Novel approach to space-survivable polyimides: Liquid phase deposition of Titania coating on Kapton

Polyimides are widely used as spacecraft external surfaces materials. They are exposed to atomic oxygen (AO), which is the main constituent of a residual atmosphere at low Earth orbit (LEO) and should be protected from the space harsh environment. This work presents an innovative approach to applying a protective coating on Kapton polyimide using liquid-phase deposition (LPD) and shows its effectiveness in reduction of AO-induced surface erosion. Titania coatings, 100 nm thick, were deposited on Kapton films by LPD using an aqueous solution of a metal-fluoride complex and boric acid at near ambient conditions. Adherent, crack-free, coatings have been obtained by optimization of the deposition conditions, the substrate surface pre-treatment procedure and post-deposition drying. Characterization of the oxide coating included Atomic Force Microscopy (AFM) in tapping and nano-indentation modes, Environmental Scanning Electron Microscopy (ESEM), and X-ray Photoelectron Spectroscopy (XPS). The durability of the titania coating under AO attack was studied using a RF plasma based facility. Surface erosion was measured both gravimetrically and by in situ Quartz Crystal Microbalance (QCM) measurements of mass loss. A dramatic reduction in the erosion of titania-coated Kapton relative to uncoated Kapton was observed: the erosion yield was as low as 2% of that of uncoated Kapton after exposure to 4× 10²⁰ O-atoms-cm^-2 of LEO equivalent AO fluence. In addition, the surface hardness of coated Kapton was improved both in the as-deposited titania coatings and in the AO-exposed titania films.