Carbon-Carbon Composites -An Overview

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Defence Science Journal, Vol 43, No 4, October 1993, pp 369-383@ 1993, tesDeviand abad-500269ABSTRACTCarbon-carbon composites are a new class of engineering materials that are ceramic in naturebut exhibit brittle to pseudoplastic behaviotir. Carbon-carbon is a unique all-carbon composite withcarbon fibre embeded in carbon matrix and is known as an inverse composite. Due to their excellentthermo-structural properties, carbon-carbon composites are used in specialised application likere-entry nose-tips, leading edges, rocket nozzles, and aircraft brake discs apart from.several industrialand-biomedical applications. The multidirectional carbon-carbon product technology is versatile andoffers design flexibility. This paper describes the multidirectional preform and carbon-carbon processtechnology and research and development activities within the country .Carbon-carbon productexperience at DRDL has also been discussed. Development of carbon-carbon brake discs processtechnology using the liquid impregnation process is described- Further the test results on materialcharacterisation, thermal, mechanical and tribological properties are presented.I.INTRODUCTIONCarbon is a unique element that can exhibit differentproperties in different forms. Sollle forms of carbon areextremely hard, like diamond, while some forms areextremely soft and ductile. Thus, in addition to its welldefined . Iotropic forms ( diamond and graphite) ,carbon can take any number of quasi-crystalline formsranging from amorphous or glassy carbon to highly,crystalline gfaphitel. The latest form of carbon (C60),discovered re'Cently, is called Fullerene, named afterBuck-Minister Fuller, the renowned American architectand philosopher. Fullerene is the roundest of all roundmolecules, more like a soccer ball, and has propertieslike. ivity and is an excellent semiconducto .Full knowledge of its properties is still not acquired.The changing Defence scenario and satellitei launchings using re-usable launch vehicles led tovigorous search for light weight, ultrahigh temperaturematerials. For a long time, carbon has been known forits high temperature properties and is widely used inheating elements. In 1879, Thomas Alva Edison used-Received12september1993a crude form of carbon fibre for the first electric lamp3.Germans used graphite for the jet vanes in the V 2rocket.But its application in structures was limited because ofits failure even at low strains, thermal shock sensitivity,anisotropy and processing difficulties for large andcomplex shapes. The advent of carbon-carbon (CC)composites changed the scene drasllcally.2. CC COMPOSITESCC composites are a new class of engineeringmaterials that are ceramic in nature but exhibit cements when embedded in carbonaceous matrixmaterial results in CC composites. As in all com sites,the aim is to combine the advantage of high specificstrength and stiffness of carbon fibres with the refractoryproperties of carbon matrix. When the fibres are laidin near-net shapeswith multidirectional reinforcements,the result is an ideal high temperatu-"e structure.2.1 Unique FeaturesThese composites are the best among all hightemperature materials becausethey are thermally stable369

and do not meltconductivityup to 3000 C,and low thermalNOVOL ()fhave high thermalexpansion(thus lass industry , furnaceindustryas wellcorrosionin chemicalmechanical strength to the end. Also, these compositesinnovativeuse is as nceto thermalgoodtemperatureshock)and retaintheirfrictionalpropertiesoverthe entirerange withlow wear.Theyhave highfracturetoughnessmannerlikeand do not fractureconventionalceramics.in a ndfor metallurgicalstage forging process. Other high techare as heat exchanger tubes for as forcrucibles,'1uclearfastners,reactors,load bearinghighplates,mechanism of fracture occurs where the fibres break aswell debondl.ll. The brake discs for high speed aircraftsrods and heating elements. Elementallike Mirage 2{)()(), Concorde, Airbus-320 are Some ofthe example where the favourable frictional propertiestissues. Thus it finds use in hi,p bone endoprosthesis,were put to use. The first generationh:ld the ntover longofnozzles,thrustnozzles using CC ball andsocket joints and high performanceturbojetengines.made of graphite which eroded rapidly and had limitedpyroliticgraphite.were used. which when reinforcedgave birth to CC composites.in USA.France and formerA TJ graphiteswith carbon fibresMajorwork is going onRussia. There is a greatdemand for CC brake discs for aircrafts.and even racing cars. The pioneerstrains. trucksin this field areBendix in USA and Dunlop in UK. Nigrafitc.is the leading organisationin formerknownwhile Germanyabout its products,Moscow,USSR. Littleisand Japanare in the race for industrial applications. Israel. Taiwan.and Egypt were reported to have initiated some R&Dactivities.A study conductedon CC compositeCC COMPOSITES:ANDF ABRICASTRUCTURAL3TIONPROCESSDESIGNFabrication olid particlescarbon)as a precursorCC composites have their origin in the jet vanesused in the German V 2 rockets. The jet vanes wereSubsequently.heart valves.arefabricatedof pure carbonwith a preliminaryby Dr Robert A; Meyerresearch in the Fa! East indicates2-0 and multi-directionalpreforms are used as primarycarbon rather than particulate fillers. There are twodistinct techniques used to fill the interstices betweenthe carbonfibres.Theseare (i) gas phase using achemical vapour depositionprocess, and (ii) the liquidphase route using thermosettingresins or pitch (PIC).The fibres can be very stiff, highly orientedand dense (pitch precursor)or. relativelystronger,less orientedgraphiticprecursor).Conversely,the matrix can be highlyoriented and graphitic if produced from pitch, eitherisotropic or anisotropicif produced from CVO orusually isotropicif producedfrom thermosettingcoJlectiveassessmentresearch effortsindicatesthatin Japan and otherprcferablcfor thick partsn. A combinationin the Unitedsupporttrend continues.on17()industrialAsianStates if the present financialInternationally.applications.CCareFig. I geofPreformsmultidirectionalCCcomposites is the freedom to orient selected fibres andthe stress iscompositesof liquid andthecountries will improve and, in time. surpass the researchactivitiesresinusing phenolic resin .A general rule of thumb cmployedby manufacturers is that the gas phase route is adequatefor thin-wallcdparts and the liquid phasc route isthe general process flow sheet.aspects ofgraphiticflexihle ,and less dens.e (P ANare carrying ()ut active research on differentHisasfor secondary carbon formed during thegas phase processes is also being followcd.technology.(knownbinder which actsthat, apart from India. as many as 18 institutions inJapan, 4 in China. 3 in Taiwan, 3 in Korea, 2 in AustraliaCCbycarbonisation process. In CC composites the carbonfibres (based on rayon/P AN/pitch) in the form of U- D ,2.2 International Status of CC Compositeslife.and artificialthisfor space shuttle wings, rocketvectoring3secondcomposites,limitation was overcome. These unique features madeit the most favourite material for re- entry nosetips,leading edge materialbone plates, osteosynthesiswith blood and softCC compositesof proneness to oxidationHowever,generationto have the best biocompatibilitycarbon is "nownaamounts to accommodatestructuralcomponentthe design loads of the finalandmakethemvirtually

ROHINIDEVI & RAMARAO: CARBON-CARBONCOMPOSITES3-DIRECTIONAlARRAY/I5-D4R CnONAlfil:ureI.DensificationARRAYby carbon matrix.delaminatian-free. Multidirectionalpreform fabricationtechnology provides the means to produce tailored andnear netshape composites,property requirementwhich meet the directional3-0,4-0,5-0of an end item.Thermal, mechanical and p.hysical properties of thecomposites can be controlled by the appropriate designof substrate parameters such as fibre orientation,volume fraction of fibres in the required direction desstructuraltheidealcomposites.Thesimplest type of multidirectionalstructure is based ona three directional (3-D) orthogonal construction asshown in Fig. 2, consisting of multiple yarn bundleslocated within the structure described in cartesianco-ordinates. In any direction, fibre bundles are straightin order to obtain the maximum structural capability offibre. The type of fibre,per site, the fibredistributions, the numberbundleof fibre bul,u, sspacings, volumefractionthe woven bulk densities characterise theARRAYFigure 3. 3-0, 4-0, and 5-0 arrays,preform.typicalThese characteristicsunitcellintheare calculated 'forpreform.Severalaweavemodifications to the basic 3-0 orthogonal designs arepossible as shown in Fig. 3, to form a more isotropicstructurein 4-0,5-0,7-0composite propertiesyarns are introduced.technology,alsoand 11-0. To enhance thebetween the planes,The multidirectionalknownas theemploys formsdiagonalpreformof structuraland textilein simpleblocks,cylinders, cones, contours, surfaces of revolution andcomplex geometriesand shapes. The techniquesemployedare conventionalweavingwithdry yarns,pierced fabrics, assembly of pre-cured rods, on manual,semiautomatedand automaticbraiding anc 3-0 knitting.where this technologyloom set-ups and 3-0Countrieslike USA, Francewas pioneered,closelyadaptabilityto strategic products. With relentless guardedOROLreinforcedduehave kept thistechnologytoitshasdevelopedprefor:m technologyweave parameters.facilitiesare establishedpreformsusing, manualThe technologyto develop.\-dimen tionaland semi-automatedFigure 4 shows the possible materialpreforming8.The weaving tcchnology2theforand 6-D preforms in blocks and cylinderswith varyingfigureimmediateare developedlooms.variants toand defectto realise371

ROHINIFigure 4.MaterialvariantsDEVI& RAMARAO:CARBON-CARBONCOMPOSITESto preforming.defect-free preforms. Efforts are being made to producepreforms in near-net shapes using automationtechniques and 3-D braiding technology. Figure 5 showssix different multidirectional preforms woven at DRDL.4. CC PROCESSING TECHNOLOGYThe CC .densification process inv()lves in-depthdeposition of secondary carbon from differentprecursors using either gas phase impregnation or theliquid phase impregnation.GRAPHITISATIONrII ---,II:II 4.1 Gas Phase Impregnation (Chemical VapourDeposition, CVD)Figure 6. Carbon-carbonmanufacturingprocess.This technique uses volatile hydrocarbons such asmethane, propane, benzene and other low molecularweight units as precursors. Thermal decomposition isachieved on the heated surface of the carbon fibresubstrates resulting in a pyrolitic carbon deposit. Thistechnique can be employed to deposit carbon on to dryfibre preforms or to densify porous CC structuresproduced by the liquid impregnation route, in whichcase it is referred to as chemical vapour infiltration.This process route was widely used by the Westerncountries for the production of thinner parts like aircraftbrake discs and nozzles. CC process technology usingCVD technique is yet to be established in our country .Thisprocessinvolvesimpregnationlike coal itches and highresins.The criterion for selection of impregnates is basedon the characteristics like viscosity, carbon yield, matrixmicrostructure and matrix crystalline structure whichare considerably influenced by the time-temperaturepressure relationshipsgeneralcategoriesduringarethe d advancedresins such as phenolics,resins likeethynylpitches based on coal tar, petroleumFigure 6 shows the CC manufacturingpyrenes orand their blends.process using themultiple impregnation,carbonisation(1000 C),pressure(1000 ,(27500C).Inatmosphericthe carbon yields obtainedhighandpressurefrompitchare only around 50 per cent i.e. approximatingthosefrom high yield thermosetting resins. Yields as high as90 per cent can be obtainedby carbonisingthe pitchunder high pressure of 1000 bars, thus makingtheprocess more efficient. Pressure applied during pyrolysisalso affects the matrixmicrostructure.The higher thepressure the more. co rse and isotropic will be themicrostructure due to the suppression of gas formation4.2 Liquid Phase Impregnation Processimpregnants IIJThetworing-structured,and escape. High pressure also helps in loweringthetemperature of mesophase formation in pitch, resultingin highly oriented crystalline structure. The HIP processis the only practical route to lower the productioncostof CC composites9.DRDLhas established the state of the art facilitiesfor the prototypeproductionof CC productsup to amaximum density of 2.0 g/cc. Apart from the basicprocess equipments, DRDL has designed and fabricated373

DEF SCI J, VOL43, NO 4. OcrOBER1993temperaturerange) and melt impregnationdepth depositionor inof SiC matrix.The materials and process known to give oxidationprotectionare given in Table I.With the external protectionexpansionmismatchpossible refractorybetweenmethods, the thermalcarbonmaterialcoatings is the main problemandto beovercome. Microcracks developed in refractory layershave to be sealed with glassy coatings. The bestoxidationresistance was achieved in which CVD surfacecoatings were formed in addition to in depth protection .Internalprotectionmethodsremoval and or deactivationinclude(ii) incorporationof oxidation inhibitorspartial substitution of matrix material.A successful protectionFigu 7.cc rom ite5status in India.several auxilia. y support systems such as centralisednitrogen gas supply, closed loop process cooling station,and ventilation/pollution control devices. Also the CCtechnology group has established the machiningfacilities and standardised the machining parameters forCC composites.CC process facilities like impregnator/carboniserexist with various institutions like VSSC and NPL. Thepresent status of this technology is depicted in Fig. 7.internalOXmATION(b)374and total orand a compatiblesubstrate since CCLong-tcrmprotcctupto6()()OCPack cementationSiO2 followed2.with SiC.by impregnationImpregnationwithinorganic salts. boronwith alkali silicates to sealoxides. phosphates,the cracks.hak)gen compounds.Sintering with SiCand B4C.Up 1o 15(XIOCImpregnation(i) Ch-emicalwithtetra ethyl-orthosilicatedepositionof Rh .Solgel process for addition.Tantalum.carbjdesof ceramic powders and glasses.and nitrides ofandRESISTANCENotwithstanding the attractive mechanical andthermal properties of CC at elevated temperatures,some of the potential applications like turbine structuralcomponents which require long term exposure 10 hightemperature are restricted by the inherent reactivity ofcarbon towards oxygen beyond 5000C. A number ofdifferent oxidation protection mechanisms have beenexplored to improve the oxidation resistance of CCcompositeslo. The techniques developed can becategorised as:Surface coatings: single layer/multilayers, usingchemical vapour deposition, pack cementation,physical vapour deposition (PVD ) and plasmaspray.In depth protection includes solgel process,impregnation with inorganic salts (for limitedandsystem comprises a coating,Short-term protection5.1 Oxidation Protection Mechaaisms(a)directTable I. V -:"JS materialsknown to give ooxidationprotection3.5.inhibitor(i)of catalytic impurities,titanium.5.6.Chemical vap )urnitride.depositioncarbide.of SiCsilicontungstenSpraying of Ni and Si in(ii) Ccrmcts ofnitrocelluloserefractorymaterials such asfollowedlacquerby sintering invacuum to form Ni-Simetallic phase of SiO2ZrB2, MOSi2 or Si1N4 amcarbides, oxidessilicides. nitridesof metals liketantalum.7.Chemical reaction withmolecularsilicon to form SiC.8.Hafnium diboride, hafnium'oxide, iridium for temperaturesbeyond 17!xrC.

ROHINIDEVI& RAMARAO:CARBON-CARBONCOMPOSITEScarbon composites. DRDL has documented theprocessing methods of C/SiC and SiC/SiC composites(see Fig. 9) and initiated research activities in this .regard. l.J i l.Ju. 6.10 :: u.l.J(/):: 0.1UNPROTECTEDCARBON RCOATEDin .composites constitute a diverse class of materials witha wide range of mechanical, thermal and morphologicalproperties. Selection of appropriate fibre, ification processing method is essential if goodoxidation resistance as well as physico-chemicalcompatibility between substrate and coating is to beachieved. The progress on research on oxidationprotection is illustrated by the bar chart shown in Fig. 8.5.2 Higher Oxidation ResistanceIntroduction of a ceramic matrix like SiC instead ofcarbon matrix in the carbon fibre preform gives higheroxidation resistance than that of oxidation-resistant CC.These composites known as ClSiC composites providea good trade off between the high temperaturecapability of carbon fibres and the high oxidationresistanceof ceramic matrices. Extensive work has beencarried out by SEP FRANCE on ClSiC composites forliquid propellant rocket and air breathing engines,thrust vectonng no1Zles, hot gas valves and tubes andspace plane thermal structures. The third family ofthermo-structural composites, viz. SiCISiC, employceramic fibres (SiC) and ceramic matrix (SiC). Thesecomposites provide an excellent oxidation resistance forlong durations and capable of withstanding thermalcycling for re-usable structures. SiCISiC composites areused for liquid propellant rocket engine chambers, jetengines, gas turbine components and space thermalstructures. However, SiCISiC composites start losingthe mechanical strength beyond 12000Cunlike carbonCCPRODUCTDEVELOPMENTEXPERIENCEDRDL has initiated research and developmentactivities in different aspects of CC technology forrealising several hi-tech CC products. The main thrustof the effort is to establish the CC composite processtechnology and the study of the influence of: (i) differenttypes of carbon fibres in various fibre architectures,(ii) impregnants, and (iii) process parameters ofimpregnation, carbonisation, high pressure carbonisation and graphitisation, on microstructure, physical,thermal, mechanical, thermo-structural and tribologicalperformance of CC composites. DRDL has pioneeredthe challenging tasks of design, development andqualification of full scale prodocts like 3-D and 4-D CCcomposites and CC aircraft brake discs. Research isunder active progress for biomedical products like CCbone implants and heart valves. Collaborative researchwith NPL includes development of pitch impregnantsand oxidation-resistant CC composites. Extensiveprocess and material characterisation data have beengenerated during the development of the abovementioned products. While in multidirectional cements is a complicated and challenging task,in bi-directional composites densification is a complexprocess involving optimum selection f processparameters like heating rates, temperatures, pressuresand pressure gradients to avoid delamination due toevolution of pyrolysis gases, shrinkage and thermalstresses.DRDL has conducted systematic and plannedexperimentation to establish densification processparameters to get delamination-free 2-D composites.Development exper

thermo-structural properties, carbon-carbon composites are used in specialised application like re-entry nose-tips, leading edges, rocket nozzles, and aircraft brake discs apart from.several industrial and- biomedical applications. The multidirectional carbon-carbon product technology is versatile and offers design flexibility.

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