Fabrication of a plasma electrolytic oxidation/anodic aluminum oxide multi-layer film via one-step anodizing aluminum in ammonium carbonate

A plasma electrolytic oxidation (PEO)/anodic aluminum oxide (AAO) multi-layer film was fabricated via one-step galvanostatic anodizing of high-purity aluminum in 0.3–2.0 M ammonium carbonate ((NH₄)₂CO₃) solutions at 283–333 K and 25–400 Am⁻². Anodizing at higher concentrations and higher temperatures caused the formation of relatively uniform anodic oxide film on the aluminum substrate. Characteristic voltage-time curves with two high-plateau voltages at approximately 250 V and 375 V were obtained during galvanostatic anodizing. A multi-layer structure of an outer PEO layer with a crystalline γ-Al₂O₃ structure and an inner amorphous AAO layer with nano-cylinerical pores was formed by continuous visible sparking occurring after passing the first voltage plateau region. The whole aluminum surface was covered with the multi-layer structure after reaching the second voltage region. The thickness of the multi-layer increased with time via the further anodizing process at the second plateau voltage. Pore sealing of the inner nanoporous film was achieved by immersion post-treatment in boiling water.