The nucleation sites of γ-Al2O3 in thermal oxide films on aluminium

Electropolished aluminium specimens were thermally oxidized in air at 515°C for various periods of time. Thermal oxide films were then stripped from the aluminium substrate and examined thoroughly in the transmission electron microscope with particular attention directed at the sites at which γ-Al₂O₃ crystals nucleate and the relationship of such sites to surface features of the aluminium substrate. It was found, for the first time, that ‘easy paths’ for the diffusion of oxygen, or the nucleation sites of γ-Al₂O₃ crystals are not distributed randomly over the electropolished aluminium surface, but form preferentially in the amorphous oxide layer grown over the pre-existing metal ridges. Thus, the diffusion of molecular oxygen through cracks in the amorphous oxide layer represents the most expected and realistic basis for explaining the local growth of γ-Al₂O₃ crystals in thermal oxide films on aluminium.