Growth mechanisms for initial stages of plasma electrolytic oxidation coating on Al

Abstract

The growth of plasma electrolytic oxidation (PEO) coating at initial stages is relevant directly with coating's morphologies and properties. In this work, the PEO coating on Al was carried out at a constant current model in an electrolyte containing sodium silicate and potassium hydroxide. The morphology, the phase compositions and the structures of initial stages PEO coatings were examined by detailed microstructural characterization. The result showed the PEO coating anodizing <11 s exhibited typical characteristics of the parallel-sided pores and the concave Al/coating boundary, with a significant temperature rising. After that, the dielectric breakdown occurred, resulting in many micron-size pores formed in the PEO coating. When anodizing time over 15 s, the gas discharge appeared at the coating/electrolyte interface. A plasma-modified layer was found composed of γ-Al2O3 nanoparticles and amorphous materials covering the coating surface to prevent the coating from being breakdown by electrons in the plasma. After 10 min, the PEO coating transforms into the typical porous ceramic PEO coating. This study is helpful in the design of the new functional surface of the PEO coating, as well as the deep understand of the coating growth mechanism.