Effects of nitrogen addition and growth condition on the enhanced mechanical properties of transition metal carbides TMC (TM = Zr, Hf)

Abstract Transition metal carbides (TMC) exhibit a range of exceptional features. However, the poor oxidation resistance of TMC derivatives generally limits their industrial applications at medium-high temperatures. The problem statement relates how the elastic properties of TMC can be enhanced through nitrogen addition and how our approach can fill the gap of earlier computational works, which biasedly assumed TMCN crystals remain unchanged in cubic form. Genetic algorithm aided crystal structure screening and ab initio computations of the structural, electronic and mechanical properties of TMC-TMN and TMC-N2 alloys containing different N contents have been performed. The calculations produced an experimentally verified Zr2CN phase and four previously unreported metallic compounds, namely the Zr4C3N (I4/mmm), Zr6C4N2 (Immm), Zr4CN3 ( Pm 3 ‾ m ) and Zr2C2N4 (Amm2). Phase stabilities of HfCxN1-x and HfC-N2 were also evaluated, with the latter anomalously showing semiconducting behavior. Ground-state properties such as lattice constants and simulated XRD were analyzed and compared with experimental literature wherever possible. The predicted formation energies and lattice dynamics investigation indicate that all studied alloys are energetically and dynamically stable even at 0 K in the whole concentration range. Analyses of the calculated elastic parameters and relevant moduli show that these phases have sufficient mechanical stability, superior melting temperature, improved Vickers hardness ~30 GPa compared to their carbides and fracture toughness of ~4.0 MPa m1/2, which is comparable to most transition metal borides. Particularly, elastic moduli and fracture toughness of Zr–C–N are increasing monotonically with N-content. Further check on density of states, metallicity, Bader charge and electron localization function (ELF) demonstrate the presence of strong covalent with mild ionic and metallic bonds in all studied structures.