Does Silver Teflon Survive Beyond LEO? Lessons from JAXA’s ERG Mission

When the Japan Aerospace Exploration Agency (JAXA) began developing the ERG (Energization and Radiation in Geospace) satellite, they faced a daunting thermal challenge. Launched in 2016, ERG operates in a highly elliptical orbit (300 km to 30,000 km), repeatedly diving through the heart of the Van Allen radiation belts.

JAXA's ERG mission studies Earth's Van Allen Radiation Belts

Standard Silver Teflon (Ag/FEP) is the industry workhorse for thermal control, but its survival in high-radiation environments was a major question mark. Through a five-publication qualification campaign, JAXA answered the critical questions: How does it fail, what fails first, and what should we use instead?

1. Two Electron Energies, Two Distinct Failure Modes

During the initial development of ERG, Ag/FEP was adopted based on schedule and cost considerations. However, it was known that FEP is susceptible to damage from electron beams. Consequently, an electron beam irradiation test was conducted to verify whether the material possessed sufficient resistance to meet the requirements for ERG.

The ERG radiation environment contains electrons across a wide energy spectrum. The qualification program deliberately tested two regimes: 10 keV (low energy, short range) and 1 MeV (high energy, long range). Because electron penetration depth scales with energy, 10 keV and 1 MeV produce physically different failure modes in the Ag/FEP material stack.

10 keV — Surface-Only Degradation. At 10 keV, penetration depth is a few microns — sufficient to affect only the ITO conductivity coating and the outermost FEP surface, but unable to reach the adhesive layer beneath. Fluence was calibrated to represent approximately two years of ERG orbit, derived from Polar satellite flux observations extrapolated to the ERG orbital environment. During irradiation, white spots appeared on the Ag/FEP surface. Post-irradiation microscopy identified these as circular electrostatic discharge marks — charge accumulating on the surface discharging locally. The surface also showed slight yellowing, surface unevenness from material deformation, and hardening. Critically, thermal-optical properties changed only marginally (α: 0.10→0.12; ε: 0.79→0.76), and surface sheet resistance — while locally variable — remained within the ERG specification throughout. In short, the Ag/FEP material is acceptable for surfaces exposed predominantly to low-energy particle flux. [4]

1 MeV — Deep Penetration, Sequential Failure. At 1 MeV, electrons penetrate several microns — well through the FEP film (~127 μm), through the silver layer, and into the adhesive beneath. The failure sequence is therefore entirely different and far more consequential to thermo-optical performance. Testing was structured to simulate ERG mission durations at 1.5, 3, 6, 12, and 24 month equivalent exposures. Adhesive delamination was observed from the very first interval (1.5-month equivalent). FEP surface cracking and physical damage began at the 6-month equivalent. Across all intervals, optical properties and surface sheet resistance remained largely unchanged — a finding that mirrors NASA LDEF results and underscores a critical subtlety: Ag/FEP can appear optically intact while being mechanically compromised.[4]

2. Thickness Slows the Progression: 5-MIL vs. 10-MIL

The companion study [2] ran parallel irradiation of 5-mil and 10-mil Ag/FEP specimens across five fluence steps (2.5×10¹⁵ to 1.25×10¹⁶ e/cm²), producing a direct qualitative and quantitative comparison of degradation rate as a function of thickness. The qualitative result is clear: at every fluence level, the 10-mil specimens show less surface disruption than the 5-mil specimens.

Figure reproduced from Shibano et al. [2]. All rights reserved by JAXA/ISAS.

For the 5-mil specimen, the failure sequence is rapid and visible: bubble formation from the adhesive layer appears at the lowest fluence step, cracking appears mid-sequence, and severe surface roughening at the highest fluences. For the 10-mil specimen, the same failure types appear — bubble formation, then cracking, then crack propagation — but they arrive later in the fluence sequence and with substantially less severity at equivalent exposure levels.

∆ε vs. Total Fluence. Figure reproduced from Shibano et al. [2]. All rights reserved by JAXA/ISAS.

The emissivity change data (Δε, measured as the difference in infrared emissivity before and after irradiation) quantifies this thickness effect directly. For the 5-mil specimen, Δε increases with total fluence — driven by adhesive delamination and progressive surface roughening, both of which alter the material's infrared radiative behavior. For the 10-mil specimen, Δε remains comparatively flat and low across the same fluence range. The 10-mil's greater FEP thickness delays the point at which adhesive delamination and surface damage become significant enough to shift emissivity measurably.[2]

Practical Implication — doubling FEP thickness does not double the radiation lifetime. Both thicknesses ultimately fail at sufficient fluence, but 10-mil Ag/FEP meaningfully extends the interval before emissivity drift becomes a design concern, and before structural cracking compromises the radiator surface. For missions operating at the margin of Ag/FEP applicability, thickness is a direct and available design lever.

3. The 1.0 MGy Threshold: When Elasticity Ends

The most quantitatively precise finding across the entire program is a specific absorbed dose threshold for loss of Ag/FEP mechanical elasticity. This emerged from systematic tensile testing following stepped-dose electron irradiation, established in project 2017A-C32 and cross-validated against gamma ray irradiation.[3]

Test specimens were prepared on 2 mm aluminum plates (30 mm x 30 mm). Electron beam irradiation was 1.0 MeV with an electron flux of ~0.628 nA, under nitrogen purge at 2~10 Pa vacuum, with water cooling maintaining sample temperature below 45°C. Target doses were 0.05, 0.25, 0.5, 1.0, and 1.25 MGy.

The 1.25 MGy sample fractured during tensile test preparation — physical handling alone was sufficient to initiate failure. It was never placed in the tensile testing machine. For the four surviving dose levels, the maximum load at fracture showed almost no variation — the material's ultimate strength is relatively dose-stable. However, strain rate (elongation at failure) decreased linearly with absorbed dose. At 1.0 MGy, strain rate fell to approximately 1.0% — identified as the elasticity boundary below which the material can no longer accommodate thermal strain without cracking. A material that cannot flex through the strain imposed by orbital thermal cycling will fail structurally even without further irradiation.

4. How ITO Conductivity is Lost: XPS Evidence

XPS Finding: Physical Removal, Not Chemical Modification. After electron beam irradiation, XPS revealed a measurable decrease in the molecular count of ITO-related species at the surface. The interpretation: electron irradiation physically removes ITO from the surface — a sputtering or desorption process driven by the electron bombardment, rather than a chemical transformation of the ITO compound itself. As ITO is removed, the underlying FEP (an electrical insulator) is progressively exposed, reducing surface conductivity. [3]

This physical removal mechanism has direct design implications. The conductivity loss is not recoverable post-irradiation. It scales with total electron fluence. And ITO coating thickness is therefore a consumable parameter — a thicker ITO layer provides more material to be removed before insulating FEP is exposed, providing a direct lifetime-extension lever.

A further ITO failure mode emerged from combined-environment testing.[1] ITO coatings that survived electron irradiation alone, and survived atomic oxygen (AO) exposure alone, failed completely when AO exposure followed prior electron irradiation. The ITO layer — which cannot even be removed by IPA wiping in its pristine state — lost all surface conductivity under this sequential exposure. AO alone produced no such loss; electron irradiation alone produced no delamination. The suspected mechanism is that electron bombardment induces sub-threshold structural damage to the ITO crystal lattice, which AO then chemically exploits — a synergistic failure mode that no single-stressor ground test can predict or detect.

5. The Combined-Environment Cascade

Sequential single-stressor testing, however carefully designed, cannot capture the interaction effects between environmental stressors acting on already-degraded material. The ERG program ran a combined-environment sequence — applying UV, electron beam, thermal cycling, and atomic oxygen in the order deemed most conservative — with conditions set to represent approximately 1.5 years of ERG on-orbit exposure.[1]

UV irradiation proved largely benign for Ag/FEP — no change to thermal-optical properties at 100 equivalent sun days. Electron beam irradiation drives adhesive failure and FEP embrittlement, but surface conductivity survives this stage. Thermal cycling of the embrittled material is the catastrophic event: comprehensive cracking across the entire surface on the first thermal cycle eliminates both structural integrity and conductivity simultaneously. Subsequent AO exposure acts on an already-failed surface — producing whitening across the bulk and dark discoloration at crack locations where silver is oxidized.[1]

A parallel test using RTV adhesive (S692) instead of the 3M 9703 electrically conductive pressure-sensitive adhesives showed reduced surface irregularities after electron irradiation — bubble formation was partially suppressed. RTV does not eliminate the fundamental FEP embrittlement, but the improved adhesive-interface behavior confirms that the mounting method — not just the film specification — is a non-negligible design variable for radiation-exposed surfaces.[1]

6. The Replacement: Silver-Coated Colorless Polyimide (PI)

The most significant long-term output of the ERG material qualification program may be the development and initial qualification of a new radiator film designed to address Ag/FEP's fundamental radiation vulnerability. Silver-coated colorless polyimide (Ag/CPI) was developed because PI is substantially more resistant to ionizing radiation than FEP — the C-C and C-N bonds in the PI backbone are more robust to electron-induced chain scission than the C-F bonds in FEP.[3]

The optical design logic is identical to Ag/FEP: a transparent polymer film allows the reflective silver layer beneath to be seen, preserving the high solar reflectance (low α) that defines the second-surface mirror function. ITO coating was applied to the outer surface for conductivity. Film thickness: 1-mil (25 μm) CPI with silver layer on the back, ITO on the top surface, 50 μm pressure-sensitive adhesives below. This is significantly thinner than standard 5-mil (127 μm) FEP, but the CPI substrate carries the mechanical load more effectively under irradiation.

At 1.0 MGy — the dose that leaves Ag/FEP at its elasticity boundary — the CPI film showed almost no change in strain rate. At 10 MGy, where Ag/FEP would have long since crumbled, the CPI showed no visible cracking at all.[3]

In combined-environment testing, the adhesive tape beneath the CPI film lifted in the same manner as observed under Ag/FEP — the adhesive-interface problem is substrate-independent and not solved by switching from FEP to CPI. However, the film itself survived the full UV, electron beam, thermal cycle, and AO sequence with only a UV-induced color change in the CPI. Surface conductivity was maintained throughout.[1]

The follow-on study[5] provided the most complete optical characterization of ITO coated Ag/CPI under electron irradiation, testing the specimen to 20 MGy — the highest dose in the program. 

Optical Degradation After 20 MGy Irradiation — Solar absorptance of ITO coated Ag/CPI specimen changed from 0.141 to 0.186 (Δα = 0.045), while normal emittance (ε_N) showed almost no change.

Top 3 Engineering Decisions the JAXA Data Should Inform

1. Thermal cycling after electron irradiation is the terminal event, not irradiation itself. The Ag/FEP material survives the electron dose — it fails completely on the first thermal cycle following embrittlement. A material that passes electron beam testing at ambient temperature may fail immediately upon reaching operational temperature cycling in orbit. Combined-environment sequential testing is essential; ambient-temperature irradiation-only testing is insufficient.

2. Test the film + adhesive stack, not just the film. At 1 MeV, adhesive delamination was observed at 1.5 month equivalent — before any FEP cracking. The adhesive is the first failure point, and it directly controls the thermal conduction path from the radiator surface to the spacecraft structure. Qualification of the film in isolation systematically misses the most operationally important failure mode.

3. If your electron environment exceeds 1 MGy absorbed dose in FEP, Ag/FEP may be the wrong material. The JAXA program proved silver-coated color polyimide (CPI) specifically as a radiation-tolerant replacement — same beginning-of-life (BOL) thermo-optical properties, dramatically superior electron irradiation resistance at 10x the critical dose with no visible degradation at end-of-life (EOL). For any mission in the radiation belts, this alternative warrants qualification consideration before defaulting to Ag/FEP on flight heritage grounds.

   
   

Final Words

SQUID3 Space supplies various spaceflight-proven thermal control tapes and can help assess whether your high-radiation mission should select Ag/FEP, Ag/CPI, or borosilicate glass-based OSR. 

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Literature Source Citations 

[1] Shibano, Y., Asamura, K., Takashima, K. & ERG Project Team. "Degradation of Thermal Control Materials for the ERG Mission [ERGにおける熱制御材劣化]." JAXA Technical Poster, document SA6000046149. Institute of Space and Astronautical Science / Japan Aerospace Exploration Agency (ISAS/JAXA). 

[2] Okazaki, S., Shibano, Y., Sugimoto, R., Nishiro, D., Ogawa, H. (ISAS/JAXA); Nagano, M. (Nagoya University); Nagai, H. (Tohoku University); Miyazaki, Y. (Reinetsu-ken). "Development of Thermal Control Technologies for Future Spacecraft [将来宇宙機に適用する熱制御技術の開発]." JAXA Technical Poster, document SA6000060229. ISAS/JAXA. 

[3] Shibano, Y., Tachikawa, S. & Ogawa, H. (ISAS/JAXA). "Performance Evaluation for Thermal Control Materials [熱制御材の材料評価]." JAXA facility use report, Project No. 2017A-C32. Academic publication category (成果公開・学術利用). JAEA Takasaki facility. Report form revised H29.5. 

[4] Shibano, Y., Asamura, K., Toyoda, H. & Ogawa, H. (JAXA). "Degradation of Ag-Deposited Teflon by Low-Energy Electron Beam Irradiation [低エネルギー電子線照射によるAg蒸着テフロンの劣化について]." JAXA. Referenced works: Miyoshi et al., AGU (10.1029/2012GM001304, 2012); Dever et al., AIAA-98-0895/NASA-TM-1998-206618; Gilmore, D.G. (ed.), Spacecraft Thermal Control Handbook, 2nd ed., Aerospace Press, 2002; Pippin, H.G. et al., J. Spacecraft and Rockets, Vol. 41, No. 3, May–June 2004, pp. 322–325. 

[5] Shibano, Y. & Tomihiro K.(ISAS/JAXA). "Evaluation of Electron Beam Degradation of Thermal Control Materials Used in Spacecraft [宇宙機に使用する熱制御材の電子線劣化評価]." JAXA facility use report, Project No. 2019A-C16. Academic publication category (成果公開・学術利用). JAEA Takasaki facility. 

All documents translated from Japanese with AI assistance. See disclaimer. Originally published in Japanese.

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AI-Assisted Translation & Content — Reader Advisory

This technical blog post was researched and drafted with the assistance of an AI language model, working from five JAXA technical documents written in Japanese. Translation from Japanese to English was performed by AI and has not been independently verified by a qualified translator or by the original authors. Readers are strongly advised to consult the original Japanese source documents before using any technical value, threshold, test condition, or finding in engineering decisions, publications, or mission-critical design work. AI translation can introduce errors in numerical values, units, causal relationships, and scientific context — all of which are consequential in a materials qualification setting.