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

Abstract

The 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 SiO2 overcoat 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 SiO2 layer.

Deterioration of Novel Silver Coated Mirrors on Polycarbonate Used for Concentrated Solar Power

“…This solution first damages the protective layer, then permeates the layer, and finally damages the silver. This mechanism is demonstrated by evaluating experiments with bare silver and silver coated with a SiO 2 layer [31]. Subsequently, water vapor penetrates the interface between the silver film and the substrate along the edges of the coating or through permeable defects, which causes delamination and blistering of the coating.…”

“…Limam et al [31] studied the formation of channels that connect the surroundings of the silver layer, through the protective layer, with the surface pores, which allows the entry of H 2 S and the growth of columns, causing surface tarnishing. Mishra et al [32] investigated the behavior of multilayer degradation concerning reflectance in conditions of accelerated weathering degradation and low salt fog.…”