Model of Stress Generation in Anodic Aluminum Oxide Films: Part I. Origin of Stress at the Film Interfaces

Abstract

Stress builds up near the interfaces of anodic aluminum oxide layers prior to formation of self-organized arrays of pores. Pore initiation may result from a viscous flow instability driven by interfacial stress gradients. We examined the mechanism of interfacial stress generation through a model of coupled viscous flow and ionic migration during growth of planar anodic aluminum oxide layers. In the model, diffusion-induced stress is produced at the oxide-solution interface because deposition of new oxide is constrained by strong adsorption of electrolyte oxyanions. Stress distributions were calculated for barrier oxide growth in phosphoric acid, using values of ion migration parameters and oxide viscosity from independent experimental results. Both the stress layer thickness of a few nanometers and the magnitude of the interfacial stress agreed with those found experimentally. The model also correctly predicted transitions between compressive and tensile stress at the oxide-solution and metal-oxide interfaces at critical current densities. The thin stress layer at the solution interface can be viewed as effective surface tension dependent on the local current density. The current distribution on a nonplanar interface would induce surface tension gradients resulting in shear and normal stress in the oxide, and these in turn could drive oxide flow leading to self-organized porous films.