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

摘要

Highlights•Silver stacks grown on SiC by cathodic magnetron sputtering for space mirror application.•Role of SiC surface on local tarnishing by H2S studied by combined surface analysis.•Suppression of high aspect ratio SiC surface defects decreases susceptibility to tarnishing.•Pathways short circuiting the SiO2 protection layer enable H2S ingress to Ag mirror layer.•Residual local sulfidation proceedsviaAg2S growth as columns erupting at stack surface.AbstractThe role of the SiC substrate surface on the resistance to the local initiation of tarnishing of thin-layered silver stacks for demanding space mirror applications was studied by combined surface and interface analysis on model stack samples deposited by cathodic magnetron sputtering and submitted to accelerated aging in gaseous H2S. It is shown that suppressing the surface pores resulting from the bulk SiC material production process by surface pretreatment eliminates the high aspect ratio surface sites that are imperfectly protected by the SiO2overcoat after the deposition of silver. The formation of channels connecting the silver layer to its environment through the failing protection layer at the surface pores and locally enabling H2S entry and Ag2S growth as columns until emergence at the stack surface is suppressed, which markedly delays tarnishing initiation and thereby preserves the optical performance. The results revealed that residual tarnishing initiation proceeds by a mechanism essentially identical in nature but involving different pathways short circuiting the protection layer and enabling H2S ingress until the silver layer. These permeation pathways are suggested to be of microstructural origin and could correspond to the incompletely coalesced intergranular boundaries of the SiO2layer.