CERAMICSINTERNATIONAL

Available online at www.sciencedirect.com

http://dx.doi.org/0272-8842/& 20

nCorrespondinE-mail addre

Please cite thiInternational (2

Ceramics International ] (]]]]) ]]]–]]]www.elsevier.com/locate/ceramint

A fractographical approach to the sintering process in porous ZrB2–B4Cbinary composites

Mehdi Shahedi Asla, Mahdi Ghassemi Kakroudia,n, Behzad Nayebib

aDepartment of Materials Science and Engineering, Faculty of Mechanical Engineering, University of Tabriz, 29 Bahman Blvd., Tabriz, IranbSchool of Metallurgy and Materials Engineering, Iran University of Science and Technology, Tehran, Iran

Received 25 July 2014; received in revised form 30 July 2014; accepted 19 August 2014

Abstract

Due to their high temperature stability and hardness, ZrB2–B4C composites have drawn attention of researchers in recent years. Althoughseveral studies have been conducted on the properties of ZrB2–B4C composites at high sintering temperatures (42200 1C), there is littleinformation about their sintering process and densification at temperatures below 1850 1C, which is industrially more attractive and could result indesirable porosity. In this paper, sintering process and porosity of ZrB2–B4C binary composites containing 0 to 100 vol% B4C (hot pressed at1800 1C for 30 min under a uniaxial load of 12 MPa) have been studied. The effect of volume fraction of components on sintering process andmicrostructure has been investigated by Scanning Electron Microscopy of fracture surfaces of samples. The amount and the type of porosity andrelative density of samples have also been determined. Results show that the density of samples decreases inversely with the amount of B4C.Also, the type of porosity changes from closed to open in high B4C volume fractions. SEM micrographs show that the sintering process iscontrolled by moss-like ZrO2 transient phases formed during the heating process.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: A. Hot pressing; B. Composites; D. Borides; D. Carbides

1. Introduction

Zirconium diboride (ZrB2) is remarkable for its ultra-highmelting temperature (3245 1C), as well as its hardness and elasticmodulus, low electrical resistivity, and resistance to chemicalattack. As a result, this material has been proposed for a varietyof structural applications at room and elevated temperatures,including armor, cutting tools, molten metal containment, steelprocessing, and electrodes. Zirconium diboride is also consideredto be an ultra-high-temperature ceramic and is a candidate for usingas leading edges and propulsion components in hypersonic aero-space and advanced reusable atmospheric reentry vehicles. Boroncarbide (B4C), as the third hardest natural material, is well-knownfor its interesting properties such as hardness (35–45 GPa) andabrasion resistance. This material is the most ideal high-temperature wear-resistant material. Due to its hardness and low

10.1016/j.ceramint.2014.08.08114 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author. Tel.: þ98 411 339 2470.ss: mg_kakroudi@tabrizu.ac.ir (M. Ghassemi Kakroudi).

s article as: M. Shahedi Asl, et al., A fractographical approach014), http://dx.doi.org/10.1016/j.ceramint.2014.08.081

density (2.52 g/cm3), B4C excels in ballistic performance.Other important features of boron carbide are its relativelyhigh neutron absorption ability, low cost, and ample sources ofraw material [1–7]. Some physical and mechanical propertiesof monolithic ZrB2 and B4C ceramics are summarized inTable 1.Interesting properties of both ZrB2 and B4C and their

possibility of covering the weaknesses of each other, haverecently attracted researchers to investigate the ZrB2–B4Csystems. In the last few years, research works have beencarried out considering one of these materials as matrix and theother as reinforcement. Pressurless sintering, hot pressing andrecently, microwave sintering have been used in manufacturingof ZrB2/B4C composites [11,14–17].The relative density of B4C based composites containing 20–

40 vol% ZrB2, hot pressed at 2100 1C for 60 min under 30 MPaload, ranged from 95% to 98%. With increased volume fractionof ZrB2, the B4C composites changed from insulator to conductorbecause ZrB2 is a good conducting material. Hence, the precision

to the sintering process in porous ZrB2–B4C binary composites, Ceramics

Table 1Summary of some physical and mechanical properties of monolithic ZrB2 and B4C ceramics.

Property ZrB2 References B4C References

Density (g/cm3) 6.12 [8, 9] 2.52 [1]Young's modulus (GPa) 489 [8, 9] 441–472 [1, 3, 10]Poisson's ratio 0.11 [9] 0.17–0.21 [2, 3]Coefficient of thermal expansion (K-1) 5.9–6.8� 10-6 [9, 11] 4.2–4.5� 10-6 [2, 11]Electrical conductivity (S/m) 1� 107 [8, 9] 140 [2]Melting temperature (1C) 3245 [8, 9] 2540 [1]Hardness (GPa) 18–23 [8] 31.7–45 [1, 12]Fracture toughness (MPa m1/2) 3.5 [13] 2–4.9 [1, 2, 10]

Fig. 1. SEM images of morphologies and XRD patterns of as-received powders; (a) ZrB2 and (b) B4C.

M. Shahedi Asl et al. / Ceramics International ] (]]]]) ]]]–]]]2

electromachining method can be used to form complicated shapesof B4C–ZrB2 composites which are hard to produce by routinemanufacturing processes [11].

High relative density ZrB2–B4C composites have beenmanufactured by microvawe technique, but the process justfocused on small-sized components. A ZrB2-4 wt% B4Cceramic composite densified by microwave sintering method,reached 498% relative density at processing temperatures aslow as 1720 1C. Vickers hardness and fracture toughness of

Please cite this article as: M. Shahedi Asl, et al., A fractographical approachInternational (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.081

the microwave sintered specimens have been measured about17.5 GPa and 3.8 MPa m1/2, respectively. It is reported that insuch composites, B4C additive not only promotes the sinteringprocess in microwave heating by absorbing the microwaveenergy, but also improves the densification of compositesfabricated by pressureless sintering [14].ZrB2 was densified below 2000 1C by pressureless sintering

with different additives such as B4C and C. Enhanceddensification was attributed to activation of sintering by

to the sintering process in porous ZrB2–B4C binary composites, Ceramics

Fig. 2. Density of ZrB2–B4C binary composites consolidated at 1800 1C for30 min under 12 MPa load.

Fig. 3. Amount and type of porosities in hot pressed ZrB2–B4C binarycomposites.

Fig. 4. SEM micrograph of fracture surface of B4C sample hot pressed at1800 1C, shows poor sinterability of B4C powder.

M. Shahedi Asl et al. / Ceramics International ] (]]]]) ]]]–]]] 3

surface oxide impurities removal from ZrB2 particle surface[7,18]. It was indicated that 4 wt% B4C additive promotes thechemical reaction in which surface oxides (B2O3 and ZrO2) areconverted into ZrB2 and some gaseous products. Thus,sintering process progresses by chemical reaction, calledchemical sintering, could be introduced as the dominantsintering mechanism in mentioned case [18]. ZrB2 was alsosintered by a combination of 2 wt% B4C and 1 wt% Cadditives. Besides grain growth inhibition, results showed thatusing this composition may promote the densification ofcomposite by removing surface oxide impurities [7].

Although several research works have been carried out onZrB2/B4C composites, there is not any comprehensive researchwhich covers the whole mentioned binary system. This studyfocuses on the sintering behavior of ZrB2–B4C binary compo-sites with various compositions from monolithic B4C tomonolithic ZrB2 ceramics, consolidated by hot pressing at1800 1C under a relatively low pressure of 12 MPa. The aim ofthis research is to understand the densification behavior and therole of volume fraction of each component in sintering process,via SEM investigations and porosity measurements.

2. Experimental procedure

2.1. Processing

ZrB2 (particle size �2 μm, Leung Hi-tech Co., China) andB4C (particle size �20 μm, Mudanjiang Jingangzuan BoronCarbide Co., China) powders were the starting materials. Fig. 1shows scanning electron microscopy (SEM) images ofmorphologies and X-ray diffraction patterns (XRD) of as-received ZrB2 and B4C powders. ZrB2 powders were thenmixed with 0, 20, 40, 60, 80 and 100 vol% B4C by ball millingfor 1 h at 120 rpm in zirconia cups and balls. Then sampleswere loaded into a graphite die and boron nitride spray as highseparator lubricant was applied to all the graphite surfaces. Hotpressing was completed in a graphite resistance-heated vacuumhot press furnace (made by Shenyang Weitai Science &Technology Development Co. Ltd., China). In each hotpressing experiment, 12 MPa pressure was applied as soonas the final isothermal temperature cycle started. Samples wereinitially heated at a rate of 15 1C/min up to 1000 1C, given adwell isotherm at 1000 1C for 30 min in order to removevolatile compounds contained in the samples, then were heatedagain at a rate of 12 1C/min up to the designated temperature.Above 1000 1C, the temperature of the graphite die wasmonitored using an infrared temperature sensor (Model IT-6).Hot pressing was carried out at 1800 1C, given a dwell isothermfor 30 min. Finally, the hot press furnace was cooled downnaturally. One pellet, with a diameter of 25 mm and thickness of5 mm, was prepared for each experiment.

2.2. Characterization

X-ray diffraction analysis (XRD: Cu lamp, λ¼1.54 Å,40 kV, 30 mA, Siemens D5000 model) was carried out onthe samples. Bulk density of the sintered specimens was

Please cite this article as: M. Shahedi Asl, et al., A fractographical approach to the sintering process in porous ZrB2–B4C binary composites, CeramicsInternational (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.081

M. Shahedi Asl et al. / Ceramics International ] (]]]]) ]]]–]]]4

measured using Archimedes' method with distilled water as theimmersing medium, and the relative density was calculatedwith respect to theoretical density. The theoretical density wasestimated using rule of mixtures calculations, based on startingcompositions of the samples and following pure componentdensities ZrB2: 6.1 g/cm3 and B4C: 2.5 g/cm3. Microstructurecharacterization was carried out by scanning electron micro-scopy (SEM: Mira3 Tescan, Czech Republic). Chemicalanalysis was performed simultaneously with SEM usingenergy dispersive spectroscopy (EDS: DXP-X10P DigitalX-Ray Processor).

3. Results and discussion

Fig. 1 presents the XRD patterns of starting materials. As itseems, the only crystalline phases detected were ZrB2 andB4C. As shown in Fig. 2, densities decreased with increasing

Fig. 5. SEM micrographs of fracture surface of (a) B4C-20 Vol% ZrB2 sample hot pand (c) EDS spectra of moss-like phase.

Please cite this article as: M. Shahedi Asl, et al., A fractographical approachInternational (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.081

B4C content due to lower density of B4C than ZrB2. Thehighest relative density belonged to monolithic ZrB2 ceramic(�84%). This observation means that the bulk and apparentdensity values are coinciding in the monolithic ZrB2 ceramic.The open-type porosity arises with increase in B4C content,

but the amount of closed-type porosity shows a reversebehavior (Fig. 3). The monolithic ZrB2 ceramic has theminimum amount of porosity, as the highest portion of closedpores exists in this sample. Reversely, open pores are dominanttype of porosity in other samples. These variations areremarkable in composites which have less than 40 vol% B4Cadditive, but the samples containing 40 to 100 vol% B4C haveapproximately the same amount and type of porosity.Densification behavior of ZrB2–B4C binary composites was

improved by decreasing B4C content. Since the progress ofsintering process is followed by the elimination of openchannels between starting powders and sealing off of the open

ressed at 1800 1C, (b) closer view of (a) showing the moss-like transient phase

to the sintering process in porous ZrB2–B4C binary composites, Ceramics

M. Shahedi Asl et al. / Ceramics International ] (]]]]) ]]]–]]] 5

pores, the nature of porosity changes gradually from open toclosed form at final densification steps. Hence, negligibleamount of open pores in the monolithic ZrB2 ceramic verifiesthat sintering process of this sample proceeded better than theothers. In ZrB2-richer samples of current study, using finerstarting powder particles intensified the elimination of voids.By decreasing the particle size, solid state diffusion becomesmore pronounced, which leads to mass transportation towardscontact areas of powder particles, and subsequently seals offthe open channels between particles.

SEM micrographs of fracture surfaces of samples arepresented in Figs. 4–9. As displayed in Fig. 4, no significantsintering occurs in monolithic B4C sample. High strengthcovalent bond in B4C is known as the main cause of lowsinterability of boron carbide. In addition to the approximatelylow temperature, relatively large size of B4C particles is alsoan important controlling parameter in sintering, due to its

Fig. 6. SEM micrographs of fracture surface of (a) B4C-40 Vol% ZrB2 sample hot prZrO2 phases and (c) EDS spectra of semi-spherical ZrO2.

Please cite this article as: M. Shahedi Asl, et al., A fractographical approachInternational (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.081

effect on insufficient contact areas between B4C particles.Marked areas in Fig. 4 show sintering bond between B4Cparticles. Clearly visible, only particles with dimensions abouta quarter of main particle's size (20 μm), are sintered and thereis no significant sintering boundary between large particles.Comparison of SEM micrographs and porosity results of the

samples (Fig. 3) indicates that relatively large particle size andlow sinterability of B4C particles leads to high amounts ofoverall and open-type porosity, contrary to the amount ofclosed-type porosity. To form a closed-type porosity (seamlessvoid), the particles around it have to be completely joined andtherefore, low sinterability of B4C particles negatively affectsthe possibility of closed pores' formation. The amount of openpores which is directly controlled by densification and sinter-ing of particles reasonably increases.SEM micrograph of fracture surface of B4C-20 vol% ZrB2

composite is shown in Fig. 5, which displays the effective role

essed at 1800 1C, (b) closer view of (a) showing the formation of semi-spherical

to the sintering process in porous ZrB2–B4C binary composites, Ceramics

Fig. 7. SEM micrographs of fracture surface of (a) ZrB2-40 Vol% B4C sample hot pressed at 1800 1C, (b) closer view of (a) showing the formation and growth ofmoss-like ZrO2 phases from two neighbor ZrB2 particles and (c) line scan analysis of the showed line in Fig. 7a.

M. Shahedi Asl et al. / Ceramics International ] (]]]]) ]]]–]]]6

of ZrB2 particles in sintering behavior. In addition to thesmaller particles size which increases the densification ofsamples in comparison with monolithic B4C sample, it isclearly visible that presence of ZrB2 significantly improves thesinterability of composite. Joining between B4C and ZrB2

particles is more visible in Fig. 5b, in which the dark/largeparticles are B4C and the bright/small particles are ZrB2.

In the contact areas between B4C and ZrB2 particles,presence of moss-like particles seems to be the main causeof sintering. These particles are not indicated in pure B4Cpowder and seem to be formed during the heating process.EDS spectra of these moss-like particles, which seem to act as

Please cite this article as: M. Shahedi Asl, et al., A fractographical approachInternational (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.081

binders or joints on ZrB2–B4C contact surfaces presented inFig. 5c, indicates high concentration of zirconium and oxygen.This confirms the results of other literatures [7,18], in whichthe formation of ZrO2 as a transient phase during the heating,is believed to promote the sintering process.Fig. 6 shows the fracture surface of B4C-40 Vol% ZrB2

composite. In addition to moss-like particles, nucleation andgrowth of semi-spherical phases on B4C particles is signifi-cantly visible (especially in Fig. 6b). EDS spectra of suchphases (Fig. 6c) shows the same composition as moss-likephases and so, it seems that the formation of ZrO2 intensifieswith the increase of ZrB2 volume fraction. Consolidation of

to the sintering process in porous ZrB2–B4C binary composites, Ceramics

Fig. 8. SEM micrograph of fracture surface of ZrB2-20 Vol% B4C sample hotpressed at 1800 1C showing a fully sintered ZrB2/B4C interface.

Fig. 9. SEM micrographs of fracture surface of (a) monolithic ZrB2 ceramics hot pZrO2 phase and (c) EDS spectra of (b).

M. Shahedi Asl et al. / Ceramics International ] (]]]]) ]]]–]]] 7

ZrB2 with B4C and the interface of sintering bonds can beclearly seen in marked areas in Fig. 6a.According to Fig. 7, same sintering mechanism can be

observed in ZrB2-40 vol% B4C. The contact areas betweenB4C/ZrB2 and ZrB2/ZrB2 particles, respectively presented inFig. 7a and Fig. 7b, obviously approve the key role of ZrB2

particles in sintering and densification of B4C–ZrB2 compo-sites. In comparison with monolithic B4C samples, ZrB2

particles not only attach to B4C particles and provide a goodbonding between B4C grains, but also join each other andincrease the density of samples.ZrB2/ZrB2 sintering seems to be controlled by formation and

growth of moss-like ZrO2 phases (Fig. 7b). Actually, it seemsthat at initial stages of sintering process, moss-like ZrO2

phases form on the surface of two neighbor ZrB2 particles.Therefore, the consequent step of sintering process starts bydiffusion of C and B atoms through moss-like phases towardsparticles. Line scan analysis of the contact area (Fig. 7c)presents the increase in the carbon concentration on points 7, 8and 9, as a sign of its diffusion from B4C towards ZrB2

ressed at 1800 1C, (b) closer view of a showing the morphology of moss-like

M. Shahedi Asl et al. / Ceramics International ] (]]]]) ]]]–]]]8

particle. Zhang et al. [18] showed that presence of ZrO2 on thesurface of ZrB2, although negatively affects the sinterability ofmonolithic ZrB2, increases the density of sintered products inZrB2–B4C system.

The chemical reaction suggested by Fahrenholtz et al. [8](Eq. 1), confirms the effects of C in B4C on ZrB2 formation inthe interface of ZrB2/B4C particles and consequently, con-tribution of the sintering process via chemical sinteringmechanism.

7ZrO2þ5B4C-7ZrB2þ3B2O3(g)þ5CO (g) (1)

Eq. (1) occurs at temperatures more than 1281 1C. It reducesZrO2 to ZrB2 (chemical sintering mechanism). Gaseous phasesare also evaporated and exhausted due to the implied vacuum.Another mechanism, as suggested by Zhu et al. [7], isa chemical reaction between the surface oxides of particlesand the free carbon normally existing in B4C powder (Eq. 2).

ZrO2þB2O3(l)þ5C-ZrB2þ5CO (g) (2)

The mentioned chemical reaction, which occurs at tempera-tures over 1508 1C, also leads to formation of ZrB2 at thecontact areas of ZrB2/B4C particles which provides a chemicalbonding between them (chemical sintering mechanism). Theprocess progresses until all of the ZrO2 phase turns into ZrB2.Fig. 8 shows a ZrB2 particle fully sintered to a B4C particle.

Fig. 10. Schematic view of several steps in sintering process of ZrB2–B4C binarytransient phase, (c) growth of moss-like ZrO2 transient phase leads to (d) joining bemoss-like ZrO2 phases and formation of ZrB2 in contact area of two particles via Eqphase and formation of ZrB2 in the contact areas of B4C/ZrB2 particles, leads to c

Please cite this article as: M. Shahedi Asl, et al., A fractographical approachInternational (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.081

As it can be clearly seen, all the moss-like ZrO2 phase iseliminated (final step).Zhang et al. [18] reported that B2O3 is also found on the

surface of both B4C and ZrB2 particles and may partly beformed during chemical reactions such as Eq. (1). This phasehas a high vapor pressure and is mostly evaporated during theheating process. The remaining amount of B2O3, may con-tribute in Eq. 2 chemical reaction and assist the chemicalsintering process.In monolithic ZrB2 (Fig. 9a), the formation of moss-like

ZrO2 phase is clearly visible. The morphological structure ofsuch phases is presented in Fig. 9b. EDS spectra of suchphases are also presented in Fig. 9c, which shows the presenceof little amount of B. It may be due to the formation orexistence of B2O3 together with ZrO2. Regarding the mor-phology of moss-like phases, it seems that such phases aremade up of two parts; (1) fine grains entrapped in (2) a matrix.Once again, referring to EDS spectra of such phases, it seemsthat the moss-like phase is a mixture of fine ZrO2 grainsbonded together with a B2O3 matrix. This result is adaptedwith the results of Zhu et al. [7] and clarifies the contributionof B2O3 in chemical sintering process of ZrB2–B4C binarycomposites, in spite of previous theories which offered thatwhole B2O3 phase evaporates during the heating process.Results from previous research together with present dis-

cussions could be used to offer a schematic model of several

composites (a) formation of surface oxides, (b) formation of moss-like ZrO2

tween two ZrB2 particles, (e) diffusion of B atoms from ZrB2 particles through. (1), (f) diffusion of C and B atoms from B4C particles through moss-like ZrO2

hemical sintering between B4C and ZrB2 particles via Eq. (2).

to the sintering process in porous ZrB2–B4C binary composites, Ceramics

M. Shahedi Asl et al. / Ceramics International ] (]]]]) ]]]–]]] 9

steps of sintering process in ZrB2–B4C binary composites.Such a model is shown in Fig. 10.

4. Conclusion

ZrB2–B4C binary composites containing 0 to 100 vol% B4Cwere consolidated at 1800 1C for 30 min under 12 MPa loadby hot pressing. The density of samples decreased inverselywith B4C content and the type of porosity changed from closedto open in high B4C volume fractions. The monolithic ZrB2

ceramic had the minimum amount of porosity, as the highestportion of the closed pores was formed in it. SEM micrographsfrom fracture surfaces of the samples showed that in thecontact areas between B4C and ZrB2 particles, the presence ofmoss-like ZrO2 particles was the main cause of their joining.Sintering process was under the control of such moss-likeZrO2 transient phases formed during hot pressing. Reductionof ZrO2 to ZrB2 at temperatures as high as the hot pressingcondition, led to the formation of fine ZrB2 at the contact areasof main ZrB2/B4C particles and provided a chemical sinteringmechanism.

Acknowledgment

This work was supported by the Advanced CeramicResearch Group, University of Tabriz, Iran. Authors wouldlike to appreciate Arta Pajouhesh Kaveh Engineering Com-pany for its assistance during the microstructural investigationsand the designing of schematic drawings.

References

[1] B.M. Moshtaghioun, A.L. Ortiz, D.G. García, A.D. Rodríguez, Toughen-ing of super-hard ultra-fine rained B4C densified by spark-plasmasintering via SiC addition, J. Eur. Ceram. Soc. 33 (2013) 1395–1401.

[2] F. Thevenot, Boron carbide- a comprehensive review, J. Eur. Ceram. Soc.6 (1990) 205–225.

[3] V. Domnich, S. Reynaud, R.A. Haber, M. Chhowalla, Boron carbide:structure, properties, and stability under stress, J. Am. Ceram. Soc. 94(11) (2011) 3605–3628.

Please cite this article as: M. Shahedi Asl, et al., A fractographical approachInternational (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.081

[4] Z.D. Kovziridze, Z. Mestvirishvili, G. Tabatadze, N.S. Nizharadze,M. Mshvildadze, E. Nikoleishvili, Improvement of boron carbidemechanical properties in B4C–TiB2 and B4C–ZrB2 systems, J. Electron.Cool. Therm. Control 3 (2013) 43–48.

[5] Z. Zuoguang, W. Mingchao, L. Min, S. Zhijie, Sintering and mechanicalproperties of mullite-reinforced boron carbide matrix composite, J. Am.Ceram. Soc. 92 (5) (2009) 1129–1132.

[6] W.G. Fahrenholtz, G.E. Hilmas, S.C. Zhang, S. Zhu, Pressurelesssintering of zirconium diboride: particle size and additive effects,J. Am. Ceram. Soc. 91 (5) (2008) 1398–1404.

[7] S. Zhu, W.G. Fahrenholtz, G.E. Hilmas, S.C. Zhang, Pressurelesssintering of zirconium diboride using boron carbide and carbon additions,J. Am. Ceram. Soc. 90 (11) (2007) 3660–3663.

[8] W.G. Fahrenholtz, G.E. Hilmas, I.G. Talmy, J.A. Zaykoski, Refractorydiborides of zirconium and hafnium, J. Am. Ceram. Soc. 90 (5) (2007)1347–1364.

[9] E.Z. Solvas, D.D. Jayaseelan, H.T. Lin, P. Brown, W.E. Lee, Mechanicalproperties of ZrB2- and HfB2-based ultra-high temperature ceramicsfabricated by spark plasma sintering, J. Eur. Ceram. Soc. 33 (7) (2013)1373–1386.

[10] S. Hayun, V. Paris, M.P. Dariel, N. Frage, E. Zaretzky, Static anddynamic mechanical properties of boron carbide processed by sparkplasma sintering, J. Eur. Ceram. Soc. 29 (2009) 3395–3400.

[11] H. Wenbo, G. Jiaxing, Z. Jihong, Y. Jiliang, Microstructure and proper-ties of B4C–ZrB2 ceramic composites, Int. J. Eng. Innov. Technol. 3 (1)(2013) 163–166.

[12] T.K. Roy, C. Subramanian, A.K. Suri, Pressureless sintering of boroncarbide, Ceram. Int. 32 (2006) 227–233.

[13] S.Q. Guo, Densification of ZrB2-based composites and their mechanicaland physical properties: a review, J. Eur. Ceram. Soc. 29 (2009)995–1011.

[14] S. Zhu, W.G. Fahrenholtz, G.E. Hilmas, S.C. Zhang, E.J. Yadlowsky,M.D. Keitz, Microwave sintering of a ZrB2–B4C particulate ceramiccomposite, Compos. A 39 (2008) 449–453.

[15] X.G. Wang, W.M. Guo, Y.M. Kan, G.J. Zhang, Hot-pressed ZrB2

ceramics with composite additives of Zr and B4C, Adv. Eng. Mater. 12(9) (2010) 893–898.

[16] H. Zhang, Y. Yan, Z. Huang, X. Liu., D. Jiang, Pressureless sintering ofZrB2–SiC ceramics: the effect of B4C content, Scr. Mater. 60 (2009)559–562.

[17] J. Yin, H. Zhang, Y. Yan, Z. Huang, X. Liu, D. Jiang, High toughness inpressureless densified ZrB2-based composites co-doped with boron–titanium carbides, Scr. Mater. 66 (2012) 523–526.

[18] S.C. Zhang, G.E. Hilmas, W.G. Fahrenholtz, Pressureless densification ofzirconium diboride with boron carbide additions, J. Am. Ceram. Soc. 89(5) (2006) 1544–1550.

to the sintering process in porous ZrB2–B4C binary composites, Ceramics