The feasibility of using the Fused Deposition of Ceramics (FDC) process to rapidly fabricate
functional quality advanced ceramic components has been demonstrated multiple extrusion
heads enable the deposition of spatially engineered ceramic microstructures on the scale of 250
um. This unique capability of FDC allows components to be built with combinations of materials
and properties that are difficult or impossible to produce using conventional fabrication
processes. Some concepts will be presented, along with examples of multiple material laminates
produced using FDC. Strength data will be presented which demonstrates the performance
improvement possible using spatially engineered microstructures.Mechanical Engineerin

Clancy, R.

Gasdaska, C.

Jamalabad, V.

Ortiz, M.

Popovitch, Dragan

Virkar, Anil

English

Texas ScholarWorks

Functionally Optimized Ceramic StructuresC. Gasdaska, R. Clancy, M. Ortiz# and V. lamalabadAlliedSignal, Inc. Research and TechnologyMorristown, Nl 07962Anil VirkarDepartment of Materials Science and Engineering, University of Utah,Dragan PopovitchAdvanced Ceramics ResearchTucson, AZThe feasibility of using the Fused Deposition of Ceramics (FDC) process to rapidly fabricatefunctional quality advanced ceramic components has been demonstrated Multiple extrusionheads enable the deposition ofspatially engineered ceramic microstructures on the scale of250pm.. This unique capability ofFDC allows components to be built with combinations ofmaterialsand properties that are difficult or impossible to produce using conventional fabricationprocesses. Some concepts will be presented, along with examples ofmultiple material laminatesproduced using FDC. Strength data will be presented which demonstrates the performanceimprovement possible using spatially engineered microstructures.IntroductionBy suitably combining materials, it is possible to achieve improvements in propertiessuch that the strength and toughness of the combination is greater than that for either materialalone. Two multi-material approaches used for strengthening and flaw tolerance in brittlematerials are development of residual surface compressive stresses due to coefficient of thermalexpansion (CTE) mismatchl , and fabrication of silicon nitride/ boron nitride fibrous monolithcomposite structures2.The concept of using combinations of ceramic materials with different coefficients ofthermal expansion (CTE) to produce residual stresses on cooling from the fabricationtemperature is well documented in the literature l . Systems which rely on phase changes toproduce residual stresses have also been studied3. Strength increases of 300% have beenobserved some of these systems. Reliability and flaw tolerance is also improved. In thepresent work, we are illterested in pursuing multi-material combinations which are com.patibleand co-sinterable with silicon nitride, the leading candidate for high temperature structuralapplications such as gas turbine engine components. The I-dimensional aspect ofpart buildingusing FDC allows for.structures more complicated than the simple laminates prepared.usingtraditional methods4•Fibrous monolith (FM) materials are essentially unidirectional composites in which thevolume fraction ofthe matrix material is < 20%. The most common example consists ofa# Located at AlliedSignal Engines Division, Phoenix, AZ705Cell boundarySilicon nitridepolycrystalline cellFigure 1. Cross-section ofan idealizedfibrousmonolith microstructure.structure containing silicon nitride filaments surrounded by a sheath of boron nitride (see Figure1).Cracking in these materials tends to be confined to the boron nitride layer, resulting inextensive delamination under bending stresses2. Alternatives to boron nitride are beinginvestigated which would allow a greater amount of fiber pullout and work offtacture undertensile loading conditions. Typically, FM materials are produced using filament-windingtechniques using fibers which have the FM structure. The FDC process offers a more costeffective method for producing these materials.The solid freeform fabrication process we are using is based on the Stratasys machine.This technique uses solids loaded filament to deposit roadsofa binder/ceramic powder mixture4•For multi-material parts, we are using a Stratasys 1650 dual-liquefier rapid prototyping machine.LaminatesIn order to take advantage of the capability of FDC to spatially distribute materials, systemsmust be found which not only result in desirable material property combinations, such as residualstress production, but which are also capable ofbeing co-sintered without excessive cracking ordelamination. Toward this end, we have been examining the use of particulates dispersed in asilicon nitride matrix to increase the CTE. Two candidate material systems for use in siliconnitride are listed in Table 1.Material Melting/Decomposition Coefficient of Young's Density (g/cmJ )Tempertaure (OC) thermal ModulusexpanSIOn (GPa)(fC x 106)SiC 2700 4.3 448 3.22TiN 2940 9.35 260 5.22Table 1. Propertles ofcandldate partlculate dlspersants.706In order to evaluate the effectiveness of these materials in strengthening silicon nitride,bi-materiallaminates were prepared using slip-casting. The materials consisted of silicon nitrideand silicon nitride plus up to 30 weight% of either TiN or SiC. The samples were sintered anddense samples were successfully prepared. Test bars were machined and room temperaturebending strength measured. Results for samples containing SiC appear in Figure 2. Thegeometry of the samples is also illustrated. The resultsindicate that average strength can beincreased by up to ~27%. In addition~ the decrease in the arnountofstrengthening at 800 °c isconsistent with the drop in residual stress as the test temperature approaches the temperature atwhich stresses start to set on cooling.Verification that the increase in strength was due to residual stress was accomplished by amethod of progressive material removal and strain measurement using strain gauges. In thistechnique3 a strain gauge is mounted on a laminated sample and the build-up of strain ismonitored as material is progressively removed ftom the opposite surface. A residual stress of~157 MPa was measured using this technique. The differential strain between laminate materialsSN-20/80-SN.1~rnSEIteSN•'V..... '1.'V'V'V~" · .. ········0······f·30170SN-30/70-SNlaminate............ " " .SN-30170-SN~ laminate.. (800°C)ooo10/90SN-10/90-SNlaminate.............".....~"·°1180170- 160'(jj::::=.-..c:.... 150Clc:(J)...1il 140-cc:(J).c 130-c:'0a.120IC')110100SN= Si3N4 + sintering aids (see Table 1)10/90 =90wt% SN+ 10wt% SiC (composition 1)20/80 = 80 wt% SN + 20wt% SiC (comp. 2)30170 = 70wt% SN + 30 wt% SiC (comp. 3)k::::-23 mm >1Geometry of 3-point specimenused for testing laminates.Figure 2 Strength data obtainedin 3-pointbendingfor laminates preparedfrom the indicated compositions. Datafor boththe laminates and the "core"materials are presented and connectedby the lines. Individual data points areplotted, with the average strength for each group represented by the filleddiamond symbol. In all cases" the average strength ofthe core (10/90, 20/80 or30/70) is less than. thirlaminate·strength,if1dica;t~ngistr¢ngthenirtgduetosurfacecompressive stresses. The drop in strength at 800c>C is also consistent with thepresence ofresidual stress due to thermal expansion mismatchbetween the coreand outer skins. The strength ofthe outer skin by itselfis shown by the grouplabeledSN707was measured as 5.4 x 10-4 • Ifwe assume that on cooling from the sintering temperaturepermanent stresses begin to set in at 1300 °c, then a difference in CTE of 0.42 x 10-61°C wouldproduce a differential strain of ~ 5.4 x 10-4.Using combinations ofmaterials·with CTE mismatch to produce residual stresses raises anumber of issues. For example, residual stresses must not be so large as to induce<spalling ordelamination between materials. In addition, the bonding at the interface and the presence ofinterface defects are important characteristics in determining the ultimate degree ofstrengthening. The spalling and delamination question is strongly dependent on the interfacialbonding and Mode II fracture energy at the interface. Techniques for measuring interfacialfracture energy are under development. However, for the problem of failure from the surfaceversus failure at the interface - where residual tensile stresses develop - has been outlined for thecase of bending. As an example, assuming a three layer laminate stressed in bending thecorresponding fracture strengths (in flexure) are:o dzE~EoO'P(s) = O'sP + (1 - v)dfor failure from the surface, andd ° 2dlE~EoO'P(I) = dzO'IP - (1 - v)dzfor failure from the interface. Where, O'~F and O'YF are the true fracture strengths for failurefrom surface and interface flaws, respectively, dl is the outer layer thickness, d2 is the inner layerthickness, d = 2dl + d2 and ~E is the free strain difference between materials (related to thedifference in CTE and ~T). E is Young's modulus.The preceding equations show that to maximize the strength as well as the damageresistance, the following are required: (1) high O'°IF: This can be achieved by improvingprocessing such that interior flaws are kept to a minimum. (Failure from the interior may not bebad either); (2) the inner layer should be fully encapsulated in the outer layer such that there is nochance of introducing damage in the inner layer during service (complete confinement); (3)enhance ~Eo to as high a value as possible such that a high surface compressive stress exists.This provides damage resistance. However, the surface compressive stress should not be so largeas to cause delamination; (4) decrease d1 to a value small enough (relative to d2) such thatinterior tensile stress is kept toa minimum, yet dl is large enough to prevent contact-induceddamage from penetrating too deep into the.inner region. Preliminary calculations show an outerlayer of thickness ~150 to 250 Jlm to be more than adequate for most applications. For acomponent of 5 mm thickness, the d2/2dl ratio is then between 9 and 15.67. For a component of20 mm in thickness, with an outer layer thickness of~500 Jlm, the ratio is 19. The correspondingtensile stress in the inner layer is very small. This latter example corresponds to the case of theSiC insert being examined by AlliedSignal Engines (see next section), where the insert is located~ 380 microns below the silicon nitride surface and the overall thickness is ~ 20 mm.708Design using FDCWhile the preceding work has demonstrated the feasibility of using compatible, co-sinterablematerial combinations for strengthening, the ultimate goal is to use the unique spatial controlcapabilities of FDC to design and build components. As part of the project on solid freeformfabrication, the Engines division at AlliedSignal (ASE) has been working on designing a multi-material turbine blade which uses residual stress strengthening.The design used for modeling consists of a ceramic blade inserted in an AF-2 alloy disk (Figure3). In this initial work, design analysis was focussed onthe attachment area since this region isknown to experience the highest stresses and contains the area most likely to fail. Stresses in theattachment area were analyzed for a rotational speed of 33,500 RPM and a uniform temperatureof 1450 OF . Based on these conditions a uniform stress of24.1 ksi was imposed outside theattachment area. The coefficient of friction between blade and disk was assumed to be 0.6. Thiswas based on previous experience with ceramic blade attachment systems. Using these inputs, abaseline condition was analyzed. The peak stress was calculated to be 105 ksi and located justoutside the contact area between the blade and disk. It arises from the high frictional loadbetween blade and disk.Various design strategies were evaluated to reduce this peak stress and are illustrated inFigure 3. Although a silicon carbide material has been modeled as the higher CTE component,particulate based silicon nitride composites would provide nearly equivalent behavior. Three ofthe four geometries examined utilized combinations of SiC and SbN4 to produce residualcompressive stresses in the contact area. The geometry illustrated in Figure 3d - a bladecontaining a central cavity to reduce weight - actually caused a 77% increase in peak stress.Figure 3c illustrates the most promising approach. A silicon carbide insert in the contact areasignificantly reduces the peak stress., //\';'1.015I-t- s" '!~SiC /1<;-+-- /01 I[~"j AF-2 lJAP-2a b c dFigure 3. Various geomerties evaluatedfor reducing the peak stress in the contact area.The amount of the reduction is a function of set temperature; i.e., the temperature at whichsufficient rigidity is obtained to start developing residual stress. The higher the set temperature,the larger the residual stress which can be produced by differential contraction. This is shown in709Figure 4, where the maximum stress at 787 C is given as a function of set temperature. Based ona set temperature of 1300 C, the peak stress is reduced from 105 ksi to 72.2 ksi.120.0~ 100.0,nU)80.0e...U).9l 60.0'u;CS 40.0><C'll 20.0E~r-------. ---- -------i1-- .-i ------ --0.0700 900 1100 1300 1500 1700Set temp., deg CFigure 4. Variation in the maximum tensile stress in the blade attachment area asa function ofset temperature.Fabrication of a complete blade using multi-material FDC is expected to begin shortly.In the meantime, the feasibility of the process has been demonstrated in sub-element testing.Figure 5 shows a cross-section of a turbine blade with a narrow band of insert material at theattachment area.Figure 5. Cross-section ofb a e built with insert material in theattachment area (black line at right).Fibrous MonolithsA major concern regarding the fused deposition of filaments with fibrous monolith structurewas whether or not the fibrous monolith structure would be retained as the filament was meltedin the liquefier and extruded. In order to address these concerns, Advanced Ceramics Research(ACR) fabricated a fibrous monolith filament containing a single core and sheath, using a modelsystem containing alumina and carbon black for this evaluation. The core material was RU9710(Stratasys, Inc.) binder containing 55 vol% alumina while the sheath consisted ofRU9 ~indercontaining 42.75 vol% alumina and 2.25 vol% carbon ?lack.. Th~c?re and sheath constituted82.5 and 17.5vol% oftb.~ 0.070 inch diameterfilament,.r~$pectiyely.A number of test pieces were produced and a crpss-sectionof one piece is shown inFigure 6. In ordertotnor¢clearly discern the stmcture,the roads inthispiec~were laid down on0.44 mm42.75v/o A1203, 2.25 v/o C-blackFigure 6. Cross-section ofsingle-cell fibrous monolith sample prepared byfuseddeposition offibrous monolith filament.top of e(lchother, resulting in the voids which appear in Figure 6. As seen in Figure 6, thefibrous monolith microstructure Was retained during the deposition process. A 0.015 inchdiameter nozzle was used for deposition, resulting in a road width of~ 0.017 inches. ACR alsoprepared a 0.070 inch diameter filament containing 12 cells, each with a core and sheath, ofalumina and carbon black respectively.A micrograph of the. filament cross-section iS$hown in Figure 7. The compositi?n of thecell and cell walls was again alumina and aluminalC-black, respectively. The composition of thecell boundary material was adjusted slightly: the solidsloading was increased from 45 to 54volume % solids. Stearic acid waS used as the dispersantfor both cells and cell walls.The cell structure retention is excellent after extrusion. Also shown in Figure? is across-section of a sample built with some overlap between adjacent rows... Thisoverlapiscommonly used in FDCto avoid the introduction ofstrength limiting voids. Thoughsome lossof structure occurs when roads overlap during a build, the overall retentioll of the FMstructure isquite good. Deposition parameters will need to be controlled closely to avoid excessive.smearing under these conditions. However, it appears that the microstructural scale o],tained (5micron cell wall thickness) using single cell filament will be sufficient to get theadvatltages ofthe fibrous monolith microstructure.711.-- 1.9mm-+... 1.78 rom ----..Figure 7. Micrographs ofmulti--cellfibrous monolithmodel material showing thestructure inthe as-preparedfilament (left), after extrusion through a 0.3 mm nozzle(center) and after deposition with a O.076mm overlap between roads (right).The work on FM structures is being extendedto materials based on silicon nitride as the corematerial and lanthanide glasses· for the cell walls. Some promising lanthanide·glass compositionshave been prepared and will be used to manufacture filament in the next year.AcknowledgementsThe support of DARPA under contract NOOO14-97-C-0115 is gratefully acknowledged.Also, we would like to thank Sriram Rangarajan ofRutgers University for fabricating the partillustrated in Figure 6, and Don Baskin for his assistance in the preparation ofthe laminates.References1. R. Sathyamoorthy, A. V. Virkar, and R. A. Cutler,J. Am. Ceram. Soc., 75 [5] 1136-1141 (1992).2. D. Kovar, B. King, R. Trice and J. Halloran, J. Am. Ceram. Soc., 80 [10] 2471-87 (1997)3. A. Virkar et.al., J. Am. Ceram. Soc., 71 [3] C-148-C-151 (1988)4. M.K. Agarwala et.a!., Am. Ceram. Soc. Bull., 75 [11] 60-65 (1996)712

Functionally Optimized Ceramic Structures

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Functionally Optimized Ceramic Structures

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