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Materials Science and Engineering A 528 (2011) 1421–1424

Contents lists available at ScienceDirect

Materials Science and Engineering A

journa l homepage: www.e lsev ier .com/ locate /msea

ffect of solid content on pore structure and mechanical properties of porousilicon nitride ceramics produced by freeze casting

eng Ye ∗, Jingyi Zhang, Limeng Liu, Haijiao Zhanchool of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

r t i c l e i n f o

rticle history:eceived 13 June 2010eceived in revised form 12 October 2010

a b s t r a c t

Porous Si3N4 ceramics were prepared by freeze casting using liquid N2 as refrigerant. The pore structure,porosity, � → �-Si3N4 transformation and mechanical properties of the obtained materials were stronglyaffected by the solid contents of the slurries. Increasing the solid content would reduce the porosity,

ccepted 21 October 2010

eywords:orous Si3N4

reeze casting

decrease the pore size and change the pore structure from the aligned channels with dendrites to theround pores with decreased pore size. The formation of this round pores impeded the � → �-Si3N4 phasetransformation, but was beneficial to the mechanical properties of the obtained porous Si3N4 due to itsunique pore structure.

© 2010 Elsevier B.V. All rights reserved.

ore structureechanical properties

. Introduction

Porous ceramics with open-pore structure have been widelysed as molten metal filters, diesel engine exhaust filters, industrialot-gas filters and catalyst supports [1–3]. The open-pore ceramicsan also be used as performs for metal-impregnated ceramic–metalomposites [4]. Porous Si3N4 ceramics with self-reinforced elon-ated �-Si3N4 grains are desirable for wide applications due toheir excellent mechanical and thermal properties. Among porouseramics with different pore structures, the porous ceramicsith interconnected pore channels are attracting much attention

ecause they are expected to have higher fluid permeability [5].ecently, the freeze-casting process has been developed to pro-uce highly porous ceramics, as it can produce interconnected porehannels in a controllable manner, which offers superior mechan-cal properties and functions [6].

For freeze casting technique, slurries are first frozen to obtainehicle crystals, usually ice, followed by freeze drying to sublimatehe solidified phase from the solid to the gas state under a reducedressure, in which the pore structure is a replica of the solventrystals [7]. The final microstructures and properties of the porous

eramics produced by freeze casting depend on the slurry concen-ration, freezing rate and sintering conditions [8–11]. However, tillow, the reports on the microstructural control and mechanicalroperties of porous silicon nitride ceramics fabricated by freeze

∗ Corresponding author. Tel.: +86 45186413921; fax: +86 45186413922.E-mail address: yf306@hit.edu.cn (F. Ye).

921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2010.10.066

casting are very scarce. The purpose of this study is to clarify theinfluence of solid content on the microstructure and mechanicalproperties of porous Si3N4 ceramics produced by freeze casting.

2. Experimental procedures

Slurries were prepared by mixing distilled water with a smallamount (0.3 wt%) of ammonium polymethacrylate anionic disper-sant (Kermel Chemical Reagent Co., Ltd., Tianjin, China), and 6 wt%Y2O3 (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China),2 wt% Al2O3 (Sinopharm Chemical Reagent Co., Ltd., Shanghai,China) and 92 wt% Si3N4 (Junyu Ceramic Co., Ltd., Shanghai, China).The Si3N4 powder consisted of 95 wt% �-Si3N4 and 5 wt% �-Si3N4and had a mean particle size of 0.5–1.0 �m. The suspensions withdifferent solid contents from 30 to 50 vol% were prepared to inves-tigate the effect of solid content on the microstructure of theobtained porous ceramics. Slurries were ball-milled for 20 h withalumina balls and de-aired by stirring in a vacuum desiccator.The resultant slurries were then poured into polyethylene molds(Ø70 mm × 15 mm) using a brass bottom plate immersed in liquidN2. The top of the container was open so that the upper surface ofthe slurry would expose to the atmosphere at room temperature.Immediately after casting, the ice crystals grow unidirectionallyfrom the bottom to the upper surface of the slurry. After the frozen

samples were completely freeze-dried under vacuum for 2 days,the green compacts were placed in a graphite crucible with a sili-con nitride-based powder bed and sintered in a graphite resistancefurnace at 1800 ◦C for 1.5 h under a 0.1 MPa nitrogen atmosphere.Both the heating and cooling rates were 5 ◦C/min.

1422 F. Ye et al. / Materials Science and Engineering A 528 (2011) 1421–1424

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ig. 1. Backscattered electron mode SEM micrographs of the porous Si3N4 ceramicn epoxy resin. (a) 30 vol%; (b) 40 vol%; (c) 50 vol%.

The densities of the samples were calculated from the dimen-ions and weight of the samples. Pore size distribution waseasured by mercury porosimetry (Model Autopore III 9420,icrometics Co., USA). Crystalline phases of the sintered porous

i3N4 ceramics were characterized by X-ray diffraction (XRD).

uantitative analysis of �- and �-Si3N4 phase content wasetermined by comparing the peak intensity ratios [12]. Theicrostructure of the cross section was observed by scanning elec-

ron microscopy (SEM; Model JSM-5600, JEOL Ltd., Tokyo, Japan).n order to evaluate the pore structure, the obtained porous Si3N4

ig. 2. Microstructures of the pores in the resultant porous Si3N4 ceramics with differenttructure and a great number of fibrous Si3N4 grains protruding from the internal walls o

different solid contents parallel to the ice growth direction after infiltrating with

ceramics were also infiltrated with an epoxy resin and analyzed bySEM using backscattered electron mode (BSE).

Flexural strength and fracture toughness were measured inair at room temperature. All flexural bars were machined withthe tensile surface perpendicular to the freezing direction. Flex-

ural strength measurements were performed on bar specimens(3 mm × 4 mm × 36 mm) using a three-point bend fixture witha span of 30 mm. Fracture toughness measurements were con-ducted at room temperature, performed on single-edge-notchbeam specimens (2 mm × 4 mm × 30 mm) with a span of 16 mm,

solid contents, indicating that the increased solid content strongly affect the poref pores. (a), (b) 30 vol%; (c), (d) 40 vol%; (e), (f) 50 vol%.

Engineering A 528 (2011) 1421–1424 1423

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F. Ye et al. / Materials Science and

nd a half-thickness notch was made using a 0.1 mm thick dia-ond wafering blade. At least, six specimens were tested for each

est condition.

. Result and discussion

The porosities of the obtained porous Si3N4 ceramics with differ-nt solid contents are listed in Table 1. It reveals that the increasedolid content could obviously decrease the porosity of the resultantorous material. The porosities measured by mercury porosimetryre almost the same as those calculated from the dimensions andeights of the samples, indicating that most of the pores in the

esultant porous composites are open ones.Fig. 1 shows the pore structures of the obtained porous Si3N4

eramics with different solid contents from 30 vol% to 50 vol%. Withhe increase of solid content, the spindle pores interconnected withendrites (Fig. 1a and b) were changed into round ones (Fig. 1c), andhe pore size decreased. For freeze casting, the pore structure is aeplica of the morphologies of the ice crystals. During the solidi-cation of the suspension, the particles may be either engulfed orjected by the advancing ice fronts. During the interaction betweenarticles and an advancing ice front, the particles are pushed byplanar ice front [13]. It is proposed that particles are concen-

rated by this pushing until particles are concentrated enough toesist further concentration [14]. The force created by the particleoncentration is the osmotic force from the osmotic pressure ofhe suspension, �. The osmotic pressure of the suspension can be

odeled with a modified Carnahan–Starling equation [15,16]:

(�) = kT

Vp

(�(

1 + � + �2 − �3)

(�m − �)3

)(1)

here �m is the volume fraction particles at maximum packing,is the solid content of the suspension, k is Boltzmann’s constant

nd Vp is the volume of a single particle.It should be noted that �(�) is to increase slowly with �, and

(�) becomes infinite at � = �m. When this force exceeds theapillary pressure (Pcap) required to pushing the particles with theolid/liquid interface, further concentration is impossible [14]. Theapillary pressure (Pcap) can be estimated as follows [17]:

cap = 3��

(1 − �)R(2)

here � is the solid–liquid interfacial energy for the suspensionedium and R is the surface-area equivalent spherical radius for

he particles. So for a given suspension, a critical solid content �chould exist. When the solid content exceeds �c, the particles can-ot be ejected by the advancing ice fronts.

As shown in Fig. 1, unidirectional aligned channels were formedlmost uniformly over the samples with 30 vol% and 40 vol% solidontents (Fig. 1a and b). Just because the pore structure is a replica

f the morphology of the ice crystal, it could be concluded that thearticles were ejected by the advancing ice front, indicating thatheir solid contents were below the critical solid content �c. Whenhe solid content reached 50 vol%, the aligned channels disappearednd were replaced by the round pores (Fig. 1c). This revealed that

able 1roperties of the porous Si3N4 with different solid contents.

Solid content ˇ/(˛ + ˇ), % Porositya, % Porosityb

30 vol% 100 66.3 64.140 vol% 100 53.2 49.450 vol% 74.4 42.7 40.2

a Calculated from the sample dimension and weight.b Measured by mercury porosimetry.

Fig. 3. Pore size distribution of the obtained porous Si3N4 ceramics with differentsolid content of the slurries. (a) 30 vol%; (b) 40 vol%; (c) 50 vol%.

the solid content of 50 vol% exceeded the critical solid content (�c)and hence resulting that the particles were engulfed by the ice front.

The microstructures of the obtained porous Si3N4 ceramics areshown in Fig. 2. It further revealed the effect of the solid content ofthe suspension on the pore structure. Increasing the solid contentchanged the pore structure from the aligned channels with den-drites (Fig. 2a–d) to the round pores (Fig. 2e and f). A great numberof fibrous Si3N4 grains protruding from the internal walls of pores.The increased solid content also obviously decreased the pore sizeand promoted the densification of the internal walls of pores.

The effect of the solid content on the pore size distribution deter-mined by mercury porosimetry is shown in Fig. 3. With increasingthe solid content increased from 30 vol% to 50 vol%, the pore sizedecreased and the pore size distribution changed from multi peaksto single peak. The sample 30 vol% solid content had four peaksin the pore size distribution, as shown in Fig. 3a. The large poresize peaks of a1 and a2 plausibly correspond to the main chan-nel open pores and the dendritic ones, respectively. The peak a3should be from the pores produced between the fibrous grains,and the smallest pore size peak of a4 is very likely to correspondto the pores in the undense internal walls. The average pore sizedecreased with increasing the solid content increased from 30 vol%to 40 vol%, and the smallest pore size peak disappeared due to thealmost complete densification of the internal walls of pores, asshown in Fig. 3b. The pore size peaks of b1, b2 and b3 correspond tothe main channel pores, dendritic pores and the pores between thefibrous grains. Further increasing the solid content to 50 vol%, onlysingle peak was detected in the pore size distribution (Fig. 3c). It isattributed to the disappearance of the aligned channel pores andinstead, the round pores appeared. The widening pore size peakwas undoubtedly from the combination of two kinds of pores fromthe macroscopically round open pores and the ones between thefibrous grains.

The results of XRD analysis revealed that only �-Si3N4 and �-

Si3N4 were detected in the resultant porous materials. The contentsof �-Si3N4 phase were about 100%, 100%, and 74.4% for sampleswith solid content of 30 vol%, 40 vol% and 50 vol%, respectively,indicating that the increased solid content inhibited the � → �-

, % Flexural strength, MPa Fracture toughness, MPam1/2

57 ± 3 1.1 ± 0.182 ± 4 2.3 ± 0.2

189 ± 5 3.5 ± 0.2

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[[14] N.O. Shanti, K. Araki, J.W. Halloran, J. Am. Ceram. Soc. 89 (2006) 2444–2447.

424 F. Ye et al. / Materials Science and

i3N4 transformation during sintering. It is generally acceptedhat the � → �-Si3N4 phase transformation and the growth of-Si3N4 grains are via a liquid phase through the solution-iffusion–reprecipitation mechanism. The additives reacted withiO2 on the surface of the Si3N4 grains to produce a liquid glassyhase composed of Y2O3, Al2O3 and SiO2. The growth rate of �-i3N4 grains is a function of the rate of dissolution of �-Si3N4articles. On the other hand, the obtained materials are highlyorous and most of pores are open, so vapor phase transport arelso contributed to anisotropic growth of �-Si3N4 grains [18]. Theiquid phase Y2O3–Al2O3–SiO2 decomposed at elevated tempera-ures and produced SiO vapor. This SiO vapor would continuouslyondense on the surface of existed �-Si3N4 grains in the inter-al wall of the aligned pores and hence resulting in the growthf the rod-like �-Si3N4 grains. For the materials studied here, dueo the high viscosity of the liquid glassy phase, it is difficult toompletely realize � → �-Si3N4 transformation through solution-iffusion–reprecipitation mechanism. For the samples with solidontent of 30 vol% and 40 vol%, both of them have higher porosityhan that of the sample with 50 vol% solid content (Table 1), andossess highly interconnected channels and dendritic pores, whichould favor the anisotropic �-Si3N4 grain growth through vaporhase transport. In contrast, the pore structure of the sample with0 vol% solid content obviously changed from the aligned channelsith dendrites to round pores, and therefore resulting in the hin-rance of �-Si3N4 grain growth due to the reduction in vapor phaseransport.

The flexural strength and fracture toughness of the porous Si3N4re shown in Table 1. The results indicated that the mechanicalroperties of obtained porous ceramics were strongly affected byhe solid content due to the different porosity and pore structure.ncreasing porosity and pore size are detrimental to the flexuraltrength and fracture toughness, and the individual round poretructure would be beneficial to the mechanical properties. All flex-ral bars were machined with the tensile surface perpendicular tohe freezing direction, so the aligned channel pores formed in theamples with a lower solid content may act the origin of the frac-uring and result in decrease in mechanical properties. As shownn Table 1, the sample with 30 vol% solid content shows the lowestexural strength and fracture toughness because it has the highest

orosity and the biggest pore size in the microstructure. In con-rast, for the sample with 50 vol% solid content, the pore structurehanged from the aligned channels with dendrites pores to roundores with decreased pore size, and therefore could increase theesistance of crack propagation and hence the mechanical prop-

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eering A 528 (2011) 1421–1424

erties. The flexural strength and fracture toughness could reach189 MPa and 3.5 MPam1/2, respectively.

4. Conclusions

Highly porous Si3N4 ceramics were prepared by freeze cast-ing process using liquid N2 as refrigerant. The solid content ofthe suspensions have strong influence on pore structure, porosity,� → �-Si3N4 phase transformation and mechanical properties ofthe resultant porous ceramics. Increasing the solid content of theslurries from 30 vol% to 50 vol% would decrease the porosity andpore size, and change the pore structure from the aligned chan-nels with dendrites into the round pores with decreasing pore size.The increased solid content inhibited � → �-Si3N4 phase transfor-mation, but was beneficial to the mechanical properties due to itsunique pore structure. The flexural strength and fracture toughnessof the sample with 50 vol% solid content could reach 189 MPa and3.5 MPam1/2, respectively.

Acknowledgment

This work was supported by National Natural Science Founda-tion of China (Grant No. 90716022).

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