Wrinkling of α-alumina films grown by thermal oxidation—I. Quantitative studies on single crystals of Fe–Cr–Al alloy

Quantitative measurements of the wrinkling morphology of the alpha-alumina scales formed by thermal oxidation at 1000°C and its evolution with oxidation time have been made using atomic force microscopy. The wrinkling wavelength and amplitude, and from these the surface area of the scale, for (110), (100) and (111) oriented single crystals of Fe–Cr–Al alloy as a function of oxidation time are presented. The residual stress in the scales is determined using piezospectroscopy. Together these results provide the basis for determining the lateral strains associated with the growth of the oxide. Crystallographic orientation of the alloy surface has a pronounced effect on both the wrinkling morphology and stress in the scale during the first 20–30 h of oxidation. The presence of the reactive element, yttrium, suppresses wrinkling and restricts lateral growth but it does not decrease the stress in the oxide.