Systematic study on mechanical and electronic properties of ternary VAlN,TiAlN and WAlN systems by first-principles calculations

Transition-metal aluminium nitrides widely used as protective tool coatings are a class of materials with a combination of high hardness, outstanding wear resistance as well as good chemical stability. In this work, through a well developed structure searching method, the ground-state phase of V_0.5Al_0.5N is verified and systematically studied on its mechanical and electronic properties by comparing with Ti_0.5Al_0.5N and W_0.5Al_0.5N via first-principles calculations. Our results show that the ground-state phase of V_0.5Al_0.5N adopts a hexagonal structure of P6₃/mmc symmetry. Mechanical property studies demonstrate the hexagonal phase has a surprisingly improved hardness of about 38 GPa and enhanced ideal strengths relative to its well-known metastable cubic B1 phase whose hardness is only about 20 GPa. This mechanical enhancement greatly expands the upper limit of the strength and hardness for this type of Al-containing ternary systems. Meanwhile, detailed analysis on strength and elastic anisotropy indicates it also exhibits much better mechanical isotropy. Underlying mechanism of the mechanical enhancement is explored by the electronic analysis in-depth. The position of the E_F is tuned by the introduction of the Al element and this electronic tuning leads to a metallic-to-semiconductor transformation from B1 to the hexagonal phase and the strengthening of the bonds between the metal elements and the N atoms.