Stress Induced by Electrolyte Anion Incorporation in Porous Anodic Aluminum Oxide

The geometry of porous anodic aluminum oxide films depends sensitively on the type of electrolyte anion in the anodizing bath. It has been proposed that near-surface compressive stress due to anion incorporation induces an oxide flow instability that results in the self-ordered patterns of pores. Experimental evidence for anion incorporation-induced stress in Al oxide films, and accumulation of such stress during anodic oxidation, was sought through in situ stress measurements supplemented by X-ray photoelectron spectroscopy (XPS). Compressive stress generated during open-circuit dissolution in phosphoric acid was found to agree quantitatively with the elastic stress, calculated assuming that phosphate ions detected by XPS are incorporated into the oxide. Incorporation of phosphate ions may be driven by the high electric field in the film. Aluminum samples with varying levels of anion incorporation-induced stress were anodized in phosphoric acid, and the near-surface residual stress in the anodic oxide was determined by stress monitoring as the film dissolved at open circuit. The surface stress tracked closely with the stress introduced at open circuit, thereby demonstrating that surface stress induced by anion incorporation accumulates during barrier oxide growth. Finally, it is shown that anion incorporation prior to anodizing can induce large tensile stress relaxations during the barrier to porous oxide transition, which are ascribed to tensile stress generation at the metal-oxide interface.