NASA/TM-- 1998-208819

Fabrication Routes for Continuous

Fiber-Reinforced Ceramic

Composites (CFCC)

James A. DiCarlo and Narottam P. Bansal

Lewis Research Center, Cleveland, Ohio

November 1998

https://ntrs.nasa.gov/search.jsp?R=19990009284 2018-05-11T16:58:38+00:00Z
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NASA/TM-- 1998-208819

Fabrication Routes for Continuous

Fiber-Reinforced Ceramic

Composites (CFCC)

James A. DiCarlo and Narottam P. Bansal

Lewis Research Center, Cleveland, Ohio

National Aeronautics and

Space Administration

Lewis Research Center

November 1998
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FABRICATION ROUTES FOR CONTINUOUS FIBER-REINFORCED

CERAMIC COMPOSITES (CFCC)

James A. DiCarlo and Narottam P. Bansal

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

ABSTRACT

The primary approaches used for fabrication of continuous fiber-reinforced ceramic composite

(CFCC) components have been reviewed. The CFCC fabrication issues related to fiber, interface,

and matrix have been analyzed. The capabilities, advantages and limitations of the five matrixinfiltration routes have been compared and discussed. Today, the best fabrication route for the CFCC

end-user is not clear and compromises need to be made depending on the details of the CFCC

application. However, with time, this problem should be reduced as research continues to develop

advanced CFCC constituents and fabrication routes.

I. INTRODUCTION

The microstructure of a ceramic matrix composite (CMC) consists of distinct ceramic phases

imbedded within a host ceramic matrix material. The advantage of forming such a multiphase system

is that by the judicious selection of the ceramic constituent compositions, sizes, geometric forms, and

volume fractions, the CMC can display technically desirable properties which are better than those

of competing state-of-the-art materials. For instance, one area of current high interest for CMC is that

of structural application in hot- section components of advanced propulsion and power systems. Here

CMC have the capability for operating at higher temperatures than metallic alloys and thus can

provide significant system payoff in terms of efficiency and performance. For these applications,

which typically extend well above 1000 °C, CMC can also outperform other high-temperature

structural materials such as carbon-carbon composites and monolithic ceramics. For example, if the

CMC constituents are chosen to be oxides or silicon-based compounds, CMC service lives in

oxidizing environments will be much longer than those of carbon-carbon composites. Also, if the

CMC interfacial phases are designed to be weak enough to deflect and inhibit the growth of strengthlimiting matrix cracks generated during system operation, CMC will display higher fracture

toughness and consequently longer service life than competing monolithic ceramics. This strength

and toughness advantage over monolithic ceramics is further enhanced if one of the composite phases

consists of strong continuous-length fibers which act to maintain material integrity by bridging

cracks that form in the host matrix (ref. 1). Also, as will be discussed here, the use of continuous fiber

reinforcement with small diameter permits the formation of woven fiber structures which can then

be infiltrated with matrix material to fabricate complex composite shapes much larger in volume than

currently available for monolithic ceramics. It is these structural, shape, and size advantages that have

effectively focused the majority of current CMC developmental efforts toward continuous fiberreinforced ceramic composites (CFCC).

NASA/TM--1998-208819 1
AlthoughCFCCcanout-performotherhigh-temperaturestructuralmaterials,thereareother

process-relatedrequirementsthat mustbemetbefore CFCCgainfull technicalandcommercial

acceptance.Primeamongtheseis afabricationroutethatwill yieldacompositemicrostructurethat

displaysoptimum,tailorable,andreproducibleperformance.Optimumperformance typically implies

that the fabrication route should not only maintain the structural properties of the fiber and matrix

phases, which are generally strong, tough, and creep-resistant, but also allow these phases to interact

in a synergistic manner. Tailorable performance implies that the fabrication route should allow

the constituent phases to be assembled in versatile configurations so that the CFCC microstructure

can be designed to best meet the structural, shape, and volume requirements for a wide range of

components. Reproducible performance generally implies that the fabrication route should allow

the interfacial and matrix phases to be well controlled and to be formed uniformly within a part and

consistently from part to part. In addition to these microstructural requirements, the fabrication route

must be able to make complex-shaped components in a cost-effective manner, that is, at a process

cost competitive with those of other high-temperature structural materials.

Today, there exists many CFCC fabrication routes, but only _ general approaches have

achieved to a large degree the performance and fabrication requirements needed to produce a wide

variety of complex shaped components with microstructures capable of structural operation for long

times at high temperatures. All of these five approaches are similar in that they first assemble smalldiameter continuous-length fibers into configurations or architectures that have been de signed to best

meet both the performance and shape requirements of the final CFCC parts. After using various

methods for applying interfacial coatings on the fibers, these particular approaches then form the

constituent phases of the host matrix by fluid infiltration; that is, by gas, slurry, or liquid infiltration

or combinations of these. Using nomenclature based on the matrix infiltration process, these general

fabrication approaches include: (1) Chemical Vapor Infiltration (CVI), (2) CVI plus Directed Metal

Oxidation (CDMO), (3) CVI plus Slurry (or polymer) infiltration plus Liquid Metal infiltration

(CSLM), (4) Polymer (or sol gel) Infiltration and Pyrolysis (PIP), and (5) PIP plus Liquid Metal

Infiltration (PLM). Table I lists these fabrication routes plus typical fiber lay-up approaches (see

section II) and matrix compositions (see Section III) used by some of the current CFCC fabrication

vendors. The use of continuous-length fiber architectures not only optimizes the strength and toughness of the composite, but also permits the formation of near-net shaped parts in a wide range of sizes,

thereby minimizing the need for costly machining. Thus, in comparison to other fabrication approaches,

these routes have important advantages in terms of CFCC component versatility and cost. However,

the five routes do differ in the particular process steps that they employ, which can have a significant

effect on the performance, tailorability, reproducibility, and cost of the CFCC components.

The scope of this paper is to examine in general detail the physical, chemical, and mechanical

aspects of the five general matrix-infiltration routes for CFCC fabrication listed in table I. For each

route, emphasis is placed on understanding issues related to its current ability for achieving low-cost

complex-shaped CFCC parts with microstructures that display good performance, tailorability, and

reproducibility. Because of the high technical payoff, these issues are examined in particular for

CFCC application in advanced engineering systems that will operate at high temperature under

aggressive environments. Thus emphasis will be placed on obtaining strong and creep-resistant fiber

and matrix phases that are stable to temperatures well above 1000 °C. To form a basis for this

discussion, Section II describes the general steps that are currently being employed and the key issues

that need to be considered for a good matrix infiltration route. This not only includes matrix

formation, but also the very important initial steps of fiber assembly and interface coating deposition.

In Section III, the five general fabrication routes are then examined in terms of the process steps used

for matrix infiltration around the fibers and interfaces. Besides fabrication details, advantages and

NASA/TM--1998-208819 2
limitations inherentin eachroutearediscussed.Finally, in SectionIV, thecurrentstatusof the

variousprocessingroutesin achievingthegeneralfabricationgoalsof performance,tailorability,

reproducibility,andcosteffectivenessaresummarizedandcompared.

II. CFCCFABRICATION ISSUES

Forjudging the technicalcapabilitiesof variousCFCCfabricationroutes,focusis typically

placedonthefinal stepsof matrixformation.However,theperformanceandtechnicalviability of

theCFCCfabricationroutealsodependstronglyontheinitial stepsof fiberarchitectureformation

andinterfacecoatingdeposition.Indeed,theseinitial stepscanimpactandlimit thematrixformation

stepssothatCFCCfabricationis acomplexintegralprocessthatrequiresoptimizationatall steps.

Thepurposeof this sectionis toexaminein ageneralmannerthesekeyfabricationstepssothatone

canbetterunderstandtheir effectson the microstructureand propertiesof CFCCcomponents

obtainedfrom thefive matrix-infiltrationroutes.

II.1. Fiber-RelatedIssues

Keyto aCFCCfabricationrouteforproducingcomplexfiberarchitecturesandcompositeshapes

is theability of thereinforcingfiber to beshapedwith bendradii assmallaspossible.However,in

bendinga fiber with diameterD, the minimumradius,Rmin, that can be achieved without fiber

fracture is given by Rmi n = ED/2c_ where E and c_ are the fiber modulus and bend strength,

respectively. For commercially available ceramic fibers suitable for long-term high-temperature use

in oxidizing environments (SiC and A1203-based compositions), modulus values typically are

N400 GPa and bend and tensile strengths do not generally exceed N4 GPa due to process-related flaws

(ref. 2). Thus, the practical limit for shaping these fibers is Rmi n = 50D, indicating a strong need to

utilize fibers with the smallest diameters available to allow the most versatility in shape forming.

Additional fabrication advantages of the smallest diameter fibers are their ability to reinforce CFCC

parts with thin sections and to be easily assembled and shaped using conventional weaving and

braiding processes (ref. 3). In order that these processes form complex architectures, it has been

empirically determined that the fibers should display Rmi n values no >_2 mm. This in turn implies

that CFCC fabrication routes should only use commercial fibers with diameters <_40 gm if their

strengths are consistently near 4 GPa, and <_20 gm if their strengths are consistently near 2 GPa.

Fortunately, there exists today a variety of commercially available SiC- and A1203-based fibers

that have diameters in the range 10 to 20 gm and tensile strengths equal or greater than 2 GPa (ref. 2).

These are typically made from Si-based polymers or oxide slurries, respectively, which are formed

in the "green" state by the continuous spinning of multifilament tows with fiber counts of _500. The

"green" tows are then passed through high temperature furnaces to form the final ceramic fiber

product. For small diameter fibers, the use of multifilament tows has the advantage of reducing the

risk of fiber breakage during handling in the weak "green" state or in subsequent fiber architecture

formation in the stronger ceramic state. It also has the advantage of reducing fiber process costs by

allowing a high mass-throughput during the production processes. In this last regard, it would appear

desirable to spin fiber counts much higher than N500 as is the typical case with carbon-based fiber

tows. However, as will be discussed, higher filament counts can inhibit subsequent CFCC fabrication

steps in which it is desirable to infiltrate interfacial and matrix materials uniformly around all fibers

within the tow.

NASA/TM--1998-208819 3
Currentlyall matrixinfiltrationroutesto structuralCFCCpartsbegintheirproductionbytheuse

of smalldiameterfibersin multifilamenttowform.Generallyfiberarchitecturesareemployedthat

areasdenseaspossiblein orderto maximizethefibervolumefractionandload-carryingability of

thecompositeif thematrixshouldcrack.Toformthesearchitectures,twodifferentlay-upapproaches

aretypicallychosen:pre-pregandpreform.In oneversionofthepre-pregapproach(pre-pregA),the

towsaretreatedin a mannersimilarto that typically usedfor commercialformationof filament

woundpolymermatrixcomposites.Thatis, aftertheindividualtowsandtheirfibersarepre-coated

withinterfacialcoatings(tobediscussed),thetowsarelaidupinunidirectionalsheetsormattesthat

arethenimpregnatedwith amatrix-precursoresinto formpre-preggedplies.Thesepliesarethen

laidupatselectedanglesto eachotherandpressedtogetherin anautoclaveto form thefinal CFCC

shapeasatwo-dimensionalcross-pliedlaminate.The"green"laminateis thensubjectedtovarious

infiltration andhigh-temperaturetreatmentmethodsto form theceramicmatrixaroundthecrosspliedfiber architecture.

Anotherversionof thepre-pregapproach route (pre-preg B) is to have the tows woven into a

two-dimensional fabric cloth using weave patterns suitable for the CFCC application. Figure 1 shows

an example of a two-dimensional fabric in which the tows have been woven in two directions (0 and

90 °) with a five-harness satin weave pattern. After fabric formation, the interface coatings are applied

in a separate step. The coated fabric is then impregnated with a matrix-precursor resin, laid up as plies

into the final CFCC shape, subjected to autoclave processing to form a green two-dimensional woven

laminate, and processed further to form the ceramic matrix. Thus the pre-preg A and B approaches differ

primarily in cross-plied versus woven 2-D fiber architecture and in the manner in which the fiber

interfacial coatings are applied.

For the preform approach to fiber lay-up, the multifilament tows are first formed into two or

three-dimensional fiber architectures using weaving or braiding. One common method (preform A)

is to pre-cut uncoated or dry fabric plies and stack the plies into a two-dimensional preform which

is then placed into tooling that has the shape of the final CMC part. The stacked fabric could be held

together dry or with a fugitive binder. The tooling typically has predrilled holes which allow one or

more interfacial coatings to be subsequently applied by chemical vapor infiltration on all fibers in

the preform. The matrix material is then infiltrated into the porosity remaining in the preform.

As discussed in Section III, this can be accomplished by gas, liquid, or slurry infiltration or by any

combination of these methods. An example of a matrix infiltrated 8-ply 0/90 ° SiC/SiC CFCC produced

by the preform A approach is shown in by the micrograph in figure 2.

A second preform approach (preform B) is to avoid the need for tooling by using weaving or

braiding approaches which shape the fibers into a near-net three-dimensional shape of the final CMC

part. Such architecture may be needed, for example, to assure fiber reinforcement in many directions

because of the structural requirements of the CFCC part. This preform can then be infiltrated with

interface and matrix material as just described. A third possible preform approach is to first coat

individual tows as in pre-preg A and then form the preform by weaving or braiding processes that

minimize the risk of damaging or degrading the coating. This approach is rarely practiced because

it generally results in low-density fiber architectures or in architectures with a high incidence of

interfacial coating damage. Thus the currently viable preform approaches differ from the pre-preg

approaches in that the interfacial fiber coatings are applied through a denser, more complex network

of fibers, but the available fiber architectures can be considerably more versatile.

NASA/TM--1998-208819 4
II.2. Interface-RelatedIssues

Theinterfacialcoatingphasesorinterphases between the fiber and matrix must provide a variety

of functions if CFCC are to meet their performance and fabrication requirements. Prime among these

is the ability to provide a weak fiber-matrix interfacial bond after CFCC fabrication and during service,

even in the presence of matrix cracks and aggressive environments. In addition, the interphase

composition and formation processes should be protective and not degrading to the fiber properties,

again either during CFCC fabrication or service. It is also desirable that the interphase performs its

functions in a consistent and reproducible manner throughout the CFCC microstructure so that

performance is the same for all similar CFCC parts. For some thermostructural applications, it may

also be desirable that the interphase provides high thermal conductivity to reduce thermal stresses

within the CFCC. Finally, the interphase formation processes should be cost effective.

Today, essentially all matrix-infiltration routes using nonoxide fibers are employing interfacial

fiber coatings based on carbon or on boron nitride with thickness typically ranging from 0.1 to

1.0 _tm. These compositions have demonstrated the ability to provide weak interfacial fiber-matrix

bonds for crack deflection and also to be nondegrading to fiber properties. However, for stability in

aggressive environments, both are lacking in terms of providing long CFCC lives in the presence of

matrix cracks. Research efforts are ongoing to find methods to eliminate this deficiency by examining

new coating concepts based on multilayers, alternate compositions, and/or weak porous phases

(refs. 4 to 8).

Nevertheless, the current use of environmentally unstable carbon and BN interphases generally

necessitates the application of protective over-coatings on these materials. The typical approach is

to use CVI to provide thin SiC coatings (N0.5 to 5 _tm) over the carbon or BN coatings. Thus, in effect,

each fiber in a tow consists of a single-fiber microcomposite with a thin C or BN interface and a thin

SiC over-coating which then becomes part of the matrix microstructure (fig. 3). These over-coatings

are generally chosen to protect the interfacial coatings and fibers from reactions with the other matrix

phases that are introduced during CFCC fabrication. Since the C and BN can be affected by long

exposures to ambient environments, it is important that the over-coating be deposited as soon as

possible on the interfacial coatings, preferably before exposure to the ambient. The over-coatings

may also act to protect the fibers and interphases from aggressive gases such as oxygen and water

vapor during CFCC service. In this regard, the interphases and over-coatings are sometimes repeated

in multilayer concepts in order to provide environmental protection layers in the presence of servicegenerated matrix cracks (refs. 4, 7 and 8).

From the above discussion, it should be clear that an important requirement for a good CFCC

fabrication route is to provide reliable and uniform interphase coatings and over-coatings throughout

the CMC microstructure. All of the five general matrix-infiltration routes used today provide both

types of coatings by CVI. Three methods are generally employed: (1) coating fibers in multifilament

tows by passing one or more tows continuously through reaction furnaces; (2) coating woven fiber

fabric in batch or continuous modes; and (3) coating fiber preform architectures in a batch mode.

Clearly there are various cost-related issues with each of these methods, which will not be discussed

here. However, one can examine the tailorability and reliability issues for each case. The importance

of such an examination is that coating methods 1,2, and 3 generally correspond, respectively to those

used for the pre-preg A, B, and the preform A-B fiber lay-up approaches.

NASA/TM--1998-208819 5
Forexample,amajorissuewiththeCVI processforfibercoatingsiscontrolof theuniformity of

thecoatingthickness(andperhapsof thecoatingcomposition).Sincethecoatingsareformedby

penetrationof theprecursorgasesintothearchitecturefollowedby reactionanddepositiononthe

fiber surfaces,thegasesbecomedepletedin reactionspeciesfrom theoutsidetoinsideof thearchitecture.Sinceall fiber surfacesin acomplexarchitecturearegenerallyatthesametemperature,the

CVI depositionprocessalwaysresultsin thickercoatingsontheoutsidefiberscomparedtothoseon

theinnerfibers.Thisratioofoutertoinnercoatingthicknessgenerallyincreaseswiththicknessof the

fiberarchitectureandwithdepositiontemperatureandpressure.Thuslowertemperaturesandpressures

canimproveuniformity,butsincetheseresultin lowerdepositionrates,theyaretypicallynotcosteffectiveoptions.Thereforeinterphaseuniformitygenerallydecreasesfromcoatingmethods1to 3

asdescribedabove,sothatthepre-pregA architectureapproachtypicallyyieldsthemostconsistent

andreproducibleinterphasecoatings.

However,intermsof assuringinterphasereliability throughoutCFCCfabrication,it isimportant

to understandthatthepre-pregapproachesrequirethecoatedtow orfabric to behandledandthen

subjectedto bendingstressesin orderto formthefinal CFCCshape.Sincetheinterphaseandovercoatingareonthefiber surface,theseconstituentscanbesubjectedto sufficientstressto losetheir

integrityandfunctionality.Forthisreason,CFCCfabricationroutesthatemploypre-pregA andB

approachestypicallydonotattempttofabricatecomplexshapeswith smallradii.Thus,althoughthe

pre-pregapproacheshave a considerableadvantagein that they can draw on the commercial

experienceandfacilitiesusedfor polymercompositefabrication,theycannotgenerallyaccommodatecomplexandversatilefiberarchitecturesaswell asthepreformapproach.(Thisarisessimply

from thefactthatpolymermatrix compositesdonot requirethecomplexinterphasecoatingsthat

CFCCdo.) Ontheotherhand,theuniformity of fiber coatingsremainsanissuewith thepreform

approaches,oftennecessitatingits useonly for thin sectionarchitecturesandparts.

It shouldbenotedthatothermethodsmightbedevelopedto replaceCVI depositionof CFCC

interphasecoatingsandthusminimizetherisk of coatingnonuniformityforthepreformapproach.

Onepotentialmethodis to developceramicfiber reinforcementsthat containsomeor all of the

chemicalingredientsrequiredfor thedesiredcoating.Thenby somespecialpre-treatmentof the

preformarchitecture,thesecoatingingredientscouldbemadeto migrateto thefiber surfacetoform

anin-situ interfacial coating. If the ingredients are controlled and limited within the fiber, coating

thickness should be uniform on all fibers. Indeed, in-situ carbon-rich interfacial coatings have been

grown on ceramic-grade Nicalon SiC fibers, but at the expense of reducing fiber strength due to the

formation of new flaws on the fiber surface (ref. 9). Thus, although the in-situ interfacial coating

concept appears promising, considerable research efibrts would appear to be required to develop

fibers that are not only structural, but also can provide the proper interphase chemistry without

degrading in strength during the interphase formation process.

II.3. Matrix-Related Issues

When dealing with complex fiber architectures, it generally follows that the matrix formation

process will require development of innovative processes that will allow infiltration of matrix

precursor materials into the architecture. Section III will discuss in more detail the infiltration

processes currently being used by the five general approaches listed in table I. The purpose here is

to examine the general property requirements of the final CFCC product and how these can influence

the matrix formation steps.

NASA/TM--1998-208819 6
Like theinterfaceover-coating,whichisin effecttheinitial matrixphase,aprimerequirement

for thefinal matrixphasesis to protectthefibersandinterphasesfrom aggressivegasessuchas

oxygenandwatervaporduringCFCCservice.Clearlythematrixcanperformthisfunctionif, inits

most denseform, it displays long-term durability in theseenvironments.Thus silicon and

oxide-basedcompoundsarepreferredforthematrixcomposition.Assuringhighmatrixdensitythen

requires(1)matrixformationprocesseswhichdonotresultinporosityopento theCFCCexternal

surfaces,and(2)matrixstructuralpropertiesthatminimizetherisk ofmatrixcrackingduringCFCC

service.AveryimportantadditionalreasonforbothrequirementsisthattheCFCCpartwill typically

be subjectedto small stressesin directionswherethereis no or only a small amountof fiber

reinforcement(for example,interlaminarregions).Thuszeroporosityandhigh structuralperformancewill minimizetherisk of matrix crackingin thesedirections.In effectthen,theserequirementstogetherdictatethatthematrix compositionandmicrostructurebechosento besimilarto

thoseofthebeststructuralmonolithicceramicthatcouldbeusedfortheintendedserviceconditions.

However,this CFCCmatrix mustbeformedby infiltration into acomplexfiber architectureby

processingapproacheswhich aretypically nonconventionalfor monolithicceramics.

Basedontheneedforthebestmonolithic-typeperformancefrom thematrixmaterial,onemight

selectmatrixcompositionsbasedonthefollowing compounds:siliconnitride,siliconcarbide,and

alumina.Here it is assumedthat the applicationswith the highestpayoff for CFCC are at

temperaturesabove1000°Candthatif theCFCCcanmeettheseapplications,theycanalsomeet

thoseat lower temperatures.Although thesethreecompositionsarepractically possible,the

performancerequirementhatthematrixmaterialhaveathermalexpansionverycloseto thatof the

commerciallyavailableSiCandA1203-basedfiberseffectivelyeliminatessiliconnitrideandA1203

asmatrixchoicesfortheSiCfibers,andsiliconnitrideandSiCasmatrixchoicesfortheA1203-based

fibers.Thus,aswill bediscussed,thefive CFCCmatrix-infiltrationroutesprimarily produceSiCandA1203-basedmatrices,eventhoughthereexistsexpansion-drivenmatrix crackingin the

as-fabricatedSiC/A1203CFCC(CDMOroute).TheSiC/SiCCFCCcanalsodisplaygoodthermal

conductivity,aperformanceadvantageforreducingthermalstressesduringservice.Thelackofhigh

thermalconductivityandoxide-basedfibersthatarecreep-ruptureresistantfor long timesabove

1000°C aretwo factorscurrentlylimiting thecommercialavailabilityof oxide/oxideCFCC.

Anotherimportantconcernduringmatrix infiltration is propertyretentionfor thefiber and

interphases.Thepotentialfor degradationcanariseeitherfromintrinsicinstabilitiesinthefiberand

interphasesduetothetemperaturerequirementsfor matrixformation,orfromchemicalandphysical

interactionbetweenthematrixprecursorsandtheseconstituents.Thefirstpossibilityarisesbecause

manycommerciallyavailablesmall-diameterfibersareonlyprocessedto maximumtemperatures

near1200°CandmostCVI interphasesto maximumtemperaturesnear1000°C.Ontheotherhand,

thenewerCFCCfabricationroutesusingliquid siliconinfiltration requirematrixformationsteps

above1400°C. The existenceof SiC over-coatingson thefiber architecturecanminimize the

interactionconcern,butthereis still somerisk thatthemetalsusedin theliquid metalfabrication

routescandiffusethroughtheover-coatinggrainboundariesandattackthefibersandinterphases.

One final concernfor matrix formation is the fact that, althoughthe matrix-infiltration

fabricationroutescanproducenearnetshapeparts,somematrixprocessesresultin open-porosity.

Also all routeswill probably requiresomeminor machiningthat will also exposefibers and

interphases.Thusthereis ageneralneedto providethe CFCCmatrix andfinishedpartwith an

environmentallyresistantover-coating.Again thetypical approachwould be chemicallyvapor

depositedSiCfortheSiC/SiCcomposites.However,therecanbeissuesconcerningtheover-coating

causingcritical changesin thedimensionsof theCFCCpart,aswell aswith theadherenceof the

NASA/TM--1998-208819 7
over-coatingespeciallyif theenvironmentwereto enterthematrix porosity.Clearly theserisks

wouldbesignificantlyreducedif thematriceswerefully denseandthefibersandinterphaseswere

stablein theCFCCserviceenvironment.

III. MATRIX-INFILTRATION ROUTESFORCFCCFABRICATION

A variety of methodssuchas chemicalvaporinfiltration, directed metal oxidation, slurry

infiltration, liquid metal infiltration, polymer infiltration and pyrolysis, reaction forming/bonding,

sol-gel, gel-casting, electrophoresis, glass-ceramic, etc. have been used for fabrication of ceramic

matrix composites. Of these, only the first five approaches and their combinations (cf. table I) are

being used to make a large variety of CFCC components. These fabrication routes will be briefly

described in this section. The advantages and limitations of each fabrication method will also be

discussed. All of these approaches involve infiltration of matrix precursor materials by gas or slurry

or liquid processes and thus are suitable for various kinds of continuous fiber architectures. These

techniques are also amenable to fabrication of components having large and complex shapes with

near net-shape capability.

III. 1. Chemical Vapor Infiltration (CVI)

In CVI, gaseous precursors of the ceramic matrix are infiltrated into a porous fiber preform where,

by chemical decomposition, they react in situ to form the matrix on all exposed surfaces within the

preform. In the most commonly used isothermal approach the preform is held at constant temperature

of about 900 to 1100 °C, a range required for a good combination of gas infiltration rate and

decomposition rate. Almost any ceramic matrix can be deposited by appropriate choice of the reactant

gases or vapors. The most important example is the production of SiC matrix composites using the

decomposition of methyltrichlorosilane (CH3SiC13) in the presence of hydrogen at about 1000 °C:

CH3SiC13 (g) --+ SiC(s) + 3HCI(g).

Various experimental parameters such as temperature, pressure, and flow rate of the gases as well

as the preform temperature need to be optimized in order to attain the desired microstructure and

properties of the matrix. Also, these variables need to be adjusted so as to delay the closing of the

porosity at the preform surface until an appropriate high density has been obtained in the interior.

A number of variations of the CVI process, depending upon whether the preform is uniformly

heated or not and how the reacting gases contact the preform, are available (refs. 10 and 11). These

are isothermal, thermal gradient, isothermal-forced flow, thermal gradient-forced flow, and pulsed

flow CVI. Detailed descriptions of these CVI processes for matrix fabrication are beyond the scope

of this section. Each has its own advantages and disadvantages in terms of matrix porosity, matrix

microstructure, and/or matrix processing time. As indicated in table I, there are many organizations

using the CVI method for CFCC components, but most use the preform A approach for forming the

fiber architecture. Table II shows typical property data for three CFCC systems fabricated by CVI

with three different fiber types.

The CVI method for matrix infiltration has several advantages. It is amenable to the fabrication

of CFCC components having irregular, large, and complex shapes with near-net shape capability.

High temperature properties are generally good because high purity matrices with controlled

NASA/TM--1998-208819 8
microstructurescan be obtained.This results typically in a SiC material with high thermal

conductivityandhigh creepresistance.Also CVI istypically carriedoutatrelatively low temperatures,so that thereis little risk of fiber damage.In addition,the fiber-matrix interfacecanbe

tailoredby pre-coatingthefiberpreformduringaninitial CVI stepusingdifferentreagentsbutthe

same reactor. Porous CFCC components fabricated by other techniques can be further densified using

CVI as long as the pores are open to the surface allowing the infiltration of the gaseous reactants.

One of the major limitations of the CVI method is the long reaction times needed for the

densification process (typically greater than 100 hr). This raises the cost of the CFCC in comparison to

processes that may require only minutes to complete the matrix infiltration. Also, it is not possible to

obtain fully dense matrices using CVI. The presence of residual closed porosity of 10 to 15 percent

may shorten the useful life of the CMC component particularly after matrix cracking in an oxidizing

environment at elevated temperatures. In relation to fully dense CFCC, the porosity can also reduce

the CFCC thermal conductivity and proportional limit stress (and strain) required for matrix cracking

(cf. table II).

III.2. CVI plus Directed Metal Oxidation (CDMO)

The CDMO process is based on the DIMOX process developed by Lanxide Corporation. In the

approach most often used (refs. 12 and 13), a fiber preform, which has been precoated with a BN

interphase coating and a CVI SiC protective over-coating, is brought in contact with molten

aluminum alloy held in a suitable container in air for the growth of an alumina matrix according to

the following reaction:

2 AI(1) + 302(g) --_ A1203 (s).

When the aluminum metal is infiltrated, the alumina matrix slowly builds up within the fiber

preform and fills up the space in between the fibers, even within the closely packed fiber tows. In the

presence of certain minor alloying elements such as Mg, Cu, Zn, Fe, etc, alumina matrices can grow

very rapidly from the surface of the melt. The reaction continues even after the build up of an A1203

layer because the reaction product is not continuous, but contains microscopic channels through

which the molten metal wicks to the surface and reacts with the gaseous atmosphere. All preform

surfaces, except the one in contact with the molten metal or alloy, are coated with a gas-permeable

barrier material by spraying or dipping. The barrier material is not easily wet by molten aluminum.

When the growth front comes in contact with the barrier layer, the reaction product formed is

impervious to the flow of molten A1, thus stopping the reaction and further growth. Proper control of

the reaction kinetics during this process is very important. Growth rates of matrix in the preform is

N2 cm/day.

One advantage of the CDMO route is that it is a low-cost process with near-net shape capabilities.

Only a minor dimensional change occurs during the process since the matrix fills the pores within

the preform without disturbing the placement of the reinforcing fiber phase. Complex-shaped large

CFCC components can be fabricated. One drawback of this method is the presence of residual

aluminum phase (N5 to 10 percent) in the matrix which must be removed if the CFCC is to be used

above the aluminum melting point (660 °C). This residual metal can be leached away by treatment

with an acid leaving behind open porosity. Matrix cracking also occurs during the fabrication process

due primarily to a thermal expansion mismatch between the alumina matrix and the protective CVI

NASA/TM--1998-208819 9
SiCover-coating,whichcanprovideupto 50percentof thetotalmatrixvolume.Thiscrackingand

theresidualporosityfrom theremovedaluminumphasereducethe proportionallimit stressand

thermalconductivityof theCFCCsystem(cf. tableII). Anotherdrawbackis thatthetotalCDMO

processrequirestheinitial CVI stepfor SiC,whichcantakemanyhoursto deposit.At thepresent

time,AlliedSignal Composites(formerlyDuPontLanxide Corporation)is the solefabricatorof

CFCCcomponentsusingtheCDMO route.

III.3. CVI plusSlurryandLiquid Metal infiltration (CSLM)

The CSLM matrix infiltration route is similar to the CDMO route in that the matrix is formed by

a liquid metal infiltration and a CVI SiC interphase over-coating is needed to protect the fibers and

interphase coating from metal attack. In this case, however, the liquid metal is typically silicon or one

of its low-melting alloys, which form reaction-bonded SiC matrices that are more compatible with

SiC fibers and the SiC interphase over-coating in terms of thermal expansion. Before metal infiltration,

fine-grained SiC particulates are slurry cast into a fiber preform that has already been processed with

interphase coatings and the SiC interphase over-coating (ref. 14). After removal of the slurry carrier

liquid, silicon or a silicon alloy is infiltrated into the particulate-filled preform at a temperature of

above 1400 °C. The melt infiltration is complete within a few hours or less. The infiltrated silicon or

its alloy bonds the SiC particulates together and forms a matrix material that is stronger and denser

than that obtained by the CVI route. The volume fraction of free silicon in the reaction-bonded matrix

may range up to 50 percent, but due to its high melting point, long-term matrix use temperatures near

1200 °C are possible.

To reduce the free silicon content, some versions of the CSLM route also infiltrate carbon slurries

or carbon-yielding polymers prior to metal infiltration (ref. 15). Although effective in converting

excess silicon into SiC, it is generally difficult to eliminate all free silicon throughout a CFCC

component. That is, some excess carbon and silicon are typically left unreacted in the matrix. If the

unreacted carbon is exposed at the CFCC surface, porosity in the matrix could increase by carbon

reaction with oxidative environments.

One advantage of the CSLM route is the production of a fairly dense SiC-based matrix. This

typically results in CFCC with better proportional limit strength and thermal conductivity than CFCC

fabricated by the complete CVI route. This can be seen in table II, particularly for CFCC with the

same fiber and interphase types. The higher density and closed porosity of the CSLM route are also

useful for eliminating the need for a final seal coat on the outer CFCC surfaces. Another advantage

over the CVI route is reduced matrix processing time at high temperatures. However, this is

compensated somewhat by the extra step of slurry infiltration.

One possible disadvantage of the CSLM route is the use of temperatures above 1400 °C for liquid

silicon infiltration. This can be further exasperated by local temperature rises caused by the

exothermic reaction between silicon and any excess carbon. These high temperatures can cause

strength loss in those SiC fiber types that are produced at temperatures below 1400 °C (e.g., Nicalon

and Hi-Nicalon). The temperatures can also increase the risk of fiber and interphase attack by

diffusion of excess free silicon through the grain boundaries in the CVI SiC protective over-coating

on the fibers. The free silicon may also cause problems during CFCC service such as enhanced matrix

creep. Also if hot spots should exist in a CFCC component, possible changes in the CFCC properties

could occur due to silicon diffusion to free surfaces on the component (silicon sweating).

NASA/TM--1998-208819 10
III.4. Polymer/Sol-GelInfiltration plusPyrolysis(PIP)

Generalaspectsof thepolymerprecursorapproachto ceramicsandceramicmatrixcomposites

havebeendescribedearlier(ref. 16).Thepolymericprecursorshouldhavecertaincharacteristics.

It shouldshowhighceramiccharyieldin orderto obtainhighmatrixdensityin theleastnumberof

PIP cycles.Highly branchedand cross-linkedpolymericprecursorsor thosecontaininga high

contentofringorcagedstructuresshowhighceramicyields.Ontheotherhand,longchainpolymers

resultin low ceramicyieldbecauseof theirtendencyto breakupintolow molecularweightvolatile

speciesduringpyrolysis.Theimportantcharacteristicsof thepolymericprecursorsarethecorrect

elementalstoichiometry,controllablemolecularweight,goodchemicalstabilityduringstorageand

use,low toxicity, low cost,andability to cross-linkatlow temperatures.

Anumberof stepsareinvolvedinthefabricationofCFCCviaPIPprocessing.Thefirststeptypically

involvesthestackingof fibermattesthathavebeenindividuallyinfiltratedwithpolymer(pre-pregB

approach).Thesearethenpressure-moldedin anautoclave,beforebeingthermallytreatedthroughcure

andpyrolysistemperatures. ThisresultsinaCFCCpreformthatcontainsanamorphousandhighlyporous

ceramicmatrix.Thenextstepistoinfiltratethisstructurewithliquidpolymerandrepeathecuringand

pyrolysiscycles.Thislastinfiltrationandsomeorall of thethermaltreatmentstepistypicallyrepeated

over10timesto achieveaCFCCwith sufficientmatrixdensitytoprovideadequateCFCCphysicaland

mechanicalproperties.TheresultingCFCCmaybeheattreatedathighertemperaturesforcrystallization

of theamorphousmatrix,relaxationofresidualstresses,andfinal consolidation.Thismethodhasalso

beenextensivelyusedforfabricationof carbon/carboncomposites.

Themainadvantageof thePIPmethodistheuseof typicalmethodsemployedin thefabrication

of polymermatrix compositesin orderto fabricatecomplex-shapedCFCCcomponents.Themain

drawbackof this methodis thelongCFCCprocessingtimesandrepeatedimpregnation/pyrolysis

cyclesin orderto obtainsufficientlyhigh matrix density.Also dueto polymer shrinkageduring

pyrolysis,matrixcrackinggenerallyresultswithin theCFCC.Re-impregnationfills thesecracksto

somedegree,butmatrixstrengthandthermalconductivitygenerallyremainlow.Anotherproblem

is thehigh costof polymerprecursors.

III.5. PIPplus Liquid MetalInfiltration (PLM)

ThePLMmatrix-infiltrationroutehasbeendevelopedtoeliminatethefabricationtimesandcosts

associatedwith themultiplere-impregnationstepsinherentin thePIProute.After theinitial stepof

usingthepre-pregBapproachto formafiberarchitecturefilled with ahighlyporouscarbonmatrix,

liquid siliconisinfiltratedintothestructurein amannersimilarto theCSLMroute.Thecarbonand

siliconreactto form adensematrixconsistingof SiCplusexcesssiliconandcarbon.In thepresent

form practicedby theGermanAerospaceCenter(ref. 17),low-costcarbonfibersareusedwith no

interphasecoatingsor interphaseover-coatings.Thissignificantlyreducesthehigh coststypically

associatedwiththefiber andinterphaseproduction.Thusthisprocesshastheprimaryadvantageof

beingthelowestcostapproachfor CFCC.However,theliquid silicondoesreactwith thecarbon

fibersat theouteredgesof thereinforcingtowssothatthestructuralpropertiesof CFCCfrom the

PLMroutearenotasgoodastheothermatrixinfiltrationmethods.Alsothepooroxidationresistance

of thecarbonfibersnecessitatestheapplicationof aSiCover-coatingontheCFCC,therebylimiting

thismaterialto only low-stressand/orshort-termapplications.

NASA/TM--1998-208819 11
IV. SUMMARY

Theprimary routesusedtodayfor thefabricationof CFCCcomponentsarebasedonthefive

generalmatrix-infiltration approacheslistedin tableI anddiscussedin SectionIII. Eachapproach

hasits advantagesandlimitationsin termsof achievingthefourimportantprocess-relatedgoalsof

(1)optimumperformance,(2)tailorableorversatilestructures,(3)reproduciblemicrostructuresand

properties,and(4)cost-effectivefabricationsteps.As discussedin SectionII, thereareavarietyof

factorsthatinfluencetheattainmentof eachof thesegoals.Forexample,for optimum performance,

the route should have a low risk for degrading the fiber and interphase properties while providing a

matrix material that is dense, strong, creep resistant, and thermally conductive. For tailorable

structures, the route should be able to achieve good CFCC performance and net-shape capability

using the wide variety of fiber architectures that may be required for complex-shaped components

of various sizes. For reproducible properties, the route should repeatedly produce the same local

microstructures throughout the entire volume of the CFCC component. Finally, for cost-effectiveness, the route should minimize the number of process steps and carry on these steps in the shortest

times and at the lowest temperatures possible.

With process-related criteria such as these, it is possible to assess and compare in a very qualitative

manner the capabilities of the five matrix-infiltration routes. This is done in table III where the

process and material details for each route are judged against the four key fabrication goals. In this

table, the rankings high (H), medium (M), and low (L) refer to the route' s relative ability to achieve

the various aspects of the fabrication goals. In general, it can be seen that no route ranks high in all

categories. Typically those routes yielding high performance matrices are not cost-effective; while

those that are low cost do not show good performance. Also, although the matrix infiltration routes

are conducive to net and complex shapes, attainment of high microstructural uniformity throughout

a CFCC component is generally not possible. Thus, today, the best fabrication route for the CFCC

end-user is not clear and compromises need to be made depending on the details of the CFCC

application. However, with time, this problem should be reduced as research continues to develop

advanced CFCC constituents and fabrication routes.

V. REFERENCES

1. A.G. Evans and F.W. Zok, Review: The Physics and Mechanics of Fiber-Reinforced Brittle

Matrix Composites, J. Mater. Sci., 29 [15]: pp. 3857-3896 (1994).

2. J.A. DiCarlo and S. Dutta, Continuous Ceramic Fibers for Ceramic Matrix Composites, in

Handbook on Continuous Fiber-Reinforced Ceramic Matrix Composites (R.L. Lehman, S.K.

E1-Rahaiby and J.B. Wachtman, Jr., Eds.), Ceramics Information Analysis Center, West

Lafayette, IN; Chap. 4 (1995).

3. F.K. Ko, Preform Fiber Architecture for Ceramic Matrix Composites, Am. Ceram. Soc. Bull.,

68[2]: pp. 401-414 (1989).

4. H.W. Carpenter, Fiber Coatings for Ceramic Matrix Composites, Ceram. Eng. Sci. Proc., 13

[7-8]: pp. 238-256 (1992).

5. P.E.D. Morgan and D.B. Marshall, Functional Interfaces for Oxide/Oxide Composites, Mater.

Sci. Eng., A162 [1-2]: pp. 15-25 (1993).

6. L.U.J.T. Ogbuji, Evaluation of a Porous Fiber Coating in SiC-Si3N 4 Minicomposite, J. Mater.

Res., 12 [5]: pp. 1287-1296 (1997).

NASA/TM--1998-208819 12
7.R. Naslain,TheConceptof LayeredInterphasesin SiC/SiC,in CeramicTransactions,Vol. 58,

High-Temperature Ceramic Matrix Composites II: Manufacturing and Material Development

(A.G. Evans and R. Naslain, Eds.), Am. Ceram. Soc., Westerville, OH); pp. 23-29 (1995).

8. S. Bertrand, O. Boisron, R. Pailler, J. Lamon and R. Naslain, (PyC/SiC) n and (BN/SiC) n

Nanoscale-Multilayered Interphases by Pressure Pulsed-CVI, in High Temperature Ceramic

Matrix Composites III(K. Niihara, K. Nakano, T. Sekino, and E. Yasuda, Eds.), Trans Tech,

Switzerland; pp. 357-360 (1998).

C. Droillard, Characterization of SiC Matrix Composites with C/SiC Interphase Sequence, Ph.

D. Thesis No. 913, University of Bordeaux, Bordeaux, France, June 1993.

R.A. Lowden, T.M. Besmann, and D.P. Stinton, Ceramic Matrix Composites Fabrication and

Processing: Chemical Vapor Infiltration, in Handbook on Continuous Fiber-Reinforced

Ceramic Matrix Composites (R.L. Lehman, S.K. E1-Rahaiby and J.B. Wachtman, Jr., Eds.),

CIAC, Purdue Univ., West Lafayette, IN; Chap. 6 (1995).

P.J. Geoghegan, Chemical Vapor Infiltrated Composites, in Ceramics and Ceramic Matrix

Composites (S.R. Levine, Ed.), Am. Soc. Mech. Engrs., New York; Chap. 7 (1992).

A.S. Fareed, Ceramic Matrix Composites Fabrication and Processing: Directed Metal Oxidation, in Handbook on Continuous Fiber-Reinforced Ceramic Matrix Composites (R.L.

Lehman, S.K. E1-Rahaiby and J.B. Wachtman, Jr., Eds.), CIAC, Purdue Univ., West Lafayette,

IN; Chap. 8 (1995).

G.H. Schiroky, A.S. Fareed, B. Sonuparlak, C.T. Lee, and B.W. Sorenson, Fabrication and

Properties of Fiber-Reinforced Ceramic Composites Made by Directed Metal Oxidation, in

Ceramics and Ceramic Matrix Composites (S R. Levine, Ed.), Am. Soc. Mech. Engrs.,

New York; Chap. 9 (1992).

S.K. Lau, S. Calandra and R. Ohnsorg, Development of Slurry Casting/Melt Infiltration Process

for CMC Fabrication, Presented at the 19 th Annual Conference on Composites, Advanced

Ceramics, Materials, and Structures, Cocoa Beach, FL, January (1995).

G. Corman and K. Luthra, Improved Properties of gas Turbine Engine Components Utilizing

CFCC and Melt Infiltration, Industrial Heating, pp. 39-40, December (1996).

J.E. French, Ceramic Matrix Composites Fabrication and Processing: Polymer Pyrolysis, in

Handbook on Continuous Fiber-Reinforced Ceramic Matrix Composites (R.L. Lehman, S.K.

E1-Rahaiby and J.B. Wachtman, Jr., Eds.), CIAC, Purdue Univ., West Lafayette, IN; Chap. 7

(1995).

J. Fabig and W. Krenkel, Principles and New Aspects in LSI-Processing, Proc. 9th CIMTEC -

World Ceramics Congress and Forum on New Materials, Florence, Italy; June 14-19 (1998).

Y. Gowayed, Auburn University, Auburn, AL (private communication)

13.

14.

NASA/TM--1998-208819 13
TABLE I.--FIVE GENERAL MATRIX-INFILTRATION APPROACHESFOR

CFCCCOMPONENTFABRICATION

Matrix-infiltration

approach

ChemicalVapor

Infiltration

(CVI)

Typicalfiber

lay-up

approacha

CVI plusSlurry/Polymer

plus

LiquidMetalInfiltration

(CSLM)

PreformA

Typicalmatrix

phasesb

SiC

Somecurrentpracticing

organizations

• AlliedSignalCompositesc

• B.F.Goodrich

• Hypertherm

• ManTechnologie

• RefractoryComposites

• SEP

CVI plusDirectedMetal PreformA SiC+ A1203 + A1 • AlliedSignal Composites c

Oxidation (CDMO)

Preform A SiC + Si

Preform A

Polymer/Sol-Gel

Infiltration plus Pyrolysis

(PIP)

Pre-preg A

SiC + Si + C

Si-O-N-C

A1203 + SiO 2

Pre-preg B

Pre-preg B

• AlliedSignal Composites c

• B.F. Goodrich

• AlliedSignal Composites c

• General Electric

• Dow Coming

• Composite Optics

• General Electric

PIP plus Liquid Metal Pre-preg B SiC + Si + C • German Aerospace Center

Infiltration (PLM)

aSee Section II for details.

bSee Section III for details.

CFormerly DuPont Lanxide Corporation.

TABLE II.--TYPICAL PROPERTIES OF 2-D CFCC FABRICATED BY DIFFERENT

MATRIX-INFILTRATION ROUTES

PropeJ_y at morn

temperature

CFCC matrix infiltration mute

CVI CVI CVI CDMO CSLM PIP

Fiber type T300 Nicalon Hi-Nicalon Nicalon Hi-Nicalon Nicalon

(vol %) Carbon SiC SiC SiC SiC SiC

(45%) (40%) (40%) (35%) (40%) (50%)

Interface C C B N B N B N Propfietaxy

Matrix SiC SiC SiC SiC + Si Si-N-C-O

PmpoJ_ional limit

strength, MPa

Intedaminax sheax

strength, MPa

62

26

6.5

Thermal conductivity

through thickness,

W/m-°K

75

32

9.5

130

43

10

AI?O 3

42

63

8.7

140

50

19

60

14

1.3

NASA/TM--1998-208819 14
TABLE III.--COMPARISON OFMATRIX-INFILTRATION

ROUTESFORCFCCFABRICATION: RELATIVE

ABILITY FOR ACHIEVING FABRICATION

CRITERIA: H - HIGH, M - MEDIUM, L - LOW

Fabricationcriteria CFCCmatrixinfiltrationmethod

CVI ICDMO

Optimumperformance

Fiber-interphasestability H M

Structuralmatrix: M/H L/L

High strength/creepresistance

High matrixthermal M L

conductivity

Tailorablestructures

Nearnetshapecapability H H

Complexshapecapability H H

Conduciveto 3-Darchitecture H H

Reproducibleproperties

Uniform interphasecoating L L

Uniform matrixmicrostructure L L

Cost-effectiveness

Low-costinterphasesteps H H

Low-costmatrix steps L M

CSLM I PIP IPLM

M H L

H/M M/L H/M

H L M

H

H

H

L

L

H

M

H H

M M

L L

M M

L L

M M

L H

Figure 1.--0/90 ° 2-D fabric composed of multifilament

tows woven in a 5-harness satin pattern [18].

NASA/TM--1998-208819 15
Figure 2.--SEM micrograph showing polished crosssection of a matrix infiltrated 8-ply 2-D SiCf/SiC

composite.

Figure 3.--High magnification SEM micrograph

showing fibers within a tow, each covered with

a BN interface coating (dark) which are in turn

covered by a CVI SiC overcoating.

NASA/TM--1998-208819 16
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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

November 1998 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Fabrication Routes for Continuous Fiber-Reinforced Ceramic

Composites (CFCC)

6. AUTHOR(S)

James A. DiCarlo and Narottam P. Bansal

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135- 3191

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546- 0001

WU-523-21-13-00

8. PERFORMING ORGANIZATION

REPORTNUMBER

E-11424

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--1998-208819

11. SUPPLEMENTARY NOTES

Responsible person, Narottam P. Bansal, organization code 5130, (216) 433-3855

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12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

The primary approaches used for fabrication of continuous fiber-reinforced ceramic composite (CFCC) components have

been reviewed. The CFCC fabrication issues related to fiber, interface, and matrix have been analyzed. The capabilities,

advantages and limitations of the five matrix-infiltration routes have been compared and discussed. Today, the best

fabrication route for the CFCC end-user is not clear and compromises need to be made depending on the details of the

CFCC application. However, with time, this problem should be reduced as research continues to develop advanced CFCC

constituents and fabrication routes.

14. SUBJECT TERMS

Ceramic composites; Fabrication methods; Interfaces

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