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Pressureless sintering of silicon-nitride composites
Composites Science and Technology 51 (1994) 265-269
I PRESSURELESS SINTERING OF SILICON-NITRIDE COMPOSITES Takao Yonezawa JMC New Materials Inc., 8-4 Koami-cho, Nihonbashi, Chuo-ku, Tokyo, 103, Japan
Shin-ichi Saitoh, Masatoshi Minamizawa & Toshitsugu Matsuda Ceramics Development Center, Japan Metals & Chemicals Co. Ltd, 1-4-63, Ohama, Sakata-shi, Yamagata-ken, 998, Japan (Received 31 July 1992; accepted 12 February 1993)
particular are attracting a great deal of attention. 1 There are very few reports, however, which describe the mechanical properties of pressureless sintered WRC 2-5 and the reported data suggest that the materials are adversely affected by the network of whiskers in the green body. Because the whiskers do not shrink during pressureless sintering, whisker networks inhibit the movement of the matrix material, so that porosity persists in sintered composite and significantly impairs its strength (Fig. l(a)). Hot pressing is used conventionally in the production of high-density W R C 6'7 and during this process the pressure destroys the whisker networks and re-orientates whiskers two-dimensionally (Fig. l(b)). Although WRC can be made fully dense by hot pressing, the process is limited to simple shapes and the fabrication cost is high. Densification of WRC by a pressureless sintering route is therefore needed if the practical applications for these materials are to be extended. If a green compact is made in which the whiskers are orientated one- or two-dimensionally, shrinkage is considerably hindered in the orientated direction but not in the direction perpendicular to the orientated direction (Fig. l(c)). Various compaction techniques were examined and compared with this in mind and it is believed that the slip casting method is the best one for orientating whiskers one- and two-dimensionally and facilitating the production of shaped parts. By this route a dense WRC has been produced and used in a number of practical applications. 8-1~
Abstract A fabrication method for whisker-reinforced siliconnitride ceramics involving pressureless sintering is reported together with the mechanical properties of the resulting composite materials. Silicon-nitride composites containing up to 20wt% of silicon carbide whiskers were slip cast to orientate the whiskers twodimensionally, parallel to the plaster mould surfaces, and then pressureless sintered at 1750 or 1825°C. The sintered body containing 20wt% of whiskers of average diameter 1.3 Izm displayed a relative density of 98%, a mean flexural strength of 950MPa and a fracture toughness of 7.0MPaVm. Furthermore, the thermal shock resistance was found to be as high as 1400°C. To date, components successfully produced from this composite material include aluminum diecasting ladles and crucibles.
Keywords: pressureless sintering, slip casting, silicon nitride, silicon carbide whisker, composite 1 INTRODUCTION
Silicon-nitride ceramics display an excellent combination of material properties, e.g. high strength, high toughness, thermal shock resistance, and oxidation resistance. Applications of silicon-nitride ceramics now being developed include automobile engine parts and parallel material developments have resulted in a progressive improvement in the properties of the silicon-nitride ceramics themselves. Studies of composite ceramics are also now in progress in order to enhance further material properties. Whisker-reinforced ceramics (WRC) in
2 EXPERIMENTAL PROCEDURE
The silicon-nitride powder (grade: SNP-85, Japan Metals & Chemicals Co., Japan), silicon-carbide
Composites Science and Technology 0266-3538/94/$0%00 ~) 1994 Elsevier Science Limited. 265
Takao Yonezawa, et al.
266 r .......
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o 6 ~ _ ~
•c+. . . . . . . . I Pressureless sintering
io
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b)
c)
°g pore
a) Fig.
1. Shrinkage
mechanisms of ceramics.
Green
whisker-reinforced
whiskers (grade: TWS-100 and TWS-400, Tokai Carbon Co., Japan), and sintering aids were mixed together with de-ionized water and some deflocculant in an alumina ballmill for 46 h. The average diameters and lengths of the TWS-100 and TWS-400 whiskers are, respectively (#m): 0.4 and 30; 1.3 and 50. They were added in amounts of 0, 10, 15 and 20wt%. Mixtures of yttria (99-99% pure, Shin-etsu Chemical Co., Japan), alumina (grade: AKP-30, Sumitomo Chemical Co., Japan) and Cordierite (Mg2ALSisOI~, grade: SS600, Marusu Yuyaku Co., Japan) were used as the sintering additives in amounts of 12 and 15 wt%. After adjusting the viscosity to approximately 0.05 Pas by adding water, the slips with solids content typically 70 wt% were cast in plaster of Paris moulds. After drying, the green compacts (50 m m x 50 mm × 6-10mm) were sintered at 1750°C/0.1 MPa nitrogen for 5 h or 1825°C/1 MPa nitrogen for 2-4 h. The bulk density of samples was measured by the Archimedean displacement method using toluene. The relative density was determined by dividing the bulk density by the true density calculated from the composition of the starting materials. The flexural strength was determined by three-point flexure tests on bars having dimensions of 3.0ram × 4.0mm × 40-0 mm, with polished tensile surfaces (see standard JIS-R-1601). Material fracture toughness was measured by the Single Edge Pre-cracked Beam method (see standard JIS-R-1607). Thermal shock resistance was determined as follows: flexure bars were inserted into a resistance heated tube furnace, soaked for 10-15min at a given temperature, quenched into a waterbath at 25°C, broken in three-point flexure at room temperature and the flexural strength recorded. The microstructures of the green and sintered bodies were studied using an optical microscope and a scanning electron microscope (SEM). 3 RESULTS A N D DISCUSSION
By controlling the viscosity of the slips, a green compact was obtained in which the whiskers were
Plaster mold
Fig. 2. Optical micrographs of the polished surface showing the alignment of whiskers in the slip cast composite. orientated two-dimensionally. The relative densities of green compacts were in the range of 65-69%. Figure 2 shows the optical micrographs which demonstrate the orientation of the whiskers in the composite material. Seen from the direction parallel to the mould surface with the optical microscope, the whiskers are aligned in the same direction, but seen from the direction perpendicular to the mould surface, the whiskers are randomly distributed. Figure 3 shows the anisotropic shrinkage and the 2.5 o d 2.0
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/X TWS-100 - - - 1750°C, 5h 0 TWS-400 1825°C, 2h • TWS-400, 1825 °C, 4h
Fig. 3. Effect of the whisker content on the shrinkage anisotropy and on the composite relative density.
Pressureless sintering of silicon-nitride composites
267
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Fig. 4. Effect of the whisker content on the flexural strength and the fracture toughness of a composite sintered at 1825°C. relative density of the composite. As the whisker content was increased, the degree of shrinkage anisotropy increased and the relative density decreased. The shrinkage in the transverse direction was 1.5-2.3 times larger than that in the parallel direction. The relative density increased with increasing sintering temperature, and was higher for the composite containing TWS-400 than for the one containing TWS-100. As the sintering time increased from 2 to 4 h, the relative density of the composite with 20 wt% TWS-400 increased to 98%. Figure 4 shows the flexural strength and the fracture toughness of a composite containing TWS-400 which was sintered at 1825°C. As the whisker content increased, the fracture toughness increased, but when the whisker loading reached 20wt% the flexural strength reduced because of the low relative density. The sintered body with 20 wt% TWS-400 sintered for 4h showed a flexural strength of 950MPa and a fracture toughness of 7.0 MPaVm. Figure 5 shows SEM micrographs of the fracture surface of a composite with 20wt% of TWS-400. Pull-out of the whiskers can be seen in several places. Figure 6 shows a crack induced by indentation on the polished surface of the composite with 20wt% of TWS-400. It can be seen that the crack deflected at the whiskers' location. The whisker pull-out and the crack deflection both improve the fracture toughness of the composite. Figure 7 shows the thermal shock resistance of the composite. Monolithic silicon nitride sintered at 1750°C exhibited a critical thermal shock temperature difference (ATc) of 800°C whereas the composite sintered at 1750°C, with 10 wt% of TWS-100, achieves a A Tc of 950°C. Furthermore, the flexural strength of
Fig. 5. SEM micrographs of the fracture surface of the composite showing the whisker pull-out. the composite which was sintered at 1825°C for 4 h with 20wt% of TWS-400 did not decrease after quenching from 1425°C into a waterbath at 25°C. This A Tc needs to be measured again after exchanging the quenching liquid from water to oil or a low melting
Fig. 6. Optical micrograph of the composite polished surface showing a crack induced by indentation.
268
Takao Yonezawa, et al.
100( 80(
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Quenching Temperature ( C )
Fig. 9. Samples manufactured from the composite. r'l Monolithic /x TWS-100, 10wt% 0 TWS-400, 20wt%
Fig. 7. Thermal shock resistance of the composite measured by water quenching.
point metal in order to prevent a water vapour layer covering the test parts and influencing the results. Nevertheless, an apparent A T c of 1400°C is an excellent result and amounts to a 100% improvement over the monolithic silicon nitride. Figure 8 shows photographs of the test bars which were subject to a quench of 1000°C and tested in three-point bending to determine flexural strength. The monolithic silicon-nitride test bar fractured in the centre as shown in the upper photograph, whilst the composite with 10 wt% of TWS-100 fractured along the orientation of the whiskers as shown in the lower photograph. The thermal expansion coefficients of silicon nitride and silicon carbide are 3.0 x 1 0 - 6 and 4.3 x 10-6, respectively, and when the test bars are quenched this difference in thermal expansion could induce microcracks between silicon-nitride matrix and silicon carbide whiskers along the whisker orientation.
4 APPLICATIONS These new composite materials, which have been named KRYPTONITE, have superior thermal shock resistance compared to the conventional silicon nitride and various applications are being developed for severe working environments, e.g. where rapid or local heating is necessary. Heater tubes, stokes and ladles used in the aluminium casting industry are potential new applications. A ladle for 1.5 kg molten aluminium and crucibles for 3 kg molten aluminium have been manufactured and are shown in Fig. 9. Other applications, such as high-temperature components for automobile engines and gas turbines, are also being investigated. 5 CONCLUSIONS By orientating whiskers two-dimensionally in a silicon-nitride matrix using the slip casting method, a high-density WRC can be produced by pressureless sintering. The sintered composites have good mechanical properties and excellent thermal shock resistance and have been used to fabricate parts for molten aluminium casting.
REFERENCES
Fig. $. Fracture conditions of test bars following thermal shock: (A) composite (B) monolithic silicon nitride.
1. Yamada, S., Recent research on SiC whisker-reinforced ceramic composites in Japan. ONRFE Sci. Bull., 12(4) (1987) 17-44. 2. Tamari, N., Kondoh, I., Kamioka, S., Ueno, K. & Toibana, Y., Sintering of Si3N4-SiC whisker composite. Yogyokyokai-shi, 94(11) (1986) 1177-79. 3. Kandori, T., Kobayashi, S., Wada, S. & Kamigaito, O., SiC Whisker reinforced SiaN4 composite. J. Mater. Sci. Lea., 6(11) (1987) 1356-8. 4. Lundberg, R., Kahlman, L., Pompe, R., Carlsson, R. & Warren, R., SiC-whisker-reinforced SigN4. Am. Ceram. Soc. Bull., 66(2) (1987) 330-3. 5. Hoffman, M. J., Nagel, A., Greil, P. & Petzow, G., Slip casting of SiC-whisker-reinforced Si3N4. J. Am. Ceram. Soc., 72(5) (1989) 765-9. 6. Ueno, K. & Toibana, Y., Mechanical properties of
Pressureless sintering of silicon-nitride composites
silicon nitride ceramic composite reinforced with silicon carbide whisker. Yogyokyokai-shi, 91(11) (1983) 491-7. 7. Buljan, S. T., Baldoni, J. G. & Huckabee, M. L., SiaN4-SiC composite. Am. Ceram. Soc. Bull., 66(2) (1987) 342-52. 8. Saitoh, S., Minamizawa, M., Yonezawa, T., Matsuda, T. & Sakai, C., Normal sintering of SiC whisker/silicon nitride composite. Synopsis, The 27th Basic Forum of the Ceram. Soc. Japan, 30-31 January 1989, Tokyo, Japan, p. 208. 9. Saitoh, S., Minamizawa, M., Yonezawa, T. & Matsuda, T., Sore properties of pressureless-sintered SiC
269
whisker/silicon nitride composite fabricated by slip casting. Synopsis, The 2nd Autumn Syrup. of the Ceram. Soc. Japan, 2-4 October 1989, Kyoto, Japan, pp. 508-9. 10. Yonezawa, T., Saitoh, S., Minamizawa, M. & Matsuda, T., Pressureless sintering of SiC-whisker-reinforced Si3N4. Proc. 1st Int. SAMPE Symp. & Exhibition. Nikkan Kogyo Shimbun, Tokyo, 1989, pp. 1131-6. 11. Yonezawa, T., Study of silicon nitride composite. Proc 91 Fall Meeting of MMIJ 1-3 October 1993, Morioka, Japan, pp. 11-14.
I PRESSURELESS SINTERING OF SILICON-NITRIDE COMPOSITES Takao Yonezawa JMC New Materials Inc., 8-4 Koami-cho, Nihonbashi, Chuo-ku, Tokyo, 103, Japan
Shin-ichi Saitoh, Masatoshi Minamizawa & Toshitsugu Matsuda Ceramics Development Center, Japan Metals & Chemicals Co. Ltd, 1-4-63, Ohama, Sakata-shi, Yamagata-ken, 998, Japan (Received 31 July 1992; accepted 12 February 1993)
particular are attracting a great deal of attention. 1 There are very few reports, however, which describe the mechanical properties of pressureless sintered WRC 2-5 and the reported data suggest that the materials are adversely affected by the network of whiskers in the green body. Because the whiskers do not shrink during pressureless sintering, whisker networks inhibit the movement of the matrix material, so that porosity persists in sintered composite and significantly impairs its strength (Fig. l(a)). Hot pressing is used conventionally in the production of high-density W R C 6'7 and during this process the pressure destroys the whisker networks and re-orientates whiskers two-dimensionally (Fig. l(b)). Although WRC can be made fully dense by hot pressing, the process is limited to simple shapes and the fabrication cost is high. Densification of WRC by a pressureless sintering route is therefore needed if the practical applications for these materials are to be extended. If a green compact is made in which the whiskers are orientated one- or two-dimensionally, shrinkage is considerably hindered in the orientated direction but not in the direction perpendicular to the orientated direction (Fig. l(c)). Various compaction techniques were examined and compared with this in mind and it is believed that the slip casting method is the best one for orientating whiskers one- and two-dimensionally and facilitating the production of shaped parts. By this route a dense WRC has been produced and used in a number of practical applications. 8-1~
Abstract A fabrication method for whisker-reinforced siliconnitride ceramics involving pressureless sintering is reported together with the mechanical properties of the resulting composite materials. Silicon-nitride composites containing up to 20wt% of silicon carbide whiskers were slip cast to orientate the whiskers twodimensionally, parallel to the plaster mould surfaces, and then pressureless sintered at 1750 or 1825°C. The sintered body containing 20wt% of whiskers of average diameter 1.3 Izm displayed a relative density of 98%, a mean flexural strength of 950MPa and a fracture toughness of 7.0MPaVm. Furthermore, the thermal shock resistance was found to be as high as 1400°C. To date, components successfully produced from this composite material include aluminum diecasting ladles and crucibles.
Keywords: pressureless sintering, slip casting, silicon nitride, silicon carbide whisker, composite 1 INTRODUCTION
Silicon-nitride ceramics display an excellent combination of material properties, e.g. high strength, high toughness, thermal shock resistance, and oxidation resistance. Applications of silicon-nitride ceramics now being developed include automobile engine parts and parallel material developments have resulted in a progressive improvement in the properties of the silicon-nitride ceramics themselves. Studies of composite ceramics are also now in progress in order to enhance further material properties. Whisker-reinforced ceramics (WRC) in
2 EXPERIMENTAL PROCEDURE
The silicon-nitride powder (grade: SNP-85, Japan Metals & Chemicals Co., Japan), silicon-carbide
Composites Science and Technology 0266-3538/94/$0%00 ~) 1994 Elsevier Science Limited. 265
Takao Yonezawa, et al.
266 r .......
]
o 6 ~ _ ~
•c+. . . . . . . . I Pressureless sintering
io
~ H o t pressing
L Pressureless sintering
b)
c)
°g pore
a) Fig.
1. Shrinkage
mechanisms of ceramics.
Green
whisker-reinforced
whiskers (grade: TWS-100 and TWS-400, Tokai Carbon Co., Japan), and sintering aids were mixed together with de-ionized water and some deflocculant in an alumina ballmill for 46 h. The average diameters and lengths of the TWS-100 and TWS-400 whiskers are, respectively (#m): 0.4 and 30; 1.3 and 50. They were added in amounts of 0, 10, 15 and 20wt%. Mixtures of yttria (99-99% pure, Shin-etsu Chemical Co., Japan), alumina (grade: AKP-30, Sumitomo Chemical Co., Japan) and Cordierite (Mg2ALSisOI~, grade: SS600, Marusu Yuyaku Co., Japan) were used as the sintering additives in amounts of 12 and 15 wt%. After adjusting the viscosity to approximately 0.05 Pas by adding water, the slips with solids content typically 70 wt% were cast in plaster of Paris moulds. After drying, the green compacts (50 m m x 50 mm × 6-10mm) were sintered at 1750°C/0.1 MPa nitrogen for 5 h or 1825°C/1 MPa nitrogen for 2-4 h. The bulk density of samples was measured by the Archimedean displacement method using toluene. The relative density was determined by dividing the bulk density by the true density calculated from the composition of the starting materials. The flexural strength was determined by three-point flexure tests on bars having dimensions of 3.0ram × 4.0mm × 40-0 mm, with polished tensile surfaces (see standard JIS-R-1601). Material fracture toughness was measured by the Single Edge Pre-cracked Beam method (see standard JIS-R-1607). Thermal shock resistance was determined as follows: flexure bars were inserted into a resistance heated tube furnace, soaked for 10-15min at a given temperature, quenched into a waterbath at 25°C, broken in three-point flexure at room temperature and the flexural strength recorded. The microstructures of the green and sintered bodies were studied using an optical microscope and a scanning electron microscope (SEM). 3 RESULTS A N D DISCUSSION
By controlling the viscosity of the slips, a green compact was obtained in which the whiskers were
Plaster mold
Fig. 2. Optical micrographs of the polished surface showing the alignment of whiskers in the slip cast composite. orientated two-dimensionally. The relative densities of green compacts were in the range of 65-69%. Figure 2 shows the optical micrographs which demonstrate the orientation of the whiskers in the composite material. Seen from the direction parallel to the mould surface with the optical microscope, the whiskers are aligned in the same direction, but seen from the direction perpendicular to the mould surface, the whiskers are randomly distributed. Figure 3 shows the anisotropic shrinkage and the 2.5 o d 2.0
"~.
0/
O
~
////
~.5
~/
~.0 ]00
7 i
-a.\\ "-.:0
95
~x
90!
6
2'o Whisker Content (%)
/X TWS-100 - - - 1750°C, 5h 0 TWS-400 1825°C, 2h • TWS-400, 1825 °C, 4h
Fig. 3. Effect of the whisker content on the shrinkage anisotropy and on the composite relative density.
Pressureless sintering of silicon-nitride composites
267
! 1000
{ 9oo ~ 8oc 70C 8
e
I
I
I
I
I
6
b
g
lb l's 2b
Whisker Content (%) O TWS-400, 2h • TWS-400, 4h
Fig. 4. Effect of the whisker content on the flexural strength and the fracture toughness of a composite sintered at 1825°C. relative density of the composite. As the whisker content was increased, the degree of shrinkage anisotropy increased and the relative density decreased. The shrinkage in the transverse direction was 1.5-2.3 times larger than that in the parallel direction. The relative density increased with increasing sintering temperature, and was higher for the composite containing TWS-400 than for the one containing TWS-100. As the sintering time increased from 2 to 4 h, the relative density of the composite with 20 wt% TWS-400 increased to 98%. Figure 4 shows the flexural strength and the fracture toughness of a composite containing TWS-400 which was sintered at 1825°C. As the whisker content increased, the fracture toughness increased, but when the whisker loading reached 20wt% the flexural strength reduced because of the low relative density. The sintered body with 20 wt% TWS-400 sintered for 4h showed a flexural strength of 950MPa and a fracture toughness of 7.0 MPaVm. Figure 5 shows SEM micrographs of the fracture surface of a composite with 20wt% of TWS-400. Pull-out of the whiskers can be seen in several places. Figure 6 shows a crack induced by indentation on the polished surface of the composite with 20wt% of TWS-400. It can be seen that the crack deflected at the whiskers' location. The whisker pull-out and the crack deflection both improve the fracture toughness of the composite. Figure 7 shows the thermal shock resistance of the composite. Monolithic silicon nitride sintered at 1750°C exhibited a critical thermal shock temperature difference (ATc) of 800°C whereas the composite sintered at 1750°C, with 10 wt% of TWS-100, achieves a A Tc of 950°C. Furthermore, the flexural strength of
Fig. 5. SEM micrographs of the fracture surface of the composite showing the whisker pull-out. the composite which was sintered at 1825°C for 4 h with 20wt% of TWS-400 did not decrease after quenching from 1425°C into a waterbath at 25°C. This A Tc needs to be measured again after exchanging the quenching liquid from water to oil or a low melting
Fig. 6. Optical micrograph of the composite polished surface showing a crack induced by indentation.
268
Takao Yonezawa, et al.
100( 80(
O O-----O
0-.-¢,~ ~-.-./t
A A
[]
0--4:
6oc i.
-~ 4oc r,
2oc /x
0
6"
'
860
'
lObO
'
12'00
'
14'00
Quenching Temperature ( C )
Fig. 9. Samples manufactured from the composite. r'l Monolithic /x TWS-100, 10wt% 0 TWS-400, 20wt%
Fig. 7. Thermal shock resistance of the composite measured by water quenching.
point metal in order to prevent a water vapour layer covering the test parts and influencing the results. Nevertheless, an apparent A T c of 1400°C is an excellent result and amounts to a 100% improvement over the monolithic silicon nitride. Figure 8 shows photographs of the test bars which were subject to a quench of 1000°C and tested in three-point bending to determine flexural strength. The monolithic silicon-nitride test bar fractured in the centre as shown in the upper photograph, whilst the composite with 10 wt% of TWS-100 fractured along the orientation of the whiskers as shown in the lower photograph. The thermal expansion coefficients of silicon nitride and silicon carbide are 3.0 x 1 0 - 6 and 4.3 x 10-6, respectively, and when the test bars are quenched this difference in thermal expansion could induce microcracks between silicon-nitride matrix and silicon carbide whiskers along the whisker orientation.
4 APPLICATIONS These new composite materials, which have been named KRYPTONITE, have superior thermal shock resistance compared to the conventional silicon nitride and various applications are being developed for severe working environments, e.g. where rapid or local heating is necessary. Heater tubes, stokes and ladles used in the aluminium casting industry are potential new applications. A ladle for 1.5 kg molten aluminium and crucibles for 3 kg molten aluminium have been manufactured and are shown in Fig. 9. Other applications, such as high-temperature components for automobile engines and gas turbines, are also being investigated. 5 CONCLUSIONS By orientating whiskers two-dimensionally in a silicon-nitride matrix using the slip casting method, a high-density WRC can be produced by pressureless sintering. The sintered composites have good mechanical properties and excellent thermal shock resistance and have been used to fabricate parts for molten aluminium casting.
REFERENCES
Fig. $. Fracture conditions of test bars following thermal shock: (A) composite (B) monolithic silicon nitride.
1. Yamada, S., Recent research on SiC whisker-reinforced ceramic composites in Japan. ONRFE Sci. Bull., 12(4) (1987) 17-44. 2. Tamari, N., Kondoh, I., Kamioka, S., Ueno, K. & Toibana, Y., Sintering of Si3N4-SiC whisker composite. Yogyokyokai-shi, 94(11) (1986) 1177-79. 3. Kandori, T., Kobayashi, S., Wada, S. & Kamigaito, O., SiC Whisker reinforced SiaN4 composite. J. Mater. Sci. Lea., 6(11) (1987) 1356-8. 4. Lundberg, R., Kahlman, L., Pompe, R., Carlsson, R. & Warren, R., SiC-whisker-reinforced SigN4. Am. Ceram. Soc. Bull., 66(2) (1987) 330-3. 5. Hoffman, M. J., Nagel, A., Greil, P. & Petzow, G., Slip casting of SiC-whisker-reinforced Si3N4. J. Am. Ceram. Soc., 72(5) (1989) 765-9. 6. Ueno, K. & Toibana, Y., Mechanical properties of
Pressureless sintering of silicon-nitride composites
silicon nitride ceramic composite reinforced with silicon carbide whisker. Yogyokyokai-shi, 91(11) (1983) 491-7. 7. Buljan, S. T., Baldoni, J. G. & Huckabee, M. L., SiaN4-SiC composite. Am. Ceram. Soc. Bull., 66(2) (1987) 342-52. 8. Saitoh, S., Minamizawa, M., Yonezawa, T., Matsuda, T. & Sakai, C., Normal sintering of SiC whisker/silicon nitride composite. Synopsis, The 27th Basic Forum of the Ceram. Soc. Japan, 30-31 January 1989, Tokyo, Japan, p. 208. 9. Saitoh, S., Minamizawa, M., Yonezawa, T. & Matsuda, T., Sore properties of pressureless-sintered SiC
269
whisker/silicon nitride composite fabricated by slip casting. Synopsis, The 2nd Autumn Syrup. of the Ceram. Soc. Japan, 2-4 October 1989, Kyoto, Japan, pp. 508-9. 10. Yonezawa, T., Saitoh, S., Minamizawa, M. & Matsuda, T., Pressureless sintering of SiC-whisker-reinforced Si3N4. Proc. 1st Int. SAMPE Symp. & Exhibition. Nikkan Kogyo Shimbun, Tokyo, 1989, pp. 1131-6. 11. Yonezawa, T., Study of silicon nitride composite. Proc 91 Fall Meeting of MMIJ 1-3 October 1993, Morioka, Japan, pp. 11-14.