Anomalous mechanical strengths and shear deformation paths of Al2O3 polymorphs with high ionicity

Abstract

Alumina (Al₂O₃) formed by selective oxidization provides an effective way to protect aluminide alloys against corrosion for sustainable applications. Despite a broad interest and investigations on Al₂O₃ polymorphs such as α-Al₂O₃ and θ-Al₂O₃, their intrinsic mechanical strengths and atomic deformation mechanisms are not yet fully understood. In this research, density functional theory is used to show that the calculated shear moduli and mechanical strengths of θ-Al₂O₃ are substantially lower than those of α-Al₂O₃, and this explains why θ-Al₂O₃ is much weaker than α-Al₂O₃. An analysis of shear deformation paths and electronic structure indicates that the longest Al–O ionic bonds are responsible for the lattice instability of both polymorphs during shear, showing they have different anisotropic features. This study gives a novel view on the failure of thermally grown α-Al₂O₃ and θ-Al₂O₃, and it should help to improve the performance of thermal barrier coatings.