Leveling Coatings for Reducing Atomic Oxygen Defect Density in Graphite Fiber-Epoxy Composites

Pinholes or other defect sites in a protective oxide coating provide pathways for atomic oxygen in low-Earth orbit to reach underlying material. Onc concept for enhancing the lifetime of materials in low-Earth orbit is to apply a leveling coating to the material prior to applying any reflective and protective coatings. Using a surface-tension-leveling coating concept, a low-viscosity epoxy was applied to the surface of several composite coupons. A protective layer of 1000 Å of SiO₂ was deposited on top of the leveling coating, and the coupons were exposed to an atomic oxygen environment in a plasma asher. Pinhole populations per unit area were estimated by counting the number of undercut sites observed by scanning electron microscopy. Defect density values of 180,000 defects/cm² were reduced to about 1000 defects/cm² as a result of the applied leveling coating. These improvements occur at a mass penalty of about 2.5 mg/cm².