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 alpha-Al2O3 and theta-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 theta-Al2O3 are substantially lower than those of alpha-Al2O3, and this explains why theta-Al2O3 is much weaker than alpha-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 alpha-Al2O3 and theta-Al2O3, and it should help to improve the performance of thermal barrier coatings.