First-principles investigation of the effect of Cr/(Ti+Cr) ratio on the structure and mechanical properties of Al-rich Al0.6875Ti0.3125-Cr N coatings

Aluminum-rich Al0.6875Ti0.3125-xCrxN coatings with different Cr/(Ti + Cr) ratios (0%, 20%, 40%, 60%, 80%, 100% respectively) are investigated using first principles methods based on density functional theory. The results show that the structural stability of the Al0.6875Ti0.3125-xCrxN coating decreases with the increase of Cr/(Ti + Cr) ratio. Al0.6875Ti0.125Cr0.1875N (Cr/(Ti + Cr) = 80%) and Al0.6875Ti0.0625Cr0.25N (Cr/(Ti + Cr) = 100%), with maximum bulk modulus, have the highest hardness (44 GPa). In addition, the analysis of band structure, density of states and charge density difference reveals that, with the increase of Cr/(Ti + Cr) ratio, the Al–N covalent bond strength is weakened while the metallic bond strength of the coating is enhanced, indicating that toughness of the Al0.6875Ti0.3125-xCrxN coating is enhanced. With respect to the Al0.6875Ti0.125Cr0.1875N (Cr/(Ti + Cr) = 60%) coating, the effect of metallic bond strength is the most evident. Thus Al0.6875Ti0.125Cr0.1875N coating, with the largest H3/E2 value (0.54), owns the best toughness and plastic deformation resistance.