The Degradation of Optical Properties in a Sulfuric Acid Anodized Aluminum Coating System Exposed to Vacuum or Vacuum-Ultraviolet Radiation

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Abstract
A model for the optical property changes of a sulfuric acid anodized aluminum coating system was formulated to include the effects of water loss and cracking of the coating when the coating is exposed to either vacuum or vacuum-ultraviolet (VUV) radiation. The coating system was modeled as a single layer on an aluminum substrate and a discontinuous layer on an aluminum substrate in the case of cracks being present. Multiple scattering interactions were neglected because of thick film approximations. The effects of the water loss-induced densification and cracking of the coating were modeled independently to produce relationships which can be used to model the reflectance spectra. Densification of the coating with water loss theoretically reduces coating transmittance and total reflectance; cracks tend to reduce the effective reflecting area, resulting in a decrease in total reflectance. The overall models correlate well with experimental reflectance data which show significant changes at the short wavelength region.