Advanced Materials and Processes for Large, Lightweight, Space ...

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esistivity. They typically show in-plane isotropy on a plane perpendicularto the foaming direction [17]. Research is currentlyunderway to engineer a better microstructure to improve thecompressive, tensile, and shear strengths of these foams.Additionally, work is ongoing to identify methods for closing offthe porous surfaces, adding a polishable substrate, strengthenattachment locations, and protecting the surfaces from atomicoxygen. Possible solutions to this problem include functionallygrading the materials, adding a facesheet to cover up the pores,or adding a cladding material to fill up the surface pores. Figure7 shows two examples of carbon foam, including one withfacesheets of ordinary composite laminate with a topcoat ofmulti-walled nanofibers bonded by an epoxy matrix. Hybridmirror designs using low CTE glass sols as the close-off andoptical substrate materials are also being considered.Figure 8. A 2-Meter PolymerMatrix Composite Mirror Producedfor NASA JPL.Composite MirrorsOrganic Matrix Composite Mirrors: Organic matrix compositesmade with conventional carbon fibers have been consideredfor use in space-based mirrors for quite some time. Theyexhibit many properties that are extraordinarily good for mirrorsystems. They have a high specific stiffness and dependingon fiber choice, they canhave good in-plane thermalconductivity. Most importantly,carbon fiber compositescan also be designedwith near zero CTE for aspace-based mirror. Manyproblems exist, however,with using these materialsfor mirrors. Polymer resinsystems can be sensitive toa variety of environmentalfactors that put into questiontheir long-term stability.Highly uniform andconsistent raw materials(composite prepreg –uncured fiber/matrixsheets) can be difficult to achieve. Finally, the multiphasenature of the composite causes a fiber-print-through phenomenon,which deleteriously affects the surface finish of theoptic**. It is important to note that this problem affects allclasses of composite materials, not just organic matrix composites.A large PMC mirror (with an areal density of 10kg/m 2 ) produced for NASA Jet Propulsion Laboratory (JPL) isshown in Figure 8.Ceramic Matrix Composite Mirrors: The desirable features ofCMC mirrors are that their CTEs and areal densities are lowwhile their strength, modulus, and fracture toughness are high.High strength and fracture toughness will be needed for durabilityand robustness in unshielded space-based mirror systems,such as the James Webb Telescope and the Space-Based Laser.Survivability and longevity requirements will demand fieldedmirrors be made of highly durable and stable materials.Therefore, as a community we need to continue to invest inscaling up these compositemirrors to very large sizes whileassuring thermal-mechanicalstability.Many DOD and NASAorganizations have sponsorednumerous businesses to developcarbon and SiC fiber-reinforcedSiC and siliconized SiC matrixcomposites. These ceramicmatrix composites (CMCs)have been fabricated in bothcontinuous and discontinuousfashion by numerous companies using a variety of processingroutes. As in glass composites, carbon fiber reinforcementsallow for CTE and strength tailorability, but polishability problemsarise when a weak carbon interfacial coating is depositedon the fibers to promote crack deflection and improve fracturetoughness. Polishability is improved by adding a thick claddingof dense CVD β-SiC to form an optical substrate.Many processes can be used to fabricate structural CMCsubstrates using chopped carbon fibers coated with a carboninterfacial layer and bonded together in a SiC matrix. Forexample, AIBG and ECM from Germany, molds a mixture ofSiC and C particles, polymer binder, and chopped carbonfibers (with and without interfacial coatings) into a blank. Themirror blanks are then heat treated under vacuum to form aporous graphitized green body. The green body is easilymachined into a lightweight shape using standard CNCmilling equipment. It is then infiltrated under vacuum withliquid silicon to form a C/SiC composite structure. [6,7],(Figure 9). It is claimed that there is no noticeable shrinkage inthe infiltration/conversion process with virtually no residualstresses left in the mirror. The composite is then rough groundand cladded with slurry to form a SiC+Si surface layer that ispolished to a surface roughness of 2 nm. Other fabricationmethods use chemical vapor infiltration to densify a preform ofcarbon or silicon carbide fibers. These composites are then cladwith either CVD β-SiC, PVD Si, or melt replication with Si toform the optical substrate.Metal Matrix Composite Mirrors: Continuous and discontinuouscarbon fiber-reinforced magnesium and aluminummatrix composites are currently being studied as possiblemirror structural substrates[7,18,19].As previouslymentioned, the volumefraction of carbon fibers canbe varied to tailor the netCTE in a composite material.It can also increase theCourtesy of NASAFigure 9. The Back Side of the500 mm C/SiC Mirror with anAreal Density of 8 kg/m 2.Courtesy of NASAFigure 10. Three-Inch DiameterC/Al and C/Mg Mirrors.strength, modulus, thermalconductivity, and fracturetoughness of the base material.Carbon-reinforced aluminumand magnesium matrix composites are highlymachineable to a lightweight configuration without warpage.These structural substrates are then coated with a CVD-Si72The AMPTIAC Quarterly, Volume 8, Number 1
cladding and polished to a surface roughness of 10 angstromsRMS (Figure 10).CONCLUSIONSFor many situations, monolithic glass remains the material ofchoice for mirrors, due to its ease of fabrication, high precisionon curvature, roughness amplitudes much less than the wavelengthrange of light, and very low distortions arising fromthermal fluctuations due to low thermal expansion. TheAMSD program has shown that monolithic glass mirrordesigns can be lightened to around 15 kg/m 2 and their costreduced substantially compared to Hubble-based technology.However, many believe that current glass technology is fastapproaching its theoretical limit, and that any further substantialreduction in areal density would require hybrid mirrordesigns. It is anticipated that an order of magnitude increase infracture toughness, compared to monolithic glass, would berequired in large segmented mirror designs for unshieldedspace-based applications. The high fracture toughness is desiredto avoid catastrophic mirror failure due to assembly accidentsand micrometeoroid impact damage.The future needs of the Air Force and the Department ofDefense will also require reductions in lead times, processingcosts, and launch load/weights. New materials and processingmethods for providing mirrors are needed. Mirror structuralsubstrates made out of advanced composite materials (metal,ceramic, and polymer), foams, and microsphere arrays do allowfor CTE and modulus tailorability, low-density, high strength,stiffness, and toughness. Small-size mirror structural substrateshave been fabricated from these new emerging materials, butscaling to 2- or 3-meter segments will require new resources,both in time and funding. Producing the surface finish and figurerequirements needed for visible quality optics from thesemulti-phase complex systems will be difficult. New methods ofpolishing, replication, and sol-gel or polymer spinning will berequired to produce quality optical substrates. Finally, researchwill be required to produce uniform, stress free reflective coatingsand dielectric stacks on such large mirror systems.In this article we have addressed the majority of existing mirrortechnologies as well as the current research areas and theirrelated technologies. While not all-inclusive, our discussionhighlights the important issues relating to constructing largemirrors for use in space. For additional information on specificprograms and materials approaches we refer the reader to thereferences.NOTES AND REFERENCESFor further information on this topic, please forward inquiriesto the authors at either Lawerence.Matson@wpafb.af.mil orDavid.Mollenhauer@wpafb.af.mil.* λ denotes the wavelength of incident electromagnetic radiation(mainly visible light in the case of optical mirrors)† Root Mean Square (RMS) – a statistical averaging functionthat characterizes the relative roughness of a surface.§ Thermal Stability (k/α) is defined as the thermal conductivity(k) divided by the coefficient of thermal expansion (α). InSI units, k is expressed in W/m-K, α in ppm/K.‡ Powder ceramics (and powder metals) are frequentlyprocessed by pressing them into a preformed shape known as ablank, or a green body. Green bodies are subsequently processedto final form by densifying them (typically by sintering or vitrification).**Fiber print-through is caused by a mismatch in propertiesbetween the fibers and the resin matrix binding them together.The resin will shrink upon cure while the fibers do not changeshape. The resin shrinkage and CTE/CME mismatch combineto form valleys in the resin-rich zones between adjacent fibers,resulting in significant surface roughness. This fiber printthroughphenomenon affects a composite mirror whether itwas polished or replicated except in the case where the use temperatureis exactly the same as the manufacturing/or polishingtemperature.[1] Private communication with Dr. Robert Jungquist ofBrashear LP; 615 Epsilon Drive, Pittsburgh, PA 15238[2] Optical Materials, Vol. 1; Edited by S. Musikant; MarcelDekker Inc.; New York, 1990[3] Air Force Research Laboratory, Materials DirectorateContract F33615-01-C-5800 with MSNW Inc., San Marcos,CA[4] Schott Glass Technologies, Inc. Zerodur Glass Ceramics,Doryea, PA (1982)[5] NASA Tech Days website at http://optics.nasa.gov/tech_days/tech_days_2002/index.html[6] Private communications with Steve Kendric of BallAerospace and Technologies. ‘skendric@ball.com’[7] “AFRL Workshop on Advanced Mirrors for DODApplications” Sept. 2002, Kirtland AFB, NM[8] R.B. McIntosh, Jr. and R.A. Paquin, “Chemical-mechanicalpolishing of low-scatter optical surfaces”. Applied Optics 19, p.2329 (1980)[9] Stahl P., Presentation at the Optical Society of AmericaConference, Optical Fabrication and Testing Session, TucsonAZ, 2002[10] “Silicon Carbide Technology for Large SubmillimeterSpace Based Telescopes”, by F. Safa, F. Levallois, M. Bougoin,and D. Castel, International Conference of Space Optics.ICSO97, Toulouse, Dec. 1997[11] “Silicon Carbide Technology for Submillimeter SpaceBased Telescope”, by F. Safa, F. Levallois, M. Bougoin, and D.Castel; 48th International Astronautical Congress, Turin,October 1997[12] Private communications with Eric Davenport of CoorsTek, Boulder Colorado. [edavenport@CoorsTek.com][13] Private communications with Mark Earley of Xinetics Inc.Devens MA[14] M.A. Earley and S.M. Daigneault, “Silicon DeformableMirror Technology,” Proceedings of the Topical Meeting onHigh Power Laser Optical Components, Oct 1989. Report No.NWC TP 7080 Dec. 1990[15] Private communications with William Goodman ofSchafer Corporation, Albuquerque NM. ‘wgoodman@schaferalb.com’The AMPTIAC Quarterly, Volume 8, Number 1 73
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