PEO Coatings

PEO Coatings Plasma Electrolytic Oxidation (PEO), also known as Electrolytic Plasma Oxidation (EPO), is a surface treatment process that involves the conversion of a metal surface into a ceramic coating using electrical discharge in an electrolyte solution. Oxidation (oxidizing reaction) refers to the loss of electrons that occurs during a reaction by a molecule, atom or ion. Oxidation occurs when the oxidation state of a molecule, atom or ion increases. The opposite process, which occurs when there is an electron gain or when the oxidation state of an atom, molecule or ion decreases, is called reduction. The PEO process produces a hard, wear-resistant coating with high corrosion resistance and improved mechanical properties of the surface. The coating is an oxidized chemical transformation of the substrate metal and grows both inward and outward from the original metal surface. Because it grows inward into the base layer, it adheres very strongly to the base layer metal. A wide variety of substrate alloys can be coated, including all processed aluminum alloys and most cast alloys, although high silicon levels can reduce coating quality. PEO uses environmentally friendly weak alkaline and acidic electrolytes in which oxide coatings are formed under the application of high electrical voltages. PEO coating formation is a complex process involving three simultaneous processes: electrochemical reactions, plasma chemical reactions and thermal oxygen diffusion reactions. Coatings obtained from these reactions typically consist of three-layer structures: a porous outer layer, a dense intermediate layer and a thin dense inner layer. The PEO process is affected by different parameters such as the nature of the substrate material, electrolyte components, current density, current type, voltage, frequency, duty cycle, additives, incorporated particles, coating time and working temperature.

Applications of PEO Coatings

PEO coatings are widely used in sectors such as aerospace, automotive, biomedical and defense due to their superior wear and corrosion resistance properties. For example, the U.S. Army has applied PEO coating technology to piston heads and fuel pumps in Humvees. These parts have gained heat resistance, extending the service life of materials. In the medical field, PEO technology is used to create better bone replacement prostheses. Technology companies also benefit from PEO technology when manufacturing the outer parts of mobile phones. PEO coatings can differ in thickness, composition and surface morphology depending on the specific parameters used during the process. Coating thickness can vary from a few micrometers to several hundred micrometers, depending on the application.

Advantages of PEO Coatings

PEO coatings offer various advantages compared to other coating types. These coatings provide higher hardness than many other coatings, including anodic and electroless coatings. Additionally, they have excellent wear resistance, which is particularly beneficial in applications where parts are exposed to high stress and friction. PEO coatings also stand out with strong thermal stability and electrical insulation properties. Numerous coating approaches have been developed for valve metals and alloys. Examples of these coatings include chemical conversion coating, anodizing, electroless/galvanic coating, organic coating, laser surface treatment, vapor (physical vapor and chemical vapor) based deposition and thermal/cold spray. Many of these techniques use toxic or harmful chemicals for the purpose of forming a coating on the substrate and require proper maintenance and monitoring during operation. The coating processes mentioned above may have certain limitations related to the substrate or base material, primarily due to low adhesion forces, delamination of the coating surface, porosity and crack growth that can lead to potential equipment failure. In contrast, PEO is less complex because it generally uses a safe and environmentally friendly solution in forming the coating and provides maximum efficient oxide coating with lower costs and high durability. However, the PEO coating method does have certain disadvantages compared to conventional processes. The process requires very high temperatures and pressures. This affects the amount of power required to operate the equipment and shortens the life of the electrolytes.

Differences Between PEO and Anodic Coating

PEO coatings, despite their similarities to conventional anodizing (anodization), differ significantly from the latter in terms of applied electrical signals and electrolytes. PEO typically forms under conditions that exceed the dielectric breakdown voltage of the oxide film and its associated gas (100-600 V) and uses dilute, environmentally friendly alkaline electrolytes. A key characteristic of PEO is the presence of numerous short-lived (from several to several tens of milliseconds) plasma microdischarges produced by gas ionization and located in the breakdown channels within the coating. The high temperatures and pressures of the plasma facilitate rapid growth of the coating (1-4 micrometers/min) containing substrate and electrolyte-derived components that form crystalline and amorphous phases. Unlike conventional anodizing, PEO does not require any special surface preparation and produces coatings of exceptional hardness (1000-2000 HV in aluminum alloys and above 400 HV in Mg and Ti alloys) and wear resistance. The porous outer part of PEO coatings serves as an excellent base for primers and paints, providing superior corrosion resistance and a range of distinctive surface coatings for industrial requirements. In summary, PEO coatings are a type of surface treatment produced by converting a metal surface using electrical discharge in an electrolyte solution. PEO coatings, which have high wear and corrosion resistance, are widely used in the aerospace, automotive, biomedical and defense industries. Sources https://www.dekmake.com/guide-to-plasma-electrolytic-oxidation/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8224744/ https://www.sciencedirect.com/science/article/pii/S0257897221005132, https://www.ucm.es/ccrm/anodizing-and-plasma-electrolytic-oxidation-of-magnesium,-aluminium-and-titanium-alloys https://www.thoughtco.com/definition-of-oxidation-in-chemistry-605456