Protected silver coatings for reflectors

For ground- and spaced-based applications, Ag-coated reflectors are indispensable because of their high reflectivity. The transport, assembly, and storage of these reflectors take places over a long period before they are finally commissioned for application. To endure this period without a decrease of reflectivity, protective coatings with a final layer, which offers a high resistance to aqueous solutions, and a low mechanical stress should be used. These criteria were taken into account for the selection of a final layer for a protected Ag coating, which was applied for reflectors utilized in the CRIRES+-instrument (an IR spectrograph used at the VLT). Reactively sputtered Al2O3, SiO2 and Si3N4 layers were investigated with regard to these criteria. In aqueous (alkaline) solutions, the investigated Si3N4 layers are more stable than the SiO2 layers and the SiO2 layers more stable than the Al2O3 layers. This shows the influence of the intrinsic material properties. The mechanical stress of the sputtered layers depends on the deposition conditions and thus on the selected parameters. A Si3N4 layer with a high resistance to aqueous (alkaline) solutions also offers low and stable mechanical stress. Therefore, the deposition parameters used for this layer were applied for sputtering the final layer of the protected Ag coating for the reflectors.