Mechanical and optical degradation of flexible optical solar reflectors during simulated low earth orbit thermal cycling. (October 2020) - Putz, B.
Abstract: Multilayer thin film systems on flexible polymer substrates are used as flexible optical solar reflectors or thermal insulation of satellites and spacecraft. During one year of operation, a satellite in low earth orbit typically encounters 6000 thermal cycles of ±100 °C. Due to the different coefficients of thermal expansion between the individual layers and the substrate it is important to investigate the thermo-mechanical stability of the multilayers as a function of the cyclic heat load. Scanning electron microscopy and focused ion beam cross-sectioning revealed that Inconel-Ag bilayers on fluorinated ethylene propylene (FEP) substrate severely degrade during thermal cycling of ±150 °C in a gaseous N2 atmosphere. After only 100 cycles through thickness cracks and subsurface voids in the Ag layer form as a result of equi-biaxial thermal stresses caused by the large difference in thermal expansion between film and substrate. Transmission Kikuchi Diffraction (TKD) before and after thermal cycling also revealed grain growth and twin widening in the Ag layer. Cracking and void formation are detrimental to application relevant material properties including corrosion protection (Inconel) and reflectivity (Ag). Reflectance measurements revealed that the amount of reflected energy as well as the reflection mode (specular vs. diffuse) significantly change during the first 100 cycles. Saturation of reflection characteristics was observed after 25 cycles, which correlates to a turning point in the evolution of Ag voids. Results of this study indicate that special focus should be directed towards thermal stress control (Δα) and tailoring of the metal-polymer interface to improve resistance of versatile metal-polymer systems against thermal cycling. Graphical abstract: Image 1 Highlights: Flexible optical solar reflectors are subjected to thermal cycling of ±150 °C. Severe mechanical and optical degradation occurs at low cycle numbers. Through thickness cracks are observed in the Inconel-Ag coating after 3 cycles. Progressive sub-surface voiding of the Ag film is observed after 6 thermal cycles. Reflectivity decreases and unfavorable changes in the reflection mode are observed.