Local Degradation Mechanisms by Tarnishing of Protected Silver Mirror Layers Studied by Combined Surface Analysis.

In this work, we addressed the local degradation mechanisms limiting the prelaunch environmental durability of thin-layered silver stacks for demanding space mirror applications. Local initiation and propagation of tarnishing were studied by combined surface and interface analysis on model stack samples consisting of thin silver layers supported on lightweight SiC substrates and protected by thin SiO overcoats, deposited by cathodic magnetron sputtering and submitted to accelerated aging in gaseous HS. The results show that tarnishing is locally initiated by the formation of AgS columns erupting above the stack surface. AgS growth is promoted at high aspect ratio defects (surface pores) of the SiC substrate as a result of an imperfect protection by the SiO overcoat. Channels most likely connect the silver layer to its environment through the protection layer, which enables local HS entry and AgS growth. The silver sulfide columns grow in number and size eventually leading to coalescence with increasing HS exposure. In more advanced stages, tarnishing slows down owing to saturation of all pre-existing imperfectly protected sites of preferential sulfidation. However, it progresses radially at the basis of the AgS columns underneath the protection layer, consuming the metallic silver layer and deteriorating the protecting overcoat.