Published by Maney Publishing (c) IOM Communications LtdSynthesis and consolidation of boron carbide:areviewA.K.Suri,C.Subramanian*,J.K.SonberandT.S.R.Ch.MurthyBoron carbide is a strategic material, finding applications in nuclear industry, armour forpersonnel andvehiclesafety, rocket propellant, etc. Its highhardnessmakes it suitableforgrinding and cutting tools, ceramic bearing, wire drawing dies, etc. Boron carbide iscommerciallyproducedeither bycarbothermicreductionof boricacidinelectricfurnacesorbymagnesiothermyinpresenceofcarbon.Sincemanyspecialtyapplicationsofboroncarbiderequiredensebodies, itsdensificationisof great importance. Hot pressingandhot isostaticpressing are the main processes employed for densification. In the recent past, variousresearchers have made attempts to improve the existing methods and also invent new processesfor synthesis and consolidation of boron carbide. All the techniques on synthesis andconsolidationofboroncarbidearediscussedindetail andcriticallyreviewed.Keywords:Synthesis,Densification,Boroncarbide,Sintering,Hardmaterial,NeutronabsorberIntroductionBoron carbide is a suitable material for many highperformanceapplicationsduetoitsattractivecombina-tionof properties suchas highhardness (29?1GPa),1low density (2?52gmcm23),1high melting point(2450uC),2highelastic modulus (448GPa),3chemicalinertness,2,4highneutronabsorptioncross-section(600barns),4,5excellent thermoelectric1,4properties, etc. Ithasfoundapplicationintheformof powder, sinteredproduct as well as thin lms. Boron carbide (also knownas blackdiamond) is the thirdhardest material afterdiamond and cubic boron nitride. Its outstandinghardness makes it a suitable abrasive powder forlapping, polishingandwater jet cuttingof metals andceramicmaterials.4Tools with boron carbide coating are used for cuttingofvariousalloyssuchasbrass, stainlesssteel, titaniumalloys, aluminiumalloys, cast iron, etc.1In sinteredform, it is usedas blastingnozzles,6ceramic bearingsand wire drawing dies due to good wear resistance.1Thecombinationof lowspecicweight, highhardnessandimpact resistancemakesit asuitablematerial asbodyandvehiclearmour.Modulustodensityratioofboroncarbideis1?86107m, whichishigherthanthatofthemost of the hightemperature materials andhence itcouldbeeffectivelyusedas astrengtheningmedium.7Thin lms of boron carbide nd application asprotectivecoatinginelectronicindustries.8,9Boron carbide is extensively used as controlrod, shielding material and as neutron detector innuclear reactors due to its ability to absorb neutronwithoutforminglonglivedradionuclide.7,1017Neutronabsorptioncapacityof boroncarbidecanbeincreasedby enriching B10isotope. Composite material containingboron carbide with good thermal conductivity andthermalshockresistancearefoundsuitableasrstwallmaterial of nuclearfusionreactors.1821Boroncarbidebasedcompositesarepotentialinertmatrixforactinideburning.22Boroncarbideisalsousedfortreatment ofcancerbyneutroncapturetherapy.23As it is a p-type semiconductor, boron carbide isfound to be a potential candidate material for electronicdevicesthat canbeoperatedat hightemperatures.24,25Owing to its high Seebeck coefcient (300mVK21),boroncarbideisanexcellentthermoelectricmaterial.26Boroncarbide is ndingnewapplications as thermo-couple, diode and transistor devices as well. Boroncarbideisanimportant component fortheproductionof refractory and other metal borides.2729The lowdensity, high stiffness and low thermal expansioncharacteristics of B4Cmake it attractive Be/Be alloyreplacementcandidateforaerospaceapplications.30Thevenot has compileda comprehensive reviewonboroncarbide1in1990, inwhichsynthesis, consolida-tion, analytical characterisation, phase diagrams, crystalstructure, properties and applications are discussed. Thispaper criticallyexamines various methods of synthesisandconsolidationofboroncarbideanddiscussestheirmerits and demerits along with structure, properties andapplications.StructureofboroncarbideThe bond between B-B atoms and B-C atoms play a keyroleindecidingthecrystal structureandpropertiesofboron carbide. Knowledge of these will help us inunderstandingthe complexities involvedinprocessingandachievingthedesiredproperties.Boroncarbideisacompositionally disordered material that exists asMaterials Group, Bhabha Atomic Research Centre, Mumbai 400085, India*Correspondingauthor,email csubra@barc.gov.in 2010Instituteof Materials, MineralsandMiningandASMInternationalPublishedbyManeyfortheInstituteandASMInternational4 International MaterialsReviews 2010 VOL 55 NO 1 DOI 10.1179/095066009X12506721665211Published by Maney Publishing (c) IOM Communications Ltdrhombohedral phase inawide range of composition,which extends from B10?4C (8?8at.-%C) to B4C(20at.-%C).3134Among them, B4C is superior inproperties suchas hardness, thermal conductivityetc.SinceB4Cisin equilibriumwithfreecarbon andisonlyboundary between BnC and BnCzC (where 4,n,10),35synthesis of B4C without free carbon is a great challenge.Carboncontentofboroncarbidegreatlyinuencesthestructure and the properties of the compound and hencetheexact knowledgeof B/Cratioof thephaseisveryimportant. But the analytical study of B-Csystemisdifcultduetoextremehardnessandchemical stabilityof boron and boron carbide phases.33Different limits ofhomogeneityrange are reportedbyresearchers at thecarbon rich side of boron carbide, corresponding toB4?3C (18?8%C),36B4?0C (20%C),33,34and B3?6C(21?6%C).37Difculties associatedwiththe estimationoffreeandcombinedcarboncouldbeaccountablefortheseinconsistentresults.36B-Cphasediagramshowinghomogeneityrangefrom8?8to20at.-%C,asgeneratedbyBouchacourtetal.34ispresentedinFig.1.CrystalstructureThe most widely accepted crystal structure of boroncarbide is rhombohedral, consisting of 12-atom icosahe-dralocatedat the corners of the unit cell. Schematicdiagram of the structure of boron carbide is presented inFig.2.38The longest diagonal of the rhombohedral unitcellcontainsthreeatomlinearchain(C-B-C).Eachendmember of the chain is bonded covalently to an atom ofthree different icosahedra.31In general, icosahedraconsist of 11boronatoms andonecarbonatom. Thelocations of carbonatoms withindifferent icosahedraare not ordered relative to one another. The icosohedralcongurationistheresultofatendencytoformthree-centre covalent bonds due to deciency of valenceelectrons of boron.39Two crystallographically in-equivalent sites exist in the icosahedron. Six atomsresideintwopolartrianglesattheoppositeendsoftheicosahedron and the remaining six atoms occupyequatorial sites. The atoms at polar sites are directlylinked to neighbouring icosahedra via strong two-centrebonds along the cell edges. The atoms in equatorial siteseitherbonddirectlytoothericosahedrathroughthree-centre bonds or tochainstructures.40,41Most of theicosahedra have a B11C structure with the C atom placedin a polar site, and a few percent have a B12 structure oraB10C2structurewiththetwoCatomsplacedintwoantipodalpolarsites.41Threetypesofthree-atomchainareenvisioned:C-B-C, C-B-B and B-B-B. Variation in carbon concentrationchanges the distribution of three-atomchains.31B4C(20%C) structure consists of B11C icosahedra and C-B-Cchains. As the composition becomes rich in boron,carbonoftheB11Cicosahedraisretained,whileoneofthecarbonatoms ontheC-B-Cchains is replacedbyboron. Near the composition B13C2,the structureconsists of B11C icosahedra and C-B-B chains. Onfurthercarbonreduction, someoftheB11Cicosahedraare replaced by B12icosahedra retaining the C-B-Bchain.40,42Carbon-boron bonds present in the threeatomchainsaremuchstrongerthanboron-boronbondinicosahedra.40Theinter-icosahedrabonds arestifferthantheintra-icosahedrabonds.43Conictingviewsstill exist concerningthenatureofsite occupancies. Amodel based on early X-ray dif-fractiondata44,45proposedthattheB4CcompositionismadeupofB12icosahedraandC-C-Cchains.Howeverlater studies38,4042,46,47basedonimprovedX-rayandneutron diffraction, nuclear magnetic resonance studies,theoretical calculationsandvibrational spectraindicatethat the structure consist of B11C icosahedra and C-B-Cchains.EvenamongthosewhofavourB11Cicosahedraand CBC chain model for 20at.-%(B4C), there isdisagreement onthe structural changes that occur inboron carbides, as the carbon content is decreasedtowards 13at.-%(B13C2). Some workers46,48proposethatcarbonatomsareremovedfromtheicosahedratoformB12icosahedra, while others40,42,49propose thatcarbon atom is replacedfrom threeatom chains. Owingto similarity of boron and carbon in electron density andnuclear cross-section (B11and C12), both X-ray andneutron diffraction studies are not very successful in1 Phase homogeneity range in B-Cphase diagram: rep-rinted with permission fromElsevier, J. Less Comm.Met., 1979, 67, Fig.2inp.3292 Schematic diagram of structure of boron carbideRhombohedral unit cell, consisting of 12-atomicosahe-dra located at the corners and C-B-C linear chain atthediagonal of theunit cell isshown. Withintheicosa-hedron, six atoms reside in two polar triangles at theopposite ends of the icosahedron and the remainingsix atoms occupy equatorial sites: reprinted with per-missionfromthe AmericanPhysical Society, Phy. Rev.Lett., 1999, 83, (16), Fig.1inp.3230Suri etal. Synthesisandconsolidationofboroncarbide: areviewInternational MaterialsReviews 2010 VOL 55 NO 1 5Published by Maney Publishing (c) IOM Communications Ltdunambiguouslyassigning the exact site occupancies.48The concentration of B12 and B11C icosahedra and C-B-CandC-B-Bchains varyandchainless unit cells alsooccur.50,51Variation ofstructure elements B12, B11C,C-B-C and C-B-B in boron carbide unit cell with C contentis showninFig.3.51The carbonrichlimit of homo-geneity range which was assumed to contain B11Cicosahedra and C-B-C chains only, also contains 19% C-B-B chains. The composition of B6?5C which wasattributedtobethemost representativestructure(B12,C-B-C)andusedformanymodelcalculationshasbeenproved to be the least dened structure containing60%B11C and 40%B12icosahedra. These structuralchanges could also explain the abrupt decrease inthermal conductivity between B4C to B6?5C. Saalet al.46have recently appliedabinitiocalculations toevolve the structure of boron carbide for the entirecomposition range. The enthalpy of formation andlattice parameters werecalculatedandcomparedwiththe experimental data. For carbon rich composition(20%C), B11C-CBCstructureandfor13?33%Ccompo-sition B12-CBCstructure were found most stable. Itsuggests that carbon atomis gradually replaced byboroninicosahedra. This result is contradictorywithother researchers who suggest that carbon atomisreplacedfromchain. At boronrichend, enthalpyandlattice parameters of B12BVaC(Vadenotes vacancy)structure is ingoodagreement withthe experimentalvaluesforboroncarbidehaving7?14at.-%C. Sincetheenthalpy of formation of B12BVaCis positive it ispredicted that B12BVaCs composition cannot bereachedbyboroncarbideandinstead, pureboronwillprecipitate out, which is in agreement with experimentalphaseequilibrium.Radevetal.43havefoundthatmetalcationscanreplaceapartofboronatomsinicosahedrapositionandthusimprovesthestiffness, hardnessandwearresistanceofboroncarbide.RecentobservationsbyRamanspectroscopysuggeststructural phase transformationandthe formationoflocalisedamorphous phase whichis weaker thantheoriginalcrystallinephaseunderconditionsofloading.52First principle molecular dynamics simulations haverevealedthatthedepressurisationamorphisationresultsfrompressure induced irreversible bending of C-B-Cchains.53StructuresensitivepropertiesThermal and electrical conductivity, heat capacity,hardness, etc. strongly dependonstructure of boroncarbide and the variations are brought out in thefollowinglines. LatticeparameteraRof rhombohedralunit cell decreaseswithincreaseincarboncontent butthe plot is discontinuous at the composition B13C2(13?33at.-%C). Density of boron carbide increaseslinearly with carbon content within the homogeneityrange of the phase according to the relation d(gcm23)52?422z0?0048[C] at.-% (r50?998) with8?8at.-%([C]>20?0at.-%. The number of atoms perunit cell is exactly15for B4C, but increases withtheboroncontentandapproaches15?3fortheboronrichlimitB10C.35Hardnessofboroncarbideincreaseswithcarbonwithinthehomogeneityrangeas thestructurebecomes stiffer.1Shear modulus of boron carbideincreaseswithcarbonfrom185GPaforB6?5C(13%C)to 198GPa for B4?3C (20%C).54Fracture toughness andYoungs modulus also increases with the carboncontent.1Heat capacity of boron carbide increases withdecreaseincarbonwithinthehomogeneityrange. Thisincreaseisduetothechangeinlatticevibrationmodeproduced by reduction of the stiffness of the three-atomchainaccompaniedwithachangefromC-B-CtoC-B-B.55Thermal conductivity of B4C(20%C) falls withtemperatureinthemanner characteristicof crystallineceramics. However, thermal conductivity of boroncarbidewithlowcarbonisrelativelylessandtempera-ture independent, a behaviour more characteristic ofamorphous materials. These differences of thermaltransport can be explained if it is assumedthat, thethermal conductivity is dominatedby the transfer ofvibrational energythroughtheinter-icosahedral chainsratherthanwithinthesoftericosahedra. AstheC-B-Cchains are inhomogeneously replaced by C-B-B chains atransition takes place fromcrystal-like transport toglass-like transport. Moreover thermal conductivity fallsbecause B-B bonds are much softer than C-Bbonds.40,55,56Gilchrist etal.57havefoundthatthermalconductivityofB4Cfallsfrom29Wm21K21atroomtemperature to 12Wm21K21at 1000uC. Thermalconductivityincreaseswhen10Bisotopereplaces11Binboroncarbide. Thisisattributedtotheincreaseinthebondingenergyperunitmassandthephononvelocityas a lighter isotope is substituted for a heavier isotope.58Electrical conductivityinboroncarbidewasstudiedbyWoodetal.49andMatusietal.55Chargecarriersinboroncarbideareholeswhichformsmallpolaronsandmove by phonon assisted hoping between carbon atomslocated at geometrically non-equivalent sites.49The non-equivalence arises from two sources. First, carbon atomscanbe distributedamong non-equivalent sites withinB11C icosahedra. Second, only a fraction of the availablepositionsofinter-icosahedralchainisgenerallylledbyC-B-Cchains.Thecarbonrichlimit(B4C)resemblesanideal crystal and therefore has the lowest electricalconductivity.55Electrical conductivity increases withtemperature, which is the sign of behaviour of asemiconductor. Density of small polaron holes is3 Composition of structure elements (B12and B11C ico-sahedra, C-B-CandC-B-Bchains) inboroncarbideunitcell and chainless unit cells with variation of C con-tent: reprinted with permission from Elsevier, SolidStateCommun., 1992, 83, Fig.4inp.850Suri etal. Synthesisandconsolidationofboroncarbide: areview6 International MaterialsReviews 2010 VOL 55 NO 1Published by Maney Publishing (c) IOM Communications Ltdindependent of temperature but the mobilityof holesincreases with temperature. Within the homogeneityrange,chargecarrierdensityandelectricalconductivitydecreases with increase in carbon content. The tempera-turedependenceof electrical conductivityisessentiallyindependentofcarbonconcentration.49IrradiationresponseNeutron irradiation of boron carbide results in extensiveintergranularcrackingduetotheformationof heliumbubbleasperthefollowingequation59615B10z0n1?3Li7z2He4z2:6 MeV (1)Formationofthesecracks,whichpreventheatconduc-tionandtheatomicdisorderresultinginhighphonondispersion, decrease the thermal conductivity duringirradiation.59The anisotropic precipitation of heliumnot onlychanges the microstructure but degrades themechanicalandphysicalpropertiesaswell.Whengrainboundarycrackingoccurs, alargeamount of trappedhelium is released simultaneously with the occurrence ofbulk swelling.62,63Considerable amount of tritiumisproducedinB4Cbyfast neutronirradiation, whichisretainedupto700uCevenonannealingandisreleasedonlyat temperature higher than900uC.62,64Copelandet al.65,66have reported that irradiation of boron carbidewithneutroncauseslatticestrainsduetotheformationof lithiumandheliumas reactionproduct as well assomeatomicdisplacements. Inui et al.67havereportedthat a complete crystalline to amorphous transitiontakes place by electron irradiation withenergy .2MeVandat temperature,163K. Theyalsofoundthat theamorphousboroncarbideremainsinamorphousstateonannealingat 1273K. Theysuggestedapossibility,that, inboroncarbide, the individual B12icosahedrathemselvesare notdestroyedby electron irradiationbuttheirregularspatial arrangementintheB12C3latticeisperturbed and is gradually put in disorder withincreasingelectrondosage, resultinginanamorphousstate.67Froment et al.59have noticedthat boronrichB8CismoreresistanttoradiationdamagecomparedtoB4Candhencebecomes apossiblecandidate for newabsorbingmaterials.11B4Cis foundtobe verystableafter fast neutronirradiationinreactors. Dimensionalchangesandthermalconductivityof11B4Caresubstan-tiallysmallerthanthatof10B4C.68SynthesisofboroncarbideBoron carbide was discovered in nineteenth century as abyproduct of reaction involving metal borides. Thepurityof boroncarbideproducedbyearlyresearcherswaslessthan75%andin1933, Ridgway69claimedtohave produced crystalline B4C of 90% purity bycarbothermicprocess. Lipp4haspresentedareviewofboroncarbideproduction, properties andapplicationsin 1965. Spohn70has also mentioned the synthesis routesforboroncarbideproductionanditsusesinhisarticle.In this section, different routes for B4C synthesis will bediscussed. Themethodsof boroncarbidesynthesisareclassiedas:(i) carbothermicreduction(ii) magnesiothermicreduction(iii) synthesisfromelements(iv) vapourphasereactions(v) synthesisfrompolymerprecursors(vi) liquidphasereactions(vii) ionbeamsynthesis(viii) VLSgrowth.CarbothermicreductionofboricacidCarbon reduction of boric acid and boron trioxide is thecommercial method for the production of boroncarbide. The overall carbothermic reduction reactioncanbepresentedasfollows4H3BO3z7C?B4Cz6COz6H2O (2)Thisreactionproceedsinthefollowingthreesteps704H3BO3?2B2O3z6H2O (3)B2O3z3CO?2Bz3CO2(4)4BzC?B4C (5)Boric acid on heating converts to B2O3by releasingwater. The reductionof B2O3withcarbonmonoxidebecomes thermodynamically feasible above 1400uC. Thefurnacetemperatureisusuallymaintainedat .2000uCtoenhancetherateof overall reaction. Theprocessishighlyendothermic,needing16800kJmol21B4C.71Three types of electric heating furnaces, namelytubular, electricarcandAchesontype(graphiterodasresistance element) are used for the production of boroncarbide. Tubularelectricfurnacesusinggraphitetubesasheatingelementareinuseforcarryingoutreactionsfor scienticstudyonly. Thesefurnaces arelimitedinsize, dependent on the size of the availability of graphitetubes. Hence large scaleproductionis notfeasible usingtubularfurnaces.ArcfurnaceprocessElectricarcfurnaceprocessfor makingboroncarbidehasbeenpatentedbySchroll et al.72intheyear1939,wherein the mixture of boric acid and petroleum coke ismelted in an arc furnace followed by crushing theresultant product andmixingit withsubstantiallythesamequantity ofboricacid andremeltingthe mixtureasecond time. The design and operation of the electric arcfurnaceforthelargescaleproductionofboroncarbidehas been explained by Scott.73In the arc furnaceprocess, the temperatures are generallyveryhighdueto localised electric arcs, which are responsible for heavyloss of boron by evaporation of its oxides. Moreover theproduct obtainedis chunks of meltedboroncarbide,which needs subsequent laborious crushing and grindingoperations.AchesontypefurnaceAchesontypefurnaces,whereagraphiterodisusedasheating element, surrounding whichthe reactants arechargedis alsousedfor productionof boroncarbide.Early patents by Ridgway69,74give the details of thefurnace and the process. Operational details of theAchesonfurnaceareexplainedbelow. Partiallyreactedchargefromthepreviousrunisassembledaroundthenewgraphiteheatingrod. Abovethis, thenewchargemixture consisting of boric acid and carbon is added. Onheating,thereactioninitiatesnearthegraphiterodandcarbondioxideescapestotheatmospherethroughthechargeabove.Asthereactionproceeds,thechargegetsheated by conduction as well as by the heat of theSuri etal. Synthesisandconsolidationofboroncarbide: areviewInternational MaterialsReviews 2010 VOL 55 NO 1 7Published by Maney Publishing (c) IOM Communications Ltdescaping CO. Boric acid initially loses its water andconvertstoB2O3. Onfurtherheating, B2O3melts andforms a glassy lm preventing the escape of CO from thereductionzone. The product gases formbubbles andgrowinsizenearorjustabovethereactionsiteandasthe pressure increases, the bubbles burst pushing thechargeabove.Duringtheseburstssomeofthepartiallyreactedchargeisthrownoutofthefurnaceandboronalsoescapes tothe atmosphere inthe formof boronoxide vapours. These bubble bursts and evaporationlossesaffecttheefciencyoftheprocessconsiderably.Aftercompletionoftherun, thetopisbrokenopenandtheboroncarbidesurroundingthegraphiterodismanuallycollected. Operatorexperienceplaysamajorrole in identifying the completely reacted product so thatlessamountofoxidesentertheboroncarbideportion.The reacted product is crushed in jawcrushers andfurthergroundtonersize. Groundpowderiswashedinwaterandleachedinacidtoremovethecontamina-tionduetogrindingmediaandalsotheaccompanyingunreduced or partially reduced oxides of boron from thereduced product. In each run, only a small portion of thecharge gets converted to carbide and the balancematerial is recirculatedinfurther runs. Somequantityof boronoxidesescapestotheatmospherealongwithcarbonmonoxide. Henceinthisprocess, conversionineachrun is lowandboron loss is high. As the rawmaterialsusedarecheapandtheprocessissimple,thisprocess has beenadoptedfor commercial production.Though the method of raw material charging andcollectionof reactedproduct couldbedifferent inarcfurnace and Acheson processes, the reaction sequence isverysimilar.Astemperatureisanimportantprocessparameterincarbothermic reductionprocess andthe heat transferplays an important role in the formation of boroncarbideRaoet al.75,76havedevisedamethodof coretemperature measurement in boron carbide manufactur-ing process. They have analysed the heat transferprocess inside the reactor andthe effect of it ontheformation of boron carbide based on the recorded data.ProcesskineticsKineticsofthereactionandalsotheproductqualityisstronglyinuencedbyporosityof the charge, type ofcarbonusedforreduction,rateofheatingandthenalcoretemperature. Processkinetics, inuenceofprocessparameters and the means of improving the productqualityandconversionefciencyhavebeen investigatedbymanyresearchers. Petroleumcokeisfoundtobeabetter reducing agent than graphite, charcoal andactivatedcharcoal.77Boric acidtocoke ratioof 3?33?5 is found optimum and at higher ratios, though boroncarbide free of carboncouldbe obtained, recoveryispoor.Alizadehetal.78haveoptimisedtheboronoxide/carbon(petroleumcoke) ratiotoyieldboroncarbidewithlow(0?65%)carbon. Additionofsmall amountofsodiumchloride (1?5%) is found to be effective inincreasingtheyield.71,79StartofformationofB4Chasbeen noticed by Subramanian et al.80at 1200uCbythermogravimetric studies on the reduction of boric acidby petroleumcoke in vacuum. Figure480shows theweight change of the charge withtemperature up to1400uC. Formationof boroncarbidebycarbothermicreductionishighlydependent onthephasechangesofreactant boronoxide fromsolidtoliquidtogaseousboron sub oxides and the effect of reaction environment(heatingrateandultimatetemperature).81Slowheating(,100Ks21)ofthechargeresultsintheformationofboron carbide by a nucleation and growth mechanism asthe reaction proceeds through a liquid boron oxide path.Intermediateheatingrates (103to105Ks21) result inthe formation of both large and small crystallites,indicating the reaction of carbon with both liquid boronoxide and gaseous boron suboxides. Rapid heating rates(.105Ks21)resultinsmallercrystallitesize,indicatingthe occurrence of reaction through gaseous boronsuboxides.Dacic et al.82have studied the thermodynamics of gasphase carbothermic reduction of boron anhydride. B2O2andBOareformedbycarbothermicreductionofB2O3according to reactions(6) and (7) and then reduced to BorB4C.4 Thermogravimetricanalysisplot of carbothermicreductionof boricacid80Suri etal. Synthesisandconsolidationofboroncarbide: areview8 International MaterialsReviews 2010 VOL 55 NO 1Published by Maney Publishing (c) IOM Communications LtdB2O3zC?B2O2zCO (6)B2O3zC?2BOzCO (7)Theeffectofthefeedcompositionandtemperatureonthe product compositionincarbothermic reductionisshowninFig.5.82Adecreaseinthepartial pressureofCO facilitates synthesis of B4C by boosting thegenerationofB2O2andBO.Production of boron carbide by carbothermy has beenessentiallya batch process.Tumanov83has reportedthedevelopment of a continuous process for the productionof boron carbide, by direct inductive heating of a chargemade of boronoxide andcarbonblack. AnalternatereductionmethodpatentedbyRafaniello84explainstheprocess for producing submicrometre size boron carbidepowders. Thetypeofcarbonused, methodofprepara-tion of the charge mixture and the fast heating rates (7010000uCmin21) are responsible in obtaining nepowders. Weimer et al.85,86have designed a verticalapparatus comprising of cooled reactant transportmember, reaction chamber, heating element and coolingchamber for the continuous productionof submicro-metre B4Cpowder. Modellingof carbothermic reduc-tion process for the production of boron carbide has notbeenattemptedbyanybodysofar.Preparationofdensearticlesneedneboroncarbidepowders in micrometre size. The product of conven-tional process has toundergoseries of size reductionprocessestoobtainsuchpowders.Suchgrindingopera-tions contaminatetheproduct necessitatingadditionalpurication steps. Availability of nanosized powders willnotonlyavoidthegrindingoperationsbutalsoreducethetemperatureofdensicationsubstantially.NanocrystallineboroncarbidePreparation of nanosized particles of boron carbide is ofrecent interest. B4C particles in the nanosize range(260nm)canbepreparedbyreductionofB2O3vapourbycarbonblackat 1350uC.87Abovethis temperature,yieldislowduetolossofB2O3fromreactionmixture.Additionofcobaltascatalystisfoundtobehelpful inyield of nanorods.88Ma et al.89have prepared highpurity boron carbide nanowires frommixed powderprecursor containing boron, boronoxide andcarbonblack. The mixture is heated quickly to 1650uC and heldat that temperature for 2h under owing argon.Vapours of B2O3, B2O2 and CO react to form B4C solidnanowires with a mean diameter of y50nm and lengthsof several hundreds of micrometres (Fig.6).89Largescale boron carbide nanowires of size 80100nmdiameterand510mminlengthhavebeensynthesisedusingB/B2O3/Cpowderprecursorunderargonowat1100uC.90Xu et al.91have synthesised nanostructures ofboroncarbidebyheatingB2O3powderto1950uCinagraphite crucible covered with a boron nitride disc.Majorityof thecrystallites depositedonboronnitridedisc showabelt-like morphology withaverage widthand length of about 510 and 50100mm, while thethickness was inthe nanoscale range (20100nm). Anumber of perfect icosahedral quasicrystal particles(Fig.7a)91andmultiplytwinnedparticles normallyinrodshapewerealsopresent(Fig.7b).91Theseparticleshaveverylargesizes(y20mm).Thusitwasfoundthatwhen the reaction takes place in gas phase or theproduct could be nanocrystalline B4C. Presence of somecatalystalsopromotestheformationofnanopowders.Although carbothermic reduction results in loweryieldduetolossofboronintheformofitsoxides,thisrouteisadoptedascommercialmethodmainlybecauseof the simple equipments and cheap raw materials whichmakethisroutethemosteconomical. Thisrouteisnotonlyusefulforcommercialpowderproductionbutalsofor the productionof nanocrystalline B4C. Details ofexperimental studiesoncarbothermicreduction, givingchargecomposition, processingconditionsandproductquality of boron carbide obtained by various researchersaresummarisedinTable1.69,72,7779,8385,92MagnesiothermicreductionofB2O3An alternate method for the production of boroncarbide is by magnesiothermic reduction of boronanhydrideinpresenceofcarbonasgivenbelow2B2O3z6MgzC?B4Cz6MgO (8)Thisreactiontakesplaceintwosteps:step12B2O3z6Mg?4Bz6MgO (9)step24BzC?B4C (10)The reaction is exothermic (DH51812kJmol21) innature. As the vapour pressure of magnesiumis highat the reactiontemperature of .1000uC, a cover gassuchasargonorhydrogenisusedandalsothesystempressuremaintainedhigh. Theproductsofthereactionare processedbyaqueous methods toremove magne-siumoxide fromboron carbide. The carbide is stillcontaminatedwithmagnesiumboridesformedasstablecompounds. This reductiontechnique yields veryneamorphouspowder, whichiswell suitedforuseinthefabrication of sintered products. One method ofcontrollingthetemperatureandtheparticlesizeoftheproduct is bychoosingtheright sizeof thereactants.Post reductive sintering at temperatures 200300uC5 Effect of feed composition and temperature on calcu-lated product composition in carbothermic reduction of1 mol B2O3(l) asper Dacic et al.:82reprintedwithper-mission from Elsevier, J. Alloys Compd, 2006, 413,Fig.2inp.200Suri etal. Synthesisandconsolidationofboroncarbide: areviewInternational MaterialsReviews 2010 VOL 55 NO 1 9Published by Maney Publishing (c) IOM Communications Ltdhigher than the reaction temperature increases theparticlesizeoftheproduct. Seedingofthechargewitha small quantity (12%) of boron carbide has been foundtoincrease the growthof B4Cparticles andthe yieldsignicantly.93AnearlypatentbyGray94explainstheprocessfortheproduction of boron carbide powders by magnesiothermicreduction of B2O3 or alkali Na2B4O7 in presence of carbonat 16501700uC. Addition of metallic sulphates as catalysthas beenfoundtoreduce the reactiontemperature to700uC.95The heat of magnesiothermic reaction is sufcientenough for self high temperature synthesis route.Formationof ultraneB4Cpowderfromthestoichio-metricmixtureofH3BO3,MgandCbyself-propagatinghightemperature synthesis (SHS) has beenstudiedbyZhang et al.96and Khanra et al.95,97The ignitiontemperatureof this mixturewas foundtobe670uCbythermal analysis method. Mechanical alloyinghas alsobeenutilisedas ameans of synthesisingsubmicrometreB4C particles by magnesiothermic reduction.98Wang et al.99have studied the synthesis of B4C breMgOcomposites bycombustionof B2O3zMgzCbresamples in an argon lled chamber. The degree ofconversionwas inuencedbypressure of the ambientargongas whichinuences theevaporationof magne-sium and thereby the combustion temperature andconversion. Calciumcanalsobe usedas reductant inplaceofMg. Berchmanetal.100haverecentlyreportedsynthesis of boron carbide powder by calciothermicreductionof borax(Na2B4O7) or B2O3inpresenceofcarbonat1000uCinargon.Thoughboroncarbidehasbeenproducedbymagne-siothermicreductionandusedforapplicationsdenedbyitshighcaloricvalue, thehighcost of magnesiumwill soon make this process obsolete for regularproduction. Table29399presentsasummaryofstudieson synthesis of boron carbide by magnesiothermicreduction.SynthesisfromelementsSynthesis of boron carbide from its elements isconsidered uneconomical due to the high cost ofa,blongstraightsegments; c,dcurlytufts6 Image(SEM) of highpuritysinglecrystallineboroncarbidenanowiresformedbythermal evaporationof B/B2O3/Cpow-der precursor at 1650uCunder argon atmosphere:89reprinted with permission fromElsevier, Chem. Phy. Lett., 2002,364, Fig.1inp.3157 a perfect icosahedral B4C particle and b rod shapedtwinned particles by carbothermic reduction of B2O3(scale bars: 10mm):91reprinted with permission fromAmerican Chemical Society, J. Phys. Chem. B, 2004,108B, Fig.4inp.7653Suri etal. Synthesisandconsolidationofboroncarbide: areview10 International MaterialsReviews 2010 VOL 55 NO 1Published by Maney Publishing (c) IOM Communications Ltdelemental boron and hence employed for specialisedapplications101,102only, suchas B10enrichedor verypure boroncarbide. For synthesis of enrichedboroncarbide, carbothermicreductionis not suitableduetolossof boronaswell as boronhold-upinthefurnaceandhencethisprocess istheonlysuitableeconomicalmethod.Althoughformationofboroncarbidefromitselements is thermodynamically possible at roomtem-perature, the heat of reaction (239kJmol21) is notsufcient to carry out in a self-sustaining fashion.103Formationof boroncarbidelayer slows downfurtherreaction, due to slow diffusion of reacting speciesthroughthislayer, thusnecessitatinghightemperatureand longer duration for complete conversion of theelements into the compound. For synthesis fromelements, boronandcarbonare thoroughlymixedtoformuniformpowdermixture, whichisthenpelletisedand reacted at high temperatures of .1500uC in vacuumor inert atmosphere. The partially sintered pellet ofboroncarbideis thencrushedandgroundtoget neB4Cpowder. ToachieveahighpurityproductofB4C,highpurityelementalboronpowderproducedbyfusedsaltelectrolyticprocess104,105isoftenused.Mechanical alloying of BCmixtures followed byheattreatmentisoneofthemethodsbeinginvestigatedforthesynthesisof boroncarbide. Roomtemperaturemillingis carriedout inplanetarymills for prolongedduration to activate the powders and the alloyed mixtureisthenannealedtoobtainboroncarbide.Sparkplasmasynthesisisanewtechnique,inwhichapulsedhighdccurrentispassedthroughthechargemixturecontainedin a cavity along with the application of uniaxialpressure. Inthis process, the start andcompletionofformationhas beennotedat 1000and1200uCrespec-tively.Combinationofmechanicalalloyingfollowedbyspark plasma sintering has been studied by Hian et al.106toobtain95%pureboroncarbide.Shockwave technique has alsobeenattemptedforboron carbide synthesis fromamorphous boron andgraphite powder107using trimethyl enetrinitramine asdetonator. The resultant product exhibited severaldifferent morphologies, such as laments, distortedellipsoid, plates andpolyhedronparticles of nanosize.In this technique reactants are kept inside a steelcontainer whichisplacedinplastictube. Adetonatoris placed between container and the plastic tube.Table1 Charge composition, processing conditions and product quality on synthesis of boron carbide by carbothermicreduction*Serialno. Reactants Processtype Processparameters Productquality Ref.(year)1 B2O3zPC Batch(resistancefurnace)2400uC CrystallineB4C90%pure 69(1933)2 H3BO3zcharcoal Batch(arcfurnace) MeltingtemperatureofchargeBoroncarbidewith15%C 72(1939)3 H3BO3zPC Batch 1470uC;HR:100uCmin21,5h,ArCrystallineB4C2530mm 79(2004)4 B2O3zPC/carbonactive Batch 1470uC;HR:100umin21,15h,ArCrystallineB4C2530mm 78(2006)5 B2O3andcarbon Batch 1800uC,20300min,Ar CrystallineB4Cwithoutfreecarbon92(2004)6 H3BO3zPC Batch(Acheson) .2000uC PartiallysinteredanddenseproductB4Cconversion:6973%77(1986)7 B2O3zcarbonblack/graphite/activatedcharcoalContinuous(inductionheating)2227uC CrystallineB4C 83(1979)8 H3BO3zVulcanXC-72carbonblackContinuous 1820uC;HR:900uCs21,3min,ArEquiaxedcrystalsof0.5mm 84(1989)H3BO3zacetylenecarbonblack2000uC;HR:10002000uCs21,3min,ArSubmicrometreparticles0.10.2mmH3BO3zactivatedcarbon 1580uC;HR:755uCs21,3min,ArSubmicrometreanduniformsizedcrystals9 H3BO3zcornstarch Continuous 1950uC,Ar Submicrometreparticles0.1mm85(1992)Boricoxideandcarbon 1850uC,Ar 0.020.1mm*PC:petroleumcoke,HR:heatingrate.Table2 Charge composition, processing conditions and product quality on synthesis of boron carbide bymagnesiothermicreduction*Serialno. Reactants Processtype Processparameters QualityofB4C Ref.(year)1 B2O3zMgzC Tubularfurnace 9501200uC,H2Finepowder 93(2002)2 B2O3zMgzC SHS Finepowder98%pure 96(2003)3 B2O3zMgzC Batch 700uC,Ar,1hcatalyst:K2SO4Boron:74.6%Carbon:25.2% 95(1967)4 B2O3zMgzC Mechanical alloying Rotationspeed:200revmin21Submicrometreparticles 88(2006)Ball toloadratio:5 : 172h5 B2O3zMgzCFibreCombustionsynthesis Ar B4CfibrezMgOcomposites 99(1994)6 H3BO3zMgzC SHS 670uC,Ar 824mmsize,8%freecarbon 97(2005)7 Na2B4O7zMgzC Continuous 16501700uC,H2Powderboron:77.5%Carbon:21.3%94(1958)*SHS:self-propagatinghightemperaturesynthesis.Suri etal. Synthesisandconsolidationofboroncarbide: areviewInternational MaterialsReviews 2010 VOL 55 NO 1 11Published by Maney Publishing (c) IOM Communications LtdInitiationofexplosivedetonationwascarriedoutbyanelectricdetonator. Aftertheshocktreatments, sampleswere recovered by shaving off the container with a lathe.In this technique very high heating and cooling rates areachieved along with high pressure. The chemicalreactionis completedinmicrotomilliseconds. Hencethisissuitableforthepreparationofcrystalsofvariousmorphology and non-equilibrium phases which are hardtobeproducedinthermalequilibriumconditions.Afewattemptshavebeennoticedonthepreparationofnanostructureboroncarbidefromitselements. Weiet al.108have prepared boron carbide nanorods byreacting carbon nanotubes (CNT) with boron powder at1150uCunder argonatmosphere. Chenet al.109havesynthesised boron carbide nanoparticles by reactingmultiwallCNTwithmagnesiumdiborideat1150uCfor3h in vacuum. At this temperature, magnesium diboridedecomposes and gives elemental boron. Recently Changetal.110hasattempted the preparationof boron carbidenanoparticles (200nm) by direct reaction betweenamorphous boronandamorphous carbonat 1550uC.The crystals obtained had a high density twin structureswithvariationofB/Cratiofromparticletoparticle.Table3106108,110113givesacomparativesummaryofstudies reported on the synthesis of B4C from itselements. SynthesisofboroncarbidefromitselementsissuitablefortheproductionofpureB4C. Thoughthecost of production is high due to the high cost ofelemental boron, forspecialisedapplicationssuchasinnuclearindustrythismethodispreferred.VapourphasereactionSynthesis of boron carbide by carrying out reactionbetweenboronandcarboncontaininggaseous specieshas beenextensivelystudied. This methodis gainfullyadopted for the formation of boron carbide coatings andsynthesis of powders and whiskers in submicrometresizes. Boronhalides suchas BCl3, BBr3andBI3aresuitable boron source but BCl3 is the most preferred dueto its ready availability and low cost. Apart fromhalides,borane(B6H6)andoxide(B2O3)arealsousefulboronsources. HydrocarbongasessuchasCH4, C2H4,C2H6, C2H2and carbon tetra chloride (CCl4) areemployedascarbonsource.Synthesisofboroncarbidetakesplaceinthereactionchamber, whichiskeptatadesired temperature, pressure and atmosphere.Generallyhydrogenispresentintheatmosphere,whichreacts withthe halogenforminghydrogenchloride asperthefollowingreactions4BCl3zCCl4z8H2?B4Cz16HCl (11)4BCl3zCz6H2?B4Cz12HCl (12)4BCl3zCH4z4H2?B4Cz12HCl (13)The owof reactants and other process parametersdecide the composition and structure of the productformed.One such set-up for vapour phase reaction isdescribedbyBourdeauinhispatent.114Theprocessofproducingboroncarbidebyreactingahalideofboroninvapour phasewithhydrocarboninthetemperaturerange15002500uChasbeenexplained.Cliftonetal.115described a process for producing boron carbidewhiskers inthe size range of 0?05to0?25mmbythereaction of B2O3vapours with the hydrocarbon gasbetween 700 and 1600uC. James et al.116have patented aprocess for the productionof boroncarbide whiskersandtheuseof catalyticelements toenhancetheyieldof thegasphasereactionprocess. Dieteret al.117havedescribeda processfortheproductionofboroncarbidepowder of ne size withasurface area>100m2g21.MacKinnon et al.118have reported that when borontrichlorideis reactedwithCH4H2mixtureinaradiofrequency argon plasma, boron carbides of variable B/Cratios are obtained as submicrometre powders, theproduct stoichiometry depending on the reactantcomposition.Chemicalvapourdeposition(CVD)Deposition of different types of boron carbide lms(B13C2, B4C, metastable phases, highlystrainedstruc-tures, etc.) by CVDtechniques has beenreportedinliterature. Theactual depositionis controlledbymasstransferandsurfacekinetics,whichaffectsthestoichio-metry and properties of the boron carbide phases grown.Graphite, singlecrystal silicon, carbonbreandboronarethesubstratematerialsusedforthinlmsynthesis.Generallytheprocess is carriedout invacuumintheTable3 Chargecomposition, processingconditionsandproduct qualityonsynthesisof boroncarbidefromelements*Serialno. Reactants Processtype Processparameters Productquality Ref.(year)1 Amor.boronzAmor.carbonSolidstatethermalreaction1550uC,4h,Ar Nanoparticles15350nm110(2007)2 BzC Hotpressing 18002200uC,34h Articlesofneartheoreticaldensity111(1975)3 BzC MAzannealing MAfor90hAnnealingat1200uCB4Cwithsomeunknownpeaks112(2006)4 BzC MAzsparkplasmasintering1650uC,16min 95%densepelletofhighpurityboroncarbide106(2004)5 Amor.boronzcarbonblackSparkplasmasynthesis .1200uC,10min SinteredB4C,disorderedfinecrystalline113(2005)6 Amor.boronzgraphite Shockwavetechnique Detonator:trimethylenetrinitramineDetonationvelocity:6.4kms21NanosizedparticlesofcrystallineB4C107(1996)7 Amor.boronzCNT Solidstatereaction 1150uC,Ar Straightnanorods 108(2002)*Amor.:amorphous;MA:mechanical alloying;CNT:carbonnanotube.Suri etal. Synthesisandconsolidationofboroncarbide: areview12 International MaterialsReviews 2010 VOL 55 NO 1Published by Maney Publishing (c) IOM Communications Ltdtemperature range of 450 to 1450uC. Substrate tempera-ture has stronginuence onthe process andproductquality. High substrate temperature results in pooradhesion whereas deposition rate is low at lowtemperature. Amorphousboroncarbidecoatingcanbeobtained at a lowtemperature of y500uC whereascrystalline lm is obtained at higher temperatures.1100uC. Amorphous boroncarbide coatings onSiChave beenobtainedby CVDfromCH4BCl3H2Armixturesatlowtemperature(9001050uC)andreducedpressure(10kPa).119Preparation of boron carbide bres by the reaction ofboron halides with woven cloth of carbonisable materialinhydrogenatmospherehasbeenpatentedbyWaineret al.120,121Jaziehpour et al.122have prepared boroncarbide nanorods ongraphite substrate at 1400uCbyCVDusing charge mixture of boronoxide, activatedcarbonandsodiumchloride. Shu-Fang et al.123havegrown novel boron carbide nanoropes by CVD using o-carborane (C2H12B10) as precursor and ferrocene(C10H10Fe)ascatalyst. Karamanetal.124havestudiedthekineticsofCVDofB4ContungstensubstrateusingBCl3CH4H2gas mixture. They proposed that twomajorreactionstakeplaceduringtheprocessBCl3g z14CH4g zH2g ?14B4C s z3HCl g (14)BCl3g zH2g ?BHCl2g zHCl g (15)Reactionrateofboroncarbideformationislowerthanthatofdichloroboraneformationovertheentirerangeof temperatures (1000 to 1400uC) studied. Schouleretal.125obtainedBCx(x>3)phasehavingwhisker-likemorphologybyreactingBCl3andB6H6at 1000uConquartz substrate in presence of hydrogenandnickel.Sezer and Brand126have written a comprehensive reviewon CVD of boron carbide. The mechanical, thermal andelectrical properties of CVD boron carbides are compar-able to other important refractory materials and promisea wide range of applicationareas, particularlyinthenuclearindustry. Theyhavealsodiscussedthethermo-dynamicmodellingusedbymanyresearchersandhaveconcluded that the process takes place far fromequilibriumandthat, thermodynamicmodellingisnotsufcient to represent experimental deposition condi-tions. Table4114118,127147presents a summary ofstudiesreportedonvapourphasereactionsynthesisofboroncarbide.Many modications such as laser CVD(LCVD),plasma enhancedCVD(PECVD), hot lament CVD(HFCVD), etc. havebeentestedfor the formationofboroncarbidelms.LaserCVDInthistechniquetheenergyofalaserbeamisusedtoheat the surface of a substrate to the temperaturerequired for chemical deposition. It allows superbspatial resolution(y5mm) becausethechemical reac-tions are restrictedtothe heatedzone createdbythefocusedlaser spot, incontrast tothetraditional CVDfurnace which heats the entire surface of the sub-strate.148Laser CVD results in deposits with high purity,high degree of crystallinity, low porosity, excellentmechanical properties and thermal stability. Theseattributes are the result of deposition occurring oneatomat atime. DepositionratesinLCVDtechniquesareordersofmagnitudehigherthanthatintraditionalCVD. The depositionrate andsurface microstructurestrongly depend on laser power and hydrogen content inthegasphase.127Control oflaserpowerdensityallowsfor codeposition of r-(B4C) and disordered graphite,whichcanbe benecial for tailoringthe thermal andelectronicproperties of boroncarbide.128Thereactiveatmosphere composition is the most important para-meter in laser CVD. When the relative amount ofcarbontoboroninthegasphaseishigh, adisorderedgraphiticphaseis depositedalongwithboroncarbideandwhenthecarbonislow, tetragonal andmetastableboron rich phase, B25C is codeposited with boroncarbide.129Patterneddepositscanbeobtainedbydirectwriting process, in which a pattern of thin linesdeposited on the substrate by moving the substrateperpendicular to the axis of the laser beam. Fibredepositions are also possible by moving away thesubstrateparallel tothelaserbeamaxisatarateequaltothedepositionrateof thebre. Direct writingandbre growth methods can be combined to produce three-dimensionalstructures.148PlasmaenhancedCVDIn PECVDchemical reaction takes place after thecreation of plasma of reacting gases. The plasma isgenerally created by radio frequency (ac) or dc dischargebetween two electrodes, the space between which is lledwiththe reactinggases. The necessaryenergyfor thechemical reaction is not introduced by heating the wholereactionchamberbutjustbyheatedgasorplasma.Thedeposition takes place at lower temperature as comparedtotraditionalCVD.Sincetheformationofthereactiveand energetic species in the gas phase occurs by collisioninthegasphase, thesubstratecanbemaintainedat alowtemperature. Hence, lmformationcanoccur onsubstratesatalowertemperaturethanispossibleintheconventionalCVDprocess,whichisamajoradvantageofPECVD.149Plasma enhanced CVD has been used by manyresearchers for thefabricationof boroncarbide(B-C)diodes whichcouldaccurately detect single neutrons,giving very high efciencies. These diodes have beenusedtofabricatetherstreal time, solidstateneutrondetectorswhicharemoreefcientandreliablethananyother neutron detecting semiconductor reported todate.150Leeet al.25,151havefabricatedphotoactivep-nhetrojunctiondiodebyPECVDof boroncarbidethinlmsfromnido-pentaborane(B5H9)andmethane(CH4)on Si (111). AB5C/Si(111) hetrojunction diode by asynchrotronradiationinduceddecompositionofortho-carborane fabricated by Byun et al.152has been found tobe comparable withPECVDdiodes. Hwang et al.153havesuccessfullyfabricatedandtestedaboroncarbide/boron diode on aluminiumsubstrates and a boroncarbide/boron junction eld affect transistor.Robertson et al.154have fabricated real time solidstate neutron detector by PECVD using closo-1,2-dicarbadodecaborane. Adenwalla et al.155have reportedthefabricationandcharacterisationof boroncarbide/siliconcarbide hetrojunction diodes by PECVD. TheliteratureisabundantonvariouspossiblecombinationsSuri etal. Synthesisandconsolidationofboroncarbide: areviewInternational MaterialsReviews 2010 VOL 55 NO 1 13Published by Maney Publishing (c) IOM Communications Ltdof source, methodof fabrication, uses, etc. andonlyafewexamplesaregivenabove.HotfilamentCVDHotlamentCVDisanattractivetechniqueduetoitssimple design and its amenability to fundamentalchemicalkineticmodellingin understandingthe processchemistry. Hot lament CVDsystems are based onthermal catalytic cracking of the precursors on thesurfaceof ahightemperaturelament usuallyrangingfrom 1000 to 2500uC. The substrate materials are usuallyheated by radiation fromthe hot lament and theTable4 Chargecomposition, processingconditionsandproduct qualityonvapourphasesynthesisof boroncarbide*Serialno. Process Reactants Processparameters Productquality Ref.(year)1 VapourphasereactionBCl3zCH41900uC;vacuum:5mmofHgBoroncarbidecrystals 114(1967)2 VapourphasereactionB2H6zC2H2ExothermicreactionandneedstobeignitedonlybysparkplugAmor.porousboroncarbidepowderofsubmicrometresize117(1977)3 VapourphasereactionB2O3zCH41075uC,18h Whiskerslength:0.54inch;diameter:0.050.25mm115(1970)4 VapourphasereactionBCl3zCH4zH21650uC,5h;vacuumcatalyst:VCl4Whiskerslength:50mm;diameter:10mm116(1969)5 RFplasmaassistedsynthesisBCl3zCH4zH2Arplasma Submicrometresizepowder 118(1975)6 CVD BCl3zCH4zH21350uC;Sub.:carbonfibreCrystallineB4Ccoating 130(1996)7 CVD BCl3zCH4zH211271227uC;Sub.:boroncoatedMo,vacuumMetastablephases,highlystrainedmicrostructure131,132(1989)8 CVD BCl3zCCl4zH21550uC,45h,Sub.:graphitePurelongcrystallineB4C;hardness:412.7GPa133(1965)9 CVD BCl3zC3H8zH28501000uC,36h;Sub.:graphitecloth;vacuum:1525torrAmor.coating 134(1981)10 CVD BCl3zCH4zH21300uC,6h;Sub.:graphite;vacuum:10mmofHgB4Ccoating 135(1968)11 CVD BCl3zCH4zH21300uC,3h;Sub.:tungsten,graphiteB4Ccoating(B:74to76%)specificgravityof2.32gmcm23136(1974)12 CVD BCl3zCH4zH28001050uC,vacuum Amor.boroncarbide 137(2006)13 CVD BCl3zCH4zH210001400uC;Sub.:tungstenCrystallineB4C 124(2006)14 LaserCVD BCl3zCH4zH2Laser:CO2;Sub.:fusedsilica;Arpressure:atmosphericCrystallineB4C 128(1999)15 LaserCVD BCl3zCH4zH2Laser:CO2;Sub.:fusedsilicaCrystallineB4C 138(1996)16 LaserCVD BCl3zCH4/C2H4zH2Laser:CO2UltrafineandcrystallineB4C 139(1990)17 LaserCVD BCl3zC2H4Laser:CO2;Sub.:fusedsilica,ArAdherent,crystallineB4C,1522%C127(2002)18 LaserCVD BCl3zCH4zH2Laser:CO2;Sub.:fusedsilica,CrystallineB4CandB25C 129(1997)19 PulsedlaserinducedCVDC6H6zBCl3Nd:YAGlaser 14to33nmB4Ccrystalsencapsulatedingraphite140(1999)20 PlasmaenhancedCVDC2B10H12(orthocarborane)11001200uC;Sub.:Si (100)B4Cnanowiresdiameter:18150nm;length:13mm141(1999)21 MicrowaveplasmaassistedCVDBBr3zCH4zH2500600uC;Sub.:graphiteAmor.boroncarbidelargecompositionrange(0to40at.-%C142(1990)22 SupersonicplasmajetCVDBCl3zCH4500600uC;Sub.:Si(100),ArzH2Microcrystallinefilmhardness:2232GPa143(1998)23 Thermal CVD BCl3zCH4zH21600uC,36hBCl3:615mLmin21;CH4:25mLmin21;H2:500mLmin21VariouscompositionbetweenB4CandB13C2144(1992)24 Hotwall CVD BCl3zCH4zH21000to1400uC;Sub.:graphite,vacuumCrystallineB13C2,longcolumnargrains145(1998)25 HotfilamentactivatedCVDBCl3zCH4zH22100uC(filament)450uC(substrate);Sub.:Si (100),vacuumAmor.boroncarbide,highpurityandgoodadhesion146(1994)26 ElectronbeamevaporationBzC Roomtemperature;Sub.:Si (100)Thinfilmsofcrystallineboroncarbide147(2008)*CVD:chemical vapourdeposition;Sub.:substrate;Amor.:amorphous;RF:radiofrequency.Suri etal. Synthesisandconsolidationofboroncarbide: areview14 International MaterialsReviews 2010 VOL 55 NO 1Published by Maney Publishing (c) IOM Communications Ltdsubstrate surface temperature is usually ,500uC.146Thedepositioniscarriedoutunderhighvacuumconditionstoavoidoxygencontaminationof the boroncarbidephase. Deshpande et al.146have obtained adhesivecoatingof boroncarbideonsiliconsubstrate andthewear resistanceof thecoatedsurfacewas foundtobeextremelyhighwhentestedusingaWC/Coball asthepin.Vapourphasesynthesismethodsaresuitableforthinlmcoatingof boroncarbideandpreparationof nepowder, bres, whiskers, etc. However the powdersproduced by this process are generally non-stoichiometricandnotsuitableforfabricationofdenseproducts. Thesemethodsarebestsuitedforlaboratorystudies.SynthesisfrompolymerprecursorsAs analternativetohightemperaturereactiontechni-ques, thereisgreatinterestindevelopmentofpolymerprecursorstoproduceceramicmaterialsat lowertem-peratures. Some of the boron loaded organic com-pounds such as carborane (C2BnHnz2), triphenylborane,polyvinylpentaboraneandborazinesonpyrolysisyieldB4C. Generallythis process is carriedout inthetem-peraturerange10001500uCinvacuumorinert atmo-sphere. A US patent156describes a process for making afreeowingboroncarbidepowderfromboricacidandsugar. Themixturedissolvedinethyleneglycol isdriedinairat180uCandthenheatedinhydrogenat700uC.Thisreactionproductisgroundandredat1700uCfor7h to yield ne boron carbide powder. Mondal et al.157describealowtemperaturesyntheticrouteinwhichapolymeric precursor is synthesisedby the reactionofboricacidandpolyvinylalcohol, whichonpyrolysisat400/800uC gives crystalline boron carbide. Sinha et al.158have presented a process in which, a stable gel is formedfrom aqueous solution of boric acid and citric acid. Thisgel is further processedtoyieldaprecursor whichonheating under vacuum to 1450uC produces B4C.Economy et al.159have preparedboroncarbide breby heating amine treated B2O3bre in inert atmo-sphere at 20002350uC. Cihangir et al.160have devel-opedamethodbasedonsulphuricaciddehydrationofsugar to synthesise a precursor material which onheating to temperature between 1400 and 1600uCyields crystallised B4C and B4C/SiC composites.Table5156159,161167givesthecomparativesummaryofstudiesreportedonthesynthesisofB4Cusingpolymerprecursors.LiquidphasereactionSynthesis of ultra ne boron carbide powder using liquidprecursorshasbeenattemptedbyafew.Thismethodisalso known as solvothermal process or coreductionmethod. Unlike conventional methods, this can beoperatedat much lower temperatures toyield boroncarbideofdesiredproperties. Shi etal.168havestudiedtheformationof ultraneboroncarbidepowders bycoreduction of boron tri bromide and carbon tetra-chloride using sodiumas reducing agent as per thefollowingreaction4BBr3zCCl4z16Na?B4Cz4NaClz12NaBr (16)Thereactionwascarriedoutinanautoclaveat450uC.B4C crystals obtained were composed of uniformspherical (80nmdia) and rod-like (200nmdiameterand2?5mmlong)particles(Fig.8).168Guetal.169haveobserved the formation of nanocrystalline B4C bysolvothermal reduction of CCl4using lithium inpresenceof amorphousboronpowderinanautoclaveat600uC.4BzCCl4z4Li?B4Cz4LiCl (17)Hexagonal B4Ccrystalswithaparticlesizeofapproxi-mately1540nmdiameterswereobtained.IonbeamsynthesisBoron carbide thin lms can be grown by directdeposition of Bzand Czions. In this process,parameters such as ion energy, ion ux ratio of differentTable5 Charge composition, processing conditions and product quality on synthesis of boron carbide using polymerprecursorSerialno. PolymerprecursorsTemperature,uC AtmosphereHoldingtime,h Productquality Ref.(year)1 Polyvinyl borate 1300 Argon 5 Crystalline 161(2009)2 ReactionproductofH3BO3andcitricacid1500 Vacuum 2.5 Crystalline,micrometresized,freecarbon:2.38%162(2006)3 ReactionproductofH3BO3andpolyvinyl alcohol400800 Air 3 Crystalline(orthorhombic),boricacidasimpurity157(2005)4 ReactionproductofH3BO3andcitricacid1450 Vacuum 2 Crystalline,freecarbon11.1wt-%158(2002)5 SolutionproductofH3BO3andglucose1400 B/CcompositecontainingcrystallineB4C163(2002)6 CondensationproductofH3BO3and2-hydroxybenzyl alcohol (HBA)1500 Ar 4 Crystalline 164(1999)7 Polyvinyl pentaborane 1000 Ar 8 Amorphous,blackandshiny165(1988)1450 Ar 48 crystalline 165(1988)8 CondensationproductofH3BO3and1,2,3propanetriol1400 Ar 2 Crystalline 166(1985)9 Ethyl Decaborane(C2H5B10H13) 1215 Vacuum ,1 CrystallineB4Ccoating(0.005inch)ontungstenwire167(1969)10 SolutionproductofH3BO3,sugarandethyleneglycol1700 H2 CrystallineB4Cpowder 156(1975)11 AminetreatedB2O3fibre 20002350 Inertatmosphere Boroncarbidefibre 159(1974)Suri etal. Synthesisandconsolidationofboroncarbide: areviewInternational MaterialsReviews 2010 VOL 55 NO 1 15Published by Maney Publishing (c) IOM Communications Ltdionspeciesandthesubstratetemperature,whichcanbeindependentlycontrolled, could be advantageously usedforobtainingthepreferredcompositionandnatureofthe boroncarbide lm. Ronninget al.170have grownthinlmof boroncarbide (BxC) by direct ionbeamdepositiononsiliconusinganionenergyof 100eVatroom temperature. Todorovic et al.171have observed theformation of amorphous boron carbide (BxC) bybombardmentofBzandB3zionsonfullerene. Beamenergieswereintherangeof15keVforBzto45keVfor B3zand uences were between 261014and261016cm22.Vapourliquidsolid(VLS)growthBoroncarbidewhiskerscanbegrownbycarbothermalVLSgrowthmechanism. This mechanisminvolves thetransportof boronand carbon as gas phasespeciesto aliquidcatalyst metal (Fe, Ni or Co) inwhichwhiskerconstituents get dissolved. Whenthe catalyst becomessupersaturatedwithboronandcarbon, boroncarbidewhiskersprecipitateoutofthemetaldroplets.Carlssonetal.172havepreparedB4Cwhiskersandplateletsusingthis technique. B2O3and carbon black were used assourceofboronandcarbonrespectively. NaCl andCowereaddedtofacilitatethegrowthof whiskers. B2O3reacts withNaCl toform BCl, which along withcarbondissolveinliquidcobalt andthenprecipitateasboroncarbide whiskers. Rao et al.173have studied theformationofboroncarbidewhiskersusingK2CO3andNiCl2 as a low melting liquid and catalyst to enhance theformationof B4Cwhiskers andplatelets. Anet al.174have used gallium oxide and sodium chloride to prepareboroncarbidenanobeltshavingalengthofaround1to10mmandthicknessofaround80to150nm, whichisshowninFig.9.174Maet al.175have investigatedthegrowthof boroncarbidenanowiresbytheadditionofirontotheprecursormixturecontainingcarbon,boronand boron trioxide. This resulted in reduction ofdiameter of nanowires from50200nmto1030nm.Scanning electron micrograph of the nanowires is showninFig.10.175AcomparativestudyofvariousmethodsofboroncarbidesynthesisispresentedinTable6.Some of the attempts to produce boron carbidecannot fall into any of the classications discussedabove. Thakkar et al.176have synthesisedhighpurityultraneboroncarbidepowdersbyreactingB2O3withmethane ina nontransferredarc dc thermal plasmareactor. A recent article177explains the process ofmaking boron carbidecarbon eutectic containing39wt-%C by melting B2CNin graphite crucible at2600uC.Boron carbide powder is either utilised directly orconsolidated to dense bodies. Various methods ofdensication,themechanismsinvolvedandtheproductquality are discussed in the following pages. Densi-cation techniques can be broadly classied as pressure-less sintering and pressurised sintering. Atmospheric/reaction/microwave and thermal plasma sintering aretermed as pressureless sintering techniques. The nuancesof densication of powder compacts, complexity and thereasons for incomplete densication by pressurelesssintering are discussed in detail by Lange.178Pressurisedsinteringcanbeclassiedas solidandgascompaction methods. Solid compaction methods are hot8 Image (TEM) of B4Crod-like particles (200 nmdiameterand 2?5mmlong) prepared at 450uCby sodiumreduc-tion of BBr3and CCl4:168reprinted with permissionfrom Elsevier, Solid State Commun., 2003, 128,Fig.3(c) inp.79 Boroncarbide nanobelts preparedby VLSgrowthfromcharge of boron oxide, activated carbon, galliumoxideand sodiumchloride at 1400uC:174reprinted with per-mission from Trans. Tech. Publications, Key Eng.Mater., 2007, 336338, (III), Fig.1inp.216710 Boron carbide nanowires prepared by VLS growthwithhelpof ironaddition:175reprintedwithpermissionfromAmerican Chemical Society, Chem. Mater., 2002,14, Fig.5(b) inp.4405Suri etal. Synthesisandconsolidationofboroncarbide: areview16 International MaterialsReviews 2010 VOL 55 NO 1Published by Maney Publishing (c) IOM Communications Ltdpressing, spark plasma sintering and super high pressuresintering. Gas compaction methods are hot isostaticpressingandhighpressuregasreactionsintering.DensificationofboroncarbideInspiteofitshightemperaturestrength,applicationofB4Cisratherlimited, inreal life, duetodifcultiesindensication,lowfracturetoughnessandlowoxidationresistance beyond 1000uC. Consolidation of B4C iscomplicated due to its high melting point, lowself-diffusion coefcient and high vapour pressure. Very highsintering temperatures are required for densication dueto the presence of predominantly covalent bonds in B4C.Boroncarbideparticlesgenerallyhaveathincoatingofsurface oxide layer which hinders the densicationsprocess. At temperatures ,2000uC, surface diffusionandevaporationcondensationmechanismoccur,whichresults in mass transfer without densication. Densi-cationisachievedonlyat temperatures.2000uC, bygrainboundaryandvolumediffusionmechanisms. Athighertemperatureexaggeratedgraingrowthalsotakesplace resulting in poor mechanical properties. One moreobservation at temperatures .2150uC is volatilisation ofnon-stoichiometric boroncarbide, leavingminute car-bonbehindatthegrainboundaries.Dole et al.179have observed the microstructure of B4Ccompacts red above 2000uC to be highly porousinterconnected structure with clusters of grains con-nected by small neck like regions and separated by large,channelledporosity. Grabchuket al.180182havefoundthatshrinkagestartsat 1500uC, recrystallisationabove1800uC and rapid grain growth above 2200uC. Attemperaturesabove2250uC, thesinteredbodycontains,5%residual porosity. Leeet al.183,184haveobservedthe start of densicationat 1800uC, rapidincrease indensication18702010uCandaslowdownindensi-cation rate 20102140uC. The surge in densication18702010uCis attributed tothe presenceof oxide layerwhich helps in precipitation of B4C through liquid B2O3or due to evaporation and condensation of rapidlyevolvingoxidegases(BOandCO).Slowerdensicationattemperaturesabove2010uCisattributedtoevapora-tionandcondensationof B4C. Figure11183showsthechanges in weight, dimension and grain size whilesinteringofboroncarbide.Densicationofboroncarbidewithoutdeteriorationof mechanical properties can be achieved either by usinga suitable sintering aid and/or applying the externalpressure (e.g. hot pressing, hot isostatic pressing).Selectionoftheadditiveandthemethodofconsolida-tion are generally dictated by the end use of the productandthe properties that are required. The additive byitself or due to in situ reaction with boron carbide wouldformanon volatilesecondphaseaidingin densicationand property enhancement. Hence, selection of theadditiveshouldbedirectedtowardstheformationofasuitable structure providingthe correct properties foruse.Recentoradvancedtechniquessuchasmicrowave/sparkplasmasintering, explosivecompaction, etc. helpto obtain dense products without microstructuralcoarsening. These techniques are presently limited tolaboratory scale only. Detailed literature survey onpressurelesssinteringwithorwithout sinteringaids,hotpressing, hot isostatic pressing, spark plasma andmicrowavesinteringof boroncarbidearepresentedinthefollowingsessions.PressurelesssinteringPressurelesssinteringisasimpleandeconomicprocessto produce dense compacts. This operation is carried outin two steps. In the rst step green compacts withTable6 Comparisonof boroncarbidesynthesismethods*Method Boronsource Carbonsource Advantage DisadvantageCarbothermicreduction H3BO3orB2O3PC,graphite,activatedcarbonCheaprawmaterial,suitableforcommercialproductionHighboronlosses,obtainedinlumpform,needgrindingforpowderproductionMagnesiothermicreduction B2O3orNa2B4O7PC,graphite,activatedcarbonFinepowder,exothermicreaction,suitableforSHSprocessProductcontaminatedwithMg,MgB2Synthesisfromelements Boron PC,graphite,activatedcarbonNolossofboron,goodcontrol overpurityandcarboncontentofproductHighcostofelementalboronVapourphasesynthesis BCl3,BBr3,BI3,B6H6,B2O3CH4,C2H4,C2H6,C2H2,CCl4Suitableforthinfilms,finepowder,fibers,whiskersDifficulttoproduceB4Cpowdersuitablefordensification,notamenableforlargescaleproductionSynthesisfrompolymerprecursorsBoricacid,B2O3,polyvinyl pentaborane,polyvinyl borate,ethyldecaboranePolyvinyl alcohol,citricacid,hydroxylbenzylalcohol,sugar,ethyleneglycolLowtemperatureprocess Highfreecarboncontent,still inlaboratorystageLiquidphasereaction BBr3,boron CCl4Lowtemperatureprocess,suitablefornanoparticlesNeedofreactivemetalsuchasNaorLi,newmethodofsynthesisIonbeamsynthesis Boron Carbon SuitableforBxC Onlyforthinfilms,ofacademicinterestonlyVapourliquidsolidgrowth B2O3Carbonblack Suitableforwhisker Needofmoltenmetalcatalyst,ofacademicinterestonly*PC:petroleumcoke;SHS:self-propagatinghightemperaturesynthesis.Suri etal. Synthesisandconsolidationofboroncarbide: areviewInternational MaterialsReviews 2010 VOL 55 NO 1 17Published by Maney Publishing (c) IOM Communications Ltdsufcienthandling strength are prepared by uniaxial diecompaction.These green pellets arethen red atchosenhightemperaturesincontrolledatmosphere.Arecentlydevelopednewtechnique, combustiondrivencompactprocess, yieldsmuchhighergreendensityandstrengththan the normal die compaction.185In this process, highpressure generated by ignition of a combustion gasmixture which raises the pressure in the chamberdramaticallyinaveryshortperiodoftimeandpushesdownthetopramonthepowderatanextremelyhighspeedrealisingthecompaction.Sintering of B4C powder compacts is commonlyperformedinaninertgasmedium. Buttheapplicationof vacuumhelps inevaporationof the surface oxidelayer and also prevents further oxidation, there bypromoting the sintering mechanisms. Removal of theoxidelayerbyheatinginareducingatmospherebeforesintering alsohas a similar effect. Literature data onpressurelesssinteringofboroncarbideandtheproductevaluation are presented in Table7.179,183,186204Increase inparticle surface area(9to17m2g21) andsintering temperature (2100 to 2190uC) give higherdensities (56 to 71%TD).187Densities of .90%TDareachievedbysinteringatatemperatureof.2200uCwithparticlesclosetoor,1mmsize.Graincoarseningisthecommonfeatureincompactswithhighdensitiesobtained by pressureless sintering.191,193Microstructuresofsampleswith87and93%TD,obtainedbypressure-less sintering of 0?8mmmedian diameter powders at2300 and 2375uC are presented in Fig.12.193Grain sizesare in the range 40100mm indicating large graingrowth. Surface to surface mass transport which isactive at temperatures belowwhich densication canproceedis responsible for the coarsening process. Athigher temperatures, vapour phase diffusionof boroncarbide is the important transport mechanism forcoarsening.Rapidheatingishelpfulinachievinghigherdensitieswithnemicrostructure, asthecompactscanbe heatedtoa temperaturewhere densicationcan takeplace before the microstructure becomes highly coar-sened.179,183,188,205Appearanceoftwinsinthegrainsischaracteristic of boron carbide. These twins slowlyvanish during high temperature annealing. Vickershardness and exural strength of the pressurelesssintered boron carbide samples are in the range 1824GPaand 120200MPa respectively,which are lowerthan theoretical values. One can conclude that, withpureB4C, adensication.90%TDispossibleonlyatvery high sintering temperatures of y2300uC. Suchcompacts have a coarse grained microstructure of11 Sinteringof boroncarbide compact: change inweight, dimension, grainsizeandcoefcient of thermal expansionupto2300uC:183reprintedwithpermissionfromWiley-Blackwell, J. Am. Ceram. Soc., 2003, 86, (9), Fig.8inp.147212 Microstructure of pressureless sintered boron carbide(0?8mm) at a 2300uCandb2375uCshowinggrains inrange 40100mmindicatinglarge grain growth:193rep-rinted with permission from Elsevier, Ceram. Int.,2006, 32, Fig.2(b) and(g) inp.230231Suri etal. Synthesisandconsolidationofboroncarbide: areview18 International MaterialsReviews 2010 VOL 55 NO 1Published by Maney Publishing (c) IOM Communications LtdTable7Powderdetails,sinteringparametersandcharacteristicsofsinteredboroncarbidebypressurelesssintering*Serialno.Materialcomposition,wt-%StartingpowderdetailsProcessingconditionsSintereddensityrth,%Microstructure,mmVickershardness,GPaKIC,MPam1/2Flexuralstrength,MPaRef.(year)1B4CFC:24.9;D5052.0to10.52170to2230uC,15min,Ar94.095.612.025.52.913.19160180186(1988)2B4CStarckmakeB/C53.7to3.8;D5050.8;SS:15202190uC,1h,Ar(upto2000uCinvacuum)9495188(2004)3B4C2250uC65%Coarse179(1989)B4CD50512300uC7072%CoarseB4Cz6wt-%CSS:122300uC.95%Fine4B4CD50(12150uC,15min,Ar78B4C:6189(1981)B4Cz3wt-%C(phenolicresin)SS:2296B4C:43.23535B4CD50,52175uC,15min,ArB4C:105190(1987)B4Cz(polycarbosilanezphenolicresin510%)SS:10.595B4C:28B/C:4.32SiC:,36B4CD50(0.842250uC91.392.7B4C:2.583.11183(2003)B4Cz3wt-%C(phenolicresin)SS:18.82250uC98.498.6B4C:2.262.4B/C53.767B4C2200uC,1h78.6B4C:28174191(2003)B4C2250uC,1h82.5B4C:50B4Cz3wt-%CSS:2.532250uC,1h92B4C:13350B4Cz5wt-%C2250uC,1h93B4Cz7.5wt-%C2250uC,1h89B4Cz9wt-%C2250uC,1h868B4CD502100uC and apressure of 34?4MPaare necessarytoobtaindensitycloseto100%TD. Slowcoolingafterdensicationhasbeenfoundtoberesponsibleforreductioninthenaldensityduetotheformationofporeswhilecooling.Asboroncarbidereactswiththediematerial, innerliningofthegraphitedieisessential topreventthisreaction.BNlining has been found to be most suitable. Themicrostructureofhotpressedspecimensshownograingrowth (1?52?0mm) up to 1950uC, a steady even growthupto2050uC(thenalgrainsize5mm)andanunevensizedgrowthandthepresenceoflargenumberoftwinsat 21502200uC.239Fast heating rates and application ofhighpressure(40MPa)havebeenhelpful inobtainingfull densicationat alower temperatureof 1900uC.179Samples obtained under these conditions show amicrostructure, freeofgrainboundaryphases, withanaveragegrainsizeof 2mmandfacetedsubmicrometreporesaccountingfor ,1vol.-%porosity.Jianxin250hasprepared boron carbide nozzles by hot pressing at2150uC in an inert atmosphere with a pressure of36MPausingstartingpowders of ,3mmsize withadensity, hardness, fracture toughness and exuralstrength of 95?5%TD, 32?5GPa, 2?53?0MPam1/2and300400MParespectively. HehasalsostudiedtheerosionwearofthisbyabrasiveairjetsusingSiO2,SiCand Al2O3 powders. While studying the densication byhot pressing, Ostapenkoet al.239have foundthat thedensication of boron carbide is controlled by a processleadingtonon-linearcreep, whoserateisafunctionofthe square of stress. Experimenting on the activatedsinteringkineticsbytheadditionofiron,Kovalchenkoet al.238havenotedthat, dislocationclimbisthemainmechanismleadingtocreep; whoserateisaquadraticfunctionof stress. Properties of dense B4Ccompactsprepared by hot pressing generally have the bestproperties withthe following values:260hardness, 2935GPa; fracture toughness, 2?82?9MPam1/2; elasticmodulus, 450470GPa; thermal conductivity, 3042Wm21K21; coefcient of thermal expansion,18 Vacuumhot press withfront door openshowinggra-phiteheatersandinsulation19 Boron carbide pellets of various sizes compacted byhot pressingSuri etal. Synthesisandconsolidationofboroncarbide: areview26 International MaterialsReviews 2010 VOL 55 NO 1Published by Maney Publishing (c) IOM Communications LtdTable8Powderdetails,sinteringparametersandcharacteristicsofsinteredboroncarbidebyhotpressing*Serialno.Materialcomposition,wt-%StartingpowderdetailsHotpressingconditionsSintereddensityrth,%Microstructure,mmVickershardness,GPaIndentationtoughness,MPam1/2Flexuralstrength,MPaReference(year)1B4CFC:1.4%34.4MPa,2.7ks,heatingrate:330Kks21;coolingrate:1665Kks211775K65.16.0240(1983)D5052.91975K72.47.0SS:0.246.22175K80.212.12325K98.513.52375K98.415.22475K99.616.42B4CD5051.5synthesisfromtheelements2200uC,22MPa,10min98.5,alargenumberoftwins239(1979)3B4CD5051;SS:122100uC,40MPa,30min,Ar.9523179(1989)4B4CD50,32150uC,36MPa,60min95.54832.52.53.0300400250(2005)5bBzCD5052.0;SS:1.271950uC,30MPa87241(2000)AmorphousBzCD5050.2;SS:12.761800uC,30MPa99aBzCD5052.74;SS:0.891800uC,30MPa916B4CB4C:D50510;O:0.11.0wt-%2200uC,34.5MPa,1h,HR:0.5Ks21,CR:1.7Ks212.5gcc211050240242(1979)B4Cz5%BB:D505202.49gcc21190B4Cz15%B2.42gcc2110402007B4CzTiO2zCB4C:D5050.50;SS:21.52000uC,50MPa,1h,Ar100B4C:3.8,TiB23.1870243(2005)TiO2:D50,nanosizeB4C:D5050.44;SS:15.5100B4C:3.4,TiB22.8720B4C:D550.41;SS:22.5100B4C:3.9,TiB23.28158B4CzTiO2zC2100uC,35MPa,8min.950%TiB2273.1200257(1990)5%TiB2304.035010%TiB2325.050020%TiB2345.163030%TiB2304.653040%TiB2284.34009B4Cz23.4%TiO2z5.28%carbonblackB4C:D5050.63;SS:19.8;TiO299.9%pure,submicrometre2000uC,20MPa,Ar,1h,HR:1525uCmin2115vol.-%TiB2;rest:B4C6.1621244(2000)10B4CB4C:D5053to52150uC,35MPa,65min,Ar95.0B4C:610292.5245246(2002)B4Cz10%(W,Ti)C(W,Ti)C:D5051to21850uC,35MPa,50min,Ar98.512,TiB2,W2B5282.8400B4Cz30%(W,Ti)C1850uC,35MPa,40min,Ar99.20.51.5263.9550B4Cz50%(W,Ti)C1850uC,35MPa,30min,Ar99.5,1234.569011B4CB4C:D5053to5;TiC:D5051to2;Mo:D5051to32150uC,35MPa,65min,Ar95.058212.60540247(2009)B4Cz5.3%Mo1950uC,35MPa,50min,Ar96.512B4Cz5%TiCz5%Mo1950uC,35MPa,50min,Ar99.112223.40550B4Cz10%TiCz4.7%Mo1950uC,35MPa,50min,Ar99.212254.25695B4Cz15%TiCz4.5%Mo1950uC,35MPa,50min,Ar99.01224.53.75625B4Cz20%TiCz4%Mo1950uC,35MPa,50min,Ar98.51223.53.60550Suri etal. Synthesisandconsolidationofboroncarbide: areviewInternational MaterialsReviews 2010 VOL 55 NO 1 27Published by Maney Publishing (c) IOM Communications LtdSerialno.Materialcomposition,wt-%StartingpowderdetailsHotpressingconditionsSintereddensityrth,%Microstructure,mmVickershardness,GPaIndentationtoughness,MPam1/2Flexuralstrength,MPaReference(year)12B4C:29.557%;B:18.335.5%;Si:3.87.5%;WCzTiC:045.3%;Co:02.8%D50,166MPa,3min,heatingrate:40Kmin21;coolingrate:100Kmin211620uC3.5gcc21B4CTiB2W2B512550245(1986)1670uC3.6gcc21266201720uC3.59gcc21307101820uC3.5gcc21288301920uC3.45gcc21257102120uC3.2gcc212658013B4CB4C:D5050.43;SS:15.3;O2:2wt-%;Fe:140ppm,Al:50ppm2050uC,5MPa,1h,Ar79.3EutecticliquidofCrB2zB4C1.9180256(2003)B4Cz10%CrB2CrB2:D5053.5862.2350B4Cz15%CrB2942.5500B4Cz20%CrB2952.8550B4Cz22.5%CrB2963.2620B4Cz25%CrB2983.2684953.266014B4CB4C:D5050.431900uC,50MPa,1h,Ar99.02.5675248(2003)B4Cz5%CrB2CrB2:D5053.599.62.6551B4Cz10%CrB299.72.7580B4Cz15%CrB299.02.8630B4Cz20%CrB299.03.5630B4Cz25%CrB298.63.458015B4Cz50%SiCB/C54.1;B:D5051.5;O:,4%1900uC,30MPa,30min,Ar;HR:15150uCmin212.50gcc21B4C,SiC273(1993)SizBzCC,SS:80;Si:D100552.74gcc21B4C,SiC16B4Cz8to13%siloxane/phenolic2275uC,28MPa,1h,Ar9799.7B4C,SiC/C234(1996)17B4CzsodiumsilicatezmagnesiumnitratezFe3O4B4C:D5050.1to11750uC,24MPa,10minto4h456825249(1974)4598.418B4CB4C:D5053.5;BN:D50,nanosized1850uC,30MPa,1h,N299.5B4C,nano-h-BN215.4410254(2008)B4Cz10wt-%BN99.2156.0420B4Cz20wt-%BN99.0115.2410B4Cz30wt-%BN98.585.0360B4Cz40wt-%BN98.174.331019B4Cz60%Al2O3B4C:D5051.01700uC,35MPa,1h,Ar3.53284.20.753030258(2005)B4Cz70%Al2O3Al2O3:D5050.23.653.0274.20.456030B4Cz80%Al2O33.773.2234.30.560070B4Cz90%Al2O33.863.4213.50.55504520B4CzAl2O3(B4C/Al2O3518:1)2150uC,35MPa,65min,Ar95.5B4C:2522.41.23.20.530034271(2008)B4CzAl2O3z5%TiCB4C:D5053to5,.95%1950uC,35MPa,60min,Ar98.7B4C,TiB224.11.14.10.543031B4CzAl2O3z10%TiCAl2O3:D5051to2,.99%1950uC,35MPa,60min,Ar98.9B4C:0.51.5,TiB224.71.04.70.444530B4CzAl2O3z15%TiCTiC:D5051to2,.99%1950uC,35MPa,60min,Ar98.7B4C,TiB224.31.04.50.438629B4CzAl2O3z20%TiC1950uC,35MPa,60min,Ar98.5B4C,TiB223.21.04.20.432332Table8ContinuedSuri etal. Synthesisandconsolidationofboroncarbide: areview28 International MaterialsReviews 2010 VOL 55 NO 1Published by Maney Publishing (c) IOM Communications Ltd561026K21; exural strength, 350MPa; compressivestrength,14003400MPa.Fractography of fully dense boron carbide compact isshown in Fig.20.193The mode of fracture appears to betransgranular.Fabricationofboroncarbideshapesbyhotpressingthe mixture of particulate boron and carbon is alsopractised.111Kalandadzeetal.241compactedboronandcarbonpowdersinampoulesupto30%TD, byshockcompression, as a result of an explosive detonation.Thesepelletsweredensiedbyhotpressingattempera-tures19002100uCandpressures2040MPainboronnitridelinedgraphite moulds.A comparisonbetween a-rhombohedral,b-rhombohedral,andamorphousboronindicatedthatsinteringintotheb-rhombohedral phaseat the nal stage cangive higher densities as follows:BbRBaRBamorphous,which is attributed to the phasetransformation occurrence from amorphous boron to b-rhombohedral boron through a-rhombohedral boronmodication.Compactswithdensitieshigherthanthat achievablebypressureless sinteringprocess are producedbyhotpressingof boroncarbidepowders. Theaddedadvan-tages of hot pressed compacts are ne grained structure,verylowporosityandimprovedmechanical properties.Larger size powders in the range 310mmcan besinteredtoneartheoretical densitiesbyhotpressingaty2000uC and 3040MPa pressure. For applicationsuch as in nuclear industry, where pure boron carbide isessential andimpurities/additives cannot be tolerated,hotpressingisthepreferredmethodtoproducedense,purecompacts.RoleofsinteradditivesEarlier we have seen that carbon additive greatlyenhancesthesinteringkineticsinpressurelesssintering.Such an effect is not expected in the case of hot pressingas the sintering mechanisms are different. In theliterature also one does not nd any report on hotpressing of B4C with carbon addition. However additionof boron would consume the free carbon available in theboroncarbide.ItisseenthatsmalladditionsofB(1to5%) improves the strength of boron carbide specimens at Serialno.Materialcomposition,wt-%StartingpowderdetailsHotpressingconditionsSintereddensityrth,%Microstructure,mmVickershardness,GPaIndentationtoughness,MPam1/2Flexuralstrength,MPaReference(year)21B4Cz6%La2O3z12%Al2O3z12%C1850uC,20MPa,1h,vacuum92.5B4C,Al8B4C7,LaAlO3,97HRA156.76251(2008)2290to99%B4Cz0to1%BN/AlNzrestREoxide-Al2O3B/C53.8to4.5;D5050.7to3.018252000uC,10.3MPa,Ar99.6100B4C:1225273.03.9700800252(2007)23B4Cz30%AlB4C:260mesh600,16.3MPa,40min1.75gcc21194(1978)Al:2325mesh24B4C/Cu578:22B4C:D5055to40;Cu:D5051to81050uC,39MPa,1h81.9253(1999)B4C/Cu592:873.6*FC:freecarboninB4C;HR:heatingrate;CR:coolingrate;D50:meanparticlediameter,mm;SS:specificsurfacearea,m2g21;RE:rareearth.Table8Continued20 Microstructure of hot pressed boron carbide showingtransgranular fracture:193reprinted with permissionfromElsevier, Ceram. Int., 2006, 32, Fig.4(a) inp.232Suri etal. Synthesisandconsolidationofboroncarbide: areviewInternational MaterialsReviews 2010 VOL 55 NO 1 29Published by Maney Publishing (c) IOM Communications Ltdlowerhotpressingtemperatures.242Similartoprepara-tion of boron carbide based cermets for nuclearapplicationsboron carbideringswith adequatestrengthhave been prepared by hot pressing technique withy30wt-%aluminiumas binder for possible use asneutron absorber.194A high density B4C/Cu cermet with70vol.-%B4Cexhibitinghighthermal conductivityhasbeen prepared by hot pressing of Cu coated B4Cpowders for the application of absorber materials inliquid metal cooled fast breeder reactor.253Thoughboron carbide in various forms is used in nuclearindustry, literaturedataontheproductionmethods isscarce. An US patent261explains a process for producingB4C armour plates with improved ballistic properties bytheadditionofCr,B,ormixturesthereof.RoleofTiB2Reaction sintering of boron carbide with the addition oftitanium oxide and carbon as per reaction(19) producesextremelynehighsurfaceareaparticlesofTiB2whichpromotedensicationandlimitthegraingrowthoftheboron carbide matrix. This microstructure with TiB2particles uniformly distributed in a ne grained B4Cmatrix is responsible for the increase in fracturetoughness andstrength. B4C15vol.-%TiB2compositewith a exural strength of 621MPa and fracturetoughnessof 6?1MPam1/2havebeenpreparedbyhotpressingat2000uCandapressureof20MPainargonatmospherefor 1hbySkorokhodet al.244Theyhaveobservedthatfactorsfortheincreasedstrengthareduetothe healing of the cracks duringsintering andthepresence of TiB2particle which force the crack topropagateinanon-planar fashionthus enhancingtheenergy dissipation at the cracktip. AnUS patent243explainsaprocesstoprepareboroncarbidecompositescontaining5to30mol.-%titaniumdiboridewithveryhigh exural strength (870MPa) and fracture toughness(3?4MPam1/2). Addition of Fe in small amounts (0?5%)has beenfoundtobe effective inincreasingthe naldensities of B4CTiB2 composite due to the formation ofFeTirichliquidphaseatthegrainjunctions.262RoleofmixedboridesAs seen in the previous lines addition of TiO2hasbrought downthehot pressingtemperatureof B4Cby>100uC. Furtherattemptstoreducethesinteringtem-peraturewithout compromisingthestrengtharegivenbelow.ReactionsinteringofB4Cwith30wt-%(W,Ti)Cat 1850uC for 30min showed increase in fracturetoughnessandexural strengthupto50wt-%(W,Ti)Ccontent.Finegrainsof(0?52mm)TiB2andW2B5wereseenin themicrostructure.The sinteringtemperatureofthis compositeis300uClowerthanthat of monolithicB4C. Flexural strength and fracture toughness forcompositewith40to50%additivewere700MPaand4?5MPam1/2. The increase in fracture toughness isattributedtothe residual stresses generatedbydiffer-encesinthethermalexpansioncoefcientbetweenB4C,TiB2 and W2B5. The effectof TiB2/W2B5 on thepath ofcrack and deection in the composite is shown inFig.21.246Further reductioninsintering temperaturewas achievedbythe additionof B, Si andCototheabovereferredmixture. ReactionsinteringofB4CwithWC,TiC,B,SiandCobyattritionmillingfollowedbyhot pressing at 1720uC for 2h gave a compact with threedistinctphases ofB4C,W2B5,andTiB2,hardnessintherange 2833GPa and exural strength of 830MPamax.245US patent by Petzow et al.263describes a processfor the preparationof boroncarbide/transitionmetalboride moulded articles comprising of B4C, Si, WC and/or TiCand Co by hot pressing between 1550 and1850uC. Effect of variation of TiCaddition on hotpressing of B4C/TiB2/Mo composite has been studied byJianxinet al.247andthe maximumvalues of fracturetoughness, exural strengthandhardness reportedare4?3MPam1/2, 695MPa and 25?0GPa respectively.Duringball milling/mixingof B4Cwithadditives, thepowders get contaminated and the microstructure of thecompositeappearsverycomplicatedafterhot pressingdue to the diffusion of W, Co, Ni, Cr, etc. either into theTiB2 grains to form (Ti,M)B2 or (Ti,M)B2 coated grains,and Ti, Fe, Co, Ni, and Cr into W2B5 to form boron richborideortheinterfaciallayer.264Roleofcarbides/nitridesLi etal.265havepreparedacompositecontainingB4C,SiC, TiB2and BNby reactive hot pressing of B4C,Si3N4, a-SiC and TiC powders and the hardness,bending strength, fracture toughness and relative densityof the composite were 88?6HRA, 554MPa,5?6MPam1/2and 95?6%respectively. Microstructureanalysis showed the presence of laminated structure anda clubbed frame dispersion phase and bunchy dispersionphase among the matrix. Fractography and crackpropagationsuggestedthat crackdeectionandbrid-gingarethepossibletougheningmechanisms.Hanet al.266havesynthesisedahighstrength(400570MPa, 69?5MPam1/2) B4CTiB2SiCgraphitecompositebyreactivehotpressingusingB4C, TiCandSiC powders. The crack deection at the phaseboundarybetweenB4Cmatrixanddispersionsconsist-ing of SiC and TiB2, which occur by residual stresses duetothe differences inthermal expansioncoefcients ofB4C, TiB2andSiCwhilecoolingfromthefabricationtemperature is responsible for the enhanced fracturetoughness values. Similar very highstrengthmaterial(four point bend strength: 850MPa; fracture toughness:6?1MPam1/2) has been prepared by the addition of530vol.-%Mo to B4C/(W,Mo)B2by hot pressing at1900uC.267Fractography of this sample showing thecrackpropagationpathisdepictedinFig.22.267When21 Crack path produced by Vickers indentation onpolished surface of hot pressed B4C30wt-%(W,Ti)Ccomposite:246reprinted with permission fromElsevier,Ceram. Int., 2002, 28, Fig.10inp.429Suri etal. Synthesisandconsolidationofboroncarbide: areview30 International MaterialsReviews 2010 VOL 55 NO 1Published by Maney Publishing (c) IOM Communications Ltdboronis addedtotheabovemixture, hardness of thecompactincreasesduetotheinhibitionofB4Cdecom-position but the bending strength and the fracturetoughness reduce.268A composite B4CVB2C obtainedbyreactionsynthesiswithhotpressinghasbeenfoundtoexhibit highhardnessandbendingstrengthsuitablefor applicationas wear andshockresistance compo-nents.269Cr and V carbides are also found to be effectivein obtaining high densities and ne grained structure.270AprocessinwhichapreceramicorganosiliconpolymerwhichonpyrolysisyieldsSiCandfreecarbonhasbeenpatented for preparation of dense bodies of boroncarbide (.97% TD) by hot pressing in inert atmosphereat a temperature of 2275uC and a pressure of 28MPa.234Reaction sinteringof boroncarbide withthe additionof oxides/carbides/nitrides has been successfullyemployedtoobtaina microstructure of ne particlesof reaction product (borides/carbides/nitrides) in B4Cmatrix.Theseadditionslowerthesinteringtemperaturethan that of monolithic B4C. Flexural strength andfracture toughness of these composites are very high duetotheresidual stresses generatedbydifferences inthethermal expansion coefcient between B4C and reactionproducts, crackbridginganddeectionmechanismsattheinterface, etc. Small quantitiesofB, Si, Ti, Fe, Co,Ni, Cr, W, etc. either intentionally added or accidentallyacquiredduringthegrinding/mixingoperationsarealsofound to be effective in marginally reducing the sinteringtemperature and improving the exural strength/fracturetoughness due to the formation of complex boridephasesandmultiinterfaces.LiquidphasesinteringAddition of CrB2aids in lowering the hot pressingtemperatureduetotheformationofCrB2B4Ceutecticat 2150uC. B4C20mol.-%CrB2composite fabricatedby hot pressing at 1900uCshows a high strength of630MPa and a modest fracture toughness of3?5MPam1/2. The veryne grainedmicrostructure isresponsible for high exural strength and residualstresses caused by thermal expansion mismatch ofCrB2and B4C for increasing toughness.243,248,256Similarly addition of Al2O3enhances the sinteringkinetics of boron carbide due to a liquid phaseformation at 1950uC.249Jianxin and Junlong271havestudied the effect of TiC content on the micro-structure, mechanical propertiesandsanderosionrateof B4C/Al2O3/TiC composites. Addition of TiCincreased the hardness of the composite and thehardnesshaddirectinuenceontheerosionrateofthenozzles.Theadditionofrareearth(RE)oxidessuchasY2O3, La2O3reduces thesinteringtemperatureduetothe formation of a liquid phase near the yttriumaluminate composition (60wt-%Y2O340wt-%Al2O3,melting point 1870uC).251Pore free sintered boroncarbide materials with high strength (700800MPa)andfracture toughness (3?63?9MPam1/2) have beenpreparedby lowpressurehotpressingwiththeadditionofBN/AlNandoxidebinder(REoxideAl2O3).252Additives such as CrB2, Al2O3, Y2O3, La2O3 etc. bringdownthe sintering temperature of B4Cdue toliquidphase formation. The reaction products formed areboride of the respective oxide, which enhances themechanical properties. Addition of TiC with otheroxidesincreasesthehardnessanderosionresistanceofB4Ccomposite.A number of new processing methods are envisaged toproducematerialswithdesignedstructureandproper-ties. Amachinable B4C/BNnanocompoisite has beenfabricated by hot pressing microsized B4C particlescoatedwithamorphousnanosizedBNparticles.254Thehardnessof compositedecreasedwithincreasecontentof BN while the machinability improved signicantly. Acomposite with.20wt-%BNcontent exhibitedexcel-lent machinability.272The surface hardness andwearresistance of this composite has been improved bysilicon inltration process.255Combination of SHS tech-nique with hot pressing (called combustion hot pressing)hasbeenusedtoprepareacomposite, containingB4CandSiCformedbyreactionamongSi, BandC,intheform of interlocked matrices with very low porosity anduniformmicrostructure.273GradedporosityB4Cmate-rials can be produced by a layering approach usingdifferentsizedistributionsofB4Cpowdersinthegreenstate, andthendensifyingthelayeredassemblybyhotpressingat1900uC.274Cobaltassinteradditivehasalsobeen attempted for hot pressing of boron carbidepowders with 5wt-%TiC at temperatures ,1500uCandahighpressureof56GPa.275Hotisostaticpressing(HIP)The HIP process, originally known as gas pressurebonding, usesthecombinationofelevatedtemperatureand high pressure to form/densify rawmaterials orpreformedcomponents.Theapplicationofthepressureiscarriedoutinsideapressurevessel,typicallyusinganinert gas as thepressuretransmittingmediumwithorwithout glass encapsulationof the part. Aresistanceheated furnace inside the vessel is the temperaturesource. Partsareloadedintothevessel andpressurisa-tion occurs usually simultaneously with the heating. Thehigh pressure provides a driving force for materialtransport during sintering which allows the densicationto proceed at considerably lower temperature incomparisontothatoftraditionalsintering.Inaddition,particularlyduringtheinitial stagesoftheprocess, thehighpressureinduces particlerearrangement andhighstressesat theparticlecontact points. Avirtuallyporefree product can be produced at a relatively lowtemperature.ThepressurelevelusedintheHIPprocesstypically is 100300MPa, as compared to 3050MPa inuniaxial hot pressing, and the isostatic mode ofapplicationof pressureisgenerallymoreefcient than22 Crack propagation path with considerable deectionin hot pressed B4C/30W20Mo composite:267reprintedwith permission fromJapan Society of Powder andPowder Metallurgy, J. Jpn Soc. Powder PowderMetall., 1999, 47, (1), Fig.8inp.28Suri etal. Synthesisandconsolidationofboroncarbide: areviewInternational MaterialsReviews 2010 VOL 55 NO 1 31Published by Maney Publishing (c) IOM Communications Ltdthe uniaxial one.276,277Larsson et al.278have studied theeffect of additionof boron, siliconandsiliconcarbidewhile hot Isostatic pressing boron carbide at 1850uC for1h under a pressure of 160MPa. Addition of boron wasfound effective in reducing the pores and graphiteinclusions and improved particle erosion resistance.Boron carbide (100%TD) could be obtained by acombinationof pressureless sinteringandpost-HIPat2150uCfor 125min under 310MPa of argon pres-sure.279,280The combination of pressureless sinteringandpost-HIPisgainingimportancefor fabricationofdense bodies with higher densities, lower graphitecontentsandsignicantlyhigherVickershardnessthancommercially hot pressed B4C.279281Elimination ofresidual porosity and signicant improvements inexural strength, elasticconstants andwear resistancewere observed with the addition of 1 and 3wt-%C in theabove process.282Fully dense and very ne grainedboron carbide has been prepared by the fabricationroute, injectionmoulding/pressurelesssintering(2175uC)/post-HIP(200MPa, Ar)fromB4Cdopedwith4wt-%carbon black.283Near net shape with full density can beachievedbyHIP.284286Figure23279shows themicro-structure of post hipped boron carbide to full theoreticaldensity. Equiaxeduniformsize grains andthingrainboundaryarethespecial featuresof thismaterial withveryhighhardness.279281Apatentedprocess explainsthe preparation of boron carbide shapes containingmetallic diborides (of Ti, Zr, Hf, V, Nb and Ta), sinteredinthetemperature2100to2200uCtogiveadensityof2?47gcc21, whichonfurther hot isostaticpressingat2100uCunderanargonpressureof200MPatoachieveatheoreticaldensityof2?56gcc21.219Porosityseverelydegrades theballisticproperties ofceramicarmourasitactsasacrackinitiator. Sinteringaidsgenerallydegradehardnessandballisticproperties.Therefore,boroncarbideprotectiveinsertsforpersonalarmour is hot pressed to minimise porosity (y98%relativedensity),yieldingacceptableperformance.Post-hipping of pressu