Plasma electrolytic oxidation (PEO) coats metal objects with a ceramic oxide layer, providing resistance to corrosion and wear. PEO uses higher electrical potentials than anodizing, creating plasma discharges that form thick oxide layers. The process dates back to the 1950s and involves inducing metals to form a protective oxide coating under the correct conditions. PEO coatings can be more than several hundred micrometers thick, while anodizing can only create oxide layers up to about 150 micrometres thick. The properties of a PEO coating can be altered by adding chemicals to the electrolyte or varying the timing of voltage pulses.
Plasma electrolytic oxidation (PEO) is one of several processes that coat the surface of a metal object with a protective layer of ceramic. Materials that can be treated in this way include metals such as aluminum and magnesium, and the ceramic coating is typically an oxide. The process bears a similarity to anodizing but uses substantially higher electrical potentials, which can cause plasma discharges to form. This tends to create very high temperatures and pressures along the surface of a part, which can result in somewhat thicker ceramic coatings than traditional anodizing is capable of. The protective layer created by plasma electrolytic oxidation can provide benefits such as resistance to corrosion and wear.
The first experiments with plasma electrolytic oxidation date back to the 1950s and since then various techniques have been developed and refined. Each of the PEO techniques work on the same basic principle, which is that some metals can be induced to form a protective oxide coating under the correct conditions. Many metals will naturally form an oxide layer in the presence of oxygen, but it is usually not very thick. To increase the thickness of the oxide coating, anodizing and other techniques must be used.
At the most basic level, plasma electrolytic oxidation bears a resemblance to traditional anodizing. The piece of metal is dropped into an electrolytic bath and connected to a source of electricity. In most cases the piece of metal will function as one electrode, while the pan that holds the electrolyte is the other. Electricity is applied to the electrodes, which causes hydrogen and oxygen to be released from the electrolyte solution. When oxygen is released, it reacts with the metal and forms an oxide layer.
Traditional anodizing uses about 15-20 volts to grow an oxide layer on a piece of metal, while most plasma electrolytic oxidation techniques use pulses of 200 or more volts. This high voltage is able to overcome the dielectric strength of the oxide, which is what leads to the plasma reactions upon which the technique depends. These plasma reactions can create temperatures of approximately 30,000°F (approximately 16,000°C), necessary for the formation of the thick oxide layers that PEO processes are capable of forming.
The oxide coatings that can be created through the plasma electrolytic oxidation process can be more than several hundred micrometers (0.0078 inches) thick. Anodizing can also be used to create oxide layers up to about 150 micrometres (0.0069 in) thick, although that process requires a strong acid solution as opposed to the dilute base electrolyte usually used for plasma electrolytic oxidation. The properties of a PEO coating can also be altered by adding various chemicals to the electrolyte or by varying the timing of the voltage pulses.
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