Fabrication of multifunctional black PEO coatings on AA7075 for spacecraft applications

- Black coatings fabricated by PEO with (NH4)6Mo7O24, K2TiF6 and Na2WO4 additives.
- Additives increase the coating growth rate, K2TiF6 shows higher thickness (30 μm).
- Coating with K2TiF6 shows higher absorptance (0.9) and emittance (0.79).
- Black coatings exhibited 103-105 times lower icorr value than substrate.

High emittance black coating with acceptable corrosion resistance and adhesion strength is important for most of the internal components of the spacecraft. Thus, the present study focused on fabricating the black ceramic composite coating on AA7075 by plasma electrolytic oxidation using three different coloring additives namely (NH4)6Mo7O24, K2TiF6 and Na2WO4 in the silicate and KOH based electrolyte system. The surface morphology, cross-sectional microstructure, elemental composition, and phase composition of the fabricated black coatings were characterized by SEM, EDS and XRD. The thermal control properties, corrosion behavior and adhesion strength of the fabricated coatings were also assessed. It was found that the coating deposition rate increased with the 0introduction of additives and the highest thickness of 30 μm was obtained for coating produced with K2TiF6 additive. The elemental and phase analysis confirmed the incorporation of Mo, Ti and W species into the coating structure. Study on thermal control properties of the coatings displayed that the black PEO coating formed in K2TiF6 containing electrolyte possessed the higher absorptance (0.9) and emittance (0.79) values. Electrochemical corrosion study in 3.5% NaCl and salt spray corrosion test in 5% NaCl showed the drastic enhancement in corrosion resistance with the addition of additives. The scratch test results revealed that all the PEO coatings possess good adherence to the substrate and comparing the critical loads of the PEO treated samples, the samples treated with additives exhibited higher critical loads due to their higher thicknesses.