Atomic Oxygen Testing of MgF2 Coatings

Spacecraft orbiting below about 300 nm are exposed to a high flux of atomic oxygen (AO) that can severely degrade surface materials, yet some coatings used on solar cell coverglass have never been qualified in this environment. Combined effects tests comprising atomic oxygen exposure, illumination, and temperature were carried out to qualify magnesium fluoride coated cover-glasses. Unexpectedly severe degradation in the form of optical transmission loss was observed upon exposure to high AO fluences. It was determined that the high intensity vacuum ultraviolet (VUV) light emitted from the AO source used acts in an unexpectedly synergistic fashion with AO to accelerate the optical degradation of these coatings. Since the excess VUV from the ground test AO source is not representative of flight conditions, its unexpected synergistic effect emphasizes the need to define proper ground test conditions when qualifying materials for AO environments