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|The following was taken from Analysis of Materials Flown on the Long Duration Exposure Facility: Summary of Results of the Materials Special Investigation Group, NASA CR, 1995.|
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Extremely thin, patchy films of silicon-based contamination were distributed all around the circumference of LDEF. On leading edge surfaces only, it was found that silicones had been oxidized into silicates by the atomic oxygen. However, analysis of LDEF tray clamp bolt heads from a variety of exterior LDEF locations, including the leading edge, revealed a molecular film that contained silicone. Since this silicone film had not reacted with atomic oxygen to form silicates, it would have had to have been deposited after the surfaces were shielded from atomic oxygen, such as would have occurred when LDEF was re-berthed in the Shuttle. In addition, the silicone found on the leading edge locations had to originate from non-LDEF surfaces as all LDEF surfaces were thoroughly outgassed by the time LDEF was retrieved. Any small amounts outgassed toward the end of the mission would have been quickly oxidized by the high flux of atomic oxygen at the end of the mission.
LDEF results are mostly from postflight analysis from hardware which underwent exposure to a variety of conditions. The on-orbit effects must be separated from ground, launch and reentry effects. An on-orbit spacecraft will not experience the turbulence of reentry, with its potential for redistributing or removing particluate contaminants, the postflight re-adsorption of moisture, deposition of a ground contamination layer, and/or changes in thermal control paints due to reaction with molecular oxygen in air, as did LDEF. Such postflight processes are all artifacts which may disguise processes which occurred on-orbit and are significant for satellite performance. Preflight ground exposures and the launch environment are also important because these environments may affect the in-service operations of a spacecraft. The solutions available for ground-based problems could be quite different than for problems induced by the space environment.
Particulate contaminants were identified from preflight exposure sources, from Space Shuttle sources, from on-orbit material degradation, and from post-flight exposures. Except for introduction of sources of stray light scattering on optical materials, small particulates had no effect on materials performance. Where large-scale failure of blanket materials occurred, with subsequent distribution of small particulates, effects on nearby surfaces were often significant. The part of the black radiator panel on tray F09 which was covered by the failed aluminum backing from an adjacent thermal blanket underwent large color changes. Lexan exposed by failure of the adhesive tape on Experiment M0001 and subsequent flexing of the blanket material was severely discolored.
Failed blanket materials from trays D09 and D10 distributed small particles of aluminum over many nearby surfaces. It is hard to quantify the effects of these occurrences because the time of failure is not known, and the particle distribution probably changed with time.
The consideration of molecular contaminant films should be separated into what happens to carbon-based (organic) material and what happens to silicon-containing material. Organics exposed to atomic oxygen are removed rather rapidly. Exposure to atomic oxygen will cause silicone surfaces to oxidize to silicates. The carbon-based functional groups of the silicones are easily oxidized and removed by abstraction processes, leaving the Si-O portion of the polymer chain. Subsequent oxygen atoms add to the Si-O chains, producing a glassy, non-volatile surface. Silicone remaining trapped beneath the surface will darken under UV exposure.
Within the consideration of each material type there are the questions of defining the sources; their quantity, location, and thermal/environmental exposure history. Deposition surfaces; their temperature, and their locations relative to the sources, must be identified. Post-deposition processes must be characterized relative to their exposure. Investigation of contaminant deposits on LDEF is a continuing activity.
Silicon-Based Molecular Contamination
Collectively, there were many sources of silicon contamination on LDEF. Some areas were contaminated prior to flight, silicone-based materials outgassed during flight, and the Space Shuttle was a potential source during both deployment and retrieval. Post-flight contamination must also be considered. Each previous source is in addition to the processes which occurred during the 69 months in orbit.
In addition to silicone-based coatings and adhesives used on several experiments, a number of other materials used have silicon present as a basic constituent, or left from the manufacturing process. The S13 type and A276 paints contain silicon, the stainless steel bolts have 1 to 2% silicon by weight in the alloy, and the copper grounding straps have a silicone release film. These facts mask the attempt to evaluate silicon contamination on these particular surfaces. The silverized Teflon® (Ag/FEP) and chromic acid anodized surfaces, which together covered >78% of the spacecraft exposed surfaces, are materials which contain no silicon. These materials are excellent witness plates when attempting to determine the quantity of silicon deposited.
The gasket seals used on the ground handling tray covers were preflight sources of silicones and compromised certain areas of the anodized tray surfaces. This material was not space qualified and outgassed extensively, with a total mass loss (TML) of over 3%. A solvent extraction process removed about 2.5 % (by weight) of material from a sample of gasket material, showing the presence of many potentially volatile species. The tray covers were on the experiment trays for an extended period of time prior to flight (months) allowing sufficient time for volatile species in the gasket material to diffuse to the surface, volatilize, and deposit on the (line-of-sight) adjacent external surfaces of the trays.
Postflight photos of external tray surfaces from leading edge trays (some with tray covers reinstalled so the close proximity is obvious) show contamination patterns, which are just the width of the gasket material, on the external tray lips and sides, at the tray corners where these surfaces were in direct line-of-sight to the gasket material. These patterns appear both on trays which were known to contain sources of silicone and on trays which were known not to contain silicone sources. The discoloration patterns show distinct orientation effects which correlate to their respective environmental exposures. Trailing edge locations do not show the same intense darkening, implying that the mechanism involves both solar exposure and atomic oxygen. Faintly discolored areas also appear along the tops of the tray lips where the gaskets were in contact with the tray surface prior to flight.
Electron Spectroscopy for Chemical Analysis (ESCA) measurements of surface silicon on a variety of materials, which did not originally contain silicon, show silicon present in amounts ranging from 0.1% (lower limit of detection) to over 30%. The amounts present are extremely exposure and location dependent. Interpretation of results are complicated by local conditions, i.e., multiple silicone sources within the same tray, vents, outgassing by adjacent specimens, and changes in exposure conditions over time (such as failed materials moving and covering or partially shielding surfaces). Tray clamp surface studies at Virginia Polytechnic Institute and State University generally showed larger percentages of silicon on leading edge clamps relative to trailing edge clamps. Clamps from space and Earth end locations also tended to be relatively high in silicon surface content.
Examination of multiple locations on an individual space end clamp (H6-11) and on an Earth end clamp (G6-5), which were each selected for close proximity to a vent, showed very high surface silicon content.
Light scattering measurements using the bi-directional reflectance distribution function (BRDF) were used on selected clamps to try to correlate BRDF measurements with silicon content. Light of a chosen wavelength is directed onto a surface at a given angle and the intensity of reflected light is measured at selected scattering angles. The locations of the ESCA measurements on clamps G6-5 and H6-11 were chosen to provide a sampling of visually different areas. Surface analysis showed that the range of silicon contents was not sufficient to develop a correlation. Bolt heads on the clamps also influenced the deposition patterns. Auger studies on selected specimens have all demonstrated that any contamination layer is extremely thin, 50 to 200 angstroms.
ESCA measurements on black chrome plated panels from the Earht end of LDEF indicated significant silicon content. Reported results from Experiments M0003 and AO133 show significant deposition of silicon due to space exposure. X-ray photoelectron spectra of Kapton from AO133, located on the space end, show 8.8% silicon on the surface. Control specimens for this experiment showed no silicon present.
Solar absorptance changes on fused silica mirrors on both the leading and trailing edges are relatively small (< 0.04 in all cases).
Carbon(organic)-Based Molecular Contamination
Characterization of individual material surfaces by ESCA gave an indication of the extent of carbon-based (organic) contamination on LDEF. In virtually every case where auger profiles were made, the thickness of the film was quite thin, a few hundred angstroms or less. Any material surface exposed to a ground environment, even in a clean room, will show some organic material (carbon) in an ESCA analysis. Those surfaces exposed to atomic oxygen showed relatively less carbon in the ESCA profiles than surfaces not exposed to atomic oxygen during flight. Preflight and on-orbit deposited carbon on leading edge surfaces were removed by oxidation. Only the postflight contributions were left on such surfaces. Examination of tray clamp stainless steel bolt heads shows very clearly the pattern of organic contamination around the LDEF. Bolts from locations between the rows 3 and 4 longeron to the longeron between row 5 and 6 show the highest percent carbon in their ESCA spectra. For all other locations, even those which received the minimal doses of atomic oxygen during the brief postretrieval attitude excursion, the percent carbon in the ESCA spectra is substantially lower.
Except for deposition thick enough to reduce transmission on optical windows and mirrors, organic-based deposits on ram facing surfaces should generally not be a problem. The possible exception is organics trapped under silicone deposits which could contribute to darkening by photoreactions with solar radiation. This possibility has not been thoroughly investigated using thin films from LDEF. For wake side surfaces, not exposed to atomic oxygen, all contaminant exposures are accumulated; preflight, on-orbit, and postflight. The brief atomic oxygen exposure during the post-retrieval attitude excursion was sufficient to remove most of the organic based contamination on surfaces not previously exposed to atomic oxygen.
Bi-Directional Reflectance Distribution Function (BRDF), Reflectance, and Surface Analysis Measurements on Selected Hardware
A portable, fixed wavelength bi-directional reflectance distribution function (BRDF) device was used to measure surface characteristics on selected tray clamps and an FEP surface from LDEF. An attempt was made to correlate the BRDF and reflectance measurements results with the amount of Si present on the surface, however, the silicon contamination levels did not vary enough to allow a correlation to be made.
1. E.R. Crutcher and W.W. Wascher, "Particle Types and Sources Associated with LDEF", NASA CP-3134, Part 1, Kissimmee, FL, June 2-8, 1991, pp 101-120.
2. E.R. Crutcher, L.S. Nishimura, K.J. Warner, and W.W. Wascher, "Migration and Generation of Contaminants from Launch through Recovery: LDEF Case History", LDEF-69 Months in Space: First LDEF Post-Retrieval Symposium, NASA CP-3134, Part 1, Kissimmee, FL, June 2-8, 1991, pp 121-140.
3. J.P. Wightman and H.L Grammer, Surface Characterization of LDEF Materials, Final Technical Report, NASA Contract # NAG-1-1186, September 1990 - October 1993.
4. H.G. Pippin, Effects of Space Exposure on Metals Flown on the Long Duration Exposure Facility, NASA Contractor Report, 1995.
5. M. Lee, W.D, Rooney, and J.B. Whiteside, "An XPS Study of Space-Exposed Polyimide Film", LDEF-69 Months in Space: Second LDEF Post-Retrieval Symposium, NASA CP-3194, Part 3, San Diego, CA, June 1-5, 1992, pp 957.
6. W.K. Stuckey, G. Radhakrishnan, and D. Wallace, "Post-Flight Analyses of the Crystals from the M0003-14 Quartz Crystal Microbalance Experiment", LDEF-69 Months in Space: Second LDEF Post-Retrieval Symposium, NASA CP-3194, Part 3, San Diego, CA, June 1-5, 1992, pp 1269.
7. W.K. Stuckey, "An Overview of the On-Orbit Contamination of the Long Duration Exposure Facility (LDEF)", LDEF Results for Spacecraft Applications, NASA CP-3257, Huntsville, AL, October 27-28, 1992, pp 533.