Polymers exposed in the low-Earth-orbit (LEO) environment are subject to erosion by atomic oxygen that is present in the Earth’s upper atmosphere through photo dissociation of molecular oxygen by ultraviolet radiation from the Sun. The polymers typically fail in their performance earlier than their desired mission lifetime because of this erosion. Thus, protective coatings are used to slow or prevent the erosion from taking place. In order to determine how well protective coating candidates perform, they are often exposed in ground-based atomic oxygen facilities, where the sample turnaround time is shorter and less expensive than for actual flight testing. The difficulty in performing these tests is that there are no ground-based facilities that can exactly duplicate the space environment. The energy of the atomic oxygen arriving and/or the directionality of the arrival are not the same. Therefore, the question arises when performing ground-based atomic oxygen testing as to how long a protected polymer should be exposed to provide equivalent damage as that which would be observed in LEO. To provide an answer to this question, an experiment developed at the NASA Glenn Research Center was flown on the International Space Station as part of the Materials International Space Station Experiment (MISSE). Results of the experiment showed that for silicon-dioxide-coated Kapton, the erosion in ground-based atomic oxygen exposure facilities ranged from approximately 40 to 160 times greater than that observed in LEO, depending upon the type of ground facility exposure used.
The experiment consisted of two samples of Kapton HN (DuPont) polyimide film coated with approximately 1300 Å of silicon dioxide on both sides. One sample was exposed in a ground-based isotropic atomic oxygen plasma, and the other was exposed to an atomic oxygen directed beam. Both samples were carefully weighed prior to and after exposure to determine the amount of erosion that took place. Both samples were then flown on MISSE as part of a group of samples on the atomic-oxygen-facing side of the passive experiment carrier (PEC) 2 tray, which was mounted outside of the Quest Airlock. The photograph shows an astronaut with a PEC tray. After nearly 4 years on orbit, the samples were returned to Earth and again carefully weighed. The samples were then exposed to atomic oxygen in the facilities to which they had been exposed prior to flight to determine if the same mass loss resulted as had prior to flight. This was performed to verify that no damage occurred during flight that could have changed the erosion rate.
Astronaut working with a MISSE PEC tray mounted on the International Space Station.
The graph plots the mass loss per unit area versus effective atomic oxygen fluence (dose) for all three exposures for the atomic oxygen directed-beam-exposed sample. Results of the experiment show that the erosion for the directed-beam-exposed sample was approximately 160 times higher than that observed in LEO and that for the isotropic plasma was approximately 40 times higher than that observed in LEO, although the latter value is probably higher because the number of defects had increased between preflight and postflight exposure, as evidence indicates. This experiment provided validation that ground-based atomic oxygen exposure facilities can provide meaningful durability data for coated polymers without conducting experiments to full mission fluence.
Mass loss per unit area versus effective atomic oxygen fluence (dose) for samples exposed preflight in the atomic oxygen directed beam, exposed on orbit on MISSE, and exposed postflight in the atomic oxygen directed beam.
Long description of figure 2.
Find out more about the research of Glenn’s Electro-Physics Branch: http://www.grc.nasa.gov/WWW/epbranch/ephome.htm
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