Annealing effects on Al/polyimide adhesion in flexible optical solar reflectors

Abstract

Flexible optical solar reflectors are made of single and multi-layered metal thin films on polymer substrates and will encounter around 6000 thermal cycles exceeding +/-100°C during one year of operation in low earth orbit. The candidate thin film system of Inconel/silver (Ag)/Teflon (FEP) recently demonstrated early damage formation (cracks and voids) after only a few thermal cycles, most likely due to the poor interfacial properties between Ag and FEP. An alternative material system that could be used is colourless polyimide, Tormed, instead of FEP. Additionally, aluminium (Al) has demonstrated very good interfacial properties with polyimide even after thermal cycling and has suitable optical properties. The adhesion of the Al/Tormed and Ag/Tormed interfaces were evaluated with tensile induced delamination. Generally, Al/Tormed has a much higher interface adhesion energy compared to Ag/Tormed, and there is no significant degradation after bake-out in vacuum (200°C, 10-6 mbar) for 10 and 24 hrs. Thus, the Al/Tormed system could be a more robust coating system for future flexible solar reflectors.