Studies of White Ceramic Coatings for ESA’s BepiColombo Mission to Mercury

摘要

BepiColombo is a mission planned by the European Space Agency (ESA) to reach Mercury, the least explored planet in our Solar System. The mission has been launched on October 20, 2018, and will face long-term exposure to temperatures up to 400 degrees C, thermal cycling between extreme temperature values, and high levels of radiation (UV, protons). A new thermal "Nanovation" coating (V14) was developed by CeraNovis in cooperation with ESA to deal with the harsh conditions. The coating comprises a sintered mixture of oxidesitrides using a silicate binder, deposited on a grade-5 or grade-2 titanium substrate. It is organic-free after sintering and distinguished by its low solar absorptance of 0.2 with IR emittance of 0.87-0.89, as well as very good thermoshock resistance and excellent adhesion on titanium grade 2 and 5, combined with a low layer thickness of 20-35 mu m. In the presence of contaminants, though, V14 is inclined of trapping the mostly organic contaminants. Upon UV-irradiation in vacuum black carbon clusters are formed from these contaminants, resulting in a higher solar absorptance value. Eventually this might lead to a higher surface temperature of the coated satellite parts. To decompose the contaminants, a cleaning procedure involving gaseous ozone has been developed, which effectively cleans contaminated surfaces involving only volatile gaseous materials and decomposition products. To minimize contamination, a sealing with a very thin vitreous layer can be applied as well. Results of tests conducted by ESA are presented. The coating's structural integrity was followed using powder X-ray diffractometry. Impact of sealing of the a priori porous coating with additional agents, batch variability, environmental testing, and in situ thermal cycling are assessed, employing also other techniques, such as scanning electron microscopy/energy dispersive X-ray spectroscopy, atomic force microscopy, and optical microscopy.