Titles:

• Fast Three-Dimensional Modeling of Atomic Oxygen Undercutting of Protected Ploymers  May-June 2004
  

• Fast Three-Dimensional Method of Modeling Atomic Oxygen Undercutting of Protected Polymers  May 2002
   
• Modeling of Transmittance Degradation Caused by Optical Surface Contamination by Atomic Oxygen Reaction With Adsorbed Silicones  June 2001
   
• Prediction and Measurement of the Atomic Oxygen Erosion Yield of Polymers in Low Earth Orbital Flight  June 2000
   
• The Dependence of Atomic Oxygen Undercutting of Protected Kapton® H Upon Defect Size  June 2000
   
• Monte Carlo Computational Modeling of the Energy Dependence of Atomic Oxygen Undercutting of Protected Polymers  April 1998
   
• Atomic Oxygen Erosion Phenomena  September 1997
   
• Monte Carlo Computational Techniques for Prediction of Atomic Oxygen Erosion of Materials  May 1996
   
• Monte Carlo Computational Modeling for Simulation of Atomic Oxygen Interactions with Composites at Defect Sites in Protective Coatings  August 1995
   
• Atomic Oxygen Interaction at Defect Sites in Protective Coatings on Polymers Flown on LDEF  1994
   
• Atomic Oxygen Durability Evaluation of Protected Polymers Using Thermal Energy Plasma Systems  February 1993
   
• Atomic Oxygen Interactions with Protected Organic Materials on the Long Duration Exposure Facility (LDEF)  1993

Snyder, Aaron and Banks, Bruce, ““Fast Three-Dimensional Modeling of Atomic Oxygen Undercutting of Protected Polymers”, Journal of Spacecraft and Rockets, Vol. 41, Number 3, pp. 340-344, May-June 2004

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.

Snyder, A., Banks, B. A., “Fast Three-Dimensional Method of Modeling Atomic Oxygen Undercutting of Protected Polymers,” presented at the Sixth International Conference on Protection of Materials and Structures from the Space Environment, NASA TM-2002-211578, Toronto, Canada, May 1-3, 2002.

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.
 

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.

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.

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.

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 dependance 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.

Banks, B. A., Stueber, T. J., Snyder, S. A., Rutledge, S. R., and Norris, M. J., "Atomic Oxygen Erosion Phenomena", presented at the American Institute of Aeronautics and Astronautics Defense and Space Programs Conference, Huntsville, Alabama, September 23-25, 1997.

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.

Banks, B. A. and Stueber, T. J., "Monte Carlo Computational Techniques for Prediction of Atomic Oxygen Erosion of Materials", presented at the NATO Advanced Research Workshop on Computer Modeling of Electronic and Atomic Processes in Solids, Wroclaw, Poland, May 20-23, 1996.

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.

Banks, B. A., Stueber, T. J., and Cales, M. R., "Monte Carlo Computational Modeling for Simulation of Atomic Oxygen Interactions with Composites at Defect Sites in Protective Coatings", proceedings of the Second International Conference on Compostites Engineering, New Orleans, Louisiana, August 21-24, 1995.

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.

Banks, B. A., de Groh, K. K., Auer, B. M., Gebauer, L., and LaMouraux, C., "Atomic Oxygen Interaction at Defect Sites in Protective Coatings on Polymers Flown on LDEF", LDEF Materials Results for Spacecraft Applications, NASA CP-3257, 1994, p.143-158.

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.
 

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.

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.

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