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Processing of Oxide Composites with Three-DimensionalFiber Architectures

James Y. Yang, Jared H. Weaver, and Frank W. Zokw

Materials Department, University of California, Santa Barbara, California 93106

Julia J. Mack

Teledyne Scientific Company, Thousand Oaks, California 91360

Fabrication of oxide fiber composites is accompanied by thedevelopment of drying cracks in the matrix following slurry in-filtration. The cracks are a result of the inherent shrinkage inparticle compacts during drying coupled with the mechanicalconstraints imposed on the matrix by the fibers. The effects aremost pronounced in systems with three-dimensional fiber archi-tectures. A mitigation strategy based on the addition of coarsematrix particles to the fine particulates has been devised and

demonstrated. Among the various implementation strategies ex-plored, the most effective involves combining the two particletypes (coarse and fine) into a single slurry and coinfiltrating theslurry through sequential vibration- and vacuum-assisted pro-cesses. Regardless of the infiltration route, the SiC particleshave no apparent detrimental effect on the fiber bundle proper-ties. Additionally, they increase the through-thickness thermaldiffusivity by 50%100%.

I. Introduction

CONTINUOUS fiber ceramic composites (CFCCs) with two-di-mensional (2D) fiber architectures are vulnerable to delam-ination when subjected to out-of-plane thermal or mechanicalstress.17 The vulnerability is particularly acute in compositeswith porous matrices.1,2,5,7 One obvious remedy is to incorporatefibers in the third (out-of-plane) direction. Indeed, the use of thisreinforcement strategy has impressive precedents in polymermatrix composites reinforced by glass and carbon fibers.812

Implementation of this strategy for CFCCs, however, presentsunique fabrication challenges:

(i) Weaving complex architectures with ceramic fibers isproblematic because of the high stiffness of these fibers, espe-cially relative to that of glass. Nevertheless, with appropriatehandling precautions and attention to architectural design, theweaving problems can be overcome. Indeed, the preforms usedin the present study (detailed below) were of exceptional qualitywith minimal fiber damage.

(ii) Processing of ceramic matrices usually involves somecombination of chemical vapor infiltration, particle slurry infil-tration and precursor solution impregnation and pyrolysis (thenotable exception being melt infiltration of Si alloys into SiCfiber-reinforced systems). Each of these routes on its own is ca-pable of only partially filling the available void space within the

fiber preform. Even with multiple infiltration cycles, the me-chanical integrity of the resulting matrices is inferior to that ofmonolithic ceramics. Consequently, matrix-dominated compos-ite properties rarely achieve their full potential.

(iii) When slurry infiltration is used, shrinkage of the greenmatrix during drying coupled with the constraints imposed bythe fibers invariably lead to the formation of matrix crackswith large opening displacements (ca. 10 mm).1315 Once formed,

these cracks are irreparable by subsequent impregnation andpyrolysis of ceramic precursor solutions. Their presence has im-plications in hermeticity, thermal conductivity, and matrix-dominated mechanical properties. Additionally, when subjectedto thermal gradients, the cracks produce nonuniform tempera-ture distributions which can, in turn, lead to further matrixdamage.16 Strategies for the mitigation of these cracks is thefocus of the present article.

The nature of the cracking problem is illustrated in Fig. 1.The figure shows a typical cross-section through an oxide CFCCwith Nextel 720t fibers in an 8-harness satin weave (8 HSW)and a mullite/alumina matrix. (Processing details are summa-rized in a subsequent section.) Drying cracks are oriented in thethrough-thickness direction, i.e., crack plane normals lying inthe plane of the fiber weave. Similar cracks have been observed

in virtually all oxide CFCCs with 2D architectures.13,14,1722Cracks are not formed in-plane because of the absence ofthrough-thickness constraint.

Drying cracks are inherently more problematic in 3D CFCCsbecause of the additional constraint imposed by fibers in thethird principal direction. Figure 2 shows an example of a com-posite with a 3D orthogonal weave of Nextel 720t fibers and thesame mullite/alumina matrix. In this case, the through-thicknessdrying cracks are somewhat longer than those in the 2D material(because of the larger intertow spaces) and exhibit larger open-ing displacements. In-plane drying cracks are also evident. Theseare concentrated near the panel mid-plane but are also presentalong fiber tow/matrix boundaries at other through-thickness

Fig. 1. Backscatter electron image of a composite with a mullite/alumina matrix and NextelTM 720 fibers in an 8-harness satin weave.Drying cracks are oriented in the through-thickness direction, perpen-

dicular to the plane of the fibers.

R. Haycontributing editor

This work was financially supported by the U.S. Air Force Research Laboratorythrough a subcontract from Siemens Power Generation (award number 44249059855,monitored by Drs. M. Cinibulk and G. Fair at AFRL and Drs. J. Lane and G. Merrill atSiemens) and the Air Force Office of Scientific Research (award number F49550-05-1-0134,monitored by Dr. B. L. Lee).

wAuthor to whom correspondence should be addressed. e-mail: zok@engineering.

ucsb.edu

Manuscript No. 25363. Received October 15, 2008; approved February 11, 2009.

Journal

J. Am. Ceram. Soc., 92 [5] 10871092 (2009)

DOI: 10.1111/j.1551-2916.2009.03036.x

r 2009 The American Ceramic Society

1087
http://i/BWUS/JACE/03036/[email protected]://i/BWUS/JACE/03036/[email protected]://i/BWUS/JACE/03036/[email protected]://i/BWUS/JACE/03036/[email protected]



locations. The latter cracks (not present in the 2D weave) areattributable to the constraint imposed by the through-thicknessZ-yarns.

Two strategies for mitigating drying cracks have been pro-posed. The first is to fill the large matrix-rich pockets betweentows with high aspect ratio chopped fibers. In one implementa-tion, 2D cloths were coated with a paste of chopped aluminafibers before stacking the cloths and infiltrating the matrix con-stituents.13 Although somewhat successful in reducing the pro-pensity for matrix cracking, the use of pastes in this manneris inherently restricted to 2D laminates. This approach also re-duces the fiber content that can be achieved. The second is basedon freeze-drying. Specifically, the slurry carrier fluid is frozenimmediately after infiltration into the fiber preform and re-moved by sublimation.23 Cracking is prevented because of the

absence of capillary forces during carrier removal. Althoughsome encouraging progress has been made using camphene asthe carrier fluid (selected for its high sublimation temperature), acritical assessment of the viability of this route for fabrication ofoxide CFCCs has yet to be performed.

The objective of the present article is to present an alternatestrategy for mitigating shrinkage cracks in 3D CFCCs. Two as-pects of the fabrication process feature prominently: (i) incor-poration of coarse (410 mm) matrix particles in addition to thesmaller particulates (r1 mm); and (ii) use of vibration to facil-itate infiltration. Secondary goals include identifying possiblefiber degradation mechanisms associated with the processingroute and probing the effects of the coarse particles on through-thickness thermal diffusivity.

The remainder of the article is organized in the following way.First, the rationale for combining coarse and fine particles in thematrix is described. Then, an assessment is made of the efficacyand the deficiencies of coinfiltration of these particles into fiberpreforms using an established vacuum-assisted route. With lim-itations of the vacuum-assisted route exposed, an augmentationbased on vibration-assisted infiltration is devised and demon-strated. Finally, measurements of the in-plane tensile propertiesand through thickness diffusivity are presented.

II. Strategy for Crack Mitigation

The distribution of matrix particle size plays a central role incontrolling drying shrinkage. Following infiltration of a dis-persed slurry into a fiber preform, the matrix particles remain

surrounded by the dispersing liquid, even at points of particlecontact. Assuming a liquid layer of thickness t at the contacts

and spherical particles of radius R, the unconstrained shrinkagestrain would be t/R. Particle rearrangement during drying maylead to additional shrinkage. The implication is that, providedt is independent of R, shrinkage can be reduced by use of largerparticles.

The requirements that the particles be large enough to reducethe shrinkage to an acceptable level yet small enough to pene-trate between the fiber tows indicate a need for a bimodal sizedistribution: coarse particles for filling the large intertow spacesand fine ones for infiltration both into the fiber tows and be-

tween the coarse particles. But, even with a bimodal distribu-tion, the size of the coarse particles is limited by the size of thechannels available for transport between tows (typically, B100mm). Provided the concept is implemented properly, only thefine particulates will be present within the tows and hence thefiber bundle properties should be unaffected by the presence ofthe coarse particles.

A secondary benefit of the bimodal distribution is an en-hancement in the efficiency of particle packing within the largematrix pockets.24 As a consequence, if subsequent matrix dens-ification is desired, less additional material is required to achievethe targeted density. But, as demonstrated by porosity measure-ments presented below, this benefit is small (about 3%).

III. Implementation Via Vacuum-Assisted Infiltration

The preceding concept was implemented using an establishedvacuum-assisted slurry infiltration method, described else-where13,14,21 and summarized in the flowchart in Fig. 3 (alongroute labeled I). The baseline method begins with preparation ofa dispersed slurry ofB1 mm mullite (MU-107, Show DenkoK.K., Tokyo, Japan) and 0.2 mm alumina particulates (AKP-50,Mitsui Mining Co., Tokyo, Japan), in a ratio of 4:1 (by volume),and with a total solids loading of 30% in deionized water. Nitricacid was added to achieve a pH of 3, thereby assuring a well-dispersed slurry. Agglomerates were broken down through acombination of stirring and ultrasonication for about 1 h. Theslurry was poured onto the preform, vacuum degassed, and theninfiltrated via a vacuum-assisted process. Once infiltration was

complete (typically 34 h), the green composite was dried andfired in air in a box furnace (1 h at 9001C). With the mullite/alumina proportions used here, the mullite particles form a con-tiguous network that inhibits densification during subsequent

Fig. 2. (a) Extensive matrix cracking in an as-processed three-dimensional oxide continuous fiber ceramic composites. (b)(c) In-plane drying cracks resulting from the constraint of the Z-yarns. (Fiberweave illustrated in Fig. 4.)

Fig. 3. Flowchart summarizing the standard processing method (routeI) and those used to incorporate coarse SiC particles (routes IIIV). M,mullite, A, alumina.

1088 Journal of the American Ceramic SocietyYang et al. Vol. 92, No. 5



heat treatment whereas the alumina particles form sinteredbridges between neighboring mullite particles. For additionalmatrix strengthening, the composite was impregnated with analumina precursor solution (aluminum hydroxychloride) andpyrolyzed in air (2 h at 9001C). In the present implementation,the precursor solution concentration was selected to produced amass yield of 8% alumina after pyrolysis (higher concentrationsproducing solutions with unacceptably high viscosity25). Twoprecursor impregnation and pyrolysis cycles were used. Theplates were subjected to final heat treatment of 2 h at 12001C

in air. The heating and cooling rates were 101

C/min for all heattreatments.To assess the efficacy of large matrix particles in mitigating

cracks, SiC particles (Norton Company, Worcester, MA) withan average size of 1373 mm (500 grit) were added to slurrieswith a 4:1 mullite/alumina ratio and the same processes used forslurry infiltration and subsequent matrix strengthening (routeIV in Fig. 3). The SiC content was selected to ensure that all ofthe intertow interstices could be filled. The total solids contentwas initially set at 30 vol% (the same as that of the mullite/alumina slurries), although slight variations were explored, asdescribed below. The initial assessment was made using a seven-layer stack of 3000 denier 8 HSW Nextelt 720 cloths (3MCompany, St. Paul, MN), about 3 cm 9 cm and 3 mm thick.The same process was then applied to comparably-sized pre-

forms of the 3D orthogonal weave illustrated in Fig. 4.Polished cross-sections through a series of 8 HSW composites

are presented in Fig. 5. The particles filled virtually all of theintertow spaces. Additionally, these regions were devoid of dry-ing cracks, consistent with the improvement expected from in-creased particle size. In regions where packing of the SiC wasnot complete, drying cracks were present, but exhibited loweropening displacement than that in the SiC-free composites. Themitigation of matrix cracking is directly attributable to the roleof the coarse SiC particles in reducing the (unconstrained) dry-ing shrinkage. A secondary benefit derived from combiningcoarse and fine particles was a slight increase in the packingdensity of particles in the matrix-rich regions. The latter effectwas inferred from measured porosities: 22.5%70.5% and19.0%70.8% for the SiC-free and SiC-containing composites,

respectively. (Despite the improved packing, the bulk density of

the SiC-containing composites was slightly reduced, from 2.8 to2.65 g/cm3, a result of the lower mass density of SiC relative tothe oxide constituents.)

Upon closer inspection of the cross-sections, subtle differ-ences are evident in the packing efficiency of the mullite/aluminaparticulates. Using slurries with comparatively high solids load-ing (35%), the oxide particles pack exceptionally well betweenthe SiC particles in the matrix-rich regions but experience somedifficulty in penetrating the spaces between fibers, manifestedas fine intratow pores. Evidently the degree of this porosity di-minishes as the slurry solids loading decreases. Concurrently,however, the efficiency of packing of the oxide particulates be-

tween the SiC particles decreases slightly, resulting in increased

Fig. 4. Three-dimensional orthogonal weave used in the present study.Top, perspective schematic; Bottom, plan view of preform. Weave con-sists of six layers of straight warp yarns, seven layers of straight fill yarnsand interlacing Z-yarns. Fiber volume fractions: 12.5% warp, 12.4%

weft, 4.2% Z-yarns (d, denier; ppi, picks per inch; epi, ends per inch; dpi,dents per inch). Weaving by 3TEX, Cary, NC.

Fig. 5. Effects of solids loading on particle packing in the two-dimensional (8-harness satin weave) fabric. The particles had been coinfiltrated by thevacuum-assisted route. Drying cracks are almost completely eliminated in all cases. Subtle changes in inter- and intratow porosity are evident.

May 2009 Processing of Oxide Composites with Three-Dimensional Fiber Architectures 1089



intertow porosity. An optimal condition appears to be obtainedfor solids loading in the range 28%30%. Although the mech-anisms associated with the formation of these defects and theirdependence on solids loading are not presently understood, noadditional effort was made to probe them further, largely be-cause of problems encountered in fabricating 3D CFCCs withthis route (described below) as well as the rather low concen-tration of such defects.

Use of the same vacuum-assisted coinfiltration procedurewith the 3D preform proved partially successful (Fig. 6); onlyabout half of the intertow volume was filled with SiC particles.Incomplete infiltration is attributable to two related factors.First, although in principle slurry infiltration is not a line-of-sight deposition process, the large size of the SiC particlesprevent them from remaining suspended in solution for timeperiods needed for infiltration. As a result, there is a greatertendency for the particles to settle into positions directly beneaththeir settling trajectory. Secondly, shadowing effects in the 3Dpreform (especially those associated with the crowns of theZ-yarns) are more pronounced than those in the 2D fabric.Notwithstanding these limitations, matrix cracking had beensuppressed in the SiC-rich regions. The inference is that thecracking could be further reduced if the SiC particles were to

completely infiltrate all available space.

IV. Vibration-Assisted Infiltration

Two alternatives were devised to enhance space filling by the SiCparticles. In the first (route II in Fig. 3), infiltration was initiallyperformed with only SiC in the slurry. After pouring the slurryonto the preform and degassing, the infiltration fixture wasplaced on a commercial vibrating table (78-RK Vibrator, Han-dler Mfg. Co., Westfield, NJ) for about 20 min. Visual inspec-tion revealed that, during this period, virtually all SiC particleshad settled. Furthermore, the thickness of the SiC cake on top ofthe preform remained unchanged for the last 10 min of vibra-tion, indicating that the infiltration process had gone to com-

pletion. Following drying of the SiC/fiber preform under a heatlamp, the mullite/alumina matrix was vacuum infiltrated andstrengthened with the alumina precursor solution (route I inFig. 3). In some cases, the green SiC/fiber preform was infil-trated with a low viscosity epoxy, for subsequent sectioningand polishing and assessment of the SiC particle distribution.Experiments were conducted with particle ranging ranging inaverage size from 9 to 30 mm (600320 grit).

Vibration assistance proved to be extremely effective in pack-ing the SiC particles into all large intertow spaces (Fig. 7),regardless of particle size. Subsequent infiltration of the mullite/alumina slurry, however, was problematic in cases where smallerSiC particles (r17 mm diameter) had been used. Evidently thetight packing within the SiC/fiber preform prevents ingress of themullite/alumina slurry. In contrast, infiltration of the mullite/

alumina slurry was largely successful with the coarser SiC par-ticles ( ! 23 mm diameter), as illustrated in Figs. 8 and 9.

The second alternative (route III in Fig. 3) was essentiallya hybrid of routes II and IV in the sense that it involved coin-filtration of all particles and used vibration to facilitate infiltra-tion. In this case, a slurry containing the desired mix of mullite,alumina and SiC was poured onto the preform and degassed,and the assembly then vibrated for about 20 min. This wasfollowed by vacuum-assisted infiltration, drying, firing andstrengthening with the precursor-derived alumina. This process-ing route proved to be the most effective and reproducible.Regardless of their size, the SiC particles packed well within theintertow spaces while the mullite and alumina particles infil-trated virtually all of the fine interstices both in the SiC-rich re-gions and within the fiber tows (Fig. 10). Consequently, the levelof extraneous porosity (caused by poor packing) and the degreeof cracking were minimal.

A further assessment of the efficacy of the SiC particles inproducing high quality matrices was made by producing com-posite panels following an identical vibration- and vacuum-assisted process, but using only the oxide particulates. In thiscase, there was no apparent benefit of the vibration in suppress-ing cracking (Fig. 11). This result reaffirms that the reducedpropensity for cracking in the preceding cases is indeed due tothe presence of the coarse SiC particles; using vibration withonly fine particulates does not yield an improvement.

Fig. 6. Vacuum-assisted coinfiltration of the mullite/alumina/SiC slurryinto the three-dimensional preform. Inset shows section orientation withrespect to the fiber weave.

Fig. 7. Two orthogonal sections through a three-dimensional preformfollowing vibration-assisted infiltration of SiC slurry (without mullite/alumina) and epoxy impregnation. Light gray regions are SiC.

Fig. 8. Three-dimensional continuous fiber ceramic composites pro-duced by vibration-assisted infiltration of 23-mm-diameter SiC particlesfollowed by vacuum infiltration of the mullite/alumina slurry. Dashedlines in (b) and (c) denote boundaries between weft tows and adjacentmatrix-rich pockets.




V. Property Assessment

A preliminary assessment was made of the fiber bundle strengthin the 3D CFCCs, with a view to identifying possible degrada-

tion mechanisms associated with SiC infiltration (caused byabrasion of the fiber surfaces during vibration). Because of lim-ited material volume, tensile tests were performed on hourglass-shaped specimens. Each specimen had a gauge width of 9 mmand a radius of curvature of 75 mm. Test specimens includedones with and without SiC particles and tested in both principalfiber directions. From about ten such tests, the average fiberbundle strength (taken as the ratio of composite strength to thefiber volume fraction aligned with the loading direction) was780750 MPa. This range is comparable to that obtained withthe same fibers in the 8 HSW weave (690760 MPa).26 Therewas no apparent detrimental effect of the SiC particles.

The through-thickness thermal diffusivity was measured us-ing the laser flash method with a Netzsch Instruments LaserFlash Apparatus 427. Circular samples, 12.6 mm in diameter,were cut from the composite panels using a core drill. A thingraphite coating was applied to the broad surfaces in order toincrease opacity and enhance absorption of the laser on the in-cident face. To enable measurements at high temperatures, the

tests were performed in an Ar atmosphere (because the graphitecoating oxidizesB6001C). Measurements were made on the 2Dand 3D composites both with and without SiC particles, up totemperatures of 1000112001C.

The test results are summarized in Fig. 12. With the 2D ar-chitecture, the addition of the coarse SiC particles yielded anelevation in diffusivity of about 50% over the entire temperaturerange. The magnitude of the elevation was even greater in com-posites with the 3D weave: almost 100% at ambient tempera-ture, diminishing to about 60% at 10001C.

VI. Conclusions

The addition of coarse matrix particles to the fine particulatesnormally used for oxide CFCCs is effective in mitigating dryingcracks. The two particle types can be coinfiltrated into 2D fiber

Fig. 9. (a) Three-dimensional continuous fiber ceramic composites pro-duced by vibration-assisted infiltration of 30 mm diameter SiC particlesfollowed by vacuum infiltration of the mullite/alumina slurry. (b, c)Higher magnification images of regions indicated in (a).

Fig.10. (a) Coinfiltration of 23-mm-diameter SiC particles and mullite/alumina mixture by sequential vibration- and vacuum-assisted infil-tration. (b, c) Higher magnification images of regions indicated in (a).

Fig.11. All-oxide three-dimensional composite fabricated by sequen-tial vibration- and vacuum-assisted slurry infiltration of the mullite/al-umina slurry.

0.0

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Fig.12. Effects of fiber architecture and presence of coarse SiC parti-cles on through-thickness thermal diffusivity of (a) 2D and (b) 3Dcomposites.

May 2009 Processing of Oxide Composites with Three-Dimensional Fiber Architectures 1091



weaves using a vacuum assisted route. The same process leads toincomplete filling of the 3D preform used in the present studybecause of rapid particle settling coupled with shadowing by thewarp weavers. For this purpose, vibration of the slurry and pre-form during infiltration yields significant packing improve-ments. Sequential infiltration of slurries with coarse and fineparticles is viable provided the coarse particles are large enoughto allow subsequent ingress of the fines. For the present system,the critical particle size is about 25 mm. A more convenient routeis to combine the two particle types into a single slurry and co-

infiltrate them through sequential vibration- and vacuum-assisted processes. Regardless of the infiltration route, the SiCparticles have no apparent detrimental effect on the fiber bundleproperties: an essential condition for successful composite de-sign. Additionally, they significantly increase the thermal diff-usivity (by ! 50%).

Demonstrations of the proposed concept have been per-formed using SiC as the coarse particle constituent. This selec-tion was based largely on the availability of a wide range ofparticle sizes as well as the chemical compatibility of SiC withthe oxide constituents. But, in principle, other particle types canbe used to serve the same function. This option would allow fortailoring of certain matrix properties, including the thermal ex-pansion coefficient and the sinterability, while still achieving acrack-free matrix system.

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