A facile preparation of ceramic coatings on Ti alloys for thermal protection systems

In order to reduce the effects of solar energy on the instruments and the apparatus in the outer space, two different kinds of ceramic coatings were prepared on Ti6Al4V alloys by plasma electrolytic oxidation (PEO) in zirconate electrolytes. The phase composition, microstructure, thickness, and roughness of coatings were examined by XRD, SEM, EDS, thickness measurement gauge, and roughness measuring instrument, respectively. The infrared emissivity and solar absorbance of the ceramic coatings were studied with the UV-vis-NIR spectrophotometer and infrared reflectometer. The results show that ZrO2 coatings are dense and deep gray-green, and mainly composed of m-ZrO2, t-ZrO2. In addition, a small amount of TiO2, ZrTiO4, and the amorphous compound containing P exists in the coating. The intrinsic solar absorbance and emissivity of ZrO2 coatings are 0.92 and 0.76, respectively. The KZr2(PO4)3 coating is rough, thick, and white with the main crystalline of KZr2(PO4)3. The intrinsic solar absorbance and emissivity of the KZr2(PO4)3 coating are 0.62 and 0.91, respectively. For both coatings, the absorbance decreases while the emissivity increases due to an increase in the real surface area of the coatings by increasing thickness and roughness of the coatings as the PEO time progresses. The porous structure of the KZr2(PO4)3 is proved to be ideal for high emissivity and low absorbance materials with the lowest solar absorbance (0.239), the highest infrared emissivity (0.99), and a corresponding balance temperature of the coating of 276 K.