First-principles calculations on the mechanical properties of niobium nitrides

We have calculated the mechanical properties of cubic δδ-NbN and hexagonal δ′δ′-NbN with density functional theory (DFT). It is found that the calculated ideal strength of δ′δ′-NbN is higher than that of δδ-NbN, which is consistent with the experimental findings. The tensile strength perpendicular to the polar plane in δδ’-NbN is comparable to the weakest bonding direction in diamond, which provides huge potential technological and industrial applications. The electronic origins of mechanical properties are discussed.