The Role of Defects in the Fracture of Oxide Films on Metals

Abstract

The extent of fracture of anodic oxide films during tensile deformation of aluminum is shown to be controlled by the density of defects in the oxide. The density of defects was monitored by the “leakage” resistance of the film, while the extent of fracture of the oxide was obtained from measurements of reanodization current transients. Other experiments in a photoelectron microscope showed that these oxides are ruptured at emerging slip steps. This provided the basis of a simple model for oxide fracture, which requires the presence of a defect in the oxide at the site of an emerging slip step. A relationship between the leakage resistance and fracture susceptibility of the oxide was derived and confirmed by experiment, showing that both properties are controlled by the same type of defect. The density of these defects may be controlled by the surface preparation prior to anodization. If the density of defects is greatly reduced, even a thin (28 nm) oxide becomes strong enough to suppress slip step formation during tensile deformation.