Air-Induced Recovery of Proton-Exposed Space Materials

T HE fact that materials degrade in space-radiation environments has been studied for decades [1–19] and continues to be an issue of significant importance for space-based systems. Consequently, ground-based simulation of space environmental effects remains an invaluable method for ensuring the performance of any orbital asset, particularly as newmaterials, coatings, and processes are introduced. High-fidelity simulation of space environmental effects requires accurate definition and modeling of the orbital mission exposure, appropriate calculation of the absorbed radiation dose as a function of material thickness, proper simulation of this dose-depth profile in the laboratory, and precise characterization of test-induced changes in material properties. When performing material characterization, in vacuomeasurements are absolutely essential to avoid atmospheric effects on tested materials. Failure to do this can lead to results that are skewed by the too-often-overlooked dynamic of air-induced recovery of radiation-induced degradation, also known as bleaching. For some materials, ignoring the effects of bleaching can easily lead to a significant under estimation of radiation-induced losses and an inaccurate prediction of on-orbit performance. Bleaching is known to occur primarily for thermal control materials [20], and recent work by The Aerospace Corporation has added to the evidence that somematerials, which have been degraded from exposure to simulated space environments, display air-induced recovery behavior. In particular, samples of silverized Teflon, aluminized Kapton, and Z-93P ceramic white paint that were exposed to simultaneous ultraviolet (UV) and electron radiation in a test designed to simulate the conditions present at geosynchronous Earth orbit (GEO) exhibited noticeable bleaching upon subsequent exposure to air. In each case, a portion of the test-induced optical losses was recovered after a brief period ( 2 min) of air exposure. The behavior is material-specific, exhibiting differences in affected spectral region and magnitude of change. The recovery can occur rapidly or over long timescales, with the rate of recovery appearing to depend on material porosity and/or ambient temperature. The spectral response of the three materials is exemplified in Fig. 1. Although air-induced bleaching of optical degradation in some thermal control materials is relatively common, and the effect is not typically observed for materials that are more resistant to the effects of space-radiation exposure, such as solar-cell coverglass.Additional data generated by The Aerospace Corporation, however, indicated that certain coated coverglass materials, namely, those with thin-film coatings of magnesium fluoride (MgF2) and indium tin oxide (ITO), might also be susceptible to bleaching. In another recent GEO simulation, samples of MgF2/ITO-coated 0214 coverglass (0214 refers to the manufacturer-specific type of coverglass substrate) exhibited slight, yet discernible, air-induced recovery of spectral transmittance following simultaneous exposure to UV and electron radiation. Figure 2 shows the material’s spectral response to a short ( 2 min) postirradiation air exposure. Primarily, because of facility limitations, proton exposures at The Aerospace Corporation historically have been performed, usually precedingUV/electron irradiation, in a separate exposure facility: the low-energy accelerator facility (LEAF). The LEAF has a vacuum chamber attached to a linear ion accelerator; the accelerator’s beam is passed through a 30 deg magnetic turn to select the appropriate ion species, and the beam is rastered across the exposure target. The LEAF lacks in vacuo optical measurement capability; thus, air exposure is unavoidable when characterizing proton-induced material changes. Because of this, it is unknown whether, or to what degree, bleaching occurs for proton-irradiated coverglass materials, although it seems reasonable to suspect that somematerials degraded by proton exposure would also exhibit air-induced recovery. Given the apparent bleaching ofUV/electron-induced degradation in MgF2/ITO-coated coverglass and the lack of in vacuo optical measurement capability of the LEAF, this research effort was undertaken to investigate the air-induced response of protonirradiated space materials. Making this possible is The Aerospace Corporation’s newest space environmental effects exposure facility, which had already possessed in vacuo optical measurement capability and now features a variable-energy proton flood gun. This recently enhanced facility makes possible measurements of protonirradiated materials that are free from atmospheric effects and allows Received 21 September 2011; revision received 5 December 2011; accepted for publication 9 January 2012. Copyright © 2012 by The Aerospace Corporation. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. All rights reserved. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0022-4650/12 and $10.00 in correspondence with the CCC. Research Scientist, Materials Science Department, Space Materials Laboratory, 2310 East El Segundo Boulevard, M2-248. Distinguished Scientist, Space Materials Laboratory, 2310 East El Segundo Boulevard, M2-248. JOURNAL OF SPACECRAFT AND ROCKETS Vol. 49, No. 4, July–August 2012