1. The stillsuit problem

Frank Herbert’s Dune gives the Fremen stillsuits as the solution to a hostile environment. The suit does not stop the desert; it manages the boundary between the human body and the sand. Spacecraft polymers need the same kind of boundary management. The polymer itself may not survive LEO, but a thin protective layer can make it survive long enough.

This entry reads about protective coatings and polymer choices for AO and UV resistance.

2. Thin-film protective coatings

The most common mitigation is a thin inorganic coating applied directly to the polymer surface. The NASA materials selection guide and degradation handbook list the usual candidates:

  • Silicon dioxide (SiO₂): perhaps the most widely used. ISS solar array blankets use ~130 nm of sputtered SiO₂ on Kapton. The coating is transparent, AO-durable, and electrically insulating.
  • Aluminum oxide (Al₂O₃): similarly durable, used on Kapton and other films.
  • Indium tin oxide (ITO): provides AO protection plus electrical conductivity, useful for preventing charge buildup.
  • Metals such as aluminum or gold: opaque but highly protective; used where optical transmission is not required.

Coatings around 100 nm thick are typical. Thinner coatings may have pinholes; thicker coatings tend to crack or spall due to intrinsic stress and mismatch in thermal expansion with the polymer substrate. A single pinhole is not fatal immediately, but AO enters through it and undercuts the coating, eventually linking cavities and causing mechanical failure.

3. Surface modification and alternative polymers

Two other approaches compete with coatings:

  • Surface modification: implanting silicon or metal atoms into the polymer surface, or chemically modifying the surface to incorporate inorganic groups. The durability depends on how many protective atoms are placed per unit area.
  • Alternative polymers: formulating polymers that already contain silicon, phosphorus, or metal atoms. Examples include silicone co-polymers, polysilsesquioxanes, and phosphorous-containing polymers. These materials can form a self-protecting oxide layer when exposed to AO.

The trade-off is usually between process maturity and performance. Coatings are well-understood and flight-proven. Alternative polymers may offer better intrinsic durability but require more qualification.

4. The pinhole problem

The dominant failure mode for coated polymers is not bulk coating erosion but defect-driven undercutting. A scratch, a particle impact, or a pinhole becomes an entry point. AO oxidizes the polymer under the coating, creating a cavity. The coating remains intact above the cavity until enough material is removed that the overlying film collapses.

This is why surface preparation matters. A smooth, clean polymer surface with a leveling undercoat can reduce defect density by orders of magnitude. The NASA degradation handbook cites a study where a leveling coating reduced pinhole density from 180,000 defects/cm² to about 1,000 defects/cm².

5. What this changes

  • Coated polymers are viable in LEO, but the coating system must be treated as a whole: substrate, leveling layer, protective layer, and handling.
  • A protective coating is not a one-time fix; it must survive launch, thermal cycling, and handling before it ever sees AO.
  • The next entry will synthesize what this means for the desktop’s material choices.