Development and Evaluation of Plasma Electrolytic Oxidation Coatings for Spacecraft Applications

Abstract

Spacecraft structures are often fabricated out of light weight materials in order to reduce the overall mass, thereby reducing the launch cost and/or increasing the payload capability. Aluminium alloys, especially AA6061, is one of the most commonly used metallic structural materials owing to its high strength to weight ratio. A wide range of surface modification techniques such as painting, powder coating, chemical conversion coating, sol-gel deposition, electrophoretic deposition, physical or chemical vapour deposition, electrochemical deposition etc. are often employed to alter one or more surface properties of the alloy for specific functional applications in the spacecraft. Among them, electrochemical methods such as anodization, electroplating, electroless plating etc. are identified as simple, cost-effective and versatile. Plasma Electrolytic Oxidation (PEO) is one such emerging electrochemical surface modification technology suitable to develop thick ceramic coatings on metals like aluminium, magnesium, titanium etc. The PEO process is a conversion coating process similar to anodizing in which the part (job) to be coated is made as the anode of a suitable electrolytic cell. Unlike anodization, where strong mineral acids are employed as electrolyte, the PEO process utilizes mild alkaline solutions as electrolytes and are hence quite environmentally friendly. The process utilizes high voltage, sufficient enough to reach the breakdown potential of the thin insulating oxide layer initially formed on the metal surface, resulting in the formation of large number of short-lived micro discharges which appear as numerous sparks distributed uniformly throughout the surface. The high temperature and pressure generated inside the discharge channel melt the substrate element as well as the already formed oxide, and the molten material gets ejected out of the channel. The anionic species in the electrolyte enter the discharge channel by electrophoresis and get involved in complex plasma assisted chemical reactions...