Phase transition engineering for toughness improvement of Ta(C,N) coating: First-principles calculations

The effect of nitrogen element solid solution on the toughness improvement of cubic-B1 Ta(C,N) coating and the associated phase transition process during tensile were investigated through first-principles calculations. The computational results exhibited that the solid solubility of nitrogen element in the cubic-B1 Ta(C,N) phase was up to 50 at.%. If nitrogen content was smaller than 30 at.%, Ta(C,N) showed Poisson's ratio lower than 0.26 and brittle fracture feature during tensile simulations. As for Ta(C,N) with nitrogen content in the range from 30 to 50 at.%, both Poisson's ratio larger than 0.26 and structure evolution during tensile signified their ductile characteristics, which monotonically rose with the increase of nitrogen content. The calculated maximum toughness of Ta(C_50%N_50%) was 384.3 GPa%, which was 92% larger than that of TaC. Structural analysis revealed that solid solution of nitrogen element enhanced distortion ability of bond length and angle, and transformed anisotropic distortion in TaC to the isotropic one in Ta(C,N), which led to phase transition from face-centered cubic to hexagonal close packed. It was found that the spatial distribution characteristics change of orbital hybridization from isotropy in TaC to anisotropy in Ta(C,N) was the underlying reason for phase transformation, which was even facilitated after loading tensile strain. This indicates that phase transition engineering motivated by orbital hybridization regulation is an effective routine to improve material toughness.