Examples of Degradation from the Degradation of International Space Station, Low Earth Orbit flight Experiments, and the Hubble Space Telescope The ISS was one of the first examples of Spacecraft designing for long-term durability in the low Earth orbit (LEO) environment. Early shuttle missions experienced texturing and thinning of polyimide Kapton and other Materials in the carbo~ntaining materials from exposure in LEO.I The primary reason for the degra dation was found to be oxidation by the atomic oxygen in LEO.I At the time, poly Space Environment imide Kapton H. a specific type of Kapton. was under consideration as the prime can didate for the solar array support structure for ISS. Kapton was attractive for this appli Sharon K.R. Miller and Bruce Banks cation because the solar cclIs needed to be bonded to a flexible and strong surface that was lightweight and had the right thermal properties, so the array could be stowed Abstract and unfolded like a sail when installed on When we think of space. we typically thInk of a vacuum contaIning wry lI\11e mailer the ISS. Kapton H atomic oxygen erosion that lies between the Earth and other planetary and stellar bodies. However. the space data from shuttle flight information caused abolle Earth's breathable atmosphere and beyond contaIns many thIngs that make concern for the survivability of Kapton for designing dural:te spacecraft a challenge. DependIng on where the spacecraft is flyfng. it use as a solar array blanket.2 High levels of may encounter atomic oxygen. ullJavidet and other forms of radiation. charged particles. erosion for Kapton also were observed on mIcrorneteoro!ds and debris. and temperatura extremes. lhese envlronmants on their the Long Duration Exposure Facility + opwainn tas nadn idn octohmerb minaaltelornta clsa uns ceadu osne tdheeg Jeaxdtealriioorn oafn sdp afacielucrrea fot ff opro Ityhmeramrsa.l ccoomntprools. ites, o(lfD eExFp)e reixmpeenritms elnatu,n ac hfreede -fflryoimng aconldle clatitoenr + structure. and power generation. This article brlefty discusses and gives examples of retrieved by the Space Shuttle to look at the some of the degradallon experienced on spacecraft and night experiments as a resun of space environment and its effect on a vari the space environment and the use of ground and space data to predict durability. ety of materials (see the Edwards et aI. arti cle in this issue).l FlglllV 1 shows on-orbit photos of a Kapton b1anket-covered panel on LDEF at deployment and after 69 months of space exposure, showing that the outer layer of Kapton on the blanket is Introduction missing because it had been totally oxi Each mission into space poses its own retrievable flight hardware. Our first dized into gaseous products.J Materials challenges with regard to spacecraft detailed knowledge of the Earth's orbital that are already in their highest oxidation durability. Materials that are used on the environment effects on spacecraft and state such as silicon dioxide and aluminum outside of spacecraft are chosen to serve associated hardware, especially that oxide, however. do not react with the a specific function to enable the space caused by atomic oxygen, came from the atomic oxygen in the LEO environment.~ craft to regulate heat, generate power. or early Shuttle missions.1 What we learn Since Kapton had aU of the desired proper support other important components. about other planetary and galactic envi ties for use on ISS other than its interaction Each mission is unique in not only the ronments will most likely also be through with the environment, different types of types of materials that are selected for direct experience. Most of our focus thin-film metal oxide coatings applied by use but also the environment that they to date has been associated with the physical vapor deposition were tried as a will encounter during flight. Spacecraft environment above Earth's breathable protective barrier.~ Metal oxide coatings designers must take into account not atmosphere where the International proved to be very effective barriers, but only the physical and mechanical proper Space Station (ISS), Space Shuttle. where there were defects in the coating ties of the materials with regard to their Hubble Space Telescope (HST), and satel caused by surface imperfections or dust intended function but also what the lites operate, but our knowledge of this particles or scratches from handling post expected properties will be at the end of environment and experience in develop coating, the atomic oxygen could stUl reach the mission due to degrildation by the ing flight experiments. ground tests, and the Kapton and create areas of undercut environment to which they will be correlations will enable a better predic ting on the surface.' Undercutting. such as subjected. Because spacecraft are used tion of performance in other environ that shown in 1;lgUIl'::! from LDEF, does not often for exploration and discovery to ments beyond Earth. This article focuses pose too much of a problem for maintain unknown environments, the first indica on some of the degradation that has been ing durability unless there are so many tion of an environmental problem is usu observed in Earth's orbit and what space defects or scratches that the undercut areas ally through experiencing a change in craft designers have done and are doing can connect and cause sections of the poly hardware conditions or performance as to enable spacecraft to survive and to mer to be removL'<i or initiate tearing across observed through telemetry, system fail predict performance for future missions the surface of the polymer. The array blan ure, or observation of degradation on in these types of environments. ket that is on ISS currently is protected with MRS BULLETIN· VOLUME 35 • JANUARY 201 D• www.mlurglbullelin Degradation of Spacecraft Materials in the Space Environment a thin-film sputter-deposited silicon diox ide coating that has allowed the array to survive longer than the I5-year mission life for which it was originally designed, but it is important to note that there was substan tial ground testing and qualification that occurred prior to fabrication to ensure that the number of defects in the coating were low enough such that less than hall of the thickness of the array blanket would be removed in order to enable the blanket to survive for the desired mission lifetime.6.7 Unfortunately, not all hardware that is flown undergoes such careful testing. Sometimes assumptions are made that a solution that initially looks good will work without being tested due to a lack of time FIgure 1. Long Duration Exposure FacUlty tray F9. (a) Pre-flight photo and (b) on-orblt and funding to verily its performance photo taken during STS-32, showing complete loss of the outer Kaplon layer of the before flight. An example of this is the blanke\. 18 The aluminized layer Is still Intact. solar array blanket box cover on ISS. Much work went into the array blanket itself, but the thermal blanket covering the box in which the array blanket was stowed was not tested in the environment pre-flight. The thermal blanket was composed of polyimide Kapton, which was aluminized on both sides.S The assumption was made that since the Kapton already had an alu minum coating, which would serve as a + barrier to the atomic oxygen and react with I it to make aluminum oxide on the surface, t the thermal blanket would not degrade in the environment. Figure ;\.1 shows the damage caused by atomic oxygen on the blanket box cover after only a year of expo sure on ISS, while the expected lifetime was supposed to have been 15 years.8 Figure 2. Photos of aluminized Kaplon flown on the Long Duration Exposure Facility with the Evaporated aluminum coatings typically aluminum-coated side lacing the space environment. (a) Defect areas in the aluminum contain more defects than sputter coating and (b) the same sample with the aluminum coating chemically removed to show the deposited coatings, but the failure of the undercut cavities under the defect sites caused by atomic oxygen erosion of the Kapton.I9 blanket box cover was not so much caused by the number of coating defects but by the fact that the Kapton was coated on both sides.S It was found later through a combination of observation of samples flown on flight experiments and Monte CarlcH>ased computational modeling that Aluminized on both sicles atomic oxygen upon impact with a surface can react, recombine, or bounce off of the surface.s When it bounces off of the sur face, the energy of the atomic oxygen is reduced to thermal energy, but it is still Aluminized on exposed side only very reactive and free to react with another surface.8 In the case of the double alu minized Kapton blanket, the atomic oxy Figure 3. Damage and damage modeling of the solar array blanket box COIIIIr on the gen could enter a defect and react with International Space Station. (a) Photo showing the damage caused by atomic oxygen on the the Kapton until reaching the second alu Kapton (aluminum coating on both sides) blanket box COIIIIr after one year in orbit. (b) Monte Carlo modeling comparison 01 undercutting patterns for KaplOn coated with aluminum on minum coating underneath. At the surface both sides compared to a single layer aluminum protective coating on Kapton. of the aluminum coating, once the alu minum is oxidized, the atomic oxygen would no longer be able to react with it, so box cover, the atomic oxygen became bounce, trapped atomic oxygen eventually it could only bounce off of the surface to trapped between the two layers of alu resulted in the removal of most of the react with the surrounding Kapton or minum causing greater opportunity for Kapton to create two freHtanding alu recombine with other oxygen atoms. erosion of the Kapton between the alu minum films.8 If the aluminum barrier had Essentially, in the case of the ISS blanket minum layers. The erosion from multiple been only on the space-facing side of the 2 MRS BULLETIN. VOLUME 35 • JANUARY 2010 • www.ml1.alQl.bullalin Degradation of Spacecraft Materials in the Space Environment blanket, the atomic oxygen would have mined in a ground-based atomic oxygen reacted at defect sites to create a narrow exposure facility.II.U Ash content is impor channel to the back side and then would tant in the prediction because the ash pres have been able to scatter through the open ent in a material is the part that cannot be pore polyimide underneath rather than oxidized and can accumulate on the sur being trapped in the Kapton blanket face during exposure shielding the under where it could react further with the poly lying material from further oxidation. In mer.s This is illustrated by the Monte Carlo essence, it develops its own protective coat atomic oxygen undercutting modeling ing that can make the observed erosion results for both double-coated and single lower than what would be expected based coated Kapton shown in Figure 3b.8 Pre on the erosion of the bulk material. Because flight qualification testing could have most of the damage observed in LEO is avoided this degradation problem. from atomic oxygen attack, most of the Not all testing, however, is straightfor work to date has focused on atomic oxygen ward. Unfortunately. there is no easy way erosion. It is still not totally understood to exactly replicate the environment of what role the other environments play in space on Earth. There are physical limita material degradation, as some components tions as to what can be done to generate of the environment appear to playa syner atomic oxygen, vacuum ultraviolet (VUV) gistic role in degradation for some materi (100-200 nm) and near UV (200-400 nm) als but not for others. radiation, ionizing radiation, and thermal A good example of synergistic environ extremes in the exact conditions and pfOo ment effects is the severe on-orbit cracking portions as in space. So flight experiments, of the outer layer of the vapor-deposited such as those flown on the Space Shuttle,9 aluminum-coated fluorinated ethylene on free carriers such as IDEF,IO and on the propylene (FEP) Teflon MLI blanket outside of ISS,II are used to provide mate observed on the solar-facing side of the rial degradation information that can be HST during the second servicing mis Agure 4. Photograph of the Hubble used to help correlate what is happening in sion.13 l'igure ·1 shows the embrittled MLI Space Telescope on orbit during the space with what is occurring in ground blanket after 6.8 years in space. A review second servicing mission showing + based testing facilities. One of the experi board that investigated the severe FEP cracking and tearing of the multilayer + ments on the Materials International Spare degradation on HST concluded that elec Insulation after 6.8 years 01 space Station Experiment (MISSE) 2 and 4 was tron and proton radiation combined with exposure due to what is believed to be a combination 01 ionizing radiation, designed to measure the erosion of a coated on-orbit thermal cycling caused the ultlaviolet radiation, x-rays, and thermal polyimide in spare, compared to its erosion observed cracking of the MU at stress cycling. in a ground-based test facility to develop a concentrated locations and that damage corre1ation factor that could be used to increased with the combined total dose of more reliably predict the in-space durabil ionizing radiation, ultraviolet radiation, ity of coated polymers tested in a ground and x-rays with thermal cycling.13 The of VUV bond breakage of the deposited based system,ll The correlation factor that conclusion was that radiation-induced volatile silicone fragments, hydrocarbon was determined (-18 times more erosion chain scission was the primary mechanism deposition, and atomic oxygen conver for coated Kapton in a ground plasma sys of degradation, and the damage rate was sion of the contaminant surface layer to tem than in LEO) along with ground significantly affected by on-orbit tempe silica allowed the build-up of a darkened system testing of some of the Kapton mul rature. Although damage was observed contaminant layer. There is a light-col tilayer insulation (MU) blanket from HSr in accelerated ground-based exposures, ored "shadowed" region directly to the was used to predict the condition of the it did not simulate the extent of damage right of the samples. This is where MLI for Servicing Mission 4 (SM4).1I The observed on HST.1l Calibration of atomic oxygen arrival was shielded, and prediction that the MU would not be ground-based accelerated exposure using hence the deposited contaminant frag degraded to the extent that it would need space data is needed to obtain more ments were able to re-evaporate.u to be covered by the astronauts during HST accurate simulation of this effect of the Because a darkened contaminant layer SM4 allowed this task to be moved to the environment. can lead to significant changes in optical end of the schedule and listed as optional, Another example of a combined envi and thermal properties of the material on thereby freeing time for other more critical ronmental effect on materials, first which it is deposited, silicones used in tasks.1I It was observed on SM4 that the observed on LDEF, was the formation of space currently have to meet stricter condition of the Kapton MU was accept a darkened contaminant layer on sur guidelines with regard to the volatile able, so the task to cover this bay was per faces due to atomic oxygen and VUV condensable products that they give off formed as one of the last servicing tasks. radiation.14 Figure 5 shows a close-up of during heating and under vacuum so as Another experiment flown on MlSSE was a tray of materials that was 38· from the not to cause Significant power losses, designed to measure the erosion rates of 40 ram (or orbital ) direction on LDEF on changes in performance of optics, or different polymers and graphite to gain the Solar-Array Materials Passive exper overheating. better understanding of polymer erosion in iment.14 The light-colored samples (rec Combined environments of atomic oxy LEO and to develop a predictive tool to tangular, square, and tensile shaped) are gen and micrometeoroid and debris enable estimation of the erosion of new silicones that contained a lot of volatiles impacts also can lead to greater damage polymers under consideration for use in that, when in space, produced large than by either alone. Figure liil shows a space, based on knowing the chemical quantities of contaminants on neighbor photo of a silver-FEP Teflon blanket flown structure, density, and ash content deter- ing surfaces and samples. A combination on the ram-facing side of IDEF, showing MRS BULLETIN. VOLUME 35. JANUARY 2010. www.ml1.org/buIlBlin 3 $ Degradation of Spacecraft Materials in the Space Environment ':' ..... :,.. :': .';. ... : .. : ': ': .. ~ \,',=, ,.. •• •. ~ ~ .... ;&.;.._~. :~.~t.:,~ , . Figure 7. Impact crater on alumigold (chromate conversion coating) on aluminum lrom the Long Duration Exposure FaCility. Frgure 5. Section 01 the Long Duration Exposure Facility Solar Array Materials Passive Conclusions Experiment (tray AS) showing dark contemlnation to the right 01 the silicone (light gold This article provided a few examples of colored) rectengular and dog bone-shaped samples. degradation that have been observed on spacecraft as a result of the Earth orbital environment. The experience gained through observing damage on spacecraft and verification of the causes of damage through a combination of spaceflight + experiment, ground-based testing. and + modeling has led to better spacecraft design. In spite of the wealth of knowl edge that has been gained, there are still '. many unknowns with regard to the role each environment plays in the damage . t, '. and in the presence of, or lack of, synergis . tic effects with other components of the environment. What has been learned, however, has resulted in better methods of testing. evaluation, and analysis that are not only valuable for enabling design of more durable spacecraft for the Earth's orbital environment but also may provide guidance for durable spacecraft design for other environments beyond Earth. Figure 6. Long Duration Exposure Facility tray A1 0 showing micrometeorold and debris impacts. (a) On-orbit photo of tray A1 0 and (b) post-flight close-up photo of the impact site showing the impact hole, delamination area, and the darkened rings caused by oxidation of Acknowledgments the silver. The authors would like to thank Kim de Groh of NASA GRC for providing useful references and infonnation for this article. micrometeoroid and debris impacts_ The aluminum from LDEF. When micromete impacts were found to cause delamination oroids and debris impact spacecraft sur of the silver-FEP Teflon composite, which faces, they generally vaporize along with References then allowed atomic oxygen to enter the some of the material they impact, which 1. LJ. Leger, Oxygtll Atom Reaction with SImI/Ie impact hole to oxidize the underlying then can condense on the surface. This Mattrin/s at Orbital A1li1udts. NASA lM-58246 exposed silver. IS This resulted in the dark condensate and the condensed molten (1982). ened rings observed around the impact material created by the impact fonn a 2. A.F. Whitaker. paper presented at the AIAA holes. An example is shown in the close-up wave or splash pattern around the crater. Shuttle Environment and Operations Meeting. image in Figure 6b.15 LDEF provided a Fortunately, the size distribution and num Washington, DC,31 October-2 November 1983, AlAA-83-2632-CP. large collection of impact data that covered ber are currently low enough that the 3. R.L O'Neal, A.S. Levine, C.c. Kiser, a wide range of impact craters from below proper design (multiple spaced layers for PllOtograpllic Survey of tIlt LDEF Mission. NASA 0.Q1 mm (10 JUll) to 5.25 mm.I6.17 Fi);uI"L' i piping. multilayer shielding. or redundant SP 531 (1996). provides an example of an impact in alu systems) can reduce the damage that 4. BA Banks, M.J. Mirtlch. S.K. Rutledge, O.M. migold (a chromate conversion coating) on impacts can causc.1l Swec, H.K. Nahra. 23rd Aerospace Sciences 4 MRS BULLETIN. VOLUME 35 • JANUARY 2D1D • www.mrl.llrg/hulletln Degradation of Spacecraft Materials in the Space Environment Meeting. Reno, NV, 14-17 January 1985, NASA 10. LDEF·69 Mont/IS in SptlCt, First Post·Rrtritval 14. BA Banks, J.A. Dever, L Gebauer, C.M. TM-87051. Symptlsium, AS. Levine, Ed., NASA Conferenro Hill, in Proceedings of tIre LDEF·69 MOllt11S in 5. B.A. Banks, B.M. Auer, S.K Rutledge, C.M. Publication 3134, Parts 1-3, Kissimmee, Fl., 2-8 Sptlce. First Post·Retrieval Symptlsi'4m, NASA Cpo Hill, Proceedings of the 4tl. Annual WorksllOp June 1991. 3134, Part 2, 2-8 June 1991, pp. 801-815. on Sptlce Optrations Automntion and Robotics 11. KK. de Groh, B.A. Banks, J.A. Dever, D.A. 15. H.G. Pippin, Analysis of Silveriud Tejloll (SOAR 90), Albuquerque, NM, 26-28 June 1990. Jaworske, S.K Miller, E.A. Sechkar, S.R. Prutko, Tlrenrral eontrol Material FIOWII on tIre LolIg 6. S.K Rutledge, RM. DUe, Proceedings of tire NASA Glenn Researc/. CfI.ter·s Materials Dltratioll Exposure Facility, NASA CR-4663 38tl. Intmtlltional SAMPE Symposium. 1()"'13 Intmtlltional SptlCf Station Experiments (MISS" Uuly 1995). May 1993, pp. 679-#J3. l-7), Intmtlltional Symposium on SM/MPAC and 16. aM. silvermrut, SptlCt Environmental EJftds 7. M.J. Forkapa, c.R. Stidham. BA Banks, S.K. SEED Experiments (JAXA). Epochal Tsukuba, on SpactCrtlfl: LEO Materials Selection Guidt. Rutledge, D.H. Ma, E.A. Sechkar, Third Japan, 1()"'11 March 2008, NASA TM·2008- NASA CR-4661 (August 1995). International Conference on Protection of 215482. 17. T.See,M. Allbrooks, D. Atkinson, C. Simon, Materials and Structures from the low Earth 12. B.A. Banks. JA Backus, M.V. Manno, D.L M. Zolensky. uMeteoroid and Debris Impacts Orbit Space Environment. Toronto. Canada. Walef5, KC. Cameron, K.K. de Groh, in Atomic Features Documented on the Long Duration 2S-26 April 1996. NASA TM·I07212. O:tygrn Erosion Yreld PrrJidion for Sprurmtfl Exposure Facility, (Preliminary Report U 8. B.A. Banks, R. Demko, Atomic Oxygm Polymers in Low Eartl. Orbit, 11 th International Compiled by Members of the LDEF Meteoroid Protmion of Materials in l.ow fATth Orbit, NASA Symposium on Materials in the Space rutd Debris specia1lnvestigation Group, NASA TM·2002·211360 (February 2002). Environment (ISMSE·ll). 15-18 September JSC 24608, August 1990). 9. B.A. Banks, K.K. de Groh, S.K. Rutledge, 2009. 18. R.L. O'Neal, A.S. Levine, c.c. Kiser, C.A. Haylas, 44th Annual Meeting by the 13. J. Dever, B. Banks, K. de Groh, S. Miller, Photagraphic Survey of the LDU Mission, NASA International Society of Optical Engineering. in Handbook of Environmental Degrodation SP·531 (1996). Denver, CO, 21 July 1999, NASA TM·I999- of Materials, M. Ku~, Ed. (William Andrew 19. K.K. de Groh, B.A. Banks, J. Sptlcecr. Rockets 209179. PubUshing. Norwich, NY. 2005), pp. 465-501. 31 (4),656 (1994). 0 + + MRS BULLETIN. 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