Plasma electrolytic oxidation, or microarc oxidation, is an electrochemical surface treatment process for 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 aluminum, 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.

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 eb coated, including all wrought aluminium alloys and most cast alloys, although high levels of silicon can reduce coating quality.

Process

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Metals such as aluminum naturally form a passivating oxide layer which provides moderate protection against corrosion. The layer is strongly adherent to the metal surface, and it will regrow quickly if scratched off. In conventional anodizing, this layer of oxide is grown on the surface of the metal by the application of electrical potential, while the part is immersed in an acidic electrolyte.

In plasma electrolytic oxidation, higher potentials are applied. For example, in the plasma electrolytic oxidation of aluminum, at least 200 V must be applied. This exceeds the dielectric breakdown potential of the growing oxide film, and discharges occur. These discharges sinter and densify the growing oxide and also partially convert it from amorphous alumina into crystalline forms such as corundum. As a result, mechanical properties such as hardness and toughness are enhanced.

Equipment used

The part to be coated is immersed in a bath of electrolyte which usually consists of a dilute alkaline solution such as KOH. It is electrically connected, so as to become one of the electrodes in the electrochemical cell, with the other, being a stainless steel counter-electrode, often the wall of the bath itself.

Potentials of over 200 V are applied between these two electrodes. These may be continuous or pulsed DC (in which case the part is simply an anode in Direct current operation), or alternating pulses (alternating current operation).

Coating properties

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Plasma electrolytic oxide coatings are generally recognized for high hardness, wear resistance, and corrosion resistance. However, the coating properties are highly dependent on the substrate used, as well as on the composition of the electrolyte and the electrical regime used (see 'Equipment used' section, above).

Even on aluminum, the coating properties can vary strongly according to the exact alloy composition. For instance, the hardest coatings can be achieved on 2XXX series. Extensive work is being pursued by Prof. T. W. Clyne at the University of Cambridge to assess the quality of PEO coatings.

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