalt phase homogeneity ranges, with structural vacancies accommodating off-stoichiometry. Yet they exhibit surprisingly varied hard- ness behaviors: some, like TiC, are seen to lose hardness with decreasing carbon content; others, like TaC, are found to increase in hardness. Additionally, alloying of the rocksalt carbides and nitrides has been claimed to produce optimized melting temperatures and hardnesses. This work seeks to develop computational models to describe the hardness behavior of the TMCs, TMNs and TMCNs, with a focus particularly on dislocations—the low-temperature carriers of plastic defor- mation. This work will also investigate the electronic mechanisms underpinning the anomalous hardness behaviors in these materials, seeking to provide plausible explanations centered on the chemical bonds within these crystalline solids.