Degradation of Spacecraft’s Temperature-Control Coatings during Deposition of Thin Au Films

Abstract

Degradation of temperature-control coatings (TCC) of “solar reflector” type caused by deposition of Au thin films was studied. “Mirror” OSO-S type coating and film PM-OA type coating were investigated. Au films with thickness of 1–24 nm were deposited on the samples by means of cathode sputtering of a pure Au target with the use of 300 eV Xe ions. A quartz microbalance was used for controlling the mass of the film deposited. The process was stopped when the frequency variation of the quartz resonator became equal to the given value. The film thickness was determined according to frequency variation of the quartz resonator. During the experiments, the thermo-optical properties (solar radiation absorptance AS and emissivity e) of TCC were measured at various film thicknesses. AFM investigations of the surface of samples were carried out too. As a result, the dependences of the values of AS and e on the film thickness d are ascertained. It is found that a significant variation of the AS/e ratio is established under film thickness less than 3.5 nm. Such films are characterized by granular structure and low conductivity. In this case, when e variation is insufficient, changes in AS value cause the main contribution to degradation of TCC properties. The permissible levels of contamination for OSO-S and PM-OA coatings are 1.2 × 10–6 and 6.7 × 10–6 g/cm2 respectively. After deposition of a thick Au film, TCC properties are changed catastrophically: emissivity e decreases to 0.2–0.3, absorptance increases by 2–3 times, and the AS/e ratio can increase by 5–10 times.