Protection of Space Materials from the Space Environment

Irradiation effects of 12 eV oxygen ions on polyimide and fluorinated ethylene propylene

ABSTRACTAtomic oxygen, formed in Earth's thermosphere, interacts readily with many materials on spacecraft flying in low Earth orbit (LEO). All hydrocarbon based polymers and graphite are easily oxidized upon the impact of ∼4.5 eV atomic oxygen as the spacecraft ram into the residual atmosphere. The resulting interactions can change the morphology and reduce the thickness of these materials. Directed atomic oxygen erosion will result in the development of textured surfaces on all materials with volatile oxidation products. Examples from space flight samples are provided. As a result of the erosive properties of atomic oxygen on polymers and composites, protective coatings have been developed and are used to increase the functional life of polymer films and composites that are exposed to the LEO environment. The atomic oxygen erosion yields for actual and predicted LEO exposure of numerous materials are presented. Results of in-space exposure of vacuum deposited aluminum protective coatings on polyimide Kapton indicate high rates of degradation are associated with aluminum coatings on both surfaces of the Kapton. Computational modeling predictions indicate that less trapping of the atomic oxygen occurs, with less resulting damage, if only the space-exposed surface is coated with vapor deposited aluminum rather than having both surfaces coated.

Atomic oxygen and molecular nitrogen are the major constituents in the residual atmosphere at low Earth orbital altitudes, and they collide with spacecraft surfaces at relative velocities of ∼7.8 km s−1. The energy associated with these hyperthermal collisions is in excess of many bond dissociation energies and may help promote materials degradation by allowing barriers to reaction or to collision‐induced dissociation (CID) to be overcome. Spacecraft in low Earth orbit (LEO) are also exposed to high fluxes of vacuum ultraviolet (VUV) radiation, which may degrade materials through various photochemical mechanisms. Fluorinated ethylene‐propylene copolymer (FEP Teflon®) is commonly used on spacecraft. Many researchers have studied the individual and/or combined effects of atomic oxygen, VUV light, and CID on FEP Teflon®, yet the detailed degradation mechanisms of FEP Teflon® in LEO are still a subject of debate. Although not as ubiquitous as a spacecraft material, polymethylmethacrylate (PMMA) has been studied as a model and control polymer because of its well known propensity to “unzip” upon exposure to VUV radiation. A combination of beam‐surface scattering, quartz crystal microbalance (QCM), and surface‐recession experiments were conducted to study the effects of various combinations of O atoms (in the ground O(3P) state), Ar atoms, and VUV light on FEP Teflon® and PMMA. A laser‐breakdown source was used to create hyperthermal beams containing O and O2 or argon. A deuterium lamp provided a source of VUV light. O atoms with 4 eV of translational energy or less did not react with a pristine FEP Teflon® surface. Volatile O‐containing reaction products were observed when the O‐atom energy was higher than 4.5 eV, and the signal increased with O‐atom energy. Significant FEP Teflon erosion (∼20% of Kapton® H) was observed when it was exposed to the hyperthermal O/O2 beam with an average O‐atom energy of 5.4 eV. FEP Teflon® and PMMA that were exposed to VUV light alone yielded volatile products and mass loss. Similarly, CID by Ar also yielded volatile products and mass loss, when the Ar energy was higher than 8 eV. However, the erosion caused by VUV light and/or CID is not significant compared to that caused by O/O2. There were no observed synergistic effects of VUV light and O/O2 exposure.

The environment encountered by space vehicles in very low Earth orbit (VLEO, 180 – 350 km altitude) contains predominantly atomic oxygen (AO) and molecular nitrogen (N2), which collide with ram surfaces at relative velocities of ~7.5 km s-1. Structural, thermal-control, and coating materials containing organic polymers are particularly susceptible to AO attack at these high velocities, resulting in erosion, roughening, and degradation of function. Copolymerization or blending of a polymer with polyhedral oligomeric silsesquioxane (POSS) yields a material that can resist AO attack through the formation of a passivating silicon-oxide layer. Still, these hybrid organic/inorganic polymers become rough through AO reactions as the passivating layer is forming. Surface roughness may enhance satellite drag because it promotes energy transfer and scattering angle randomization during gas-surface collisions. As potential low-drag and AO-resistant materials, we have investigated POSS-containing films of clear and Kapton-like polyimides that have an atomically smooth AO-resistant coating of Al2O3 that is grown by atomic layer deposition (ALD). Coated and uncoated films were exposed to hyperthermal molecular beams containing atomic and molecular oxygen to investigate their AO resistance, and molecular beam-surface scattering studies were conducted to characterize the gas-surface scattering dynamics on pristine and AO-exposed surfaces to inform drag predictions. The AO erosion yield of Al2O3 ALD-coated films is essentially zero. Simulations of drag on a representative satellite structure that are based on the observed scattering dynamics suggest that the use of the Al2O3 ALD-coated POSS-polyimides on external satellite surfaces have the potential to reduce drag to less than half that predicted for diffuse scattering surfaces. These smooth and AO-resistant polymer films thus show promise for use in the extreme oxidizing and high-drag environment in VLEO.

The degradation of materials exposed in LEO (low earth orbit) is one of the most important problems in space technology because the predominant component of the atmosphere in LEO is AO (atomic oxygen) which degrades some kinds of materials. This paper presents an experimental simulation on the ground for the degradation of carbon-based materials. In the experiment, AO produced at a plasma torch is accelerated fluid-dynamically with working gas. Graphite, C/C (carbon/carbon) composite and Si-C. C.(silicon impregnated C/C composite) were used as the target materials. The mass loss per unit area depends strongly on the temperature of the material surface. The eroded surfaces of materials were observed with SEM (scanning electron microscope) and laser focus displacement meter. In case of C/C composite, the matrix region erodes more than carbon fiber. The mass loss of Si-C. C. is less than that of corresponding C/C composite and only the C/C region erodes deeply with almost no erosion in Si and SiC regions. In order to investigate the degradation caused by the impact of AO, numerical simulation was also conducted for graphite using MD (molecular dynamics) method.

Very low Earth orbit (VLEO) altitude, defined as altitudes below 350 km, offers significant advantages for high-resolution Earth observation, as well as low-cost launches and communications. However, developing long-lived VLEO satellites remains technically challenging due to the highly dense residual atmosphere, primarily composed of atomic oxygen (O, or AO) and molecular nitrogen (N2). At VLEO altitudes, dense AO collides with the ram face of satellites at a relative velocity of ~ 8 km s−1, causing significant erosion, roughening, and degradation of organic materials used for thermal control, structural components, and coatings. In addition, the highly dense atmosphere may induce drag on satellites, complicating orbit maintenance and affecting the propulsion system. This study investigates a polyimide film coated with a photocurable silsesquioxane (SQ, manufactured by Toagosei Co., Ltd.) as a potential AO-resistant material. Through the reaction with AO, the SQ coating forms a passivating silica layer on the surface, protecting the underlying polyimide film from further AO attack. To investigate the potential use of SQ-coated polyimide films as a material for the external surfaces of VLEO satellites, the films were exposed to AO in low Earth orbit (LEO) via Material Degradation Monitor 2 and Carbon Nanotube missions on the International Space Station, as well as a lab-based laser-detonation AO source. The mass losses and erosion yields of LEO-exposed films were higher than those of the lab-exposed films. The erosion of SQ-coated polyimides exhibited sensitivity to the environmental differences between the LEO and the laser-detonation AO source (e.g., the velocity distributions and compositions of O and O2, UV, and thermal cycling). Regardless of the exposure environment, cracks that penetrated the polyimide layer formed when AO exposure exceeded ~ 1 × 1021 atoms cm−2. The cracks formed were more severe under LEO exposure than in the lab. Similar cracking phenomena were observed at LEO and VLEO altitudes during the Material Degradation Monitor mission on Super-Low-Altitude Test Satellite, as indicated by a marked increase in visible diffusion above ~ 1 × 1021 atoms cm−2. Cross-sectional laser Raman spectroscopy of lab-exposed films revealed that the reaction of the coating with AO induced compressive stress, which was relieved by the formation of cracks. These cracks also affect thermo-optical and mechanical properties and may increase satellite drag through multiple-bounce scattering. To enable long-term VLEO missions, the molecular design of the coating must be improved to maintain nanoscale surface smoothness and prevent crack formation under high-fluence AO exposure.

Accurate ground-based simulation of low Earth orbit (LEO) conditions experienced by a satellite has proven to be a challenge. Recent progress that has been made toward the development of means for simulating portions of the LEO environment will be reviewed. A satellite in LEO has a speed relative to the atmosphere of approximately 8km∕s. The neutral atmospheric molecules exchange momentum upon collision with the surfaces of the satellite, leading to drag, lift, and moments, but ground facilities still have trouble simulating pure beams of ground state atmospheric gases at this speed. The most important atmospheric species, atomic oxygen, collides with ram-direction satellite surfaces with a relative energy of 5eV. The solar UV spectrum in LEO is a composite of many emission lines and continuum, which must be simulated using special lamp systems. Thruster plumes and outgassing molecules result in a contamination atmosphere close to the spacecraft surface. Energetic atomic oxygen atoms in the presence of solar UV radiation and contamination gases can produce synergistic effects that result in many chemical reactions on or in the vicinity of the outer satellite surfaces. These reactions can lead to structural or operational damage to various satellite components, such as a reduction in the effectiveness of thermal control paints, cleanliness of optical lenses, and the reduction of the output of solar cells. Several techniques for generating beams of 5eV atomic oxygen in the ground state will be reviewed as well as special diagnostic instrumentation developed to detect atomic oxygen. These are combined in space simulation chambers for a more complete study of materials degradation. Some electric thrusters exit directly to the vacuum of space. Their exhaust and plume simulation presents unique vacuum chamber pumping problems. Specially designed cryogenic pumps to simulate the conditions that these electric thrusters will experience in orbit will be described. Reference to inventories of space simulation chambers will be given.

The Skylab D024 Thermal Control Coatings and Polymeric Films Experiment was designed to determine the effects of the external Skylab space environment on the performance and properties of a wide variety of selected thermal control coatings and polymeric films. Three duplicate sets of thermal control coatings and polymeric films were exposed to the Skylab space environment for varying periods of time during the mission. The specimens were retrieved by the astronauts during extravehicular activities (EVA) and placed in hermetically sealed return containers, recovered, and returned to the Wright Laboratory/Materials Laboratory/WPAFB, Ohio for analysis and evaluation. Postflight analysis of the three sets of recovered thermal control coatings indicated that measured changes in specimen thermo-optical properties were due to a combination of excessive contamination and solar degradation of the contaminant layer. The degree of degradation experienced over-rode, obscured, and compromised the measurement of the degradation of the substrate coatings themselves. Results of the analysis of the effects of exposure on the polymeric films and the contamination observed are also presented. The D024 results were used in the design of the LDEF M0003-5 Thermal Control Materials Experiment. The results are presented here to call to the attention of the many other LDEF experimenters the wealth of directly related, low earth orbit, space environmental exposure data that is available from the ten or more separate experiments that were conducted during the Skylab mission. Results of these experiments offer data on the results of low altitude space exposure on materials recovered from space with exposure longer than typical STS experiments for comparison with the LDEF results.

ZrTiAlV alloy has potential applications in space mechanisms. The change of wear and the friction coefficient of the ZrTiAlV alloy subjected to atomic oxygen (AO) exposure were investigated aiming to evaluate the tribological properties influenced by the atomic oxygen (AO) environment of the low Earth orbit (LEO). The AO testing employing a 5 eV AO beam was performed under various AO fluencies. The mass loss and roughness of the exposed to AO surfaces were measured. The exposed surfaces and the worn surfaces were analyzed using scanning electron microscopy (SEM) and X-ray photoelectron spectrometry (XPS) to explore the mechanism of AO effect.

Forty-one different polymers are being exposed to the low Earth orbit (LEO) environment on the exterior of the International Space Station (ISS) for one year as part of MISSE (Materials International Space Station Experiment). MISSE is a materials flight experiment sponsored by the Air Force Research Lab/Materials Lab and the National Aeronautics and Space Administration (NASA). A second set of the same polymers is planned to be flown as part of PEACE (Polymer Erosion And Contamination Experiment), a short duration shuttle flight experiment, and therefore these forty-one polymers on ISS are collectively called the MISSE PEACE Polymers. The objective of the MISSE PEACE Polymers experiment is to accurately determine the atomic oxygen (AO) erosion yield of a wide variety of polymeric materials. The polymers range from those commonly used for spacecraft applications, such as Teflon FEP, to more recently developed polymers, such as high temperature polyimide PMR (polymerization of monomer reactants). Additional polymers were included to explore erosion yield dependence upon chemical composition. Details on the specific polymers being flown, flight sample fabrication, and pre-flight characterization techniques will be discussed. The MISSE PEACE Polymers experiment was placed on the exterior of ISS during a spacewalk on August 16, 2001 and is planned to be retrieved in the fall of 2002. The erosion yield data obtained from this experiment will be compared with data from the future short duration experiment PEACE and with predicted results from models developed by a Canadian group that predicts the AO erosion yield of organic materials based on their chemical structure. Having flight data, and comparing flight data with the predictive model results, will be valuable for spacecraft design purposes.

Atomic oxygen is formed in the low Earth orbital environment (LEO) by photo dissociation of diatomic oxygen by short wavelength (< 243 nm) solar radiation which has sufficient energy to break the 5.12 eV O2 diatomic bond in an environment where the mean free path is sufficiently long (~ 108 meters) that the probability of reassociation or the formation of ozone (O3) is small. As a consequence, between the altitudes of 180 and 650 km, atomic oxygen is the most abundant species. Spacecraft impact the atomic oxygen resident in LEO with sufficient energy to break hydrocarbon polymer bonds, causing oxidation and thinning of the polymers due to loss of volatile oxidation products. Mitigation techniques, such as the development of materials with improved durability to atomic oxygen attack, as well as atomic oxygen protective coatings, have been employed with varying degrees of success to improve durability of polymers in the LEO environment. Atomic oxygen can also oxidize silicones and silicone contamination to produce non-volatile silica deposits. Such contaminants are present on most LEO missions and can be a threat to performance of optical surfaces. The LEO atomic oxygen environment, its interactions with materials, results of space testing, computational modeling, mitigation techniques, and ground laboratory simulation procedures and issues are presented.

Temperature dependence of atomic‑oxygen irradiation mechanism of hydrogenated diamond-like carbon film

Evaluation of atomic oxygen fluences by analyzing transmitted light images of polyimide obtained at SLATS/MDM

Piezoelectric energy harvesting from the atomic oxygen hypervelocity impact in low Earth orbit

Reactive molecular dynamics simulations on the mechanism of the different protection methods of KAPTON during atomic oxygen impact