Aqueous Gelcasting and Pressureless Sintering ofZirconium Diboride Ceramics

Jie Yin†

State Key Laboratory of High Performance Ceramics and Superfine Microstructure,Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China

University of the Chinese academy of sciences, Beijing, 100039 China

Zhaoquan Zhang, Zhengren Huang, Hui Zhang, Yongjie Yan, Xuejian Liu,Yan Liu, and Dongliang Jiang

State Key Laboratory of High Performance Ceramics and Superfine Microstructure,Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China

Dense (97.3%) zirconium diboride (ZrB2) ceramics were obtained via gelcasting and pressureless sintering. Four wt%B4C was used as sintering aid. ZrB2, SiC, and B4C can codisperse well in the alkaline region, using a polyacrylate dispersant.

Compared with monolithic ZrB2 (Z), the mechanical properties of ZrB2-SiC (ZS) were enhanced. The Vickers hardness andfracture toughness of ZS were (13.1 � 0.6) GPa and (2.5 � 0.4) MPa m1/2, respectively.

Introduction

Zirconium diboride (ZrB2), as a member of ultra-high-temperature ceramics (UHTCs), is expected to be

used in the thermal protection systems for hypersonicaerospace vehicles, high-temperature electrodes, andcrucibles for molten metal contact potentially.1–3

Die Pressing, hot pressing, and spark plasma sin-tering are traditional ways for forming dense ZrB2-based ceramics. Nevertheless, advanced wet-formingmethods, typically gelcasting, are known for their

†jieyin@student.sic.ac.cn

© 2013 The American Ceramic Society

Int. J. Appl. Ceram. Technol., 1–6 (2013)DOI:10.1111/ijac.12120

ability to prepare large scale and near-net-shape compo-nents, thus are attracting increasing attentions in recentyears.4 Up till now, investigations concerning gelcastingand pressureless sintering of ZrB2-based ceramics arerare. Slurry preparation plays a critical role in successfulwet-forming process. He et al.5,6 applied gelcasting andpressureless sintering to obtain dense ZrB2-SiC ceram-ics. The dispersant polyacrylic acid adsorbed onto theparticle surface and helped to change the surface chargebehavior. However, first, the ZrB2-SiC slurry theyprepared was too viscous for casting (the viscosity was~3 Pa�s at a shear rate of 60 s); and secondly, carbonwas added into powder mixtures, which is hard to dis-perse in water and might lead to shear-thickeningbehavior. Secondly, investigations concerning gelcastingof pure ZrB2 ceramic are scarce, which has potentialapplications in several aspects as mentioned above. Heet al.7 also investigated the effect of polyethyleneimine(PEI) on highly concentrated and well dispersed ZrB2-SiC aqueous suspensions. In addition, Zhang et al.8

studied the dispersion and interaction of ZrB2 nano-powders in n-butanol, using gallic acid as dispersant.

In our earlier work, an ammonium salt of a poly-acrylate polymer was used to prepare highly concen-trated ZrB2 gelcasting slurry.9 The dispersingmechanism was the adsorption of PAA� ions onto par-ticle surfaces, which can alter surface charge density.Moreover, polyacrylate dispersants had already beenproved to be effective in dispersing ZrB2, SiC, andB4C in aqueous media. L€u et al.10–12prepared ZrB2-based ceramics using tape casting and hot pressing. Thedispersant they used was Lopon 885, an ammoniumpolyacrylate solution. Besides, Medri et al.13,14preparedZrB2-SiC ceramics by slip casting and pressureless sin-tering, and they also used ammonium salt of acrylichomopolymer as dispersant.

The purpose of this article is to develop gelcastingtechnique followed by pressureless sintering for prepar-ing high performance ZrB2 ceramics.

Experimental Procedure

Commercially available powders were used as rawmaterials: ZrB2 (Dandong Chemical, China,D50 = 2.5 lm, purity>90%), SiC (FCP 15C, SIKATECH., Norway, D50 < 0.5 lm, purity>99%), andB4C (Mudanjiang Jingangzuan Boron Carbide, China,D50 < 1 lm, purity>90%). Polyacrylate dispersant

(SD-07, 30 wt% aqueous solution, Nanjing XiaokeNano Ceramics Tec, Nanjing, China) was selected asdispersant on the basis of our earlier work.9

Organics were introduced for gelcasting process.N,N’-Dimethylacrylamide (DMAA, analytical pure,Shandong Wenchang Petrochemical, China), N,N’-Methylenebisacrylamide (MBAM, analytical pure, FlukaChemika, Switzerland), and Ammonium persulfate(APS, Shanghai Degussa-aj initiator, China) were cho-sen as monomer, cross-linker and initiator, respectively.

Initially, ZrB2 powder was milled (Model 01-HD,Union Process Precision Machinery, Qingdao, China)at 300 rpm for 2 h in ethanol medium, with cobaltbonded tungsten carbide media.15 The particle sizedecreased down to ~0.4 lm, and its BET surface areawas 8.3 m2/g.9 The as-milled ZrB2 powder was addedinto a hydrochloric acid solution (pH ~2) to eliminateexcess impurities and then washed by de-ionized waterfollowed by freeze-drying. In our research, 0 and 4 wt%B4C were added to ZrB2(�20vol% SiC) batches, whichwere designated as Z, Z-4B (ZrB2-9 vol%B4C), ZS,and ZS-4B (ZS-8 vol%B4C) for simplicity. The pow-der mixtures were dispersed into a premix solution (de-ionized water together with monomer, cross-linker, andinitiator) with 0.60 mg�m�2 pure polyacrylate (0.5 wt% pure polyacrylate of the total solid masses). The sus-pensions were prepared by constant stirring using amechanical agitator at 400 rpm in an ice water bathfor an hour. Subsequently, the slurry was degassed at avacuum level of ~2.0 9 103 Pa before pouring intomolds. The solidification proceeded at 60°C for anhour. The green bodies experienced further drying at90°C for 4–5 h and at room temperature for 2 daysafterward before binder removal (1°C/min heating to600°C, with 1 h hold at 600°C). Finally, sintering wascarried out in a high-temperature graphite resistancefurnace (High-Multi 10000, Fujidempa kogyo, Saitam-a, Japan). ZrB2(-SiC) ceramics were fired at a rate of10°C/min to 1600°C in vacuum (~8 Pa), then back-filled with argon atmosphere and continued heating to2100°C (2 h holding) before cooling down to roomtemperature.

The suspensions were characterized in terms of zetapotential and rheological measurements. For zetapotential analysis (Zetaplus, Brookhaven Instruments,Holtsville, NY), slurries of 0.01 vol% solids loading inthe presence of 0.5wt% polyacrylate dispersant wereprepared. The pH values of the suspensions wereadjusted by diluted HCl and NaOH solutions. The

2 International Journal of Applied Ceramic Technology—Yin, et al. 2013

shear-dependent behavior of suspensions was investi-gated using a parallel-plate system on Physica MCR301 (Anton Paar, Austria) at a constant temperature of25°C. It was evaluated at increasing and decreasingshear rates between 0 and 200 s (with 200 s main-tained for 50 s). The suspensions prepared for the zetapotential and rheological characterizations were free ofmonomer, cross-linker, or initiator, and the volumeratios of the solid compositions for all the suspensionswere ZrB2/B4C = 91:9 (Z-4B), ZrB2/SiC = 4:1 (ZS)and ZrB2/SiC/B4C = 73.6:18.4:8.0, respectively.

Density of green bodies and sintered ceramics wasmeasured by the Archimedes method using ethanol andwater as immersing media. The theoretical density wascalculated according to rule of mixtures (ZrB2: 6.09g/cm3, SiC: 3.21 g/cm3 and B4C: 2.52 g/cm3), and thenominal composition contents were used for calcula-tion. Specimen with dimensions of 3 9 4 9 36 mmwas polished to 1 lm finish using diamond abrasives.The three-point flexural strength was tested with auniversal tester (Instron-1195, Instron, Canton, MA)over a 30-mm span, using a cross-head speed of0.5 mm/min. At least five samples were measured toget an average flexural strength value. Vickers hardnessand fracture toughness were measured by means ofVickers indentation method (Model 300, Tukon, Can-ton, MA) with five samples each, using a load of 5 kgand dwell time of 10 s. The microstructure of polishedsurface was observed by scanning electron microscopy(SEM, JXA 8100, JEOL, Tokyo, Japan).

Results and Discussion

Favorable codispersion is important in realizingchemical homogeneity of the final materials. The sur-face charge behavior of ZrB2, B4C, and SiC powders inthe presence of 0.5wt% dispersant in water mediumwas characterized by zeta potential measurement(Fig. 1). Without dispersant, the highest negative zetapotential values of B4C and SiC were ~�30 mV and~�45 mV, respectively, and both appeared in the alka-line region of pH 10–11. Obvious decreases in fpotential values (decreases of >20 mV at pH 11) wasfound. Similar effect as ZrB2 was presumably to beresponsible for the dispersion behavior of SiC and B4Cpowders: the adsorption of PAA� ions onto particlesurfaces to improve their surface charge behavior. Theresult clearly shows these powders can codisperse well

in the alkaline region (pH 10–11), with the presence of0.5wt% dispersant.

The flow curves of concentrated slurries are shownin Fig. 2. The slurries exhibited a decrease in viscosityin the descending curve (after constant shearing at200 s for 50 s) compared with the ascending curve.For instance, the viscosities of Z suspension at 100 scorresponding to ascending and descending curve were0.90 and 0.68 Pa�s, respectively. The decrease inviscosity was attributed to the breakup of agglomeratedstructures. Besides, all suspensions displayed shear-thin-ning behavior. The addition of 4 wt% B4C increased

(a)

(b)

Fig. 1. Zeta potential of ZrB2, B4C, and SiC (a) withoutdispersant and (b) with the presence of 0.5 wt% polyacrylatedispersant.

www.ceramics.org/ACT Gelcasting and Sintering of ZrB2 3

the solids content of ZrB2 slurry (52.4 vol%). The vis-cosity increased significantly (Fig. 2a,b, Z: 0.68 Pa�swhile Z-4B: 1.97 Pa�s, both at 100 s in the descendingcurve) as the suspension became more concentrated.Yet, the Z-4B suspension showed more apparent time-dependent behavior, suggesting the presence of flocstherein. The viscosity level of ZS suspensions was closeto Z suspensions (Fig. 2c,d; ZS: 0.74 Pa�s while ZS-4B: 1.33 Pa�s, both at 100 s in the descending curve).The increase in viscosity due to the incorporation ofB4C was lower in ZS (1.33 Pa�s) compared with Z(1.97 Pa�s, both in the descending curve). It was pre-sumably because of the lower volume addition of B4Cin ZS suspension than Z. As evidenced by the previousstudy,9 the incorporation of organic additives couldhelp to decrease the viscosity of ZrB2 concentrated sus-

pension. The presence of organic components in thepremix solution is beneficial to the dispersion behavior.Thus, the conclusion can be drawn that the suspensionsprepared were suitable for gelcasting.

Due to its strong covalent bond as well as the lowvolume and grain-boundary diffusion rate, the sinter-ability of ZrB2 is poor. Without sintering aids, the rela-tive density of Z and ZS materials was lower than 75%(Table I). The presence of B4C could remove the sur-face ZrO2 impurity by involving in the reactionbelow,12

5B4CðsÞ þ 7ZrO2ðsÞ ¼ 7ZrB2ðsÞ þ 5COðgÞþ 3B2O3ðgÞ

The Gibbs-free energy corresponding to the reac-tion was as follows,5

(a) (c)

(b) (d)

Fig. 2. Rheological properties of (a–b) Z and Z-4B suspensions, and (c–d) ZS and ZS-4B suspensions.

4 International Journal of Applied Ceramic Technology—Yin, et al. 2013

DGoT ¼ 1378:7� 0:9242T þ RT ln P5

CO

As the ΔGT became negative during the vacuumheating cycle till 1600°C, this reaction was thermo-dynamic favorable, hence the ZrO2 could be wellremoved. The relative density of ZS-4B (95.3%) waslower than Z-4B (97.3%) material. Previous research-ers have concluded that the presence of SiO2 as asurface oxide impurity on SiC increases the stabilityof the surface oxide compared with monolithic ZrB2

ceramic, due to its very low vapor pressure(PSiO2 = 10�6 Pa at 1450°C).16 Hence, the ZS-4Bceramic was more difficult to be densified. In theprevious work, B4C and C were codoped to effec-tively eliminate the oxide species (ZrO2, B2O3, andSiO2) and promote the densification process, while inthe current work, only B4C was introduced; thus,samples of lower relative density were obtained here.However, increases of 8%, 17%, and 47% corre-sponding to average flexural strength, Vickers hard-ness, and fracture toughness values by, respectively,were found in ZS-4B compared with Z-4B ceramic.The flexural strength, Vickers hardness, and fracturetoughness of ZS-4B composites reached (253 � 53)MPa (13.1 � 0.6) GPa and (2.5 � 0.4) MPa m1/2,respectively. These increases should be ascribed to thefiner microstructure. The microstructure of polishedsurfaces of Z-4B and ZS-4B is shown in Fig. 3. Theaverage particle size of SiC in ZS-4B was 6 lm withan aspect ratio of 2.8. The presence of platelet-likeSiC curbed the grain growth of ZrB2, so the averagesize of ZrB2 grain decreased from 18.8 lm (Z-4B)to 10.0 lm (ZS-4B). In addition, the SiC plateletscould deflect cracks during their propagation and dis-sipate the fracture energy, which was possibly con-trolled by the residual thermal stress that developsduring cooling from the processing temperature.17

Thus, toughness showed a considerable improvement.

On the other hand, the relationship between frac-ture toughness (KIC) and flexural strength (r) can beexpressed by the Irwin relationship

KIC ¼ Y rffiffiffi

ap

Table I. Properties of ZrB2(-SiC) Ceramics Sintered at 2100°C for 2 h

MaterialRelative greendensity (%)

Relativedensity (%)

Flexural strength(MPa)

Vickers hardness(GPa)

Fracture toughness(MPa m1/2)

Z 52.9 � 0.3 57.1 � 1.0 36 � 7 – –Z-4B 56.1 � 0.2 97.3 � 0.8 234 � 35 11.2 � 0.3 1.7 � 0.1ZS 53.1 � 0.5 72.7 � 1.6 172 � 22 – –ZS-4B 56.4 � 0.6 95.3 � 0.5 253 � 53 13.1 � 0.6 2.5 � 0.4

(a)

(b)

Fig. 3. Scanning electron microscopic pictures of polishedsurfaces of (a) Z-4B and (b) ZS-4B ceramics.

www.ceramics.org/ACT Gelcasting and Sintering of ZrB2 5

where Y is a geometric constant, and a is the criticalflaw size. From Table I, the variation of critical flawsize between Z-4B and ZS-4B materials,

aZS�4B

aZ�4B¼ 1:9

Therefore, the critical defect size of ZS-4B was muchlarger than Z-4B ceramic. Yet for ZS-4B, the improve-ment on mechanical properties was limited, typicallystrength. The presence of excess pores in ZS-4B com-posite played a crucial role in degrading its mechanicalproperties.

The mechanical properties of ZS-4B were lowerthan previous reported values.5 It should be ascribed toseveral reasons: First is the lower relative density. Poros-ity plays a critical role in deteriorating mechanicalproperties. Secondly, the grain sizes of ZrB2 and SiChere were relatively large compared with the previousresult.5 The presence of C as sintering aids helped tocurb the grain growth in their case. It might also beattributed to the variation of test methods that the frac-ture toughness values were lower than earlier reportedlevel (in the current research, toughness was calculatedbased on the Vickers indentation method, while in theprevious research the toughness was measured using thesingle-edge notched bend method).5 In addition, otherfactors, such as the presence of microcracks and artifactof the test method may also influence the mechanicalproperties. Further studies are under way.

Conclusion

ZrB2(-SiC) ceramics were prepared by aqueous gel-casting and pressureless sintering at 2100°C for 2 h.The ZrB2, B4C, and SiC powders could codisperse wellin the alkaline region with 0.5 wt% polyacrylate disper-sant, SD-07. The presence of SD-07 helped to increasethe surface charge behavior of these powders. Four wt%B4C was used to increase the sintering driving forceupon firing, and a relative density of 97.3% wasachieved for ZrB2 ceramic. The addition of 20 vol%SiC into ZrB2 matrix improved the mechanical proper-

ties as well as refined the particle sizes of ZrB2 matrix,and the average flexural strength, Vickers hardness, andfracture toughness values of ZS-4B composites were(253 � 53) MPa, (13.1 � 0.6) GPa and (2.5 � 0.4)MPa m1/2, respectively.

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