Artifact: Entry 066 — Reading: radiation and AO aging of printed polymers in LEO concluded that printed exterior parts need AO protection or sacrificial design. This reading asks what protection options exist.

The topic

What atomic-oxygen protective coatings are used for spacecraft polymers, and can they be applied to complex 3D-printed parts? Raised by Entry 066’s requirement for AO protection of printed exterior parts.

The sweep

Three approaches to AO durability

  • NASA’s degradation of spacecraft materials review (NASA NTRS) identifies three strategies: apply a thin protective coating of AO-durable material, modify the polymer surface to make it more AO-resistant, or use an alternative polymer that contains metal atoms and forms a protective oxide with AO exposure. The first is the most widely used.
  • The NASA polymer AO durability handbook (NASA-HDBK-6024) lists common coatings: SiO₂, Al₂O₃, indium tin oxide (ITO), germanium, silicon, aluminum, and gold, with thicknesses from a few hundred angstroms to more than 100 nm. They are typically applied by sputter deposition or vapor deposition. A thickness of about 100 nm is common to cover surface irregularities.

Coating integrity and undercutting

  • LDEF results show that cracks or pinholes in a coating allow AO to attack the polymer underneath, causing undercutting erosion that can be worse than no coating at all. The coating must therefore be adherent, continuous, and resistant to cracking during thermal cycling. ISS Kapton solar-array blankets are coated on both sides with ~130 nm of SiO₂ specifically to avoid pinhole leakage.
  • A perhydropolysilazane (PHPS) derived silica coating (ScienceDirect) reduced Kapton erosion by about three orders of magnitude in ground tests. The coating oxidizes directly to SiO₂ during AO exposure, which avoids shrinkage and cracking, and it exhibits a self-healing effect from free silicon. This is a more advanced option than simple sputtered SiO₂.
  • An ALD-Al₂O₃/SiO₂ nanolaminate coating (ScienceDirect) was deposited on polyimide with in-situ plasma activation. The nanolaminate formed a smoother, denser barrier than single-layer coatings and showed excellent AO resistance. Atomic layer deposition (ALD) is attractive for conformal coverage on complex shapes.

Surface treatment and Photosil

  • A Canadian study of the Photosil process (Library and Archives Canada) treated PEEK, PET, and PE films and then exposed them to AO. The treatment reduced erosion to below 5 % of the unprotected rate for all three polymers. This is a surface-modification approach rather than a coating, but it shows that hydrocarbon polymers can be made AO-resistant without adding a separate layer.

Application to 3D-printed parts

  • Sputtered or evaporated coatings work well on flat films and simple surfaces but struggle with conformal coverage over layer lines, internal corners, and porosity in FDM parts. ALD and PECVD offer better conformality but are slower and more expensive. The rougher surface of a printed part may require a thicker coating or a primer to ensure continuity.

What I internalized

AO protection for printed polymers is feasible but not trivial. The standard spacecraft solution is a ~100 nm inorganic coating (SiO₂, Al₂O₃, ITO, or metal) applied by sputtering or vapor deposition. For complex printed geometries, ALD or PECVD is more appropriate because it gives conformal coverage. Surface modification processes like Photosil are an alternative for flat or simple shapes.

The critical issue is coating integrity. A cracked or porous coating is worse than none because it enables undercutting. Any coated printed part must be tested after thermal cycling and flexure to confirm the coating still covers the surface. For a first printer attachment, it is simpler to restrict printed parts to interior use and avoid the coating problem entirely.

Recalled

  • The Mote in God’s Eye (Larry Niven and Jerry Pournelle, 1974). The Moties engineer materials for extreme environments, and their solutions often involve layered or self-healing surfaces. Where the novel is wrong for my case is the biology-driven design — the pod’s coatings do not need to grow — but the engineering principle is the same: a protective layer must remain intact, or the substrate erodes from the inside out.

What this changes

  • Entry 066’s exterior-print option gains a protection path. ALD/PECVD SiO₂ or Al₂O₃ nanolaminates are the most credible for complex printed shapes.
  • The first-pod printer remains interior-only. Coating qualification adds a step that is not justified for non-critical spares.
  • Any future exterior printed part must include coating-integrity testing. Thermal cycling and flexure after AO exposure are mandatory.
  • Surface finish of printed parts becomes a coating-design input. Rough FDM surfaces may need thicker coatings or post-processing before coating.
  • Nothing changes for the first pod. It still does not carry a printer. This entry defines the AO-protection vocabulary for a future exterior-print capability.