Effects of Thickness and Field Strength of Anodic Oxide Film on Aluminum on its Compressive Rupture

The rupture behavior of an anodized aluminum surface was investigated in a borate buffer solution using an in situ micro-indentation test, in which transients of the indentation load and flowing current were monitored during loading and unloading of a sapphire indenter. A pair of anodic current peaks emerged during both downward and upward drives of the indenter due to rupture and repair of an anodic oxide film. The total area of film ruptured during both drives, as well as the area of radial cracks, increased with an increase in film thickness and with a decrease in electric field applied while an area of circular cracks was almost constant. It was suggested that the film–rupture owing to radial cracks formation was enhanced by tensile stress accumulated in the anodic oxide film by thickening of the oxide film and lowering of the electrostriction.