Plasma electrolytic oxidation

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Plasma electrolytic oxidation (PEO), or microarc oxidation (MAO), is an electrochemical surface treatment process for generating oxide coatings on metals. It is similar to anodizing, but it employs higher potentials, so that discharges occur.

This process can be used to grow thick (tens or hundreds of micrometers), largely crystalline, oxide coatings on metals such as aluminium, magnesium and titanium. Because they can present high hardness and a continuous barrier, these coatings can offer protection against wear, corrosion or heat as well as electrical insulation.

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A typical PEO surface on aluminium, as viewed in an SEM.

The coating is a chemical conversion of the substrate metal into its oxide, and grows both inwards and outwards from the original metal surface. Because it is a conversion coating, rather than a deposited coating (such as a coating formed by plasma spraying), it has excellent adhesion to the substrate metal. A wide range of substrate alloys can be coated, including all wrought aluminium alloys and most cast alloys, although high levels of silicon can reduce coating quality.

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