Leak Rate Performance of Three Silicone Elastomer Compounds after Ground-Simulated and On-Orbit Environment Exposures

摘要

Three silicone elastomer compounds were evaluated to determine the effect of exposure to a spacecraft's external environment and of exposure to atomic oxygen in terrestrial facilities. Air leak rate was used as the metric to quantify the degradation that occurred on S0383-70, S0899-50, and XELA-SA-401 elastomers formed into O-rings. Specimens were positioned external to the International Space Station (1SS) for 18 months aboard M1SSE-6 where they were exposed to atomic oxygen, ultraviolet and particle radiation, vacuum pressure, and temperature fluctuations. The flight specimens were separated into two groups and positioned in ram-facing and in wake-facing directions, where the atomic oxygen fluence was 1.2× 10~(22) and 6.0×10~(20) atoms/in.~2, respectively. Select specimens were pretreated with atomic oxygen prior to flight and their performance was quantified separately. Additional specimens were exposed to atomic oxygen generated by introducing air to radio frequency plasma. Yet another set of specimens was reserved as a control group to which the others were compared. The leak rates of the specimens were determined using a mass-point leak rate technique with uncertainty analysis and showed that the performances of the three elastomers degraded at different rates. The leak rate of XELA-SA-401 was eliminated from the study due to its poor relative flight performance. The ratio of the ram-facing to wake-facing leak rates was 1.2 and 2.9 for the S0383-70 and S0899-50 compounds, respectively, indicating the former compound was not appreciably affected by atomic oxygen while the later was. After receiving atomic oxygen fluence of 1.2×10~(22)22 atoms/in.2 during flight, the leak rate of the S0899-50 was 5 times that of the S0383-70 specimens; however, the leak rate of the S0383-70 was 1.4 times that of the S0899-50 after terrestrial exposure to 1.2×10~(22) atoms/in.~2, indicating the terrestrial facilities do not accurately represent the orbital environment external to ISS.