Michael Jacobson, Ruth Kneale, Fred Gillett, Keith Raybould, John Filhaber, Charles Carniglia, Ronald Laird, Dennis Kitchens, Ric Shimshock, Donald Booth
Acting as a prime contractor to the Gemini Project, the Optical Data Associates (ODA), with its major subcontractors, BOC Coating Technologies (BOCCT) and Deposition Sciences, Inc. (DSI), developed options for depositing protected silver coatings on the 8-M primary mirrors. The project began with a study that identified sputtering as the preferred deposition technique, defined a set of candidate adhesor and protective coatings for the silver layer, and simulated stack performance. The next phase involved pilot magnetron sputtering studies by BOCCT and DSI of designs involving silicon nitride and hafnia, respectively. ODA also developed mid-IR reflectance standards at (lambda) equals 10.6 micrometers to control the silver coating measurements in the critical 8 - 12 micrometers atmospheric window. The study results were successful, with both BOCCT and DSI producing Ag coatings with R equals 0.9920 +/- 0.0001 and protected Ag coatings with R equals 0.9910 +/- 0.0001. The Gemini Project coating plants are designed to sputter bare and protected Al and Ag coatings.
Modifying the emissivity of non-planar surfaces and objects with complex geometries has proven
to be a difficult task. Optical interference coatings have been successfully used to change the
spectral emissivity of a surface. However, typical deposition processes for these coatings, such
as evaporation, are line-of-sight processes that require complex masking and/or rotation systems
in order to coat non-planar surfaces. Objects with very complex geometries cannot be coated at all
by line-of-sight processes. In addition, evaporative processes often do not provide films with
good resistance to thermal cycling to high temperatures.
Low Pressure Chemical Vapor Deposition (LPCVD) is a non-line-of-sight process that unifonnly
coats all exposed surfaces of objects with complex shapes. DSI has developed an LPCVD
process that is capable of the routine manufacture of optical interference coatings. We have used
this process to deposit high quality, uniform, conformal coatings for the modification of the
emissivity of a variety of materials including ceramics and metals. Coatings of these substrates in
very complex, non-planar forms have been demonstrated. The coatings show excellent durability
and withstand repeated cycling to high temperatures. In this paper we will briefly describe the
coating process, show examples of some of the substrate forms, and describe the spectral and
environmental performance of these emittance control coatings.
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