J O U R N A L O F M A T E R I A L S S C I E N C E L E T T E R S 2 0, 2 0 0 1, 2217 – 2220

Microstructure control of liquid-phase sintered β-SiC by seeding

YOUNG-WOOK KIM∗Department of Materials Science and Engineering, The University of Seoul, Seoul 130-743, South KoreaE-mail: ywkim@uoscc.uos.ac.kr

MAMORU MITOMONational Institute for Research in Inorganic Materials, Ibaraki 305-0044, Japan

GUO-DONG ZHANDepartment of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA

The control of microstructure is one of primary con-cerns in liquid-phase sintered silicon carbide because“in situ-toughened or self-reinforced SiC ceramics”with a microstructure containing anisotropically grown,plate-shaped SiC grains are known to exhibit highfracture toughness [1–5]. A fracture toughness of∼8 MPa · m1/2 has been reported in toughened SiC ce-ramics with yttrium aluminum garnet (Y3Al5O12, YAG)as a grain boundary phase [1] and a higher fracturetoughness of ∼9 MPa · m1/2 in toughened SiC ceram-ics with Al-B-C [6].

Attempts to introduce the plate-shaped SiC grainsinto the microstructure can be summarized as the fol-lowing two strategies: (i) taking advantage of the β → α

phase transformation at high temperatures, which us-ually accelerates the grain growth [1–6], and (ii) use ofsolution-reprecipitation process [7–10]. The latter ef-forts include the seeding for favoring preferential graingrowth without phase transformation [7, 8] and the useof fine α-SiC starting powders for triggering abnormalgrain growth [9, 10]. These plate-shaped grains can actas a reinforcing phase that promotes crack bridging anddeflection, resulting in the improved toughness [1, 11].

The objective of this study is to investigate the ef-fect of seed shape on the microstructural develop-ment in liquid-phase sintered SiC. In order to sim-plify the analysis, the β → α phase transformationwas avoided completely by using an oxynitride glass(Y0.124Mg0.160Si0.414Al0.302O1.4N0.151) as a sinteringadditive, which enlarges the stability region of β-SiCup to 2000 ◦C [12].

Ultrafine β-SiC powder (Sumitomo-Osaka CementCo., Tokyo, Japan) was oxidized at 600 ◦C for 2 h un-der air to eliminate free carbon, and hydrofluoric acid-treated to remove SiO2. The particle size was ∼90 nmas calculated from the specific surface area. A mixtureof SiO2 (Reagent Grade, Kanto Chemical Co., Inc.,Tokyo, Japan), MgO (High-Purity Grade, Wako PureChemical Industries, Ltd., Osaka, Japan), Y2O3 (99.9%pure, Shin-Etsu Chemical Co., Tokyo, Japan), Al2O3(99.9% pure, Sumitomo Chemical Co., Tokyo, Japan),and AIN (Grade F, Tokuyama Soda Co., Tokyo, Japan)powders was prepared to an oxynitride composition

∗Author to whom all correspondence should be addressed.

Y0.124Mg0.160Si0.414Al0.302O1.400N0.151 by ball millingin hexane for 3 h using SiC media and SiC container.The oxynitride composition had an appreciable SiC so-lubility at high temperatures and a good potential forcrystallization control [13, 14].

Three batches of powder were prepared, each con-taining 90 wt% SiC and 10 wt% the oxynitride com-position (see Table I). To prepare a powder composi-tion without seeds, 90 wt% ultrafine β-SiC powder and10 wt% powder mixture of the oxynitride compositionwere ball milled in ethanol for 3 h using SiC grind-ing balls. To prepare powder compositions containingseeds, 80 wt% ultrafine β-SiC powder and 10 wt% pow-der mixture of the oxynitride composition were ballmilled in ethanol for 2.5 h using SiC balls and a jar.Then, 10 wt% large β-SiC powder (0.44 µm, B-1 grade,Showa Denko, Tokyo, Japan) or β-SiC whisker (0.1–1.0 µm in diameter and 10–30 µm in length, Tokamax,Takai Carbon Co., Ltd., Tokyo, Japan) (seeds) wasadded, and the mixture was milled for 0.5 h. Aftermilling, the slurry was dried and hot-pressed. SiC ce-ramics without seeds (designated as SC1) and SiC

TABLE I Batch composition, relative density, and fracture toughnessof hot-pressed materials

FractureBatch Relative toughness

Material composition (wt%) density (%) (MPa · m1/2)

SC1 90% fine β-SiCa 99.8 2.6 ± 0.2+ 10% oxynitride glassb

SC2 80% fine β-SiC 99.8 3.7 ± 0.2+ 10% large β-SiCc (seeds)+ 10% oxynitride glass

SC3 80% fine β-SiC 99.7 6.7 ± 0.2+ 10% β-SiC whiskerd (seeds)+ 10% oxynitride glass

a The average particle size is ∼90 nm.b The composition of oxynitride glass is Y0.124Mg0.160Si0.414Al0.302-O1.400N0.151.c The average particle size is ∼0.44 µm (B-1 grade, Showa Denko,Tokyo, Japan).d The diameter and length are 0.1–1.0 µm and 10–30 µm, respectively(Tokamax, Takai Carbon Co., Ltd., Tokyo, Japan).

0261–8028 C© 2002 Kluwer Academic Publishers 2217

Figure 1 Microstructures of hot-pressed materials: (a) SC1, (b) SC2, and (3) SC3 (refer to Table I).

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ceramics seeded with large β-SiC powder (designatedas SC2) were hot-pressed at 1850 ◦C for 1 h undera pressure of 20 MPa in a nitrogen atmosphere. SiCceramics seeded with β-SiC whisker (SC3) were hot-pressed at 1900 ◦C for 1 h under a pressure of 20 MPain a nitrogen atmosphere.

Sintered density was determined by the Archimedesmethod. The theoretical density of the materials,3.207 g/cm3, was calculated according to the rule ofmixtures (theoretical density of the oxynitride glass was3.18 g/cm3) [14]. The hot-pressed and annealed mate-rials were cut and polished, then etched by a plasmaof CF4 containing 7.8% O2. The microstructures wereobserved by scanning electron microscopy (SEM).X-ray diffraction (XRD) using Cu Kα radiation wasperformed on ground powders. The fracture toughnesswas estimated by measuring crack lengths generated bya Vickers indenter [15].

Relative densities of >99.5% were achieved by hot-pressing for all compositions. Typical microstructuresof the hot-pressed materials are shown in Fig. 1. Themicrostructure of SC1 material consists of very fineequiaxed grains (average size ∼120 nm). In contrast,the growth of large grains in small matrix grains, i.e., bi-modal microstructure, is shown in materials with seeds(SC2 and SC3). The microstructure of the SC2 mate-rial consisted of large, equiaxed grains and fine matrixgrains while that of SC3 consisted of large, rod-likegrains and fine matrix grains. Relatively large grains inFig. 1b and c show the core/rim structure, indicatingthat grain growth through solution-reprecipitation oc-curred [16]. Large grain size difference between seedsand matrix grains was a driving force for abnormal graingrowth of some grains [17].

X-ray diffraction spectra for ground powders of thematerials with seeds are shown in Fig. 2. As shown, themajor phase is β-SiC for all of the materials, indicatingno phase transformation during sintering. It is consis-tent with the previous results that the oxynitride glassenlarges the stability region of β-SiC [12, 18].

As shown in Fig. 1, the morphology of large grainsdepends on the shape of seeds; i.e., equiaxed parti-cles (seeds) lead to equiaxed grains and rod-likewhiskers (seeds) lead to rod-like grains after sintering.Previous results [8, 19, 20] showed that the addition

Figure 2 X-ray diffraction spectra in (a) SC2 and (b) SC3.

of large, equiaxed α-SiC seeds into fine β-SiC matrixleads to elongation of some grains due to both the pres-ence of α/β interfaces in a grain and the occurrence ofβ → α phase transformation. Present results suggestthat grain morphology in liquid-phase sintered SiC canbe controlled by manipulating the seed shape, if thereis no phase transformation during sintering. Therefore,It can be generally established, based on microstruc-tural observation and previous results [8, 17–20], thatthe requirements for microstructure control by seedingare as follows: (1) appreciable solubility of the solidfor solution-reprecipitation mechanism; (2) apprecia-ble size difference between seeds and matrix particles;and (3) no phase transformation during sintering andannealing.

The microstructure of SC3 is a new kind of “self-reinforced microstructure,” consisting of large, rod-like β-SiC grains and fine β-SiC matrix grains. Pre-viously reported self-reinforced microstructures werecomposed of plate-shaped α-SiC grains or plate-shapedα/β composite grains [1–8]. But the present resultsshowed that self-reinforced β-SiC with rodlike β-SiCgrains could be obtained by seeding SiC whiskers andsintering in β-SiC stable region.

Preliminary results of the toughness measurement forthe materials are listed in Table I. It shows that seed-ing increases the toughness and rodlike morphology isbetter than equiaxed one for toughening.

It can be summarized that the material with largeβ-SiC particles as seeds had a bimodal microstructurewith large equiaxed grains dispersed in small grain ma-trix. In contrast, the material with β-SiC whiskers asseeds had a bimodal microstructure of small matrixgrains and large elongated (rod-like) grains. Presentresults suggest that grain morphology in liquid-phasesintered β-SiC can be controlled by manipulating theseed shape, if there is no phase transformation duringsintering.

AcknowledgmentThis work was supported partially by Korea Ministryof Science & Technology under Grant No. 1-3-069.

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Received 18 Apriland accepted 18 October 2001

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