SRI International in collaboration with Saint-Gobain/Norton Industrial Ceramics
Corporation and Allison Engine Company is developing a new SFF process, Direct Photo
Shaping, for the fabrication of functional ceramic components. The process is based on the layerby-layer
photocuring of ceramic slurries containing monomers curable by visible light. Each layer
is photoimaged by a liquid crystal display (LCD) or a digital light processing (DLP) projection
system. After binder removal and sintering, the properties of silicon nitride tiles prepared according
to the Dire~t Photo Shaping process were found to be comparable to properties of tiles formed by
conventional methods.Mechanical Engineerin

Annavajjula, Durga

Chen, Eric

Groseclose, Lance

Hardy, Anne

Ho, Lung-Hua

Liu, Chunling

Liu, Susan

Lombardo, Steven J.

Mangaudis, Mike

Narang, Subhash C.

Sharma, Sunity

Stotts, John

Ventura, Susanna C.

English

Texas ScholarWorks

A NEW SFF PROCESS FOR FUNCTIONAL CERAMIC COMPONENTSSusanna C. Ventura, Subhash C. Narang, Sunity Sharma, John Stotts,Chunling Liu, Susan Liu, Lung-Hua Ho, and Durga AnnavajjulaSRI International, Menlo Park, CASteven 1. Lombardo, Anne Hardy, Mike Mangaudis, and Eric ChenSaint-Gobain/Norton Industrial Ceramics Corporation, Northboro, MALance GrosecloseAllison Engine Company, Indianapolis, INABSTRACTSRI International in collaboration with Saint-Gobain/Norton Industrial CeramicsCorporation and Allison Engine Company is developing a new SFF process, Direct PhotoShaping, for the fabrication of functional ceramic components. The process is based on the layer-by-layer photocuring of ceramic slurries containing monomers curable by visible light. Each layeris photoimaged by a liquid crystal display (LCD) or a digital light processing (DLP) projectionsystem. After binder removal and sintering, the properties of silicon nitride tiles prepared accordingto the Dire~t Photo Shaping process were found to be comparable to properties of tiles formed byconventional methods. .INTRODUCTIONRapid prototyping is presently being applied to the fabrication of nonstructural materials bymeans of computer-aided design/computer-aided manufacturing (CAD/CAM) technology, whereincomputer files descriptive of the object are used to create parts made from materials such as UV-curable polymers. The components fabricated. in this fashion are considered nonfunctional, andtheir main application is for iterative design evaluation in molding making. There is an increasingdemand to extend rapid prototyping technology to the fabrication of functional components withengineering properties and dimensional tolerances comparable to those of conventionally producedcomponents. _.The development of solid freeform fabrication (8FF) manufacturing technology is expectedto allow the fabrication of functional prototypes from advanced ceramic, metallic, and multiphasematerials for structural and electronic applications. The SFF approach to net shape forming holdsgreat promise for rapid prototyping of ceramic components through simplification of theprocessing cycle (e.g., elimination of the time consuming steps of pattern making and moldfabrication).In this paper we describe a new SFF process, Direct Photo Shaping, and its use for thefabrication of silicon nitride components.327DIRECT PHOTO SHAPING PROCESSThe Direct Photo Shaping process is described in Figure 1. A liquid crystal display (LCD)or a digital light processing (DPL) projection system are used as an electronic mask to buildlithographic images onto a photopolymerizable slurry containing ceramic powder. A visible lightsource is coupled with the projection system. The image to be projected is changed according tothe CAD model infoITIlation. The photopolymerizable slurry contains the ceramic powder~ adispersant~ photocurable monomers~ and a visible light initiator. The slurry is dispensed onto thebuild platform and leveled with a doctor blade. Each film is photoexposed by visible lightprojection. After each exposure~ a new film of photopolymerizable dispersion is applied on thebuild platform. When the fabrication is complete~ the green ceramic body is removed from theplatform and rinsed with a suitable solvent to dislodge any uncured material.The ceramic forming process is based on photo-gelcasting~ since the green body is formedby photocuring of a slurry of the ceramic powder in a solution of photopolymerizable monomers.Gelcasting [1] has been shown to produce complex-shaped~ near-net-shape parts. Gelcast greenbodies are generally strong and machinable~ and after sintering, highly dense ceramic parts areobtained./_computerI =1/Dispenser/Coater~//CM-320581-331Figure 1. Schematic illustration of the Direct Photo Shaping process.328EXPERIMENTALPowder ProductionSilicon nitride powder (Ube Industries, Yamaguchi, Japan) and 4 wt% Y203 sintering aidwere coprocessed by aqueous milling to reduce the particle size and to homogeneously distributethe sintering aid. Milled slurries were passed through a magnetic separator to remove ironcontamination and were then filtered and concentrated. The concentrated slurries were dried byfreeze drying. Details of the powder processing have been previously described [2].Ceramic slurryCexamic slurries were prepared by ball-milling the silicon nitride powder in thephotoeurable monomers, after addition of a suitable dispersant, a solvent/plasticizer and thephotoinitiator. The viscosity of the ceramic slurries was determined with a Brookfield DV-II+viscometer.Component FabricationTiles (3"x3"xO.5") were formed according to three different methods to compare theirmechanical properties. Tiles were prepared by multilayer deposition and photoexposure to a quartztungsten-halogen lamp. The photoeurable slurry comprised of 68 wt% silicon nitride, a suitabledispersant, photocurable monomers, plasticizer, and a visible light photoinitiator. Each layerapplied was 2 mil thick. Additionally, tiles were formed by either uniaxially pressing the drypowder followed by isostatic pressing (30 ksi) or by theImally curing (120°C) monolithic tilesfOImed from a slurry of the same composition as used for the SFF process.Other complex shape samples were prepared by direct photoexposure using a 1000 wattxenon arc lamp from Oriel Corporation, Stratford, CT with a filter that allows only thetransmission of radiation from 420 om to 630 om, and reduces theffilal effects.Binder solvent was removed from the photoeured samples by heating in air to 450°C over48 hours and then heating to 650°C. Samples were presintered to improve their handling strengthafter the binder removal and were then densified by HIPing using glass encapsulation.Sample CharacterizationSamples for microstructural evaluation were prepared by polishing fracture surfaces andthen plasma etching using a 95% CF4/5% 02 carrier gas [3]. This etching technique removes theSi3N4 and leaves the intergranular phases. Sintered densities were measured in deionized waterusing the Archirp.edes method. .Specimens for flexure testing were machined either to 1.5 mm x 2 mm x 25 mm or 3 mm x4 mm x 50 mm size. Specimens were cut both parallel and perpendicular to the depositiondirection. In both cases the lamination direction was parallel to the long dimension. Aexurestrength was measured by four-point bending in accordance with ASTM method C1161.Hardness was measured with a Vickers diamond indenter at a load of 0.5 kg. Fracture toughnesswas measured by both the indentation method and the controlled flaw method.329RESULTSBy pretreating the silicon nitride powder with a suitable dispersant, we were able to prepareslurries containing silicon nitride up to 55 vol%. These slurries were photocured as thin strongfilms. The typical working thickness of each layer used in our multilayer deposition process wasabout 2 to 4 mil. The sl~ composition was optimized to improve interlayer adhesion andeliminate any possible delamination during debinderization. This was achieved by reducing thelayer stiffness and using additives to produce a tacky top layer. This optimization allowed us toprepare freeformed tiles, that after debinderization and densification, were found to havemechanical properties comparable with that of silicon nitride pressed samples.After binder removal, the samples showed no blistering or discoloration and tiles preparedwith the optimized slurry composition showed minimal delamination. Carbon analysis showed noresidual carbon. After HIPing the samples were >98% dense, as shown in Table 1, and noevidence of the original layers was observed by SEM examination of cross-sectional surfaces ofthe SFF tile. Figure 2 shows representative etched surfaces for a sample prepared by the SFFmethod and for a tile formed by dry pressing. Both samples show a pronounced bimodaldistribution of grain sizes and as is typical of in situ reinforced Si3N4 show a pronounced acicularmicrostructure.Figure 3 shows two representative silicon nitride green parts, SRI and DARPA logos, thatwere initially selected to demonstrate the feasibility of Direct Photo Shaping to build multilayerswith variable cross-section and contours.Mechanical PropertiesTable 1 summarizes the mechanical property data for tiles formed by dry pressing, thermalgel casting a monolithic tile and layered tiles formed by the solid free form method. The threeforming techniques were used to assess the effect of the forming technique on the mechanicalproperties and to verify that the solvent and polymers could be removed without deleteriouslyaffecting the densification and the mechanical properties. In many cases, the gelled monolithic tilescould be processed to near full density and the mechanical properties were comparable to thosemeasured for tiles formed by dry pressing. This indicated that the organic solvents/polymers couldbe removed without affecting the mechanical properties of the Si3N4 • In some cases themonolithic samples showed lower densities and strengths than the dry pressed and layeredsamples; this was attributed to slight differences in the HIPing cycles which were used for thesesamples. The sample formed by the solid free form technique showed similar properties to the drypressed samples with MOR values slightly in excess of 1 GPa. The flexure strengths for thelayered tiles were not dependent on the orientation relative to the casting direction. FracturetoiIghness measured by Vickers indentation was 4.0.tv1Pa m1l2 for the dry pressed sam~les and ~.5tv1Pa m1/2 for the solid freeform sample. Fracture toughness as measured by controlled IndentatIonwas slightly higher, as is typically observed, and was comparable to values previously reported onoptimized slip~ material of the same composition [l].330Table 1SUMMARY OF MECHANICAL PROPERTY DATA FOR TILES FORMED BYUNIAXIAL PRESSING, THERMAL GELATION AND LAYER-BY-LAYERFABRICATION METHODDensity Hardness KIC(MPa './m) MOR(MPa) WeibullIparametersSample Forming (glcc) (GPa) Indent Indent 3x4x50 1.5x2x beta nFracture rom 25mmC012 Press 3.22 17.0 4.0 5.9 844 937 23 30L08 Press 3.22 15.5 4.0 6.1 832 976 24 30L02 Gelled 3.22 NA NA 5.2 839 NA NA NAL08 Gelled 3.10 15.8 3.8 4.1 580 779 13 13#202a SFF 3.21 16.5 4.5 NA NA 1018 16 15#202b SFF NA NA NA NA NA 1052 25 15Note: Specimens for 202a were cut so that the casting planes were parallel to specimenfaces; specimens for 202b were cut so that the casting planes were Perpendicular tothe spe(~~men faces. In both cases the lamination direction was parallel to the longdimension of the specimen.CONCLUSIONSWe have described a new SFF process, Direct Photo Shaping, based on theadditive fabrication of layers photoimaged by a liquid crystal display (LCD) or a digitallight processing (DLP) projection system. By optimizing the slurry composition wefabricated layers that adhered to each other and showed minimal delamination afterremoving the binder. Mechanical properties of silicon nitride tiles prepared according to theDirect Photo Shaping process were comparable to properties of tiles formed by drypressing and were slightly in excess of 1 GPa.ACKNOWLEDGMENTSThis work has been done under the support of DARPAlONR contract No.NOOO1495COO19. The authors would like to thank Dr. William Coblenz of DARPA andDr. Steven Fishman of ONR for their continuous support and valuable suggestions.331REFERENCES1. 0.0. Omatete, M.A. Janney, and R.A. Strehlow, "Gel Casting- A New Ceramic FormingProcess", Ceram. Bull. 70(10), 1991.2. V.K. Pujari, D.M. Tracey, M.R. Foley, N.!. Paille, P.J. Pelletier, L.C. Sales, C.A. Wilkensand R.L. Yeckley, "Reliable Ceramics for Advanced Heat Engines," Am. Ceram. Soc. Bull., 74(4), 86-90, 1995.3. K.N. Seibein and W.M. Lovington, "Plasma Etching of Si3N4"; pp 319-329 in MicrostructuralScience, vol. 16. Edited by H.J. Cialoni, M.E. Blum, G.W.E. Johnson, and G.F. VanderVoort,ASM International, Metals Park, OH, 1985. .332(b)Figure 2. Representative microstructures of (a) a sample prepared according toDirect Photo Shaping process and (b) a sample prepared by uniaxially pressing.333Figure 3. Representative green silicon nitride parts built by Direct Photo Shaping.334

A New SFF Process for Functional Ceramic Components

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A New SFF Process for Functional Ceramic Components

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