Colloidal processing of silicon carbide

Materials Letters North-Holland

MAmmALs LmWms

16 ( 1993) 295-299

Colloidal processing of silicon carbide Yoshihiro Hirata, Sadahiro Yamada and Yasuo Fukushige Department ofApplied Chemistry and Chemical Engineering, Faculty qfEngineering, I-21-40 Korimoto, Kagoshima 890, Japan Received

13 February

Kagoshima University,

1993

Rheological properties of silicon carbide aqueous suspensions and sintering behavior of consolidated powder compacts were studied on two kinds of submicrometer-sized powders (powder A: median size 0.1 urn, 0.38 wt% SiOz, powder B: median size 0.7 urn, 1.75 wt% SiO*). Highly concentrated fluid suspensions ( > 50 ~01%) and high-density green compacts ( > 65% TD (theoretical density) ) were prepared from electrostatically stabilized powder B in the solutions at pH = 10. Addition of tine y-alumina particles ( z 20 nm) as a sintering additive (O-5 wtW) lowered both the green densities of powders A and B to about 50% TD. The density of Sic with alumina hot-pressed at 2 100-2 150°C was higher for powder B than for powder A.

1. Introduction

particle sizes to making high-density by hot-pressing [ 14- 17 1.

Controlling the density and microstructure of a green compact is very important in designing the resultant sintered microstructure of a dense ceramic material. In the synthesis of high-performance ceramics, colloidal processing (dispersion, consolidation and sintering of colloidal particles) is recommended owing to better agglomerate control than dry pressing or other forming methods [l-3]. Consolidation of ceramic particles in a well-dispersed suspension gives a high-density green compact with a uniform structure [ 4-81. Dispersibility and stability of colloidal particles are usually controlled by electrostatic repulsive forces among the charged particles or by electrosteric interaction of polymer adsorbed on the charged particles [4,9-l 31. The rheological properties of the colloidal suspensions are influenced by the interaction between electrostatically or electrosterically stabilized particles and affect the densities and microstructures of the consolidated powder compacts, which in turn control the properties of sintered ceramics. The objectives of this study are to understand the rheological behavior of aqueous suspensions and the properties of consolidated powder compacts of two kinds of silicon carbide powders with different oxygen contents and 0167-577x/93/$

06.00 0 1993 Elsevier

Science Publishers

silicon carbide

2. Experimental procedure The following two kinds of submicrometer-sized SIC powders were used in this study: (A) B-Sic powder with a median size of 0.1 urn and a specific surface are of 20 m2/g, chemical composition: Sic > 98 wt%, Si02 0.38 wt%, C 0.60 wt%, Al 0.0 1 wt%, Fe 0.03 wt% (Ibigawa Electric Industry Co., Ltd., Gifu, Japan) and (B) a-SiC powder with a median size of 0.7 urn and a specific surface area of 20 m2/g, chemical composition: SIC 97.5%, Si02 1.75%, C 0.65%, Al 0.025%, Fe 0.027% (Yakushima Electric Industry Co., Ltd., Kagoshima, Japan). Fig. 1 shows transmission electron micrographs of these powders (H700H type, Hitachi Co., Tokyo, Japan). The surface of particles was coated by thin layer (may be Si02 film) with thickness about 20 and 30 nm for powders A and B, respectively [ 18,19 1. The SIC powders were mixed with y-alumina powder as a sintering additive with a specific surface area of 80 m2/g (diameter of equivalent spherical particle: 21 nm, chemical composition: A1203 > 99.9 wt%, Na 40 ppm, Fe < 10 ppm, Si < 10 ppm, Asahi Chemical Industry Co., Ltd., Tokyo, Japan). Electrophoretic

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Fig. 1. Transmission electron micrographs of submicrometer-sized (powder B).

mobility of the colloidal particles in an aqueous suspension was measured at room temperature after stirring by a magnetic stirrer for 30 min (zeta-meter, Zeta-Meter Inc., USA). Dilute HCl and NH,OH solutions were used for pH adjustment. After the Sic/ A1203 mixed powders were dispersed in aqueous solutions at pH= 10 for 24 h, air bubbles in the colloidal suspensions were eliminated in a bell jar connected to a vacuum pump. The shear rate versus shear stress relation of the aqueous suspensions was measured by a cone and plate viscometer at room temperature (Visconic EMD type, Tokyo Keiki Co., Tokyo, Japan). The colloidal solids in the aqueous suspensions were consolidated by filtration through gypsum molds. Green compacts were hot-pressed in a graphite die at a pressure of 39 MPa under a reduced atmosphere of 93.3 Pa (FVH-5 type, Fuji Denpa Kogyo Co., Osaka, Japan). The hot-pressing temperature was increased at a rate of 1O”C/min and then kept at 2100-2150°C for 1 h. The densities of the green compacts and hot-pressed samples were measured by the Archimedes method using kerosene and distilled water, respectively. After the polished surface of sintered Sic was etched with the mixture of NaCl/NaOH = 8 5 / 15 ( molar ratio ) at 500 ’ C for 296

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SiC powders with median sizes of 0.1 pm (powder A) and 0.7 pm

20 min in air, the microstructures were observed by use of optical microscopy.

3. Results and discussion 3. I. Properties of silicon carbide suspensions Fig. 2 shows the mobility of Sic particles (powders A and B) and y-alumina particles in the aqueous suspensions as a function of pH. The isoelectric points of powders A and B were at pH = 3. i and 4.4, respectively. The pH dependence of mobility for powder B resembles that for SiOz powder [20], suggesting that the surface of Sic was coated by a SiOz film (fig. 1) , From fig. 2, a high dispersion state of SIC powders was expected at pH = 8 to 11 due to the strong repulsive force among negatively charged particles. On the other hand, the dispersion of y-A&OS particles with an isoelectric point at pH = 9.0 was also expected at high pH above IO. That is, the SiC powders and y-AlzO, powder were mixed in the aqueous solutions at pH= 10. Fig. 3 shows the shear rate (if versus shear stress (S) relation for the aqueous suspensions of Sic powders A and B at pH = 10. The rheological behavior of

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PH Fig. 2. Electrophoretic mobility of Sic (powders y-A1203 powders in aqueous solutions.

A and B) and

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Fig. 3. Shear rate versus shear stress for Sic suspensions.

the suspensions of powder A was close to a Newtonian flow at low solid contents ( < 15 ~01%) and to a Bingham flow at high solid contents ( z=-2 1 vol%). As seen in fig. 3, the suspensions of powder B showed a Newtonian flow at higher solid contents (~48 ~01%) compared with those of powder A. The above results would reflect the difference of particle sizes because the mobilities of both powders were comparable at pH = 10 (fig. 2 ). The rheological behavior of both suspensions was evaluated by approximating the shear rate versus shear stress relation to a Bingham flow: The slopes and intercepts at j=O of the j-S plots correspond to viscosity (q) and yield stress (S,) of the suspensions, respectively. Fig. 4 shows q and S, for the suspensions of powders A and B as a function of solid content. The S, of powder A increased drastically at solid contents above 21 vol% and it was impossible to make the fluid sus-

pensions with high solid contents above 30 ~01%. On the other hand, powder B gave the fluid suspensions at about 50 ~01% solids. The average distance between particles in a suspension decreases with increasing particle size at a given solid content. This decrease of particle distance increases the interaction energy between particles (interaction energy: summation of repulsive energy and van der Waals energy) [ 4,9]. Therefore, the high S, values at relatively low solid contents for the suspensions of powder A are related to the above strong interaction of particles. Fig. 5 shows the effects of y-alumina addition on the rheological properties of Sic suspensions. The viscosity and yield stress of SIC suspensions, especially those of suspensions of powder B, were increased by the addition of fine y-alumina particles. These results may be explained by the increase of interaction energy with decreasing particle distance or 297

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formation of particle network with heterocoagulation between highly charged negative Sic particles and lowly charged alumina particles. 3.2. Consolidation and hot-pressing of Sic powders Fig. 6 shows the densities of powder compacts consolidated by filtration through gypsum molds as a function of solid content. The densities of green compacts were almost independent of the solid contents of the suspensions. The green densities were higher for powder B of larger particles than for powder A of smaller particles. Addition of y-alumina in the range 1 to 5 wt0/6 decreased both the green densities of powders A and B to about 50% TD. Fig. 7 shows the relative density of Sic hot-pressed at 2100-2150°C for 1 h. The density increase after hot-pressing of powders A and B without A120J was 12.7 and 5.7% DT, respectively. This result suggests that high-purity fine SIC powder essentially can be densitied without sintering additives. In fact, Kijima et al. [ 2 I] reported that a high-purity ultrafine Sic powder of 5 nm was densified to relative density of 85% by hot-pressing at 2300°C. On the other hand, sinterability of SIC with Al2O3 was higher for powder B than for powder A. Powder B with 3-5 wt% of A1203 could be densified to 93.6-95.8% TD and the density of powder A with 5 wt% A1203 was 90.0% TD. These results indicate high acceleration effects of the liquid phases in the Si02-A1203 system on the sintering of SIC [ 15,171. In the system Si02-A1,03 [22], liquids are formed at 1587°C by the eutectic

60 ,

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Fig. 7. Relative densities of Sic hot-pressed at 2100-2 150°C for 1 h as a function of the amount of Y-A1203.

reaction between SiO* and mullite ( 3A1203.2Si02), at 1828°C by the peritectic reaction between mullite and A1203 plus liquid, and at 2054°C of the melting point of alumina. Although SiOZ melts at 1726°C the effects of the liquid phase of SiO, on the densification of SIC would be small as seen in the density of SIC without A1203. Therefore, the difference of sinterability in powder B with a higher SiOZ content and powder A with a lower SiOZ content would be explained by the amount of liquid phases in the Si02-A&O3 system. Fig. 8 shows the polished and etched microstructures of Sic of powders A and B with 5 wt% A1203, hot-pressed at 2150°C for 1 h at 39 MPa. Both samples consisted of plate-like grains of 60 to 390 urn [ 15 1. The sizes of plate-like grains in the sintered SIC were smaller for the SiO,-rich larger Sic powder (powder B ).

I 4. Conclusions

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The rheological properties of aqueous suspensions of submicrometer-sized Sic powders were influenced by particle size rather than SiOZ content. Highly concentrated fluid suspensions were prepared from larger SIC particles. However, the green densities of consolidated powder compacts were almost independent of the solid contents of the suspensions. Addition of A1203 to SIC suspensions decreased the green densities. The sinterability of SIC with AlzOs was higher for the SiO,-rich SIC powder

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Fig. 8. Polished and etched microstructures of Sic hot-pressed with 5 wt% AlzOXat 2 150°C for I h

than for the SiO,-poor SIC powder. The sizes of platelike grains in the sintered Sic were smaller for the SiO,-rich larger Sic powder.

Acknowledgement

The Sic powder (powder B) was supplied by Yakushima Electric Industry Co., Ltd., Japan. This study was financially supported by a Grant for the Scientific Researches for Southwest Pacific Area, Kagoshima University, 1992.

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