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.