EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Flexible optical solar reflectors and multilayer insulation systems for satellites and spacecraft are made of multilayered metal thin films on polymer substrates. In low earth orbit, a satellite typically encounters 6000 thermal cycles of ± 100°C during one year of operation. Due to the different coefficients of thermal expansion between the individual metal layers and the polymer substrate it is important to investigate the thermo-mechanical stability of the multilayers as a function of the cyclic heat load. One candidate material system, a fluorinated ethylene propylene (FEP) substrate coated with silver (Ag, 150 nm) and Inconel (30 nm), was studied using simulated low earth orbit thermal cycling of ± 150°C in a gaseous N2 atmosphere. After only 100 cycles through thickness cracks in the Inconel film 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. Scanning electron microscopy characterization using Transmission Kikuchi Diffraction (TKD) before and after thermal cycling also revealed microstructural changes including grain growth and twin widening in the Ag layer. Both cracking and void formation are detrimental to application relevant material properties including corrosion protection (Inconel) and reflectivity (Ag). To assess the reflectivity degradation reflectance measurements were performed. These measurements revealed that the amount of reflected energy and the reflection mode (specular vs. diffuse) significantly change during the first 100 cycles. Of note is the observation of a saturation of reflection characteristics after 25 cycles, correlating 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 and tailoring of the microstructure and metal-polymer interface to improve resistance of versatile metal-polymer systems against thermal cycling.