Preparation of high emissivity and low absorbance thermal control coatings on Ti alloys by plasma electrolytic oxidation

In this work, plasma electrolytic oxidation (PEO) technique is used for in situ preparation of high emissivity and low solar absorbance thermal control coatings on Ti-6Al-4V alloys. The thermal control ceramic coatings were characterized by SEM, EDS and XRD, respectively. The thermal control properties (αS and ε) were investigated by ultraviolet-visible-near infrared spectrophotometer instrument and solar absorption reflectometer. Meantime, the thermal shock properties of the coated samples were investigated as well. The results show that the prepared ceramic coating is porous with some big particles stacking around like large craters. The coating is mainly composed of O, Si, Ti, P and Na. The coating is not crystallized well and there is a great number of amorphous silicate in the coatings. The concentration of silicate and the current density influence the thickness and the roughness of the coatings, and consequently influence the thermal control properties. The absorbance and the emissivity are decreased first and then increased with the increase of the concentration. The absorbance is decreased while the emissivity is increased when increasing the current density. Thermal shock tests show not only that there is a good adhesion between the substrate and the coating, but also that the coating and its thermal control properties are stable and constant. When the concentration of the silicate is 10 g/L and the current density is 10 A/dm2, the best coating was obtained with the solar absorbance of 0.39, hemispherical emissivity of 0.92, and the balance temperature of 318 K, which presents a promising application prospect in the thermal control system of Ti alloys materials.