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Multilayer four-flux model for the optical degradation of thermal control coatings in space

The aim of this paper is to generalize the four-flux radiative transfer model to the case of a multilayer medium. An application is presented with the study of the optical degradation of a white paint in simulated space environment. This paint is constituted of a mixing a zinc oxide and a silicone resin. A sample was irradiated with 45 keV protons and reflectance measurements were achieved in situ after each step of irradiation in order to see the evolution of the thermo-optical properties of the coating. These tests were completed after irradiation by Scanning Electron Microscopy (SEM) in order to characterize the structure of the material and to detect possible structural changes due to the irradiation. This experimental investigation allowed us to define hypothesis to be introduced in the model. In particular, we assume that the optical degradation centered on 410 nm is due to a variation a∓ of the imaginary part of the refractive index of zinc oxide in the damaged layer. The generalized four-flux model was validated by comparing numerical calculation with experiment.