Study by temperature induced deflection of ageing and damaging of multilayered mirrors for satellites

In order to determine the causes reducing or destroying the stability of the multilayers which consist the essential parts of satellite mirrors, an experimental study, based on deflection tests has been performed. The observed temperature induced deflections are discussed in relation to diffusion controlled mechanisms producing microstructural changes in the multilayer. The deflections were observed when annealing the multilayers up to 573 K, in a pure argon or oxygen atmosphere.The multilayers consisted in successive thin films, deposited on a 〈0 0 1〉 silicon substrate: first a 30 nm thick amorphous silica layer, then a metallic film, made of nickel or chromium, and finally a silver layer which ensures the reflective properties.It was verified that the metal glass interface remained stable during heat-treatments, while other diffusion controlled mechanisms could degrade the multilayers. Among them: the oxidation of the metallic layer, possible solid state reaction forming Ni₂Si, and well identified agglomerations of the silver layer with formation of hillocks and unwetted zones.