INFLUENCE OF GAMMA-ALUMINA ON THE STRUCTURE OF BARRIER ANODIC OXIDE-FILMS ON ALUMINUM

Barrier anodic oxide films formed on clean and smooth aluminum surfaces are generally amorphous. However, the presence of a thin layer of thermal oxide on the surface of aluminum promotes the growth of gamma prime -alumina during subsequent anodic oxidation. Thermal oxide may or may not contain gamma -alumina crystals, depending upon growth conditions. When the thermal oxide layer contains gamma -alumina crystals, these become incorporated into the growing barrier oxide layer and act as nuclei for the amorphous to gamma prime -alumina transformation. In this study, transmission electron microscopy of ultramicrotomed sections of the aluminum substrate and the barrier oxide was employed to obtain further insight into the influence of gamma -alumina on the structure of barrier anodic oxide films on aluminum. It is shown that no oxide formation is observed above the gamma -alumina crystal nor at the interface between the gamma -alumina crystal and aluminum substrate. Barrier film growth is observed over the aluminum surface surrounding the gamma -alumina crystal. Within the barrier oxide, a gamma prime -alumina crystal is observed clearly to grow into the amorphous barrier oxide.