Development and Evaluation of Plasma Electrolytic Oxidation Coatings for Spacecraft Applications
Pillai, Anju M
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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 and modifies the composition of the melt. Once the melt comes in contact with the electrolyte which is maintained at near room temperature, solidification occurs and growth of the coating happens. Thus, PEO coatings consist not only of substrate oxides but also complex oxides and compounds due to the incorporation of components present in the electrolyte. In this research, utilization of the PEO process in the development of a few functional ceramic coatings on AA6061 alloy which finds application in spacecraft will be discussed. A systematic study of the effect of various process parameters on the coating growth behaviour will be presented. A gradual transition of the microstructure of the PEO coating from crystalline to amorphous phase with change in electrolyte concentration and the effect of this transition on the mechanical property and corrosion behaviour of the coating will be discussed. Solar reflector PEO coating and two types of flat absorber PEO coatings (utilizing two different additives) are prepared for spacecraft thermal control applications. Similarly, high insulation coating for realization of indigenous bus bar for efficient power distribution in high power satellites are developed using PEO technique. The development, characterization, evaluation and space qualification of these coatings will be elaborated and the implementation of high insulation PEO coating on flight hardware will be discussed as a case study.