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
[email protected] 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.
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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)
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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)
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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