Development and Characterization of High Emittance and Low-Thickness Plasma Electrolytic Oxidation Coating on Ti6Al4V for Spacecraft Application

Plasma electrolytic oxidation (PEO) or microarc oxidation process has been carried out on Ti6Al4V substrate in electrolytic bath containing sodium silicate and sodium hypophosphate. The phase composition, microstructure and roughness of the PEO coating are examined by x-ray diffraction, scanning electron microscopy and profilometry techniques, respectively. Wavelength-dependent thermo-optical properties viz. solar absorptance ( α s ) and IR emittance ( ε ir ) are recorded by UV-VIS-NIR and FTIR spectrophotometer, respectively. Nanomechanical properties such as nanohardness ( H ) and Young’s modulus ( E ) are evaluated by well-established nanoindentation technique. The PEO coating is found to be hydrophobic (contact angle ~108°) and possesses low thickness of 5-9 μ m. Average ε ir is obtained to be 85% while average α s is recorded as 76%. The characteristics H and E values are 2.59 GPa and 81.45 GPa, respectively. The PEO coating is also characterized with respect to potentiodynamic polarization and impedance spectroscopy to evaluate corrosion resistance. The total impedance and corrosion current density ( i corr ) are in the order of 1 MΩ and 10 -8 µA.cm -2 , respectively. The results indicate superior corrosion resistance of the present PEO coating compared to similar coatings reported in the literature.