Plasma Electrolytic Oxide (PEO) Coatings are hard, dense, wear-resistant, and well-adhered oxide coatings for metals such as aluminum and magnesium. The process by which they are grown may also be referred to as micro-arc oxidation (MAO) or spark discharge anodizing. Essentially, it involves the modification of a conventional anodically grown oxide film by the application of an electric field greater than the dielectric breakdown field for the oxide. Discharges occur, and the resulting plasma-chemical reactions contribute to the growth of the coating. Rapid cooling also modifies the oxide, resulting in a complex mixture of amorphous material and nanocrystalline phases.