synthesis and consolidation of boron carbide- a review
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Synthesis and consolidation of boron carbide:a review
A. K. Suri, C. Subramanian*, J. K. Sonber and T. S. R. Ch. Murthy
Boron carbide is a strategic material, finding applications in nuclear industry, armour for
personnel and vehicle safety, rocket propellant, etc. Its high hardness makes it suitable for
grinding and cutting tools, ceramic bearing, wire drawing dies, etc. Boron carbide is
commercially produced either by carbothermic reduction of boric acid in electric furnaces or
by magnesiothermy in presence of carbon. Since many specialty applications of boron carbide
require dense bodies, its densification is of great importance. Hot pressing and hot isostatic
pressing are the main processes employed for densification. In the recent past, various
researchers have made attempts to improve the existing methods and also invent new processes
for synthesis and consolidation of boron carbide. All the techniques on synthesis and
consolidation of boron carbide are discussed in detail and critically reviewed.
Keywords: Synthesis, Densification, Boron carbide, Sintering, Hard material, Neutron absorber
IntroductionBoron carbide is a suitable material for many highperformance applications due to its attractive combina-tion of properties such as high hardness (29?1 GPa),1
low density (2?52 gm cm23),1 high melting point(2450uC),2 high elastic modulus (448 GPa),3 chemicalinertness,2,4 high neutron absorption cross-section (600barns),4,5 excellent thermoelectric1,4 properties, etc. Ithas found application in the form of powder, sinteredproduct as well as thin films. Boron carbide (also knownas black diamond) is the third hardest material afterdiamond and cubic boron nitride. Its outstandinghardness makes it a suitable abrasive powder forlapping, polishing and water jet cutting of metals andceramic materials.4
Tools with boron carbide coating are used for cuttingof various alloys such as brass, stainless steel, titaniumalloys, aluminium alloys, cast iron, etc.1 In sinteredform, it is used as blasting nozzles,6 ceramic bearingsand wire drawing dies due to good wear resistance.1 Thecombination of low specific weight, high hardness andimpact resistance makes it a suitable material as bodyand vehicle armour. Modulus to density ratio of boroncarbide is 1?86107 m, which is higher than that of themost of the high temperature materials and hence itcould be effectively used as a strengthening medium.7
Thin films of boron carbide find application asprotective coating in electronic industries.8,9
Boron carbide is extensively used as controlrod, shielding material and as neutron detector innuclear reactors due to its ability to absorb neutronwithout forming long lived radionuclide.7,10–17 Neutron
absorption capacity of boron carbide can be increasedby enriching B10 isotope. Composite material containingboron carbide with good thermal conductivity andthermal shock resistance are found suitable as first wallmaterial of nuclear fusion reactors.18–21 Boron carbidebased composites are potential inert matrix for actinideburning.22 Boron carbide is also used for treatment ofcancer by neutron capture therapy.23
As it is a p-type semiconductor, boron carbide isfound to be a potential candidate material for electronicdevices that can be operated at high temperatures.24,25
Owing to its high Seebeck coefficient (300 mV K21),boron carbide is an excellent thermoelectric material.26
Boron carbide is finding new applications as thermo-couple, diode and transistor devices as well. Boroncarbide is an important component for the productionof refractory and other metal borides.27–29 The lowdensity, high stiffness and low thermal expansioncharacteristics of B4C make it attractive Be/Be alloyreplacement candidate for aerospace applications.30
Thevenot has compiled a comprehensive review onboron carbide1 in 1990, in which synthesis, consolida-tion, analytical characterisation, phase diagrams, crystalstructure, properties and applications are discussed. Thispaper critically examines various methods of synthesisand consolidation of boron carbide and discusses theirmerits and demerits along with structure, properties andapplications.
Structure of boron carbideThe bond between B-B atoms and B-C atoms play a keyrole in deciding the crystal structure and properties ofboron carbide. Knowledge of these will help us inunderstanding the complexities involved in processingand achieving the desired properties. Boron carbide is acompositionally disordered material that exists as
Materials Group, Bhabha Atomic Research Centre, Mumbai 400085, India
*Corresponding author, email csubra@barc.gov.in
� 2010 Institute of Materials, Minerals and Mining and ASM InternationalPublished by Maney for the Institute and ASM International
4 International Materials Reviews 2010 VOL 55 NO 1 DOI 10.1179/095066009X12506721665211
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rhombohedral phase in a wide range of composition,which extends from B10?4C (8?8 at.-%C) to B4C(20 at.-%C).31–34 Among them, B4C is superior inproperties such as hardness, thermal conductivity etc.Since B4C is in equilibrium with free carbon and is onlyboundary between BnC and BnCzC (where 4,n,10),35
synthesis of B4C without free carbon is a great challenge.Carbon content of boron carbide greatly influences thestructure and the properties of the compound and hencethe exact knowledge of B/C ratio of the phase is veryimportant. But the analytical study of B-C system isdifficult due to extreme hardness and chemical stabilityof boron and boron carbide phases.33 Different limits ofhomogeneity range are reported by researchers at thecarbon rich side of boron carbide, corresponding toB4?3C (18?8%C),36 B4?0C (20%C),33,34 and B3?6C(21?6%C).37 Difficulties associated with the estimationof free and combined carbon could be accountable forthese inconsistent results.36 B-C phase diagram showinghomogeneity range from 8?8 to 20 at.-%C, as generatedby Bouchacourt et al.34 is presented in Fig. 1.
Crystal structureThe most widely accepted crystal structure of boroncarbide is rhombohedral, consisting of 12-atom icosahe-dra located at the corners of the unit cell. Schematicdiagram of the structure of boron carbide is presented inFig. 2.38 The longest diagonal of the rhombohedral unitcell contains three atom linear chain (C-B-C). Each endmember of the chain is bonded covalently to an atom ofthree different icosahedra.31 In general, icosahedraconsist of 11 boron atoms and one carbon atom. Thelocations of carbon atoms within different icosahedraare not ordered relative to one another. The icosohedralconfiguration is the result of a tendency to form three-centre covalent bonds due to deficiency of valenceelectrons of boron.39 Two crystallographically in-equivalent sites exist in the icosahedron. Six atomsreside in two polar triangles at the opposite ends of theicosahedron 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 siteseither bond directly to other icosahedra through three-centre bonds or to chain structures.40,41 Most of theicosahedra have a B11C structure with the C atom placedin a polar site, and a few percent have a B12 structure ora B10C2 structure with the two C atoms placed in twoantipodal polar sites.41
Three types of three-atom chain are envisioned: C-B-C, C-B-B and B-B-B. Variation in carbon concentrationchanges the distribution of three-atom chains.31 B4C(20%C) structure consists of B11C icosahedra and C-B-Cchains. As the composition becomes rich in boron,carbon of the B11C icosahedra is retained, while one ofthe carbon atoms on the C-B-C chains is replaced byboron. Near the composition B13C2, the structureconsists of B11C icosahedra and C-B-B chains. Onfurther carbon reduction, some of the B11C icosahedraare replaced by B12 icosahedra retaining the C-B-Bchain.40,42 Carbon-boron bonds present in the threeatom chains are much stronger than boron-boron bondin icosahedra.40 The inter-icosahedra bonds are stifferthan the intra-icosahedra bonds.43
Conflicting views still exist concerning the nature ofsite occupancies. A model based on early X-ray dif-fraction data44,45 proposed that the B4C composition ismade up of B12 icosahedra and C-C-C chains. Howeverlater studies38,40–42,46,47 based on improved X-ray andneutron diffraction, nuclear magnetic resonance studies,theoretical calculations and vibrational spectra indicatethat the structure consist of B11C icosahedra and C-B-Cchains. Even among those who favour B11C icosahedraand CBC chain model for 20 at.-% (B4C), there isdisagreement on the structural changes that occur inboron carbides, as the carbon content is decreasedtowards 13 at.-% (B13C2). Some workers46,48 proposethat carbon atoms are removed from the icosahedra toform B12 icosahedra, while others40,42,49 propose thatcarbon atom is replaced from three atom chains. Owingto similarity of boron and carbon in electron density andnuclear cross-section (B11 and C12), both X-ray andneutron diffraction studies are not very successful in
1 Phase homogeneity range in B-C phase diagram: rep-
rinted with permission from Elsevier, J. Less Comm.
Met., 1979, 67, Fig. 2 in p. 329
2 Schematic diagram of structure of boron carbide
Rhombohedral unit cell, consisting of 12-atom icosahe-
dra located at the corners and C-B-C linear chain at
the diagonal of the unit cell is shown. Within the icosa-
hedron, six atoms reside in two polar triangles at the
opposite ends of the icosahedron and the remaining
six atoms occupy equatorial sites: reprinted with per-
mission from the American Physical Society, Phy. Rev.
Lett., 1999, 83, (16), Fig. 1 in p. 3230
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unambiguously assigning the exact site occupancies.48
The concentration of B12 and B11C icosahedra and C-B-C and C-B-B chains vary and chainless unit cells alsooccur.50,51 Variation of structure elements B12, B11C, C-B-C and C-B-B in boron carbide unit cell with C contentis shown in Fig. 3.51 The carbon rich limit 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 wasattributed to be the most representative structure (B12,C-B-C) and used for many model calculations has beenproved to be the least defined structure containing60%B11C and 40%B12 icosahedra. These structuralchanges could also explain the abrupt decrease inthermal conductivity between B4C to B6?5C. Saalet al.46 have recently applied ab initio calculations toevolve the structure of boron carbide for the entirecomposition range. The enthalpy of formation andlattice parameters were calculated and compared withthe experimental data. For carbon rich composition(20%C), B11C-CBC structure and for 13?33%C compo-sition B12-CBC structure were found most stable. Itsuggests that carbon atom is gradually replaced byboron in icosahedra. This result is contradictory withother researchers who suggest that carbon atom isreplaced from chain. At boron rich end, enthalpy andlattice parameters of B12 BVaC (Va denotes vacancy)structure is in good agreement with the experimentalvalues for boron carbide having 7?14 at.-%C. Since theenthalpy of formation of B12 BVaC is positive it ispredicted that B12 BVaC’s composition cannot bereached by boron carbide and instead, pure boron willprecipitate out, which is in agreement with experimentalphase equilibrium. Radev et al.43 have found that metalcations can replace a part of boron atoms in icosahedraposition and thus improves the stiffness, hardness andwear resistance of boron carbide.
Recent observations by Raman spectroscopy suggeststructural phase transformation and the formation oflocalised amorphous phase which is weaker than theoriginal crystalline phase under conditions of loading.52
First principle molecular dynamics simulations haverevealed that the depressurisation amorphisation resultsfrom pressure induced irreversible bending of C-B-Cchains.53
Structure sensitive propertiesThermal and electrical conductivity, heat capacity,hardness, etc. strongly depend on structure of boroncarbide and the variations are brought out in thefollowing lines. Lattice parameter aR of rhombohedralunit cell decreases with increase in carbon content butthe plot is discontinuous at the composition B13C2
(13?33 at.-%C). Density of boron carbide increaseslinearly with carbon content within the homogeneityrange of the phase according to the relation d(g cm23)52?422z0?0048[C] at.-% (r50?998) with8?8 at.-%([C]>20?0 at.-%. The number of atoms perunit cell is exactly 15 for B4C, but increases with theboron content and approaches 15?3 for the boron richlimit B10C.35 Hardness of boron carbide increases withcarbon within the homogeneity range as the structurebecomes stiffer.1 Shear modulus of boron carbideincreases with carbon from 185 GPa for B6?5C (13%C)to 198 GPa for B4?3C (20%C).54 Fracture toughness andYoung’s modulus also increases with the carboncontent.1
Heat capacity of boron carbide increases withdecrease in carbon within the homogeneity range. Thisincrease is due to the change in lattice vibration modeproduced by reduction of the stiffness of the three-atomchain accompanied with a change from C-B-C to C-B-B.55 Thermal conductivity of B4C (20%C) falls withtemperature in the manner characteristic of crystallineceramics. However, thermal conductivity of boroncarbide with low carbon is relatively less and tempera-ture independent, a behaviour more characteristic ofamorphous materials. These differences of thermaltransport can be explained if it is assumed that, thethermal conductivity is dominated by the transfer ofvibrational energy through the inter-icosahedral chainsrather than within the softer icosahedra. As the C-B-Cchains are inhomogeneously replaced by C-B-B chains atransition takes place from crystal-like transport toglass-like transport. Moreover thermal conductivity fallsbecause B-B bonds are much softer than C-Bbonds.40,55,56 Gilchrist et al.57 have found that thermalconductivity of B4C falls from 29 W m21 K21 at roomtemperature to 12 W m21 K21 at 1000uC. Thermalconductivity increases when 10B isotope replaces 11B inboron carbide. This is attributed to the increase in thebonding energy per unit mass and the phonon velocityas a lighter isotope is substituted for a heavier isotope.58
Electrical conductivity in boron carbide was studiedby Wood et al.49 and Matusi et al.55 Charge carriers inboron carbide are holes which form small polarons andmove by phonon assisted hoping between carbon atomslocated at geometrically non-equivalent sites.49 The non-equivalence arises from two sources. First, carbon atomscan be distributed among non-equivalent sites withinB11C icosahedra. Second, only a fraction of the availablepositions of inter-icosahedral chain is generally filled byC-B-C chains. The carbon rich limit (B4C) resembles anideal crystal and therefore has the lowest electricalconductivity.55 Electrical conductivity increases withtemperature, which is the sign of behaviour of asemiconductor. Density of small polaron holes is
3 Composition of structure elements (B12 and B11C ico-
sahedra, C-B-C and C-B-B chains) in boron carbide unit
cell and chainless unit cells with variation of C con-
tent: reprinted with permission from Elsevier, Solid
State Commun., 1992, 83, Fig. 4 in p. 850
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independent of temperature but the mobility of holesincreases with temperature. Within the homogeneityrange, charge carrier density and electrical conductivitydecreases with increase in carbon content. The tempera-ture dependence of electrical conductivity is essentiallyindependent of carbon concentration.49
Irradiation responseNeutron irradiation of boron carbide results in extensiveintergranular cracking due to the formation of heliumbubble as per the following equation59–61
5B10z0n1?3Li7z2He4z2:6 MeV (1)
Formation of these cracks, which prevent heat conduc-tion and the atomic disorder resulting in high phonondispersion, decrease the thermal conductivity duringirradiation.59 The anisotropic precipitation of heliumnot only changes the microstructure but degrades themechanical and physical properties as well. When grainboundary cracking occurs, a large amount of trappedhelium is released simultaneously with the occurrence ofbulk swelling.62,63 Considerable amount of tritium isproduced in B4C by fast neutron irradiation, which isretained up to 700uC even on annealing and is releasedonly at temperature higher than 900uC.62,64 Copelandet al.65,66 have reported that irradiation of boron carbidewith neutron causes lattice strains due to the formationof lithium and helium as reaction product as well assome atomic displacements. Inui et al.67 have reportedthat a complete crystalline to amorphous transitiontakes place by electron irradiation with energy .2 MeVand at temperature ,163 K. They also found that theamorphous boron carbide remains in amorphous stateon annealing at 1273 K. They suggested a possibility,that, in boron carbide, the individual B12 icosahedrathemselves are not destroyed by electron irradiation buttheir regular spatial arrangement in the B12C3 lattice isperturbed and is gradually put in disorder withincreasing electron dosage, resulting in an amorphousstate.67 Froment et al.59 have noticed that boron richB8C is more resistant to radiation damage compared toB4C and hence becomes a possible candidate for newabsorbing materials. 11B4C is found to be very stableafter fast neutron irradiation in reactors. Dimensionalchanges and thermal conductivity of 11B4C are substan-tially smaller than that of 10B4C.68
Synthesis of boron carbideBoron carbide was discovered in nineteenth century as abyproduct of reaction involving metal borides. Thepurity of boron carbide produced by early researcherswas less than 75% and in 1933, Ridgway69 claimed tohave produced crystalline B4C of 90% purity bycarbothermic process. Lipp4 has presented a review ofboron carbide production, properties and applicationsin 1965. Spohn70 has also mentioned the synthesis routesfor boron carbide production and its uses in his article.In this section, different routes for B4C synthesis will bediscussed. The methods of boron carbide synthesis areclassified as:
(i) carbothermic reduction(ii) magnesiothermic reduction
(iii) synthesis from elements(iv) vapour phase reactions(v) synthesis from polymer precursors
(vi) liquid phase reactions(vii) ion beam synthesis
(viii) VLS growth.
Carbothermic reduction of boric acidCarbon reduction of boric acid and boron trioxide is thecommercial method for the production of boroncarbide. The overall carbothermic reduction reactioncan be presented as follows
4H3BO3z7C?B4Cz6COz6H2O (2)
This reaction proceeds in the following three steps70
4H3BO3?2B2O3z6H2O (3)
B2O3z3CO?2Bz3CO2 (4)
4BzC?B4C (5)
Boric acid on heating converts to B2O3 by releasingwater. The reduction of B2O3 with carbon monoxidebecomes thermodynamically feasible above 1400uC. Thefurnace temperature is usually maintained at .2000uCto enhance the rate of overall reaction. The process ishighly endothermic, needing 16 800 kJ mol21 B4C.71
Three types of electric heating furnaces, namelytubular, electric arc and Acheson type (graphite rod asresistance element) are used for the production of boroncarbide. Tubular electric furnaces using graphite tubesas heating element are in use for carrying out reactionsfor scientific study only. These furnaces are limited insize, dependent on the size of the availability of graphitetubes. Hence large scale production is not feasible usingtubular furnaces.
Arc furnace process
Electric arc furnace process for making boron carbidehas been patented by Schroll et al.72 in the year 1939,wherein the mixture of boric acid and petroleum coke ismelted in an arc furnace followed by crushing theresultant product and mixing it with substantially thesame quantity of boric acid and remelting the mixture asecond time. The design and operation of the electric arcfurnace for the large scale production of boron carbidehas been explained by Scott.73 In the arc furnaceprocess, the temperatures are generally very high dueto localised electric arcs, which are responsible for heavyloss of boron by evaporation of its oxides. Moreover theproduct obtained is chunks of melted boron carbide,which needs subsequent laborious crushing and grindingoperations.
Acheson type furnace
Acheson type furnaces, where a graphite rod is used asheating element, surrounding which the reactants arecharged is also used for production of boron carbide.Early patents by Ridgway69,74 give the details of thefurnace and the process. Operational details of theAcheson furnace are explained below. Partially reactedcharge from the previous run is assembled around thenew graphite heating rod. Above this, the new chargemixture consisting of boric acid and carbon is added. Onheating, the reaction initiates near the graphite rod andcarbon dioxide escapes to the atmosphere through thecharge above. As the reaction proceeds, the charge getsheated by conduction as well as by the heat of the
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escaping CO. Boric acid initially loses its water andconverts to B2O3. On further heating, B2O3 melts andforms a glassy film preventing the escape of CO from thereduction zone. The product gases form bubbles andgrow in size near or just above the reaction site and asthe pressure increases, the bubbles burst pushing thecharge above. During these bursts some of the partiallyreacted charge is thrown out of the furnace and boronalso escapes to the atmosphere in the form of boronoxide vapours. These bubble bursts and evaporationlosses affect the efficiency of the process considerably.
After completion of the run, the top is broken openand the boron carbide surrounding the graphite rod ismanually collected. Operator experience plays a majorrole in identifying the completely reacted product so thatless amount of oxides enter the boron carbide portion.The reacted product is crushed in jaw crushers andfurther ground to finer size. Ground powder is washedin water and leached in acid to remove the contamina-tion due to grinding media and also the accompanyingunreduced 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 recirculated in further runs. Some quantityof boron oxides escapes to the atmosphere along withcarbon monoxide. Hence in this process, conversion ineach run is low and boron loss is high. As the rawmaterials used are cheap and the process is simple, thisprocess has been adopted for commercial production.Though the method of raw material charging andcollection of reacted product could be different in arcfurnace and Acheson processes, the reaction sequence isvery similar.
As temperature is an important process parameter incarbothermic reduction process and the heat transferplays an important role in the formation of boroncarbide Rao et al.75,76 have devised a method of coretemperature measurement in boron carbide manufactur-ing process. They have analysed the heat transferprocess inside the reactor and the effect of it on theformation of boron carbide based on the recorded data.
Process kinetics
Kinetics of the reaction and also the product quality isstrongly influenced by porosity of the charge, type ofcarbon used for reduction, rate of heating and the finalcore temperature. Process kinetics, influence of processparameters and the means of improving the productquality and conversion efficiency have been investigatedby many researchers. Petroleum coke is found to be abetter reducing agent than graphite, charcoal andactivated charcoal.77 Boric acid to coke ratio of 3?3–3?5 is found optimum and at higher ratios, though boroncarbide free of carbon could be obtained, recovery ispoor. Alizadeh et al.78 have optimised the boron oxide/carbon (petroleum coke) ratio to yield boron carbidewith low (0?65%) carbon. Addition of small amount ofsodium chloride (1?5%) is found to be effective inincreasing the yield.71,79 Start of formation of B4C hasbeen noticed by Subramanian et al.80 at 1200uC bythermogravimetric studies on the reduction of boric acidby petroleum coke in vacuum. Figure 480 shows theweight change of the charge with temperature up to1400uC. Formation of boron carbide by carbothermicreduction is highly dependent on the phase changes ofreactant boron oxide from solid to liquid to gaseousboron sub oxides and the effect of reaction environment(heating rate and ultimate temperature).81 Slow heating(,100 K s21) of the charge results in the formation ofboron carbide by a nucleation and growth mechanism asthe reaction proceeds through a liquid boron oxide path.Intermediate heating rates (103 to 105 K s21) 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(.105 K s21) result in smaller crystallite size, indicatingthe occurrence of reaction through gaseous boronsuboxides.
Dacic et al.82 have studied the thermodynamics of gasphase carbothermic reduction of boron anhydride. B2O2
and BO are formed by carbothermic reduction of B2O3
according to reactions (6) and (7) and then reduced to Bor B4C.
4 Thermogravimetric analysis plot of carbothermic reduction of boric acid80
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B2O3zC?B2O2zCO (6)
B2O3zC?2BOzCO (7)
The effect of the feed composition and temperature onthe product composition in carbothermic reduction isshown in Fig. 5.82 A decrease in the partial pressure ofCO facilitates synthesis of B4C by boosting thegeneration of B2O2 and BO.
Production of boron carbide by carbothermy has beenessentially a batch process. Tumanov83 has reported thedevelopment of a continuous process for the productionof boron carbide, by direct inductive heating of a chargemade of boron oxide and carbon black. An alternatereduction method patented by Rafaniello84 explains theprocess for producing submicrometre size boron carbidepowders. The type of carbon used, method of prepara-tion of the charge mixture and the fast heating rates (70–10 000uC min21) are responsible in obtaining finepowders. Weimer et al.85,86 have designed a verticalapparatus comprising of cooled reactant transportmember, reaction chamber, heating element and coolingchamber for the continuous production of submicro-metre B4C powder. Modelling of carbothermic reduc-tion process for the production of boron carbide has notbeen attempted by anybody so far.
Preparation of dense articles need fine boron carbidepowders in micrometre size. The product of conven-tional process has to undergo series of size reductionprocesses to obtain such powders. Such grinding opera-tions contaminate the product necessitating additionalpurification steps. Availability of nanosized powders willnot only avoid the grinding operations but also reducethe temperature of densification substantially.
Nanocrystalline boron carbide
Preparation of nanosized particles of boron carbide is ofrecent interest. B4C particles in the nanosize range(260 nm) can be prepared by reduction of B2O3 vapourby carbon black at 1350uC.87 Above this temperature,yield is low due to loss of B2O3 from reaction mixture.Addition of cobalt as catalyst is found to be helpful in
yield of nanorods.88 Ma et al.89 have prepared highpurity boron carbide nanowires from mixed powderprecursor containing boron, boron oxide and carbonblack. The mixture is heated quickly to 1650uC and heldat that temperature for 2 h under flowing argon.Vapours of B2O3, B2O2 and CO react to form B4C solidnanowires with a mean diameter of y50 nm and lengthsof several hundreds of micrometres (Fig. 6).89 Largescale boron carbide nanowires of size 80–100 nmdiameter and 5–10 mm in length have been synthesisedusing B/B2O3/C powder precursor under argon flow at1100uC.90 Xu et al.91 have synthesised nanostructures ofboron carbide by heating B2O3 powder to 1950uC in agraphite crucible covered with a boron nitride disc.Majority of the crystallites deposited on boron nitridedisc show a belt-like morphology with average widthand length of about 5–10 and 50–100 mm, while thethickness was in the nanoscale range (20–100 nm). Anumber of perfect icosahedral quasicrystal particles(Fig. 7a)91 and multiply twinned particles normally inrod shape were also present (Fig. 7b).91 These particleshave very large sizes (y20 mm). Thus it was found thatwhen the reaction takes place in gas phase or theproduct could be nanocrystalline B4C. Presence of somecatalyst also promotes the formation of nanopowders.
Although carbothermic reduction results in loweryield due to loss of boron in the form of its oxides, thisroute is adopted as commercial method mainly becauseof the simple equipments and cheap raw materials whichmake this route the most economical. This route is notonly useful for commercial powder production but alsofor the production of nanocrystalline B4C. Details ofexperimental studies on carbothermic reduction, givingcharge composition, processing conditions and productquality of boron carbide obtained by various researchersare summarised in Table 1.69,72,77–79,83–85,92
Magnesiothermic reduction of B2O3
An alternate method for the production of boroncarbide is by magnesiothermic reduction of boronanhydride in presence of carbon as given below
2B2O3z6MgzC?B4Cz6MgO (8)
This reaction takes place in two steps:step 1
2B2O3z6Mg?4Bz6MgO (9)
step 2
4BzC?B4C (10)
The reaction is exothermic (DH51812 kJ mol21) innature. As the vapour pressure of magnesium is highat the reaction temperature of .1000uC, a cover gassuch as argon or hydrogen is used and also the systempressure maintained high. The products of the reactionare processed by aqueous methods to remove magne-sium oxide from boron carbide. The carbide is stillcontaminated with magnesium borides formed as stablecompounds. This reduction technique yields very fineamorphous powder, which is well suited for use in thefabrication of sintered products. One method ofcontrolling the temperature and the particle size of theproduct is by choosing the right size of the reactants.Post reductive sintering at temperatures 200–300uC
5 Effect of feed composition and temperature on calcu-
lated product composition in carbothermic reduction of
1 mol B2O3 (l) as per Dacic et al.:82 reprinted with per-
mission from Elsevier, J. Alloys Compd, 2006, 413,
Fig. 2 in p. 200
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higher than the reaction temperature increases theparticle size of the product. Seeding of the charge witha small quantity (1–2%) of boron carbide has been foundto increase the growth of B4C particles and the yieldsignificantly.93
An early patent by Gray94 explains the process for theproduction of boron carbide powders by magnesiothermicreduction of B2O3 or alkali Na2B4O7 in presence of carbon
at 1650–1700uC. Addition of metallic sulphates as catalysthas been found to reduce the reaction temperature to700uC.95 The heat of magnesiothermic reaction is sufficientenough for self high temperature synthesis route.Formation of ultra fine B4C powder from the stoichio-metric mixture of H3BO3, Mg and C by self-propagatinghigh temperature synthesis (SHS) has been studied byZhang et al.96 and Khanra et al.95,97 The ignitiontemperature of this mixture was found to be 670uC bythermal analysis method. Mechanical alloying has alsobeen utilised as a means of synthesising submicrometreB4C particles by magnesiothermic reduction.98
Wang et al.99 have studied the synthesis of B4C fibre–MgO composites by combustion of B2O3zMgzCfibre
samples in an argon filled chamber. The degree ofconversion was influenced by pressure of the ambientargon gas which influences the evaporation of magne-sium and thereby the combustion temperature andconversion. Calcium can also be used as reductant inplace of Mg. Berchman et al.100 have recently reportedsynthesis of boron carbide powder by calciothermicreduction of borax (Na2B4O7) or B2O3 in presence ofcarbon at 1000uC in argon.
Though boron carbide has been produced by magne-siothermic reduction and used for applications definedby its high calorific value, the high cost of magnesiumwill soon make this process obsolete for regularproduction. Table 293–99 presents a summary of studieson synthesis of boron carbide by magnesiothermicreduction.
Synthesis from elementsSynthesis of boron carbide from its elements isconsidered uneconomical due to the high cost of
a,b long straight segments; c,d curly tufts6 Image (SEM) of high purity single crystalline boron carbide nanowires formed by thermal evaporation of B/B2O3/C pow-
der precursor at 1650uC under argon atmosphere:89 reprinted with permission from Elsevier, Chem. Phy. Lett., 2002,
364, Fig. 1 in p. 315
7 a perfect icosahedral B4C particle and b rod shaped
twinned particles by carbothermic reduction of B2O3
(scale bars: 10 mm):91 reprinted with permission from
American Chemical Society, J. Phys. Chem. B, 2004,
108B, Fig. 4 in p. 7653
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elemental boron and hence employed for specialisedapplications101,102 only, such as B10 enriched or verypure boron carbide. For synthesis of enriched boroncarbide, carbothermic reduction is not suitable due toloss of boron as well as boron hold-up in the furnaceand hence this process is the only suitable economicalmethod. Although formation of boron carbide from itselements is thermodynamically possible at room tem-perature, the heat of reaction (239 kJ mol21) is notsufficient to carry out in a self-sustaining fashion.103
Formation of boron carbide layer slows down furtherreaction, due to slow diffusion of reacting speciesthrough this layer, thus necessitating high temperatureand longer duration for complete conversion of theelements into the compound. For synthesis fromelements, boron and carbon are thoroughly mixed toform uniform powder mixture, which is then pelletisedand reacted at high temperatures of .1500uC in vacuumor inert atmosphere. The partially sintered pellet ofboron carbide is then crushed and ground to get fineB4C powder. To achieve a high purity product of B4C,high purity elemental boron powder produced by fusedsalt electrolytic process104,105 is often used.
Mechanical alloying of B–C mixtures followed byheat treatment is one of the methods being investigatedfor the synthesis of boron carbide. Room temperaturemilling is carried out in planetary mills for prolongedduration to activate the powders and the alloyed mixtureis then annealed to obtain boron carbide. Spark plasmasynthesis is a new technique, in which a pulsed high dccurrent is passed through the charge mixture containedin a cavity along with the application of uniaxialpressure. In this process, the start and completion offormation has been noted at 1000 and 1200uC respec-tively. Combination of mechanical alloying followed byspark plasma sintering has been studied by Hian et al.106
to obtain 95% pure boron carbide.
Shock wave technique has also been attempted forboron carbide synthesis from amorphous boron andgraphite powder107 using trimethyl enetrinitramine asdetonator. The resultant product exhibited severaldifferent morphologies, such as filaments, distortedellipsoid, plates and polyhedron particles of nanosize.In this technique reactants are kept inside a steelcontainer which is placed in plastic tube. A detonatoris placed between container and the plastic tube.
Table 1 Charge composition, processing conditions and product quality on synthesis of boron carbide by carbothermicreduction*
Serialno. Reactants Process type Process parameters Product quality Ref. (year)
1 B2O3zPC Batch (resistancefurnace)
2400uC Crystalline B4C 90% pure 69 (1933)
2 H3BO3zcharcoal Batch (arc furnace) Melting temperatureof charge
Boron carbide with 15%C 72 (1939)
3 H3BO3zPC Batch 1470uC; HR:100uC min21, 5 h, Ar
Crystalline B4C 25–30 mm 79 (2004)
4 B2O3zPC/carbon active Batch 1470uC; HR: 100u min21,1–5 h, Ar
Crystalline B4C 25–30 mm 78 (2006)
5 B2O3 and carbon Batch 1800uC, 20–300 min, Ar Crystalline B4C without freecarbon
92 (2004)
6 H3BO3zPC Batch (Acheson) .2000uC Partially sintered and denseproduct B4C conversion:69–73%
77 (1986)
7 B2O3zcarbon black/graphite/activated charcoal
Continuous (inductionheating)
2227uC Crystalline B4C 83 (1979)
8 H3BO3zVulcan XC-72carbon black
Continuous 1820uC; HR: 900uC s21,3 min, Ar
Equiaxed crystals of 0.5 mm 84 (1989)
H3BO3zacetylene carbonblack
2000uC; HR: 1000–2000uC s21, 3 min, Ar
Submicrometre particles0.1–0.2 mm
H3BO3zactivated carbon 1580uC; HR: 755uC s21,3 min, Ar
Submicrometre and uniformsized crystals
9 H3BO3zcorn starch Continuous 1950uC, Ar Submicrometre particles0.1 mm
85 (1992)
Boric oxide and carbon 1850uC, Ar 0.02–0.1 mm
*PC: petroleum coke, HR: heating rate.
Table 2 Charge composition, processing conditions and product quality on synthesis of boron carbide bymagnesiothermic reduction*
Serialno. Reactants Process type Process parameters Quality of B4C Ref. (year)
1 B2O3zMgzC Tubular furnace 950–1200uC, H2 Fine powder 93 (2002)2 B2O3zMgzC SHS … Fine powder 98% pure 96 (2003)3 B2O3zMgzC Batch 700uC, Ar, 1 h catalyst: K2SO4 Boron: 74.6% Carbon: 25.2% 95 (1967)4 B2O3zMgzC Mechanical alloying Rotation speed: 200 rev min21 Submicrometre particles 88 (2006)
Ball to load ratio: 5 : 1 72 h5 B2O3zMgzCFibre Combustion synthesis Ar B4C fibrezMgO composites 99 (1994)6 H3BO3zMgzC SHS 670uC, Ar 8–24 mm size, 8% free carbon 97 (2005)7 Na2B4O7zMgzC Continuous 1650–1700uC, H2 Powder boron: 77.5% Carbon:
21.3%94 (1958)
*SHS: self-propagating high temperature synthesis.
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Initiation of explosive detonation was carried out by anelectric detonator. After the shock treatments, 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 chemicalreaction is completed in micro to milliseconds. Hencethis is suitable for the preparation of crystals of variousmorphology and non-equilibrium phases which are hardto be produced in thermal equilibrium conditions.
A few attempts have been noticed on the preparationof nanostructure boron carbide from its elements. Weiet al.108 have prepared boron carbide nanorods byreacting carbon nanotubes (CNT) with boron powder at1150uC under argon atmosphere. Chen et al.109 havesynthesised boron carbide nanoparticles by reactingmultiwall CNT with magnesium diboride at 1150uC for3 h in vacuum. At this temperature, magnesium diboridedecomposes and gives elemental boron. Recently Changet al.110 has attempted the preparation of boron carbidenanoparticles (200 nm) by direct reaction betweenamorphous boron and amorphous carbon at 1550uC.The crystals obtained had a high density twin structureswith variation of B/C ratio from particle to particle.
Table 3106–108,110–113 gives a comparative summary ofstudies reported on the synthesis of B4C from itselements. Synthesis of boron carbide from its elementsis suitable for the production of pure B4C. Though thecost of production is high due to the high cost ofelemental boron, for specialised applications such as innuclear industry this method is preferred.
Vapour phase reactionSynthesis of boron carbide by carrying out reactionbetween boron and carbon containing gaseous specieshas been extensively studied. This method is gainfullyadopted for the formation of boron carbide coatings andsynthesis of powders and whiskers in submicrometresizes. Boron halides such as BCl3, BBr3 and BI3 aresuitable boron source but BCl3 is the most preferred dueto its ready availability and low cost. Apart fromhalides, borane (B6H6) and oxide (B2O3) are also usefulboron sources. Hydrocarbon gases such as CH4, C2H4,C2H6, C2H2 and carbon tetra chloride (CCl4) areemployed as carbon source. Synthesis of boron carbidetakes place in the reaction chamber, which is kept at a
desired temperature, pressure and atmosphere.Generally hydrogen is present in the atmosphere, whichreacts with the halogen forming hydrogen chloride asper the following reactions
4BCl3zCCl4z8H2?B4Cz16HCl (11)
4BCl3zCz6H2?B4Cz12HCl (12)
4BCl3zCH4z4H2?B4Cz12HCl (13)
The flow of reactants and other process parametersdecide the composition and structure of the productformed.
One such set-up for vapour phase reaction isdescribed by Bourdeau in his patent.114 The process ofproducing boron carbide by reacting a halide of boronin vapour phase with hydrocarbon in the temperaturerange 1500–2500uC has been explained. Clifton et al.115
described a process for producing boron carbidewhiskers in the size range of 0?05 to 0?25 mm by thereaction of B2O3 vapours with the hydrocarbon gasbetween 700 and 1600uC. James et al.116 have patented aprocess for the production of boron carbide whiskersand the use of catalytic elements to enhance the yieldof the gas phase reaction process. Dieter et al.117 havedescribed a process for the production of boron carbidepowder of fine size with a surface area >100 m2 g21.MacKinnon et al.118 have reported that when borontrichloride is reacted with CH4–H2 mixture in a radiofrequency argon plasma, boron carbides of variable B/Cratios are obtained as submicrometre powders, theproduct stoichiometry depending on the reactantcomposition.
Chemical vapour deposition (CVD)
Deposition of different types of boron carbide films(B13C2, B4C, metastable phases, highly strained struc-tures, etc.) by CVD techniques has been reported inliterature. The actual deposition is controlled by masstransfer and surface kinetics, which affects the stoichio-metry and properties of the boron carbide phases grown.Graphite, single crystal silicon, carbon fibre and boronare the substrate materials used for thin film synthesis.Generally the process is carried out in vacuum in the
Table 3 Charge composition, processing conditions and product quality on synthesis of boron carbide from elements*
Serialno. Reactants Process type Process parameters Product quality Ref. (year)
1 Amor. boronzAmor.carbon
Solid state thermalreaction
1550uC, 4 h, Ar Nanoparticles15–350 nm
110 (2007)
2 BzC Hot pressing 1800–2200uC, 3–4 h Articles of neartheoretical density
111 (1975)
3 BzC MAzannealing MA for 90 h Annealingat 1200uC
B4C with someunknown peaks
112 (2006)
4 BzC MAzspark plasmasintering
1650uC, 16 min 95% dense pelletof high purity boroncarbide
106 (2004)
5 Amor. boronzcarbonblack
Spark plasma synthesis .1200uC, 10 min Sintered B4C,disordered finecrystalline
113 (2005)
6 Amor. boronzgraphite Shock wave technique Detonator: trimethylenetrinitramine Detonationvelocity: 6.4 km s21
Nanosized particlesof crystalline B4C
107 (1996)
7 Amor. boronzCNT Solid state reaction 1150uC, Ar Straight nanorods 108 (2002)
*Amor.: amorphous; MA: mechanical alloying; CNT: carbon nanotube.
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temperature range of 450 to 1450uC. Substrate tempera-ture has strong influence on the process and productquality. High substrate temperature results in pooradhesion whereas deposition rate is low at lowtemperature. Amorphous boron carbide coating can beobtained at a low temperature of y500uC whereascrystalline film is obtained at higher temperatures.1100uC. Amorphous boron carbide coatings on SiChave been obtained by CVD from CH4–BCl3–H2–Armixtures at low temperature (900–1050uC) and reducedpressure (10 kPa).119
Preparation of boron carbide fibres by the reaction ofboron halides with woven cloth of carbonisable materialin hydrogen atmosphere has been patented by Waineret al.120,121 Jaziehpour et al.122 have prepared boroncarbide nanorods on graphite substrate at 1400uC byCVD using charge mixture of boron oxide, activatedcarbon and sodium chloride. Shu-Fang et al.123 havegrown novel boron carbide nanoropes by CVD using o-carborane (C2H12B10) as precursor and ferrocene(C10H10Fe) as catalyst. Karaman et al.124 have studiedthe kinetics of CVD of B4C on tungsten substrate usingBCl3–CH4–H2 gas mixture. They proposed that twomajor reactions take place during the process
BCl3 gð Þz 1
4CH4 gð ÞzH2 gð Þ?
1
4B4C sð Þz3HCl gð Þ (14)
BCl3 gð ÞzH2 gð Þ?BHCl2 gð ÞzHCl gð Þ (15)
Reaction rate of boron carbide formation is lower thanthat of dichloroborane formation over the entire rangeof temperatures (1000 to 1400uC) studied. Schouleret al.125 obtained BCx (x>3) phase having whisker-likemorphology by reacting BCl3 and B6H6 at 1000uC onquartz substrate in presence of hydrogen and nickel.Sezer and Brand126 have 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 application areas, particularly in thenuclear industry. They have also discussed the thermo-dynamic modelling used by many researchers and haveconcluded that the process takes place far fromequilibrium and that, thermodynamic modelling is notsufficient to represent experimental deposition condi-tions. Table 4114–118,127–147 presents a summary ofstudies reported on vapour phase reaction synthesis ofboron carbide.
Many modifications such as laser CVD (LCVD),plasma enhanced CVD (PECVD), hot filament CVD(HFCVD), etc. have been tested for the formation ofboron carbide films.
Laser CVD
In this technique the energy of a laser beam is used toheat the surface of a substrate to the temperaturerequired for chemical deposition. It allows superbspatial resolution (y5 mm) because the chemical reac-tions are restricted to the heated zone created by thefocused laser spot, in contrast to the traditional CVDfurnace which heats the entire surface of the sub-strate.148 Laser CVD results in deposits with high purity,high degree of crystallinity, low porosity, excellent
mechanical properties and thermal stability. Theseattributes are the result of deposition occurring oneatom at a time. Deposition rates in LCVD techniquesare orders of magnitude higher than that in traditionalCVD. The deposition rate and surface microstructurestrongly depend on laser power and hydrogen content inthe gas phase.127 Control of laser power density allowsfor codeposition of r-(B4C) and disordered graphite,which can be beneficial for tailoring the thermal andelectronic properties of boron carbide.128 The reactiveatmosphere composition is the most important para-meter in laser CVD. When the relative amount ofcarbon to boron in the gas phase is high, a disorderedgraphitic phase is deposited along with boron carbideand when the carbon is low, tetragonal and metastableboron rich phase, B25C is codeposited with boroncarbide.129 Patterned deposits can be obtained by directwriting 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 thesubstrate parallel to the laser beam axis at a rate equalto the deposition rate of the fibre. Direct writing andfibre growth methods can be combined to produce three-dimensional structures.148
Plasma enhanced CVD
In PECVD chemical 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 filledwith the reacting gases. The necessary energy for thechemical reaction is not introduced by heating the wholereaction chamber but just by heated gas or plasma. Thedeposition takes place at lower temperature as comparedto traditional CVD. Since the formation of the reactiveand energetic species in the gas phase occurs by collisionin the gas phase, the substrate can be maintained at alow temperature. Hence, film formation can occur onsubstrates at a lower temperature than is possible in theconventional CVD process, which is a major advantageof PECVD.149
Plasma enhanced CVD has been used by manyresearchers for the fabrication of boron carbide (B-C)diodes which could accurately detect single neutrons,giving very high efficiencies. These diodes have beenused to fabricate the first real time, solid state neutrondetectors which are more efficient and reliable than anyother neutron detecting semiconductor reported todate.150 Lee et al.25,151 have fabricated photoactive p-nhetrojunction diode by PECVD of boron carbide thinfilms from nido-pentaborane (B5H9) and methane(CH4)on Si (111). A B5C/Si(111) hetrojunction diode by asynchrotron radiation induced decomposition of ortho-carborane fabricated by Byun et al.152 has been found tobe comparable with PECVD diodes. Hwang et al.153
have successfully fabricated and tested a boron carbide/boron diode on aluminium substrates and a boroncarbide/boron junction field affect transistor.Robertson et al.154 have fabricated real time solidstate neutron detector by PECVD using closo-1,2-dicarbadodecaborane. Adenwalla et al.155 have reportedthe fabrication and characterisation of boron carbide/silicon carbide hetrojunction diodes by PECVD. Theliterature is abundant on various possible combinations
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of source, method of fabrication, uses, etc. and only afew examples are given above.
Hot filament CVD
Hot filament CVD is an attractive technique due to itssimple design and its amenability to fundamental
chemical kinetic modelling in understanding the processchemistry. Hot filament CVD systems are based onthermal catalytic cracking of the precursors on thesurface of a high temperature filament usually rangingfrom 1000 to 2500uC. The substrate materials are usuallyheated by radiation from the hot filament and the
Table 4 Charge composition, processing conditions and product quality on vapour phase synthesis of boron carbide*
Serialno. Process Reactants Process parameters Product quality Ref. (year)
1 Vapour phasereaction
BCl3zCH4 1900uC; vacuum:5 mm of Hg
Boron carbide crystals 114 (1967)
2 Vapour phasereaction
B2H6zC2H2 Exothermic reactionand needs to beignited only by sparkplug
Amor. porous boroncarbide powder ofsubmicrometre size
117 (1977)
3 Vapour phasereaction
B2O3zCH4 1075uC, 18 h Whiskers length:0.5–4 inch; diameter:0.05–0.25 mm
115 (1970)
4 Vapour phasereaction
BCl3zCH4zH2 1650uC, 5 h; vacuumcatalyst: VCl4
Whiskers length: 50 mm;diameter: 10 mm
116 (1969)
5 RF plasmaassisted synthesis
BCl3zCH4zH2 Ar plasma Submicrometre size powder 118 (1975)
6 CVD BCl3zCH4zH2 1350uC; Sub.: carbonfibre
Crystalline B4C coating 130 (1996)
7 CVD BCl3zCH4zH2 1127–1227uC; Sub.:boron coated Mo,vacuum
Metastable phases, highlystrained microstructure
131,132 (1989)
8 CVD BCl3zCCl4zH2 1550uC, 4–5 h, Sub.:graphite
Pure long crystalline B4C;hardness: 41¡2.7 GPa
133 (1965)
9 CVD BCl3zC3H8zH2 850–1000uC, 3–6 h;Sub.: graphite cloth;vacuum: 15–25 torr
Amor. coating 134 (1981)
10 CVD BCl3zCH4zH2 1300uC,6 h; Sub.:graphite; vacuum:10 mm of Hg
B4C coating 135 (1968)
11 CVD BCl3zCH4zH2 1300uC, 3 h; Sub.:tungsten, graphite
B4C coating (B: 74 to 76%)specific gravity of 2.32 gm cm23
136 (1974)
12 CVD BCl3zCH4zH2 800–1050uC, vacuum Amor. boron carbide 137 (2006)13 CVD BCl3zCH4zH2 1000–1400uC; Sub.:
tungstenCrystalline B4C 124 (2006)
14 Laser CVD BCl3zCH4zH2 Laser: CO2; Sub.:fused silica; Ar pressure:atmospheric
Crystalline B4C 128 (1999)
15 Laser CVD BCl3zCH4zH2 Laser: CO2; Sub.:fused silica
Crystalline B4C 138 (1996)
16 Laser CVD BCl3zCH4/C2H4zH2
Laser: CO2 Ultra fine and crystalline B4C 139 (1990)
17 Laser CVD BCl3zC2H4 Laser: CO2; Sub.: fusedsilica, Ar
Adherent, crystalline B4C,15–22%C
127 (2002)
18 Laser CVD BCl3zCH4zH2 Laser: CO2; Sub.: fusedsilica,
Crystalline B4C and B25C 129 (1997)
19 Pulsed laserinduced CVD
C6H6zBCl3 Nd:YAG laser 14 to 33 nm B4C crystalsencapsulated in graphite
140 (1999)
20 Plasma enhancedCVD
C2B10H12
(orthocarborane)1100–1200uC; Sub.:Si (100)
B4C nanowires diameter:18–150 nm; length: 13 mm
141 (1999)
21 Microwave plasmaassisted CVD
BBr3zCH4zH2 500–600uC; Sub.:graphite
Amor. boron carbide largecomposition range (0 to40 at.-%C
142 (1990)
22 Supersonic plasmajet CVD
BCl3zCH4 500–600uC; Sub.: Si(100), ArzH2
Microcrystalline film hardness:22–32 GPa
143 (1998)
23 Thermal CVD BCl3zCH4zH2 1600uC, 3–6 h BCl3:6–15 mL min21; CH4:2–5 mL min21; H2:500 mL min21
Various composition betweenB4C and B13C2
144 (1992)
24 Hot wall CVD BCl3zCH4zH2 1000 to 1400uC; Sub.:graphite, vacuum
Crystalline B13C2, long columnargrains
145 (1998)
25 Hot filament activatedCVD
BCl3zCH4zH2 2100uC (filament)450uC (substrate);Sub.: Si (100), vacuum
Amor. boron carbide, high purityand good adhesion
146 (1994)
26 Electron beamevaporation
BzC Room temperature;Sub.: Si (100)
Thin films of crystalline boroncarbide
147 (2008)
*CVD: chemical vapour deposition; Sub.: substrate; Amor.: amorphous; RF: radio frequency.
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substrate surface temperature is usually ,500uC.146 Thedeposition is carried out under high vacuum conditionsto avoid oxygen contamination of the boron carbidephase. Deshpande et al.146 have obtained adhesivecoating of boron carbide on silicon substrate and thewear resistance of the coated surface was found to beextremely high when tested using a WC/Co ball as thepin.
Vapour phase synthesis methods are suitable for thinfilm coating of boron carbide and preparation of finepowder, fibres, whiskers, etc. However the powdersproduced by this process are generally non-stoichiometric and not suitable for fabrication of denseproducts. These methods are best suited for laboratorystudies.
Synthesis from polymer precursorsAs an alternative to high temperature reaction techni-ques, there is great interest in development of polymerprecursors to produce ceramic materials at lower tem-peratures. Some of the boron loaded organic com-pounds such as carborane (C2BnHnz2), triphenylborane,polyvinyl pentaborane and borazines on pyrolysis yieldB4C. Generally this process is carried out in the tem-perature range 1000–1500uC in vacuum or inert atmo-sphere. A US patent156 describes a process for making afree flowing boron carbide powder from boric acid andsugar. The mixture dissolved in ethylene glycol is driedin air at 180uC and then heated in hydrogen at 700uC.This reaction product is ground and fired at 1700uC for7 h to yield fine boron carbide powder. Mondal et al.157
describe a low temperature synthetic route in which apolymeric precursor is synthesised by the reaction ofboric acid and polyvinyl alcohol, which on pyrolysis at400/800uC gives crystalline boron carbide. Sinha et al.158
have presented a process in which, a stable gel is formedfrom aqueous solution of boric acid and citric acid. Thisgel is further processed to yield a precursor which onheating under vacuum to 1450uC produces B4C.Economy et al.159 have prepared boron carbide fibre
by heating ‘amine treated B2O3 fibre’ in inert atmo-sphere at 2000–2350uC. Cihangir et al.160 have devel-oped a method based on sulphuric acid dehydration ofsugar to synthesise a precursor material which onheating to temperature between 1400 and 1600uCyields crystallised B4C and B4C/SiC composites.Table 5156–159,161–167 gives the comparative summary ofstudies reported on the synthesis of B4C using polymerprecursors.
Liquid phase reactionSynthesis of ultra fine boron carbide powder using liquidprecursors has been attempted by a few. This method isalso known as solvothermal process or coreductionmethod. Unlike conventional methods, this can beoperated at much lower temperatures to yield boroncarbide of desired properties. Shi et al.168 have studiedthe formation of ultra fine boron carbide powders bycoreduction of boron tri bromide and carbon tetra-chloride using sodium as reducing agent as per thefollowing reaction
4BBr3zCCl4z16Na?B4Cz4NaClz12NaBr (16)
The reaction was carried out in an autoclave at 450uC.B4C crystals obtained were composed of uniformspherical (80 nm dia) and rod-like (200 nm diameterand 2?5 mm long) particles (Fig. 8).168 Gu et al.169 haveobserved the formation of nanocrystalline B4C bysolvothermal reduction of CCl4 using lithium inpresence of amorphous boron powder in an autoclaveat 600uC.
4BzCCl4z4Li?B4Cz4LiCl (17)
Hexagonal B4C crystals with a particle size of approxi-mately 15–40 nm diameters were obtained.
Ion beam synthesisBoron carbide thin films can be grown by directdeposition of Bz and Cz ions. In this process,parameters such as ion energy, ion flux ratio of different
Table 5 Charge composition, processing conditions and product quality on synthesis of boron carbide using polymerprecursor
Serialno. Polymer precursors
Temperature,uC Atmosphere
Holdingtime, h Product quality Ref. (year)
1 Polyvinyl borate 1300 Argon 5 Crystalline 161 (2009)2 Reaction product of H3BO3
and citric acid1500 Vacuum 2.5 Crystalline, micrometre
sized, free carbon: 2.38%162 (2006)
3 Reaction product of H3BO3
and polyvinyl alcohol400–800 Air 3 Crystalline (orthorhombic),
boric acid as impurity157 (2005)
4 Reaction product of H3BO3
and citric acid1450 Vacuum 2 Crystalline, free carbon
11.1 wt-%158 (2002)
5 Solution product of H3BO3
and glucose1400 … … B/C composite containing
crystalline B4C163 (2002)
6 Condensation product of H3BO3
and 2-hydroxy benzyl alcohol (HBA)1500 Ar 4 Crystalline 164 (1999)
7 Polyvinyl pentaborane 1000 Ar 8 Amorphous, black andshiny
165 (1988)
1450 Ar 48 crystalline 165 (1988)8 Condensation product of H3BO3
and 1,2,3 propanetriol1400 Ar 2 Crystalline 166 (1985)
9 Ethyl Decaborane (C2H5B10H13) 1215 Vacuum ,1 Crystalline B4C coating(0.005 inch) on tungstenwire
167 (1969)
10 Solution product of H3BO3, sugarand ethylene glycol
1700 H2 … Crystalline B4C powder 156 (1975)
11 Amine treated B2O3 fibre 2000–2350 Inert atmosphere … Boron carbide fibre 159 (1974)
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Ltd ion species and the substrate temperature, which can be
independently controlled, could be advantageously usedfor obtaining the preferred composition and nature ofthe boron carbide film. Ronning et al.170 have grownthin film of boron carbide (BxC) by direct ion beamdeposition on silicon using an ion energy of 100 eV atroom temperature. Todorovic et al.171 have observed theformation of amorphous boron carbide (BxC) bybombardment of Bz and B3z ions on fullerene. Beamenergies were in the range of 15 keV for Bz to 45 keVfor B3z and fluences were between 261014 and261016 cm22.
Vapour liquid solid (VLS) growthBoron carbide whiskers can be grown by carbothermalVLS growth mechanism. This mechanism involves thetransport of boron and carbon as gas phase species to aliquid catalyst metal (Fe, Ni or Co) in which whiskerconstituents get dissolved. When the catalyst becomessupersaturated with boron and carbon, boron carbidewhiskers precipitate out of the metal droplets. Carlssonet al.172 have prepared B4C whiskers and platelets usingthis technique. B2O3 and carbon black were used assource of boron and carbon respectively. NaCl and Cowere added to facilitate the growth of whiskers. B2O3
reacts with NaCl to form BCl, which along with carbondissolve in liquid cobalt and then precipitate as boroncarbide whiskers. Rao et al.173 have studied theformation of boron carbide whiskers using K2CO3 andNiCl2 as a low melting liquid and catalyst to enhance theformation of B4C whiskers and platelets. An et al.174
have used gallium oxide and sodium chloride to prepareboron carbide nanobelts having a length of around 1 to10 mm and thickness of around 80 to 150 nm, which isshown in Fig. 9.174 Ma et al.175 have investigated thegrowth of boron carbide nanowires by the addition ofiron to the precursor mixture containing carbon, boronand boron trioxide. This resulted in reduction ofdiameter of nanowires from 50–200 nm to 10–30 nm.Scanning electron micrograph of the nanowires is shownin Fig. 10.175 A comparative study of various methodsof boron carbide synthesis is presented in Table 6.
Some of the attempts to produce boron carbidecannot fall into any of the classifications discussed
above. Thakkar et al.176 have synthesised high purityultra fine boron carbide powders by reacting B2O3 withmethane in a non transferred arc dc thermal plasmareactor. A recent article177 explains the process ofmaking boron carbide–carbon eutectic containing39 wt-%C by melting B2CN in graphite crucible at2600uC.
Boron carbide powder is either utilised directly orconsolidated to dense bodies. Various methods ofdensification, the mechanisms involved and the productquality are discussed in the following pages. Densi-fication techniques can be broadly classified as pressure-less sintering and pressurised sintering. Atmospheric/reaction/microwave and thermal plasma sintering aretermed as pressureless sintering techniques. The nuancesof densification of powder compacts, complexity and thereasons for incomplete densification by pressurelesssintering are discussed in detail by Lange.178
Pressurised sintering can be classified as solid and gascompaction methods. Solid compaction methods are hot
8 Image (TEM) of B4C rod-like particles (200 nm diameter
and 2?5 mm long) prepared at 450uC by sodium reduc-
tion of BBr3 and CCl4:168 reprinted with permission
from Elsevier, Solid State Commun., 2003, 128,
Fig. 3(c) in p. 7
9 Boron carbide nanobelts prepared by VLS growth from
charge of boron oxide, activated carbon, gallium oxide
and sodium chloride at 1400uC:174 reprinted with per-
mission from Trans. Tech. Publications, Key Eng.
Mater., 2007, 336–338, (III), Fig. 1 in p. 2167
10 Boron carbide nanowires prepared by VLS growth
with help of iron addition:175 reprinted with permission
from American Chemical Society, Chem. Mater., 2002,
14, Fig. 5(b) in p. 4405
Suri et al. Synthesis and consolidation of boron carbide: a review
16 International Materials Reviews 2010 VOL 55 NO 1
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pressing, spark plasma sintering and super high pressuresintering. Gas compaction methods are hot isostaticpressing and high pressure gas reaction sintering.
Densification of boron carbideIn spite of its high temperature strength, application ofB4C is rather limited, in real life, due to difficulties indensification, low fracture toughness and low oxidationresistance beyond 1000uC. Consolidation of B4C iscomplicated due to its high melting point, low self-diffusion coefficient and high vapour pressure. Very highsintering temperatures are required for densification dueto the presence of predominantly covalent bonds in B4C.Boron carbide particles generally have a thin coating ofsurface oxide layer which hinders the densificationsprocess. At temperatures ,2000uC, surface diffusionand evaporation condensation mechanism occur, whichresults in mass transfer without densification. Densi-fication is achieved only at temperatures .2000uC, bygrain boundary and volume diffusion mechanisms. Athigher temperature exaggerated grain growth also takesplace resulting in poor mechanical properties. One moreobservation at temperatures .2150uC is volatilisation ofnon-stoichiometric boron carbide, leaving minute car-bon behind at the grain boundaries.
Dole et al.179 have observed the microstructure of B4Ccompacts fired above 2000uC to be highly porousinterconnected structure with clusters of grains con-nected by small neck like regions and separated by large,channelled porosity. Grabchuk et al.180–182 have foundthat shrinkage starts at 1500uC, recrystallisation above1800uC and rapid grain growth above 2200uC. Attemperatures above 2250uC, the sintered body contains,5% residual porosity. Lee et al.183,184 have observedthe start of densification at 1800uC, rapid increase in
densification 1870–2010uC and a slow down in densifi-cation rate 2010–2140uC. The surge in densification1870–2010uC is attributed to the presence of oxide layerwhich helps in precipitation of B4C through liquid B2O3
or due to evaporation and condensation of rapidlyevolving oxide gases (BO and CO). Slower densificationat temperatures above 2010uC is attributed to evapora-tion and condensation of B4C. Figure 11183 shows thechanges in weight, dimension and grain size whilesintering of boron carbide.
Densification of boron carbide without deteriorationof mechanical properties can be achieved either by usinga suitable sintering aid and/or applying the externalpressure (e.g. hot pressing, hot isostatic pressing).Selection of the additive and the method of consolida-tion are generally dictated by the end use of the productand the properties that are required. The additive byitself or due to in situ reaction with boron carbide wouldform a non volatile second phase aiding in densificationand property enhancement. Hence, selection of theadditive should be directed towards the formation of asuitable structure providing the correct properties foruse. Recent or advanced techniques such as microwave/spark plasma sintering, explosive compaction, etc. helpto obtain dense products without microstructuralcoarsening. These techniques are presently limited tolaboratory scale only. Detailed literature survey onpressureless sintering with or without sintering aids, hotpressing, hot isostatic pressing, spark plasma andmicrowave sintering of boron carbide are presented inthe following sessions.
Pressureless sinteringPressureless sintering is a simple and economic processto produce dense compacts. This operation is carried outin two steps. In the first step green compacts with
Table 6 Comparison of boron carbide synthesis methods*
Method Boron source Carbon source Advantage Disadvantage
Carbothermic reduction H3BO3 or B2O3 PC, graphite,activated carbon
Cheap raw material,suitable for commercialproduction
High boron losses,obtained in lump form,need grinding for powderproduction
Magnesiothermic reduction B2O3 or Na2B4O7 PC, graphite,activated carbon
Fine powder, exothermicreaction, suitable for SHSprocess
Product contaminatedwith Mg, MgB2
Synthesis from elements Boron PC, graphite,activated carbon
No loss of boron, goodcontrol over purity andcarbon content of product
High cost of elementalboron
Vapour phase synthesis BCl3, BBr3, BI3,B6H6, B2O3
CH4, C2H4, C2H6,C2H2, CCl4
Suitable for thin films, finepowder, fibers, whiskers
Difficult to produce B4Cpowder suitable fordensification, not amenablefor large scale production
Synthesis from polymerprecursors
Boric acid, B2O3,polyvinyl pentaborane,polyvinyl borate, ethyldecaborane
Polyvinyl alcohol,citric acid, hydroxylbenzyl alcohol,sugar, ethyleneglycol
Low temperature process High free carbon content,still in laboratory stage
Liquid phase reaction BBr3, boron CCl4 Low temperature process,suitable for nanoparticles
Need of reactive metalsuch as Na or Li, newmethod of synthesis
Ion beam synthesis Boron Carbon Suitable for BxC Only for thin films, ofacademic interest only
Vapour liquid solid growth B2O3 Carbon black Suitable for whisker Need of molten metalcatalyst, of academicinterest only
*PC: petroleum coke; SHS: self-propagating high temperature synthesis.
Suri et al. Synthesis and consolidation of boron carbide: a review
International Materials Reviews 2010 VOL 55 NO 1 17
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sufficient handling strength are prepared by uniaxial diecompaction. These green pellets are then fired at chosenhigh temperatures in controlled atmosphere. A recentlydeveloped new technique, combustion driven compactprocess, yields much higher green density and strengththan the normal die compaction.185 In this process, highpressure generated by ignition of a combustion gasmixture which raises the pressure in the chamberdramatically in a very short period of time and pushesdown the top ram on the powder at an extremely highspeed realising the compaction.
Sintering of B4C powder compacts is commonlyperformed in an inert gas medium. But the applicationof vacuum helps in evaporation of the surface oxidelayer and also prevents further oxidation, there bypromoting the sintering mechanisms. Removal of theoxide layer by heating in a reducing atmosphere beforesintering also has a similar effect. Literature data onpressureless sintering of boron carbide and the productevaluation are presented in Table 7.179,183,186–204
Increase in particle surface area (9 to 17 m2 g21) andsintering temperature (2100 to 2190uC) give higherdensities (56 to 71% TD).187 Densities of .90% TDare achieved by sintering at a temperature of .2200uCwith particles close to or ,1 mm size. Grain coarseningis the common feature in compacts with high densitiesobtained by pressureless sintering.191,193 Microstructuresof samples with 87 and 93% TD, obtained by pressure-less sintering of 0?8 mm median diameter powders at2300 and 2375uC are presented in Fig. 12.193 Grain sizesare in the range 40–100 mm indicating large graingrowth. Surface to surface mass transport which isactive at temperatures below which densification canproceed is responsible for the coarsening process. Athigher temperatures, vapour phase diffusion of boroncarbide is the important transport mechanism forcoarsening. Rapid heating is helpful in achieving higherdensities with fine microstructure, as the compacts canbe heated to a temperature where densification can takeplace before the microstructure becomes highly coar-sened.179,183,188,205 Appearance of twins in the grains ischaracteristic of boron carbide. These twins slowly
vanish during high temperature annealing. Vickershardness and flexural strength of the pressurelesssintered boron carbide samples are in the range 18–24 GPa and 120–200 MPa respectively, which are lowerthan theoretical values. One can conclude that, withpure B4C, a densification .90% TD is possible only atvery high sintering temperatures of y2300uC. Suchcompacts have a coarse grained microstructure of
11 Sintering of boron carbide compact: change in weight, dimension, grain size and coefficient of thermal expansion up
to 2300uC:183 reprinted with permission from Wiley-Blackwell, J. Am. Ceram. Soc., 2003, 86, (9), Fig. 8 in p. 1472
12 Microstructure of pressureless sintered boron carbide
(0?8 mm) at a 2300uC and b 2375uC showing grains in
range 40–100 mm indicating large grain growth:193 rep-
rinted with permission from Elsevier, Ceram. Int.,
2006, 32, Fig. 2(b) and (g) in p. 230–231
Suri et al. Synthesis and consolidation of boron carbide: a review
18 International Materials Reviews 2010 VOL 55 NO 1
Pub
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d by
Man
ey P
ublis
hing
(c)
IOM
Com
mun
icat
ions
Ltd
Ta
ble
7P
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de
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505
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,A
r94. 0
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2B
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;D
505
0. 8
;S
S:
15–20
2190uC
,1
h,
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invacuum
)94–95
……
……
188
(2004)
3B
4C
2250uC
65%
Coars
e…
……
179
(1989)
B4C
D505
12300uC
70–72%
Coars
eB
4C
z6
wt-
%C
SS
:12
2300uC
.95%
Fin
e4
B4C
D50(
12150uC
,15
min
,A
r78
B4C
:6
……
…189
(1981)
B4C
z3
wt-
%C
(phenolic
resin
)S
S:
22
96
B4C
:4
3. 2
353
5B
4C
D50,
52175uC
,15
min
,A
r…
B4C
:105
190
(1987)
B4C
z(p
oly
carb
osila
nez
phenolic
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)S
S:
10. 5
95
B4C
:28
B/C
:4. 3
2S
iC:
,3
6B
4C
D50(
0. 8
42250uC
91. 3
–92. 7
B4C
:2. 5
8–
3. 1
1…
……
183
(2003)
B4C
z3
wt-
%C
(phenolic
resin
)S
S:
18. 8
2250uC
98. 4
–98. 6
B4C
:2. 2
6–2. 4
……
…B
/C5
3. 7
67
B4C
2200uC
,1
h78. 6
B4C
:28
174
191
(2003)
B4C
2250uC
,1
h82. 5
B4C
:50
…B
4C
z3
wt-
%C
SS
:2. 5
32250uC
,1
h92
B4C
:13
350
B4C
z5
wt-
%C
2250uC
,1
h93
…B
4C
z7. 5
wt-
%C
2250uC
,1
h89
…B
4C
z9
wt-
%C
2250uC
,1
h86
…8
B4C
D50<
32250uC
,2
h,A
r95. 5
19–21
……
192
(2005)
B4C
z4%
CS
S:
18. 8
;O
:1. 2
3%
;N
:0. 4
%2250uC
,2
h,
Ar
97. 7
19–21
……
B4C
z4%
BC
arb
on
bla
ck:
D505
20
nm
;S
S:
120
2250uC
,2
h,A
r87. 0
18–20
……
B4C
z4%
SiC
SiC
(SS
):11. 5
92250uC
,2
h,
Ar
90. 8
19–23
……
B4C
z4%
TiB
2TiB
2:
D90<
42250uC
,2
h,
Ar
95. 4
21–25
9B
4C
2375uC
,1
h,
vacuum
93
B4C
:50–120
24–25
……
193
(2006)
B4C
z1
wt-
%C
D505
0. 5
to2
2325uC
,1
h,
vacuum
91
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:y
20
B4C
z3
wt-
%C
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2325uC
,1
h,
vacuum
90
B4C
:y
10
B4C
z5
wt-
%Z
rO2
B:
78%
;C
:19. 0
5%
;O
:0. 5
%2275uC
,1
h,
vacuum
93
B4C
:y
532
B4C
z5
wt-
%TiB
22375uC
,1
h,
vacuum
82
B4C
:y
15
10
B4C
D505
1. 3
32150uC
,vacuum
86
B4C
:30
22
2. 2
220
213
(2008)
B4C
z5
wt-
%TiB
2S
S:
6. 6
490
…26
2. 6
260
B4C
z10
wt-
%TiB
2B
/C5
3. 8
to3. 9
93
B4C
:17
29
2. 8
290
B4C
z15
wt-
%TiB
296
…31
2. 8
5360
B4C
z20
wt-
%TiB
296
B4C
:14
29
2. 9
320
B4C
z25
wt-
%TiB
298
…26
3. 0
5280
B4C
z30
wt-
%TiB
298. 5
B4C
:10
23
3. 4
270
Suri et al. Synthesis and consolidation of boron carbide: a review
International Materials Reviews 2010 VOL 55 NO 1 19
Pub
lishe
d by
Man
ey P
ublis
hing
(c)
IOM
Com
mun
icat
ions
Ltd
Seri
al
no
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co
mp
osit
ion
,w
t-%
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Sin
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hard
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GP
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Fle
xu
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str
en
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Pa
Ref.
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11
B4C
2050uC
,1
h,
inert
74
18
1. 9
195
(2006)
B4C
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1. 3
32150uC
,1
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82
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B4C
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inert
85
23
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B4C
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wt-
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lcTalc
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47. 7
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gO
2150uC
,1
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inert
90
25
2. 3
B4C
z20
wt-
%ta
lc2150uC
,1
h,
inert
92
26
2. 4
B4C
z25
wt-
%ta
lc2150uC
,1
h,
inert
95
27
2. 6
B4C
z30
wt-
%ta
lc2150uC
,1
h,
inert
98
23
2. 7
12
B4C
2275uC
,1
h,
vacuum
86. 6
3B
4C
26. 8
5221
(2008)
B4C
z2. 5
wt-
%Z
rO2
B4C
:D
50<
189. 5
6B
4C
,Z
rB2
…B
4C
z5
wt-
%Z
rO2
B/C
54. 0
993. 8
6B
4C
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rB2
31. 6
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4C
z10
wt-
%Z
rO2
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rg
rad
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4C
z15
wt-
%Z
rO2
95. 5
B4C
,Z
rB2
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9B
4C
z20
wt-
%Z
rO2
93. 0
8B
4C
,Z
rB2
30. 9
5B
4C
z25
wt-
%Z
rO2
95. 8
2B
4C
,Z
rB2
31. 7
1B
4C
z30
wt-
%Z
rO2
92. 3
4B
4C
,Z
rB2
31. 0
7B
4C
,Z
rB2
13
B4C
2050uC
,1
h,
Ar
72
19
2. 0
190
196
(2006)
B4C
2150uC
,1
h,
Ar
75
22
2. 1
200
B4C
z5
wt-
%Z
rO2
D505
12150uC
,1
h,
Ar
79
25
2. 2
205
B4C
z10
wt-
%Z
rO2
SS
:14
2150uC
,1
h,
Ar
86
27
2. 6
210
B4C
z15
wt-
%Z
rO2
ZrO
2z
3w
t-%
Y2O
32150uC
,1
h,
Ar
97
28
2. 9
220
B4C
z20
wt-
%Z
rO2
D505
0. 8
2150uC
,1
h,
Ar
97
29
3. 0
235
B4C
z25
wt-
%Z
rO2
SS
:16
2150uC
,1
h,
Ar
97
27
3. 0
260
B4C
z30
wt-
%Z
rO2
2150uC
,1
h,
Ar
98
26
3. 1
340
14
B4C
2190uC
,1
h,
Ar
62
50
197
(1999)
B4C
z5
wt-
%Ti
2190uC
,1
h,
Ar
67
…B
4C
z10
wt-
%Ti
2190uC
,1
h,
Ar
73
…B
4C
z15
wt-
%Ti
D50
,1
2190uC
,1
h,
Ar
78
…B
4C
z20
wt-
%Ti
SS
:9–17
2190uC
,1
h,
Ar
86
…TiO
2:
D50,
2(u
pto
1500uC
:vacuum
)…
B4C
z5
wt-
%TiO
22160uC
,1
h,
Ar
72
B4C
:10
200
B4C
z10
wt-
%TiO
22160uC
,1
h,
Ar
77
TiB
2:
5–7
300
B4C
z15
wt-
%TiO
22160uC
,1
h,
Ar
88
350
B4C
z20
wt-
%TiO
22160uC
,1
h,
Ar
95
420
15
B4C
2190uC
,1
h,
Ar
(up
to1500uC
:vacuum
)71
120
187
(2000)
B4C
z10
wt-
%TiO
2D
505
5to
773
B4C
:10
200
B4C
z20
wt-
%TiO
2S
S:
17
75
TiB
2:
5300
B4C
z30
wt-
%TiO
2TiO
2:
D100(
280
330
B4C
z40
wt-
%TiO
293
400
Ta
ble
7C
on
tin
ue
d
Suri et al. Synthesis and consolidation of boron carbide: a review
20 International Materials Reviews 2010 VOL 55 NO 1
Pub
lishe
d by
Man
ey P
ublis
hing
(c)
IOM
Com
mun
icat
ions
Ltd
Seri
al
no
.M
ate
rial
co
mp
osit
ion
,w
t-%
Sta
rtin
gp
ow
der
deta
ilsP
rocessin
gco
nd
itio
ns
Sin
tere
dd
en
sit
yr
th,
%M
icro
str
uctu
re,
mm
Vic
kers
hard
ness,
GP
aK
IC,
MP
am
1/2
Fle
xu
ral
str
en
gth
,M
Pa
Ref.
(year)
16
B4C
z0
to30
wt-
%TiO
2z
1to
6w
t-%
CB
4C
:D
505
0. 6
39
1900–2050uC
,1
h,
Ar
Up
to99%
B4C
:22¡
2…
3. 2
–3. 7
1350–513
216
(1996)
SS
:19. 8
TiB
2:
6–10
Purity
:99. 8
9%
TiO
2and
C:
sub
mic
rom
etr
e17
B4C
D505
1. 6
(B/C
53. 9
)2180uC
,2
h,
Ar
83
……
198
(2007)
B4C
z22
vol.-%
ZrO
2O
2:
1. 7
%98
27–31
350
B4C
z28
vol.-%
TiO
2Z
rO2:
sub
mic
rom
etr
e98
28–33
375
B4C
z8
vol.-%
Y2O
3TiO
2:
sub
mic
rom
etr
e97. 5
27–30
180
Y2O
3:
sub
mic
rom
etr
e18
B4C
2150uC
,15
min
,A
r85
B4C
:3. 6
3199
(1992)
B4C
z1
wt-
%A
l 2O
3D
50<
0. 9
(B/C
53. 9
)92
B4C
:5. 3
5B
4C
z2
wt-
%A
l 2O
3(1
wt-
%O
2and
0. 4
%N
2)
93
B4C
z3
wt-
%A
l 2O
3A
l 2O
3:
99. 9
9%
pure
96
B4C
:7. 0
7B
4C
z4
wt-
%A
l 2O
388
B4C
z5
wt-
%A
l 2O
386
B4C
:8. 9
219
B4C
z1. 5
%TiC
D100:
sub
mic
rom
etr
e2175uC
,2
h,
Ar
96. 3
B4C
:12,
TiB
24. 0
286
200
(1998)
B4C
z1. 5
%TiC
SS
:15. 0
2200uC
,2
h,
Ar
94. 9
B4C
:18,
TiB
231
3. 7
266
B4C
z3. 0
%TiC
O2:
0. 9
8%
;N
2:
0. 3
2%
2200uC
,2
h,
Ar
96. 6
B4C
:11,
TiB
2,
C29
B4C
z4. 5
%TiC
FC
:0. 6
4%
2200uC
,2
h,
Ar
98. 1
B4C
:8,
TiB
2,
C28
B4C
z6. 0
%TiC
TiC
:D
505
1. 5
2200uC
,2
h,
Ar
98. 4
B4C
:5,
TiB
2,
C28
O:
0. 5
3%
;N
:0. 1
0%
20
B4C
D505
0. 0
82260uC
,15
min
,A
r–N
271. 9
201
(1977)
B4C
z1. 0
wt-
%B
eC
SS
:16. 1
2130uC
,15
min
,A
r–N
280. 1
B4C
z1. 0
wt-
%B
eC
B/C
:3. 5
72230uC
,15
min
,A
r–N
285. 5
B4C
z1. 0
wt-
%B
eC
BeC
:sub
mic
rom
etr
e2280uC
,15
min
,A
r–N
294. 0
21
B4C
z10
wt-
%S
iCz
3w
t-%
Al
B4C
:D
505
9;
SiC
:D
505
2. 5
2150uC
,10
min
,A
r94
202
(1982)
22
B4C
z25
wt-
%C
rB2
D505
0. 4
32015uC
,1
h,
Ar
90. 2
B4C
:20–50
2. 6
404
203,
204
(2002,
2002)
B4C
z27. 9
wt-
%C
rB2
SS
:15. 3
2015uC
,1
h,
Ar
93. 3
B4C
:20
2. 7
400
CrB
2:
20
B4C
z25
wt-
%C
rB2
O2:
2%
max.
2030uC
,1
h,
Ar
98. 1
B4C
:100–150
3. 7
525
B4C
z27. 9
wt-
%C
rB2
Fe:
140
pp
m2030uC
,1
h,
Ar
96. 3
5. 8
108
Al:
50
pp
mC
rB2:
D505
3. 5
*FC
:fr
ee
carb
on
inB
4C
;D
50:
mean
part
icle
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;S
S:
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ecific
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ace
are
a,
m2
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1.
Ta
ble
7C
on
tin
ue
d
Suri et al. Synthesis and consolidation of boron carbide: a review
International Materials Reviews 2010 VOL 55 NO 1 21
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y50 mm, high amount of intragranular porosity andpoor flexural strength (y200 MPa).
Carbon as sinter additive
Various sinter additives have been tested to increase therate of densification, control grain growth and improvemechanical properties of boron carbide. Carbon hasbeen found to be very effective, primarily in reducing theoxide layer of the boron carbide powders, there bypromoting sintering and hindering grain growth.Carbon, well distributed between the particles reactswith B2O3 coating according to the reaction
2B2O3z6C?B4Cz6CO (18)
Removal of oxide layer allows direct contact betweenB4C particles, permitting sintering to initiate at sig-nificantly lower temperature (y1350uC). In addition,carbon at the grain boundaries enhances diffusion,facilitating accelerated solid state sintering. Varioustypes of carbon such as petroleum coke, carbon black,graphite, glucose and phenolic resin (e.g. phenolformaldehyde) can be used as sintering aid. If carbonin solid form such as coke, graphite or carbon black ischosen, it is mandatory that very fine size is used and themixing carried out thoroughly, in mixtures such asplanetary mill/attritor to form an intimate contact suchas a fine coating on boron carbide particles. An additivesuch as phenol formaldehyde resin plays two roles;namely as binder while cold pressing and as carbonprecursor which is uniformly distributed on the surfaceof the grains. Any carbon which is not consumed by thereduction reaction is left in the compact as excesscarbon. High densities of 98?65% TD with a fine grainsize of 2?34 mm have been achieved by the addition of3%C in the form of phenolic resin.183 Compactsexhibiting a finer, less faceted grain structure withsmaller and more uniformly distributed pores wereprepared by the addition of 6 wt-%C (in the form of athermoset resin).179 Carbon addition inhibits the coar-sening process, thereby preventing the formation oflarge unsinterable pores. As a result, carbon doped B4C
undergoes normal sintering and nearly full density canbe achieved. Much of the carbon remains in the B4Cmicrostructure as graphite particle. Phenolic resin ascarbon addition is found better than carbon black andglucose.191 With 3 and 5 wt-%C the density obtainedwas 92 and 93% TD respectively. With higher amountsof carbon (>7?5%C), a reversal in densification wasobserved. Yin et al.206 have studied the sintering kineticsof pure and carbon doped boron carbide with 0?42 mmsized B4C powders in the temperature range 1900–2200uC and a period of 5–45 min. They have deduced,the main sintering mechanisms to be volume and grainboundary diffusion for pure boron carbide and grainboundary diffusion for carbon doped boron carbideshowing activated sintering. Schwetz et al.189,207 haveobtained compacts of 97–98% TD with the addition of1–3% phenolic type of carbon using a powder of largesurface area (22 m2 g21). A fine grained (7–8 mm)compact with superior mechanical properties (flexuralstrength: 351–353 MPa and fracture toughness:3?3 MPa m1/2) was obtained.
Very fine powders in the range of nanosizes wouldincrease the rate of sintering due to very large surfacearea and particle to particle contact. Zorzi et al.192 haveused carbon black of 20 nm size (specific surface area:120 m2 g21) with boron carbide powder of surfacearea18?8 m2 g21 to obtain 97?7% TD compacts havingKnoop hardness in the range 19–21 GPa by sintering at2250uC for 2 h. While studying the densificationbehaviour of nanosized boron carbide, it has beenfound that solid state sintering (1500 to 1850uC) startsonly after the evaporation of B2O3. The formation ofeutectic (B4C-C) liquid droplets appears on the surfaceof graphite coated B4C particles above 1920uC, whichform the weaker regions in the sintered product.208
Sample sintered at 2300uC, showed the grain boundaryto be free from carbon and the excess carbon present asfine graphite crystals at the triple point resulting inexcellent mechanical properties (Fig. 13a).209 Highresolution TEM image of the sample shows noboundary or amorphous layer (Fig. 13b).209 Large,
13 a TEM image of pressureless sintered boron carbide with 7?9 wt-% phenolic resin at 2250uC showing graphite crystal
at triple points and b high resolution TEM image of grain boundary of same sample:209 reprinted with permission
from the Ceramic Society of Japan, J. Ceram. Soc. Jpn, 2004, 112, (5), Figs. 2 and 3 in p. S400
Suri et al. Synthesis and consolidation of boron carbide: a review
22 International Materials Reviews 2010 VOL 55 NO 1
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complex boron carbide shapes of density .94% arefabricated by slip casting method using a binder andsintering at 2280uC for 2 h.210
Thermosetting phenolic types of resins are mostsuitable as carbon precursors as they give a completeuniform coating of carbon on the surface of carbideparticles. Carbon can be replaced by boron to react withsurface oxide layer of B4C to achieve similar effects. Inaddition boron will also react with free carbon of B4C toform carbide.192
Addition of a small amount (3–5 wt-%) of carbon toB4C plays an important role in eliminating the surfaceoxide layer, thereby achieving higher densities (y97%TD) with fine grains and improved mechanical proper-ties (hardness: 19–25 GPa; flexural strength: 160–350 MPa). The quantity and the method of carbonaddition have to be carefully chosen to avoid free carbonin the sintered body.
Role of carbide/boride additives
In addition to carbon, other grain refinement agents forB4C are Si and Al. Although the strength can be signi-ficantly improved by grain size reduction, the toughnessremains low. Addition of carbides and borides have beenfound to increase the flexural strength and fracturetoughness of B4C by grain refinement and crack pro-pagation influencing mechanisms such as crack deflec-tion, micro crack interaction and crack impediment.211
Carbides/borides can either be directly added or formedby in situ reaction with B4C while sintering. Faberand Evans212 have predicted that fracture toughnessincreases due to crack deflection around second phaseparticles in two-phase ceramic materials. The idealsecond phase, in addition to maintaining chemicalcompatibility, should be present in amounts of 10 to20 vol.-%. Greater amounts may diminish the toughnessincrease due to overlapping particles. Particles with highaspect ratios are most suitable for maximum tougheningespecially particles with rod shaped morphologies.
Zorzi et al.192 have reported that addition of4 wt-%TiB2 to B4C lead to good results in final density,hardness and wear resistance. Baharvandi andHadian213 have reported the addition of TiB2 onsinterability and mechanical properties of B4C. Densityand fracture toughness values were found to increasewith TiB2 fraction in the entire range of 0 to 30%,whereas bending strength and hardness improve tocertain amount (15%TiB2) and then start decreasing.With 30%TiB2 the highest density and fracture tough-ness of 98?5% TD and 3?4 MPa m1/2 were achievedrespectively. At 15%TiB2 the highest hardness of 31 GPaand flexural strength of 360 MPa were obtained. TiB2
also acted as a grain growth inhibitor. TiB2 is formed byreaction sintering of boron carbide with titanium oxideat y1500uC as per the following reaction
B4Cz2TiO2z3C?2TiB2z4CO: (19)
Levin et al.197 have found that TiO2 is reduced bycarbon originated from the carbide phase. This leads tothe formation of substoichiometric boron carbide, whichis responsible for increased rate of sintering. Addition of40 wt-%TiO2 to B4C powder (17 m2 g21 specific area)gives a compact of 95% TD after sintering for 1 h at2160uC.187,197 Presence of TiB2 results in lowering ofactivation energy for sintering and hence very high
densities of 99% could be achieved without pressure attemperatures of 2050–2100uC.214,215 Metallographicexamination revealed a two phase microstructure withB4C grains of 10 and TiB2 of 5–7 mm size. Grain sizeof B4C was found inversely proportional to the quantityof carbon in the sample. Increase in volume fraction ofTiB2 led to an increase in flexural strength and fracturetoughness to a maximum of 513 MPa and 3?7 MPa m1/2
respectively at 15 vol.-%.216,217 The observed increase instrength and fracture toughness are due to the interac-tion between the propagating crack front and localthermal mismatch stress associated with TiB2 particles.
Titanium carbide also reacts with boron carbide toform TiB2 as per the reaction
B4Cz2TiC?3Cz2TiB2 (20)
The amount of second phase TiB2 and excess carbonplay a distinct role in sintering. Densities higher than95% TD were obtained by sintering at temperaturesabove 2150uC.200 As the carbon content increased from1?5 to 6?0%, the grain size of B4C decreased from 10 to3 mm, flexural strength increased from 292 to 502 MPa,and toughness decreased from 4?2 to 2?9 MPa m1/2.
Addition of various transition metal (Ti, Zr, Hf, V,Nb and Ta) carbides/borides for preparing dense boroncarbide pellets have been patented.218,219 Boron carbideis reacted at approximately 1500uC with the transitionmetal oxide/carbide to form a mixture of boroncarbidezmetallic carbide/boride, which is sintered attemperatures .2000uC to obtain densities .95%.Goldstein et al.198 have studied the reaction betweenB4C and MeO mixtures (MeO–TiO2, ZrO2, V2O5,Cr2O3, Y2O3 and La2O3) fired up to 2180uC for 2 h inargon. The main solid reaction products are found to beborides. Such composites exhibited a sintering aptitudehigher than that of monolithic B4C, increasing with theamount of metal oxide in the initial mixture. Thehardness and strength of composite were comparable tothat of hot pressed B4C. Khazai et al.220 have patented aprocess for the preparation of boron carbide/titaniumdiboride composite with uniform distribution of both.
Baharvandi et al.196 have studied the effect of additionof Yttria doped zirconia on sintering behaviour(between 2050 and 2150uC in argon for 1 h) andmechanical properties of B4C. Densities of 97% TDwere obtained in samples with >15%ZrO2 addition.Boron carbide reacts with ZrO2 during sintering to formZrB2 as per the following reaction
B4Cz2ZrO2?2ZrB2zB2O3zCO: (21)
Fracture toughness and flexural strength of the com-pacts increased from 2?1 to 3?1 MPa m1/2 and 200 to340 MPa respectively with the increase of ZrO2 contentfrom 0 to 30%. Processing in vacuum and highertemperature (2275uC) increased the hardness to.30 GPa.221 Figure 14 presents the variation in hard-ness of pressureless sintered B4C with ZrO2 addition.Hardness in the entire range of composite is y30 GPacompared to 27 GPa for pure B4C. Backscattered imageof this sample shows a white phase containing up to1?2%Zr distributed in B4C matrix (Fig. 15).Fractography (Fig. 16)221 shows the mode of fractureto be a combination of transgranular and intergranular.
The in situ reactions for the formation of carbide/boride consume some of the carbon from boron carbide,
Suri et al. Synthesis and consolidation of boron carbide: a review
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thereby generating substoichiometric boron carbide.This defective structure enhances diffusion kineticsthereby improving the rate of sintering. The improvedmechanical properties are attributed to the presence offine distribution of secondary phase particles in thematrix. If the second phase is not as hard as boroncarbide and then presence of large volume is likely todeteriorate the mechanical properties and hence one hasto optimise the level of second phase addition to obtainthe best properties.
Liquid phase sintering
Liquid phase sintering is carried out either by theaddition of a substance with low melting point or by theformation of a low melting substance by in situ reactionof the additive with boron carbide. Wettability, capillaryaction, dissolution and reprecipitation are the importantparameters, which decide the ability to sinter and themechanical properties. Wetting of polycrystalline B4Cby molten aluminium between 900 and 1200uC havebeen studies by Lin et al.222 They found the Wettabilityof B4C to be strongly depend on temperature and theformation of different reaction products such asAl2?1B51C8, Al3BC, AlB2, Al3B48C2 at the interface.
Danny et al.223 have studied the problems of B4C–Alparticulate composite fabrication and explained thatchemical reactions between B4C and aluminium occurbetween 800 and 1400uC much faster than capillaryinduced liquid rearrangement, inhibiting the densifica-tion process. It is suggested that the application ofexternal pressure during sintering to accelerate densifi-cation faster than the kinetics of phase formation.Attempts have been recently made to prepare B4C–CeB6/Al composite with improved strength and tough-ness by pressureless infiltration technology.224
Silicon with a melting point of 1410uC, when added toboron carbide acts as a liquid sinter additive and inaddition reacts with carbon of boron carbide to formsilicon carbide, thus helping the sintering process andalso strengthening the matrix. Silicon carbide hasattractive properties, similar to that of boron carbidesuch as high hardness (28 GPa), low specific gravity(3?1 g cm23) and good wear resistance and henceconsidered an attractive sinter addition to boroncarbide. Taylor et al.225 describe a process where siliconis infiltrated into a porous body of boron carbide andthen sintered in the temperature range 1500–2200uC toobtain dense (2?57 g cm23) non-porous boron carbidebody which is extremely hard (modulus of rupture235 MPa, modulus of elasticity 353 GPa). X-ray dif-fraction pattern obtained correspond to normal B4C,second boron carbide type having an expanded lattice,alpha and beta silicon carbides and silicon. The chemicalanalysis showed 16?5%C, 32?6% total silicon, 50?9%total boron, 0?16% free carbon and 12?6% free Si. Thepresence of unreacted, free silicon lowers mechanicalproperties of reaction bonded B4C. The fraction of freeSi can be reduced by increasing the green density of theinitial boron carbide preforms.226 Mallick et al.227 havedemonstrated the possibility of net shape production viainfiltration of Si melt into porous preform containingB4C and carbon. Chen et al.228 have studied theformation and sintering mechanisms of reaction bondedsilicon carbide boron carbide composites. In the productsintered at 1450uC, a non-stoichiometric boron carbide
14 Variation in hardness of pressureless sintered B4C
with ZrO2 addition (hardness in entire range of com-
posite is y30 GPa compared to 27 GPa for pure
B4C):221 reprinted with permission from Elsevier,
Ceram. Int., 2008, 34, Fig. 3 in p. 1545
15 Backscattered image of sintered boron carbide with
ZrO2: white regions analysed to contain 1?2%Zr
16 Image (SEM) of fractured surface of sintered boron
carbide with ZrO2 addition: mode of fracture seen as
combination of trans- and intergranular:221 reprinted
with permission from Elsevier, Ceram. Int., 2008, 34,
Fig. 6 in p. 1547
Suri et al. Synthesis and consolidation of boron carbide: a review
24 International Materials Reviews 2010 VOL 55 NO 1
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phase B12(C,Si,B)3 and a minor phase rich in B10C orB49C1?82 were seen. Microcracks were also observed inboron carbide grains. B4C cermets, for application asneutron absorber have been prepared by sintering with aeutectic alloy of Al–Si at a low temperature of 550uC.229
The role of iron in the sinterability of B4C has beenstudied by contact angle measurement and dilatometricdensification. The interaction zone consisted of a finemixture of FeB and graphite. The sintering kineticsconfirmed the liquid phase sintering leading to improvedmass transfer.230 Mizrahi et al.231 have also attributedthe formation of liquid phase promoting the masstransfer mechanism in B4C–Fe mixtures. Iron additionsalso provide the desired porosity for successful infiltra-tion of preforms.
Addition of carbon to react with infiltrated silicon forforming silicon carbide rather than with boron carbide isbeneficial as silicon carbide has a lower hardness andhigher specific gravity compared to boron carbide.232
Hayun et al.233 have found that presence of a largefraction of plate-like SiC in samples formed due to initialhigher porosity, are responsible for increased flexuralstrength and fracture toughness on account of strength-ening effect of the high aspect ratio of the second phaseparticles. The dynamic strength and the dynamicfracture toughness K1d are significantly higher than thecorresponding static properties and are insensitive to theresidual silicon fraction and to the strain rate (up to5103 s21). Properties of the B4C material prepared bythis method are given as: density, 2?57 g cm23; young’smodulus, 382 GPa; flexural strength, 278 MPa andfracture toughness, 5?0 MPa m1/2. Organosilicon poly-mers such as polycarbosilane,190 polysiloxanes, poly-silazanes, polysilanes, metallopolysiloxanes andmetallopolysilanes,234 which upon pyrolysis yield SiCand free carbon have been found to be very effective inpressureless sintering of boron carbide. Weaver202 haspatented a process where in a mixture of boron carbideand silicon carbide are mixed in an aluminium mill to acomposition of 87B4C–10SiC–3Al which is sintered at1800–2300uC to densities up to 94% TD.
Chromium boride forms a eutectic liquid phase withB4C at a temperature of 2150uC. The fractured surfaceof the B4C specimen with 20 mol.-%CrB2 prepared at2000uC showed the CrB2 particles to be partiallymelted and wetted with the adjacent B4C particles(Fig. 17).203,204,235 There was no significant growth ofB4C grains and the densification was mainly attributedto B4C particles rearrangement caused by the CrB2–B4Ceutectic liquid formation. However at temperaturesabove 2200uC, abnormal grain growth of B4C occurred.Specimen with 98?1% TD (20 mol.-%CrB2, sintered at2030uC) showed a high flexural strength of 525 MPa anda moderate fracture toughness of 3?7 MPa m1/2. Theimprovement in fracture toughness is due to theformation of microcracks and the deflection of propa-gating cracks due to the thermal mismatch of CrB2 andB4C.
Addition of alumina improves the densification ofboron carbide with the formation of liquid phaseAlB12C2 by the reaction between B4C and Al2O3.Maximum density of 96% TD was achieved by theaddition of 3 wt-% alumina doped B4C sintered at2150uC for 15 min.199 Exaggerated grain growth wasobserved in the specimen containing .4% alumina.
Baharvandi et al.195 have studied the effect of highalumina talc (26?62Al2O3–47?78SiO2–25?6MgO) powderas a sintering aid. The sample sintered at 2050uC for 1 hshowed the formation of MgB2, SiC and Al2O3. Themechanism of sintering is due to the formation of liquidphase anstatite at 2000uC and its reaction with B4C toform SiC, MgB2 and Al2O3. Though the density (78–98%) and fracture toughness (2–2?8 MPa m1/2) of thecompact increased with the quantity of talc in the chargeup to 30%, microhardness attained a peak value of26 GPa at 25% and then fell down.
A US patent236 explains a process wherein boroncarbide powder is mixed with aqueous sodium silicate(to give SiO2 equivalent of 0?75–1?5 wt-%B4C) andalumina (1–3 wt-%), compacted and sintered at 2100uCto give compacts exceeding 90% TD. As per another USpatent by Prochazka201 addition of 0?5–3 wt-%BeC toboron carbide helps in achieving a density of 85–94%TD by sintering at 2200–2280uC. Weaver in his patent202
has mentioned a process to obtain refractory bodiescomposed of 60–98 wt-% boron carbide, 2–40 wt-%silicon carbide and 0–10 wt-% aluminium with densitiesexceeding 94% TD by cold pressing followed by apressureless heat treatment.
Low melting metals/alloys such as Al and Al–Si havebeen used to bind boron carbide particles to differentshapes for neutron absorber applications. Liquid phasesintering of boron carbide is carried out by in situreaction with alumina, silica, BeC, CrB2, etc. Thisreduces the sintering temperature by a few hundreddegrees and the sintered product has a fine microstruc-ture and moderate mechanical properties. Silicon isfound to be an excellent additive which is introducedinto a sintered porous body of B4C by infiltrationtechnique. This helps in two ways: by liquid phasesintering and by SiC formation which greatly improvesthe mechanical properties. Owing to fine grain structureand high fracture toughness of the end product, this isthe method chosen for the manufacture of B4C basedarmour shields.
Hot pressingIn conventional sintering, the rate of densification isvery slow. Though higher densification can be achievedat higher temperatures, it is difficult to prevent grain
17 Fracture surface of B4C–CrB2 specimen indicating
partially melted CrB2:204 reprinted with permission
from Springer, J. Mater. Sci. Lett., 2002, 21, Fig. 4 in
p. 1446
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growth. Addition of second phase can only retard butnot completely prevent the grain growth. Pressureassisted consolidation/sintering generally involves heat-ing a powder compact, with the simultaneous applica-tion of pressure. The powder compacts are typicallyheated externally using graphite heating elements andthe pressure is applied hydraulically. A photograph of avacuum hot press with front door open showing thegraphite die and heaters is presented in Fig. 18. Hotpressing is the most common method for fabricatingdense articles of pure B4C. Without sinter additives B4Ccan be fully densified by hot pressing .2100uC with anapplied load of >30 MPa. Photographs of boroncarbide pellets of various sizes compacted by hotpressing are presented in Fig. 19. Densification bysintering during hot pressing results from three succes-sive mechanisms:
(i) particle rearrangement, where the total and openporosities decrease and the closed porosity remainsconstant (temperature range: 1800–1950uC)
(ii) plastic flow, leading to the closing of openporosity without significantly affecting the closedpores (1950–2100uC)
(iii) volume diffusion and pore elimination at the endof the hot pressing (2100–2200uC).1,186,237–239
The density, porosity and microstructure of hot pressedB4C depend on the hot pressing parameters, such astemperature, pressure, time, heating/cooling rate, etc.Though very fine particles are not a precondition, thesize of boron carbide used for hot pressing generally fallsin the range 1–10 mm. Table 8179,194,234,239–258 presentsthe literature data on hot pressing of boron carbide with/without additives.
The density of the compacts obtained under identicalhot pressing conditions (2150uC for 10 min) were 91?6
and 99?7% TD with starting powders of 3?85 and0?35 mm respectively.259 Though at lower temperatures,heating rate influences the rate of densification, attemperatures .2000uC it has no important influence ondensification.240 A temperature of >2100uC and apressure of 34?4 MPa are necessary to obtain densityclose to 100% TD. Slow cooling after densification hasbeen found to be responsible for reduction in the finaldensity due to the formation of pores while cooling. Asboron carbide reacts with the die material, inner liningof the graphite die is essential to prevent this reaction.BN lining has been found to be most suitable. Themicrostructure of hot pressed specimens show no graingrowth (1?5–2?0 mm) up to 1950uC, a steady even growthup to 2050uC (the final grain size 5 mm) and an unevensized growth and the presence of large number of twinsat 2150–2200uC.239 Fast heating rates and application ofhigh pressure (40 MPa) have been helpful in obtainingfull densification at a lower temperature of 1900uC.179
Samples obtained under these conditions show amicrostructure, free of grain boundary phases, with anaverage grain size of 2 mm and faceted submicrometrepores accounting for ,1 vol.-% porosity. Jianxin250 hasprepared boron carbide nozzles by hot pressing at2150uC in an inert atmosphere with a pressure of36 MPa using starting powders of ,3 mm size with adensity, hardness, fracture toughness and flexuralstrength of 95?5% TD, 32?5 GPa, 2?5–3?0 MPa m1/2
and 300–400 MPa respectively. He has also studied theerosion wear of this by abrasive air jets using SiO2, SiCand Al2O3 powders. While studying the densification byhot pressing, Ostapenko et al.239 have found that thedensification of boron carbide is controlled by a processleading to non-linear creep, whose rate is a function ofthe square of stress. Experimenting on the activatedsintering kinetics by the addition of iron, Koval’chenkoet al.238 have noted that, dislocation climb is the mainmechanism leading to creep; whose rate is a quadraticfunction of stress. Properties of dense B4C compactsprepared by hot pressing generally have the bestproperties with the following values:260 hardness, 29–35 GPa; fracture toughness, 2?8–2?9 MPa m1/2; elasticmodulus, 450–470 GPa; thermal conductivity, 30–42 W m21 K21; coefficient of thermal expansion,
18 Vacuum hot press with front door open showing gra-
phite heaters and insulation
19 Boron carbide pellets of various sizes compacted by
hot pressing
Suri et al. Synthesis and consolidation of boron carbide: a review
26 International Materials Reviews 2010 VOL 55 NO 1
Pub
lishe
d by
Man
ey P
ublis
hing
(c)
IOM
Com
mun
icat
ions
Ltd
Ta
ble
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ow
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rd
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rin
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g*
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al
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mp
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MP
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rate
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21;
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te:
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1775
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…240
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1975
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SS
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esis
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ments
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MP
a,
10
min
98
.5,
ala
rge
num
ber
of
twin
s…
……
239
(1979)
3B
4C
D505
1;
SS
:12
2100uC
,40
MP
a,
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min
,A
r.
95
2–3
……
…179
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4B
4C
D50,
32150uC
,36
MP
a,
60
min
95. 5
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32. 5
2. 5
–3. 0
300–400
250
(2005)
5bB
zC
D505
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S:
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,30
MP
a87
……
……
241
(2000)
Am
orp
hous
Bz
CD
505
0. 2
;S
S:
12. 7
61800uC
,30
MP
a99
aBz
CD
505
2. 7
4;
SS
:0. 8
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,30
MP
a91
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4C
B4C
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505
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O:
0. 1
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MP
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:0. 5
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CR
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Ks
21
2. 5
gcc
21
10–50
240
242
(1979)
B4C
z5%
BB
:D
505
20
2. 4
9g
cc
21
…190
B4C
z15%
B2. 4
2g
cc
21
10–40
200
7B
4C
zTiO
2z
CB
4C
:D
505
0. 5
0;
SS
:21. 5
2000uC
,50
MP
a,
1h,
Ar
100
B4C
:3. 8
,TiB
2…
3. 1
870
243
(2005)
TiO
2:
D50,
nanosiz
eB
4C
:D
505
0. 4
4;
SS
:15. 5
100
B4C
:3. 4
,TiB
22. 8
720
B4C
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55
0. 4
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SS
:22. 5
100
B4C
:3. 9
,TiB
23. 2
815
8B
4C
zTiO
2z
C2100uC
,35
MP
a,
8m
in.
95
0%
TiB
227
3. 1
200
257
(1990)
5%
TiB
230
4. 0
350
10%
TiB
232
5. 0
500
20%
TiB
234
5. 1
630
30%
TiB
230
4. 6
530
40%
TiB
228
4. 3
400
9B
4C
z23. 4
%TiO
2z
5. 2
8%
carb
on
bla
ck
B4C
:D
505
0. 6
3;
SS
:19. 8
;TiO
299. 9
%p
ure
,sub
mic
rom
etr
e2000uC
,20
MP
a,
Ar,
1h,
HR
:15–25uC
min
21
15
vol.-%
TiB
2;
rest:
B4C
6. 1
621
244
(2000)
10
B4C
B4C
:D
505
3to
52150uC
,35
MP
a,
65
min
,A
r95. 0
B4C
:6–10
29
2. 5
245
246
(2002)
B4C
z10%
(W,T
i)C
(W,T
i)C
:D
505
1to
21850uC
,35
MP
a,
50
min
,A
r98. 5
1–2,
TiB
2,W
2B
528
2. 8
400
B4C
z30%
(W,T
i)C
1850uC
,35
MP
a,
40
min
,A
r99. 2
0. 5
–1. 5
26
3. 9
550
B4C
z50%
(W,T
i)C
1850uC
,35
MP
a,
30
min
,A
r99. 5
,1
23
4. 5
690
11
B4C
B4C
:D
505
3to
5;
TiC
:D
505
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2;
Mo:
D505
1to
32150uC
,35
MP
a,
65
min
,A
r95. 0
5–8
21
2. 6
0540
247
(2009)
B4C
z5. 3
%M
o1950uC
,35
MP
a,
50
min
,A
r96. 5
1–2
……
…B
4C
z5%
TiC
z5%
Mo
1950uC
,35
MP
a,
50
min
,A
r99. 1
1–2
22
3. 4
0550
B4C
z10%
TiC
z4. 7
%M
o1950uC
,35
MP
a,
50
min
,A
r99. 2
1–2
25
4. 2
5695
B4C
z15%
TiC
z4. 5
%M
o1950uC
,35
MP
a,
50
min
,A
r99. 0
1–2
24. 5
3. 7
5625
B4C
z20%
TiC
z4%
Mo
1950uC
,35
MP
a,
50
min
,A
r98. 5
1–2
23. 5
3. 6
0550
Suri et al. Synthesis and consolidation of boron carbide: a review
International Materials Reviews 2010 VOL 55 NO 1 27
Pub
lishe
d by
Man
ey P
ublis
hing
(c)
IOM
Com
mun
icat
ions
Ltd
Seri
al
no
.M
ate
rial
co
mp
osit
ion
,w
t-%
Sta
rtin
gp
ow
der
deta
ilsH
ot
pre
ssin
gco
nd
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ns
Sin
tere
dd
en
sit
yr
th,
%M
icro
str
uctu
re,
mm
Vic
kers
hard
ness,
GP
a
Ind
en
tati
on
tou
gh
ness,
MP
am
1/2
Fle
xu
ral
str
en
gth
,M
Pa
Refe
ren
ce
(year)
12
B4C
:29. 5
–57%
;B
:18. 3
–35. 5
%;
Si:
3. 8
–7. 5
%;
WC
z
TiC
:0–45. 3
%;
Co:
0–2. 8
%
D50,
166
MP
a,
3m
in,
heating
rate
:40
Km
in2
1;
coolin
gra
te:
100
Km
in2
1
1620uC
3. 5
gcc
21
B4C
–TiB
2–W
2B
512
…550
245
(1986)
1670uC
3. 6
gcc
21
26
620
1720uC
3. 5
9g
cc
21
30
710
1820uC
3. 5
gcc
21
28
830
1920uC
3. 4
5g
cc
21
25
710
2120uC
3. 2
gcc
21
26
580
13
B4C
B4C
:D
505
0. 4
3;
SS
:15. 3
;O
2:
2w
t-%
;Fe:
140
pp
m,
Al:
50
pp
m2050uC
,5
MP
a,
1h,
Ar
79. 3
Eute
ctic
liquid
of
CrB
2z
B4C
…1. 9
180
256
(2003)
B4C
z10%
CrB
2C
rB2:
D505
3. 5
86
2. 2
350
B4C
z15%
CrB
294
2. 5
500
B4C
z20%
CrB
295
2. 8
550
B4C
z22. 5
%C
rB2
96
3. 2
620
B4C
z25%
CrB
298
3. 2
684
95
3. 2
660
14
B4C
B4C
:D
505
0. 4
31900uC
,50
MP
a,
1h,
Ar
99. 0
2. 5
675
248
(2003)
B4C
z5%
CrB
2C
rB2:
D505
3. 5
99. 6
2. 6
551
B4C
z10%
CrB
299. 7
2. 7
580
B4C
z15%
CrB
299. 0
2. 8
630
B4C
z20%
CrB
299. 0
3. 5
630
B4C
z25%
CrB
298. 6
3. 4
580
15
B4C
z50%
SiC
B/C
54. 1
;B
:D
505
1. 5
;O
:,
4%
1900uC
,30
MP
a,
30
min
,A
r;H
R:
15–150uC
min
21
2. 5
0g
cc
21
B4C
,S
iC…
……
273
(1993)
Siz
Bz
CC
,S
S:
80;
Si:
D1005
52. 7
4g
cc
21
B4C
,S
iC16
B4C
z8
to13%
silo
xane/
phenolic
2275uC
,28
MP
a,
1h,
Ar
97–99. 7
B4C
,S
iC/C
……
…234
(1996)
17
B4C
zsod
ium
sili
cate
z
mag
nesiu
mnitra
tez
Fe
3O
4
B4C
:D
505
0. 1
to1
1750uC
,24
MP
a,
10
min
to4
h45–68
…25
……
249
(1974)
45–98. 4
18
B4C
B4C
:D
505
3. 5
;B
N:
D50,
nanosiz
ed
1850uC
,30
MP
a,
1h,
N2
99. 5
B4C
,nano-h
-BN
21
5. 4
410
254
(2008)
B4C
z10
wt-
%B
N99. 2
15
6. 0
420
B4C
z20
wt-
%B
N99. 0
11
5. 2
410
B4C
z30
wt-
%B
N98. 5
85. 0
360
B4C
z40
wt-
%B
N98. 1
74. 3
310
19
B4C
z60%
Al 2
O3
B4C
:D
505
1. 0
1700uC
,35
MP
a,
1h,
Ar
3. 5
3…
28
4. 2
¡0. 7
530¡
30
258
(2005)
B4C
z70%
Al 2
O3
Al 2
O3:
D505
0. 2
3. 6
53. 0
27
4. 2
¡0. 4
560¡
30
B4C
z80%
Al 2
O3
3. 7
73. 2
23
4. 3
¡0. 5
600¡
70
B4C
z90%
Al 2
O3
3. 8
63. 4
21
3. 5
¡0. 5
550¡
45
20
B4C
zA
l 2O
3(B
4C
/Al 2
O35
18
:1)
2150uC
,35
MP
a,
65
min
,A
r95. 5
B4C
:2–5
22. 4
¡1. 2
3. 2
¡0. 5
300¡
34
271
(2008)
B4C
zA
l 2O
3z
5%
TiC
B4C
:D
505
3to
5,
.95%
1950uC
,35
MP
a,
60
min
,A
r98. 7
B4C
,TiB
224. 1
¡1. 1
4. 1
¡0. 5
430¡
31
B4C
zA
l 2O
3z
10%
TiC
Al 2
O3:
D505
1to
2,
.99%
1950uC
,35
MP
a,
60
min
,A
r98. 9
B4C
:0. 5
–1. 5
,TiB
224. 7
¡1. 0
4. 7
¡0. 4
445¡
30
B4C
zA
l 2O
3z
15%
TiC
TiC
:D
505
1to
2,
.99%
1950uC
,35
MP
a,
60
min
,A
r98. 7
B4C
,TiB
224. 3
¡1. 0
4. 5
¡0. 4
386¡
29
B4C
zA
l 2O
3z
20%
TiC
1950uC
,35
MP
a,
60
min
,A
r98. 5
B4C
,TiB
223. 2
¡1. 0
4. 2
¡0. 4
323¡
32
Ta
ble
8C
on
tin
ue
d
Suri et al. Synthesis and consolidation of boron carbide: a review
28 International Materials Reviews 2010 VOL 55 NO 1
Pub
lishe
d by
Man
ey P
ublis
hing
(c)
IOM
Com
mun
icat
ions
Ltd
561026 K21; flexural strength, 350 MPa; compressivestrength, 1400–3400 MPa.
Fractography of fully dense boron carbide compact isshown in Fig. 20.193 The mode of fracture appears to betransgranular.
Fabrication of boron carbide shapes by hot pressingthe mixture of particulate boron and carbon is alsopractised.111 Kalandadze et al.241 compacted boron andcarbon powders in ampoules up to 30% TD, by shockcompression, as a result of an explosive detonation.These pellets were densified by hot pressing at tempera-tures 1900–2100uC and pressures 20–40 MPa in boronnitride lined graphite moulds. A comparison between a-rhombohedral, b-rhombohedral, and amorphous boronindicated that sintering into the b-rhombohedral phaseat the final stage can give higher densities as follows:BbRBaRBamorphous, which is attributed to the phasetransformation occurrence from amorphous boron to b-rhombohedral boron through a-rhombohedral boronmodification.
Compacts with densities higher than that achievableby pressureless sintering process are produced by hotpressing of boron carbide powders. The added advan-tages of hot pressed compacts are fine grained structure,very low porosity and improved mechanical properties.Larger size powders in the range 3–10 mm can besintered to near theoretical densities by hot pressing aty2000uC and 30–40 MPa pressure. For applicationsuch as in nuclear industry, where pure boron carbide isessential and impurities/additives cannot be tolerated,hot pressing is the preferred method to produce dense,pure compacts.
Role of sinter additives
Earlier we have seen that carbon additive greatlyenhances the sintering kinetics in pressureless sintering.Such an effect is not expected in the case of hot pressingas the sintering mechanisms are different. In theliterature also one does not find any report on hotpressing of B4C with carbon addition. However additionof boron would consume the free carbon available in theboron carbide. It is seen that small additions of B (1 to5%) improves the strength of boron carbide specimens atS
eri
al
no
.M
ate
rial
co
mp
osit
ion
,w
t-%
Sta
rtin
gp
ow
der
deta
ilsH
ot
pre
ssin
gco
nd
itio
ns
Sin
tere
dd
en
sit
yr
th,
%M
icro
str
uctu
re,
mm
Vic
kers
hard
ness,
GP
a
Ind
en
tati
on
tou
gh
ness,
MP
am
1/2
Fle
xu
ral
str
en
gth
,M
Pa
Refe
ren
ce
(year)
21
B4C
z6%
La
2O
3z
12%
Al 2
O3z
12%
C…
1850uC
,20
MP
a,
1h,
vacuum
92. 5
B4C
,A
l 8B
4C
7,
LaA
lO3,
97
HR
A…
156. 7
6251
(2008)
22
90
to99%
B4C
z0
to1%
BN
/AlN
zre
st
RE
oxid
e-A
l 2O
3
B/C
53. 8
to4. 5
;D
505
0. 7
to3. 0
1825–2000uC
,10. 3
MP
a,
Ar
99. 6
–100
B4C
:1–2
25–27
3. 0
–3. 9
700–800
252
(2007)
23
B4C
z30%
Al
B4C
:2
60
mesh
600,
16. 3
MP
a,
40
min
1. 7
5g
cc
21
……
……
194
(1978)
Al:
2325
mesh
24
B4C
/Cu
578
:22
B4C
:D
505
5to
40;
Cu:
D505
1to
81050uC
,39
MP
a,
1h
81. 9
……
……
253
(1999)
B4C
/Cu
592
:873. 6
*FC
:fr
ee
carb
on
inB
4C
;H
R:
heating
rate
;C
R:
coolin
gra
te;
D50:
mean
part
icle
dia
mete
r,mm
;S
S:
sp
ecific
surf
ace
are
a,
m2
g2
1;
RE
:ra
reeart
h.
Ta
ble
8C
on
tin
ue
d
20 Microstructure of hot pressed boron carbide showing
transgranular fracture:193 reprinted with permission
from Elsevier, Ceram. Int., 2006, 32, Fig. 4(a) in p. 232
Suri et al. Synthesis and consolidation of boron carbide: a review
International Materials Reviews 2010 VOL 55 NO 1 29
Pub
lishe
d by
Man
ey P
ublis
hing
(c)
IOM
Com
mun
icat
ions
Ltd
lower hot pressing temperatures.242 Similar to prepara-tion of boron carbide based cermets for nuclearapplications boron carbide rings with adequate strengthhave been prepared by hot pressing technique withy30 wt-% aluminium as binder for possible use asneutron absorber.194 A high density B4C/Cu cermet with70 vol.-%B4C exhibiting high thermal conductivity hasbeen prepared by hot pressing of Cu coated B4Cpowders for the application of absorber materials inliquid metal cooled fast breeder reactor.253 Thoughboron carbide in various forms is used in nuclearindustry, literature data on the production methods isscarce. An US patent261 explains a process for producingB4C armour plates with improved ballistic properties bythe addition of Cr, B, or mixtures thereof.
Role of TiB2
Reaction sintering of boron carbide with the addition oftitanium oxide and carbon as per reaction (19) producesextremely fine high surface area particles of TiB2 whichpromote densification and limit the grain growth of theboron carbide matrix. This microstructure with TiB2
particles uniformly distributed in a fine grained B4Cmatrix is responsible for the increase in fracturetoughness and strength. B4C–15 vol.-%TiB2 compositewith a flexural strength of 621 MPa and fracturetoughness of 6?1 MPa m1/2 have been prepared by hotpressing at 2000uC and a pressure of 20 MPa in argonatmosphere for 1 h by Skorokhod et al.244 They haveobserved that factors for the increased strength are dueto the healing of the cracks during sintering and thepresence of TiB2 particle which force the crack topropagate in a non-planar fashion thus enhancing theenergy dissipation at the crack tip. An US patent243
explains a process to prepare boron carbide compositescontaining 5 to 30 mol.-% titanium diboride with veryhigh flexural strength (870 MPa) and fracture toughness(3?4 MPa m1/2). Addition of Fe in small amounts (0?5%)has been found to be effective in increasing the finaldensities of B4C–TiB2 composite due to the formation ofFe–Ti rich liquid phase at the grain junctions.262
Role of mixed borides
As seen in the previous lines addition of TiO2 hasbrought down the hot pressing temperature of B4C by>100uC. Further attempts to reduce the sintering tem-perature without compromising the strength are givenbelow. Reaction sintering of B4C with 30 wt-%(W,Ti)Cat 1850uC for 30 min showed increase in fracturetoughness and flexural strength up to 50 wt-%(W,Ti)Ccontent. Fine grains of (0?5–2 mm) TiB2 and W2B5 wereseen in the microstructure. The sintering temperature ofthis composite is 300uC lower than that of monolithicB4C. Flexural strength and fracture toughness forcomposite with 40 to 50% additive were 700 MPa and4?5 MPa m1/2. The increase in fracture toughness isattributed to the residual stresses generated by differ-ences in the thermal expansion coefficient between B4C,TiB2 and W2B5. The effect of TiB2/W2B5 on the path ofcrack and deflection in the composite is shown inFig. 21.246 Further reduction in sintering temperaturewas achieved by the addition of B, Si and Co to theabove referred mixture. Reaction sintering of B4C withWC, TiC, B, Si and Co by attrition milling followed byhot pressing at 1720uC for 2 h gave a compact with threedistinct phases of B4C, W2B5, and TiB2, hardness in the
range 28–33 GPa and flexural strength of 830 MPamax.245 US patent by Petzow et al.263 describes a processfor the preparation of boron carbide/transition metalboride moulded articles comprising of B4C, Si, WC and/or TiC and Co by hot pressing between 1550 and1850uC. Effect of variation of TiC addition on hotpressing of B4C/TiB2/Mo composite has been studied byJianxin et al.247 and the maximum values of fracturetoughness, flexural strength and hardness reported are4?3 MPa m1/2, 695 MPa and 25?0 GPa respectively.During ball milling/mixing of B4C with additives, thepowders get contaminated and the microstructure of thecomposite appears very complicated after hot 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 richboride or the interfacial layer.264
Role of carbides/nitrides
Li et al.265 have prepared a composite containing B4C,SiC, TiB2 and BN by 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?6 HRA, 554 MPa,5?6 MPa m1/2 and 95?6% respectively. Microstructureanalysis showed the presence of laminated structure anda clubbed frame dispersion phase and bunchy dispersionphase among the matrix. Fractography and crackpropagation suggested that crack deflection and brid-ging are the possible toughening mechanisms.
Han et al.266 have synthesised a high strength (400–570 MPa, 6–9?5 MPa m1/2) B4C–TiB2–SiC–graphitecomposite by reactive hot pressing using B4C, TiC andSiC powders. The crack deflection at the phaseboundary between B4C matrix and dispersions consist-ing of SiC and TiB2, which occur by residual stresses dueto the differences in thermal expansion coefficients ofB4C, TiB2 and SiC while cooling from the fabricationtemperature is responsible for the enhanced fracturetoughness values. Similar very high strength material(four point bend strength: 850 MPa; fracture toughness:6?1 MPa m1/2) has been prepared by the addition of5–30 vol.-%Mo to B4C/(W,Mo)B2 by hot pressing at1900uC.267 Fractography of this sample showing thecrack propagation path is depicted in Fig. 22.267 When
21 Crack path produced by Vickers indentation on
polished surface of hot pressed B4C–30 wt-%(W,Ti)C
composite:246 reprinted with permission from Elsevier,
Ceram. Int., 2002, 28, Fig. 10 in p. 429
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boron is added to the above mixture, hardness of thecompact increases due to the inhibition of B4C decom-position but the bending strength and the fracturetoughness reduce.268 A composite B4C–VB2–C obtainedby reaction synthesis with hot pressing has been foundto exhibit high hardness and bending strength suitablefor application as wear and shock resistance compo-nents.269 Cr and V carbides are also found to be effectivein obtaining high densities and fine grained structure.270
A process in which a preceramic organosilicon polymerwhich on pyrolysis yields SiC and free carbon has beenpatented for preparation of dense bodies of boroncarbide (.97% TD) by hot pressing in inert atmosphereat a temperature of 2275uC and a pressure of 28 MPa.234
Reaction sintering of boron carbide with the additionof oxides/carbides/nitrides has been successfullyemployed to obtain a microstructure of fine particlesof reaction product (borides/carbides/nitrides) in B4Cmatrix. These additions lower the sintering temperaturethan that of monolithic B4C. Flexural strength andfracture toughness of these composites are very high dueto the residual stresses generated by differences in thethermal expansion coefficient between B4C and reactionproducts, crack bridging and deflection mechanisms atthe interface, etc. Small quantities of B, Si, Ti, Fe, Co,Ni, Cr, W, etc. either intentionally added or accidentallyacquired during the grinding/mixing operations are alsofound to be effective in marginally reducing the sinteringtemperature and improving the flexural strength/fracturetoughness due to the formation of complex boridephases and multi interfaces.
Liquid phase sintering
Addition of CrB2 aids in lowering the hot pressingtemperature due to the formation of CrB2–B4C eutecticat 2150uC. B4C–20 mol.-%CrB2 composite fabricatedby hot pressing at 1900uC shows a high strength of630 MPa and a modest fracture toughness of3?5 MPa m1/2. The very fine grained microstructure isresponsible for high flexural strength and residualstresses caused by thermal expansion mismatch ofCrB2 and B4C for increasing toughness.243,248,256
Similarly addition of Al2O3 enhances the sinteringkinetics of boron carbide due to a liquid phaseformation at 1950uC.249 Jianxin and Junlong271 havestudied the effect of TiC content on the micro-structure, mechanical properties and sand erosion rate
of B4C/Al2O3/TiC composites. Addition of TiCincreased the hardness of the composite and thehardness had direct influence on the erosion rate of thenozzles. The addition of rare earth (RE) oxides such asY2O3, La2O3 reduces the sintering temperature due tothe formation of a liquid phase near the yttrium–aluminate composition (60 wt-%Y2O3–40 wt-%Al2O3,melting point 1870uC).251 Pore free sintered boroncarbide materials with high strength (700–800 MPa)and fracture toughness (3?6–3?9 MPa m1/2) have beenprepared by low pressure hot pressing with the additionof BN/AlN and oxide binder (RE oxide–Al2O3).252
Additives such as CrB2, Al2O3, Y2O3, La2O3 etc. bringdown the sintering temperature of B4C due to liquidphase formation. The reaction products formed areboride of the respective oxide, which enhances themechanical properties. Addition of TiC with otheroxides increases the hardness and erosion resistance ofB4C composite.
A number of new processing methods are envisaged toproduce materials with designed structure and proper-ties. A machinable B4C/BN nanocompoisite has beenfabricated by hot pressing microsized B4C particlescoated with amorphous nanosized BN particles.254 Thehardness of composite decreased with increase contentof BN while the machinability improved significantly. Acomposite with .20 wt-%BN content exhibited excel-lent machinability.272 The surface hardness and wearresistance of this composite has been improved bysilicon infiltration process.255 Combination of SHS tech-nique with hot pressing (called combustion hot pressing)has been used to prepare a composite, containing B4Cand SiC formed by reaction among Si, B and C, in theform of interlocked matrices with very low porosity anduniform microstructure.273 Graded porosity B4C mate-rials can be produced by a layering approach usingdifferent size distributions of B4C powders in the greenstate, and then densifying the layered assembly by hotpressing at 1900uC.274 Cobalt as sinter additive has alsobeen attempted for hot pressing of boron carbidepowders with 5 wt-%TiC at temperatures ,1500uCand a high pressure of 5–6 GPa.275
Hot isostatic pressing (HIP)The HIP process, originally known as gas pressurebonding, uses the combination of elevated temperatureand high pressure to form/densify raw materials orpreformed components. The application of the pressureis carried out inside a pressure vessel, typically using aninert gas as the pressure transmitting medium with orwithout glass encapsulation of the part. A resistanceheated furnace inside the vessel is the temperaturesource. Parts are loaded into the vessel and pressurisa-tion occurs usually simultaneously with the heating. Thehigh pressure provides a driving force for materialtransport during sintering which allows the densificationto proceed at considerably lower temperature incomparison to that of traditional sintering. In addition,particularly during the initial stages of the process, thehigh pressure induces particle rearrangement and highstresses at the particle contact points. A virtually porefree product can be produced at a relatively lowtemperature. The pressure level used in the HIP processtypically is 100–300 MPa, as compared to 30–50 MPa inuniaxial hot pressing, and the isostatic mode ofapplication of pressure is generally more efficient than
22 Crack propagation path with considerable deflection
in hot pressed B4C/30W–20Mo composite:267 reprinted
with permission from Japan Society of Powder and
Powder Metallurgy, J. Jpn Soc. Powder Powder
Metall., 1999, 47, (1), Fig. 8 in p. 28
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the uniaxial one.276,277 Larsson et al.278 have studied theeffect of addition of boron, silicon and silicon carbidewhile hot Isostatic pressing boron carbide at 1850uC for1 h under a pressure of 160 MPa. Addition of boron wasfound effective in reducing the pores and graphiteinclusions and improved particle erosion resistance.Boron carbide (100% TD) could be obtained by acombination of pressureless sintering and post-HIP at2150uC for 125 min under 310 MPa of argon pres-sure.279,280 The combination of pressureless sinteringand post-HIP is gaining importance for fabrication ofdense bodies with higher densities, lower graphitecontents and significantly higher Vickers hardness thancommercially hot pressed B4C.279–281 Elimination ofresidual porosity and significant improvements inflexural strength, elastic constants and wear resistancewere observed with the addition of 1 and 3 wt-%C in theabove process.282 Fully dense and very fine grainedboron carbide has been prepared by the fabricationroute, injection moulding/pressureless sintering (2175uC)/post-HIP (200 MPa, Ar) from B4C doped with 4 wt-%carbon black.283 Near net shape with full density can beachieved by HIP.284–286 Figure 23279 shows the micro-structure of post hipped boron carbide to full theoreticaldensity. Equiaxed uniform size grains and thin grainboundary are the special features of this material withvery high hardness.279–281 A patented process explainsthe preparation of boron carbide shapes containingmetallic diborides (of Ti, Zr, Hf, V, Nb and Ta), sinteredin the temperature 2100 to 2200uC to give a density of2?47 g cc21, which on further hot isostatic pressing at2100uC under an argon pressure of 200 MPa to achievea theoretical density of 2?56 g cc21.219
Porosity severely degrades the ballistic properties ofceramic armour as it acts as a crack initiator. Sinteringaids generally degrade hardness and ballistic properties.Therefore, boron carbide protective inserts for personalarmour is hot pressed to minimise porosity (y98%relative density), yielding acceptable performance. Post-hipping of pressureless sintered boron carbide is gainingimportance for this purpose. This will not only yieldlighter weight armour but also enable forming ofcomplex shapes.
Major equipments needed for pressureless sinteringare cold compaction press and sintering furnace. For hotpressing, a graphite die is assembled between the rams of
the press and heated by the furnace surrounding it.Multi cavity dies are used for increasing the rate ofproduction. In HIP the whole equipment has to bedesigned for high temperature and high pressureoperation. This puts a limitation on the size of theequipment and the number of compacts that can befabricated at a time. Compacts produced by pressurelesssintering generally needs a finishing process of surfacegrinding and end finishing to obtain desired shape andsize. Hot pressed objects also undergo surface finishingoperations at times to remove any contamination fromthe die material. Hipped compacts do not need anyfurther processing and can be directly used.Manufacturing costs and throughput of pressurelesssintering with post-HIP are attractive compared to hotpressing.281
Spark plasma sinteringConventional pressure assisted consolidation techni-ques, such as hot pressing, hot isostatic pressing, etc.,require long processing time and high temperature inorder to produce high density parts. In order tominimise expensive machining, the powder densificationprocess must be capable of near net shaping. One suchprocess and the apparatus for rapid bonding of ceramicmaterials have been patented by Yoo et al.287 Thematerials to be consolidated are placed in a graphite dieand punch assembly. The driving force for densificationis provided by passing current directly through theparticle material, with simultaneously applying highshear and high pressure in separate steps. High shearforce in combination with pulsed electric power isinitially applied to the particle material to generateelectrical discharge that activates the particle surface byevaporation of oxide film, impurities and moisture.Subsequently bonding is accomplished by resistanceheating at the contact points between the activatedparticles in the presence of high pressure. The time andtemperature required for consolidation is lowered ashigh current density is applied in addition to high shearand high pressure (up to 2000 MPa) which leads tolocalised heating and plastic deformation at interparticlecontact areas. The rapid sintering which preferably lastsfor less than a few minutes prevents grain growth andallows the particles to retain their microstructure. Sparkplasma sintering (SPS), plasma activated sintering,plasma pressure consolidation/P2C and instrumentedpulse electrodischarge consolidation are the differentnames given for the same process.
Ghosh et al.52 have consolidated submicrometre sizedcommercial boron carbide to near theoretical densitiesusing plasma pressure compaction technique. Thedensities obtained at 1750uC by the application of88 MPa pressure in 2 and 5 min were 96 and 99?2% TDrespectively. The average grain size of these compactswas 1?6 and 2 mm. Optical micrograph (Fig. 24)52
reveals nearly equiaxed fine grained microstructure.The hardness and fracture toughness values were inthe range of 25?41 to 27?45 GPa and 3?22 to3?61 MPa m1/2 under static testing conditions.Addition of graphite and TiB2 do not aid in consolida-tion of B4C by this method. Addition of small amountsof Al2O3 and Fe has been found to be effective inachieving higher densities by SPS of B4C due toformation a liquid phase.288–290
23 B4C specimen pressureless sintered and hipped at
2150uC and 310 MPa to 99?1% relative density:279 rep-
rinted with permission from Materials Research
Society, J. Mater. Res., 2005, 20, (8), Fig. 6 in p. 2115
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In situ synthesis and densification
Influence of temperature on synthesis and consolidationhas been studied by Tamburini et al.113 Formation ofB4C commences at y1000uC and is complete by 1200uC,with a hold time of 10 min. Significant densificationoccurs above 1600uC, but the material shows evidence ofextensive crystallographic disorder due to twins whichhowever decrease at higher temperatures. Koderaet al.291 while heating B-C powders by SPS haveobserved that, formation of B4C starts at 1300uC,consolidation with twins initiates at 1600uC and 98%density obtained at 1900uC. Heian et al.106 have alsonoted similar structural defects in the material preparedby mechanical activation followed by field assistedcombustion. Wang et al.292 have noticed that B and Cbegin to react from 1300uC and the product was B richboron carbide (B4zxC) and free C. With increasingtemperature C atoms diffused into B4zxC lattice,resulting in the reduction of free carbon content anddecrease in lattice parameters of B4zxC. Densification ofthe synthesised boron carbide occurred from 1700 to1900uC. Simultaneous synthesis and densification ofB3?5C, B4C and B4?5C by spark plasma sintering hasshown that carbon free boron rich compounds areformed in the temperature range 1300–1600uC andconsolidation occurs above 1700uC.293
Fabrication of functionally graded B4C cermets
Continuous functionally graded boron carbide alumi-nium cermets have been prepared by spark plasmasintering of B4C from boron and carbon followed byinfiltration of aluminium.294 This processing routeresults in a material with very promising propertiesand interesting microstructural features. Hardness pro-file of functionally graded material before and after Almelt infiltration is shown in Fig. 25.294 A B4C/Cu gradedcomposite as plasma facing component for fusionreactors with performance better than nuclear gradegraphite has been prepared by rapid self-resistancesintering under ultra high pressure.295
Microwave processingMicrowave sintering has the advantages of uniform andrapid heating since the energy is directly coupled into thespecimen rather than being conducted into the specimen
from an external heat source. Enhanced densificationand finer microstructures, not feasible in the conven-tional furnaces are possible through the use of micro-wave systems. The sample size and shape, thedistribution of the microwave energy inside the cavity,and the magnetic field of the electromagnetic radiationare all important in heating and sintering.296 Literatureon microwave sintering of boron carbide is scarce. Oneof the recent articles describes the behaviour of B4C/SiC/Al mixtures during microwave heating in air. A densecomposite with B4C skeleton and the voids filled withthe reaction products of Al and B4C were obtained. SiCwas not attacked by oxygen and was able to contributeto matrix toughness. This material with low specificgravity and high hardness is attractive for use inlightweight armour.297 One can expect more researchusing microwave heating as and when bigger size unitswith higher power ratings become available in themarket.
A novel floating zone method has been adopted forpreparing directionally reinforced B4C–TiB2 compo-site.298 Temperature dependence of the bending strengthof this composite was evaluated in the temperaturerange 25–1600uC. With increasing temperature, its valuedecreases to 120 MPa at 800uC and then increased to230 MPa at 1000–1400uC. Even at 1600uC, the bendingstrength was as high as 200 MPa.298 The development ofnon-aqueous gel casting process for preparation of B4C–Al composites has been reported by Zhang et al.299
ConclusionsThe understanding of the crystal structure of boroncarbide has been evolving over the years and even todaycannot be said to be fully elucidated. Carbothermicreduction of boric acid has been the commercial methodfor the production of boron carbide in spite of theshortcomings. Need of the hour in carbothermic reduc-tion is process modelling, which will greatly help intuning the process to achieve improved productivity anduniform quality. Though a continuous method has beenestablished for the production of SiC,300 a similarmethod has to be adopted for B4C also. Sol–gel methodappears to be a promising technique for production ofsuitable fine boron carbide powders for direct consoli-dation. Vapour phase synthesis, still in laboratory scale,
24 Microstructure of boron carbide densified by SPS at
1750uC for 5 min, showing nearly equiaxed fine
grained microstructure:52 reprinted with permission
from Wiley-Blackwell, J. Am. Ceram. Soc., 2007, 90,
(6), Fig. 4 in p. 1853
25 Hardness versus position profile of functionally
graded boron carbide with and without Al infiltra-
tion:294 reprinted with permission from Elsevier, Mater.
Sci. Eng. A, 2008, A488, Fig. 8 in p. 337
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is an attractive process for the preparation of submicro-metre sized powders, whiskers, etc. Though thermo-dynamic models based on Gibbs free energyminimisation have been used to predict experimentalconditions, models based on mass transfer and kineticsin addition are needed to understand the depositionmechanism and the growth process. Boron carbidebased coatings by CVD methods for various semicon-ducting applications such as diode, transistor, thermo-electric converter, thermocouple, neutron sensors, etc.are gaining importance and the use of boron carbide inthis field will increase manifold in the coming years. Inaddition to the established uses, the future of boroncarbide will depend on the possible production ofmicrometre/nanosized powders, whiskers and advancedcoating techniques to meet the varying demands.
On consolidation, the research so far has been focusedon reducing the sintering temperature, inhibit graingrowth and improve the mechanical properties. Carbonhas been the chief architect of these improvements. Addi-tives based on carbides/borides/nitrides are also found tobe effective in obtaining a fine grained structure withhigher fracture toughness/flexural strength. Hot pressingand HIP have remained the main densification methodsfor production of pore free pure boron carbide due to theirability to achieve near theoretical density without graincoarsening. The process of silicon impregnation into asintered porous boron carbide body produces a very highdense compact with improved mechanical properties dueto the presence of fine SiC particles. Post-hipped pres-sureless sintering is gaining importance for manufactureof complex shapes with fine grains especially for armourmaterial. Production of nanosized particles followed bySPS for consolidation will revolutionise the method ofB4C component manufacture. The mechanism, micro-structure and properties of compacts by SPS technique areyet to be fully understood. Various available methods ofpowder production and consolidation techniques havebeen exploited for fabrication of boron carbide compo-nents with tailor made properties.
B4C to be considered as high temperature structurematerial should have good thermal and oxidationresistive properties. Detailed investigations on oxidationcharacteristics of B4C at high temperatures have beencarried out by Steinbruck et al.301,302 and others.303,304
Higher oxidation stability of B4C with the addition ofZr, Cr and W borides have also been reported by Radevet al.305 Serious efforts on oxidation prevention of boroncarbide at high temperatures and improvement ofthermal properties have not been attempted. It may beworthwhile to investigate composites with silicides andRE borides. With excellent dielectric properties, thermaland chemical stability, and erosion resistance in highintensity laser beams, BN–B4C composite materialscould find a unique place for high temperature applica-tions.306 From the point of view of application, boroncarbide has established as the material for abrasiveapplications, neutron absorber and armour material. Itsuse in electronic industry and high temperature applica-tions will see a higher growth in the coming years.
A recent study regarding road map for advancedceramics provides guide lines for future investments.307
1. Development of innovative process technologies totransfer new knowledge on functional and structuralproperties into ceramic materials and devices.
2. In the long run, usage of tailor made powders(complex composition, compositional gradient in parti-cle, well defined particle shape) and reliable usage ofnanoscale powders will become more important.
3. Techniques for near net shape forming should beconsidered a field for fruitful further activity.
All these points are truly applicable to boron carbidealso.
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