Chemical and electrical interaction mechanisms during the plasma electrolytic (PEO) mixed oxide formation on magnesium

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Magnesium is the lightest metallic engineering material and therefore offers an enormous potential to save weight in mobile systems. Beyond that, magnesium is easy to recycle. Excellent casting process properties and a good damping capacity towards electromagnetic and mechanical oscillations predestine magnesium materials for the construction of machine casings as well as framework and encasements for sensible sensory, optical and entertainment electronic devices. Despite the positive processing and application properties listed here, the application spectrum of magnesium alloys is currently limited due to their low resistance towards corrosive and tribological stress. Plasma electrolytic oxidation is a promising and environmentally friendly surface treatment process to encounter these technical challenges. An already successfully finished DFG-project was concerned with the substrate/electrolyte interaction before and during the discharge initiation, as well as the aimed insertion of electrolyte components into the generated PEOcoating. It was shown that by employing highly concentrated electrolytes, very hard and chemically resistant mixed oxide (chemical compositions dominated by electrolyte components rather than substrate components) coatings are producible, which hardness exceed that of MgO layers significantly. However, due to their morphology being afflicted with local defects, such coatings will negatively affect the resulting corrosion- and wear resistance. This reveals further research needs. A characterisation of the complex coat-forming processes as well as the interactions of chemical and electrical process parameters during the plasma electrolyte coating procedure is only possible by empirical means, according to the state of the art and in lack of a consistent process model. Therefore, the proposed project is aiming towards the research of action mechanisms of interacting chemical and electrical processes during the plasma electrolytic oxidation of magnesium under formation of mixed oxides. To reach this goal, the process needs to be characterised and understood in detail. For that, the charge throughput of the pulses necessary for coating formation are to be broken down into electro- and plasma-chemical parts, and specific process stages like the coat-healing Softsparking and cathodic discharges need to be systematically recorded. On this basis, hybrid pulse patterns adjusted to the individual process stages will be developed. The data foundation generated during the project will subsequently be used to create a model concept which describes the underlying mechanisms. Based on the obtained conclusions and using environmentally friendly electrolytes, adherent and low-defect mixed oxide coatings are to be generated on magnesium substrates and to be qualified for corrosively and tribologically demanding applications.