4 Reactive hot pressing and sintering characterization of ZrB2–SiC–ZrC composites

Materials Science and Engineering A491(2008)

117–123

Reactive hot pressing and sintering characterization

of ZrB2–SiC–ZrC composites

Qu Qiang?,Zhang Xinghong,Meng Songhe,Han Wenbo,Hong Changqing,Han Jiecai

Center for Composite Materials,Harbin Institute of Technology,Harbin150001,China

Received18November2007;received in revised form21January2008;accepted21January2008

Abstract

The reactive synthesis route,microstructure and properties of ZrB2–SiC–ZrC composites prepared from a mixture of Zr/B4C/Si powders via reactive hot pressing were investigated.The reactions of the Zr/B4C/Si system commenced at800?C and completed after a60min hold at1700?C in argon atmosphere.The reacted products at1700?C were composed of?ne particles and the size was smaller than1?m,which provided a large driving force for densi?cation.The powders were sintered at1750?C,1800?C,1850?C and1900?C,respectively.The results showed that the relative density and average grain size of the composites increased with the increase of sintering temperature.The relationships between the microstructural features and mechanical properties of the composites were discussed.

?2008Elsevier B.V.All rights reserved.

Keywords:Reactive hot pressing;Sintering;Microstructure;Mechanical property

1.Introduction

Transitional metal borides and carbides are promising candi-dates for structural applications exceeding2250?C due to high percentage of covalent bonding[1].The strong covalent bonding of zirconium diboride(such as ZrB2or HfB2)leads to a high melting temperature,high hardness and high strength.Along with the properties mentioned above,the oxidation and thermal shock resistance of ZrB2also make it a candidate for thermal pro-tection systems and propulsion components on future hypersonic ?ight vehicles and reusable launch vehicles[2,3].

It is well known that the improvements to the oxidation resistance of ZrB2can be accomplished with the addition of 10–30vol%SiC particulates as a second phase[4–6].The SiC and ZrC particulates also increase the strength and fracture toughness of ZrB2-based ceramics at room temperature because a second phase may inhibit grain growth[7].Actually,because the mechanical properties of the composites,such as mechani-cal strength,hardness and fracture toughness are controlled by densi?cation and microstructure of the sintered bodies,the pro-duction of the homogeneous and densi?ed ZrB2-based ceramics is de?nitely important.

?Corresponding author.Fax:+8645186402382.

E-mail address:quqianghit@https://www.360docs.net/doc/8d10508427.html,(Q.Qiang).

Due to the high melting point and strong covalent bond,ZrB2-based composites are dif?cult to be sintered into full densities via the hot pressing sintering by using ZrB2as raw material. Attempts to densify ZrB2without sintering aids resulted in rather low?nal densities[8,9].Sintering aids such as Ni,B4C, Si3N4and Al3N4have been added to enhance the sinterability [8,10,11].These additives reduce the densi?cation temperature for ZrB2to between1650?C and1900?C.Although additives can reduce the densi?cation temperatures,the ceramics exhibit mechanical degradation at elevated temperatures[8,10].

Reactive hot pressing(RHP)is a technique in which both the chemical reactions of the starting materials and densi?ca-tion occur in single step.This technique can be economical due to cheap starting powders and relatively low sintering tem-perature,and it also leads to the possibility of densifying the materials without additives[12].Considerable works have been published concerning the in situ synthesis of ZrB2–SiC com-posites.Zhang fabricated the ZrB2–SiC through a mixture of Zr,B4C,Si powders with a?exural strength(three-point bend-ing)of506MPa and a fracture toughness of4.0MPa m1/2[13], which were both signi?cantly lower than the highest reported values for ZrB2–SiC hot pressed from commercially available ZrB2and SiC powders[14].Comparing with Zhang et al.[13], Zimmermann replaced the Zr by the ZrH2,because ZrH2can reduce the adiabatic temperature to1310?C and the reaction will be more controllable[15].A ZrB2–ZrC–SiC composite was

0921-5093/$–see front matter?2008Elsevier B.V.All rights reserved. doi:10.1016/j.msea.2008.01.053

118Q.Qiang et al./Materials Science and Engineering A491(2008)117–123

prepared through the RHP and the ZrC was bene?cial to pro-mote the densi?cation and improve the mechanical properties [7].Chamberlain investigated ZrB2–SiC composite through the RHP with Zr,B and SiC powders at1650?C,and the enhanced densi?cation was attributed to the formation of nanosized ZrB2 during the reactive process[16].

Generally,if the reactions have a high enthalpy and the val-ues of Gibbs free energy are large negative during the reactive hot pressing,it is easy to undergo the self-propagating high-temperature synthesis(SHS)for the systems,such as Ti–B4C [12,17,18]and Zr–B4C[19–21].Moreover,the SHS products are quite porous and contain a lot of cracks because of their high reactive temperature and large cooling rate[12,17,18,22]. The reactive hot pressing requires relatively mild chemical reac-tion,and the slow heating rate and holding at low temperature could prevent from SHS reaction or even thermal explosion [7,13,15,16,23,24].

The present work investigates the reaction process and sintering characterization that lead to the formation of ZrB2–20vol%SiC–6.05vol%ZrC composite from a starting powder mixture of Zr,B4C and Si through the reaction(1)(x=1, y=0.455).

x Zr+y B4C+(3y?x)Si→2y ZrB2+(x?2y)ZrC

+(3y?x)SiC(1) The densi?cation process,microstructure and mechanical properties of composites which are sintered ranging from 1750?C to1900?C will be compared to understand the effect of sintering temperature.

2.Experimental procedures

Commercially available Zr(mean particle size40.0?m, purity99.5%,impurities included Fe0.3,Ti0.1,Sn0.1,Al0.01, Ni0.01,Mg0.01,according to the supplier,Northwest Institute for Non-ferrous Metal Research,China),B4C(mean particle size8.0?m,purity98.0%,impurities included Fe2O30.4–0.8,Si 0.4–1.0,according to the supplier,Jingangzuan Boron Carbide Co.Ltd.,Mudanjiang,China)and Si powders(mean particle size40.0?m,purity99.4%,impurities included Fe0.29,Cu 0.1,Zn0.05,Sb0.05,according to the supplier,Shouchang Co. Ltd.,Shanghai,China)were used as raw materials.The pow-ders contained agglomerates up to80?m(Zr)and120?m(Si) in size.The raw powders,Zr/B4C/Si in molar ratio1/0.455/0.365 according to reaction(1)(x=1,y=0.455),were ball milled in ethanol for12h in a nylon-coated tank using ZrO2balls.Rotary evaporation was used to separate the milled powder from the slurry.This procedure also ensured that large Zr and Si agglom-erates would be dispersed.The mixture of powders was placed into a boron nitride-coated graphite die.

In order to investigate the reaction mechanisms,the samples were heated at a rate of10?C/min to various temperatures in stagnant argon atmosphere and cooled in the furnace by turning off power to achieve rapid cooling.The parameters of heating temperature and dwell time listed in Table1.

ZrB2–20vol%SiC–6.05vol%ZrC composites were fabri-cated by in situ hot pressing reaction synthesis.The samples were heated at10?C/min from room temperature to800?C and then held at this temperature for60min in argon atmosphere. After the hold,the samples were heated at3?C/min to900?C and held for60min,and then heated at3?C/min to1000?C, followed by a60min hold.Then the samples were heated at 3?C/min to1500?C and held for an additional60min.The slow heating rate and holding at low temperatures were used to pre-vent from SHS reaction or thermal explosion,and the hold at 1500?C for60min prompted the reaction process further.After this hold,the samples were?nally sintered at1750?C,1800?C, 1850?C and1900?C,respectively.The application of pressure gradually increased to30MPa and maintained for60min at each sintering temperature.Hereafter the composites are referred to as ZSZ1750,ZSZ1800,ZSZ1850and ZSZ1900,respectively.

After sintering,the phase compositions of investigated sam-ples were identi?ed by X-ray diffraction(XRD).Scanning electron microscopy(SEM)was also employed to characterize the microstructure of sintered samples.The relative density of sintered specimens was measured using the Archimedes method. Flexural strength was measured with three-point bending tests on36mm×4mm×3mm(length×width×thickness) test bars,using a span of30mm with a crosshead speed of 0.5mm/min.Fracture toughness was evaluated using single-edge notched bend(SENB)beams22mm×4mm×2mm (length×height×width),notch depth and radius of2mm and 0.2mm,respectively,using a span of16mm and a crosshead speed of0.05mm/min.At least?ve specimens were prepared for each test.Vickers hardness was measured by Vickers indentation with5kg as applied load for10s on a polished section.

3.Results and discussion

3.1.Reaction mechanisms

The speci?c feature of reactive hot pressing is that the mate-rial is synthesized via a series of reactions from raw powders

Table1

Phase compositions of samples heat-treated under different parameters(heating rate10?C/min)

Test Sintering parameters Crystalline end products Sample1Dwell at800?C for1h Zr,B4C,Si,ZrC,ZrO2

Sample2Dwell at800?C for1h,then dwell at900?C for1h Zr,B4C,Si,ZrC,ZrB2,ZrO2 Sample3Dwell at800?C for1h,then dwell at1000?C for1h,then dwell at1100?C for1h Zr,B4C,Si,ZrC,ZrB2,SiC,ZrO2 Sample4Dwell at800?C for1h,then dwell at1000?C for1h,then dwell at1300?C for1h B4C,Si,ZrC,ZrB2,SiC,ZrO2 Sample5Dwell at800?C for1h,then dwell at1000?C for1h,then dwell at1500?C for1h B4C,Si,ZrC,ZrB2,SiC Sample6Dwell at800?C for1h,then dwell at1000?C for1h,then dwell at1700?C for1h ZrC,ZrB2,SiC

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Fig.1.The XRD patterns of samples1–6which were heat-treated at different temperatures for60min in argon.Sample1,800?C;sample2,900?C;sample 3,1100?C;sample4,1300?C;sample5,1500?C;sample6,1700?C.

to the?nal.Hence,it is necessary to study the sequence of phase formation and reaction mechanism.Six samples were heat-treated at different temperatures and then the phase com-position(shown in Table1)was determined by XRD.Stacked XRD patterns from the different heat-treated conditions were presented in Fig.1.

After holding at800?C for60min,apart from Zr,B4C and Si from raw powders,X-ray diffraction identi?ed also a ZrC and ZrO2in sample1.The ZrO2phase was attributed to the oxidation of Zr powder in the air and ZrO2milling balls.The shape and position of ZrC peaks suggested that a range of non-stoichiometric ZrC1?X compounds had been formed.The formation of the Zr-rich carbide required more Zr than the sys-tem actually contained,which can be accounted for by some unreacted B4C was still present in the sample1.The presence of ZrC1?X also indicated the reaction between Zr and B4C com-menced at800?C and ZrC1?X formed preferentially,it was attributed to the diffusion of carbon being much faster than that of boron in zirconium,so carbon can reach the center of the Zr particles resulting in the presence of ZrC[23,25].However, the ZrB2peaks were detected in sample2,showing that ZrB2 formed at a relatively higher temperature of900?C due to the increment of the diffusion coef?cient of boron.

As temperature increasing,ZrB2and ZrC became the main phases based on the peak intensity in the XRD patterns(shown in sample3of Fig.1).It was worth noting that the appear-ance of weak peaks at35.61?,59.9?,71.78?and75.48?2θindicated the synthesized SiC occurred at about1100?C,which was higher than the ZrB2and ZrC because of the inertness of the silicon.In addition,it was found that holding at800?C and then1000?C for60min allowed the avoidance of thermal explo-sion during the synthesis process.Apparently,there was enough time during the slow heating and dwelling for solid state diffu-sion processes to take place resulting in the formation of strong diffusion barriers that prevented the reaction from becoming self-sustaining.

The formed ZrC decreased when the temperature exceeded 1300?C,because the ZrC phase was unstable in the presence of B4C and Si according to reaction(2),thus

2ZrC+B4C+3Si→2ZrB2+3SiC(2) ZrB2and SiC formed in the boundary of formed ZrC[15]. The XRD pattern of samples4–6also showed the decrement of ZrC and elimination of B4C and Si,accompanying with the increment both of ZrB2and SiC.The elimination of ZrO2in sam-ple5indicated that the ZrO2reacted with B4C above1300?C as reaction(3)[16,26]thoroughly,which was bene?cial to improve the densi?cation of ZrB2-based ceramics[26].

5B4C+7ZrO2→7ZrB2+5CO(g)+3B2O3(g)(3) Furthermore,a small broadening ZrC peaks at33.74?,39.14?, 56.64?and67.64?were observed in sample5indicating the substoichiometric ZrC1?X existed,which was owing to the for-mation of CO of reaction(3).However,some vacancy and defects would improve the diffusion and thus increase the den-si?cation.Here,it could be noted that the synthesis reaction had completed after holding for60min at1700?C,and the sample contained ZrB2,SiC and ZrC with no peaks indicative of unre-acted B4C or elemental Zr and Si.It can be concluded that the subsequent hot pressing at a temperature higher than1700?C only contributed to the densi?cation.

3.2.Microstructure

It can be seen in Fig.2the size of grains obtained from the reactions was less than1?m.The formation of very?ne and homogeneous grains would provide a large driving force for densi?cation at moderate low temperatures[16].Therefore temperatures of1750?C,1800?C,1850?C and1900?C were chosen as the sintering temperature to study the densi?cation behavior.The density was tested by water displacement method. Density values of6.09g/cm3for ZrB2,6.74g/cm3for ZrC and 3.21g/cm3for SiC were used in the calculation of the theoret-ical densities of the obtained composites.A relative density

of Fig.2.SEM of the particles after heat-treated at1700?C for60min.

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Table2

Relative density and mechanical properties of ZrB2–SiC–ZrC composites

Material Relative density(%TD)Flexural strength(MPa)Fracture toughness(MPa m1/2)Vickers hardness(GPa) ZSZ175091.4511±29 5.67±0.1714.3±0.7

ZSZ180097.0543±347.04±0.3718.2±0.4

ZSZ185099.1622±13 6.49±0.3519.0±0.5

ZSZ190099.6526±9 6.50±0.3019.8±0.5

Table3

Relative density and mechanical properties of ZrB2–SiC–ZrC composites reported elsewhere

Composition(vol%)Relative density(%TD)Flexural strength(MPa)Fracture toughness(MPa m1/2)References ZrB2–20SiC–6.4ZrC96.8 5.1[7]

ZrB2–25SiC97.67506 4.0[13]

ZrB2–27SiC99720 3.5[15]

ZrB2–30SiC99800 2.12[16]

91.4%was achieved by reactive hot pressing at only1750?C and increased to97%at1800?C,the relative density exceeded99% when sintering temperatures were higher than1850?C(listed in Table2).Such high relative density was dif?cult to attain from a commercial mixture of ZrB2,ZrC,SiC powders via hot pressing or SPS at a sintering temperature below1900?C,by just rely-ing on a solid sintering mechanism[27,28].It was likely that a much higher local temperature was involved during the reaction sintering due to the reaction heat,which increased the diffusion of the particles and enhanced the densi?cation.In addition,it also suggested that the driving force for sintering was attributed to the?ne grains and lattice defects.The ZrB2–SiC–ZrC com-posites all fabricated through RHP at a sintering temperature below1900?C resulting in satis?ed density(listed in Table3) [7,13,15,16].

Fig.3shows the polished cross-section of composites sin-tered at various temperatures,where dark areas represented the SiC particles and gray areas ZrB2and ZrC.It was dif?cult to distinguish ZrB2and ZrC phases due to the very little contrast between them and the EDS facility was insensitive to boron and carbon elements.It can be seen that the pore distribution coincided with the relative densities of the composites(listed in Table2).The composites sintered at1850?C and1900?C almost densi?ed with rare residual pores.

The grain size of the composites produced by reactive hot-pressing was strongly dependent on the?nal sintering

tem-Fig.3.SEM of polished cross-section of(a)ZSZ1750,(b)ZSZ1800,(c)ZSZ1850and(d)ZSZ1900.

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Fig.4.SEM of the morphology of(a)ZSZ1750,(b)ZSZ1800,(c)ZSZ1850and(d)ZSZ1900.

perature.The analysis of grain size distribution was performed on the etched surface(shown in Fig.4)of all composites.In the composite ZSZ1750,the average grain size of ZrB2and ZrC was just2?m,indicating the inhibition of grain growth at a rel-ative low temperature.The average grain size of ZrB2and ZrC increased to4?m in both the ZSZ1800and ZSZ1850,showing a little grain growth with the increasing sintering temperature.In contrast,obvious grain growth was observed in ZSZ1900,The average grain size of ZrB2and ZrC were8?m with some grains larger than10?m.The excessive grain growth would deterio-rate the mechanical properties of the composites,thus it should be retarded during the synthesized process[29].In addition,the morphology of ZrB2grains was mainly in fairly equiaxed and plate-like which was consisted with Ref.[7].The speci?c mor-phology should be related to the growth mechanism of ZrB2, but it was not clearly studied yet.

3.3.Properties

Mechanical properties of the composites were listed in Table2.The variation in the mechanical properties was well re?ected in the change of microstructural features.The ZSZ1850 possessed the highest?exural strength depending on its dense body with?ne grains and homogeneous microstructure.The decrement of?exural strength in ZSZ1800and ZSZ1750was due to the9%and3%porosity distributed in the bulks.However, the reducing?exural strength in ZSZ1900should be attributed to the obvious grain growth of the ZrB2and ZrC.The reported ?exural strength(listed in Table3)[15,16]was higher than the values in this study because the?ner raw powders used and?ner grains obtained in the?nal bulks.

The Vickers hardness was19.8,19.2and18.2GPa for ZSZ1900,ZSZ1850and ZSZ1800,respectively,showing they were in a similar level.The hardness was an inherent property and did not vary signi?cantly for the range of grain sizes,but it decreased to14.3GPa of ZSZ1750due to the obviously poros-ity.The hardness had been shown to decrease exponentially as the porosity increased for ceramic materials[16].

Fig.5shows the SEM image of the crack propagation formed by the Vickers indentation.The crack propagated preferentially along the interface which suggested the main toughening mech-anism was crack de?ection.The fracture toughness of ZSZ1800 was7.04MPa m1/2,higher than the other three,because the moderate bonding between grains formed at relative low sinter-ing temperature was bene?cial to promote crack de?ection and induce crack emergence(shown in Fig.5b),which also reduced the?exure strength.Additionally,the3%porosity distributed in ZSZ1800can release the crack tip stress and absorb an amount of energy during fracture.This fracture mode also improved the fracture toughness of the composite.The dense bulk of ZSZ1850 and ZSZ1900had a similar fracture toughness of6.49MPa m1/2 and6.50MPa m1/2,a slight lower than that of ZSZ1800.It was shown in Fig.5c and d that the crack de?ected during the prop-agation,but it hardly induced any crack emergence to consume the fracture energy,which was attributed to the stronger bond-ing between grains formed at higher sintering temperature.The poor fracture toughness of ZSZ1750was still mainly attributed to the higher porosity https://www.360docs.net/doc/8d10508427.html,paring with the reported val-

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Fig.5.SEM of the crack propagation formed by Vickers indentation of (a)ZSZ1750,(b)ZSZ1800,(c)ZSZ1850and (d)ZSZ1900.

ues (listed in Table 3)[7,13,15,16],the fracture toughness in this study was obvious higher because of the plate-like ZrB 2grains.The crack de?ection would be enhanced when the crack pene-trated the plate-like grains and a mount of energy was consumed to improve the fracture toughness.4.Conclusion

This work reported reactive synthesis route,microstructure and mechanical properties of ZrB 2–SiC–ZrC composites pre-pared from a mixture of Zr/B 4C/Si powders by in situ reactive synthesis.The reactions of the Zr/B 4C/Si system commenced at 800?C and completed after a 60min hold at 1700?C in argon atmosphere.The slow heating rate and long holding at low temperature were used to prevent from self-propagating high-temperature synthesis during the synthesis process.The products of the reaction were composed of ?ne powders smaller than 1?m which provided a large driving force for densi?ca-tion.The powders were sintered at 1750?C,1800?C,1850?C and 1900?C,respectively.The relative density of the compos-ites was 91.4%for 1750?C,97.0%for 1800?C and higher than 99%for both 1850?C and 1900?C.The average grain size kept as small as 2?m after sintering at 1750?C,it increased to 4?m for both 1800?C and 1850?C,and increased obviously to 8?m with some grains larger than 10?m after sintering at 1900?C.The composite sintered at 1850?C possessed the highest ?exural strength of 622±13MPa due to the dense body with homo-geneous microstructure and ?ne grains.The main toughening mechanism of the composites was crack de?ection and plate-like ZrB 2grains were bene?cial to improve the fracture toughness.

The moderate bonding of grains and 3%porosity also enhanced the resistance of crack extendibility of ZSZ1800,resulting in the highest fracture toughness of 7.04±0.37MPa m 1/2.Acknowledgements

This work was supported by the National Natural Sci-ence Foundation of China (Nos.90505015and 50602010),the Research Fund for the Doctoral Program of Higher Education (No.20060213031)and the Program for New Century Excellent Talents in University.References

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