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Ceramics International 40 (2014) 7245–7251www.elsevier.com/locate/ceramint

The influence of granulation on the gelcasting of pressureless-sinteredsilicon carbide ceramics

Xianhui Li, Qingzhi Yann, Meiqi Cao, Yingying Mi, Yongjun Han, Changchun Ge

Institute of Special Ceramics and Powder Metallurgy, University of Science & Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing, China

Received 23 July 2013; received in revised form 28 November 2013; accepted 16 December 2013Available online 27 December 2013

Abstract

Gelcasting combined with pressureless sintering is a promising technological process to fabricate large scale or complex-shaped silicon carbideceramics (SiC) products with excellent properties. To obtain high solid loading SiC slurry with low viscosity, two promising compact granulationand spray granulation methods were proposed to coarsen raw submicron SiC powders into a larger secondary granule. By compact and spraygranulation processes, the average particle sizes were enlarged from 0.51 μm to 40.7 μm and 8.2 μm, respectively. The enlarged particlesdecreased the volume of structural water in the slurries remarkably, resulting in the decrease in the viscosity of slurry. The viscosities of slurryprepared from spray-granulated powders and compact-granulated powders were 1/2.5 and 1/3.5 to that from submicron powder at a shear rate of100 s�1, respectively. The lower viscosities benefited to the escaped air bubble in slurries, and leaded to a dense microstructure in green andsintered bodies. The bending strengths of green bodies and sintered bodies prepared from granulated powders were about 3 times and 2.2 times tothat from submicron powders. It is worth to notice that even though the dried spray-granulated powders with sphere-like appearance displayedhigher tap density and well fluidence, the slurry viscosity was 58 Pa s higher than that of compact-granulation powders due to the hollow particlestructure. As a result, compact granulation powders showed desired slurry fluidity and dense green and sintered bodies, thus high bendingstrength.& 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: D. SiC; Gelcasting; Granulation; Pressureless sintering

1. Introduction

Pressureless sintered silicon carbide ceramics (SiC) is apromising material which possesses a series of excellentproperties, such as high hardness, high strength, high thermalshock resistance, outstanding chemical stability and good wearresistance. Specially, pressureless sintering SiC with submi-cron raw powders is crucial to reach high strength, corrosionresistance [1–3]. However, the widespread applications of SiCare limited by difficulties in forming and high cost inmachining, especially for components with complex shapeand large size [4]. Gelcasting is a new ceramic formingprocess, which was first developed at the Oak Ridge NationalLaboratory (ORNL), USA, by Janney and Omatete in the1990s. In the gelcasting process, slurry prepared from ceramicpowders and monomers solution was poured into a mold and

e front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All ri10.1016/j.ceramint.2013.12.064

g author.ss: qzyan@ustb.edu.cn (Q. Yan).

polymerized in situ with the aid of catalyst and initiator; then agreen body with uniform density and complex shape identicalto the mold cavity was obtained [5,6]. So it is an ideal choicefor manufacturing complex-shaped pressureless-sintered SiC.In the gelcasting process, the preparation of slurry with high

solid content and low viscosity is a key issue. Until now,researchers have studied the effect of solid content, powdersurface state, particle size, dispersant, pH, initiator, catalystand inhibition of oxygen on gelcasting [7–14]. According tothe previous theoretical and experimental works, the adjust-ment of particle size and its distribution can improve the solidscontent and slurry fluidity. Liu et al. [15] investigated theeffects of different particle sizes on the rheological properties.The results indicated that single submicron SiC powder led toviscous slurry, thus low solid loading. When bimodal powderscomposed of coarser and finer particle size were used as rawmaterials, the fluidity of slurry was improved significantly andhigh solid content slurry was obtained. Smith and Haber [16]and Tari et al. [17] observed the similar phenomenon when

ghts reserved.

Ceramics powders( SiC, B4C and C )

Deionized water

Ball milling

Drying

Compaction

Crushing

Sifting

Compact-granulatedpowders

Centrifugal atomized

Spray-granulated powders

Drying

Fig. 1. The flow chart of two granulating processes.

Monomers and crosslink (AM, MBAM)

Deionized water

Mechanically stirring

Ball milling

Granulated powder

Vacuum degassing

Vacuum degassing

Casting

Gelling

Demolding

Drying

Debonding

Sintered body with SGP or CGP

Dispersant ( TMAH )

Initiator and Catalyst ( APS, TEMED )

Pressureless sintering

Fig.2. The flow chart of SiC specimen fabrication process.

X. Li et al. / Ceramics International 40 (2014) 7245–72517246

they prepared slurry using two kinds of single fine aluminapowders and alumina powders with bimodal particle sizedistribution. Takai et al. [18] investigated the effects of specificsurface area on the dispersion of aqueous yttria slurries forgelcasting. The results showed that high solids content andcastable yttria slurry could be achieved more easily usingcoarser (lower specific surface area) particles.

It is clear that there are conflict requirements of SiC powdersize for dense pressureless-sintering and gelcasting with highsolid loading and low viscosity slurry. How to satisfy theconflict demands? In this work we firstly prepared SiC slurrywith two granulation techniques in the gelcasting process. Theraw submicron silicon carbide powder was coarsened into asecondary granule by spray granulation and compact granula-tion respectively. The effects of granulation on the slurryfluidity, properties of the green body and the sintered bodywere investigated in detail.

2. Experimental

2.1. Materials and processing

Silicon carbide powders with an average particle size of0.51 μm, specific surface area of 11 m2/g and purity of 98.7%were used as raw materials. Boron carbide with an averageparticle size of 1.0 μm and purity of 92�92.5% and carbon blackwith Iodine adsorption of 85 m2/g and Dibutyl phthalate (DBP)adsorption of 0.78 cm3/g were used as the sintering additives.Commercially available acrylamide (AM) and N,N0- methylenebisacrylamide (MBAM) were used as monomers. Deionizedwater was used as a medium. Tetra methyl ammonium hydroxidesolution ((CH3)4NOH, TMAH) was used as dispersant to reducethe reunion. Ammonium persulphate ((NH4)2S2O8, APS) andN,N,N0,N0-tetra methyl ethylene diamine (TEMED) were used asinitiator and catalyst respectively. The pH values of the suspen-sions were adjusted by diluted ammonia. All of the chemicalreagents above were chemically pure.

To coarsen the SiC particles, the slurry was prepared fromthe raw submicron SiC powders (denoted as SMP), additivespowders and deionized water by ball milling for 24 h. Then themilled slurry was granulated with spray-granulation andcompact-granulation methods. For the spray-granulation pro-cess, the milled slurry was atomized into droplets by a high-speed rotary centrifugal disc (7200 rpm), water in dropletsevaporated quickly by the hot air (outlet temperature 110 1C)and dried spray-granulated powders were obtained, denoted asSGP. For the compact-granulation process, the milled slurrywas dried and pressed into compacts at 60�100 MPa. Thenthe compacts were crushed and sifted through a 60-mesh sieveto obtain compact-granulation powders, denoted as CGP. Thedetailed flow chart of the granulation process is shown inFig. 1

For the gelcasting process, a premix solution of monomerswas prepared by dissolving AM, MBAM and TMAH indeionized water by mechanically stirring. Then the granulatedpowders were added into the premixed solution and stirred for6 h to get gelcasting slurry with a solid content of 50 vol%.

The as prepared slurry was degassed for 15 min and degassedagain for another 5 min after the addition of initiator andcatalyst. Afterwards, the slurry was cast into a mold to forma wet green body. The wet green body was demolded and

X. Li et al. / Ceramics International 40 (2014) 7245–7251 7247

dried completely. As obtained, the dry green body washeated to remove binders at 600 1C in a vacuum furnace andsintered at 2100 1C for 1 h in an Ar atmosphere. The detailedflow chart of the gelcasting and sintering processes is shown inFig. 2.

For comparison, the raw SMP with boron carbide andcarbon black powders were used for gelcasting under the sameprocess conditions with the two granulated powders.

2.2. Characterization

The raw SMP size distribution was analyzed by a laserparticle size analyzer (BT-9300H). The particle size distribu-tions of CGP and SGP were analyzed by an image analysissoftware package from scanning electron microscopy (SEM)images. Rheological properties of the slurries were determinedby a rotational viscometer (NXS-11B, China). The morphol-ogy of three powders and the microstructure of green body andsintered body were observed with a scanning electron micro-scope (LEO1450 SEM). Bulk densities of the samples weremeasured by the Archimedes method. The flexural strengthwas measured by a three-point bending test.

Fig. 3. SEM micrographs of three powders. (a) raw submicron powders, SMP; (b

3. Results and discussion

3.1. Powder characterization

The SEM micrographs of the SMP, SGP and CGP areshown in Fig. 3. It can be seen that the raw SBP were almostequiaxed grains with the particle size of submicrons. Both SGPand CGP presented remarkable enlarged particle size. SGPappeared to be of apple-like shapes with homogeneous particlesize of about 40 μm. It was apparent that some holes weredistributed on the particle surface. When cutting these parti-cles, hollow microstructures were observed, as shown inFig. 4. This is a typical micrograph of the sprayed dryingpowders due to the inward collapse of slurry droplet duringdrying [19]. For CGP, the particles presented an irregularpolygon shape with a rough surface. The particle size rangedfrom 1 μm to above 100 μm. The cross section micrograph inFig. 4 showed that CGP possessed a full solid microstructure.From Fig. 5, the average particle sizes of SMP, SGP and

CGP were 0.51 μm, 40.7 μm and 8.2 μm, respectively, whichwere consistent with the SEM micrographs. The absence ofparticles smaller than 15 μm in SGP was due to the dustcatcher follow-up drying process.

,c) spray-granulated powders, SGP; (c,d) compact-granulated powders, CGP.

Fig. 4. Polished cross-sections of the granulated powders. (a) spray-granulatedpowders, SGP; (b) compact-granulated powders, CGP.

X. Li et al. / Ceramics International 40 (2014) 7245–72517248

The bulk densities of SMP, SGP and CGP were 0.50 g/cm3,0.76 g/cm3 and 0.60 g/cm3, and the tap densities of SMP, SGP andCGP were 0.78 g/cm3, 0.96 g/cm3 and 0.89 g/cm3, as shown inFig. 6. As expected, granulation increased the bulk density and tapdensity of SiC powders. The higher bulk and tap densities of SGPare attributed to the sphere-like shape and large size particles.

Fig. 5. The particle size distribution of three powders.

3.2. The influence of granulation on the rheological propertiesof slurries

The viscosity of the slurries prepared from three powderswith constant solid loading of 50 vol% is shown in Fig. 7. Itcan be seen, for all shear rates, that the slurries prepared fromboth granulated powders showed obviously lower viscosityvalues than those from SBP. At a shear rate of 100 s�1, theviscosity of SMP slurry was 472 MPa s; meanwhile, theviscosities of SGP and CGP slurries were 193 MPa s and134 MPa s, which were only 1/2.5 and 1/3.5 to that from SBPrespectively. To our surprise, the CGP slurry displayed theviscosity value of 58 MPa s lower than that of SGP at the shearrate of 100 s�1.

In aqueous slurry of solid powder, a water molecule in thenear-surface region of solid particles had strong directionalarrangement tendency and translated to immovable structuralwater [20–25]. The volume of immovable structural water in

slurries prepared from granulated powders decreased obviouslydue to the increase of the particle size, leading to much lowerslurry viscosity due to the larger particle size.As mentioned above, the SGP particle displayed a hollow

structure with some loopholes in the shell layer. The free waterin slurry was likely to fill the internal cavity through thoseloopholes and was fixed in the hollow structures, leading to ahigher slurry viscosity value than that of CGP slurry.

3.3. The influence of granulation on the propertiesof green body

Fig. 8 shows morphologies of the fracture surface of greenbodies prepared from three powders. As shown, SMP sample

Fig. 7. Viscosities curves of SiC slurries prepared from three powders with 50vol% solid loading.

Fig. 8. Fracture surfaces of gel casting green bodies prepared from threepowders. (a) submicron powders, SMP; (b) spray-granulated powders, SGP;(c) compact-granulated powders, CGP.

Table 1Densities and bending strengths of gelcasting green bodies prepared from thethree powders.

Submicronpowders

Spray-granulatedpowders

Compact-granulatedpowders

Density (g/cm3) 1.29 1.63 1.64Bending strength(Mpa)

8.370.5 25.170.6 28.870.7

Fig. 6. The bulk density and tap density of three powders.

X. Li et al. / Ceramics International 40 (2014) 7245–7251 7249

presented a honeycombed structure with pore size rangingfrom 20 to 100 μm (Fig. 8a), while SGP and CGP samplespresented dense structures (Fig. 8b and c). The pores in theSMP samples were derived from the bubbles distributed inslurry, which were introduced by mechanical agitation andsurface adsorption of SMP. These bubbles failed to beeliminated in the vacuum degassing process due to highviscosity. On the contrary, the bubbles in the slurry preparedfrom the granulation powders escape easily because of the lowslurry viscosity. As a result, the green bodies from SGP andCGP were dense.

It is worth to note that sphere-like secondary particles(marked by numbers 1 and 2 in Fig. 8) and irregular secondaryparticles (marked by numbers 3 and 4 in Fig. 8) were observedin SGP green body and CGP green body respectively,indicating that granulated secondary particles possessedenough strength to avoid destruction during gelcasting.

The difference in the microstructure caused the variedproperties of green body. As shown in Table 1, the densityof SMP green body was 1.29 g/cm3, while the densities ofSGP and CGP green bodies were 1.63 g/cm3 and 1.64 g/cm3,which were approximately 1.3 times to that from SMP sampledue to the dense microstructure. The bending strength of greenbody prepared from SMP was 8.370.5 MPa, while thosefrom SGP and CGP were 25.170.6 MPa and 28.870.7 MPa,which were approximately 3 times and 3.5 times to that fromSMP respectively. This remarkable improvement was benefited

from not only increased density but also from the enhancementeffect of secondary particles.It was noted that the bending strength of green body from

CGP was unusually 3.7 MPa higher than that from SGP,though both samples had a similar density. This phenomenoncan be explained by the fact that the irregular shapes and roughsurfaces of CGP secondary particles caused a firmer combina-tion between particles than that of sphere-like and smoothsurface SGP particles, thus higher bending strength. This

Table 2Densities and bending strengths of gelcasting sintered bodies prepared from the

X. Li et al. / Ceramics International 40 (2014) 7245–72517250

higher bending strength is very important to the follow-upprocess, such as machine and sintering.

three powders.

Submicronpowders

Spray-granulatedpowders

Compact-granulatedpowders

Density (g/cm3) 2.34 3.10 3.11Bending strength(Mpa)

190740 420780 430770

3.4. The influence of granulation on the properties ofsintered body

Fig. 9 shows morphologies of the fracture surface of sinteredbodies prepared from three powders. Compared with thehoneycombed structure of SMP sample, SGP and CGPsamples were homogeneous and dense, which consisted ofgreen bodies. It was surprising that there were no secondaryparticles observed in sintered bodies of SGP and CGP. Thisproved that the secondary particles had been destroyed in thesintering process, and the raw submicron particles take effectin solid phase diffusion, particles rearrangement, volumeshrinkage, and so on.

The densities and bending strengths of sintered bodiesprepared from three powders are shown in Table 2. Thedensity of SMP sintered body was 2.33 g/cm3, while thedensities of SGP and CGP green bodies were 3.10 g/cm3 and

Fig. 9. Fracture surfaces of sintered bodies prepared from three powders.(a)submicron powders, SMP; (b) spray-granulated powders, SGP; (c) compact-granulated powders, CGP.

3.11 g/cm3, which were approximately 33% higher than that ofSMP due to the dense microstructure. The bending strength ofSMP sintered body was 190740 MPa, while the bendingstrengths of SGP and CGP sintered bodies were 420780 MPaand 430770 MPa, which were about 2.2 times to that of SMPrespectively. This remarkable improvement was mainly bene-fited from the increased density. The similar bending strengthof SGP and CGP sintered bodies was attributed to thedisappearance of secondary particles in the sintering process.

4. Conclusion

In the present study, compact granulation and spray gran-ulation were adopted for the first time in the gelcasting processto fabricate pressureless-sintering SiC. Granulations enlargedaverage particle sizes from 0.51 μm of raw SMP to 40 μm and8.2 μm of SGP and CGP. The granulated powders in slurrydecreased the viscosity of the slurry and increased the bendingstrength of green body. At a shear rate of 100 s�1, theviscosities of SGP and CGP slurries were 1/2.5 and 1/3.5 tothat of SMP slurry due to the increased particle size; thebending strengths of SGP and CGP green bodies wereapproximately 3 times and 3.5 times to that of SMP respec-tively. The granulated particles were observed in the greenbody but disappeared during the sintering process, leading to ahomogeneous microstructure and performances of sinteredbody; the bending strengths of SGP and CGP sintered bodieswere approximately 2.2 times to that of SMP.Moreover, the viscosity of CGP slurry was 58 MPa lower

than that of SGP due to the solid particle structure, and thebending strength of green body was 3.7 MPa higher than thatof SGP due to its irregular shapes and rough surface. Weconcluded that compact granulation was more suitable for thegelcasting process to acquire low viscosity slurry and highstrength green body.

Acknowledgements

This work was supported by the specialized research fundfor the Doctoral Program of Higher Education of China No.20110006110025.

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