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.
While various layer schemes have been developed to protect thin film silver mirrors from tarnish and corrosion, the mechanisms by which these protective layers improve mirror durability are not fully understood. Mixed flowing gas exposure of plasma beam sputtered silver mirrors was used to investigate how the composition of the very thin adhesion layer changes the mechanism of corrosion feature growth. Two model mirror coatings were analyzed in which the composition of the base layer below the silver and the adhesion layer above were varied. Optical measurements and microscopy, SEM, TEM, and EDS were used to characterize the compositional and chemical effects at the layer interfaces. Large circular corrosion features formed along the silver-chromium interfaces; the addition of nickel to the layers on either side of the silver limited the growth of these features, but resulted in the corrosive attack of the silver itself.
Molecular contamination degrades sensitive spacecraft surfaces and can adversely affect the useful life of a spacecraft.
In order to accurately predict spacecraft performance and end of life, an understanding of the primary mechanisms and
processes involved in the deposition and "fixing" of molecular contaminants is necessary. The objective for this research
effort has been to investigate how solar vacuum ultraviolet (VUV) radiation and surface temperature influence
photochemical reactions of molecular contaminants. This report presents the effects of VUV intensity and surface
temperature on photo-deposition and "photo-fixing" of dioctyl phthalate (DOP) films.
KEYWORDS: Picture Archiving and Communication System, Digital imaging, Particles, Cameras, Contamination, Digital cameras, Calibration, Semiconducting wafers, Aerospace engineering, Image analysis
The use of digital cameras and digital imaging software for the measurement of particle obscuration is discussed. Novel
calibration standards are used to evaluate the sensitivity and accuracy of commercially available digital cameras for
detecting microscopic dust particles and other contaminant features on surfaces. Lighting and illumination effects are
also illustrated and discussed. The digital image histogram of particles on a surface is shown to give good results for the
percent area coverage.
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