Silver mirrors protected with yttria for the 0.5-14-microm region.

The best known front surface mirror in the visible and IR region of the spectrum is a freshly deposited silver coating. However, as silver tarnishes in the atmosphere due to the formation of Ag2S, protective coating is mandatory. Most of the materials commonly used for this purpose exhibit ab­ sorption in various parts of the 0.5-14-μm region; moreover, there exists the difficulty of obtaining film with good adher­ ence to silver. Thorium fluoride is sometimes commercially used; however, it is rather soft and radioactive. Recently, Y2O3 was used as a protective layer for Al mirrors in the 8-12-μm region. We verified the results published and subjected the protected Al mirrors to environmental tests. These mirrors passed the adherence, abrasion, humidity, and salt-spray tests as specified in MIL-M-13508B. In view of these results, Ag mirrors were coated with yttria, and their optical properties and durability were studied. Silver mirrors were prepared by evaporation of 99.99% pure Ag (Balzers) on an intermediate chromium layer deposited on a highly polished and thoroughly cleaned glass substrate. Y2O3 was deposited from a heavy tungsten boat. During evaporation the pressure was maintained at 10 5 to 10 6 Torr, and the temperature of the substrate was maintained between 50 and 100°C. Y2O3 layers were deposited with thicknesses ranging from 1000 to 1400 A. However, above 1200 Å the Y2O3 film began to exhibit a yellowish hue, which deepened as the layer thickness increased. Therefore, extensive optical and environmental tests were performed on mirrors with a 1200-Å thick layer. The reflectance of the protected silver mirror was measured between 0.45 and 14 μm and at nearly normal angles of inci­ dence to the surface. The measurements were performed on Beckman Acta IV and Perkin-Elmer 283 spectrophotometers equipped with standard variable specular reflectance ac­ cessories. The reflectances of protected and unprotected silver mirrors were found to be identical between 1.5 and 14 μm; below 1.5 μm the reflectance of the protected mirror gradually decreased. The reduction of the reflectance at 0.5 μm was 1.5% as compared with the unprotected mirror (Fig. 1). The mirrors were subjected to standard environmental and durability requirements of MIL-M-13508B: adherence test with cellophane tape, humidity exposure for 24 h, and abra­ sion resistance to an eraser. No damage to the mirror or Fig. 1. Reflectance of unprotected (solid line) and protected (dashed line) silver mirrors.