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

Abstract

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.

Role of SiC substrate surface on local tarnishing of deposited silver mirror stacks

“…They confirmed that the surface porosity characterizing the SiC substrate and originating from the material production process is suppressed by the CVD surface treatment like specified by the provider. On the SiC substrate ( Figure 1(a,b)), the lateral dimensions (0.5-6 µm), depth (0.05-0.4 µm) and density (~1.6×10 6 cm -2 ) of the surface pores remain essentially unchanged after deposition of the stacks, showing that the pores remain incompletely filled and that the surface still exhibits high aspect ratio topographic sites as discussed previously [35]. On the SiC+CVD substrate (Figure 1(d…”

“…Accelerated atmospheric aging was performed in gaseous H2S selected as most corrosive indoor atmospheric agent of silver. The accelerated aging tests were performed at 1000 mbar H2S and 75°C for 24, 48 and 96 h, like previously described [35]. The sulfurized samples were transferred through air for surface analysis.…”

“…Starting from the outer surface, they allow us defining the following five regions (all marked): the protection layer region (using the SiOand 18 Oions), the adhesion interfacial layer Ia, the silver mirror layer and the adhesion interfacial layer Ib regions (all three using the Agions) and the SiC substrate region (using the SiCand Cions). The Ia and Ib interfacial layer regions can also be defined using their respective ions as reported previously [35].…”

“…It has been shown that a H2S concentration as low as 0.2 ppb is sufficient to initiate silver sulfidation [28]. Degradation studies performed on space silver mirrors mostly addressed the efficiency of the protection layer and the effect of environmental aging on the optical properties [10,15,25,[29][30][31], and more seldom their tarnishing mechanism [32][33][34][35].…”

“…In a recent work [35], we studied model stack samples consisting of thin silver layers covered by SiO2 coatings deposited by cathodic magnetron sputtering on SiC, a substrate already in use in light weight all-SiC telescopes assigned to scientific or earth observation missions. These stacks were submitted to accelerated aging in H2S gas and studied by combined surface and interface analysis using Atomic Force Microscopy (AFM), X-ray Photoelectron Spectroscopy (XPS) and Time-of Flight Secondary Ion Mass Spectrometry (ToF-SIMS).…”