Topotactically Modified Stress in Coatings

The unexpected occurrence of compressive stresses in solgel-derived alumina coatings following a densification transformation was found to be related to the topotactic transformation of boehmite (γ-AlOOH) to γ-alumina (γ-Al₂O₃) on heating. The boehmite coating showed a pronounced fiber texture with the 001] crystallographic axis perpendicular to the substrate-coating interface plane. At about 500°C, the orthorhombic boehmite transformed into tetragonal γ-alumina with very little, if any, reorientation of the crystallites. Because of the topotactic nature of the transformation, the fiber texture persisted in the γ-alumina phase. This resulted in an anisotropic change that generated compressive strains in the coating.