Skip main navigation bar NASA Glenn Research Center
  Space Environment and Experiments Branch

[GRC Hanger link to Glenn Research Center Home Page]

[NASA Logo link to NASA national site]

Atomic Oxygen Abstracts


Home
Up
Facilities
Technologies
Publications
Awards
Contact Us
Site Map

 


Lessons Learned from Atomic Oxygen Interaction with Spacecraft Materials in Low Earth Orbit


There have been five Materials International Space Station Experiment (MISSE) passive experiment carriers (PECs) (MISSE 1-5) to date that have been launched, exposed in space on the exterior of International Space Station (ISS) and than returned to Earth for analysis.  An additional four MISSE PECs (MISSE 6A, 7A, and 7B) are in various stages of completion.  The PECs are two-sided suitcase to size sample carriers that are intended to provide information on the effects of the low Earth orbital environment on a wide variety of materials and components.  As a result of post retrieval ananlyses of the retrieved MISSE 2 experiments, and numerous prior space experiments, there have been valuable lessons learned and needs identified that are worthy of being documented so that planning, design, and analysis of future space environment experiments can benefit from the experience in order to maximize the knowleged gained.  Some of the lessons learned involve the techniques, concepts, and issues associated with measuring atomic oxygen erosion yields.  These are presented along with several issues to be considered when desigining experiments, such as the uncertainty in mission duration, scattering and contamination effects on results, and the accuracy of measuring atomic oxygen erosion.

[horizontal rule]

Use of Atomic Oxygen for Increased Water Contact Angles of Various Polymers for Biomedical Applications

In the low Earth orbit (LEO) space environment, spacecraft surfaces can be altered during atomic oxygen exposure through oxidation and erosion. There can be terrestrial benefits of such interactions, such as the modification of hydrophobic or hydrophilic properties of polymers due to chemical modification and texturing. Such modification of the surface may be useful for biomedical applications. For example, atomic oxygen texturing may increase the hydrophilicity of polymers, such as Aclar, thus allowing increased adhesion and spreading of cells on textured Petri dishes. The purpose of this study was to determine the effect of atomic oxygen exposure on the hydrophilicity of nine different polymers. To determine whether hydrophilicity remains static after atomic oxygen exposure or changes with exposure, the contact angles between the polymer and a water droplet placed on the polymer’s surface were measured. The polymers were exposed to atomic oxygen in a radio frequency (RF) plasma asher. Atomic oxygen plasma treatment was found to significantly alter the hydrophilicity of non-fluorinated polymers. Significant decreases in the water contact angle occurred with atomic oxygen exposure. Fluorinated polymers were found to be less sensitive to changes in hydrophilicity for equivalent atomic oxygen exposures, and two of the fluorinated polymers became more hydrophobic. The majority of change in water contact angle of the non-fluorinated polymers was found to occur with very low fluence exposures, indicating potential cell culturing benefit with short treatment time.

 
[horizontal rule]

Comparison of the Atomic Oxygen Erosion Depth and Cone Height of Various Materials at Hyperthermal Energy

Atomic oxygen readily reacts with most spacecraft polymer materials exposed to the low Earth orbital (LEO) environment. If the atomic oxygen arrival comes from a fixed angle of impact, the resulting erosion will foster the development of a change in surface morphology as material thickness decreases. Hydrocarbon and halopolymer materials, as well as graphite, are easily oxidized and textured by directed atomic oxygen in LEO at energies of ~4.5eV. What has been curious is that the ratio of cone height to erosion depth is quite different for different materials. The formation of cones under fixed direction atomic oxygen attack may contribute to a reduction in material tensile strength in excess of that which would occur if the cone height to erosion depth ratio was very low because of greater opportunities for crack initiation. In an effort to understand how material composition affects the ratio of cone height to erosion depth, an experimental investigation was conducted on 18 different materials exposed to a hyperthermal energy directed atomic oxygen source (~70eV). The materials were first salt-sprayed to provide microscopic local areas that would be protected from atomic oxygen. This allowed erosion depth measurements to be made by scanning microscopy inspection. The polymers were then exposed to atomic oxygen produced by an end Hall ion source that was operated on pure oxygen. Samples were exposed to an atomic oxygen effective fluence of 1.0x1020 atoms/cm2 based on Kapton H polyimide erosion. The average erosion depth and average cone height were determined using field emission scanning electron microscopy (FESEM). The experimental ratio of average cone height to erosion depth is compared to polymer composition and other properties.

[horizontal rule]

MISSE PEACE   Polymers Atomic Oxygen Erosion Results

Forty-one different polymers, collectively called the PEACE (Polymer Erosion and Contamination Experiment) Polymers, have been exposed to the low Earth orbit environment on the exterior of the International Space Station (ISS) for nearly four years as part of Materials International Space Station Experiment 1 & 2 (MISSE 1 & 2).  The objective of the MISSE PEACE Polymers experiment is to accurately determine the atomic oxygen erosion yield of a very wide variety of polymeric materials.  The polymers range from those commonly used for spacecraft applications, such as TeflonÒ 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.  The MISSE PEACE Polymers experiment was flown in MISSE Passive Experiment Carrier 2 (PEC 2), Tray 1, on the exterior of the ISS Quest Airlock and was exposed to atomic oxygen along with solar and charged particle radiation.  MISSE PEC 1 & 2 were successfully retrieved during a space walk on July 30, 2005 during Discovery’s STS-114 Return to Flight mission.  Details on the specific polymers flown, flight sample fabrication, and pre-flight and post-flight characterization techniques will be discussed along with a summary of the atomic oxygen erosion yield results.  The long duration low Earth orbit erosion yield data obtained from this experiment will be compared with predictive model results for the same polymers.  The PEACE Polymers erosion yield data is unique and provides extremely valuable information for spacecraft design purposes.

[horizontal rule]

MISSE Scattered Atomic Oxygen Characterization Experiment

An experiment designed to measure the atomic oxygen (AO) erosion profile of scattered AO was exposed to low Earth orbital (LEO) AO for almost 4 years as part of the Materials International Space Station Experiment 1 & 2 (MISSE 1 & 2).  The experiment was flown in MISSE Passive Experiment Carrier 2 (PEC 2), Tray 1, attached to the exterior of the International Space Station (ISS) Quest Airlock.  The experiment consisted of an aperture disk lid of Kapton H polyimide coated on the space exposed surface with a thin AO durable silicon dioxide film.  The aperture lid had a small hole in its center to allow AO to enter into a chamber and impact a base disk of aluminum.  The AO that scattered from the aluminum base could react with the under side of the aperture lid which was coated sporadically with microscopic sodium chloride particles.  Scattered AO erosion can occur to materials within a spacecraft that are protected from direct AO attack but because of apertures in the spacecraft the AO can attack the interior materials after scattering.  The erosion of the underside of the Kapton lid was sufficient to be able to use profilometry to measure the height of the buttes that remained after washing off the salt particles.  The erosion pattern indicated that peak flux of scattered AO occurred at and angle of approximately 45o degrees from the incoming normal incidence on the aluminum base unlike the erosion pattern predicted for scattering based on Monte Carlo computational predictions for AO scattering from Kapton H polyimide.  The effective erosion yield for the scattered AO was found to be a factor of 0.214 of that for direct impingement on Kapton H polyimide.

[horizontal rule]

Comparison of Atomic Oxygen Erosion Yields of Materials at Various Energies and Impact Angles

The atomic oxygen erosion yields of various materials, measured in volume of material oxidized per incident atomic oxygen atom, are compared to the commonly accepted standard of Kapton H polyimide. The ratios of the erosion yield of Kapton H to the erosion yield of various materials are not consistent at different atomic oxygen energies. Although it is most convenient to use isotropic thermal energy RF plasma ashers to assess atomic oxygen durability, the results can be misleading because the relative erosion rates at thermal energies are not necessarily the same as low Earth orbital (LEO) energies of ~4.5 eV. An experimental investigation of the relative atomic oxygen erosion yields of a wide variety of polymers and carbon was conducted using isotropic thermal energy (~0.1 eV) and hyperthermal energy (~70 eV) atomic oxygen using an RF plasma asher and an end Hall ion source. For hyperthermal energies, the atomic oxygen erosion yields relative to normal incident Kapton H were compared for sweeping atomic oxygen arrival with that of normal incidence arrival. The results of isotropic thermal energy, normal incident, and sweeping incident atomic oxygen are also compared with measured or projected LEO values.

[horizontal rule]

Preliminary Analysis of Polymer Film Thermal Control and Gossamer Materials Experiments on Materials International Space Station Experiment (MISSE 1 and MISSE 2)

A total of 31 samples were included in the National Aeronautics and Space Administration (NASA) Glenn Research Center (GRC) Polymer Film Thermal Control (PFTC) and Gossamer Materials experiments, which were exposed to the low Earth orbit environment for nearly four years on the exterior of the International Space Station (ISS) as part of the Materials International Space Station Experiment (MISSE 1 and MISSE 2).  MISSE is a materials flight experiment sponsored by the Air Force Research Lab/Materials Lab and NASA.  This paper describes objectives, materials, and characterizations for the MISSE 1 and MISSE 2 GRC PFTC and Gossamer Materials samples.  Samples included films of polyimides, fluorinated polyimides, and TeflonÒ fluorinated ethylene propylene (FEP) with and without second-surface metalizing layers and/or surface coatings.  Also included were films of polyphenylene benzobisoxazole (PBO) and a polyarylene ether benzimidazole (TOR-LMTM).  Polymer film samples were examined post-flight for changes in mechanical and optical properties. The environment in which the samples were located was characterized through analysis of sapphire contamination witness samples and samples dedicated to atomic oxygen (AO) erosion measurements.  Results of the preliminary analyses of the PFTC and Gossamer Materials experiments are discussed.

[horizontal rule]

Effects of the Space Environment on Polymer Film Materials Exposed on the Materials International Space Station Experiment (MISSE 1 and MISSE 2)

A total of 28 polymer film samples were included in the National Aeronautics and Space Administration (NASA) Glenn Research Center (GRC) Polymer Film Thermal Control (PFTC) and Gossamer Materials Experiments, which were exposed to the low Earth orbit environment for nearly 4 years on the exterior of the International Space Station (ISS) as part of the Materials International Space Station Experiment (MISSE 1 and MISSE 2).  MISSE is a materials flight experiment sponsored by the Air Force Research Lab/Materials Lab and NASA.  This paper will describe objectives, materials, and characterizations for the MISSE 1 and MISSE 2 GRC PFTC and Gossamer Materials samples.  Samples included films of polyimides, fluorinated polyimides, and TeflonÒ fluorinated ethylene propylene (FEP) with and without second-surface metalizing layers and/or surface coatings.  Also included were films of polyphenylene benzobisoxazole (PBO) and a polyarylene ether benzimidazole (TOR-LMTM).  Polymer film samples were examined post-flight for changes in mechanical and optical properties and for  atomic oxygen (AO) erosion.  Results of the preliminary analyses of the PFTC and Gossamer Materials Experiments are discussed.

[horizontal rule]

MISSE PEACE Polymers Atomic Oxygen Erosion Results

Forty-one different polymer samples, collectively called the Polymer Erosion and Contamination Experiment (PEACE) Polymers, have been exposed to the low Earth orbit (LEO) environment on the exterior of the International Space Station (ISS) for nearly 4 years as part of Materials International Space Station Experiment 2 (MISSE 2).  The objective of the PEACE Polymers experiment was to determine the atomic oxygen erosion yield of a wide variety of polymeric materials after long term exposure to the space environment.  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.  The MISSE PEACE Polymers experiment was flown in MISSE Passive Experiment Carrier 2 (PEC 2), tray 1, on the exterior of the ISS Quest Airlock and was exposed to atomic oxygen along with solar and charged particle radiation.  MISSE 2 was successfully retrieved during a space walk on July 30, 2005, during Discovery’s STS-114 Return to Flight mission.  Details on the specific polymers flown, flight sample fabrication, pre-flight and post-flight characterization techniques, and atomic oxygen fluence calculations are discussed along with a summary of the atomic oxygen erosion yield results.  The MISSE 2 PEACE Polymers experiment is unique because it has the widest variety of polymers flown in LEO for a long duration and provides extremely valuable erosion yield data for spacecraft design purposes.

[horizontal rule]

Ground-to-Space Effective Atomic Oxygen Fluence Correlation for DC 93-500 Silicone

The objective of this research was to calibrate the ground-to-space effective atomic oxygen fluence for DC 93-500 silicone in a thermal energy electron cyclotron resonance (ECR) oxygen plasma facility. A technique has been developed at NASA Glenn Research Center to determine the equivalent amount of atomic oxygen exposure in an ECR ground-test facility to produce the same degree of atomic oxygen damage as in space. The approach used was to compare changes in the surface hardness of ground test (ECR)-exposed DC 93-500 silicone with DC 93-500 exposed to low Earth orbit (LEO) atomic oxygen as part of a shuttle flight experiment. The ground-to-space effective atomic oxygen fluence correlation was determined based on the fluence in the ECR source that produced the same hardness for the fluence in space. A nanomechanical measurement system operated in conjunction with an atomic force microscope (AFM) was used to determine the surface hardness of the silicones. Hardness vs contact depth measurements were obtained for five ECR-exposed DC 93-500 samples (ECR exposed for 18 to 40 h, corresponding to Kapton effective fluences of 4.2×1020 to 9.4×1020 atoms/cm2, respectively) and for space-exposed DC 93-500 from the Evaluation of Oxygen Interactions with Materials III (EOIM III) shuttle flight experiment, exposed to LEO atomic oxygen (2.3×1020atoms/cm2). Pristine controls for the ECR tests and for the EOIM III flight sample were also evaluated. A ground-to-space correlation value was determined based on correlation values for four contact depths (150, 200, 250, and 300 nm), which represent the near-surface depth data. The results indicate that the Kapton effective atomic oxygen fluence in the ECR facility needs to be 2.64 times higher than in LEO to replicate equivalent exposure damage in the ground test silicone as occurred in the space exposed silicone.

[horizontal rule]

Determination of Ground-Laboratory to In-Space Effective Atomic Oxygen Fluence for DC 93-500 Silicone

The objective of this research was to calibrate the ground-to-space effective atomic oxygen fluence for DC 93-500 silicone in a thermal energy electron cyclotron resonance (ECR) oxygen plasma facility.  Silicones, a commonly used spacecraft material, do not chemically erode with atomic oxygen attack like other organic materials.  Silicones react with atomic oxygen and form an oxidized hardened silicate surface layer. Therefore, the effective atomic oxygen fluence in a ground test facility should not be determined based on mass loss measurements, as they are with organic polymers, such as Kapton, a polyimide.  A technique has been developed at the Glenn Research Center to determine the equivalent amount of atomic oxygen exposure in an ECR ground test facility to produce the same degree of atomic oxygen damage as in space.  The approach used was to compare changes in the surface hardness of ground test (ECR) exposed DC 93-500 silicone with DC 93-500 exposed to low Earth orbit (LEO) atomic oxygen as part of a shuttle flight experiment.  The ground to in-space effective atomic oxygen fluence correlation was deter¬mined based on the fluence in the ECR source that produced the same hardness for the fluence in-space.  Because microhardness measurements need to be obtained on the very surface layer of a rubber substrate (with primarily elastic deformation) traditional techniques for microhardness that apply large forces and indenta¬tions based on plastic deforma¬tion, could not be used.  Therefore, a nanomechanical measurement system operated in conjunction with an atomic force microscope (AFM) was used to determine the surface hardness of the silicones.  The nanomechanical system can provide ultra light load indentations and can continuously measure force and displacement as an indent is made.  Hardness versus contact depth measurements were obtained for five ECR exposed DC 93-500 samples (ECR exposed for 18 hrs to 40 hrs, corresponding to Kapton effective fluences of 4.2 x 1020 to 9.4 x 1020 atoms/cm2, respectively) and for a space exposed DC 93-500 from the Evaluation of Oxygen Interactions with Materials III (EOIM III) shuttle flight experiment, exposed to LEO atomic oxygen for 2.3 x 1020 atoms/cm2.  Pristine controls for the ECR tests and for the EOIM III flight sample were also evaluated.  A ground-to-space correlation value was determined based on correlation values for four contact depths (150, 200, 250 & 300 nm), which represent the near surface depth data.  The results indicate that the Kapton effective atomic oxygen fluence in the ECR facility needs to be 2.64 times higher than in LEO to replicate equivalent exposure damage in the ground test silicone as occurred in the space exposed silicone.

[horizontal rule]

Fast Three-Dimensional Modeling of Atomic Oxygen Undercutting of Protected Polymers

A method is presented to model atomic oxygen erosion of protected polymers in low Earth orbit.  Undercutting of protected polymers by atomic oxygen can occur due to the presence of scratch, crack or pin-window defects in the protective coatings.  As a means of providing a better understanding of undercutting processes, a fast method of modeling atomic-oxygen undercutting of protected polymers has been developed.  Current simulation methods often rely on computationally expensive ray-tracing procedures to track the surface-to-surface movement of individual “atoms”.  To reduce the burden of time consuming calculations, the method introduced in this paper replaces computationally demanding individual particle simulations by substituting a model that utilizes both a geometric configuration-factor technique, which collectively governs the diffuse transport of atoms between surfaces, and an efficient algorithm, which rapidly computes the cumulative effects stemming from the series of atomic oxygen collisions at the surfaces of an undercut cavity.  This new method facilitates the systematic study of three-dimensional undercutting by allowing rapid simulations to be made over a wide range of erosion parameters.

[horizontal rule]

Atomic Oxygen Effects on Spacecraft Materials

Low Earth orbital (LEO) atomic oxygen cannot only erode the external surfaces of polymers on spacecraft, but can cause degradation of surfaces internal to components on the spacecraft where openings to the space environment exist. Although atomic oxygen attack on internal or interior surfaces may not have direct exposure to the LEO atomic oxygen flux, scattered impingement can have can have serious degradation effects where sensitive interior surfaces are present. The effects of atomic oxygen erosion of polymers interior to an aperture on a spacecraft is simulated using Monte Carlo computational techniques. A 2-dimensional model is used to provide quantitative indications of the attenuation of atomic oxygen flux as a function of distance into a parallel walled cavity. The degree of erosion relative is compared between the various interior locations and the external surface of an LEO spacecraft.

[horizontal rule]

Space Flight Experiments to Measure Polymer Erosion and Contamination on Spacecraft

Atomic oxygen erosion and silicone contamination are serious issues that could damage or destroy spacecraft components after orbiting for an extended period of time, such as on a space station or satellite. An experiment, the Polymer Erosion And Contamination Experiment (PEACE) will be conducted to study the effects of atomic oxygen (AO) erosion and silicone contamination, and it will provide information and contribute to a solution for these problems. PEACE will fly 43 different polymer materials that will be analyzed for AO erosion effects through two techniques: mass loss measurement and recession depth measurement. Pinhole cameras will provide information about the arrival direction of AO, and silicone contamination pinhole cameras will identify the source of silicone contamination on a spacecraft. All experimental hardware will be passively exposed to AO for up to two weeks in the actual space environment when it flies in the bay of a space shuttle. A second set of the PEACE Polymers is being exposed to the space environment for erosion yield determination as part of a second experiment, Materials International Space Station Experiment (MISSE). MISSE is a collaboration between several federal agencies and aerospace companies. During a space walk on August 16, 2001, MISSE was attached to the outside of the International Space Station (ISS) during an extravehicular activity (EVA), where it began its exposure to AO for approximately 1 1/2 years. The PEACE polymers, therefore, will be analyzed after both short-term and long-term AO exposures for a more complete study of AO effects.

[horizontal rule]

Techniques for Measuring Low Earth Orbital Atomic Oxygen Erosion of Polymers

Polymers such as polyimide Kapton® and Teflon® FEP (fluorinated ethylene propylene) are commonly used spacecraft materials due to their desirable properties such as flexibility, low density, and in the case of FEP, a low solar absorptance and high thermal emittance. Polymers on the exterior of spacecraft in the low Earth orbit (LEO) environment are exposed to energetic atomic oxygen. Atomic oxygen reaction with polymers causes erosion, which is a threat to spacecraft durability. It is therefore important to understand the atomic oxygen erosion yield (E, the volume loss per incident oxygen atom) of polymers being considered in spacecraft design. The most common technique for determining E is through mass loss measurements. For limited duration exposure experiments, such as shuttle experiments, where the atomic oxygen fluence is often so low that mass loss measurements can not produce acceptable uncertainties, recession measurements based on atomic force microscopy analyses can be used. Equally necessary to knowing the mass loss or recession depth for determining the erosion yield of polymers is the knowledge of the atomic oxygen fluence that the polymers were exposed to in space. This paper discusses the procedures and relevant issues for mass loss and recession depth measurements for passive atomic oxygen erosion yield characterization of polymers, along with techniques for active atomic oxygen fluence and erosion characterization. One active atomic oxygen erosion technique discussed is a new technique based on optical measurements. Details including the use of both semi-transparent and opaque polymers for active erosion measurement are reviewed.

[horizontal rule]

Direct C-C Bond Breaking in the Reaction of O(3P) with Flouropolymers in Low Earth Orbit (and) Degradation of Fluoropolymers by O(3P) in Low Earth Orbit

Spacecraft flying in low Earth orbit (LEO) are exposed to a harsh environment which includes frequent bombardment by fast atomic oxygen (AO) and ultraviolet (UV) radiation. As a result, many spacecraft surface materials are severely eroded. In the case of fluoropolymers, a controversy exists whether AO or UV or AO/UV synergy is responsible for the degradation. In this study, with the use of ab initio calculations, we address the question whether the most abundant species in LEO, viz., atomic oxygen in its ground state, O(3P), alone can cause the degradation in fuoropolymer materials. The smallest fluorocarbons CNF2N+2 (N = 2, 3, 5) serve as models of fluoropolymers. Since electronegativity of fluorine seems to preclude F-abstraction by O(3P), we concentrate on direct O(3P) attacks on carbon-carbon bonds. For the case of fluoroethane (N = 2), we explore the triplet potential energy surface of the following reaction:  O(3P) + CF3 – CF3 –> •O – CF3 + •CF3. Analogous reactions, where O(3P) attacks on a central carbon atom, are studied for the higher fluorocarbons. Results obtained using the Hartree-Fock method and density functional theory are reported. We conclude that O(3P) species in LEO possesses enough translational energy to degrade fluorocarbon materials.

[horizontal rule]

Issues and Consequences of Atomic Oxygen Undercutting of Protected Polymers in Low Earth Orbit

Hydrocarbon based polymers that are exposed to atomic oxygen in low Earth orbit are slowly oxidized which results in recession of their surface. Atomic oxygen protective coatings have been developed which are both durable to atomic oxygen and effective in protecting underlying polymers. However, scratches, pin window defects, polymer surface roughness and protective coating layer configuration can result in erosion and potential failure of protected thin polymer films even though the coatings are themselves atomic oxygen durable. This paper will present issues that cause protective coatings to become ineffective in some cases yet effective in others dues to the details of their specific application. Observed in-space examples of failed and successfully protected materials using identical protective thin films will be discussed and analyzed. Proposed approaches to prevent the failures that have been observed will also be presented.

[horizontal rule]

Fast Three-Dimensional Method of Modeling Atomic Oxygen Undercutting of Protected Polymers

A method is presented to model atomic oxygen erosion of protected polymers in low Earth orbit (LEO). Undercutting of protected polymers by atomic oxygen occurs in LEO due to the presence of scratch, crack or pin-window defects in the protective coatings. As a means of providing a better understanding of undercutting processes, a fast method of modeling atomic-oxygen undercutting of protected polymers has been developed. Current simulation methods often rely on computationally expensive ray-tracing procedures to track the surface-to-surface movement of individual “atoms”. The method introduced in this paper replaces slow individual particle approaches by substituting a model that utilizes both a geometric configuration-factor technique, which governs the diffuse transport of atoms between surfaces, and an efficient telescoping series algorithm, which rapidly integrates the cumulative effects stemming from the numerous atomic oxygen events occurring at the surfaces of an undercut cavity. This new method facilitates the systematic study of three-dimensional undercutting by allowing rapid simulations to be made over a wide range of erosion parameters.

[horizontal rule]

Vacuum Ultraviolet Radiation and Atomic Oxygen Durability Evaluation of HST Bi-Stem Boom Thermal Shield Materials

Bellows-type thermal shields were proposed for use on the Hubble Space Telescope (HST) solar array bi-stem booms to reduce the thermal gradient-induced jitter during orbital thermal cycling. Candidate thermal shield materials included aluminized FEP Teflon with and without protective coatings for durability to atomic oxygen (AO) and combined AO and ultraviolet (UV) radiation. NASA Lewis (now Glenn) Research Center performed vacuum ultraviolet (VUV) radiation and AO durability testing of candidate materials as part of an overall program coordinated by NASA Goddard Space Flight Center (GSFC) to evaluate the on-orbit durability of these thermal shield materials.

Coating adhesion problems were observed for samples having AO- and the combined AO/UV-protective coatings which were attributed to exposure to rapid thermal cycling used to simulate thermal cycling on orbit. Such adhesion problems led to production of coating flakes from the material which could pose a significant risk to HST optics if the coated materials were used for the bi-stem boom thermal shields. No serious degradation was observed for the uncoated aluminized Teflon as evaluated by optical microscopy, although atomic force microscopy (AFM) revealed that an embrittled surface layer would build up on the uncoated Teflon surface due to ultraviolet radiation exposure. This embrittled layer was not completely removed by AO erosion. Despite the formation of this embrittled layer, no cracks or particle flakes were produced for the uncoated material upon exposure to VUV and AO.

Uncoated aluminized FEP Teflon was determined to be the most appropriate thermal shield material and was used on the replacement solar arrays installed during the December 1993 First HST Servicing Mission.

[horizontal rule]

Atomic Oxygen Protection of Materials in Low Earth Orbit

Spacecraft polymeric materials as well as polymer-matrix carbon-fiber composites can be significantly eroded as a result of exposure to atomic oxygen in low Earth orbit (LEO). Several new materials now exist, as well as modifications to conventionally used materials that provide much more resistance to atomic oxygen attack than conventional hydrocarbon polymers. Protective coatings have also been developed which are resistant to atomic oxygen attack and provide protection of underlying materials. However, in actual spacecraft applications, the configuration, choice of materials, surface characteristics and functional requirements of quasidurable materials or protective coatings can have great impact on the resulting performance and durability. Atomic oxygen degradation phenomena occurring on past and existing spacecraft will be presented. Issues and considerations involved in providing atomic oxygen protection for materials used on spacecraft in low Earth orbit will be addressed. Analysis of in-space results to determine the causes of successes and failure of atomic oxygen protective coatings is presented.

[horizontal rule]

A Sensitive Technique Using Atomic Force Microscopy to Measure the Low Earth Orbit Atomic Oxygen Erosion of Polymers

Polymers such as polyimide Kapton and Teflon FEP (fluorinated ethylene propylene) are commonly used spacecraft materials due to their desirable properties such as flexibility, low density, and in the case of FEP low solar absorptance and high thermal emittance. Polymers on the exterior of spacecraft in the low Earth orbit (LEO) environment are exposed to energetic atomic oxygen. Atomic oxygen erosion of polymers occurs in LEO and is a threat to spacecraft durability. For example, depths of more than 0.0127 cm thickness of Kapton and Mylar were eroded away after 5.8 years in LEO on the Long Duration Exposure Facility (LDEF). It is therefore important to understand the atomic oxygen erosion yield (E, the volume loss per incident oxygen atom) of polymers being considered in spacecraft design. Because long-term space exposure data is rare and very costly, short-term exposures such as on the shuttle are often relied upon for atomic oxygen erosion determination. The most common technique for determining E is through mass loss measurements. For limited duration exposure experiments, such as shuttle experiments, the atomic oxygen fluence is often so small that mass loss measurements can not produce acceptable uncertainties. Therefore, a recession measurement technique has been developed using selective protection of polymer samples, combined with post-flight atomic force microscopy (AFM) analysis, to obtain accurate erosion yields of polymers exposed to low atomic oxygen fluences. This paper discusses the procedures used for this recession depth technique along with relevant characterization issues. In particular, a polymer is salt-sprayed prior to flight, then the salt is washed off post-flight and AFM is used to determine the erosion depth from the protected plateau. A small sample was salt-sprayed for AFM erosion depth analysis and flown as part of the Limited Duration Candidate Exposure (LDCE-4,-5) shuttle flight experiment on STS-51. This sample was used to study issues such as use of contact versus non-contact mode imaging for determining recession depth measurements. Error analyses were conducted and the percent probable error in the erosion yield when obtained by the mass loss and recession depth techniques has been compared. The recession depth technique is planned to be used to determine the erosion yield of 42 different polymers in the shuttle flight experiment PEACE (Polymer Erosion And Contamination Experiment) planned to fly in 2002 or 2003.

[horizontal rule]

MISSE PEACE Polymers: an International Space Station Environmental Exposure Experiment

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.

[horizontal rule]

Zr-Containing 4,4’-ODA/PMDA Polyimide Composites

The objective of this research is to improve the atomic oxygen resistance of KaptonTM, a polyimide (PI) made from pyromellitic acid dianhydride (PMDA) and 4,4’-oxydianiline (ODA), while retaining or enhancing the desirable properties of the pure polymer. Toward this end, zirconium-containing complexes and polymers were used to make composites and blends. Tetra(acetylacetonato)zirconium(IV), Zr(acac)4, which is commercially available, was identified as the best zirconium-containing complex for enhancing the atomic oxygen resistance of polyimide composites of the 10 complexes screened. Films prepared from the commercially-available polyamic acid (PAA) of PMDA-ODA (DuPont) have good uniformity, flexibility, and tensile strength. A 24-layer 10% (mol) Zr(acac)4/PI composite film showed significant improvement (ca. 20 fold) of atomic oxygen resistance over the pure polyimide. However, 10% (mol) Zr(acac)4 represents an upper concentration limit, above which films undergo cracking upon thermal imidization. In order to increase the Zr complex concentration in PMDA-ODA PI films, while retaining good film properties, [Zr(adsp)2-PMDA]n coordination polymer [bis(4-amino-N,N’-disalicylidene-1,2-phenylenediamino)zirconium(IV)-pyromellitic dianhydride] and [Zr(adsp)2-PMDA-ODA-PMDA]n terpolymer were synthesized and blended with commercial PAA, respectively. Several techniques were used to characterize the films made from the polymer containing Zr(acac)4. Plasma studies of films having 2% (mol) incremental concentrations of Zr in the Kapton up to 10% (mol) show that the overall rate of erosion is reduced about 75 percent.

[horizontal rule]

Low Temperature Testing of a Radiation Hardened CMOS 8-Bit Flash Analog-to-Digital (A/D) Converter

Power processing electronic systems, data acquiring probes, and signal conditioning circuits are required to operate reliably under harsh environments in many of NASA’s missions. The environment of the space mission as well as the operational requirements of some of the electronic systems, such as infrared-based satellite of telescopic observation stations where cryogenics are involved, dictate the utilization of electronics that can operate efficiently and reliably at low temperatures. In this work, radiation-hard CMOS 8-bit flash A/D converters were characterized in terms of voltage conversion and offset in the temperature range of +25 to –190 °C. Static and dynamic supply currents, ladder resistance, and gain and offset errors were also obtained in the temperature range of +125 to –190 °C. The effect of thermal cycling on these properties for a total of ten cycles between +80 and -150 °C was also determined. The experimental procedure along with the data obtained are reported and discussed in the paper.

[horizontal rule]

Modeling of Transmittance Degradation Caused by Optical Surface Contamination by Atomic Oxygen Reaction With Adsorbed Silicones

A numerical procedure is presented to calculate transmittance degradation caused by contaminant films on spacecraft surfaces produced though the interaction of orbital atomic oxygen (AO) with volatile silicones and hydrocarbons from spacecraft component. In the model, contaminant accretion is dependent on the adsorption of species, depletion reactions due to gas-surface collisions, desorption, and surface reactions between AO and silicone producing SiOx (where x is near 2). A detailed description of the procedure used to calculate the constituents of the contaminant layer is presented, including the equations that govern the evolution of fractional coverage by specie type. As an illustrative example of film growth, calculation results using a prototype code that calculates the evolution of surface coverage by specie type is presented and discussed. An example of the transmittance degradation caused by surface interaction of AO with deposited contaminant is presented for the case of exponentially decaying contaminant flux. These examples are performed using hypothetical values for the process parameters.

[horizontal rule]

Can Hydrocarbon Chains be Disrupted by Fast O(3P) Atoms?

O(3P) is a highly reactive species which may cause damage to materials on contact. In low Earth orbit (LEO) high energy collisions (~4.5 eV) of O(3P) with spacecraft materials can lead to extensive degradation. In this study we use ab initio molecular orbital calculations to investigate the possibility of chain breaking in polyethylene caused by a single O(3P) attack under LEO conditions, since the occurrence of such reactions could greatly accelerate the erosion. The smallest alkanes (N=2,3,5,7) serve as models of polyethylene. For the case of ethane (N=2), we explore the triplet potential energy surface of the following reaction:  O(3P) + CH3 ~ CH3 –> •O  ~ CH3 + •CH3. Analogous reactions, where O(3P) attacks a central carbon atom, are studied for the higher alkanes. Results obtained using the Hartree-Fock method, density functional theory, and, in the simplest case, i.e., ethane, second-order MØller-Plesset perturbation theory, Gaussian theoretical models (G1, G2, and G2MP2), and complete basis set (CBS-QB3) approach are reported. We conclude that conditions in LEO are conductive to chain breaking in polyethylene caused by a single O(3P) attack.

[horizontal rule]

Prediction and Measurement of the Atomic Oxygen Erosion Yield of Polymers in Low Earth Orbital Flight

Recently developed models of the erosion of polymeric materials by AO in low Earth orbit (LEO) have been used for predictive evaluation of the erosion resistance in LEO for a representative, comprehensive set of polymers. The established correlations of erosion yield values with the chemical composition and structure of hydrocarbon polymers, and with their flammability have been used for predictive evaluation of the behavior of those materials in LEO. Among these materials, a variety of aromatic and aliphatic hydrocarbon polymers, including homopolymers, copolymers, and terpolymers, have been considered. Predictive estimates have also been given for linear-chain fluoro- and fluoro-chloropolymers. With different degrees of fluorination, using a recently modified version of the predictive model, and the results were in good agreement with the flight data, that exist to date. Altogether, predictive evaluations have been performed for more than 40 polymers, including a few recommended materials for which the lower and higher extremes in erosion yield in LEO can be expected, based on their chemical composition and structure.

For almost half of the selected materials, there is no date from neither space nor ground-based experimental testing. For the rest, the data was collected mostly from the Long Duration Exposure Facility (LDEF) and several other flight experiments. The predicted erosion yield values ReLEO were found to be, mostly, in good agreement with the flight data for materials, already tested in LEO. A reasonable agreement between the two mentioned above predictive correlations, i.e., the one related to the chemical composition and structure of the materials, and the one related to the material’s flammability was found for ReLEO (pred) for the majority of untested materials.

A low Earth orbital space experiment entitled “Polymers Erosion and Contamination Experiment,” has been designed and is planned to allow measurement of the atomic oxygen erosion yield of a set of 40 different polymeric materials, whose erosion yields were predicted as described above. This will allow direct comparison between predicted and measured in-space atomic oxygen erosion yield. The experiment is a Get-Away-Special (GAS can) experiment to be conducted in the Shuttle bay that allows atomic oxygen to impinge on two sets of the 40 types of polymers. One set of polymer samples will be analyzed later, using weigh loss to measure atomic oxygen erosion yields, and the other set will be evaluated using erosion depth to measure atomic oxygen erosion yield. Erosion depth will be measured by means of salt or mica flake particles on the polymer surfaces to act on local protective coatings, which will result in the production of step height changes that are measured by atomic force microscopy. Using this latter technique, erosion yield measurements with uncertainties of ~3% can be achieved for typical polymers with atomic oxygen fluences of ~5 x 1019 atoms/cm2.

[horizontal rule]

The Dependence of Atomic Oxygen Undercutting of Protected Kapton® H Upon Defect Size

Understanding the behavior of polymeric materials when exposed to the low-Earth-orbit (LEO) environment is important in predicting performance characteristics such as in-space durability. Atomic oxygen (AO) present in LEO is known to be the principle agent in causing undercutting erosion of SiOx protected polyimide Kapton® H film, which serves as a mechanically stable blanket material in solar arrays. The rate of undercutting is dependent on the rate of arrival, directionality and energy of the AO with respect to the film surface. The erosion rate also depends on the distribution of the size of defects existing in the protective coating. This paper presents results of experimental ground testing using low energy, isotropic AO flux together with numerical modeling to determine the dependence of undercutting erosion upon defect size.

[horizontal rule]

International Test Program for Synergistic Atomic Oxygen and VUV Exposure of Spacecraft Materials

Spacecraft in low Earth orbit (LEO) are subject to degradation in thermal and optical performance of components and materials through interaction with atomic oxygen and vacuum ultraviolet radiation which are predominant in LEO. Due to the importance of LEO durability and performance to manufacturers and users, an international test program for assessing the durability of spacecraft materials and components was initiated. Initial tests consisted of exposure of samples representing a variety of thermal control paints and multiplayer insulation materials that have been used in space. Materials donated from various international sources were tested alongside a material whose performance is well known such as Teflon FEP or Kapton H for multiplayer insulation, or Z-93-P for white thermal control paints. The optical, thermal or mass loss data generated during the test was then provided to the participating material supplier. Data was not published unless the participant donating the material consented to the publication. This paper presents a description of the types of test and facilities that have been used for the test program as well as some examples of data that have been generated. The test program is intended to give spacecraft builders and users a better understanding of degradation processes and effects to enable improved prediction of spacecraft performance.

[horizontal rule]

Consequences of Atomic Oxygen Interaction with Silicone and Silicone Contamination on Surfaces in Low Earth Orbit

The exposure of silicones to atomic oxygen in low Earth orbit causes oxidation of the surface, resulting in conversion of silicone to silica. This chemical conversion increases the elastic modulus to the surface and initiates the development of a tensile strain. Ultimately, with sufficient exposure, tensile strain leads to cracking of the surface enabling the underlying unexposed silicone to be converted to silica resulting in additional depth and extent of cracking. The use of silicone coatings for the protection of materials from atomic oxygen attack is limited because of the eventual exposure of underlying unprotected polymeric material due to deep tensile stress cracking of the oxidized silicone. The use of moderate to high volatility silicones in low Earth orbit has resulted in a silicone contamination arrival at surfaces which are simultaneously being bombarded with atomic oxygen, thus leading to conversion of the silicone contaminant to silica. As a result of these processes, a gradual accumulation of contamination occurs leading to deposits, which at times have been up to several microns thick (as in the case of a Mir solar array after 10 years in space). The contamination species typically consist of silicon, oxygen, and carbon, which in the synergistic environment of atomic oxygen and UV radiation leads to increased solar absorptance and reduced solar transmittance. A comparison of the results of atomic oxygen interaction with silicones and silicone contamination will be presented based on the LDEF, EOIM-III, Offeq-3 spacecraft and Mir solar array in-space results. The design of a contamination pin-hole camera space experiment, which uses atomic oxygen to produce an image of the sources of silicone contamination, will also be presented.

[horizontal rule]

A Space Experiment to Measure the Atomic Oxygen Erosion of Polymers and Demonstrate a Technique to Identify Sources of Silicone Contamination

A low Earth orbital space experiment entitled, "Polymers Erosion and Contamination Experiment" (PEACE) has been designed as a Get-Away Special (GAS Can) experiment to be accommodated as a Shuttle in-bay environmental exposure experiment. The first objective is to measure the atomic oxygen erosion yields of ~40 different polymeric materials by mass loss and erosion measurements using atomic force microscopy. The second objective is to evaluate the capability of identifying sources of silicone contamination through the use of a pin-hole contamination camera, which utilizes environmental atomic oxygen to produce a contaminant source image on an optical substrate.

[horizontal rule]

Investigation of Atomic Oxygen Erosion of Polyimide Kapton H Exposed to a Plasma Asher Environment

Experimental results are presented on the erosion characteristics of the polyimide Kapton H, which serves as a blanket material in solar arrays. This polymer has a number of characteristics that make it a suitable choice for both terrestrial and space applications. In this paper attention is focused on the durability of protected Kapton hen exposed to atomic oxygen (AO) in a plasma asher. A strip of 0.025-mm thick Kapton film, coated on both sides with SiO2, was studied during a 1360-hour exposure. The erosion, located at defect sites in the protective coating and measured optically, is described in terms of volume loss as a function of AO fluence. Three simple geometric profiles are used to generate a useful array of cavity shapes to model erosion evolution. These models connect the volume erosion rate to the observed lateral expansion of the developing cavities via their diameters, measured adjacent to the upper and lower protective film, and fitted by least-squares regression to simple power law functions of fluence. The rationale for the choice of models is discussed. It was found later that growth in cavity size evolves less than linearly with fluence.

[horizontal rule]

Atomic Oxygen Durability of Graphite Epoxy Composite Silver Mirrors for Space Power Applications

Two light-weight graphite epoxy composite mirrors, each having a silver reflective layer and a silicon dioxide protection layer, were exposed to two levels of atomic oxygen fluence in a ground-based plasma asher facility for the purpose of evaluating their atomic oxygen durability. Total reflectivity and specular reflectivity were monitored during the course of atomic oxygen exposure. Optical microscope photographs were also taken during the course of exposure to document the process of atomic oxygen undercutting at pin window defect sites. Although there was evidence of atomic oxygen undercutting at pin window defects sites, functional performance of the mirrors remained fair over the course of atomic oxygen exposure.

[horizontal rule]

Monte Carlo Computational Modeling of the Energy Dependence of Atomic Oxygen Undercutting of Protected Polymers

A Monte Carlo computational model has been developed which simulates atomic oxygen attack of protected polymers at defect sites in the protective coatings. The parameters defining how atomic oxygen interacts with polymers and protective coatings as well as the scattering process which occur have been optimized to replicate experimental results observed from protected polyimide Kapton on the Long Duration Exposure Facility (LDEF) mission. Computational prediction of atomic oxygen undercutting at defect sites in protective coatings for various arrival energies was investigated. The atomic oxygen undercutting energy dependence predictions enable one to predict mass loss that would occur in low Earth orbit, based on lower energy ground laboratory atomic oxygen beam systems. Results of computational model prediction of undercut cavity size as a function of energy and defect size will be presented to provide insight into expected in-space mass loss of protected polymers with protective coating defects based on lower energy ground laboratory testing.

[horizontal rule]

Atomic Oxygen Durability of Second Surface Silver Microsheet Glass Concentrators

Second surface silver microsheet glass concentrators are being developed for potential use in future solar dynamic space power systems. Traditional concentrators are aluminum honeycomb sandwich composites with either aluminum or graphite epoxy face sheets, where a reflective aluminum layer is deposited onto an organic leveling layer on the face sheet. To protect the underlying layers, a SiO2 layer is applied on top of the aluminum reflective layer. These concentrators may be vulnerable to atomic oxygen degradation due to possible atomic oxygen attack of the organic layers at defect sites in the protective and reflective coatings. A second surface microsheet glass concentrator would be inherently more atomic oxygen durable than these first surface concentrators. In addition, a second surface microsheet glass concentrator design provides a smooth optical surface and allows for silver to be used as a reflective layer, which would improve the reflectivity of the concentrator and the performance of the system. A potential threat to the performance of second surface microsheet glass concentrators is atomic oxygen attack of the underlying silver at seams and edges or at micrometeoroid and debris (MMD) impact sites. Second surface silver microsheet glass concentrator samples were fabricated and tested for atomic oxygen durability. The samples were initially exposed to an atomic oxygen environment in a plasma asher. Samples were evaluated for potential degradation at fabrication seams, simulated MMD impact sites, and edges. Optical microscopy was used to evaluate atomic oxygen degradation. Reflectance was obtained for an impacted sample prior to and after atomic oxygen exposure. After an initial atomic oxygen exposure to an effective fluence of » 1 x 1021 atoms/cm², oxidation of that silver at defect sites and edges was observed. Exposure to an additional » 1 x 1021 atoms/cm² caused no observed increase in oxidation. Oxidation at an impact site caused negligible changes in reflectance. In all cases oxidation was found to be confined to the vicinity of the seams, impact sites, edges, or defect sites. Asher to in-space atomic oxygen correlation issues will be addressed.

[horizontal rule]

Atomic Oxygen Undercutting of Long Duration Exposure facility Aluminized Kapton Multilayer Insulation

Atomic oxygen undercutting is a potential threat to vulnerable spacecraft materials which have atomic oxygen protective coatings. Such undercutting is due to the atomic oxygen attack of oxidized materials at microscopic defects in the protective coatings. These defects occur during fabrication and handling, or from micrometeoroid and debris bombardment in space. An aluminized-polyimide Kapton multi-layer insulation sample that was located on the leading edge of the Long Duration Exposure Facility has been used to study low Earth orbit atomic oxygen undercutting. Cracks in the aluminized coating located around vent holes provided excellent defect sites for the evaluation of atomic oxygen undercutting. The experimentally observed undercut profiles were compared to predictions from Monte Carlo models for normal incident space ram atomic oxygen attack. The shape of the undercut profile was found to vary with crack width, which is proportional to the number of oxygen atoms entering the crack. The resulting profiles of atomic oxygen undercutting which occurred on the aluminized-Kapton sample indicated wide undercut cavities in spite of the fixed ram orientation. Potential causes of the observed undercutting are presented. Implications of the undercutting profiles relevant to Space Station Freedom are also discussed.

[horizontal rule]

Determination of Atomic Oxygen Fluence Using Spectrophotometric Analysis of Infrared Transparent Witness Coupons for Long Duration Exposure Tests

Atomic oxygen degradation is one of the several major threats to the durability of spaceborn systems in low Earth orbit. Ground-based simulations are conducted to learn how to minimize the adverse effects of atomic oxygen exposure. Assessing the fluence of atomic oxygen chambers such as the plasma asher over long periods of time is necessary for accurate determination of atomic oxygen exposure. Currently, an atomic oxygen susceptible organic material such as Kapton is placed next to samples as a witness coupon and its mass loss is monitored and used to determine the effective atomic oxygen fluence. However, degradation of the Kapton witness coupons occurs so rapidly in plasma ashers that for any long term test many witness coupons must be used sequentially in order to keep track of the fluence. This necessitates opening vacuum to substitute fresh coupons. A passive dosimetry technique was sought to monitor atomic oxygen exposure over longer periods without the need to open the plasma asher to the atmosphere. This paper investigates the use of spectrophotometric analysis of durable IR transparent witness coupons to measure atomic oxygen exposure for longer duration testing. The method considered would be conductive to making in situ measurements of atomic oxygen fluence.

[horizontal rule]

Space Station Freedom Solar Array Blanket Coverlay Atomic Oxygen Durability Testing Results

The power system for the Space Station Freedom used a flexible solar array for photovolatic power generation. Support for the solar cells and current carriers on the flexible array is provided by the solar array blanket. The main structural member of the array blanket is the coverlay (laminate), which is composed of Kapton, fiberglass scrim cloth and silicone adhesive. The anti-solar facing side of the laminate is protected from the atomic oxygen environment with a thin film coating of silicone dioxide. Coated Kapton and laminate samples were exposed to simulated atomic oxygen environments (plasma asher and directed beam) to determine whether the coated Kapton is durable and the degree to which the coating is damaged by the lamination process. Test results indicated that the mass loss relative to unprotected Kapton (relative reactivity) for the laminate was roughly a factor of 10 higher than for the coated Kapton possibly due in part to an increase in the number of scratches in the coating. This increase is probably due to handling during the lamination process. These results were not dependant on whether the exposure was performed in the plasma asher or the directed beam. Although atomic oxygen at thermal energies can produce results which are pessimistic indicators of in space durability, the data indicates that if surface scratching of the coating is limited and the coated Kapton is adherent to the underlying silicone, the laminate should survive for its desired lifetime of 15 years.

[horizontal rule]

Atomic Oxygen Durability Evaluation of Protected Polymers Using Thermal Energy Plasma Systems

The durability evaluation of protected polymers intended for use in low Earth orbit (LEO) has necessitated the use of large-area, high-fluence, atomic oxygen exposure systems. Two thermal energy atomic oxygen exposure systems which are frequently used for such evaluations are radio frequency (RF) plasma ashers and electron cyclotron resonance plasma sources. Plasma source testing practices such as sample preparation, effective fluence prediction, atomic oxygen flux determination, erosion measurement, operational considerations, and erosion yield measurements are presented. Issues which influence the prediction of in-space durability based on ground laboratory thermal energy plasma system testing are also addressed.

[horizontal rule]

Atomic Oxygen Durability Testing of an International Space Station Solar Array Validation Coupon

An International Space Station solar array validation coupon was exposed in a directed atomic oxygen beam for space environment durability testing. At the NASA Glenn Research Center. Exposure to atomic oxygen and intermittent tensioning of the solar array were conducted to verify the solar array's durability to low Earth orbital atomic oxygen and to the docking threat of plume loading both of which are anticipated over its expected mission life of fifteen years. The validation coupon was mounted on a specially designed rotisserie. The rotisserie mounting enabled the solar and anti-solar facing side of the array to be exposed to directed atomic oxygen in a sweeping arrival process replicating space exposure. The rotisserie mounting also enabled tensioning, in order to examine the durability of the array and its hinge to simulated plume loads. Flash testing to verify electrical performance of the solar array was performed with a solar simulator before and after the exposure to atomic oxygen and tensile loading. Results of the flash testing indicated little or no degradation in the solar array's performance. Photographs were also taken of the array before and after the durability testing and are included along with along with comparisons and discussions in this report. The amount of atomic oxygen damage appeared minor with the exception of a very few isolated defects. There was also no indications that the simulated plume loadings had weakened or damaged the array, even though there was some erosion of Kapton due to atomic oxygen attack. Based on the results of this testing, it is apparent that the International Space Station's solar array should survive the low Earth orbital atomic oxygen environment and docking threats which are anticipated over its expected mission life.

[horizontal rule]

Atomic Oxygen Interaction at Defect Sites in Protective Coatings on Polymers Flown on LDEF

Although the Long Duration Exposure Facility (LDEF) had exposed materials with a fixed orientation relative to the ambient low-Earth-orbital environment, arrival of atomic oxygen is angularly distributed as a result of the atomic oxygen's high temperature Maxwellian velocity distribution and the LDEF's orbital inclination. Thus, atomic oxygen entering defects in protective coatings on polymeric surfaces can cause wider undercut cavities than the size of the defect in the protective coating. Because only a small fraction of atomic oxygen reacts upon first impact with most polymeric materials, secondary reactions with lower energy thermally accommodated atomic oxygen can occur. The secondary reactions of scattered and/or thermally accommodated atomic oxygen also contribute to widening the undercut cavity beneath the protective coating defect. As the undercut cavity enlarges, exposing more polymer, the probability of atomic oxygen reacting with underlying polymeric material increases because of multiple opportunities for reaction. Thus, the effective atomic oxygen erosion yield for atoms entering defects above that of the unprotected material. Based on the results of analytical modeling and computational modeling, aluminized Kapton multilayer insulation exposed to atomic oxygen on row 9 lost the entire externally exposed player of polyimide Kapton, yet based on the results of this investigation, the bottom surface aluminum film must have remained in place, but crazed. Atomic oxygen undercutting at defect sites in protective coatings on graphite epoxy composites indicates that between 40 to 100 percent of the atomic oxygen thermally accommodates upon impact, and that the reaction probability of thermally accommodated atomic oxygen may range from 1.1x10-6 to 2.1x10-3, depending upon the degree of thermal accommodation upon each impact.

[horizontal rule]

Atomic Oxygen Interactions with Protected Organic Materials on the Long Duration Exposure Facility

The Long Duration Exposure Facility (LDEF) has provided an excellent opportunity to understand the nature of directed atomic oxygen interactions with protected polymers and composites. Although there were relatively few samples of materials with protective coatings on their external surfaces on LDEF which were exposed to a high atomic oxygen fluence, analysis of such samples has enabled an examination of the shape of atomic oxygen undercut cavities at defect sites in the protective coatings. Samples of front-surface aluminized (Kapton®) polyimide were inspected by scanning electron microscopy to identify and measure crack defects in the aluminum protective coatings. After chemical removal of the aluminum coating, measurements were also made of the width of the oxidized undercut cavities below the crack defects. The LDEF flight undercut cavity geometries were then compared to the Monte Carlo computational model undercut cavity predictions. The comparison of the LDEF results and computational modeling indicates agreement in specific undercut cavity geometries for atomic oxygen reaction probabilities dependant upon the 0.68 to 3.0 power if the energy. However, no single energy dependency was adequate to replicate flight results over a variety of aluminum crack widths.

[horizontal rule]

Atomic Oxygen Erosion Phenomena

The surface textures resulting from directed atomic oxygen interaction with materials which produce fully volatile oxidation products are similar to those produced by more energetic physical sputter texturing. A Monte Carlo computational model has been developed which simulated both low Earth orbital energetic atomic oxygen attack as well as isotropic thermal energy plasma atomic oxygen interactions with materials with volatile oxides. The surface roughening predicted by the model agrees with experimental observations, indicating that surface texture develops under the simplest interaction assumptions and grows in a less than linear manner with atomic oxygen fluence. The more paraxial the atomic oxygen arrival is, the greater the surface roughness for the same atomic oxygen fluence. The detailed nature of the scattering interactions appears to play a negligible role in the development of surface roughness.

[horizontal rule]

Monte Carlo Computational Modeling for Simulation of Atomic Oxygen Interactions with Composites at Defect Sites in Protective Coatings

Spacecraft orbiting the earth at altitudes below 500 kilometers are exposed to the remnants of the earth's upper atmosphere. This low Earth orbital (LEO) environment consists predominantly of atomic oxygen caused by photo-dissociation of O2 by ultraviolet radiation from the sun. Organic matrix carbon fiber composite materials exposed to this environment are oxidized at a rate which would limit the durability of many spacecraft components. As a result, atomic oxygen protective coatings consisting of metals and metal oxides are being used to protect materials from oxidation degradation in LEO. The use of Monte Carlo computational modeling to simulate the effects of atomic oxygen undercutting oxidation of composite materials both in the ground laboratory and in space can greatly assist in improving the ability to project in-space durability testing. This modeling was used to test coating materials for performance in LEO.

[horizontal rule]

Plasma and Beam Facility Atomic Oxygen Erosion of a Transition Metal Complex

Glassy residues of the complex bis (N, N1-disalicylidene-1, 2-phenylenediamino) zirconium (IV), Zr (dsp)2, on glass slides were exposed to atomic oxygen in a plasma asher or an atomic beam facility for various amounts of time in order to study the erosion process, determine the rate of erosion, and learn the chemical identity of the residue. The exposed films were characterized by weight loss, optical photography, profilometry, diffuse reflectance and total transmittance spectroscopy, scanning electron microscopy (SEM) with wavelength dispersive x-ray spectrometry (WDS), x-ray diffraction , and x-ray photoelectron spectroscopy (XPS). Results indicate that these films erode much more slowly polyimide (Kapton®) film under identical conditions, that the erosion is very non-uniform, and that the zirconium dioxide is the predominant product after extended exposure. This complex is currently being evaluated as a polymer additive.

[horizontal rule]

Leveling Coatings for Reducing Atomic Oxygen Defect Density in Graphite Fiber-Epoxy Composites

Pinholes or other defect sites in a protective oxide coating provide pathways for atomic oxygen in low Earth orbit to reach underlying material. One concept for enhancing the lifetime of materials in low Earth orbit is to apply a leveling coating to the material prior to the material prior to applying any reflective and protective coatings. Using a surface-tension-leveling coating concept, a low-viscosity epoxy was applied to the surface of several composite coupons. A protective layer of 1000 Å of SiO2 was deposited on top of the leveling coating, and the coupons were exposed to an atomic oxygen environment in a plasma asher. Pinhole populations per unit area were estimated by counting the number of undercut sites observed by scanning electron microscopy. Defect density values of 180,000 defects/cm² were reduced to about 1000 defects/cm² as a result of applied leveling coating. These improvements occur at a mass penalty of about 2.5 mg/cm².

[horizontal rule]

Prediction of In-Space Durability of Protected Polymers Based on Ground Laboratory Thermal Energy Atomic Oxygen

The probability of atomic oxygen reacting with polymeric materials is orders of magnitude lower at thermal energies (<0.1 eV) than at orbital impact energies (4.5 eV). As a result, absolute atomic oxygen fluxes at thermal energies must be orders of magnitude higher than orbital energy fluxes, to produce the same effective fluxes (or same oxidation rates) for polymers. These differences can cause highly pessimistic durability predictions for protected polymers, and polymers which develop protective metal oxide surfaces as a result of oxidation if one does not make suitable calibrations. A comparison was conducted of undercut cavities below defect sites in protected polyimide Kapton samples flown on the Long Duration Exposure Facility (LDEF) with similar samples exposed in thermal energy oxygen plasma. The results of this comparison were used to quantify predicted material loss in space based on material loss in ground laboratory thermal energy plasma testing. A microindent hardness comparison of surface oxidation of a silicone flown on the Environmental Oxygen Interaction with Materials III (EOIM-III) experiment with samples exposed in thermal energy plasmas was similarly used to calibrate the rate of oxidation of silicone in space relative to samples in thermal energy plasmas exposed to polyimide Kapton effective fluences.

[horizontal rule]

Monte Carlo Computational Techniques for Prediction of Atomic Oxygen Erosion of Materials

Materials on the surface of spacecraft in low Earth orbit (LEO) are exposed to the remnants of the Earth's upper atmosphere. Energetic solar photons cause photodissociation of O2 to produce highly reactive atomic oxygen. As spacecraft orbit through the atomic oxygen, impact energies of 4.5± 1 eV result with an arrival flux sufficient to cause polymeric materials to be oxidized at rates high enough durability concerns. To increase materials durability adequate to meet spacecraft mission lifetime requirements, atomic oxygen protective coatings have been applied over polymers. Such coatings typically consist of metal oxide thin films. The durability of such protected polymers used for solar array blankets and thermal control is limited as a result of microscopic defects in the protective films.

[horizontal rule]

Recovery of a Charred Painting Using Atomic Oxygen Treatment

A non-contact method is described which uses atomic oxygen to remove soot and char from the surface of a painting. The atomic oxygen was generated by the dissociation of oxygen in low-pressure air using radio frequency energy. The treatment, which is an oxidation process, allows control of the amount of material to be removed. The effectiveness of char removal from half of a fire-damaged oil painting was studied using reflected light measurements from selected areas of the painting and by visual and photographic observation. The atomic oxygen was able to effectively remove char and soot from the treated half of the painting. The remaining loosely bound pigment was lightly sprayed with a mist to replace the binder and then varnish was reapplied. Caution should be used when treating an untested paint medium using atomic oxygen. A representative edge or corner should be tested first in order to determine if the process would be safe for the pigments present. As more testing occurs, a greater knowledge base will be developed as to what types of paints and varnishes can or cannot be treated using this technique. With proper precautions, atomic oxygen treatment does appear to be a technique with great potential for allowing charred, previously unrestorable art to be salvaged.

[horizontal rule]

Atomic Oxygen Treatment as a Method of Recovering Smoke Damaged Paintings

Smoke damage, as a result of fire, can be difficult to remove from some types of painting media without causing swelling, leaching, or pigment movement or removal. A non-contact technique has been developed which can remove soot from the surface of a painting by use of a gently flowing gas containing atomic oxygen. The atomic oxygen chemically reacts with the soot on the surface creating gasses such as carbon monoxide and carbon dioxide which can be removed through the use of an exhaust system. The reaction is limited to the surface so that the process can be timed to stop when the paint layer is reached. Atomic oxygen is a primary component of the low Earth orbital environment, but it can be generated on Earth through various methods. This paper will discuss the results of atomic oxygen treatment of soot exposed acrylic gesso, ink on paper, and a varnished oil painting. Reflectance measurements were used to characterize the surfaces before and after treatment.

[horizontal rule]

An Atmospheric Atomic Oxygen Source for Cleaning Smoke Damaged Art Objects

Soot and other carbonaceous combustion products deposited on the surfaces of porous ceramic, stone, ivory, and paper can be difficult to remove and can have potentially unsatisfactory results using wet chemical and/or abrasive cleaning techniques. An atomic oxygen source which operates in air at atmospheric pressure, using a mixture of oxygen and helium, has been developed to produce an atomic oxygen beam which is highly effective in oxidizing soot deposit on surfaces by burning candles made of paraffin, oil, or rendered animal fat. Atomic oxygen source operating conditions and the results of cleaning soot from paper, gesso, ivory, limestone, and water color-painted limestone are presented.

[horizontal rule]

A Comparison of Atomic Oxygen Erosion Yields of Carbon and Selected Polymers Exposed in Ground Based Facilities and in Low Earth Orbit

A comparison of the relative erosion yields (volume of material removed per oxygen atom arriving) for FEP Teflon, polyethylene, and pyrolytic graphite with respect to Kapton HN was performed in an atomic oxygen directed beam system, in a plasma asher, and in space on the EOIM-III (Evaluation of Oxygen Interaction with Materials-III) flight experiment. This comparison was performed to determine the sensitivity of material reaction to atomic oxygen flux, atomic oxygen fluence, and vacuum ultraviolet radiation for enabling accurate estimates of durability in ground based facilities. The relative erosion yield of pyrolytic graphite was found not to be sensitive to these factors that for FEP was sensitive slightly to fluence and possibly ions, and that for polyethylene was found to be partially VUV and flux sensitive but more sensitive to an unknown factor. Results indicate that the ability to use these facilities for material relative durability prediction is great as long as the sensitivity of particular materials to conditions such as VUV, and atomic oxygen flux and fluence are taken into account. When testing materials of a particular group such as Teflon, it may be best to use a witness sample made of a similar material that has some available space data on it. This would enable one to predict an equivalent exposure in the ground based facility.

[horizontal rule]

Evaluation of Space Power Materials Flown on the Passive Optical Sample Assembly

Evaluating the performance of materials on the exterior of spacecraft id of continuing interest, particularly in anticipation of those applications that will require a long duration in low Earth orbit. The Passive Optical Sample Assembly (POSA) experiment flown on the exterior of Mir as a risk mitigation experiment for the International Space Station was designed to better understand the interaction of materials with the low Earth orbit environment and to better understand the potential contamination threats that may be present in the vicinity of spacecraft. Deterioration in the optical performance of candidate space power materials due to the low Earth orbit environment, the contamination environment, or both, must be evaluated in order to propose measures to mitigate such deterioration. The thirty-two samples of space power materials studied here include solar array blanket materials such as polyimide Kapton H and SiOx coated polyimide Kapton H, front surface aluminized sapphire, solar dynamic concentrator materials such as silver on spin coated polyimide and aluminum on spin coated polyimide, CV1144 silicone, and the thermal control paint Z-93-P. The physical and optical properties that were evaluated prior to and after the POSA flight include mass, total, diffuse, and specular reflectance, solar absorptance, and infrared emittance. Additional post flight evaluation included scanning electron microscopy to observe surface features caused by the low Earth orbit environment and the contamination environment, and variable angle spectroscopic ellipsometry to identify contaminant type and thickness. This paper summarizes the results of pre- and post-flight measurements, identifies the mechanisms responsible for optical properties deterioration, and suggests improvements for the durability of materials in future missions.

[horizontal rule]

Environmental Durability Issues for Solar Power Systems in Low Earth Orbit

Space solar power systems for use in low Earth orbit (LEO) environment experience a variety of harsh environmental conditions. Materials used for solar power generation in LEO need to be durable to environmental threats such as atomic oxygen, ultraviolet (UV) radiation, thermal cycling, and micrometeoroid and debris impact. Another threat to LEO solar power performance is due to contamination from other spacecraft components. This paper gives an overview of these LEO environmental issues as they relate to space solar power system materials. Issues addressed include atomic oxygen erosion of organic materials, atomic oxygen undercutting of protective coatings, UV darkening of ceramics, UV embrittlement of Teflon, effects of thermal cycling on organic composites, and contamination due to silicone and organic materials. Specific examples of samples from the Long Duration Exposure Facility (LDEF) and materials returned from the first servicing mission of the Hubble Space Telescope (HST) are presented. Issues concerning ground laboratory facilities which simulate the LEO environment are discussed along with ground-to-space correlation issues.

[horizontal rule]

Atomic Oxygen/Vacuum Ultraviolet Radiation Exposure of Z-93 and Z-93-P Coatings

Laboratory testing was conducted in order to assess the long-term atomic oxygen and vacuum ultraviolet radiation durability of the thermal control coating Z-93-P to be used on the International Space Station radiator surfaces. This testing provided atomic oxygen equivalent to approximately four years and vacuum ultraviolet radiation equivalent to approximately twenty-five years on Space Station radiator surfaces. Solar absorptance data were obtained in vacuo at various exposure increments. Facility limitations resulted in the inability to provide the appropriate atomic oxygen to vacuum ultraviolet radiation ratio that would be experienced by Space Station radiator surfaces, and unexpected sputtering of components in the vacuum chamber caused a contaminant layer to be deposited on the samples. However, some conclusions can be made from the data. First, Z-93-P samples performed comparably to the Z-93 control sample assuring that the successful flight history of the original Z-93 formulation can be applied to the reformulated Z-93-P coating. Second, solar absorptance increases of no more than 0.1 were calculated for the combined atomic oxygen and vacuum ultraviolet radiation exposure environment used in this test.

[horizontal rule]

A Technique for Synergistic Atomic Oxygen and Vacuum Ultraviolet Radiation Durability Evaluation of Materials for use in LEO

Material erosion data collected during flight experiments such as the Environmental Oxygen Interaction with Materials (EOIM)-III and the Long Duration Exposure Facility (LDEF) have raised questions as to the sensitivity of materials erosion to levels of atomic oxygen exposure and vacuum ultraviolet (VUV) radiation. The erosion sensitivity of some materials such as FEP Teflon used as a thermal control material on satellites in low Earth orbit (LEO), is particularly important but difficult to determine. This is in large part due to the inability to hold all but one exposure parameter constant during a flight experiment. This is also difficult to perform in a ground based facility, because often the variation of the level of atomic oxygen or VUV radiation also results in a change in the level of the other parameter. A facility has been developed which allows each parameter to be changed almost independently and offer broad area exposure. The resulting samples can be made large enough for mechanical testing. The facility uses an electron cyclotron resonance plasma source to provide the atomic oxygen. A series of glass plates is used to focus the atomic oxygen while filtering the VUV radiation from the plasma source. After filtering, atomic oxygen effective flux levels can still be measured which are as high as 7x1015 atoms/cm²-sec which is adequate for accelerated testing. VUV radiation levels after filtering can be as low as 0.3 suns. Additional VUV suns can be added with the use of deuterium lamps which allow the VUV level to be changed while keeping the flux of atomic oxygen constant. This paper discusses the facility, and the results from exposure of Kapton and FEP at pre-determined atomic oxygen flux and VUV sun levels.

[horizontal rule]

Modify Surfaces with Ions and Arcs

NASA originally conducted research in the field of electron bombardment because the technology involves generation of high-velocity ions, which have the potential to produce much higher propellant exhaust velocities for spacecraft than chemical propulsion. As a consequence, considerable data were collected about the effects of ion beams on a wide range of materials. Based on this information, researchers designed specialized surface modification techniques such as ion beam sputter texturing, etching, and simultaneous deposition and etching. Arc-texturing technology was developed as a result of research on high-thermal-emittance radiators. In this process, an electric arc is formed between a carbon or silicone-carbide electrode and a moving metal surface, resulting in durable, microscopically rough surfaces that emit heat more efficiently than coated materials. Atomic-oxygen texturing is a by-product of studies about the effects of atomic oxygen on the surfaces of spacecraft. The purpose of the original research was to find coatings that could withstand atomic-oxygen attack, but it evolved into deliberate bombardment of polymeric materials to increase thermal emittance or reduce coefficient of friction.

[horizontal rule]

Optical Property Enhancement and Durability Evaluation of Heat and Receiver Aperture Shield Materials

Solar Dynamic (SD) power systems have been investigated by the National Aeronautics and Space Administration (NASA) for electrical power generation in space. As part of the International Space Station (ISS) program, NASA Glenn Research Center (GRC) teamed with the Russian Space Agency (RSA) to build a SD system to be flown on the Russian Space Station MIR. Under the US/Russian SD Flight Demonstration (SDFD) program, GRC worked with AlliedSignal Aerospace, the heat receiver contractor, on the development, characterization, and durability testing of materials to obtain appropriate optical and thermal properties for the SDFD heat receiver aperture shield. The aperture shield is composed of refractory metal multi-foil insulation (MFI) attached to an aperture back plate. Because of anticipated off-pointing periods, the aperture shield was designed to withstand the extreme temperatures that 80 W/cm² would produce. To minimize the temperature that the aperture shield will reach during off-pointing, it was desired for the aperture shield exterior layer to have a solar absorptance (a s) to thermal emittance (e ) ratio as small as possible. In addition, a very low specular reflectance (r s < 0.1) was also necessary, because reflected concentrated sunlight could cause overheating of the concentrator which is undesirable. Testing was conducted at GRC to evaluate pristine and optical property enhanced molybdenum and tungsten foils and screen covered foils. Molybdenum and tungsten foils were grit-blasted using silicon carbide or alumina grit under various grit-blasting conditions for optical property enhancement. Black rhenium coated tungsten foil was also evaluated. Tungsten, black rhenium-coated tungsten, and grit-blasted tungsten screens of various mesh sizes were placed over the pristine and grit-blasted foils for optical property characterization. Grit-blasting was found to be effective in decreasing the specular reflectance and absorptance/emittance ratio of the refractory foils. The placement of a screen further enhanced these optical properties, with a grit-blasted screen over a grit-blasted foil producing the best results. Based on the optical property enhancement results, samples were tested for atomic oxygen (AO) and vacuum heat treatment (VHT) durability. Grit-blasted (Al2O3 grit) 2 mil tungsten foil was chosen for the exterior layer of the SDFD heat receiver shield. A 0.007 in. diameter, 20x20 mesh tungsten screen was chosen to cover the tungsten foil. Based on these test results, a heat receiver aperture shield test unit has been built by Aerospace Design and Development (A.D.D.) with the screen covered grit-blast tungsten foil exterior layers. The aperture shield was tested in GRC's Solar Dynamic Ground Test Demonstration (SDGTD) system to verify the thermal and structural durability of the outer foil layers during an off-pointing period.

[horizontal rule]

Advances in Optical Property Measurements of Spacecraft Materials

This report describes some of the instruments and experimental approaches available for measuring optical properties of thermal control materials. It also describes the instruments' uses in laboratory studies of the effects of combined contaminants and the space environment on these materials, and in the qualification of hardware for spacecraft. In recent years, several instruments for measurement of solar absorptance (a ) and infrared emittance (e ) have been introduced. These instruments offer improved speed, accuracy, and data-handling, all of which substantially improve the study of contaminated thermal control materials. A transient method for directly measuring material e is also described, and the results are compared with other instruments. In addition, our understanding of oxygen exposure effects on the ( of materials following contamination or exposure to simulated space conditions shows that oxygen exposure before measuring of e should be avoided.

[horizontal rule]

Indium Tin Oxide-Magnesium Fluoride Co-Deposited Films for Spacecraft Applications

Highly transparent coatings with a maximum sheet resistivity between 108 and 109 ohms/square are desired to prevent charging of solar arrays for low Earth polar orbit and geosynchronous orbit missions. Indium tin oxide (ITO) and magnesium fluoride (MgF2) were ion beam sputter co-deposited onto fused silica substrates and were evaluated for transmittance, sheet resistivity and the effects of simulated space environments including atomic oxygen (AO) and vacuum ultraviolet (VUV) radiation. Optical properties and sheet resistivity as a function of MgF2 content in the films will be presented. Films containing 8.4 wt.% MgF2 were found to be highly transparent and provided sheet resistivity in the required range. These films maintained a high transmittance upon exposure to AO and to VUV radiation, although exposure to AO in the presence of charged species and intense electromagnetic radiation cause significant degradation in film transmittance. Sheet resistivity of the as-fabricated films increased with time in ambient conditions. Vacuum heat treatment following film deposition caused a reduction in sheet resistivity. However, following heat resistivity values remained stable during storage in ambient conditions.

[horizontal rule]

Performance and Properties of Atomic Oxygen Protective Coatings for Polymeric Materials

Polymeric materials intended for use on spacecraft surfaces in low Earth orbit need protective coatings to prevent oxidation resulting from reaction with environmental atomic oxygen.  The effectiveness of atomic oxygen protective coatings relies upon the inherent atomic oxygen durability of the coating itself, and the number and area of scratch and pin window defects in the protective coating.  Highly effective coatings result in protected polymer oxidation mass losses which are a very small fraction of that of unprotected materials.  Such coatings are required for high atomic oxygen fluence missions such as Space Station Freedom. Typically, SiOx (where 1.9 < X < 2.0) coatings have been shown to be effective atomic oxygen protection.  This paper will present the results of various protective and/or electrically conductive coatings, including germanium, SiOx, and indium tin oxide, which have been exposed to atomic oxygen in RF plasma ashers and compares the results with state-of-the-art SiOx coatings.  Resulting protected polymeric material mass loss, electrical conductivity, and optical properties dependence upon atomic oxygen exposure are presented. 
 
Home ] Up ] Facilities ] Technologies ] Publications ] [ Patents ] Contact Us ] Site Map ]
[Space Processes and Experiments Division]
[Privacy Policy and Important Notices]
Curator:  Sandra.A.Zolo@nasa.gov and NASA Official Responsible For Content: Sharon.K.Miller@nasa.gov 
* an asterisk indicates an external link
Last Updated: 02/04/2014