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 SiO₂ overcoats, deposited by cathodic magnetron sputtering and submitted to accelerated aging in gaseous H₂S. The results show that tarnishing is locally initiated by the formation of Ag₂S columns erupting above the stack surface. Ag₂S growth is promoted at high aspect ratio defects (surface pores) of the SiC substrate as a result of an imperfect protection by the SiO₂ overcoat. Channels most likely connect the silver layer to its environment through the protection layer, which enables local H₂S entry and Ag₂S growth. The silver sulfide columns grow in number and size eventually leading to coalescence with increasing H₂S 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 Ag₂S columns underneath the protection layer, consuming the metallic silver layer and deteriorating the protecting overcoat.