Thin Film Oxide Barrier Layers: Protection of Kapton from Space Environment by Liquid Phase Deposition of Titanium Oxide

Polyimides are widely used for the external surfaces of spacecraft. In low Earth orbit (LEO), they are exposed to atomic oxygen (AO) and to problems of electrostatic discharge (ESD). This work demonstrates that liquid-phase deposition (LPD) of titania creates a protective coating on Kapton polyimide that is effective in reducing AO-induced surface erosion and in preventing ESD. Adherent titania coatings, 100-300 nm thick, were deposited on Kapton at near-ambient conditions by LPD using an aqueous solution of a metal-fluoride complex and boric acid. Characterization of the oxide-coated Kapton included atomic force microscopy (AFM) in tapping and nanoindentation modes, electrostatic force microscopy (EFM), scanning electron microscopy (SEM), Rutherford backscattering (RBS) and X-ray photoelectron spectroscopy (XPS). The as-deposited titania-coated Kapton can be prepared without significant changes in the original thermo-optical properties of the polymer, while preventing ESD and improving the surface hardness. The durability of the oxide coating under AO attack was studied using an oxygen RF plasma. Surface erosion was measured both gravimetrically and by in situ quartz crystal microbalance (QCM) measurements. The AO exposure caused some changes in the thermooptical properties and surface morphology. The erosion yield of titania-coated Kapton was only 2% of that observed for uncoated Kapton after exposure to 4 × 1020 O-atoms cm-2 of LEO equivalent AO fluence.