Preparation of the plasma electrolytic oxidation coating on Mg[sbnd]Li alloy and its thermal control performance

Abstract

When the spacecraft runs in orbit, wild temperature swings always affect the inner instruments under strong solar radiation. Therefore, the MgLi alloy is not suitable as aerospace pace materials before modified with thermal control coating. In this work, the plasma electrolytic oxidation (PEO) was employed to prepare thermal control coatings on MgLi alloys with various Li contents (4 wt% and 9 wt%) in silicate-based systems. The effects of Li content and electrolyte composition on the coating formation and thermal control performance of PEO coating were studied by various methods. The results show that all of PEO coatings were main composed of crystalline MgO and Mg2SiO4. With the increases of electrolyte concentration, the obtained coating had large thickness and roughness, which improved the emissivity and solar absorptance. A possible influence mechanism of the spark discharges on the surface morphologies of PEO coatings were proposed: the dominant spark discharges on Mg4Li were from the excitation of Na ions, inducing relatively more electrolyte compositions to form the coating (particle-clusters); The dominant discharges on Mg9Li were from the excitation of Na ions and Li ions, facilitating relatively more alloy ingredients to make up the coating (protuberances). When prepared in the electrolyte with high concentration, the Mg4Li coating possessed more nano-particles clusters, and Mg9Li coating possessed more protuberances, which endowed the Mg4Li coating with lower solar absorptance and the Mg9Li coating with higher emissivity.Besides, the Mg9Li coating prepared in the electrolyte containing 20 g/L sodium silicate, 2.0 g/L NaF and 2.0 g/L NaOH had the optimal thermal control performance.