Materials · July 2025
Osmotic Blistering in Parylene C Coatings
A materials investigation into coating adhesion failure on elastomer substrates, and the recovery behavior that made it hard to catch.
What went wrong
Parylene C coated elastomer components were developing blisters in service. The confusing part: parts that blistered would sometimes look fine again after sitting on a bench, which made the failure hard to reproduce and easy to dismiss as an inspection artifact.
Mechanism
The behavior is consistent with osmotic blistering. Ionic contamination trapped at the coating-substrate interface draws water through the permeable coating, the local osmotic pressure lifts the film, and when the part dries the elastomer’s elasticity pulls the blister back down. The adhesion damage persists even though the visible defect does not.
What that changed
- Inspection needed to happen wet, or immediately after exposure, not after a bench soak
- The real control variable was interface cleanliness before coating, not coating thickness
- Accelerated testing had to include a humidity cycle or it would pass parts that failed in the field
Deliverable
A technical write-up for the team explaining the mechanism, why the recovery behavior was misleading, and which process controls actually address it.