Characterizing the optical performance of AZ93 with a fluoropolymer overcoat under ultraviolet exposure

Abstract

AZ93 with a fluoropolymer overcoat is an option to simplify ground handling of space hardware. The overcoat applied on some on-orbit International Space Station (ISS) hardware provides contamination protection for optically sensitive ceramic thermal control coatings. However, if the fluoropolymer is not eroded on-orbit by atomic oxygen (AO), then it will darken. This will increase the solar absorptance resulting in possible thermal performance degradation. If the fluoropolymer overcoat was not present, optical performance would be significantly improved. To characterize the optical performance of the AZ93 with the fluoropolymer overcoat for modeling the UV degradation, laboratory testing of the coating was performed at Marshall Space Flight Center (MSFC). Sample coupons prepared by AZ Technology were exposed under vacuum to ultraviolet radiation. At periodic intervals, the samples were removed from the testing chamber to acquire images and to measure the solar absorptance. The images showed visible differences between AZ93 with the overcoat and without the overcoat as vacuum ultraviolet (VUV) exposure increased. Darkening is more pronounced in the samples with the fluoropolymer overcoat. This was also evident in the solar absorptance measurements. Optical properties of AZ93 with the fluoropolymer overcoat significantly degraded in comparison to those without the overcoat. A short period of little change followed by an exponential rise in solar absorptance was observed. The optical degradation of the fluoropolymer overcoat is described in terms of surface reaction chemistry and kinetics and is found to follow a pseudo first order reaction rate.