Sharon K. Rutledge

Ion beam sputter‐deposited diamondlike films

A single argon ion beam source was used to sputter deposit carbon films on fused silica, copper and tantalum substrates under conditions of sputter deposition alone and sputter deposition combined with simultaneous argon ion bombardment. Simultaneously deposited and ion bombarded carbon films were prepared under conditions of carbon atom removal to arrival ratios of 0, 0.36, and 0.71. Deposition and etch rates were measured for films on fused silica substrates. Resulting characteristics of the deposited films are: electrical resistivity of ≳1011 Ω cm, densities of 2.1 g/cm3 for sputter‐deposited films and 2.2 g/cm3 for simultaneously sputter‐deposited and Ar ion‐bombarded films. For ∠1700‐A thick films deposited by either process and at 5550 A wavelength light the reflectance was 0.2, the absorptance was 0.7, the absorption coefficient was 6.7×104 cm−1, and the transmittance was 0.1.

Sputtered coatings for protection of spacecraft polymers

Abstract Kapton® polyimide oxidizes at significant rates (4.3 × 10 −24 g per incident oxygen atom) when exposed in low earth orbit to the ram atomic oxygen flux. Ion-beam-sputter-deposited thin films of Al 2 O 3 and SiO 2 as well as a codeposited mixture of predominantly SiO 2 with a small amount of polytetrafluoroethylene were evaluated and found to be effective in protecting Kapton from oxidation both in laboratory plasma ashing tests as well as in space on board shuttle flight STS-8. A protective film of 96% SiO 2 or more and 4% polytetrafluoroethylene or less was found to be very flexible compared with the pure metal oxide coatings and resulted in mass loss rates that were 0.2% of that of the unprotected Kapton. The optical properties of Kapton for wavelengths investigated between 0.33 and 2.2 μm were not significantly altered by the presence of the coatings or changed by exposure of the coated Kapton to the low earth orbital ram environment.

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.

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 simulates 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 hi the development of surface roughness.

Atomic Oxygen Treatment as a Method of Recovering Smoke Damaged Paintings

Abstract The noncontact technique that is described uses atomic oxygen, generated under low pressure in the presence of nitrogen, to remove soot and charred varnish from the surface of a painting. The process, which involves surface oxidation, permits control of the amount of surface material removed. The effectiveness of the process was evaluated by reflectance measurements from selected areas taken during the removal of soot from acrylic gesso, ink on paper, and varnished oil paint substrates. For the latter substrate, treatment also involved the removal of damaged varnish and paint binder from the surface.

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.

A Comparison of Space- and Ground-Based Facility Environmental Effects for Fep Teflon

Fluorinated Ethylene Propylene (PEP) Teflon™ is widely used as a thermal control material for spacecraft. However, it is susceptible to erosion, cracking, and subsequent mechanical failure in low Earth orbit (LEO). One of the difficulties in determining whether FEP Teflon will survive during a mission is the wide disparity of erosion rates observed for this material in space and in ground-based facilities. Each environment contains different levels of atomic oxygen, ions, and vacuum ultraviolet (VUV) radiation, in addition to parameters such as the energy of the arriving species and temperature. These variations make it difficult to determine what is causing the observed differences in erosion rates.

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 Lewis 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.

Plasma and beam facility atomic oxygen erosion of a transition metal complex

Glassy residues of the complex bis(N,N′-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 lime 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 nonuniform, and that zirconium dioxide is the predominant product after extended exposure. This complex is currently being evaluated as a polymer additive.

Atomic Oxygen Protective Coatings

Atomic oxygen resident in low Earth orbit (LEO) impinges upon orbiting spacecraft such as Space Station Freedom (SSF) with sufficient flux to cause rapid oxidation and premature failure of organic spacecraft materials. Protective coatings consisting of metal oxides, fluoropolymerfilled metal oxides, and silicones can be used to minimize the reaction of atomic oxygen with organic materials. Such protective coatings are necessary for the long-term durability of polymeric films such as polyimide Kapton solar array blankets and other oxidizable materials. Defects in atomic oxygen protective coatings can enable atomic oxygen to react and oxidize the underlying polymeric material. The number and area of atomic oxygen defects is dependent upon surface irregularities, contamination during protective coating deposition, flexure or abrasion during materials processing, and micrometeoroid or debris impact in space. A combination of ground-based LEO simulation testing, in-space experiments, and Monte Carlo modeling have been utilized to forecast degradation modes of atomic oxygen protected materials exposed to sweeping atomic oxygen arrival conditions such as will occur on SSF.