Effects of Atomic Oxygen in Low Earth Orbit on Space Lubricants

Among many environmental factors in low earth orbit (LEO), atomic oxygen (AO) is one of the few neutral species that can destruct a variety of materials, especially synthetic ones. Thus, the on-ground exposure tests of potential space lubricating materials to the simulated atomic oxygen environment are of crucial importance. The paper investigated the effects of AO exposure on the chemical, physical and tribological properties of three typical lubricants, i.e. silicone oil (CPSO), perfluoropolyether oil (Fomblin Z25) and silicon hydrocarbon oil (SiCH). After exposure to AO, significant mass lose of Formblin Z25 ranging from 0.09% to 5.10% was observed, while relatively low mass losses of 0.40%~0.49% and 0.03%~0.05% were found for CPSO and SiCH, respectively. Gel permeation chromatography (GPC) measurements showed that the number-average molecule weights (Mn) of CPSO and SiCH after AO irradiation increased from 5905 to 6627 for CPSO and from 1954 to 2384 for SiCH, and the polydispersity indexes (PDI) increased from 2.09 to 2.59 for CPSO and from 1.15 to 1.37 for SiCH. The friction tests showed that the friction coefficients of the lubricants became unstable after AO irradiation for all the three lubricants, especially for Z25 that almost lost its lubricity after a 300 min irradiation. The rank of resistance to AO irradiation was SiCH, CPSO, and Fomblin Z25.