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Ceramic matrix composites (CMC) areproduced from ceramic fibers embed-

ded in a ceramic matrix. Various ceramicmaterials, oxide or non-oxide, are used forthe fibers and the matrix. Also a large variety of fiber structures is available. Soproperties of CMC can be adapted to spe-cial construction tasks. They are especial-ly valuable for components with demand-ing thermal and mechanical requirements.

MotivationWhen new components are developed,usually Finite Element simulations areused to check the loads during operationand to optimize geometry. Databases areavailable, which allow a quick selection ofthe optimum material considering materialproperties, cost and process requirements[1]. If complex requirements are to be ful-filled, different material properties, whichcontribute to the same requirement can becombined to form a material index. Differ-ent requirements can lead to conflictingobjectives, which are handled by con-structing trade-off surfaces in material index charts [2]. As an example, a lightweight and stiff plateis considered which shall be used at tem-

Ceramic Matrix Composites – an Alternative for Challenging Construction TasksCeramic Matrix Composites (CMC) are extremely valuable forapplications with demanding thermal and mechanical requirements. CMC have been developed to achieve a damage tolerant quasi-ductilefracture behavior and to maintain all other advantages of monolithicceramics at high temperatures.

peratures above 600 °C. Brittle fracture ofthe plate is not allowed. Such plates can beused as material support in heat treatmentprocesses. The first requirement leads to aminimization of specific weight ρ and amaximization of flexural modulus E. Therespective material index which combinesboth properties is the ratio I = ρ/E1/3. Thebest material for the first requirement isthe material with the smallest index I. Tofulfill the second requirement, i.e. to avoidbrittle fracture, the fracture energy shall be

maximized. Fig. 1 shows a selection ofthose materials – among a database of4000 materials – which have promising

KeywordsCeramic matrix composites (CMC), ceramic slurry infiltration,chemical vapor infiltration, alumina fibers, silicon carbide fibers

Friedrich RaetherFraunhofer Center for High Tempera-ture Materials and Design HTL95448 BayreuthGermanywww.htl.fraunhofer.de

Corresponding author:friedrich.raether@isc.fraunhofer.de

fracture energy in kPa-1m-1

inde

x I

in k

g m

-3G

Pa-

1/3

Fig. 1 Light-weight materials with maximum stiffness and maximum fracture energy selected from [1].CMC dominate in a broad range of intermediate fracture energies and indices I, whereas stainlesssteel and Ni based alloys dominate at high fracture energies and carbon foam is superior for extremely small indices I

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properties in terms of these two require-ments. The trade-off surface is shown aswell. If the focus is on high fracture en ergycustomary metals like stainless steel canbe selected, but if greater weight is at -tached to the first requirement, CMC ma -ter ials like SiC/SiC and Ox/Ox are superior. With customary materials, i.e. polymers,metals and monolithic ceramics typicallimitations occur in operational behavior.Polymers are rather weak and there use islimited to low temperatures. Apart fromthe refractory metals, which are expensiveand brittle, metals can be used up to1000 °C – some special alloys up to1200 °C. A disadvantage of these metals is their low creep resistance and their susceptibility to oxidation. Monolithic ceramics can be used up to very high tem-

toughness of CMC. This interaction iscarefully designed using two complemen-tary concepts [4, 5]: • Weak interface concept: the fibers are

coated to reduce adhesion to the matrix.During fracture fibers are pulled out ofthe matrix and absorb fracture energy(Fig. 2).

• Weak matrix concept: the stiffness of thematrix is adjusted much lower than thestiffness of the fibers. During fracturecracks arise in the matrix and are deflect-ed at the fibers, thereby increasing thefracture surface and elongation at break(Fig. 3).

Tab. 1 shows material properties of typicalCMC. Due to anisotropy and different CMCqualities a broad range is covered [1, 4].Note that the properties cannot be arbi-trarily combined within the given range.The composition and microstructure ofCMC components has to be carefully de-signed according to the respective use.Compared to metals, the most importantadvantages of CMCs are a significantlysmaller density, which is important forlightweight constructions, and a muchhigher maximum operating temperature.For many applications also its resistanceto wear and aggressive chemicals is im-portant. Costs of CMC strongly depend oncomposition and manufacturing route.They vary between some hundred andsome thousand EUR/kg. So, CMC are expensive compared to other materialsand the high price has to pay off by longerservice life or by a unique performance invalue-added products. If non-oxide CMC components are used inoxidic atmospheres at high temperatures,the components can be protected from oxidation using environmental barriercoatings (EBC). Compared to carbon, silicon carbide is less sensitive to oxi -dation because it forms a protective layerof silicon dioxide. In addition to the EBC,non-oxide fibers used in CMC are oftenprotected by a fiber coating to avoid attackof oxygen molecules diffusing through thepore channels of the matrix.

ApplicationsCMC are used in many high temperatureprocesses. They have a very high thermalshock and creep resistance, which enablesdesigns with large mechanical and thermalloads. As an example, Fig. 4 shows some

peratures in the range of 1000 °C to2000 °C. They have an excellent creep resistance and show high stiffness. Themain disadvantage of monolithic ceramicsis their low fracture toughness, whichleads to brittle fracture and detrimentalthermal shock resistance. CMC have beendeveloped to achieve a damage tolerantquasi-ductile fracture behavior and tomaintain all other advantages of mono -lithic ceramics.

CMC types and propertiesThere are many different types of CMC.Classification is usually done in terms offiber and matrix materials – separated by aslash. E.g., C/SiC is a CMC made of carbonfibers and a silicon carbide matrix. Non-oxide fibers used in CMC are mainly madeof carbon or silicon carbide, oxide fibers ofalumina, mullite or silica. Non-oxide matri-ces are mostly silicon carbide, carbon ormixtures of silicon carbide and silicon.Oxide matrices consist of alumina, zir -conia, mullite or other alumino-silicates.Usually oxide fibers are combined with oxide matrices and non-oxide fibers withnon-oxide matrices. Thus, the main CMCtypes are C/C, C/SiC, SiC/SiC and Ox/Ox,where Ox represents one of the oxide ma-terials mentioned previously [3]. In add -ition to matrix and fibers, most CMC con-tain pores – usually between 1 and 30 %.CMC are further classified according totheir fiber structure, which has a large im-pact on material properties. Ceramic fibershave a tensile strength between 1000 MPaand 7000 MPa – about an order of magni-tude higher than the strength of the matrix.Likewise, the elastic modulus of the fibers,typically between 200 GPa and 900 GPa is higher than the elastic modulus of the mat rix. The fiber type has to be carefullyselected. Fiber degradation occurs be-tween 1000 °C and 2100 °C depending onfiber material and fiber quality. It controlsthe maximum service temperature of theCMC. Continuous or short fibers are usedfor CMC manufacture. Fibers can be ori-ented unidirectional or planar to achievespecial anisotropic properties. Woven orunwoven fabrics can be used, wherebytextile techniques like breading allow for3D structures with complex load charac-teristics. The interaction between fibers and matrixduring fracture provides the high fracture

Fig. 2 Fracture surface of a CMC with fiber pull-out

Fig. 3Stress-strain diagram during fracture of anOx/Ox CMC – indicated is the respective curvefor monolithic ceramics

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Ox/Ox hot gas valves used to control thegas flow in gas fired high temperature fur-naces. Compared to metallic valves, theservice life of the CMC components ismuch longer and over-compensates theirhigher purchasing costs. CMC compon -ents, used as batch carriers in metal hard-ening are another example (Fig. 5). TheseC/C-grids have small heat capacity – thusreducing energy consumption and allowing

fast heating and cooling cycles. Differentfrom metallic batch carriers, they show nocreep deformation providing much longerservice life. Other applications of CMC inhigh temperature processes are flametubes, heat exchangers, protective tiles,and various high temperature holders.The high wear resistance and the favorablefriction properties of CMC lead to applica-tions as sliding contact bearings, brakes

and clutch-plates. As an example Fig. 6shows a C/SiC brake disk used in passen-ger cars. It has a life time longer than thelife time of the car and a much smallerweight than customary brake disks made of cast iron. So, the higher costs of theCMC brake disks are compensated by re-duced fuel consumption and elimination ofservice costs for the renewal of the brakedisks.

Tab. 1 Material properties of typical CMC at ambient temperature, the range reflects minimum and maximum of the respective property in different directionsor for different CMC qualities (Ox/Ox covers CMC with alumina fibers and alumina or alumino-silicate matrix) [1,4]

Fig. 4 Hot gas valve made of Ox/Ox CMC used tocontrol gas flow in gas fired furnaces (source: Walter E.C. Pritzkow Spezialkeramik)

Fig. 5 C/C batch carriers for metal hardening (source: Schunk group)

Property Unit SiC/SiC C/SiC C/C Ox/Ox

Fiber content vol.-% 40–60 10–70 40–60 30–50

Porosity vol.-% 10–15 1–20 8–23 10–40

Density g/cm3 2,3–2,9 1,8–2,8 1,4–1,7 2,1–2,8

Tensile strength MPa 150–360 80–540 14–1100 70–280

Bending strength MPa 280–550 80–700 120– 1200 80–630

Strain-to-failure % 0,1–0,7 0,5–1,1 0,1–0,8 0,12–0,4

Young’s modulus GPa 70–270 30–150 10–480 50–210

Fracture toughness MPa·m1/2 25–32 25–30 5,7–,3 58–69

Thermal conductivity W/m·K 6–20 10–130 10–70 1–4

Coefficient of thermal expansion ppm/K 2,8–5,2 0–7 0,6–8,4 2–7,5

Maximum service temperature °C 1100–1600 1350–2100 2000–2100 1000–1100

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fiber bundles (Fig. 7) or by textile structur-ing of continuous fibers with techniqueslike weaving, knitting and braiding (Fig. 8).Also non-woven structures like uniaxial ormultiaxial fabrics, fleeces and felts areused. Some non-woven structures canalso be produced directly from shortfibers, which are made by disk spinning –avoiding the more expensive nozzle spin-ning and cutting of endless fibers. The pre-forms have a one-, two- or three-dimen-sional fiber structure. This structure con-trols the anisotropic properties of the finalCMC. It can be designed to bear exactlythe anisotropic loads expected during useof the CMC components. The matrix material is introduced into thepreform via a fluid phase – either gaseousor liquid. Liquids are infiltrated as slurrywith ceramic particles, as polymers or asmetallic melts. Usually silicon is used formetallic melt infiltration. Silicon melts at1414 °C, is soaked into the preform by capillary forces, and reacts with carbon inthe preform to form silicon carbide. Thisprocess is called liquid silicon infiltration(LSI). Alternatively, polymers are introduced asorganometallic compounds either in dis-solved or molten state to achieve sufficientlow viscosities. External pressure can sup-port the infiltration. During a subsequentheat treatment the polymers are pyrolysedand the final structure of carbon or siliconcarbide is formed. Since the volume of thepolymers decreases during pyrolysis, thisso called polymer infiltration and pyrolysisprocess (PIP) has to be repeated typically3 to 10 times to achieve high densities ofthe final CMC. As a further variant using liquids, slurrieswith ceramic particles are infiltrated intothe preforms at ambient temperatures.This impregnation allows a quasi ductilemanufacturing of complex shapes (Fig. 9)analogous to the processing of sheet metals. After drying of the preforms, a heattreatment is required to obtain enoughstrength within the ceramic matrix by asintering process. Usually sintering is in-terrupted in the initial stage because exces-sive shrinkage of the matrix would lead tocracks in the CMC structure. Ceramic slur-ry infiltration (CSI) leads to matrices withan open porosity between 20 and 50 %. Unlike using liquids, the matrix can be intro duced via a reactive gas. This chem -

CMC can also be used in extreme environ-ments like gas turbines. Operating tem -pera tures of gas turbines have been in-creased to improve energy efficiency.More over, new designs of the turbines re-quire blades with very high rotationalspeeds. The lightweight and high-tem -perature properties of CMC are ideal forthis application. Tests have already beenperformed in working gas turbines andfirst products shall be ready in 2016 [6].Other CMC applications in aerospace arebody flaps, shrouds and thermal protec-tion systems.The anisotropic thermal expansion ofC/SiC can be used to design componentswith zero thermal expansion in one or twodirections of space. These components areused as support in precision optics, e.g. insatellite communication or microelectron-ics, or for calibration of dimensional con-trol tools.

CMC manufacturingCMC are produced using ceramic fibers ina thickness range of 3 to 20 µm. The smallfiber diameters provide flexibility of thefibers during further textile processing.Fibers are usually manufactured as yarnswith some hundreds up to several tenthousands of filaments. Prices increasefrom carbon fibers over oxide ceramicfibers to silicon carbide fibers from 20 EUR/kg for the cheapest carbon fibersup to 10 000 EUR/kg for the most expen-sive silicon carbide fiber type. CMC pre-forms are produced from the fibers eitherby cutting the yarns and forming short

Fig. 6 Brake disk made of C/SiC CMC (source: Brembo SGL Carbon Ceramic Brakes)

Fig. 7 Structure of a short fiber bundle reinforcedC/SiC CMC

Fig. 8 Cross section through an Ox/Ox CMC reinforced by a 2D woven fabric

Fig. 9 Distributor for testing exhaust systems madeof Ox/Ox CMC (source Walter E.C. Pritzkow Spezialkeramik)

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ical vapor infiltration (CVI) is similar to thewell known CVD process. It is performedin a controlled atmosphere of the reactivegases at temperatures above 800 °C andneeds infiltration times of several hours upto some days to achieve sufficient dens -ities. An advantage of the CVI process is itscapability to apply coatings on the fibersbefore introducing the matrix. Using the liq-uid infiltration routes the coatings are ap-plied either via an additional CVD processor by – cheaper wet chemical routes.

TrendsCosts of CMC are controlled to a large ex-tent by the costs of ceramic fibers. Withcarbon fibers a dramatic decrease ofprices was already achieved when massproduction started within the last decades.

The production volumes of alumina andsilicon carbide fibers are still small and theworld market is shared between few com-panies located in North America and EastAsia. With increasing use of CMC, add -itional companies, especially in Europe,will enter the market [7, 8]. So it is expect-ed that fiber prices decrease medium-term. Another source of high production costsof CMC components is the generation ofthe matrix, which needs expensive batchprocesses at high temperatures – usuallyin controlled atmosphere. Currently largeactivities in R+D consider more efficientinfiltration processes [9]. Further benefit isexpected from standardization of CMCcom ponents for serial production. Largescale production of simple shapes likeplates and pipes would eliminate manual

processing steps. More complex compon -ents can be built via joining techniques,which are currently developed.A driving force for the increasing demandfor CMC is the computer based design ofnew components. FE simulations providecomplex technical requirements like highlyanisotropic loadings. The competitive ad-vantage of CMC is highest with such com-plex demands. Currently, intense researchis done to understand CMC behavior fromits structure on micro- and meso-scale andto predict life time of CMC components un-der operating conditions (compare e.g.[9]). This will help to further improve per-formance of CMC components and to con-vince construction engineers through both,economical and technical reasons to useCMC in additional applications.

[1] CES Selector (Cambridge EngineeringSelector), Version 5.2.0 (2009)

[2] Wanner, A.; Fleck, C.; Ashby, M.F.: Materials selection in mechanical design.Elsevier, 2006

[3] Krenkel, W. (Ed.): Ceramic matrix composites. Weinheim 2008

References

[4] Bansal, N.P. (Ed.): Handbook of ceramiccomposites. Boston 2005

[5] Tushtev, K.; et al.: Deformation and failure modeling of fiber re inforced ceramics with porous matrix. AdvancedEngineering Materials 6 (2004) 664–669

[6] www.flightglobal.com/news/articles/ge-cmcs-may-yield-15-fuel-burn-cut-in-leap-engines-374337/

[7] www.sglgroup.com/cms/international/presslounge/news/2012/02/02242012_p.html?__locale=en

[8] www.cerafib.de/[9] www.forschungsstiftung.de/

index.php/Drucken/Projekt/323.html