Growth of porous anodic films on sputtering-deposited aluminium incorporating Al–Hf alloy nanolayers

Formation of porous anodic films on sputtering-deposited aluminium incorporating Al–Hf tracer layers has been examined at constant current in sulphuric and phosphoric acids. Hafnium was selected as the tracer species since the migration rates of Hf4+ and Al3+ ions are similar in barrier-type anodic alumina. The distribution of hafnium in the films was determined using ion beam analysis, scanning electron microscopy and transmission electron microscopy. Increases in the anodizing voltage and barrier layer thickness accompany the oxidation of hafnium and the migration of Hf4+ ions through the barrier layer region of the porous film. Hf4+ and Al3+ ions that migrate to the pore bases are lost to the electrolyte. Other Hf4+ ions are incorporated into the cell walls. For films formed in phosphoric acid, with relatively thick barrier layers, channelling of the ion current leads to accelerated outward transport of Hf4+ ions toward the pore base, while a U-shaped inner edge of the hafnium distribution beneath the pores is associated with more slowly transported hafnium species. The tracer behaviours for films formed in both acids are consistent with the transport of Hf4+ ions in the barrier layer regions by a combination of flow of film material and ion migration, the flow being a key factor in the development of the pores. The percentage losses of Hf4+ and Al3+ ions from the films to the electrolyte are relatively similar, correlating with their similar migration rates, and contrast with the retention in the film of slow migrating W6+ ions, found previously, due to a more dominant role of flow.