ICMCTF2007 Session C4: Optical Coatings for Aerospace Applications

Friday, April 27, 2007 8:00 AM in Room Royal Palm 4-6

Friday Morning

Start	Invited?	Item
8:00 AM	Invited	C4-1 Optical Thin-Films for Space Applications P.D. Fuqua, J.D. Barrie, C.-T. Chu (The Aerospace Corporation) Optical thin-films are used in a wide range of challenging environments, but few are as challenging as those associated with space. Optical thin-films are essential to a spacecraft's thermal stability, power generation, and attitude control. Some missions require high-performance coatings to enable astronomical or terrestrial observation. The high cost of building and launching satellites drives requirements for extreme durability in the harsh space environment. Optical thin-films may be required to survive exposure to energetic charged particles, hyperthermal monoatomic oxygen, and vacuum ultraviolet light with minimal changes in transmittance, reflectance, absorption and emittance. The need for broadband high-reflectivity mirrors has pushed the community into using silver coatings that require protective overcoats to prevent degradation in air in the years between deposition and launch. Furthermore, innovative, lightweighted mirrors can be susceptible to changes in surface figure due to a film's intrinsic and environmental stresses. Lastly, contamination is an omnipresent issue in satellite technology, and its effects on optical performance must be understood to assure mission success.
8:40 AM		C4-3 Mechanical Testing of Reaction Bonded Silicon Carbide for Space Qualification D. Witkin, I. Palusinski, J. Geis (The Aerospace Corporation) Silicon carbide has been proposed as a substrate for optical thin films in aerospace applications. The mechanical properties of one silicon carbide product, reaction-bonded silicon carbide (RBSC), are being investigated. RBSC is formed by the infiltration of molten silicon into a silicon carbide particulate green compact, forming a composite material consisting of carbide particles in a silicon matrix, with a reaction product forming at the particle-matrix interface. Mechanical testing has included four-point flexural bending (modulus of rupture), biaxial flexural strength, and fracture toughness, all conducted per appropriate ASTM standards. Samples were harvested from three separate slabs, with consistent sample locations and orientations from each slab. Weibull plots of results of four-point bend and biaxial flexure testing indicate little variation in Weibull modulus or characteristic strength between the three slabs (10-23 samples per slab). Fracture toughness testing is made difficult by the composite nature of the material because of the wide variation in fracture toughness of the two materials (1.0 and 4.5 MPa m1/2 for Si and SiC, respectively) and the fundamental question of whether the material’s fracture behavior satisfies the assumptions of linear elastic fracture mechanics. Nevertheless, for designers of optical components, conservative estimates of fracture toughness are possible based on the test results.
9:00 AM		C4-4 Impact of Plasma and Atomic Oxygen Cleaning on Optical Coatings and Substrates C. Bao (CVI Laser, LLC) This paper presented surface cleaning results of plasma and atomic oxygen cleaning methods on optical coatings and substrates. Surface cleanness is evaluated by contact angle measurement. Surface cleanness aging effect is investigated. Furthermore, impact of atomic oxygen cleaning is researched on DUV optical coatings.
9:20 AM	Invited	C4-5 Dimensionally Stable and Survivable VIS/LWIR Coatings for the Space Effects Environment W. Goodman (Schafer Coproration); D. Reicher, S. Peplinkski (AFRL Optical Components Laboratory) This paper presents background on the design of a survivable coating in the context of cryogenic and space radiation environments. Many potential users are interested in dimensionally stable, survivable optical coatings for operation in the visible and long wavelength infrared (8-12 microns), for temperatures as low as 35 K, and in the Space Effects Environment (SEE). Schafer Corporation demonstrated a VIS/NIR dielectric coating deposited by the Optical Components Evaluation Laboratory at the NASA Marshall Space Flight Center (MSFC) X-Ray Calibration Facility (XRCF). The coating reversibly changed the figure of a Schafer SLMS™ (Silicon Lightweight Mirror System) by only 0.5 nm RMS for cyclic testing between 300 and 25 Kelvin. A coefficient of thermal expansion matched C/SiC mirror mount was part of the experiment. As part of another program Schafer designed, manufactured and tested a low emissivity, high-reflectance VIS/NIR/LWIR coating designed from individual elements in the periodic table. This paper discusses the relevant environments, our unique procedure for designing coatings, and preliminary results of the coating performance. OCEL would like to work with government and commercial partners to fully space qualify the coating.
10:00 AM		C4-7 Growth and Control of Residual Stress in Optical Coatings During Rapid Mirror Assembly Processing M.Y. Chen, J.G. Jones (Air Force Research Laboratory); L.E. Matson (Air Force Research Laboraotry); T.-I. Mah, R. Bhattacharya (UES Inc.) Recent advances in fabrication of ultra-lightweight precision mirrors have demonstrated great potential of rapid mirror assembly processing via replication technology. The high cost associated with grinding and polishing steps of each mirror are eliminated. Here a mandrel is polished to an inverse figure shape, coated with the appropriate reflective layer and optical membrane and then reinforced with a structural substrate. We have successfully fabricated a magnetron sputtering approach for building replicated optical membranes from Ag-coated SiC/Si3N4 nano-laminates. The optical membranes were bonded to structural substrate using negative CTE nano-powder-reinforced geopolymer. Close control and design of optical coating stack and its bonding/release layer to reach the desired residual stress is essential to the success of this technology. The residual stress may be controlled or alleviated by careful design of the optical coating stack through material selection or adjustment of deposition parameters. AFRL Materials and Manufacturing Directorate (ML) has unique automated deposition chamber facility that allows coatings from any of three deposition sources simultaneously or sequentially. The deposition process uses process control algorithms developed by ML for gradient and multilayer coatings, and provides a programmable flexibility to vary the composition of the coatings continuously. It has been successfully demonstrated to form nanocomposite coatings comprised of specific quantities of pre-selected metal, ceramic, and carbon to form multilayer coatings, or a combination of the two. Process regulation is necessary for the deposition of multiple materials simultaneously over an extended period of time with minimal process drift. The automated deposition chamber provides this valuable regulation capacity. In this paper, we present its utilization to control the residual stress level in a coating stack consisting of reflective coatings, bonding layers, and nano-laminates for space mirrors.
10:20 AM		C4-8 Energy Sources, Dynamics, and Thermodynamic Considerations for Coatings Performance, Reactions, and Degradation Mechanisms in Space Environments G. Pippin (The Boeing Company) During flight, the thermodynamic systems of which spacecraft coatings are a part, are open, time varying systems, which may be far from equilibrium. Under flight conditions in any orbit, there are many sources which can, and do, bring energy into this system. Such sources may include neutral atomic and molecular species, ionizing radiation from the ambient low energy plasma, solar wind, trapped belt radiation, and cosmic rays, solar photons across the IR, visible, UV, and VUV wavelengths, and soft x-rays, impacts from natural micrometeoroids and man-made debris. In addition, the temperature variations during an individual orbit may be significant, changing the equilibrium point of the system. This presentation will review the potential contributions of the external influences to the coating system. The intent is to help the coating s user to select materials that best achieve the desired engineering result, given the constraints of the rather energetic environment. Achieving specific engineering performance requirements such as flexibility, ease of application, durability, lifetime, cost, thermal and/or electrical conductivity, optical and mechanical properties, is challenging for space applications. The demands placed upon coatings by the operational environment often conflict with the requirements of the engineering application. Using examples from previous flight experiments, the effects of the space environments on coatings will be described and mitigation techniques, and their limitations, discussed. The energetics of space environments will be discussed with respect to coating applications.
10:40 AM		C4-9 Post Flight Analysis of MISSE Optical Coatings A.F. Stewart (Boeing DES) The Materials on the International Space Station Experiment (MISSE, 2001-2005) was a test bed for determining the importance of the effects of the low earth orbit space environment on materials. For optical materials on board MISSE, the space environment presented some unique problems. Variations in temperature, exposure to ultraviolet radiation, particle radiation, atomic oxygen and contamination from the immediate environment were expected to play significant roles. Post flight analysis of optical coatings and windows has shown that the MISSE samples received low levels of contamination and that the samples, while degraded for some criteria, were in remarkably good condition. This study will review data from different aspects of this experiment with emphasis on optical properties such as light scatter and analysis of the contamination found.