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Various layer schemes are used to protect thin film silver mirrors from tarnish, corrosion, and degradation. One common design is based on the coating currently utilized by the twin Gemini astronomical telescopes, consisting of two very thin layers of nickel-chromium nitride (NiCrNx) on either side of the silver layer followed by a protective overcoat of silicon nitride (SiNx). Variations of this coating design have employed assorted NiCr- or Cr-based layers of varying thicknesses on either side of the silver, in efforts to achieve both high reflectivity and good durability. Generally, however, it is believed that reflectivity comes at the expense of durability. Ongoing work in our laboratory has explored the durability and corrosion behavior of various Gemini-style silver mirrors with NiCr- and Cr-based layers. Here, we present recent work on varying the Ni/Cr ratio in the NiCrNx adhesion layer above the silver and the effect on mirror durability, in terms of reflectance, scatter, and corrosion feature development during accelerated environmental exposure testing. By varying the adhesion layer Ni/Cr ratio from Ni-rich (90/10 Ni/Cr) to Cr-rich (0/100 Ni/Cr), it is possible to control the initial optical performance of the mirrors, as Ni-content is inversely related to reflectance. By varying the Ni/Cr ratio, it is also possible to control mirror durability, including the mechanism of corrosion feature development. This suggests that there may be an intermediate Ni/Cr composition with reduced Ni-content that improves reflectance without sacrificing durability.
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