Preparation of black high absorbance and high emissivity thermal control coating on Ti alloy by plasma electrolytic oxidation

It is of great significance for the research of thermal control coatings on Ti alloys due to their wide applications in aerospace and satellites and many other fields. In this work, black thermal control coatings were prepared on Ti-6Al-4V alloy by plasma electrolytic oxidation (PEO) technique. The coatings were characterized by SEM, EDS and XRD, respectively. The thermal control properties (solar absorbance, α_S and emissivity, 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 ceramic coatings are black and porous and the elements such as Fe, Co and Ni and W from the electrolyte incorporate into the coating and exist in the form of an amorphous state. The coatings are of high emissivity and high absorbance character, which are influenced by the doping of Na₂WO₄ and the PEO time. Increasing the concentration of Na₂WO₄, α_S increases firstly and then remains constant while ε increases gradually. Extending PEO time, both α_S and ε increase. The annealing treatment reduces the solar absorbance greatly, but does not influence the emissivity apparently, which may be due to the improvement of the crystallization of the coating. When the dosage of Na₂WO₄ is 7.5g/L and the reaction time is 25min, the best coating is obtained with α_S and ε of 0.93 and 0.88, respectively. This work not only provides an effective method to prepare thermal control coating for Ti alloys as the engineering materials, but also expands the application range of PEO technique in the fields of functional coatings.