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The degradation of the mechanical properties of polyimide films was evaluated by means of tensile tests after exposure to a low earth orbit (LEO) environment. Polyimide films irradiated with atomic oxygen (AO), ultraviolet (UV) light, and electron beam (EB) rays using ground simulation facilities were also evaluated similarly and compared. In these experiments tensile stress (7.0 MPa or less) was applied to the samples in order to assess its effects on mechanical properties. The mechanical properties of the flight samples decreased concomitantly with increased exposure duration. The fracture surfaces exhibited characteristic radiated patterns initiating from the exposed surfaces which showed a rough texture. In the AO-irradiated samples the mechanical properties degraded and the surface texture developed as the AO fluence increased; similar fracture surfaces appeared in the flight samples. In contrast, UV and EB irradiation had little impact on mechanical properties. Based on these results, the eroded surfaces by AO irradiation served as the starting points of the rupture, resulting in degradation of mechanical properties of polyimide films exposed to a LEO environment. The tensile stress states induced no difference in evaluations.

Mechanical properties of polyimide films are degraded by exposure to a low earth orbit environment. The main environmental factor for that degradation is atomic oxygen (AO). Using tensile tests, AO-irradiated surface topography observations, and fracture surface analyses, this study investigated the degradation behavior of polyimide films’ mechanical properties by increased AO fluence and its accompanying degradation mechanisms. Tensile strength and elongation of polyimide films were reduced concomitantly with increased AO fluence. Furthermore, AO-irradiated polyimide films fractured from the AO-irradiated surfaces, of which roughness became marked as AO fluence increased. These results reflect that reduction of mechanical properties is attributable to the roughness increase in AO-irradiated surfaces. Polyimide films coated with indium tin oxide (ITO) were also evaluated to confirm the degradation behavior of AO protective films. Surfaces of ITO-coated polyimide films remained smooth even after AO irradiation. However, undercut cavities were formed at ITO coating defect sites. Rupture of ITO-coated polyimide films initiates from the undercut cavities, engendering large reduction of tensile strength and elongation. The degradation of the mechanical properties of ITO-coated polyimide films increased substantially until the undercut cavities fully penetrated the film.

To clarify the effects of space environment on mechanical properties of polymer, exposure experiments were conducted utilizing the International Space Station Russian Service Module. Poly‐ether‐ether‐ketone (PEEK) films under tensile stress were exposed to low Earth orbit (LEO) environment, and reference samples were irradiated with atomic oxygen (AO), electron beam (EB), and ultraviolet light (UV) in ground facilities. By comparing the results of flight and ground tests, the degradation behavior and the influential factors in LEO were investigated. The following results were obtained. (1) UV was found to be the harshest factor in LEO on tensile properties, since it decreased elongation to 15% of pristine sample after 46‐months exposure. (2) AO in LEO eroded the specimen surface with a cone‐like morphology and reduced the thickness; however, it had no significant effect on tensile properties. (3) EB irradiation in LEO had no measurable effects on the material properties.

The surface textures of polyimide (PI) films exposed to a low Earth orbit environment in the “SM and JEM/MPAC&SEED experiments” and PI films irradiated by atomic oxygen (AO) using a ground facility were quantitatively evaluated. Additionally, the degradation in tensile strength of these PI films was assessed in terms of the surface texture changes. The exposed surfaces of the flight PI films were contaminated by silicon oxide layers and included some local valleys, or extremely deep depressions compared with the surrounding areas. The depth of the deepest local valleys on the exposed surfaces (a 0max ), which were predicted by extreme statistics, exceeded those of the local valleys of the AO-irradiated PI films. When the tensile strength of these PI films was plotted against AO fluence, the tensile strength reduction trends clearly differed for each PI film. However, the reduction trends were shown to closely correlate when the tensile strengths were plotted against a 0max . These results mean that the tensile strength of the PI films attacked by AO can be evaluated based on the depth of surface cracks and that fracture mechanics is considered an effective tool to predict tensile strength reduction.

Effects of atomic oxygen (AO) on mechanical properties of polyimide films were investigated using surface topography observations and tensile tests. The ITO‐coated polyimide films, which are expected to have a high durability against AO attacks, were also evaluated. Surfaces of polyimide films irradiated by AO were deeply eroded, exhibiting a rough texture. The roughness developed as the AO fluence increased. Tensile strength and elongation of the polyimide films degraded after AO irradiation tests. In addition, that degradation became marked concomitantly with increased AO fluence. These results indicate that development of surface roughness can degrade mechanical properties of polyimide films. In contrast, surfaces of ITO‐coated polyimide films remained flat and were hardly affected by AO irradiation. Nevertheless, the tensile strength and elongation of ITO‐coated polyimide films were decreased by AO irradiation. Such degradation resulted from undercut cavities formed by AO erosion at the defects sites of ITO coating.

Polymermaterials exposed to a space environment exhibit strongly degraded properties because of environmental factors, for example, atomic oxygen, vacuum ultraviolet, and radiation. In addition, the degradation of polymer materials can be accelerated because of the synergistic effects of these environmental factors. For designing highreliability spacecraft, it is important to understand precisely the polymer materials’ degradation. In this report, the synergistic effects of atomic oxygen and vacuum ultraviolet on polyimide films, Kapton H, and silver-coated fluorinated ethylene propylenefilmswere investigated through comparison of the degradation behaviors after single, sequential, and simultaneous irradiations. For both materials, there was no significant change attributed to the synergistic effects in erosion yield and thermo-optical properties. However, the surface morphology of silver-coated fluorinated ethylene propylene changed substantially depending on the irradiationmethod. Surfaces of silver-coated fluorinated ethylene propylene were eroded by atomic oxygen, but were smoothed by vacuum ultraviolet. The surface morphology after sequential irradiations differed depending on the irradiation sequence. A rougher surface with low blunt cones was produced after simultaneous irradiation because of the interaction of the erosion by atomic oxygen attacks and smoothing by vacuum ultraviolet irradiation. This report also describes the measurement methods for fluence of each beam under simultaneous irradiation.

Thermal control materials such as white paints and germanium-coated polyimide film were evaluated with respect to their space environmental tolerance for materials selection of the Dual-frequency Precipitation Radar of the Global Precipitation Measurement satellite (GPM/DPR). Though peeling off and cracking occurred in one paint material during the thermal shock test, other paints showed good tolerance against thermal shock, atomic oxygen, and ultraviolet ray irradiation. Germanium coating on polyimide film was also verified as high atomic oxygen tolerant barrier. Comparing different thickness germanium coatings, it seems that a 1000 angstrom Germanium film has fewer defects and risk of AO undercutting than a 525 angstrom Germanium film.