Advanced Materials and Processes for Large, Lightweight, Space ...
Advanced Materials and Processes for Large, Lightweight, Space ...
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, <strong>and</strong> shear strengths of these foams.Additionally, work is ongoing to identify methods <strong>for</strong> closing offthe porous surfaces, adding a polishable substrate, strengthenattachment locations, <strong>and</strong> 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 <strong>and</strong>optical substrate materials are also being considered.Figure 8. A 2-Meter PolymerMatrix Composite Mirror Produced<strong>for</strong> NASA JPL.Composite MirrorsOrganic Matrix Composite Mirrors: Organic matrix compositesmade with conventional carbon fibers have been considered<strong>for</strong> use in space-based mirrors <strong>for</strong> quite some time. Theyexhibit many properties that are extraordinarily good <strong>for</strong> mirrorsystems. They have a high specific stiffness <strong>and</strong> dependingon fiber choice, they canhave good in-plane thermalconductivity. Most importantly,carbon fiber compositescan also be designedwith near zero CTE <strong>for</strong> aspace-based mirror. Manyproblems exist, however,with using these materials<strong>for</strong> mirrors. Polymer resinsystems can be sensitive toa variety of environmentalfactors that put into questiontheir long-term stability.Highly uni<strong>for</strong>m <strong>and</strong>consistent 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 <strong>for</strong> NASA Jet Propulsion Laboratory (JPL) isshown in Figure 8.Ceramic Matrix Composite Mirrors: The desirable features ofCMC mirrors are that their CTEs <strong>and</strong> areal densities are lowwhile their strength, modulus, <strong>and</strong> fracture toughness are high.High strength <strong>and</strong> fracture toughness will be needed <strong>for</strong> durability<strong>and</strong> robustness in unshielded space-based mirror systems,such as the James Webb Telescope <strong>and</strong> the <strong>Space</strong>-Based Laser.Survivability <strong>and</strong> longevity requirements will dem<strong>and</strong> fieldedmirrors be made of highly durable <strong>and</strong> stable materials.There<strong>for</strong>e, as a community we need to continue to invest inscaling up these compositemirrors to very large sizes whileassuring thermal-mechanicalstability.Many DOD <strong>and</strong> NASAorganizations have sponsorednumerous businesses to developcarbon <strong>and</strong> SiC fiber-rein<strong>for</strong>cedSiC <strong>and</strong> siliconized SiC matrixcomposites. These ceramicmatrix composites (CMCs)have been fabricated in bothcontinuous <strong>and</strong> discontinuousfashion by numerous companies using a variety of processingroutes. As in glass composites, carbon fiber rein<strong>for</strong>cementsallow <strong>for</strong> CTE <strong>and</strong> strength tailorability, but polishability problemsarise when a weak carbon interfacial coating is depositedon the fibers to promote crack deflection <strong>and</strong> improve fracturetoughness. Polishability is improved by adding a thick claddingof dense CVD β-SiC to <strong>for</strong>m an optical substrate.Many processes can be used to fabricate structural CMCsubstrates using chopped carbon fibers coated with a carboninterfacial layer <strong>and</strong> bonded together in a SiC matrix. Forexample, AIBG <strong>and</strong> ECM from Germany, molds a mixture ofSiC <strong>and</strong> C particles, polymer binder, <strong>and</strong> chopped carbonfibers (with <strong>and</strong> without interfacial coatings) into a blank. Themirror blanks are then heat treated under vacuum to <strong>for</strong>m aporous graphitized green body. The green body is easilymachined into a lightweight shape using st<strong>and</strong>ard CNCmilling equipment. It is then infiltrated under vacuum withliquid silicon to <strong>for</strong>m 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 ground<strong>and</strong> cladded with slurry to <strong>for</strong>m a SiC+Si surface layer that ispolished to a surface roughness of 2 nm. Other fabricationmethods use chemical vapor infiltration to densify a pre<strong>for</strong>m ofcarbon or silicon carbide fibers. These composites are then cladwith either CVD β-SiC, PVD Si, or melt replication with Si to<strong>for</strong>m the optical substrate.Metal Matrix Composite Mirrors: Continuous <strong>and</strong> discontinuouscarbon fiber-rein<strong>for</strong>ced magnesium <strong>and</strong> 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 <strong>and</strong> C/Mg Mirrors.strength, modulus, thermalconductivity, <strong>and</strong> fracturetoughness of the base material.Carbon-rein<strong>for</strong>ced aluminum<strong>and</strong> 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