Development of Ca3(PO4)2 inorganic thermal-control coating used in harsh space environments

Abstract

Passive thermal control coatings (TCC's) have been widely used to tune the thermal equilibrium of spacecraft surfaces since 1970s. Development of new TCC's with improved thermo-physical properties and radiation hardness are essential to the space assets that fly in harsh environments. Herein, we fabricated calcium orthophosphate based inorganic white paint, which possess relative low solar absorptance (αs) of 0.095 and thermal emittance (εH) of 0.88. Noticeably, the Ca3(PO4)2 coating shows excellent space radiation tolerance and thermal control capacity. After performing 90 keV proton irradiation at a fluence of 4×1015 cm-2, the αs of ZnO coating increases from 0.160 to 0.213 while the Ca3(PO4)2 coating rises from 0.095 to 0112, which only degrades one third (0.017) of the ZnO coating (0.053). Furthermore, after performing simulated solar irradiation test, the surface temperature of Ca3(PO4)2 paint is 14.4 °C lower than that of ZnO coating, evidencing its prominent thermal-optical performance. Besides, we also found that Ga2O3 based liquid-glass white paint is a good candidate for thermal equilibrium of spacecraft.