Synthesis and Consolidation of Boron Carbide- A Review

Synthesis and consolidation of boron carbide: a reviewA. 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 Published by Maney Publishing (c) IOM Communications Ltd Introduction Boron carbide is a suitable material for many high performance applications due to its attractive combination 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 chemical inertness,2,4 high neutron absorption cross-section (600 barns),4,5 excellent thermoelectric1,4 properties, etc. It has found application in the form of powder, sintered product as well as thin films. Boron carbide (also known as black diamond) is the third hardest material after diamond and cubic boron nitride. Its outstanding hardness makes it a suitable abrasive powder for lapping, polishing and water jet cutting of metals and ceramic materials.4 Tools with boron carbide coating are used for cutting of various alloys such as brass, stainless steel, titanium alloys, aluminium alloys, cast iron, etc.1 In sintered form, it is used as blasting nozzles,6 ceramic bearings and wire drawing dies due to good wear resistance.1 The combination of low specific weight, high hardness and impact resistance makes it a suitable material as body and vehicle armour. Modulus to density ratio of boron carbide is 1?86107 m, which is higher than that of the most of the high temperature materials and hence it could be effectively used as a strengthening medium.7 Thin films of boron carbide find application as protective coating in electronic industries.8,9 Boron carbide is extensively used as control rod, shielding material and as neutron detector in nuclear reactors due to its ability to absorb neutron without forming long lived radionuclide.7,10–17 Neutron Materials Group, Bhabha Atomic Research Centre, Mumbai 400085, India *Corresponding author, email [email protected] absorption capacity of boron carbide can be increased by enriching B10 isotope. Composite material containing boron carbide with good thermal conductivity and thermal shock resistance are found suitable as first wall material of nuclear fusion reactors.18–21 Boron carbide based composites are potential inert matrix for actinide burning.22 Boron carbide is also used for treatment of cancer by neutron capture therapy.23 As it is a p-type semiconductor, boron carbide is found to be a potential candidate material for electronic devices 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 thermocouple, diode and transistor devices as well. Boron carbide is an important component for the production of refractory and other metal borides.27–29 The low density, high stiffness and low thermal expansion characteristics of B4C make it attractive Be/Be alloy replacement candidate for aerospace applications.30 Thevenot has compiled a comprehensive review on boron carbide1 in 1990, in which synthesis, consolidation, analytical characterisation, phase diagrams, crystal structure, properties and applications are discussed. This paper critically examines various methods of synthesis and consolidation of boron carbide and discusses their merits and demerits along with structure, properties and applications. Structure of boron carbide The bond between B-B atoms and B-C atoms play a key role in deciding the crystal structure and properties of boron carbide. Knowledge of these will help us in understanding the complexities involved in processing and achieving the desired properties. Boron carbide is a compositionally disordered material that exists as ß 2010 Institute of Materials, Minerals and Mining and ASM International 4 International Materials Reviews 2010 VOL 55 NO 1 Published by Maney for the Institute and ASM International DOI 10.1179/095066009X12506721665211 Suri et al. Synthesis and consolidation of boron carbide: a review 1 Phase homogeneity range in B-C phase diagram: reprinted with permission from Elsevier, J. Less Comm. Met., 1979, 67, Fig. 2 in p. 329 Published by Maney Publishing (c) IOM Communications Ltd 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 in properties such as hardness, thermal conductivity etc. Since B4C is in equilibrium with free carbon and is only boundary 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 the structure and the properties of the compound and hence the exact knowledge of B/C ratio of the phase is very important. But the analytical study of B-C system is difficult due to extreme hardness and chemical stability of boron and boron carbide phases.33 Different limits of homogeneity range are reported by researchers at the carbon rich side of boron carbide, corresponding to B4?3C (18?8%C),36 B4?0C (20%C),33,34 and B3?6C (21?6%C).37 Difficulties associated with the estimation of free and combined carbon could be accountable for these inconsistent results.36 B-C phase diagram showing homogeneity range from 8?8 to 20 at.-%C, as generated by Bouchacourt et al.34 is presented in Fig. 1. 2 Schematic diagram of structure of boron carbide Rhombohedral unit cell, consisting of 12-atom icosahedra located at the corners and C-B-C linear chain at the diagonal of the unit cell is shown. Within the icosahedron, six atoms reside in two polar triangles at the opposite ends of the icosahedron and the remaining six atoms occupy equatorial sites: reprinted with permission from the American Physical Society, Phy. Rev. Lett., 1999, 83, (16), Fig. 1 in p. 3230 Crystal structure The most widely accepted crystal structure of boron carbide is rhombohedral, consisting of 12-atom icosahedra located at the corners of the unit cell. Schematic diagram of the structure of boron carbide is presented in Fig. 2.38 The longest diagonal of the rhombohedral unit cell contains three atom linear chain (C-B-C). Each end member of the chain is bonded covalently to an atom of three different icosahedra.31 In general, icosahedra consist of 11 boron atoms and one carbon atom. The locations of carbon atoms within different icosahedra are not ordered relative to one another. The icosohedral configuration is the result of a tendency to form threecentre covalent bonds due to deficiency of valence electrons of boron.39 Two crystallographically inequivalent sites exist in the icosahedron. Six atoms reside in two polar triangles at the opposite ends of the icosahedron and the remaining six atoms occupy equatorial sites. The atoms at polar sites are directly linked to neighbouring icosahedra via strong two-centre bonds along the cell edges. The atoms in equatorial sites either bond directly to other icosahedra through threecentre bonds or to chain structures.40,41 Most of the icosahedra have a B11C structure with the C atom placed in a polar site, and a few percent have a B12 structure or a B10C2 structure with the two C atoms placed in two antipodal polar sites.41 Three types of three-atom chain are envisioned: C-BC, C-B-B and B-B-B. Variation in carbon concentration changes the distribution of three-atom chains.31 B4C (20%C) structure consists of B11C icosahedra and C-B-C chains. As the composition becomes rich in boron, carbon of the B11C icosahedra is retained, while one of the carbon atoms on the C-B-C chains is replaced by boron. Near the composition B13C2, the structure consists of B11C icosahedra and C-B-B chains. On further carbon reduction, some of the B11C icosahedra are replaced by B12 icosahedra retaining the C-B-B chain.40,42 Carbon-boron bonds present in the three atom chains are much stronger than boron-boron bond in icosahedra.40 The inter-icosahedra bonds are stiffer than the intra-icosahedra bonds.43 Conflicting views still exist concerning the nature of site occupancies. A model based on early X-ray diffraction data44,45 proposed that the B4C composition is made up of B12 icosahedra and C-C-C chains. However later studies38,40–42,46,47 based on improved X-ray and neutron diffraction, nuclear magnetic resonance studies, theoretical calculations and vibrational spectra indicate that the structure consist of B11C icosahedra and C-B-C chains. Even among those who favour B11C icosahedra and CBC chain model for 20 at.-% (B4C), there is disagreement on the structural changes that occur in boron carbides, as the carbon content is decreased towards 13 at.-% (B13C2). Some workers46,48 propose that carbon atoms are removed from the icosahedra to form B12 icosahedra, while others40,42,49 propose that carbon atom is replaced from three atom chains. Owing to similarity of boron and carbon in electron density and nuclear cross-section (B11 and C12), both X-ray and neutron diffraction studies are not very successful in International Materials Reviews 2010 VOL 55 NO 1 5 Suri et al. Synthesis and consolidation of boron carbide: a review First principle molecular dynamics simulations have revealed that the depressurisation amorphisation results from pressure induced irreversible bending of C-B-C chains.53 Structure sensitive properties Thermal and electrical conductivity, heat capacity, hardness, etc. strongly depend on structure of boron carbide and the variations are brought out in the following lines. Lattice parameter aR of rhombohedral unit cell decreases with increase in carbon content but the plot is discontinuous at the composition B13C2 (13?33 at.-%C). Density of boron carbide increases linearly with carbon content within the homogeneity range of the phase according to the relation d (g cm23)52?422z0?0048[C] at.-% (r50?998) with 8?8 at.-%([C]>20?0 at.-%. The number of atoms per unit cell is exactly 15 for B4C, but increases with the boron content and approaches 15?3 for the boron rich limit B10C.35 Hardness of boron carbide increases with carbon within the homogeneity range as the structure becomes stiffer.1 Shear modulus of boron carbide increases with carbon from 185 GPa for B6?5C (13%C) to 198 GPa for B4?3C (20%C).54 Fracture toughness and Young’s modulus also increases with the carbon content.1 Heat capacity of boron carbide increases with decrease in carbon within the homogeneity range. This increase is due to the change in lattice vibration mode produced by reduction of the stiffness of the three-atom chain accompanied with a change from C-B-C to C-BB.55 Thermal conductivity of B4C (20%C) falls with temperature in the manner characteristic of crystalline ceramics. However, thermal conductivity of boron carbide with low carbon is relatively less and temperature independent, a behaviour more characteristic of amorphous materials. These differences of thermal transport can be explained if it is assumed that, the thermal conductivity is dominated by the transfer of vibrational energy through the inter-icosahedral chains rather than within the softer icosahedra. As the C-B-C chains are inhomogeneously replaced by C-B-B chains a transition takes place from crystal-like transport to glass-like transport. Moreover thermal conductivity falls because B-B bonds are much softer than C-B bonds.40,55,56 Gilchrist et al.57 have found that thermal conductivity of B4C falls from 29 W m21 K21 at room temperature to 12 W m21 K21 at 1000uC. Thermal conductivity increases when 10B isotope replaces 11B in boron carbide. This is attributed to the increase in the bonding energy per unit mass and the phonon velocity as a lighter isotope is substituted for a heavier isotope.58 Electrical conductivity in boron carbide was studied by Wood et al.49 and Matusi et al.55 Charge carriers in boron carbide are holes which form small polarons and move by phonon assisted hoping between carbon atoms located at geometrically non-equivalent sites.49 The nonequivalence arises from two sources. First, carbon atoms can be distributed among non-equivalent sites within B11C icosahedra. Second, only a fraction of the available positions of inter-icosahedral chain is generally filled by C-B-C chains. The carbon rich limit (B4C) resembles an ideal crystal and therefore has the lowest electrical conductivity.55 Electrical conductivity increases with temperature, which is the sign of behaviour of a semiconductor. Density of small polaron holes is Published by Maney Publishing (c) IOM Communications Ltd 3 Composition of structure elements (B12 and B11C icosahedra, C-B-C and C-B-B chains) in boron carbide unit cell and chainless unit cells with variation of C content: reprinted with permission from Elsevier, Solid State Commun., 1992, 83, Fig. 4 in p. 850 unambiguously assigning the exact site occupancies.48 The concentration of B12 and B11C icosahedra and C-BC and C-B-B chains vary and chainless unit cells also occur.50,51 Variation of structure elements B12, B11C, CB-C and C-B-B in boron carbide unit cell with C content is shown in Fig. 3.51 The carbon rich limit of homogeneity range which was assumed to contain B11C icosahedra and C-B-C chains only, also contains 19% CB-B chains. The composition of B6?5C which was attributed to be the most representative structure (B12, C-B-C) and used for many model calculations has been proved to be the least defined structure containing 60%B11C and 40%B12 icosahedra. These structural changes could also explain the abrupt decrease in thermal conductivity between B4C to B6?5C. Saal et al.46 have recently applied ab initio calculations to evolve the structure of boron carbide for the entire composition range. The enthalpy of formation and lattice parameters were calculated and compared with the experimental data. For carbon rich composition (20%C), B11C-CBC structure and for 13?33%C composition B12-CBC structure were found most stable. It suggests that carbon atom is gradually replaced by boron in icosahedra. This result is contradictory with other researchers who suggest that carbon atom is replaced from chain. At boron rich end, enthalpy and lattice parameters of B12 BVaC (Va denotes vacancy) structure is in good agreement with the experimental values for boron carbide having 7?14 at.-%C. Since the enthalpy of formation of B12 BVaC is positive it is predicted that B12 BVaC’s composition cannot be reached by boron carbide and instead, pure boron will precipitate out, which is in agreement with experimental phase equilibrium. Radev et al.43 have found that metal cations can replace a part of boron atoms in icosahedra position and thus improves the stiffness, hardness and wear resistance of boron carbide. Recent observations by Raman spectroscopy suggest structural phase transformation and the formation of localised amorphous phase which is weaker than the original crystalline phase under conditions of loading.52 6 International Materials Reviews 2010 VOL 55 NO 1 the new charge mixture consisting of boric acid and carbon is added.66 have reported that irradiation of boron carbide with neutron causes lattice strains due to the formation of lithium and helium as reaction product as well as some atomic displacements. Moreover the product obtained is chunks of melted boron carbide. the charge gets heated by conduction as well as by the heat of the International Materials Reviews 2010 VOL 55 NO 1 7 . Ridgway69 claimed to have produced crystalline B4C of 90% purity by carbothermic process.74 give the details of the furnace and the process. Partially reacted charge from the previous run is assembled around the new graphite heating rod.73 In the arc furnace process. in boron carbide. the individual B12 icosahedra themselves are not destroyed by electron irradiation but their regular spatial arrangement in the B12C3 lattice is perturbed and is gradually put in disorder with increasing electron dosage. Spohn70 has also mentioned the synthesis routes for boron carbide production and its uses in his article.62. Inui et al. dependent on the size of the availability of graphite tubes. electric arc and Acheson type (graphite rod as resistance element) are used for the production of boron carbide. Lipp4 has presented a review of boron carbide production. The furnace temperature is usually maintained at .63 Considerable amount of tritium is produced in B4C by fast neutron irradiation.2000uC to enhance the rate of overall reaction. The overall carbothermic reduction reaction can be presented as follows 4H3 BO3 z7C?B4 Cz6COz6H2 O This reaction proceeds in the following three steps70 4H3 BO3 ?2B2 O3 z6H2 O B2 O3 z3CO?2Bz3CO2 4BzC?B4 C (3) (4) (5) (2) Irradiation response Neutron irradiation of boron carbide results in extensive intergranular cracking due to the formation of helium bubble as per the following equation59–61 7 10 1 4 : 5 B z0 n ?3 Li z2 He z2 6 MeV (1) Published by Maney Publishing (c) IOM Communications Ltd Formation of these cracks. needing 16 800 kJ mol21 B4C.59 have noticed that boron rich B8C is more resistant to radiation damage compared to B4C and hence becomes a possible candidate for new absorbing materials. wherein the mixture of boric acid and petroleum coke is melted in an arc furnace followed by crushing the resultant product and mixing it with substantially the same quantity of boric acid and remelting the mixture a second time. The process is highly endothermic. which are responsible for heavy loss of boron by evaporation of its oxides. Early patents by Ridgway69. In this section. The design and operation of the electric arc furnace for the large scale production of boron carbide has been explained by Scott. surrounding which the reactants are charged is also used for production of boron carbide. Above this.2 MeV and at temperature . Hence large scale production is not feasible using tubular furnaces. The temperature dependence of electrical conductivity is essentially independent of carbon concentration. namely tubular.72 in the year 1939. 11B4C is found to be very stable after fast neutron irradiation in reactors.62. charge carrier density and electrical conductivity decreases with increase in carbon content. that. Acheson type furnace Acheson type furnaces. Carbothermic reduction of boric acid Carbon reduction of boric acid and boron trioxide is the commercial method for the production of boron carbide. a large amount of trapped helium is released simultaneously with the occurrence of bulk swelling. Dimensional changes and thermal conductivity of 11B4C are substantially smaller than that of 10B4C. On heating.59 The anisotropic precipitation of helium not only changes the microstructure but degrades the mechanical and physical properties as well.68 Boric acid on heating converts to B2O3 by releasing water. The methods of boron carbide synthesis are classified as: (i) carbothermic reduction (ii) magnesiothermic reduction (iii) synthesis from elements (iv) vapour phase reactions (v) synthesis from polymer precursors Electric arc furnace process for making boron carbide has been patented by Schroll et al. They also found that the amorphous boron carbide remains in amorphous state on annealing at 1273 K. resulting in an amorphous state. As the reaction proceeds.Suri et al. These furnaces are limited in size. decrease the thermal conductivity during irradiation. When grain boundary cracking occurs. which needs subsequent laborious crushing and grinding operations. which prevent heat conduction and the atomic disorder resulting in high phonon dispersion. the reaction initiates near the graphite rod and carbon dioxide escapes to the atmosphere through the charge above. The reduction of B2O3 with carbon monoxide becomes thermodynamically feasible above 1400uC. Operational details of the Acheson furnace are explained below. Within the homogeneity range.163 K.64 Copeland et al. the temperatures are generally very high due to localised electric arcs. which is retained up to 700uC even on annealing and is released only at temperature higher than 900uC.71 Three types of electric heating furnaces. where a graphite rod is used as heating element. They suggested a possibility.67 Froment et al. Tubular electric furnaces using graphite tubes as heating element are in use for carrying out reactions for scientific study only.65. properties and applications in 1965. different routes for B4C synthesis will be discussed. Synthesis and consolidation of boron carbide: a review independent of temperature but the mobility of holes increases with temperature. Arc furnace process Synthesis of boron carbide Boron carbide was discovered in nineteenth century as a byproduct of reaction involving metal borides. The purity of boron carbide produced by early researchers was less than 75% and in 1933.67 have reported that a complete crystalline to amorphous transition takes place by electron irradiation with energy .49 (vi) liquid phase reactions (vii) ion beam synthesis (viii) VLS growth. After completion of the run. conversion in each run is low and boron loss is high.76 have devised a method of core temperature measurement in boron carbide manufacturing process.82 have studied the thermodynamics of gas phase carbothermic reduction of boron anhydride. recovery is poor.77 Boric acid to coke ratio of 3?3– 3?5 is found optimum and at higher ratios. only a small portion of the charge gets converted to carbide and the balance material is recirculated in further runs. The reacted product is crushed in jaw crushers and further ground to finer size. 8 International Materials Reviews 2010 VOL 55 NO 1 .80 at 1200uC by thermogravimetric studies on the reduction of boric acid by petroleum coke in vacuum. Synthesis and consolidation of boron carbide: a review Published by Maney Publishing (c) IOM Communications Ltd 4 Thermogravimetric analysis plot of carbothermic reduction of boric acid80 escaping CO. type of carbon used for reduction. rate of heating and the final core temperature. Dacic et al. this process has been adopted for commercial production. Rapid heating rates (. As the raw materials used are cheap and the process is simple. They have analysed the heat transfer process inside the reactor and the effect of it on the formation of boron carbide based on the recorded data. indicating the occurrence of reaction through gaseous boron suboxides. Operator experience plays a major role in identifying the completely reacted product so that less amount of oxides enter the boron carbide portion. the bubbles burst pushing the charge above. These bubble bursts and evaporation losses affect the efficiency of the process considerably.105 K s21) result in smaller crystallite size. Alizadeh et al. Hence in this process. The product gases form bubbles and grow in size near or just above the reaction site and as the pressure increases. B2O3 melts and forms a glassy film preventing the escape of CO from the reduction zone.100 K s21) of the charge results in the formation of boron carbide by a nucleation and growth mechanism as the reaction proceeds through a liquid boron oxide path. Formation of boron carbide by carbothermic reduction is highly dependent on the phase changes of reactant boron oxide from solid to liquid to gaseous boron sub oxides and the effect of reaction environment (heating rate and ultimate temperature). B2O2 and BO are formed by carbothermic reduction of B2O3 according to reactions (6) and (7) and then reduced to B or B4C. Boric acid initially loses its water and converts to B2O3.71. Though the method of raw material charging and collection of reacted product could be different in arc furnace and Acheson processes. As temperature is an important process parameter in carbothermic reduction process and the heat transfer plays an important role in the formation of boron carbide Rao et al. influence of process parameters and the means of improving the product quality and conversion efficiency have been investigated by many researchers. Process kinetics. Intermediate heating rates (103 to 105 K s21) result in the formation of both large and small crystallites. the reaction sequence is very similar. Addition of small amount of sodium chloride (1?5%) is found to be effective in increasing the yield. During these bursts some of the partially reacted charge is thrown out of the furnace and boron also escapes to the atmosphere in the form of boron oxide vapours. Figure 480 shows the weight change of the charge with temperature up to 1400uC.79 Start of formation of B4C has been noticed by Subramanian et al. Process kinetics Kinetics of the reaction and also the product quality is strongly influenced by porosity of the charge. indicating the reaction of carbon with both liquid boron oxide and gaseous boron suboxides. charcoal and activated charcoal. though boron carbide free of carbon could be obtained. On further heating.81 Slow heating (.75. Some quantity of boron oxides escapes to the atmosphere along with carbon monoxide. Ground powder is washed in water and leached in acid to remove the contamination due to grinding media and also the accompanying unreduced or partially reduced oxides of boron from the reduced product. Petroleum coke is found to be a better reducing agent than graphite. the top is broken open and the boron carbide surrounding the graphite rod is manually collected. In each run.Suri et al.78 have optimised the boron oxide/ carbon (petroleum coke) ratio to yield boron carbide with low (0?65%) carbon. this route is adopted as commercial method mainly because of the simple equipments and cheap raw materials which make this route the most economical.:82 reprinted with permission from Elsevier. Fig. Nanocrystalline boron carbide yield of nanorods. Alloys Compd. Preparation of dense articles need fine boron carbide powders in micrometre size. Although carbothermic reduction results in lower yield due to loss of boron in the form of its oxides. yield is low due to loss of B2O3 from reaction mixture. 7b).92 Magnesiothermic reduction of B2O3 An alternate method for the production of boron carbide is by magnesiothermic reduction of boron anhydride in presence of carbon as given below 2B2 O3 z6MgzC?B4 Cz6MgO This reaction takes place in two steps: step 1 2B2 O3 z6Mg?4Bz6MgO step 2 4BzC?B4 C (10) (9) (8) Preparation of nanosized particles of boron carbide is of recent interest. 200 B2 O3 zC?B2 O2 zCO B2 O3 zC?2BOzCO (6) (7) The effect of the feed composition and temperature on the product composition in carbothermic reduction is shown in Fig.82 A decrease in the partial pressure of CO facilitates synthesis of B4C by boosting the generation of B2O2 and BO.91 These particles have very large sizes (y20 mm).89 Large scale boron carbide nanowires of size 80–100 nm diameter and 5–10 mm in length have been synthesised using B/B2O3/C powder precursor under argon flow at 1100uC.91 have synthesised nanostructures of boron carbide by heating B2O3 powder to 1950uC in a graphite crucible covered with a boron nitride disc. while the thickness was in the nanoscale range (20–100 nm).85.83–85.Suri et al. 6). 2 in p. Availability of nanosized powders will not only avoid the grinding operations but also reduce the temperature of densification substantially. One method of controlling the temperature and the particle size of the product is by choosing the right size of the reactants. Modelling of carbothermic reduction process for the production of boron carbide has not been attempted by anybody so far. Details of experimental studies on carbothermic reduction. Tumanov83 has reported the development of a continuous process for the production of boron carbide. processing conditions and product quality of boron carbide obtained by various researchers are summarised in Table 1. giving charge composition. B2O2 and CO react to form B4C solid nanowires with a mean diameter of y50 nm and lengths of several hundreds of micrometres (Fig.90 Xu et al. Production of boron carbide by carbothermy has been essentially a batch process. Post reductive sintering at temperatures 200–300uC International Materials Reviews 2010 VOL 55 NO 1 9 . B4C particles in the nanosize range (260 nm) can be prepared by reduction of B2O3 vapour by carbon black at 1350uC. The mixture is heated quickly to 1650uC and held at that temperature for 2 h under flowing argon. reaction chamber. by direct inductive heating of a charge made of boron oxide and carbon black.87 Above this temperature. 7a)91 and multiply twinned particles normally in rod shape were also present (Fig.77–79. An alternate reduction method patented by Rafaniello84 explains the process for producing submicrometre size boron carbide powders.1000uC. 413. J. Weimer et al. The product of conventional process has to undergo series of size reduction processes to obtain such powders.88 Ma et al.86 have designed a vertical apparatus comprising of cooled reactant transport member. The type of carbon used. 2006. The carbide is still contaminated with magnesium borides formed as stable compounds. Such grinding operations contaminate the product necessitating additional purification steps. boron oxide and carbon black.89 have prepared high purity boron carbide nanowires from mixed powder precursor containing boron. Addition of cobalt as catalyst is found to be helpful in The reaction is exothermic (DH51812 kJ mol21) in nature. a cover gas such as argon or hydrogen is used and also the system pressure maintained high. Thus it was found that when the reaction takes place in gas phase or the product could be nanocrystalline B4C. Majority of the crystallites deposited on boron nitride disc show a belt-like morphology with average width and length of about 5–10 and 50–100 mm. 5. heating element and cooling chamber for the continuous production of submicrometre B4C powder. Synthesis and consolidation of boron carbide: a review Published by Maney Publishing (c) IOM Communications Ltd 5 Effect of feed composition and temperature on calculated product composition in carbothermic reduction of 1 mol B2O3 (l) as per Dacic et al. This route is not only useful for commercial powder production but also for the production of nanocrystalline B4C. A number of perfect icosahedral quasicrystal particles (Fig. Vapours of B2O3. method of preparation of the charge mixture and the fast heating rates (70– 10 000uC min21) are responsible in obtaining fine powders. This reduction technique yields very fine amorphous powder. Presence of some catalyst also promotes the formation of nanopowders. As the vapour pressure of magnesium is high at the reaction temperature of .69. The products of the reaction are processed by aqueous methods to remove magnesium oxide from boron carbide.72. which is well suited for use in the fabrication of sintered products. 2002. Formation of ultra fine B4C powder from the stoichiometric mixture of H3BO3. Calcium can also be used as reductant in place of Mg.b long straight segments. 364. B. Chem. Phy. the high cost of magnesium will soon make this process obsolete for regular production.95.93 An early patent by Gray94 explains the process for the production of boron carbide powders by magnesiothermic reduction of B2O3 or alkali Na2B4O7 in presence of carbon 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. Fig. 2004.96 and Khanra et al. c.100 have recently reported synthesis of boron carbide powder by calciothermic reduction of borax (Na2B4O7) or B2O3 in presence of carbon at 1000uC in argon. Chem. 315 higher than the reaction temperature increases the particle size of the product. Though boron carbide has been produced by magnesiothermic reduction and used for applications defined by its high calorific value. Mg and C by self-propagating high temperature synthesis (SHS) has been studied by Zhang et al. Mechanical alloying has also been utilised as a means of synthesising submicrometre B4C particles by magnesiothermic reduction. Lett.d curly tufts 6 Image (SEM) of high purity single crystalline boron carbide nanowires formed by thermal evaporation of B/B2O3/C powder precursor at 1650uC under argon atmosphere:89 reprinted with permission from Elsevier.95 The heat of magnesiothermic reaction is sufficient enough for self high temperature synthesis route. Fig. Synthesis and consolidation of boron carbide: a review Published by Maney Publishing (c) IOM Communications Ltd a. Seeding of the charge with a small quantity (1–2%) of boron carbide has been found to increase the growth of B4C particles and the yield significantly.Suri et al. 7653 at 1650–1700uC. The degree of conversion was influenced by pressure of the ambient argon gas which influences the evaporation of magnesium and thereby the combustion temperature and conversion.98 Wang et al. 1 in p. J.. 108B. Phys. Berchman et al. Addition of metallic sulphates as catalyst has been found to reduce the reaction temperature to 700uC.97 The ignition temperature of this mixture was found to be 670uC by thermal analysis method. 4 in p. Synthesis from elements Synthesis of boron carbide from its elements is considered uneconomical due to the high cost of 10 International Materials Reviews 2010 VOL 55 NO 1 . Table 293–99 presents a summary of studies on synthesis of boron carbide by magnesiothermic reduction.99 have studied the synthesis of B4C fibre– MgO composites by combustion of B2O3zMgzCfibre samples in an argon filled chamber. distorted ellipsoid. 1 h catalyst: K2SO4 Rotation speed: 200 rev min21 Ball to load ratio: 5 : 1 72 h Ar 670uC. such as filaments. carbothermic reduction is not suitable due to loss of boron as well as boron hold-up in the furnace and hence this process is the only suitable economical method. Spark plasma synthesis is a new technique. Reactants 1 2 3 4 5 6 B2O3zPC H3BO3zcharcoal H3BO3zPC B2O3zPC/carbon active B2O3 and carbon H3BO3zPC Process type Batch (resistance furnace) Batch (arc furnace) Batch Batch Batch Batch (Acheson) Process parameters 2400uC Melting temperature of charge 1470uC. Although formation of boron carbide from its elements is thermodynamically possible at room temperature.02–0. For synthesis from elements. International Materials Reviews 2010 VOL 55 NO 1 11 . H2 … 700uC. 1–5 h.2 mm Submicrometre and uniform sized crystals Submicrometre particles 0.6% Carbon: 25. product quality on synthesis of boron carbide by Process parameters 950–1200uC. elemental boron and hence employed for specialised applications101. HR: 100uC min21.1 mm 0. 20–300 min. 5 h. HR: 900uC s21. Ar 1580uC. For synthesis of enriched boron carbide.1500uC in vacuum or inert atmosphere. in which a pulsed high dc current is passed through the charge mixture contained in a cavity along with the application of uniaxial pressure. processing magnesiothermic reduction* Serial no. which is then pelletised and reacted at high temperatures of . plates and polyhedron particles of nanosize.5 mm 84 (1989) Submicrometre particles 0. Table 2 Charge composition. The partially sintered pellet of boron carbide is then crushed and ground to get fine B4C powder. Ar . Ar Crystalline B4C without free 92 (2004) carbon Partially sintered and dense 77 (1986) product B4C conversion: 69–73% Crystalline B4C 83 (1979) Equiaxed crystals of 0. thus necessitating high temperature and longer duration for complete conversion of the elements into the compound. 1 2 3 4 5 6 7 Reactants B2O3zMgzC B2O3zMgzC B2O3zMgzC B2O3zMgzC B2O3zMgzCFibre H3BO3zMgzC Na2B4O7zMgzC Process type Tubular furnace SHS Batch Mechanical alloying Combustion synthesis SHS Continuous conditions and Mechanical alloying of B–C mixtures followed by heat treatment is one of the methods being investigated for the synthesis of boron carbide. boron and carbon are thoroughly mixed to form uniform powder mixture. processing conditions and product quality on synthesis of boron carbide by carbothermic reduction* Serial no. Synthesis and consolidation of boron carbide: a review Table 1 Charge composition. high purity elemental boron powder produced by fused salt electrolytic process104.102 only. Ar 1650–1700uC.1 mm 85 (1992) *PC: petroleum coke.3% Ref. (year) 93 96 95 88 (2002) (2003) (1967) (2006) 99 (1994) 97 (2005) 94 (1958) *SHS: self-propagating high temperature synthesis.Suri et al. the heat of reaction (239 kJ mol21) is not sufficient to carry out in a self-sustaining fashion. In this technique reactants are kept inside a steel container which is placed in plastic tube. H3BO3zactivated carbon 3 min. Ar 1470uC. such as B10 enriched or very pure boron carbide. In this process. Combination of mechanical alloying followed by spark plasma sintering has been studied by Hian et al.1–0. due to slow diffusion of reacting species through this layer. Room temperature milling is carried out in planetary mills for prolonged duration to activate the powders and the alloyed mixture is then annealed to obtain boron carbide.2% Submicrometre particles B4C fibrezMgO composites 8–24 mm size. Ar H3BO3zcorn starch Boric oxide and carbon 1850uC.5% Carbon: 21.105 is often used. the start and completion of formation has been noted at 1000 and 1200uC respectively. HR: 1000– H3BO3zacetylene carbon black 2000uC s21. Ar 2000uC. H2 Quality of B4C Fine powder Fine powder 98% pure Boron: 74. To achieve a high purity product of B4C. (year) 69 (1933) 72 (1939) 79 (2004) 78 (2006) 7 8 Published by Maney Publishing (c) IOM Communications Ltd 9 Continuous (induction 2227uC B2O3zcarbon black/ graphite/activated charcoal heating) Continuous 1820uC.103 Formation of boron carbide layer slows down further reaction. HR: 100u min21. Ar 1800uC. H3BO3zVulcan XC-72 carbon black 3 min. HR: heating rate.2000uC Product quality Crystalline B4C 90% pure Boron carbide with 15%C Crystalline B4C 25–30 mm Crystalline B4C 25–30 mm Ref. HR: 755uC s21. Ar Continuous 1950uC.106 to obtain 95% pure boron carbide. The resultant product exhibited several different morphologies. Shock wave technique has also been attempted for boron carbide synthesis from amorphous boron and graphite powder107 using trimethyl enetrinitramine as detonator. A detonator is placed between container and the plastic tube. Ar. 8% free carbon Powder boron: 77. 3 min. BBr3 and BI3 are suitable boron source but BCl3 is the most preferred due to its ready availability and low cost. pressure and atmosphere. 10 min 113 (2005) 6 Shock wave technique 7 Amor. After the shock treatments.110 has attempted the preparation of boron carbide nanoparticles (200 nm) by direct reaction between amorphous boron and amorphous carbon at 1550uC. single crystal silicon. One such set-up for vapour phase reaction is described by Bourdeau in his patent. At this temperature. (year) 110 (2007) 111 (1975) 112 (2006) 106 (2004) 5 Amor. boronzcarbon black Amor.Suri et al. Clifton et al. highly strained structures. CNT: carbon nanotube. boronzAmor. MacKinnon et al.1200uC.109 have synthesised boron carbide nanoparticles by reacting multiwall CNT with magnesium diboride at 1150uC for 3 h in vacuum. In this technique very high heating and cooling rates are achieved along with high pressure. which reacts with the halogen forming hydrogen chloride as per the following reactions 4BCl3 zCCl4 z8H2 ?B4 Cz16HCl 4BCl3 zCz6H2 ?B4 Cz12HCl 4BCl3 zCH4 z4H2 ?B4 Cz12HCl (11) (12) (13) Vapour phase reaction Synthesis of boron carbide by carrying out reaction between boron and carbon containing gaseous species has been extensively studied. A few attempts have been noticed on the preparation of nanostructure boron carbide from its elements. Ar 1800–2200uC. The actual deposition is controlled by mass transfer and surface kinetics. Synthesis of boron carbide from its elements is suitable for the production of pure B4C. desired temperature. boronzgraphite .117 have described a process for the production of boron carbide powder of fine size with a surface area >100 m2 g21. The chemical reaction is completed in micro to milliseconds. C2H6.115 described a process for producing boron carbide whiskers in the size range of 0?05 to 0?25 mm by the reaction of B2O3 vapours with the hydrocarbon gas between 700 and 1600uC. Hydrocarbon gases such as CH4. which is kept at a The flow of reactants and other process parameters decide the composition and structure of the product formed. etc. This method is gainfully adopted for the formation of boron carbide coatings and synthesis of powders and whiskers in submicrometre sizes. Chemical vapour deposition (CVD) Deposition of different types of boron carbide films (B13C2. for specialised applications such as in nuclear industry this method is preferred. metastable phases. borane (B6H6) and oxide (B2O3) are also useful boron sources.) by CVD techniques has been reported in literature. B4C. 12 International Materials Reviews 2010 VOL 55 NO 1 . C2H2 and carbon tetra chloride (CCl4) are employed as carbon source. 4 h. MA: mechanical alloying. the product stoichiometry depending on the reactant composition. Apart from halides. Table 3106–108.116 have patented a process for the production of boron carbide whiskers and the use of catalytic elements to enhance the yield of the gas phase reaction process. Wei et al.4 km s21 1150uC. carbon fibre and boron are the substrate materials used for thin film synthesis. Recently Chang et al. Dieter et al. Chen et al. processing conditions and product quality on synthesis of boron carbide from elements* Serial no. samples were recovered by shaving off the container with a lathe. C2H4. 16 min Product quality Nanoparticles 15–350 nm Articles of near theoretical density B4C with some unknown peaks 95% dense pellet of high purity boron carbide Sintered B4C.110–113 gives a comparative summary of studies reported on the synthesis of B4C from its elements. boron carbides of variable B/C ratios are obtained as submicrometre powders.108 have prepared boron carbide nanorods by reacting carbon nanotubes (CNT) with boron powder at 1150uC under argon atmosphere.: amorphous. disordered fine crystalline Nanosized particles of crystalline B4C Straight nanorods Ref. Synthesis and consolidation of boron carbide: a review Published by Maney Publishing (c) IOM Communications Ltd Initiation of explosive detonation was carried out by an electric detonator. Though the cost of production is high due to the high cost of elemental boron. boronzCNT Solid state reaction Detonator: trimethyl enetrinitramine Detonation velocity: 6. 3–4 h MA for 90 h Annealing at 1200uC 1650uC. Generally the process is carried out in vacuum in the Table 3 Charge composition. Synthesis of boron carbide takes place in the reaction chamber. Generally hydrogen is present in the atmosphere.114 The process of producing boron carbide by reacting a halide of boron in vapour phase with hydrocarbon in the temperature range 1500–2500uC has been explained.118 have reported that when boron trichloride is reacted with CH4–H2 mixture in a radio frequency argon plasma. Boron halides such as BCl3. which affects the stoichiometry and properties of the boron carbide phases grown. magnesium diboride decomposes and gives elemental boron. The crystals obtained had a high density twin structures with variation of B/C ratio from particle to particle. Hence this is suitable for the preparation of crystals of various morphology and non-equilibrium phases which are hard to be produced in thermal equilibrium conditions. Ar 107 (1996) 108 (2002) *Amor. 1 2 3 4 Reactants Amor. Graphite. carbon BzC BzC BzC Process type Solid state thermal reaction Hot pressing MAzannealing MAzspark plasma sintering Spark plasma synthesis Process parameters 1550uC. James et al. Laser CVD In this technique the energy of a laser beam is used to heat the surface of a substrate to the temperature required for chemical deposition. The deposition rate and surface microstructure strongly depend on laser power and hydrogen content in the gas phase. Direct writing and fibre growth methods can be combined to produce threedimensional structures.153 have successfully fabricated and tested a boron carbide/ boron diode on aluminium substrates and a boron carbide/boron junction field affect transistor. They have also discussed the thermodynamic modelling used by many researchers and have concluded that the process takes place far from equilibrium and that.1100uC. The deposition takes place at lower temperature as compared to traditional CVD. have been tested for the formation of boron carbide films. Schouler et al. A B5C/Si(111) hetrojunction diode by a synchrotron radiation induced decomposition of orthocarborane fabricated by Byun et al. which is a major advantage of PECVD. solid state neutron detectors which are more efficient and reliable than any other neutron detecting semiconductor reported to date.122 have prepared boron carbide nanorods on graphite substrate at 1400uC by CVD using charge mixture of boron oxide. B25C is codeposited with boron carbide. Since the formation of the reactive and energetic species in the gas phase occurs by collision in the gas phase. the substrate can be maintained at a low temperature. It allows superb spatial resolution (y5 mm) because the chemical reactions are restricted to the heated zone created by the focused laser spot. Robertson et al.148 Plasma enhanced CVD In PECVD chemical reaction takes place after the creation of plasma of reacting gases. Hwang et al. When the relative amount of carbon to boron in the gas phase is high. Adenwalla et al. They proposed that two major reactions take place during the process 1 BCl3 ðgÞz CH4 ðgÞzH2 ðgÞ? 4 1 B4 C ðsÞz3HCl ðgÞ 4 BCl3 ðgÞzH2 ðgÞ?BHCl2 ðgÞzHCl ðgÞ mechanical properties and thermal stability.120. Hence. hot filament CVD (HFCVD).121 Jaziehpour et al.154 have fabricated real time solid state neutron detector by PECVD using closo-1. in contrast to the traditional CVD furnace which heats the entire surface of the substrate. film formation can occur on substrates at a lower temperature than is possible in the conventional CVD process. etc. Many modifications such as laser CVD (LCVD).152 has been found to be comparable with PECVD diodes. giving very high efficiencies.125 obtained BCx (x>3) phase having whisker-like morphology by reacting BCl3 and B6H6 at 1000uC on quartz substrate in presence of hydrogen and nickel. excellent International Materials Reviews 2010 VOL 55 NO 1 13 . These attributes are the result of deposition occurring one atom at a time.150 Lee et al. low porosity.148 Laser CVD results in deposits with high purity.Suri et al. These diodes have been used to fabricate the first real time.123 have grown novel boron carbide nanoropes by CVD using ocarborane (C2H12B10) as precursor and ferrocene (C10H10Fe) as catalyst. tetragonal and metastable boron rich phase. High substrate temperature results in poor adhesion whereas deposition rate is low at low temperature. which can be beneficial for tailoring the thermal and electronic properties of boron carbide.129 Patterned deposits can be obtained by direct writing process. Fibre depositions are also possible by moving away the substrate parallel to the laser beam axis at a rate equal to the deposition rate of the fibre. thermal and electrical properties of CVD boron carbides are comparable to other important refractory materials and promise a wide range of application areas.127 Control of laser power density allows for codeposition of r-(B4C) and disordered graphite. a disordered graphitic phase is deposited along with boron carbide and when the carbon is low.151 have fabricated photoactive p-n hetrojunction diode by PECVD of boron carbide thin films from nido-pentaborane (B5H9) and methane(CH4) on Si (111). particularly in the nuclear industry. Table 4114–118.155 have reported the fabrication and characterisation of boron carbide/ silicon carbide hetrojunction diodes by PECVD.127–147 presents a summary of studies reported on vapour phase reaction synthesis of boron carbide. Shu-Fang et al. Substrate temperature has strong influence on the process and product quality. Deposition rates in LCVD techniques are orders of magnitude higher than that in traditional CVD. Amorphous boron carbide coating can be obtained at a low temperature of y500uC whereas crystalline film is obtained at higher temperatures . activated carbon and sodium chloride.124 have studied the kinetics of CVD of B4C on tungsten substrate using BCl3–CH4–H2 gas mixture. Sezer and Brand126 have written a comprehensive review on CVD of boron carbide.149 Plasma enhanced CVD has been used by many researchers for the fabrication of boron carbide (B-C) diodes which could accurately detect single neutrons. The literature is abundant on various possible combinations (14) (15) Reaction rate of boron carbide formation is lower than that of dichloroborane formation over the entire range of temperatures (1000 to 1400uC) studied. high degree of crystallinity. The plasma is generally created by radio frequency (ac) or dc discharge between two electrodes. in which a pattern of thin lines deposited on the substrate by moving the substrate perpendicular to the axis of the laser beam.2dicarbadodecaborane. The mechanical.25. thermodynamic modelling is not sufficient to represent experimental deposition conditions. plasma enhanced CVD (PECVD). Karaman et al. The necessary energy for the chemical reaction is not introduced by heating the whole reaction chamber but just by heated gas or plasma.128 The reactive atmosphere composition is the most important parameter in laser CVD. Synthesis and consolidation of boron carbide: a review Published by Maney Publishing (c) IOM Communications Ltd temperature range of 450 to 1450uC.119 Preparation of boron carbide fibres by the reaction of boron halides with woven cloth of carbonisable material in hydrogen atmosphere has been patented by Wainer et al. Amorphous boron carbide coatings on SiC have been obtained by CVD from CH4–BCl3–H2–Ar mixtures at low temperature (900–1050uC) and reduced pressure (10 kPa). the space between which is filled with the reacting gases. 6 h.: boron coated Mo. highly strained microstructure Pure long crystalline B4C. vacuum 2100uC (filament) 450uC (substrate). Sub.: graphite cloth.: graphite. Sub.: carbon fibre 1127–1227uC. H2: 500 mL min21 1000 to 1400uC. 18 h Product quality Boron carbide crystals Amor. Sub. Sub.Suri et al. length: 13 mm Amor. Nd:YAG laser 1100–1200uC. porous boron carbide powder of submicrometre size Whiskers length: 0. 3 h.: Si (100). diameter: 0.: fused silica. Sub. 1 2 Process Vapour phase reaction Vapour phase reaction Reactants BCl3zCH4 B2H6zC2H2 Process parameters 1900uC. Sub. long columnar grains Amor. Sub. 15–22%C Crystalline B4C and B25C 14 to 33 nm B4C crystals encapsulated in graphite B4C nanowires diameter: 18–150 nm. boron carbide large composition range (0 to 40 at.7 GPa Amor. hardness: 41¡2. Sub. Sub. diameter: 10 mm Submicrometre size powder Crystalline B4C coating Metastable phases. Sub. uses. etc.: tungsten.132 (1989) Published by Maney Publishing (c) IOM Communications Ltd 6 7 8 9 CVD CVD BCl3zCCl4zH2 BCl3zC3H8zH2 133 (1965) 134 (1981) 10 CVD BCl3zCH4zH2 B4C coating 135 (1968) 11 12 13 14 CVD CVD CVD Laser CVD BCl3zCH4zH2 BCl3zCH4zH2 BCl3zCH4zH2 BCl3zCH4zH2 B4C coating (B: 74 to 76%) specific gravity of 2. Sub. (year) 114 (1967) 117 (1977) 3 Vapour phase reaction Vapour phase reaction RF plasma assisted synthesis CVD CVD B2O3zCH4 115 (1970) 4 5 BCl3zCH4zH2 BCl3zCH4zH2 BCl3zCH4zH2 BCl3zCH4zH2 1650uC.: fused silica Laser: CO2 Laser: CO2. vacuum: 5 mm of Hg Exothermic reaction and needs to be ignited only by spark plug 1075uC.: fused silica. Ar Laser: CO2. Hot filament CVD systems are based on thermal catalytic cracking of the precursors on the surface of a high temperature filament usually ranging from 1000 to 2500uC. Ar pressure: atmospheric Laser: CO2. Sub.: graphite 850–1000uC. RF: radio frequency. Sub. CH4: 2–5 mL min21. Synthesis and consolidation of boron carbide: a review of source.32 gm cm23 Amor.: Si (100). The substrate materials are usually heated by radiation from the hot filament and the Table 4 Charge composition.: amorphous.: graphite 500–600uC. vacuum 1000–1400uC.-%C Microcrystalline film hardness: 22–32 GPa Various composition between B4C and B13C2 138 (1996) 139 (1990) 127 (2002) 129 (1997) 140 (1999) 141 (1999) 142 (1990) 22 23 BCl3zCH4 BCl3zCH4zH2 143 (1998) 144 (1992) 24 25 Hot wall CVD Hot filament activated CVD Electron beam evaporation BCl3zCH4zH2 BCl3zCH4zH2 Crystalline B13C2.: Si (100) 116 (1969) 118 (1975) 130 (1996) 131. high purity and good adhesion Thin films of crystalline boron carbide 145 (1998) 146 (1994) 26 BzC 147 (2008) *CVD: chemical vapour deposition.05–0. graphite 800–1050uC. Sub. 5 h. 3–6 h BCl3: 6–15 mL min21. coating Ref. and only a few examples are given above.: substrate. Amor. 4–5 h.5–4 inch. vacuum: 15–25 torr 1300uC. method of fabrication. vacuum 1550uC. Sub. crystalline B4C. boron carbide. 3–6 h.: Si (100) 500–600uC. 14 International Materials Reviews 2010 VOL 55 NO 1 . vacuum: 10 mm of Hg 1300uC.: tungsten Laser: CO2. vacuum catalyst: VCl4 Ar plasma 1350uC. vacuum Room temperature.: graphite. Sub. Sub. boron carbide Crystalline B4C Crystalline B4C 136 (1974) 137 (2006) 124 (2006) 128 (1999) 15 16 17 18 19 20 21 Laser CVD Laser CVD Laser CVD Laser CVD Pulsed laser induced CVD Plasma enhanced CVD Microwave plasma assisted CVD Supersonic plasma jet CVD Thermal CVD BCl3zCH4zH2 BCl3zCH4/ C2H4zH2 BCl3zC2H4 BCl3zCH4zH2 C6H6zBCl3 C2B10H12 (orthocarborane) BBr3zCH4zH2 Crystalline B4C Ultra fine and crystalline B4C Adherent.25 mm Whiskers length: 50 mm. processing conditions and product quality on vapour phase synthesis of boron carbide* Serial no. Hot filament CVD Hot filament CVD is an attractive technique due to its simple design and its amenability to fundamental chemical kinetic modelling in understanding the process chemistry. Sub. ArzH2 1600uC.: fused silica. 169 have observed the formation of nanocrystalline B4C by solvothermal reduction of CCl4 using lithium in presence of amorphous boron powder in an autoclave at 600uC. A US patent156 describes a process for making a free flowing boron carbide powder from boric acid and sugar.2.1 wt-% B/C composite containing crystalline B4C Crystalline Amorphous.159 have prepared boron carbide fibre Published by Maney Publishing (c) IOM Communications Ltd The reaction was carried out in an autoclave at 450uC. In this process. by heating ‘amine treated B2O3 fibre’ in inert atmosphere at 2000–2350uC. Liquid phase reaction Synthesis of ultra fine boron carbide powder using liquid precursors has been attempted by a few. micrometre sized. The mixture dissolved in ethylene glycol is dried in air at 180uC and then heated in hydrogen at 700uC. 8). Shi et al. etc. fibres. boric acid as impurity Crystalline. This reaction product is ground and fired at 1700uC for 7 h to yield fine boron carbide powder. Some of the boron loaded organic compounds such as carborane (C2BnHnz2). this can be operated at much lower temperatures to yield boron carbide of desired properties. a stable gel is formed from aqueous solution of boric acid and citric acid. Mondal et al. Table 5156–159. Sinha et al. processing conditions and product quality on synthesis of boron carbide using polymer precursor Serial no.157 describe a low temperature synthetic route in which a polymeric precursor is synthesised by the reaction of boric acid and polyvinyl alcohol. sugar and ethylene glycol Amine treated B2O3 fibre 1700 2000–2350 H2 … 156 (1975) 159 (1974) Inert atmosphere … International Materials Reviews 2010 VOL 55 NO 1 15 .168 Gu et al. uC Atmosphere 1300 1500 400–800 1450 1400 1500 1000 1450 1400 1215 Argon Vacuum Air Vacuum … Ar Ar Ar Ar Vacuum Holding time.500uC. Synthesis and consolidation of boron carbide: a review substrate surface temperature is usually .5 3 2 … 4 8 48 2 . Vapour phase synthesis methods are suitable for thin film coating of boron carbide and preparation of fine powder.158 have presented a process in which. free carbon 11.3 propanetriol Ethyl Decaborane (C2H5B10H13) 10 11 Solution product of H3BO3. Generally this process is carried out in the temperature range 1000–1500uC in vacuum or inert atmosphere. triphenylborane. h Product quality 5 2. Deshpande et al.Suri et al. ion flux ratio of different Table 5 Charge composition. whiskers. Economy et al.168 have studied the formation of ultra fine boron carbide powders by coreduction of boron tri bromide and carbon tetrachloride using sodium as reducing agent as per the following reaction 4BBr3 zCCl4 z16Na?B4 Cz4NaClz12NaBr (16) Synthesis from polymer precursors As an alternative to high temperature reaction techniques. parameters such as ion energy. polyvinyl pentaborane and borazines on pyrolysis yield B4C. This method is also known as solvothermal process or coreduction method.160 have developed a method based on sulphuric acid dehydration of sugar to synthesise a precursor material which on heating to temperature between 1400 and 1600uC yields crystallised B4C and B4C/SiC composites.005 inch) on tungsten wire Crystalline B4C powder Boron carbide fibre Ref. Unlike conventional methods. This gel is further processed to yield a precursor which on heating under vacuum to 1450uC produces B4C. there is great interest in development of polymer precursors to produce ceramic materials at lower temperatures. black and shiny crystalline Crystalline Crystalline B4C coating (0. Ion beam synthesis Boron carbide thin films can be grown by direct deposition of Bz and Cz ions. However the powders produced by this process are generally nonstoichiometric and not suitable for fabrication of dense products. which on pyrolysis at 400/800uC gives crystalline boron carbide.146 The deposition is carried out under high vacuum conditions to avoid oxygen contamination of the boron carbide phase. Polymer precursors 1 2 3 4 5 6 7 Polyvinyl borate Reaction product of H3BO3 and citric acid Reaction product of H3BO3 and polyvinyl alcohol Reaction product of H3BO3 and citric acid Solution product of H3BO3 and glucose Condensation product of H3BO3 and 2-hydroxy benzyl alcohol (HBA) Polyvinyl pentaborane Temperature.161–167 gives the comparative summary of studies reported on the synthesis of B4C using polymer precursors. free carbon: 2.1 Crystalline Crystalline. Cihangir et al.146 have obtained adhesive coating of boron carbide on silicon substrate and the wear resistance of the coated surface was found to be extremely high when tested using a WC/Co ball as the pin. These methods are best suited for laboratory studies. 4BzCCl4 z4Li?B4 Cz4LiCl (17) Hexagonal B4C crystals with a particle size of approximately 15–40 nm diameters were obtained. (year) 161 (2009) 162 (2006) 157 (2005) 158 (2002) 163 (2002) 164 (1999) 165 (1988) 165 (1988) 166 (1985) 167 (1969) 8 9 Condensation product of H3BO3 and 1.38% Crystalline (orthorhombic). B4C crystals obtained were composed of uniform spherical (80 nm dia) and rod-like (200 nm diameter and 2?5 mm long) particles (Fig. which along with carbon dissolve in liquid cobalt and then precipitate as boron carbide whiskers. complexity and the reasons for incomplete densification by pressureless sintering are discussed in detail by Lange. Atmospheric/ reaction/microwave and thermal plasma sintering are termed as pressureless sintering techniques. 14. which can be independently controlled.172 have prepared B4C whiskers and platelets using this technique.Suri et al. 5(b) in p. Ronning et al. Mater. could be advantageously used for obtaining the preferred composition and nature of the boron carbide film.178 Pressurised sintering can be classified as solid and gas compaction methods. Publications.174 Ma et al. B2O3 and carbon black were used as source of boron and carbon respectively. 336–338. 9. Synthesis and consolidation of boron carbide: a review 8 Image (TEM) of B4C rod-like particles (200 nm diameter and 2?5 mm long) prepared at 450uC by sodium reduction of BBr3 and CCl4:168 reprinted with permission from Elsevier.175 A comparative study of various methods of boron carbide synthesis is presented in Table 6. A recent article177 explains the process of making boron carbide–carbon eutectic containing 39 wt-%C by melting B2CN in graphite crucible at 2600uC. NaCl and Co were added to facilitate the growth of whiskers.. Chem. Beam energies were in the range of 15 keV for Bz to 45 keV for B3z and fluences were between 261014 and 261016 cm22.. The nuances of densification of powder compacts. Rao et al. Solid State Commun. Fig. Scanning electron micrograph of the nanowires is shown in Fig. Fig. 3(c) in p. 2003. B2O3 reacts with NaCl to form BCl. Thakkar et al. This resulted in reduction of diameter of nanowires from 50–200 nm to 10–30 nm.176 have synthesised high purity ultra fine boron carbide powders by reacting B2O3 with methane in a non transferred arc dc thermal plasma reactor. Fig.175 have investigated the growth of boron carbide nanowires by the addition of iron to the precursor mixture containing carbon.174 have used gallium oxide and sodium chloride to prepare boron carbide nanobelts having a length of around 1 to 10 mm and thickness of around 80 to 150 nm. Ni or Co) in which whisker constituents get dissolved. Tech. boron and boron trioxide. boron carbide whiskers precipitate out of the metal droplets. Some of the attempts to produce boron carbide cannot fall into any of the classifications discussed above. gallium oxide and sodium chloride at 1400uC:174 reprinted with permission from Trans. the mechanisms involved and the product quality are discussed in the following pages. Carlsson et al. When the catalyst becomes supersaturated with boron and carbon. 10. Densification techniques can be broadly classified as pressureless sintering and pressurised sintering. which is shown in Fig. 4405 16 International Materials Reviews 2010 VOL 55 NO 1 . 128. (III). An et al. Solid compaction methods are hot 10 Boron carbide nanowires prepared by VLS growth with help of iron addition:175 reprinted with permission from American Chemical Society. Boron carbide powder is either utilised directly or consolidated to dense bodies.171 have observed the formation of amorphous boron carbide (BxC) by bombardment of Bz and B3z ions on fullerene.173 have studied the formation of boron carbide whiskers using K2CO3 and NiCl2 as a low melting liquid and catalyst to enhance the formation of B4C whiskers and platelets.170 have grown thin film of boron carbide (BxC) by direct ion beam deposition on silicon using an ion energy of 100 eV at room temperature. activated carbon. Mater. Vapour liquid solid (VLS) growth Boron carbide whiskers can be grown by carbothermal VLS growth mechanism. Todorovic et al. Various methods of densification. 7 9 Boron carbide nanobelts prepared by VLS growth from charge of boron oxide.. 2007. This mechanism involves the transport of boron and carbon as gas phase species to a liquid catalyst metal (Fe. 2002. Key Eng. 2167 Published by Maney Publishing (c) IOM Communications Ltd ion species and the substrate temperature. 1 in p. C2H6. BI3. activated carbon Magnesiothermic reduction B2O3 or Na2B4O7 PC. channelled porosity. citric acid. ethylene glycol CCl4 Low temperature process. C2H4. not amenable for large scale production Polyvinyl alcohol. MgB2 High cost of elemental boron PC. activated carbon PC.179 have observed the microstructure of B4C compacts fired above 2000uC to be highly porous interconnected structure with clusters of grains connected by small neck like regions and separated by large. The additive by itself or due to in situ reaction with boron carbide would form a non volatile second phase aiding in densification and property enhancement. Low temperature process High free carbon content. In the first step green compacts with Advantage Cheap raw material. surface diffusion and evaporation condensation mechanism occur. C2H2.184 have observed the start of densification at 1800uC. in real life.g. At higher temperature exaggerated grain growth also takes place resulting in poor mechanical properties.Suri et al. Lee et al. selection of the additive should be directed towards the formation of a suitable structure providing the correct properties for use. Selection of the additive and the method of consolidation are generally dictated by the end use of the product and the properties that are required. Detailed literature survey on pressureless sintering with or without sintering aids. suitable for commercial production Fine powder. Hence. SHS: self-propagating high temperature synthesis. B2O3. etc. dimension and grain size while sintering of boron carbide. due to difficulties in densification. Densification is achieved only at temperatures . Gas compaction methods are hot isostatic pressing and high pressure gas reaction sintering. hot isostatic pressing). At temperatures . BBr3. of Boron Carbon Suitable for BxC academic interest only Carbon black Suitable for whisker Need of molten metal B2O3 catalyst. CCl4 Synthesis from elements Boron Vapour phase synthesis Synthesis from polymer precursors Liquid phase reaction Ion beam synthesis Vapour liquid solid growth Difficult to produce B4C powder suitable for densification. This operation is carried out in two steps. obtained in lump form. the sintered body contains . fibers. exothermic reaction.5% residual porosity. fine powder. Recent or advanced techniques such as microwave/ spark plasma sintering. Densification of boron carbide without deterioration of mechanical properties can be achieved either by using a suitable sintering aid and/or applying the external pressure (e. The surge in densification 1870–2010uC is attributed to the presence of oxide layer which helps in precipitation of B4C through liquid B2O3 or due to evaporation and condensation of rapidly evolving oxide gases (BO and CO). application of B4C is rather limited. activated carbon CH4. B6H6. suitable for SHS process No loss of boron. These techniques are presently limited to laboratory scale only.183. which results in mass transfer without densification. good control over purity and carbon content of product Suitable for thin films. of academic interest only BCl3. explosive compaction. Synthesis and consolidation of boron carbide: a review pressing. spark plasma sintering and super high pressure sintering. Very high sintering temperatures are required for densification due to the presence of predominantly covalent bonds in B4C. Boric acid. hot pressing. need grinding for powder production Product contaminated with Mg. Grabchuk et al. whiskers Disadvantage High boron losses. spark plasma and microwave sintering of boron carbide are presented in the following sessions. Boron carbide particles generally have a thin coating of surface oxide layer which hinders the densifications process. At temperatures above 2250uC. graphite.180–182 have found that shrinkage starts at 1500uC. Densification of boron carbide In spite of its high temperature strength. Figure 11183 shows the changes in weight. polyvinyl pentaborane.2000uC. rapid increase in Table 6 Comparison of boron carbide synthesis methods* Method Carbothermic reduction Boron source H3BO3 or B2O3 Carbon source Published by Maney Publishing (c) IOM Communications Ltd densification 1870–2010uC and a slow down in densification rate 2010–2140uC. Slower densification at temperatures above 2010uC is attributed to evaporation and condensation of B4C. B2O3 *PC: petroleum coke. low selfdiffusion coefficient and high vapour pressure. One more observation at temperatures . low fracture toughness and low oxidation resistance beyond 1000uC. Consolidation of B4C is complicated due to its high melting point.2150uC is volatilisation of non-stoichiometric boron carbide. hydroxyl still in laboratory stage polyvinyl borate. Need of reactive metal BBr3. by grain boundary and volume diffusion mechanisms. International Materials Reviews 2010 VOL 55 NO 1 17 . boron suitable for nanoparticles such as Na or Li. hot pressing. new method of synthesis Only for thin films. graphite. ethyl benzyl alcohol. Pressureless sintering Pressureless sintering is a simple and economic process to produce dense compacts. help to obtain dense products without microstructural coarsening. leaving minute carbon behind at the grain boundaries. graphite. decaborane sugar. recrystallisation above 1800uC and rapid grain growth above 2200uC. hot isostatic pressing.2000uC. Dole et al. high pressure generated by ignition of a combustion gas mixture which raises the pressure in the chamber dramatically in a very short period of time and pushes down the top ram on the powder at an extremely high speed realising the compaction. combustion driven compact process. Grain coarsening is the common feature in compacts with high densities obtained by pressureless sintering. 1472 sufficient handling strength are prepared by uniaxial die compaction. These twins slowly vanish during high temperature annealing.191. vapour phase diffusion of boron carbide is the important transport mechanism for coarsening. a densification .179. Int.Suri et al. yields much higher green density and strength than the normal die compaction. 12. Am. Fig.193 Microstructures of samples with 87 and 93% TD. Fig. Such compacts have a coarse grained microstructure of 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 reprinted with permission from Elsevier. A recently developed new technique.186–204 Increase in particle surface area (9 to 17 m2 g21) and sintering temperature (2100 to 2190uC) give higher densities (56 to 71% TD). Rapid heating is helpful in achieving higher densities with fine microstructure. Ceram. But the application of vacuum helps in evaporation of the surface oxide layer and also prevents further oxidation. Vickers hardness and flexural strength of the pressureless sintered boron carbide samples are in the range 18– 24 GPa and 120–200 MPa respectively. Literature data on pressureless sintering of boron carbide and the product evaluation are presented in Table 7.188. Surface to surface mass transport which is active at temperatures below which densification can proceed is responsible for the coarsening process. (9).. 8 in p. 230–231 18 International Materials Reviews 2010 VOL 55 NO 1 . Removal of the oxide layer by heating in a reducing atmosphere before sintering also has a similar effect..183. grain size and coefficient of thermal expansion up to 2300uC:183 reprinted with permission from Wiley-Blackwell. dimension. One can conclude that. These green pellets are then fired at chosen high temperatures in controlled atmosphere. as the compacts can be heated to a temperature where densification can take place before the microstructure becomes highly coarsened.179. there by promoting the sintering mechanisms.1 mm size. Soc. Synthesis and consolidation of boron carbide: a review Published by Maney Publishing (c) IOM Communications Ltd 11 Sintering of boron carbide compact: change in weight. At higher temperatures.183. Ceram.193 Grain sizes are in the range 40–100 mm indicating large grain growth.185 In this process. 32.2200uC with particles close to or . 2003.187 Densities of . obtained by pressureless sintering of 0?8 mm median diameter powders at 2300 and 2375uC are presented in Fig. 86.90% TD is possible only at very high sintering temperatures of y2300uC. which are lower than theoretical values.205 Appearance of twins in the grains is characteristic of boron carbide. J. 2006.90% TD are achieved by sintering at a temperature of . with pure B4C. 2(b) and (g) in p. Sintering of B4C powder compacts is commonly performed in an inert gas medium. Ar 2170 to 2230uC. vacuum 1 h.4–98.0–25.58– 3. C: 19. 15 min. 2275uC.Ar 78.3–92. D5050.0 to 10.5 wt-%C B4Cz9 wt-%C B4C B4Cz4%C B4Cz4%B 2200uC. 2250uC. 2325uC.8.19 Microstructure. GPa Flexural strength.4 93 91 90 93 82 86 90 93 96 96 98 98.2 … 353 65% 70–72% . Ar 2 h.6 82.91–3. O: 1.0–95. 1 1 1 1 1 1 2 2 2 h h h h h h h.5 97.23%.7 87. 2250uC.59 TiB2: D90<4 192 (2005) Suri et al.9 3.8. (year) 186 (1988) 188 (2004) 179 (1989) 1 B4C FC: 2–4.8 to 3.Ar h.5 92 93 89 86 95. 2250uC. % Vickers hardness.4% Carbon black: D50520 nm.7 to 3.84 SS: 18.8 95. 2250uC.6 160–180 … … 94–95 … … … … … 12.8 B/C53. vacuum 1 h. 2250uC.85 2.2 2. vacuum 1 h.4 … … D5051 SS: 12 D50(1 SS: 22 D50. 2250uC. Ar h. 2250uC.7 98. Ar 2190uC.6 2.05 3. MPa m1/2 Material composition. 2325uC.05%. 2375uC.5 2 B4C Starck make B/C53.9.0 19–21 19–21 18–20 19–23 21–25 24–25 … … … … … 174 … 350 … … … … … … … … D50<3 SS: 18. 2150uC.Published by Maney Publishing (c) IOM Communications Ltd Table 7 Powder details.5 B4C: B4C: B4C: B4C: B4C: B4C: … B4C: … B4C: … B4C: 22 26 29 31 29 26 23 2. Ar … … 3.5 to 2 (Acheson) B: 78%. Ar 1 h.5% 193 (2006) 10 D5051. sintering parameters and characteristics of sintered boron carbide by pressureless sintering* Serial no. Processing conditions 94. 15 min.33 SS: 6.5 SS: 10.8 2. 2250uC. 2250uC. N: 0.5 2. SS: 120 SiC (SS): 11. wt-% Starting powder details Sintered density rth. 1 h.9 2250uC.32 D50(0.53 8 B4C B4C B4Cz3 wt-%C B4Cz5 wt-%C B4Cz7. O: 0. MPa Ref. 15 min. vacuum vacuum 50–120 y20 y10 y5 y15 30 17 14 10 32 213 (2008) Synthesis and consolidation of boron carbide: a review 2010 VOL B4Cz4%SiC B4Cz4%TiB2 B4C B4Cz1 wt-%C B4Cz3 wt-%C B4Cz5 wt-%ZrO2 B4Cz5 wt-%TiB2 B4C B4Cz5 wt-%TiB2 B4Cz10 wt-%TiB2 B4Cz15 wt-%TiB2 B4Cz20 wt-%TiB2 B4Cz25 wt-%TiB2 B4Cz30 wt-%TiB2 90.4 220 260 290 360 320 280 270 55 NO 1 19 . 2 h.95% 78 96 … 95 Coarse Coarse Fine B4C: 6 B4C: 4 B4C: 105 B4C: 28 189 (1981) 190 (1987) 5 B4C B4C B4Cz6 wt-%C B4C B4Cz3 wt-%C (phenolic resin) B4C B4Cz(polycarbosilanez phenolic resin510%) 2250uC 2250uC 91.26–2. vacuum 1 h.76 B4C: 28 B4C: 50 B4C: 13 … … 183 (2003) 7 191 (2003) SS: 2. SS: 15–20 3 4 2175uC.6 … … SiC: . mm KIC. D5052. 2375uC. Ar (up to 2000uC in vacuum) 2250uC 2300uC 2300uC 2150uC.3 B4C: 2.64 B/C53. 9 International Materials Reviews D5050.5 6 B4C B4Cz3 wt-%C (phenolic resin) B/C: 4.8.11 B4C: 2. 07 13 196 (2006) D5051 SS: 14 ZrO2z3 wt-%Y2O3 D5050. h.34 26 27 23 26. 2150uC.62%Al2O3. Ar (up to 1500uC: vacuum) 187 (2000) B4Cz10 B4Cz20 B4Cz30 B4Cz40 wt-%TiO2 wt-%TiO2 wt-%TiO2 wt-%TiO2 D5055 to 7 SS: 17 TiO2: D100(2 . h.1 2. B4C. 2150uC.4 2. h. mm Ref. Ar 2160uC. inert inert inert vacuum 1 1 1 1 1 h.86 95. h.9 3.7 221 (2008) Flexural strength. (year) 195 (2006) International Materials Reviews 11 B4C B4C B4Cz5 wt-% talc B4Cz10 wt-% talc B4Cz15 wt-% talc D5051. B4C.71 31. h. wt-% Starting powder details Sintered density rth. B4C. 2190uC. 25.6 2. 2190uC.1 14 B4C B4C B4Cz5 wt-%ZrO2 B4Cz10 wt-%ZrO2 B4Cz15 wt-%ZrO2 B4Cz20 wt-%ZrO2 B4Cz25 wt-%ZrO2 B4Cz30 wt-%ZrO2 B4C B4Cz5 wt-%Ti B4Cz10 wt-%Ti B4Cz15 wt-%Ti B4Cz20 wt-%Ti D50 . inert inert inert inert inert 74 78 82 85 90 Material composition. MPa m1/2 1. h.19 30.9 Talc: 26. 2190uC. 2190uC.2 2. 2150uC.3 2.8 to 3. h.2 2.85 … 31. 2150uC. 2150uC.56 93. Processing conditions 2050uC. 1 1 1 1 1 1 1 1 1 1 1 1 1 h. h. GPa KIC.2 (up to 1500uC: vacuum) 2050uC.68 31.0 2.0 2.8 SS: 16 19 22 25 27 28 29 27 26 2. h.64 B/C53.5 wt-%ZrO2 B4Cz5 wt-%ZrO2 B4Cz10 wt-%ZrO2 B4Cz15 wt-%ZrO2 B4Cz20 wt-%ZrO2 B4Cz25 wt-%ZrO2 B4Cz30 wt-%ZrO2 B4C B4C. 2150uC.Published by Maney Publishing (c) IOM Communications Ltd Suri et al. h.78%SiO2. MPa 18 21 22 23 25 Serial no.09 ZrO2 reactor grade ZrB2 ZrB2 ZrB2 ZrB2 ZrB2 ZrB2 ZrB2 ZrB2 NO 1 B4Cz20 wt-% talc B4Cz25 wt-% talc B4Cz30 wt-% talc B4C B4Cz2. B4C. h. 2150uC.0 3. 47. h. 2150uC.18 30. h.3 95. 2150uC. 2190uC. B4C.9 2. h.5 93. Ar 2160uC. h.95 31.1 SS: 9–17 TiO2: D50.6 2. 1 1 1 1 h. h. 1 h. Ar Ar Ar Ar Ar Ar Ar Ar Ar Ar Ar Ar Ar 72 75 79 86 97 97 97 98 62 67 73 78 86 190 200 205 210 220 235 260 340 50 … … … … … 72 77 88 95 71 73 75 80 93 B4C: 10 TiB2: 5–7 200 300 350 420 120 B4C: 10 TiB2: 5 200 300 330 400 197 (1999) 15 B4Cz5 wt-%TiO2 B4Cz10 wt-%TiO2 B4Cz15 wt-%TiO2 B4Cz20 wt-%TiO2 B4C 2160uC. 2150uC. Ar 2160uC. h. B4C. 1 h.0 3.6%MgO Synthesis and consolidation of boron carbide: a review 2010 12 VOL 55 B4C: D50<1 B/C54. B4C. 92 95 98 86. 2275uC. % Microstructure.82 92. 2150uC. 2150uC.08 95. 1 h. 2150uC. Ar 2190uC.33 SS: 6. 20 Table 7 Continued Vickers hardness. 1 h. h. h.63 89.1 2. 2150uC. 1 h. 7 286 266 200 (1998) Suri et al. 2015uC.0 wt-%BeC B4Cz10 wt-%SiCz3 wt-%Al B4Cz25 wt-%CrB2 D100: submicrometre SS: 15. Ar 2030uC.10% D5050. h. 2 h.53%. MPa … Serial no. TiB2. m2 g21.-%ZrO2 B4Cz28 vol. TiB2.0 wt-%BeC B4Cz1. 15 min.43 B4C: 20–50 B4C: 20 CrB2: 20 B4C: 100–150 2.08 SS: 16.35 SS: 19.5%TiC B4Cz6. C 8.3 202 (1982) 203. 204 (2002. mm.7 5. Ar Ar Ar Ar Ar B4C: B4C: B4C: B4C: B4C: B4C: 8.0 94 90.3 15 min. h.7 3.98%.7% ZrO2: submicrometre TiO2: submicrometre Y2O3: submicrometre 2180uC.2 2 2 2 2 2 h. 1 h. 2015uC. 15 min.71 350–513 Flexural strength. 2200uC. TiB2.1 85. 10 min. 1 h. Fe: 140 ppm Al: 50 ppm CrB2: D5053. % Microstructure. 2230uC. mm Ref.4 4.92 12.5 VOL *FC: free carbon in B4C. (year) 216 (1996) 16 B4Cz0 to 30 wt-%TiO2z1 to 6 wt-%C B4C: D5050. TiB2 18. 2200uC. SS: specific surface area. 2200uC.4% N2) Al2O3: 99.32% FC: 0.5%TiC B4Cz3.0 wt-%BeC B4Cz1. Ar TiB2: 6–10 Up to 99% B4C: 22¡2 Material composition.5 94.9) O2: 1.9) (1 wt-%O2 and 0.-%Y2O3 2150uC. 1 h. 1 h. Ar B4C: 7. 15 min. 55 NO 1 21 . Ar 83 98 98 97. 2002) B4Cz25 wt-%CrB2 B4Cz27. Ar 2030uC.63 5.07 B4C: B4C: 3.1 B/C: 3.9 wt-%CrB2 SS: 15.Published by Maney Publishing (c) IOM Communications Ltd Table 7 Continued Vickers hardness.0 O2: 0. D50: mean particle diameter.0 3.639 17 B4C B4Cz22 vol. C 31 29 28 28 85 92 93 96 88 86 96.9 wt-%CrB2 Synthesis and consolidation of boron carbide: a review 2010 O2: 2% max.5 O: 0.6 (B/C53.5%TiC B4Cz1. N2: 0. C 5.5 D5050. Ar Ar–N2 Ar–N2 Ar–N2 Ar–N2 Ar 71. N: 0.9 (B/C53.9 96. 2280uC.64% TiC: D5051. 2200uC.8 404 400 525 108 International Materials Reviews B4Cz27.-%TiO2 B4Cz8 vol.57 BeC: submicrometre B4C: D5059. TiB2 11.0%TiC B4Cz4. 2150uC. SiC: D5052. 15 min. h. 1 h.0%TiC 2175uC.9 80.8 Purity: 99. h.6 98.89% TiO2 and C: submicrometre D5051. wt-% Starting powder details Sintered density rth. GPa KIC.2–3. 2130uC. 2260uC.6 2. Processing conditions 1900–2050uC.5 … 27–31 28–33 27–30 … 350 375 180 198 (2007) 18 D50<0.3 98.1 98. Ar 93. 20 201 (1977) 21 22 B4C B4Cz1.1 96. MPa m1/2 3.99% pure 199 (1992) 19 B4C B4Cz1 wt-%Al2O3 B4Cz2 wt-%Al2O3 B4Cz3 wt-%Al2O3 B4Cz4 wt-%Al2O3 B4Cz5 wt-%Al2O3 B4Cz1.3 94. Zorzi et al.206 have studied the sintering kinetics of pure and carbon doped boron carbide with 0?42 mm sized B4C powders in the temperature range 1900– 2200uC and a period of 5–45 min.179 Carbon addition inhibits the coarsening process. it has been found that solid state sintering (1500 to 1850uC) starts only after the evaporation of B2O3. Various types of carbon such as petroleum coke. Carbon. which form the weaker regions in the sintered product. carbon black. Carbon as sinter additive Various sinter additives have been tested to increase the rate of densification. less faceted grain structure with smaller and more uniformly distributed pores were prepared by the addition of 6 wt-%C (in the form of a thermoset resin). They have deduced.191 With 3 and 5 wt-%C the density obtained was 92 and 93% TD respectively. in mixtures such as planetary mill/attritor to form an intimate contact such as a fine coating on boron carbide particles. 2 and 3 in p. primarily in reducing the oxide layer of the boron carbide powders.g. An additive such as phenol formaldehyde resin plays two roles. Yin et al. High densities of 98?65% TD with a fine grain size of 2?34 mm have been achieved by the addition of 3%C in the form of phenolic resin. facilitating accelerated solid state sintering. While studying the densification behaviour of nanosized boron carbide. J. carbon at the grain boundaries enhances diffusion. phenol formaldehyde) can be used as sintering aid. thereby preventing the formation of large unsinterable pores.192 have used carbon black of 20 nm size (specific surface area: 120 m2 g21) with boron carbide powder of surface area18?8 m2 g21 to obtain 97?7% TD compacts having Knoop hardness in the range 19–21 GPa by sintering at 2250uC for 2 h. Soc. 13a).Suri et al. it is mandatory that very fine size is used and the mixing carried out thoroughly. 2004. well distributed between the particles reacts with B2O3 coating according to the reaction 2B2 O3 z6C?B4 Cz6CO (18) Removal of oxide layer allows direct contact between B4C particles.207 have obtained compacts of 97–98% TD with the addition of 1–3% phenolic type of carbon using a powder of large surface area (22 m2 g21). Figs. 22 International Materials Reviews 2010 VOL 55 NO 1 . control grain growth and improve mechanical properties of boron carbide. namely as binder while cold pressing and as carbon precursor which is uniformly distributed on the surface of the grains. Jpn. glucose and phenolic resin (e. Carbon has been found to be very effective. Schwetz et al. A fine grained (7–8 mm) compact with superior mechanical properties (flexural strength: 351–353 MPa and fracture toughness: 3?3 MPa m1/2) was obtained. showed the grain boundary to be free from carbon and the excess carbon present as fine graphite crystals at the triple point resulting in excellent mechanical properties (Fig. 13b). In addition. With higher amounts of carbon (>7?5%C). a reversal in densification was observed. (5). Ceram. high amount of intragranular porosity and poor flexural strength (y200 MPa). carbon doped B4C undergoes normal sintering and nearly full density can be achieved. Synthesis and consolidation of boron carbide: a review Published by Maney Publishing (c) IOM Communications Ltd 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. Phenolic resin as carbon addition is found better than carbon black and glucose.209 High resolution TEM image of the sample shows no boundary or amorphous layer (Fig. Any carbon which is not consumed by the reduction reaction is left in the compact as excess carbon. the main sintering mechanisms to be volume and grain boundary diffusion for pure boron carbide and grain boundary diffusion for carbon doped boron carbide showing activated sintering. graphite. As a result. Much of the carbon remains in the B4C microstructure as graphite particle. If carbon in solid form such as coke. 112. Very fine powders in the range of nanosizes would increase the rate of sintering due to very large surface area and particle to particle contact. graphite or carbon black is chosen. The formation of eutectic (B4C-C) liquid droplets appears on the surface of graphite coated B4C particles above 1920uC.208 Sample sintered at 2300uC.209 Large.183 Compacts exhibiting a finer.189. permitting sintering to initiate at significantly lower temperature (y1350uC). S400 y50 mm. there by promoting sintering and hindering grain growth. 192 Addition of a small amount (3–5 wt-%) of carbon to B4C plays an important role in eliminating the surface oxide layer. other grain refinement agents for B4C are Si and Al.211 Carbides/borides can either be directly added or formed by in situ reaction with B4C while sintering. Hf. Synthesis and consolidation of boron carbide: a review complex boron carbide shapes of density . Addition of carbides and borides have been found to increase the flexural strength and fracture toughness of B4C by grain refinement and crack propagation influencing mechanisms such as crack deflection. Baharvandi and Hadian213 have reported the addition of TiB2 on sinterability and mechanical properties of B4C.214. flexural strength increased from 292 to 502 MPa. V2O5. This leads to the formation of substoichiometric boron carbide.198 have studied the reaction between B4C and MeO mixtures (MeO–TiO2.2000uC to obtain densities . Khazai et al. Densities of 97% TD were obtained in samples with >15%ZrO2 addition. micro crack interaction and crack impediment.192 have reported that addition of 4 wt-%TiB2 to B4C lead to good results in final density. 15). The hardness and strength of composite were comparable to that of hot pressed B4C. Addition of various transition metal (Ti. Densities higher than 95% TD were obtained by sintering at temperatures above 2150uC. 16)221 shows the mode of fracture to be a combination of transgranular and intergranular. Boron carbide reacts with ZrO2 during sintering to form ZrB2 as per the following reaction B4 Cz2ZrO2 ?2ZrB2 zB2 O3 zCO: (21) Published by Maney Publishing (c) IOM Communications Ltd (19) Levin et al. Grain size of B4C was found inversely proportional to the quantity of carbon in the sample. Such composites exhibited a sintering aptitude higher than that of monolithic B4C. Role of carbide/boride additives densities of 99% could be achieved without pressure at temperatures of 2050–2100uC.220 have patented a process for the preparation of boron carbide/titanium diboride composite with uniform distribution of both. Nb and Ta) carbides/borides for preparing dense boron carbide pellets have been patented. At 15%TiB2 the highest hardness of 31 GPa and flexural strength of 360 MPa were obtained. Particles with high aspect ratios are most suitable for maximum toughening especially particles with rod shaped morphologies. TiB2 is formed by reaction sintering of boron carbide with titanium oxide at y1500uC as per the following reaction B4 Cz2TiO2 z3C?2TiB2 z4CO: 197 The amount of second phase TiB2 and excess carbon play a distinct role in sintering.Suri et al.210 Thermosetting phenolic types of resins are most suitable as carbon precursors as they give a complete uniform coating of carbon on the surface of carbide particles.221 Figure 14 presents the variation in hardness of pressureless sintered B4C with ZrO2 addition. ZrO2. The main solid reaction products are found to be borides. hardness and wear resistance. The in situ reactions for the formation of carbide/ boride consume some of the carbon from boron carbide. Carbon can be replaced by boron to react with surface oxide layer of B4C to achieve similar effects. Increase in volume fraction of TiB2 led to an increase in flexural strength and fracture toughness to a maximum of 513 MPa and 3?7 MPa m1/2 respectively at 15 vol. Zorzi et al. Greater amounts may diminish the toughness increase due to overlapping particles. Titanium carbide also reacts with boron carbide to form TiB2 as per the reaction B4 Cz2TiC?3Cz2TiB2 (20) In addition to carbon. increasing with the amount of metal oxide in the initial mixture. which is sintered at temperatures . Processing in vacuum and higher temperature (2275uC) increased the hardness to . Although the strength can be significantly improved by grain size reduction. in addition to maintaining chemical compatibility. which is responsible for increased rate of sintering. and toughness decreased from 4?2 to 2?9 MPa m1/2. V. International Materials Reviews 2010 VOL 55 NO 1 23 . whereas bending strength and hardness improve to certain amount (15%TiB2) and then start decreasing. Density and fracture toughness values were found to increase with TiB2 fraction in the entire range of 0 to 30%. Goldstein et al. Addition of 40 wt-%TiO2 to B4C powder (17 m2 g21 specific area) gives a compact of 95% TD after sintering for 1 h at 2160uC.30 GPa. have found that TiO2 is reduced by carbon originated from the carbide phase. In addition boron will also react with free carbon of B4C to form carbide. flexural strength: 160– 350 MPa). Cr2O3. The quantity and the method of carbon addition have to be carefully chosen to avoid free carbon in the sintered body.200 As the carbon content increased from 1?5 to 6?0%.218. TiB2 also acted as a grain growth inhibitor.219 Boron carbide is reacted at approximately 1500uC with the transition metal oxide/carbide to form a mixture of boron carbidezmetallic carbide/boride. Fractography (Fig. Baharvandi et al.216.187.95%. the grain size of B4C decreased from 10 to 3 mm. thereby achieving higher densities (y97% TD) with fine grains and improved mechanical properties (hardness: 19–25 GPa.197 Presence of TiB2 results in lowering of activation energy for sintering and hence very high Fracture toughness and flexural strength of the compacts increased from 2?1 to 3?1 MPa m1/2 and 200 to 340 MPa respectively with the increase of ZrO2 content from 0 to 30%. Hardness in the entire range of composite is y30 GPa compared to 27 GPa for pure B4C. The ideal second phase. the toughness remains low. Faber and Evans212 have predicted that fracture toughness increases due to crack deflection around second phase particles in two-phase ceramic materials.217 The observed increase in strength and fracture toughness are due to the interaction between the propagating crack front and local thermal mismatch stress associated with TiB2 particles. With 30%TiB2 the highest density and fracture toughness of 98?5% TD and 3?4 MPa m1/2 were achieved respectively.94% are fabricated by slip casting method using a binder and sintering at 2280uC for 2 h. Zr. should be present in amounts of 10 to 20 vol.215 Metallographic examination revealed a two phase microstructure with B4C grains of 10 and TiB2 of 5–7 mm size.-%.196 have studied the effect of addition of Yttria doped zirconia on sintering behaviour (between 2050 and 2150uC in argon for 1 h) and mechanical properties of B4C. Y2O3 and La2O3) fired up to 2180uC for 2 h in argon.-%. Backscattered image of this sample shows a white phase containing up to 1?2%Zr distributed in B4C matrix (Fig. a non-stoichiometric boron carbide 24 International Materials Reviews 2010 VOL 55 NO 1 .and intergranular:221 reprinted with permission from Elsevier. Int. This defective structure enhances diffusion kinetics thereby improving the rate of sintering. Silicon carbide has attractive properties. 32?6% total silicon. 2008. Liquid phase sintering 16 Image (SEM) of fractured surface of sintered boron carbide with ZrO2 addition: mode of fracture seen as combination of trans. Synthesis and consolidation of boron carbide: a review 14 Variation in hardness of pressureless sintered B4C with ZrO2 addition (hardness in entire range of composite is y30 GPa compared to 27 GPa for pure B4C):221 reprinted with permission from Elsevier. It is suggested that the application of external pressure during sintering to accelerate densification faster than the kinetics of phase formation.223 have studied the problems of B4C–Al particulate composite fabrication and explained that chemical reactions between B4C and aluminium occur between 800 and 1400uC much faster than capillary induced liquid rearrangement. Ceram. similar to that of boron carbide such as high hardness (28 GPa). Al3BC. low specific gravity (3?1 g cm23) and good wear resistance and hence considered an attractive sinter addition to boron carbide.225 describe a process where silicon is infiltrated into a porous body of boron carbide and then sintered in the temperature range 1500–2200uC to obtain dense (2?57 g cm23) non-porous boron carbide body which is extremely hard (modulus of rupture 235 MPa. Fig. 50?9% total boron. Ceram. Al3B48C2 at the interface.224 Silicon with a melting point of 1410uC. capillary action.226 Mallick et al.227 have demonstrated the possibility of net shape production via infiltration of Si melt into porous preform containing B4C and carbon. X-ray diffraction pattern obtained correspond to normal B4C. 1545 Published by Maney Publishing (c) IOM Communications Ltd thereby generating substoichiometric boron carbide. Chen et al. Attempts have been recently made to prepare B4C– CeB6/Al composite with improved strength and toughness by pressureless infiltration technology. 15 Backscattered image of sintered boron carbide with ZrO2: white regions analysed to contain 1?2%Zr Danny et al. Wettability. Taylor et al. Fig. when added to boron carbide acts as a liquid sinter additive and in addition reacts with carbon of boron carbide to form silicon carbide. 2008. dissolution and reprecipitation are the important parameters. The fraction of free Si can be reduced by increasing the green density of the initial boron carbide preforms. AlB2. 1547 Liquid phase sintering is carried out either by the addition of a substance with low melting point or by the formation of a low melting substance by in situ reaction of the additive with boron carbide. If the second phase is not as hard as boron carbide and then presence of large volume is likely to deteriorate the mechanical properties and hence one has to optimise the level of second phase addition to obtain the best properties.Suri et al. which decide the ability to sinter and the mechanical properties. thus helping the sintering process and also strengthening the matrix. inhibiting the densification process. 0?16% free carbon and 12?6% free Si. 34. Wetting of polycrystalline B4C by molten aluminium between 900 and 1200uC have been studies by Lin et al. second boron carbide type having an expanded lattice. The presence of unreacted. 3 in p.222 They found the Wettability of B4C to be strongly depend on temperature and the formation of different reaction products such as Al2?1B51C8. modulus of elasticity 353 GPa).. The improved mechanical properties are attributed to the presence of fine distribution of secondary phase particles in the matrix.228 have studied the formation and sintering mechanisms of reaction bonded silicon carbide boron carbide composites. free silicon lowers mechanical properties of reaction bonded B4C. 34. The chemical analysis showed 16?5%C.. 6 in p. Int. In the product sintered at 1450uC. alpha and beta silicon carbides and silicon. This helps in two ways: by liquid phase sintering and by SiC formation which greatly improves the mechanical properties. metallopolysiloxanes and metallopolysilanes. As per another US patent by Prochazka201 addition of 0?5–3 wt-%BeC to boron carbide helps in achieving a density of 85–94% TD by sintering at 2200–2280uC.235 There was no significant growth of B4C grains and the densification was mainly attributed to B4C particles rearrangement caused by the CrB2–B4C eutectic liquid formation. Though higher densification can be achieved at higher temperatures. The sample sintered at 2050uC for 1 h showed the formation of MgB2. The mechanism of sintering is due to the formation of liquid phase anstatite at 2000uC and its reaction with B4C to form SiC. 5?0 MPa m1/2. are responsible for increased flexural strength and fracture toughness on account of strengthening effect of the high aspect ratio of the second phase particles. The improvement in fracture toughness is due to the formation of microcracks and the deflection of propagating cracks due to the thermal mismatch of CrB2 and B4C. B4C cermets. 1446 Baharvandi et al. polysilanes. Lett.199 Exaggerated grain growth was observed in the specimen containing . the rate of densification is very slow. Mater. 17).204. Low melting metals/alloys such as Al and Al–Si have been used to bind boron carbide particles to different shapes for neutron absorber applications. this is the method chosen for the manufacture of B4C based armour shields. Silicon is found to be an excellent additive which is introduced into a sintered porous body of B4C by infiltration technique. Liquid phase sintering of boron carbide is carried out by in situ reaction with alumina. Iron additions also provide the desired porosity for successful infiltration of preforms. 17 Fracture surface of B4C–CrB2 specimen indicating partially melted CrB2:204 reprinted with permission from Springer. Though the density (78– 98%) and fracture toughness (2–2?8 MPa m1/2) of the compact increased with the quantity of talc in the charge up to 30%.229 The role of iron in the sinterability of B4C has been studied by contact angle measurement and dilatometric densification. abnormal grain growth of B4C occurred. 2002. 278 MPa and fracture toughness. silica. it is difficult to prevent grain International Materials Reviews 2010 VOL 55 NO 1 25 .234 which upon pyrolysis yield SiC and free carbon have been found to be very effective in pressureless sintering of boron carbide.-%CrB2. 4 in p. CrB2. compacted and sintered at 2100uC to give compacts exceeding 90% TD. Properties of the B4C material prepared by this method are given as: density.230 Mizrahi et al.Si. Specimen with 98?1% TD (20 mol. J.233 have found that presence of a large fraction of plate-like SiC in samples formed due to initial higher porosity. 382 GPa.232 Hayun et al. SiC and Al2O3. for application as neutron absorber have been prepared by sintering with a eutectic alloy of Al–Si at a low temperature of 550uC. Published by Maney Publishing (c) IOM Communications Ltd Hot pressing In conventional sintering. This reduces the sintering temperature by a few hundred degrees and the sintered product has a fine microstructure and moderate mechanical properties.231 have also attributed the formation of liquid phase promoting the mass transfer mechanism in B4C–Fe mixtures. 2–40 wt-% silicon carbide and 0–10 wt-% aluminium with densities exceeding 94% TD by cold pressing followed by a pressureless heat treatment. The dynamic strength and the dynamic fracture toughness K1d are significantly higher than the corresponding static properties and are insensitive to the residual silicon fraction and to the strain rate (up to 5103 s21). The sintering kinetics confirmed the liquid phase sintering leading to improved mass transfer. Addition of carbon to react with infiltrated silicon for forming silicon carbide rather than with boron carbide is beneficial as silicon carbide has a lower hardness and higher specific gravity compared to boron carbide. Weaver202 has patented a process where in a mixture of boron carbide and silicon carbide are mixed in an aluminium mill to a composition of 87B4C–10SiC–3Al which is sintered at 1800–2300uC to densities up to 94% TD. The fractured surface of the B4C specimen with 20 mol.-%CrB2 prepared at 2000uC showed the CrB2 particles to be partially melted and wetted with the adjacent B4C particles (Fig. flexural strength. A US patent236 explains a process wherein boron carbide powder is mixed with aqueous sodium silicate (to give SiO2 equivalent of 0?75–1?5 wt-%B4C) and alumina (1–3 wt-%). MgB2 and Al2O3.190 polysiloxanes. Chromium boride forms a eutectic liquid phase with B4C at a temperature of 2150uC. microhardness attained a peak value of 26 GPa at 25% and then fell down. Maximum density of 96% TD was achieved by the addition of 3 wt-% alumina doped B4C sintered at 2150uC for 15 min. However at temperatures above 2200uC.B)3 and a minor phase rich in B10C or B49C1?82 were seen.Suri et al. 21. Addition of alumina improves the densification of boron carbide with the formation of liquid phase AlB12C2 by the reaction between B4C and Al2O3. Owing to fine grain structure and high fracture toughness of the end product. polysilazanes.4% alumina. BeC. sintered at 2030uC) showed a high flexural strength of 525 MPa and a moderate fracture toughness of 3?7 MPa m1/2.. Organosilicon polymers such as polycarbosilane. Sci. young’s modulus.203. 2?57 g cm23. Fig. Synthesis and consolidation of boron carbide: a review phase B12(C.195 have studied the effect of high alumina talc (26?62Al2O3–47?78SiO2–25?6MgO) powder as a sintering aid. Microcracks were also observed in boron carbide grains. The interaction zone consisted of a fine mixture of FeB and graphite. etc. Weaver in his patent202 has mentioned a process to obtain refractory bodies composed of 60–98 wt-% boron carbide. The powder compacts are typically heated externally using graphite heating elements and the pressure is applied hydraulically.238 have noted that.234. Properties of dense B4C compacts prepared by hot pressing generally have the best properties with the following values:260 hardness.2100uC with an applied load of >30 MPa. 30– 42 W m21 K21. elastic modulus.239–258 presents the literature data on hot pressing of boron carbide with/ without additives.240 A temperature of >2100uC and a pressure of 34?4 MPa are necessary to obtain density close to 100% TD. a steady even growth up to 2050uC (the final grain size 5 mm) and an uneven sized growth and the presence of large number of twins at 2150–2200uC. with an average grain size of 2 mm and faceted submicrometre pores accounting for . Slow cooling after densification has been found to be responsible for reduction in the final density due to the formation of pores while cooling. Though very fine particles are not a precondition. A photograph of a vacuum hot press with front door open showing the graphite die and heaters is presented in Fig. 450–470 GPa. BN lining has been found to be most suitable. 29– 35 GPa. While studying the densification by hot pressing. at temperatures . fracture toughness. Densification by sintering during hot pressing results from three successive mechanisms: (i) particle rearrangement. with the simultaneous application of pressure.1. porosity and microstructure of hot pressed B4C depend on the hot pressing parameters. the size of boron carbide used for hot pressing generally falls in the range 1–10 mm.194. Addition of second phase can only retard but not completely prevent the grain growth. As boron carbide reacts with the die material.186. Synthesis and consolidation of boron carbide: a review 19 Boron carbide pellets of various sizes compacted by hot pressing 18 Vacuum hot press with front door open showing graphite heaters and insulation growth. Hot pressing is the most common method for fabricating dense articles of pure B4C.239 have found that the densification of boron carbide is controlled by a process leading to non-linear creep. Ostapenko et al. dislocation climb is the main mechanism leading to creep. hardness. etc.239 Fast heating rates and application of high pressure (40 MPa) have been helpful in obtaining full densification at a lower temperature of 1900uC. Table 8179.-% porosity. 32?5 GPa. Experimenting on the activated sintering kinetics by the addition of iron. pressure.259 Though at lower temperatures. Photographs of boron carbide pellets of various sizes compacted by hot pressing are presented in Fig. thermal conductivity. Published by Maney Publishing (c) IOM Communications Ltd 26 International Materials Reviews 2010 VOL 55 NO 1 . inner lining of the graphite die is essential to prevent this reaction. 19.1 vol. Without sinter additives B4C can be fully densified by hot pressing . heating rate influences the rate of densification. time. The microstructure of hot pressed specimens show no grain growth (1?5–2?0 mm) up to 1950uC. 18.Suri et al. whose rate is a quadratic function of stress.2000uC it has no important influence on densification. He has also studied the erosion wear of this by abrasive air jets using SiO2. 2?8–2?9 MPa m1/2. SiC and Al2O3 powders.3 mm size with a density.179 Samples obtained under these conditions show a microstructure. whose rate is a function of the square of stress. free of grain boundary phases. Koval’chenko et al. 2?5–3?0 MPa m1/2 and 300–400 MPa respectively. The density of the compacts obtained under identical hot pressing conditions (2150uC for 10 min) were 91?6 and 99?7% TD with starting powders of 3?85 and 0?35 mm respectively. Pressure assisted consolidation/sintering generally involves heating a powder compact. where the total and open porosities decrease and the closed porosity remains constant (temperature range: 1800–1950uC) (ii) plastic flow.237–239 The density. heating/cooling rate. such as temperature. Jianxin250 has prepared boron carbide nozzles by hot pressing at 2150uC in an inert atmosphere with a pressure of 36 MPa using starting powders of . coefficient of thermal expansion. fracture toughness and flexural strength of 95?5% TD. leading to the closing of open porosity without significantly affecting the closed pores (1950–2100uC) (iii) volume diffusion and pore elimination at the end of the hot pressing (2100–2200uC). 0 … 2 .1 6.5 2.5 87 99 91 2. 1 h. 30 MPa 1800uC.5 MPa. submicrometre B4C: D5053 to 5 (W.7 K s21 240 242 (1979) B: D50520 2000uC. CR: 1.5 g cc21 10–50 B4C 34.5 95. sintering parameters and characteristics of sintered boron carbide by hot pressing* Vickers Indentation Flexural hardness.5 … … … 239 (1979) … 179 (1989) 300–400 250 (2005) … 241 (2000) 3 4 5 6 B4C B4C bBzC Amorphous BzC aBzC B4C D5051.5 … 3. 22 MPa.5 B4C: D5050. 15–25uC min21 2150uC.Ti)C B4Cz30%(W.4% D5052. Ar 2150uC.5 23. 34. SS: 0.1 99. 30 min.Ti)C B4C B4C: D5053 to 5. heating rate: 1775 330 K ks21. 1 h.4 80.75 3. 30 MPa 2200uC. Mo: D5051 to 3 … 10–40 B4C: 3.74. 20 MPa.9 SS: 0.60 … 550 695 625 550 NO 1 27 .89 B4C: D50510. MPa.5 1–2 1–2 1–2 1–2 1–2 … 22 25 24. TiO2 99. 8 B4CzTiO2zC 2.5 99. a large number … of twins 2–3 … 4–8 32.1 2. MPa. SS: 15. 8 min B4C: D5050.2 99. SS: 22.8.60 190 200 870 720 815 200 350 500 630 530 400 621 245 400 550 690 540 244 (2000) 246 (2002) International Materials Reviews 10 B4C: D5050. 35 MPa.-%TiB2.25 3.Ti)C: D5051 to 2 2000uC. 1850uC. 1950uC.1–1. TiB2 B4C: 3. TiB2.5–1.5 synthesis from the elements D5051. wt-% Hot pressing conditions K 65. 10 min 72. 2. min.8 3.2 98.1 5–8 247 (2009) VOL 55 B4Cz5.40 4.4 .7%Mo B4Cz15%TiCz4.9% pure. GPa MPa m1/2 MPa Reference (year) … … … 240 (1983) Serial Material composition. 1950uC. 1850uC. 1 h.2 99.8. 40 MPa.1 4.0 5. min. SS: 21. SS: 12. TiB2 0%TiB2 5% TiB2 10%TiB2 20%TiB2 30%TiB2 40%TiB2 15 vol. MPa.76 D5052. 36 MPa. min. nanosize 2100uC.0 wt-% 2100uC. 35 MPa.0 Starting powder details Microstructure. 35 MPa.5. O: 0. Ar Ar Ar Ar Ar 96.5 .5 15.5 99.Published by Maney Publishing (c) IOM Communications Ltd Table 8 Powder details. min.9 4.4 MPa.2 K K K K K … … 2.50.28% carbon black B4C B4Cz10%(W.7 ks. HR: 0. strength.3 D5052.2.0 12.49 g cc21 2. rest: B4C B4C: 6–10 1–2.5–3.0. min.3%Mo B4Cz5%TiCz5%Mo B4Cz10%TiCz4. % mm 1 B4C FC: 1.5%Mo B4Cz20%TiCz4%Mo 1950uC. 35 MPa.63.0 98.5 Suri et al.5 K s21. Ar 7 B4Cz5%B B4Cz15%B B4CzTiO2zC TiO2: D50.41.4 99.0 98.3 6.6 98 7. 1850uC.95 … 243 (2005) 27 30 32 34 30 28 257 (1990) 9 65 50 40 30 65 min.0 5. SS: 1.5 98.4.1 29 28 26 23 21 2. no.95 95.6 4. 60 min 1950uC. W2B5 0. TiC: D5051 to 2. MPa. min. 50 MPa. TiB2 B4C: 3. HR: 95.5 2.27 D5050.Ti)C B4Cz50%(W. cooling rate: 1665 K ks21 1975 2175 2325 2375 2475 2200uC. Ar Ar Ar Ar Ar Ar. min.2 3.44. 35 MPa.0 3. 35 MPa.24–6. Sintered density rth. 35 35 35 35 35 MPa.4%TiO2z5. 50 50 50 50 50 min. 30 MPa 1800uC. 2150uC.9.5 B4C: D550. SS: 19. min. 1950uC. toughness.42 g cc21 100 100 100 . 1950uC.1 13. SS: 12 D50.2 16.1 4.8 3. Synthesis and consolidation of boron carbide: a review 2010 11 B4Cz23. O2: 2050uC.43.2 5.1¡0. nanosized 1850uC.6 2. MPa. toughness. B: 18. Fe: 140 ppm. WCz TiC: 0–45.6 g cc21 3. 10 min to 4 h 45–68 2. 50 MPa.7¡1.2¡1.5 2.59 g cc21 3.9 B4C–TiB2–W2B5 12 26 30 28 25 26 … … 550 620 710 830 710 580 180 Starting powder details Microstructure. Ar.5¡0. 28 Table 8 Continued Serial Material composition.2 3. HR: 15–150uC min21 2275uC. 5 MPa. no.4¡1. cooling rate: 100 K min21 1620uC 1670uC 1720uC 1820uC 1920uC 2120uC Eutectic liquid of CrB2zB4C 1. 2–5 TiB2 0. MPa. MPa. N2 B4C.53 3.0 4.6 2.3%. 35 35 35 35 35 MPa. 1 h.86 95.3¡0. SiC/C … … 25 … … … … 234 (1996) 249 (1974) 17 SizBzC C. Ar NO 1 14 B4C: D5050.0 99. 30 MPa.4 410 254 (2008) 420 410 360 310 530¡30 258 (2005) 560¡30 600¡70 550¡45 300¡34 271 (2008) 430¡31 445¡30 386¡29 323¡32 .1 to 1 B4Czsodium silicatez magnesium nitratezFe3O4 18 B4C: D5053. SiC B/C54.8–7.1¡1. 35 MPa.2 2. SiC B4C. wt-% Hot pressing conditions 66 MPa.5. 30 MPa. min.4 … 350 500 550 620 684 660 675 551 580 630 630 580 … 248 (2003) 273 (1993) 16 B4C.Published by Maney Publishing (c) IOM Communications Ltd Suri et al.2¡0.4 99. Co: 0–2.0 23.2 24. 30 min.1.8% 13 B4C B4C: D5050.5 99.5 3. 3 min.3 4. Ar Ar Ar Ar Ar (B4C/Al2O3518 : 1) B4C: D5053 to 5. MPa.5. B4C: B4C.0 24.2 g cc21 79.77 3. 1 h. BN: D50.9 98. SS: 80.5¡0. 1 h. O: .5 3.7 98.2 99.5 4.5–1. % mm Vickers Indentation Flexural hardness.1 24. .7 98. Sintered density rth.2 3.95% Al2O3: D5051 to 2. min. nano-h-BN 19 B4C: D5051.5 2.0 3.74 g cc 97–99.0 99. 1950uC.8 3.3. Si: 3.5%CrB2 B4Cz25%CrB2 1900uC.2¡0.0 98. min.3 256 (2003) Synthesis and consolidation of boron carbide: a review 2010 VOL 55 B4Cz10%CrB2 B4Cz15%CrB2 B4Cz20%CrB2 B4Cz22.2 3.65 3.5 … 3. 24 MPa.45 g cc21 3. Si: D10055 B4Cz8 to 13% siloxane/ phenolic B4C: D5050.4% 86 94 95 96 98 95 99.0 98.7¡0. 1950uC.99% 45–98.5%.5–57%.4 4.7 4.5 98. Ar 2 wt-%.5 98.5 g cc21 3.99% TiC: D5051 to 2.5.2 2. . Al: 50 ppm CrB2: D5053. 1950uC.0 5.1 B4C: 29.5 g cc21 3. 1 h. .7 99. min.1 3.4 6. strength.5 3.2 1700uC.50 g cc21 … 2.3¡1.0 Al2O3: D5050.6 99. SS: 15.8 3.0 5. Ar 20 B4C B4Cz10 wt-% BN B4Cz20 wt-% BN B4Cz30 wt-% BN B4Cz40 wt-% BN B4Cz60%Al2O3 B4Cz70%Al2O3 B4Cz80%Al2O3 B4Cz90%Al2O3 B4CzAl2O3 B4CzAl2O3z5%TiC B4CzAl2O3z10%TiC B4CzAl2O3z15%TiC B4CzAl2O3z20%TiC 2150uC.4 4.2¡0.5 3.2¡0.5 15 21 B4C B4Cz5%CrB2 B4Cz10%CrB2 B4Cz15%CrB2 B4Cz20%CrB2 B4Cz25%CrB2 B4Cz50%SiC 1900uC.4 4.7 B4C.7 2.5%. TiB2 TiB2 TiB2 21 15 11 8 7 28 27 23 21 22. 1950uC.3– 35.5 4. GPa MPa m1/2 MPa Reference (year) 245 (1986) International Materials Reviews 12 D50. B: D5051. 1 h. Ar 1750uC.4 B4C: B4C. 28 MPa. B4C. heating rate: 40 K min21. 65 60 60 60 60 min.43 CrB2: D5053. Larger size powders in the range 3–10 mm can be sintered to near theoretical densities by hot pressing at y2000uC and 30–40 MPa pressure. 350 MPa. 10. b-rhombohedral. which is attributed to the phase transformation occurrence from amorphous boron to brhombohedral boron through a-rhombohedral boron modification.Suri et al. 1 h. Fig. m2 g21. 2006.5. The added advantages of hot pressed compacts are fine grained structure.6 92. where pure boron carbide is essential and impurities/additives cannot be tolerated.6–100 1850uC. HR: heating rate.111 Kalandadze et al. % mm Published by Maney Publishing (c) IOM Communications Ltd B4C. Such an effect is not expected in the case of hot pressing as the sintering mechanisms are different.7 to 3. Fabrication of boron carbide shapes by hot pressing the mixture of particulate boron and carbon is also practised. D5050. strength. very low porosity and improved mechanical properties.5 … Earlier we have seen that carbon additive greatly enhances the sintering kinetics in pressureless sintering. Compacts with densities higher than that achievable by pressureless sintering process are produced by hot pressing of boron carbide powders. 20 MPa. Ar Starting powder details Serial Material composition. However addition of boron would consume the free carbon available in the boron carbide.75 g cc21 99. 1 h … B/C5 3.9 97 HRA … 25–27 … *FC: free carbon in B4C. 40 min Al: 2325 mesh B4C: D5055 to 40. compressive strength. vacuum Hot pressing conditions 1825–2000uC. hot pressing is the preferred method to produce dense. A comparison between arhombohedral. … … 253 (1999) Microstructure. 16..193 The mode of fracture appears to be transgranular.9 73.0–3. and amorphous boron indicated that sintering into the b-rhombohedral phase at the final stage can give higher densities as follows: BbRBaRBamorphous. as a result of an explosive detonation. pure compacts.76 251 (2008) 3.3 MPa. It is seen that small additions of B (1 to 5%) improves the strength of boron carbide specimens at Table 8 Continued 21 22 23 24 B4C/Cu578 : 22 B4C/Cu592 : 8 International Materials Reviews 2010 VOL 55 NO 1 29 . D50: mean particle diameter. 39 MPa. These pellets were densified by hot pressing at temperatures 1900–2100uC and pressures 20–40 MPa in boron nitride lined graphite moulds. mm. Role of sinter additives … 1. 1400–3400 MPa. LaAlO3. flexural strength. 20. Int. CR: cooling rate. For application such as in nuclear industry. toughness.0 81. SS: specific surface area. wt-% B4Cz6%La2O3z 12%Al2O3z12%C 90 to 99%B4Cz0 to 1% BN/AlNzrest RE oxide-Al2O3 B4Cz30%Al B4C: 260 mesh 600. Fractography of fully dense boron carbide compact is shown in Fig. Al8B4C7. GPa MPa m1/2 MPa Reference (year) 700–800 252 (2007) 194 (1978) … … … 156.8 to 4. Ceram. Sintered density rth. Synthesis and consolidation of boron carbide: a review Vickers Indentation Flexural hardness. 32. Cu: D5051 to 8 1050uC. no.241 compacted boron and carbon powders in ampoules up to 30% TD. RE: rare earth. 232 561026 K21. by shock compression. 4(a) in p.3 MPa. B4C: 1–2 20 Microstructure of hot pressed boron carbide showing transgranular fracture:193 reprinted with permission from Elsevier. In the literature also one does not find any report on hot pressing of B4C with carbon addition. -%B4C exhibiting high thermal conductivity has been prepared by hot pressing of Cu coated B4C powders for the application of absorber materials in liquid metal cooled fast breeder reactor. Si and Co by attrition milling followed by hot pressing at 1720uC for 2 h gave a compact with three distinct phases of B4C. An US patent261 explains a process for producing B4C armour plates with improved ballistic properties by the addition of Cr. During ball milling/mixing of B4C with additives. 21. Effect of variation of TiC addition on hot pressing of B4C/TiB2/Mo composite has been studied by Jianxin et al. Int. Si. 28. The crack deflection at the phase boundary between B4C matrix and dispersions consisting of SiC and TiB2. Addition of Fe in small amounts (0?5%) has been found to be effective in increasing the final densities of B4C–TiB2 composite due to the formation of Fe–Ti rich liquid phase at the grain junctions. Si and Co to the above referred mixture.244 They have observed that factors for the increased strength are due to the healing of the cracks during sintering and the presence of TiB2 particle which force the crack to propagate in a non-planar fashion thus enhancing the energy dissipation at the crack tip.266 have synthesised a high strength (400– 570 MPa. Reaction sintering of B4C with 30 wt-%(W. An US patent243 explains a process to prepare boron carbide composites containing 5 to 30 mol. B. hardness in the Li et al. The sintering temperature of this composite is 300uC lower than that of monolithic B4C. fracture toughness and relative density of the composite were 88?6 HRA. fracture toughness: 6?1 MPa m1/2) has been prepared by the addition of 5–30 vol.247 and the maximum values of fracture toughness.M)B2 coated grains. etc.M)B2 or (Ti. which occur by residual stresses due to the differences in thermal expansion coefficients of B4C. 429 range 28–33 GPa and flexural strength of 830 MPa max.Suri et al. TiC and SiC powders.246 Further reduction in sintering temperature was achieved by the addition of B. Cr. Fine grains of (0?5–2 mm) TiB2 and W2B5 were seen in the microstructure.262 Role of mixed borides 21 Crack path produced by Vickers indentation on polished surface of hot pressed B4C–30 wt-%(W. SiC. Co. B4C–15 vol. Reaction sintering of B4C with WC. TiB2 and BN by reactive hot pressing of B4C. Ni. Han et al. flexural strength and hardness reported are 4?3 MPa m1/2. Further attempts to reduce the sintering temperature without compromising the strength are given below. the powders get contaminated and the microstructure of the composite appears very complicated after hot pressing due to the diffusion of W. Synthesis and consolidation of boron carbide: a review lower hot pressing temperatures. Similar very high strength material (four point bend strength: 850 MPa. W2B5.242 Similar to preparation of boron carbide based cermets for nuclear applications boron carbide rings with adequate strength have been prepared by hot pressing technique with y30 wt-% aluminium as binder for possible use as neutron absorber.265 have prepared a composite containing B4C. 6–9?5 MPa m1/2) B4C–TiB2–SiC–graphite composite by reactive hot pressing using B4C. Fe. Microstructure analysis showed the presence of laminated structure and a clubbed frame dispersion phase and bunchy dispersion phase among the matrix.267 When 30 International Materials Reviews 2010 VOL 55 NO 1 .-%TiB2 composite with a flexural strength of 621 MPa and fracture toughness of 6?1 MPa m1/2 have been prepared by hot pressing at 2000uC and a pressure of 20 MPa in argon atmosphere for 1 h by Skorokhod et al. TiC. or mixtures thereof.263 describes a process for the preparation of boron carbide/transition metal boride moulded articles comprising of B4C. 695 MPa and 25?0 GPa respectively. Co. This microstructure with TiB2 particles uniformly distributed in a fine grained B4C matrix is responsible for the increase in fracture toughness and strength. 10 in p.-%Mo to B4C/(W. Si3N4.245 US patent by Petzow et al. and Cr into W2B5 to form boron rich boride or the interfacial layer. and Ti.Ti)C at 1850uC for 30 min showed increase in fracture toughness and flexural strength up to 50 wt-%(W. 22. The effect of TiB2/W2B5 on the path of crack and deflection in the composite is shown in Fig. either into the TiB2 grains to form (Ti.253 Though boron carbide in various forms is used in nuclear industry. a-SiC and TiC powders and the hardness. The increase in fracture toughness is attributed to the residual stresses generated by differences in the thermal expansion coefficient between B4C. literature data on the production methods is scarce. B.Mo)B2 by hot pressing at 1900uC.. and TiB2. 5?6 MPa m1/2 and 95?6% respectively. TiB2 and W2B5. Flexural strength and fracture toughness for composite with 40 to 50% additive were 700 MPa and 4?5 MPa m1/2.267 Fractography of this sample showing the crack propagation path is depicted in Fig.Ti)C composite:246 reprinted with permission from Elsevier. bending strength. 2002. Fig. 554 MPa. Ceram. WC and/ or TiC and Co by hot pressing between 1550 and 1850uC.-% titanium diboride with very high flexural strength (870 MPa) and fracture toughness (3?4 MPa m1/2). Role of TiB2 Published by Maney Publishing (c) IOM Communications Ltd Reaction sintering of boron carbide with the addition of titanium oxide and carbon as per reaction (19) produces extremely fine high surface area particles of TiB2 which promote densification and limit the grain growth of the boron carbide matrix. Fractography and crack propagation suggested that crack deflection and bridging are the possible toughening mechanisms.264 Role of carbides/nitrides As seen in the previous lines addition of TiO2 has brought down the hot pressing temperature of B4C by >100uC. Ni.194 A high density B4C/Cu cermet with 70 vol. TiB2 and SiC while cooling from the fabrication temperature is responsible for the enhanced fracture toughness values.Ti)C content. The reaction products formed are boride of the respective oxide.234 Reaction sintering of boron carbide with the addition of oxides/carbides/nitrides has been successfully employed to obtain a microstructure of fine particles of reaction product (borides/carbides/nitrides) in B4C matrix.273 Graded porosity B4C materials can be produced by a layering approach using different size distributions of B4C powders in the green state. Addition of TiC with other oxides increases the hardness and erosion resistance of B4C composite. The pressure level used in the HIP process typically is 100–300 MPa. J. 1999. Co.20 wt-%BN content exhibited excellent machinability. The high pressure provides a driving force for material transport during sintering which allows the densification to proceed at considerably lower temperature in comparison to that of traditional sintering. particularly during the initial stages of the process.275 Hot isostatic pressing (HIP) The HIP process. the high pressure induces particle rearrangement and high stresses at the particle contact points.1500uC and a high pressure of 5–6 GPa. A composite with .256 Similarly addition of Al2O3 enhances the sintering kinetics of boron carbide due to a liquid phase formation at 1950uC. Al2O3. 8 in p. B and C. These additions lower the sintering temperature than that of monolithic B4C.270 A process in which a preceramic organosilicon polymer which on pyrolysis yields SiC and free carbon has been patented for preparation of dense bodies of boron carbide (. Si. as compared to 30–50 MPa in uniaxial hot pressing..272 The surface hardness and wear resistance of this composite has been improved by silicon infiltration process. crack bridging and deflection mechanisms at the interface. Flexural strength and fracture toughness of these composites are very high due to the residual stresses generated by differences in the thermal expansion coefficient between B4C and reaction products.-%CrB2 composite fabricated by hot pressing at 1900uC shows a high strength of 630 MPa and a modest fracture toughness of 3?5 MPa m1/2. La2O3 etc. Ni. Fig. containing B4C and SiC formed by reaction among Si. (1). mechanical properties and sand erosion rate International Materials Reviews 2010 VOL 55 NO 1 31 . uses the combination of elevated temperature and high pressure to form/densify raw materials or preformed components.268 A composite B4C–VB2–C obtained by reaction synthesis with hot pressing has been found to exhibit high hardness and bending strength suitable for application as wear and shock resistance components.254 The hardness of composite decreased with increase content of BN while the machinability improved significantly. in the form of interlocked matrices with very low porosity and uniform microstructure. A virtually pore free product can be produced at a relatively low temperature. etc. hardness of the compact increases due to the inhibition of B4C decomposition but the bending strength and the fracture toughness reduce. In addition. either intentionally added or accidentally acquired during the grinding/mixing operations are also found to be effective in marginally reducing the sintering temperature and improving the flexural strength/fracture toughness due to the formation of complex boride phases and multi interfaces.269 Cr and V carbides are also found to be effective in obtaining high densities and fine grained structure. Liquid phase sintering Published by Maney Publishing (c) IOM Communications Ltd of B4C/Al2O3/TiC composites. W. The very fine grained microstructure is responsible for high flexural strength and residual stresses caused by thermal expansion mismatch of CrB2 and B4C for increasing toughness. Cr. La2O3 reduces the sintering temperature due to the formation of a liquid phase near the yttrium– aluminate composition (60 wt-%Y2O3–40 wt-%Al2O3. bring down the sintering temperature of B4C due to liquid phase formation.Suri et al. A machinable B4C/BN nanocompoisite has been fabricated by hot pressing microsized B4C particles coated with amorphous nanosized BN particles. Small quantities of B.243. The addition of rare earth (RE) oxides such as Y2O3. originally known as gas pressure bonding. A number of new processing methods are envisaged to produce materials with designed structure and properties.249 Jianxin and Junlong271 have studied the effect of TiC content on the microstructure. and the isostatic mode of application of pressure is generally more efficient than Addition of CrB2 aids in lowering the hot pressing temperature due to the formation of CrB2–B4C eutectic at 2150uC. etc.255 Combination of SHS technique with hot pressing (called combustion hot pressing) has been used to prepare a composite. melting point 1870uC). which enhances the mechanical properties. B4C–20 mol.248. The application of the pressure is carried out inside a pressure vessel.251 Pore free sintered boron carbide materials with high strength (700–800 MPa) and fracture toughness (3?6–3?9 MPa m1/2) have been prepared by low pressure hot pressing with the addition of BN/AlN and oxide binder (RE oxide–Al2O3). 47. typically using an inert gas as the pressure transmitting medium with or without glass encapsulation of the part. Powder Powder Metall. Jpn Soc. Fe. A resistance heated furnace inside the vessel is the temperature source. Y2O3. 28 boron is added to the above mixture. Ti.97% TD) by hot pressing in inert atmosphere at a temperature of 2275uC and a pressure of 28 MPa. Synthesis and consolidation of boron carbide: a review 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.274 Cobalt as sinter additive has also been attempted for hot pressing of boron carbide powders with 5 wt-%TiC at temperatures .252 Additives such as CrB2. and then densifying the layered assembly by hot pressing at 1900uC. Parts are loaded into the vessel and pressurisation occurs usually simultaneously with the heating. Addition of TiC increased the hardness of the composite and the hardness had direct influence on the erosion rate of the nozzles. Equiaxed uniform size grains and thin grain boundary are the special features of this material with very high hardness. J.288–290 the uniaxial one. Addition of boron was found effective in reducing the pores and graphite inclusions and improved particle erosion resistance. 6 in p. The hardness and fracture toughness values were in the range of 25?41 to 27?45 GPa and 3?22 to 3?61 MPa m1/2 under static testing conditions. hot isostatic pressing.281 Spark plasma sintering Conventional pressure assisted consolidation techniques. The driving force for densification is provided by passing current directly through the particle material. sintered in the temperature 2100 to 2200uC to give a density of 2?47 g cc21. which on further hot isostatic pressing at 2100uC under an argon pressure of 200 MPa to achieve a theoretical density of 2?56 g cc21. Fig. In order to minimise expensive machining. Synthesis and consolidation of boron carbide: a review 23 B4C specimen pressureless sintered and hipped at 2150uC and 310 MPa to 99?1% relative density:279 reprinted with permission from Materials Research Society.279–281 A patented process explains the preparation of boron carbide shapes containing metallic diborides (of Ti. (8). 20. 24)52 reveals nearly equiaxed fine grained microstructure. In HIP the whole equipment has to be designed for high temperature and high pressure operation. elastic constants and wear resistance were observed with the addition of 1 and 3 wt-%C in the above process. High shear force in combination with pulsed electric power is initially applied to the particle material to generate electrical discharge that activates the particle surface by evaporation of oxide film. silicon and silicon carbide while hot Isostatic pressing boron carbide at 1850uC for 1 h under a pressure of 160 MPa.279.282 Fully dense and very fine grained boron carbide has been prepared by the fabrication route. the powder densification process must be capable of near net shaping.. yielding acceptable performance. plasma activated sintering. with simultaneously applying high shear and high pressure in separate steps. The densities obtained at 1750uC by the application of 88 MPa pressure in 2 and 5 min were 96 and 99?2% TD respectively. injection moulding/pressureless sintering (2175uC)/ post-HIP (200 MPa. Major equipments needed for pressureless sintering are cold compaction press and sintering furnace.Suri et al.280 The combination of pressureless sintering and post-HIP is gaining importance for fabrication of dense bodies with higher densities.279–281 Elimination of residual porosity and significant improvements in flexural strength. Manufacturing costs and throughput of pressureless sintering with post-HIP are attractive compared to hot pressing. This will not only yield lighter weight armour but also enable forming of complex shapes. plasma pressure consolidation/P2C and instrumented pulse electrodischarge consolidation are the different names given for the same process. Hot pressed objects also undergo surface finishing operations at times to remove any contamination from the die material. require long processing time and high temperature in order to produce high density parts. impurities and moisture.52 have consolidated submicrometre sized commercial boron carbide to near theoretical densities using plasma pressure compaction technique. lower graphite contents and significantly higher Vickers hardness than commercially hot pressed B4C. Hf. V. For hot pressing. Nb and Ta). The time and temperature required for consolidation is lowered as high current density is applied in addition to high shear and high pressure (up to 2000 MPa) which leads to localised heating and plastic deformation at interparticle contact areas.. This puts a limitation on the size of the equipment and the number of compacts that can be fabricated at a time. Hipped compacts do not need any further processing and can be directly used. Addition of graphite and TiB2 do not aid in consolidation of B4C by this method. Ar) from B4C doped with 4 wt-% carbon black. Spark plasma sintering (SPS). Zr. Ghosh et al.278 have studied the effect of addition of boron. Optical micrograph (Fig.219 Porosity severely degrades the ballistic properties of ceramic armour as it acts as a crack initiator.287 The materials to be consolidated are placed in a graphite die and punch assembly. such as hot pressing. Mater. One such process and the apparatus for rapid bonding of ceramic materials have been patented by Yoo et al. Addition of small amounts of Al2O3 and Fe has been found to be effective in achieving higher densities by SPS of B4C due to formation a liquid phase. Therefore. The rapid sintering which preferably lasts for less than a few minutes prevents grain growth and allows the particles to retain their microstructure. boron carbide protective inserts for personal armour is hot pressed to minimise porosity (y98% relative density). Multi cavity dies are used for increasing the rate of production. Sintering aids generally degrade hardness and ballistic properties. Posthipping of pressureless sintered boron carbide is gaining importance for this purpose. 2115 the press and heated by the furnace surrounding it. The average grain size of these compacts was 1?6 and 2 mm. Boron carbide (100% TD) could be obtained by a combination of pressureless sintering and post-HIP at 2150uC for 125 min under 310 MPa of argon pressure.277 Larsson et al.284–286 Figure 23279 shows the microstructure of post hipped boron carbide to full theoretical density. a graphite die is assembled between the rams of Published by Maney Publishing (c) IOM Communications Ltd 32 International Materials Reviews 2010 VOL 55 NO 1 .283 Near net shape with full density can be achieved by HIP. etc. 2005.276. Res. Compacts produced by pressureless sintering generally needs a finishing process of surface grinding and end finishing to obtain desired shape and size. Subsequently bonding is accomplished by resistance heating at the contact points between the activated particles in the presence of high pressure. still in laboratory scale. 2008. consolidation with twins initiates at 1600uC and 98% density obtained at 1900uC. A dense composite with B4C skeleton and the voids filled with the reaction products of Al and B4C were obtained. Need of the hour in carbothermic reduction is process modelling. 1853 25 Hardness versus position profile of functionally graded boron carbide with and without Al infiltration:294 reprinted with permission from Elsevier.295 Conclusions The understanding of the crystal structure of boron carbide has been evolving over the years and even today cannot be said to be fully elucidated. not feasible in the conventional furnaces are possible through the use of microwave systems.292 have noticed that B and C begin to react from 1300uC and the product was B rich boron carbide (B4zxC) and free C.294 A B4C/Cu graded composite as plasma facing component for fusion reactors with performance better than nuclear grade graphite has been prepared by rapid self-resistance sintering under ultra high pressure. Heian et al. Densification of the synthesised boron carbide occurred from 1700 to 1900uC. the bending strength was as high as 200 MPa. with a hold time of 10 min. B4C and B4?5C by spark plasma sintering has shown that carbon free boron rich compounds are formed in the temperature range 1300–1600uC and consolidation occurs above 1700uC.294 This processing route results in a material with very promising properties and interesting microstructural features. With increasing temperature. its value decreases to 120 MPa at 800uC and then increased to 230 MPa at 1000–1400uC. Enhanced densification and finer microstructures.297 One can expect more research using microwave heating as and when bigger size units with higher power ratings become available in the market.298 Temperature dependence of the bending strength of this composite was evaluated in the temperature range 25–1600uC. Eng. J.296 Literature on microwave sintering of boron carbide is scarce. (6). This material with low specific gravity and high hardness is attractive for use in lightweight armour. Wang et al. A novel floating zone method has been adopted for preparing directionally reinforced B4C–TiB2 composite. Ceram. Vapour phase synthesis. Even at 1600uC.300 a similar method has to be adopted for B4C also. which will greatly help in tuning the process to achieve improved productivity and uniform quality. Mater. Simultaneous synthesis and densification of B3?5C. the distribution of the microwave energy inside the cavity. Sol–gel method appears to be a promising technique for production of suitable fine boron carbide powders for direct consolidation. A. but the material shows evidence of extensive crystallographic disorder due to twins which however decrease at higher temperatures.299 Continuous functionally graded boron carbide aluminium cermets have been prepared by spark plasma sintering of B4C from boron and carbon followed by infiltration of aluminium. The sample size and shape. Carbothermic reduction of boric acid has been the commercial method for the production of boron carbide in spite of the shortcomings.293 Fabrication of functionally graded B4C cermets from an external heat source.113 Formation of B4C commences at y1000uC and is complete by 1200uC. 90. Significant densification occurs above 1600uC. A488. One of the recent articles describes the behaviour of B4C/SiC/ Al mixtures during microwave heating in air. Soc. 4 in p. and the magnetic field of the electromagnetic radiation are all important in heating and sintering. showing nearly equiaxed fine grained microstructure:52 reprinted with permission from Wiley-Blackwell. Sci. 337 Published by Maney Publishing (c) IOM Communications Ltd In situ synthesis and densification Influence of temperature on synthesis and consolidation has been studied by Tamburini et al. Hardness profile of functionally graded material before and after Al melt infiltration is shown in Fig. formation of B4C starts at 1300uC. With increasing temperature C atoms diffused into B4zxC lattice. 2007. resulting in the reduction of free carbon content and decrease in lattice parameters of B4zxC.298 The development of non-aqueous gel casting process for preparation of B4C– Al composites has been reported by Zhang et al.106 have also noted similar structural defects in the material prepared by mechanical activation followed by field assisted combustion.Suri et al.291 while heating B-C powders by SPS have observed that. Synthesis and consolidation of boron carbide: a review 24 Microstructure of boron carbide densified by SPS at 1750uC for 5 min. Microwave processing Microwave sintering has the advantages of uniform and rapid heating since the energy is directly coupled into the specimen rather than being conducted into the specimen International Materials Reviews 2010 VOL 55 NO 1 33 . SiC was not attacked by oxygen and was able to contribute to matrix toughness.. Kodera et al. Am. 25. Fig. 8 in p. Fig. Though a continuous method has been established for the production of SiC. 4072–4077. Li: ‘Boron carbide and boron carbonitride thin films as protective coatings in ultra-high density hard disk drives’. J. Cree. H. J. inhibit grain growth and improve the mechanical properties. Jacquemain: ‘Investigation on boron carbide oxidation for nuclear reactor safety: experiments in highly oxidizing conditions’. ASM International. R. Mater. Gotoh. 2006. F. Mater.. P. 2005. Aselage: ‘A propeosed boron-carbide-based solid-state neutron detector’. A. 2000. K. Ann.303. 185–194. Lee and C. 3. 013529?1– 013529?3 15. Chen. Drouan. Springler-Verlag Berlin. well defined particle shape) and reliable usage of nanoscale powders will become more important. Eng. Technische Rundschau. Jimbou. Thevenot: ‘Boron carbide – a comprehensive review’. General electric Co. It may be worthwhile to investigate composites with silicides and RE borides. A. Valentine. Coat. J. Constant and D. Its use in electronic industry and high temperature applications will see a higher growth in the coming years. C. K. Lee. Eur. Philadelphia. Trester. I. W. 272–276.. Chung and S. 19. J. Missile and Space Div. thermal and chemical stability. E. Morita and B. 13. Suzuki. A408. 20. Nakagawa. Mater. 2005. Mater.304 Higher oxidation stability of B4C with the addition of Zr. boron carbide has established as the material for abrasive applications. E. K. W. N. Post-hipped pressureless sintering is gaining importance for manufacture of complex shapes with fine grains especially for armour material. 2. Boron carbide based coatings by CVD methods for various semiconducting applications such as diode. Matkovich: ‘Boron and refractory borides’. 205–225. Marchand. J. Y. 10. On consolidation. 2008. Techniques for near net shape forming should be considered a field for fruitful further activity.. Nucl. 4. Dominguez. M. N. Though thermodynamic models based on Gibbs free energy minimisation have been used to predict experimental conditions. Y. In addition to the established uses. 374. Energ. Dunner. A.ntis. Y. OH.. Y. Kosuling and G. BN–B4C composite materials could find a unique place for high temperature applications. etc. T. 28. A. J. J. Gavillet: ‘SIMS imaging analyses of inreactor irradiated boron carbide control rod samples’. 2197– 2199. Murthy: ‘Elastic properties of boron carbide’. V. Ceram. neutron sensors. Risovany. 1484–1491. J.. Phys. Suzuki. Committ. http://www. Akiba. 2008. 2nd edn. etc. thermoelectric converter. M. P. D. J. and erosion resistance in high intensity laser beams. 2004. Mikhailichenko: ‘Reprocessing of the irradiated boron carbide enriched by boron-10 isotope and its reuse in the control rods of the fast breeder reactors’. With excellent dielectric properties. Materials Park. Wallura and V. Technol. Lett. Mater.307 1. 368–371.iaea. 3–7. K. Nucl. 6. Nakagawa. Vols. Mater. O. Magn. Park. Saidoh. 5. B. J. Detailed investigations on oxidation characteristics of B4C at high temperatures have been carried out by Steinbruck et al. International Atomic Energy Agency. 3. C. P. 18. 11. IAEA-TECDOC--884. Divan. Y. 2008. 124. 14. Seiler. Alagon and P. Repetto and S. E. A. M. models based on mass transfer and kinetics in addition are needed to understand the deposition mechanism and the growth process. Sci. R. R. Jul 64–Jun 65. Design. 473–481.. Techn. Gatti. 1146–1152. usage of tailor made powders (complex composition. compositional gradient in particle. 233–237. the future of boron carbide will depend on the possible production of micrometre/nanosized powders. Mater.pdf 17. Tsuchiya: ‘Thermal conductivity and retention characteristics of composites made of boron carbide and carbon Published by Maney Publishing (c) IOM Communications Ltd 34 International Materials Reviews 2010 VOL 55 NO 1 . L. 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