Preparation of thermal control coatings on Ti alloy by plasma electrolytic oxidation in K2ZrF6 solution

Ceramic coatings with high emission and low absorptance were prepared on Ti6Al4V alloy by plasma electrolytic oxidation (PEO) in zirconate electrolyte. The effects of current density and working frequency on the structure and thermal control properties of the coatings were investigated. The phase composition, microstructure, thickness and roughness of coatings were examined by XRD, SEM, EDS, thickness measurement gauge and roughness measuring instrument, respectively. Thermal control properties of the coatings were studied with a UV-VIS-NIR spectrophotometer and an infrared reflectometer. The results show that the coatings are porous and composed of a large amount of KZr2(PO4)3, and a little monoclinic ZrO2, tetragonal ZrO2 and ZrP2O7 as well. The thickness of the coatings increases with the increase of the current density or the decrease of the working frequency while the roughness of the coatings increases with the increase of the current density and the working frequency. The increase of current density reduces the absorptance, but improves the emissivity; the increase of working frequency improves the absorptance, but does not change emissivity apparently. The coating prepared at 10 A/dm2, 50 Hz and 50 min, has the lowest solar absorptance (0.34) and the highest infrared emissivity (0.9).