Preparation of thermal control coatings on Mg–Li alloys by plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO) has been becoming an important method to prepare thermal control coating on valve metals. The aim of this study is to prepare high emissivity and low absorptance thermal control coatings on Mg–Li alloys in phosphate electrolyte system. The effects of technique parameters and electrolyte concentration on the coating structure and thermal control properties were investigated. The composition and structure of the coatings were examined using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive analysis (EDX). The thermal control properties containing absorptance (α_S) and emissivity (ε) were investigated by ultraviolet–visible–near infrared spectrophotometer instrument and solar absorption reflectometer. The results show that the PEO coatings have typical porous structure with some bulges on the surface and are composed of crystallized Mg₃(PO)₄ and MgO. With the increases of the current density and the reaction time, the coating's thickness and R_a are increased because of the increase of the amount of micro-bulges and micro-cracks on the surface. As the concentration of the electrolytes increases, the number of micro-pores are decreased and the coating surface become uneven due to the increase of the conductivity of the electrolyte which enhances the spark discharging during the PEO process. The thermal control properties are related to the coating thickness and R_a, which can be adjusted by the technique parameters and the electrolyte concentration. In general, increasing the current density, extending the reaction time or modifying the electrolyte concentration, the absorptance is reduced whereas the emissivity is improved. The coating prepared at 10 A/dm², 20 min in 8.250 g/L phosphate solution, presents the lowest solar absorptance (0.33) and the highest infrared emissivity (0.85) due to the large specific surface area and great thickness of the coating.