Everything about Plasma Electrolytic Oxidation totally explained
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.
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's 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.
Process
Metals such as aluminium naturally form a
passivating oxide layer which provides moderate protection against corrosion. The layer is strongly
adherent to the metal surface, and it'll 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
aluminium, 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 (α-Al
2O
3). 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
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 aluminium, the coating properties can vary strongly according to the exact
alloy composition. For instance, the hardest coatings can be achieved on
2XXX series aluminium, where the highest proportion of crystalline phase
corundum (α-Al
2O
3) is formed, resulting in hardnesses of ~2000
HV, whereas coatings on the
5XXX series have less of this important constituent and are hence softer. Extensive work is being pursued by Prof. T. W. Clyne at the
University of Cambridge to investigate microstructural (& pore architectural), mechanical and thermal characteristics of PEO coatings.
Further Information
Get more info on 'Plasma Electrolytic Oxidation'.
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