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Pressureless sintering of silicon carbide with additives of samarium oxide and alumina
Materials Letters 17 ( 1993) 27-30 bosh-Holland
Pressureless sintering of silicon carbide with additives of samarium oxide and alumina Zhao Chen Institutefor Materials Research, Tohoku University, Sendui 980, Japan
Received 1 April 1993
Additions of AlzOs and Sm,O, have been found to be effective in densifying Sic. The highest density of 92.6% of the theoretical density was achieved with the Smz03 and AlzOJ eutectic composition. It is suggested that the improvement in density is due to the liquid phase formed between A1203and SmzOl. Some mechanical properties of samples with additives ofeutectic composition are also measured.
1. Introduction Sic has been recognized as a promising structural ceramic because of its excellent oxidation resistance, strength retention to high temperature and high wear resistance. The excellent properties are due to the highly covalent Si-C bond, which on the other hand, makes it diff~cuIt to sinter Sic to a high density. In 1975, Prochazka [ 11 discovered that small additions of boron and carbon result in high densities without the application of external pressure. This development has made it possible to produce Sic components with complex shapes economically. Later in 198 1, it was reported that simultaneous addition of Y,O, and A1203 could be used to improve the densification of Sic. This has been proved by other researchers [ 2-41. It was also reported that the addition of Y203 and A&O3 could improve the fracture toughness via a crack deflection mechanism [41. The aim of this paper is to present the effect of simultaneous addition of Sm,03 and A1203 on the densi~cation of Sic by pressureless sintering. The additive composition was chosen in the region with Sm,03 between 0 and 50 mol%, because a liquid phase is expected to emerge at a temperature of 175 5 OC (see fig, 1) _This temperature allows the Sic to be sintered to high density while keeping a highly refractory grain boundary phase which will be a ben-
2400
,
2300
_
2200
-
1500
,
,
, 2324
i
-
IO
20
40
60
80
Al203
100 smzos
Mel (%) Fig. 1.Phase diagram of the system Al,O,-Sm,O,
[ 5 1.
efit for the high temperature behavior. It is found that appropriate content of SmzOl and A1203 additives can improve the densification considerably.
2. Experimental A commercially available P-Sic powder (Betarundum, Ibiden Co., Ltd. Japan) was used for this study. The powder had an average diameter of 2 pm. The
016%577x/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
27
Volume
17, number
I,2
MATERIALS
Smz03 (Nippon Yttrium Co., Ltd., Japan) and A1203 (Asahi Chemical Ind. Co., Ltd., Japan) powders were used as additives. The total mole percentage of additives is 6 mol%, but varies in mole ratio of Sm203/ AllO (see table 1). The composition of additives in sample 3 corresponds to the eutectic composition between Smz03 and A1203. The theoretical densities are calculated according to the rule of mixtures. Mixing was performed by wet milling in ethanol using nylon-coated steel balls for 24 h. After drying in air, a water solution of 5 wt% polyethanol glycol was added as binder, and the mixtures were dried and milled again. Then the powders were compacted into bars by uniaxial pressing at a pressure of 100 MPa followed by isostatic pressing at 600 MPa to obtain green bodies with uniform high density. The bars were heated to 500’ C at a rate of 2’ C/min and kept for 2 h in order to remove the polyethanol glycol. The samples were sintered in a graphite resistance furnace in an atmosphere of flowing argon at a heating rate of 35”C/min to 1000°C and at a rate of lO’C/ min between 1000’ C and 2000’ C. After holding at 2000°C for 30 min, the samples were furnace cooled. Density was measured by means of the Archimedes law using distilled water. Microstructure was observed by SEM with both fractured and polished surfaces. The polished surface was thermally etched at 1200°C for 10 min. The hardness was determined by Vickers indentation. Elastic modulus was measured by using the resonance method. Strength measurement was conducted on a material test machine with the three-point bending method. Fracture toughness was calculated from the equation
LETTERS
July 1993
3. Results and discussion Fig. 2 shows the change of density with the change of additive composition. The highest density of 92.6% of the theoretical density is obtained with sample 3. As can be seen from fig. 2, samples 2-4 have relatively high densities. This is probably a result of the liquid phase which is formed between A1203 and Sm,03 in this range of composition above the temperature of 1755°C [ 61. The fact that sample 3 has the largest density supports this conclusion, because additives with eutectic composition produce the largest amount of liquid phase in this experiment. In the case of sample 1, it is shown that Al2O3 alone is not as effective in improving densification as the combination of A1203+SmzOj; in this case, no liquid phase occurs in a pure system temperature of 2071 “C. Despite the unavoidable existence of SiOZ in Sic powder, the amount of liquid phase between SiOZ and A1203 is quite moderate, hence the effect of liquid phase sintering is limited. In the case of sample 5, however, the composition of additives corresponds to the compound SmA103 which does not melt until 2 104°C [ 61. This temperature is even higher than that of A&O3 alone and far greater than the sintering temperature employed in this research; thus this also leads to a less densilied sintered body. The SEM micrographs are presented in fig. 3. Figs. 3A and 3E show the neck growth and large pore size. They indicate that the sintered bodies are still in their early stages of sintering. While figs. 3B, 3C and 3D show an increase in average grain size and a decrease
[51:
Table 1 Compositions
and theoretical
densities
of various
mixtures
Sample no.
Sm2Q (mol I)
Al203 (mol%)
Theoretical
1
0
2 3 4 5
1
6 5 4.56 4 3
3.3043 3.4296 3.4760 3.5505 3.6634
1.44 2 3
density
(g/cm’)
0.00
0.20
0.40
SmzOdAb03
0.60
(mole ratio)
Fig. 2. Change of density with additive
28
0.80
composition.
1 .oo
Volume 17, number 1,2
MATERIALS LETTERS
July 1993
Fig. 3. SEM micrographs: (A) 0 mol% Sm,03, 6 mol% Al,03; (B) I mol% Sm*O,, 5 mol% A1,03; (C) and (F) 1.44 mol% Sm201. 4.56 mol% AlzOs; (D) 2 molI Smz03, 4 mol% Al,O,; (E) 3 mol% Sm20,, 3 mol% A&Ox.
and size of pores, which result in densification. They also show that most of the grains are uniform and that no exaggerated grain growth was observed. Fig. JE shows the polished surface of a in the amount
sample with additive of eutectic composition. The bright contrast is the secondary phase because of its higher average atomic number. The secondary phase was observed primarily at multiple grain junctions 29
Volume 17, number I,2
MATERIALS LETTERS
Table 2 Mechanical properties of Sic with eutectic composition additive Hardness (GPa)
Modulus (GPa)
Strength (MPa)
Toughness ( MPa ml/*)
17.1
373.5
344.6
4.6
and between the grains. The majority of Sic grains had rounded edges. This micrograph clearly reveals that the secondary phase becomes liquid at high temperature and wets the surface of the Sic grains, thus resulting in the liquid phase sintering. The microstructure examination supported the result of fig. 2 and the existence of a liquid phase sintering mechanism. Some mechanical properties of the samples with eutectic composition are shown in table 2. The hardness and strength are not very high due to the relatively low density, which can be expected to be improved by further optimizing the composition and sintering conditions. The fracture toughness however is higher than the reported 3.09-3.44 MPa rn1j2 for pressureless sintering of Sic with B and C as additives [ 71. The same result had been reported in a similar system of Sic with Y203 and A1203 as additives [4]. It was concluded that the mismatch in thermal expansion built up residual strain fields around the secondary phase, which improved fracture toughness by a crack deflection process [ 8 1.
4. Conclusions The simultaneous
30
addition
of A1203 and Sm203 is
July 1993
effective in promoting the densification of pressureless sintered Sic. The samples could be sintered to 92.6% of the theoretical density at a temperature of 2000°C and with a holding time of 30 min. The reason for this is suggested to be that the simultaneous addition of Sm20, and A1203 decreases the temperature at which the liquid phase occurs, thus promoting the densification of Sic by a liquid phase sintering mechanism. The sintered body has a fracture toughness of 4.6 MPa m ‘I2 . This method can be expected to be an alternative way of economically producing complex shaped Sic components.
Acknowledgement The author thanks Mr. A. Okubo preparing the samples.
for helping
in
References [ 1] S. Prochazka, in: Proceedings of the Conference on Ceramics for High-Performance Applications, eds. J.J. Burke, A.E. Gorum and R.M. Katz (BrookHill, 1975) p. 239. [2] M. Omori and H. Takei, J. Am. Ceram. Sot. 65 (1982) C92. [3] L. Cordrey, D.E. Niesz and D.J. Shanetield, in: Sintering of advanced ceramics, eds. CA. Handerker, J.E. Blendell and W.A. Kaysser (American Ceramic Society, Columbus, OH, 1990) p. 618. [4] D.H.KimandC.H.Kim,J.Am.Ceram.Soc.73 (1990) 1431. [ 51G.R. Anstis, P. Chantikul, B.R. Lawn and D.B. Marshall, J. Am. Ceram. Sot. 64 ( 198 1) 533. [ 61 M. Mizuno, T. Yamada and T. Noguchi, Yogyo Kyokai Shi (J. Japan. Ceram. Sot.) 85 (1985) 374. [ 71 G. Orange, H. Tanaka and G. Fantozzi, Ceramics Intern. 13 (1987) 159. [8] K.T. Faber and A.G. Evans, Acta Metall. 31 (1983) 565.
Pressureless sintering of silicon carbide with additives of samarium oxide and alumina Zhao Chen Institutefor Materials Research, Tohoku University, Sendui 980, Japan
Received 1 April 1993
Additions of AlzOs and Sm,O, have been found to be effective in densifying Sic. The highest density of 92.6% of the theoretical density was achieved with the Smz03 and AlzOJ eutectic composition. It is suggested that the improvement in density is due to the liquid phase formed between A1203and SmzOl. Some mechanical properties of samples with additives ofeutectic composition are also measured.
1. Introduction Sic has been recognized as a promising structural ceramic because of its excellent oxidation resistance, strength retention to high temperature and high wear resistance. The excellent properties are due to the highly covalent Si-C bond, which on the other hand, makes it diff~cuIt to sinter Sic to a high density. In 1975, Prochazka [ 11 discovered that small additions of boron and carbon result in high densities without the application of external pressure. This development has made it possible to produce Sic components with complex shapes economically. Later in 198 1, it was reported that simultaneous addition of Y,O, and A1203 could be used to improve the densification of Sic. This has been proved by other researchers [ 2-41. It was also reported that the addition of Y203 and A&O3 could improve the fracture toughness via a crack deflection mechanism [41. The aim of this paper is to present the effect of simultaneous addition of Sm,03 and A1203 on the densi~cation of Sic by pressureless sintering. The additive composition was chosen in the region with Sm,03 between 0 and 50 mol%, because a liquid phase is expected to emerge at a temperature of 175 5 OC (see fig, 1) _This temperature allows the Sic to be sintered to high density while keeping a highly refractory grain boundary phase which will be a ben-
2400
,
2300
_
2200
-
1500
,
,
, 2324
i
-
IO
20
40
60
80
Al203
100 smzos
Mel (%) Fig. 1.Phase diagram of the system Al,O,-Sm,O,
[ 5 1.
efit for the high temperature behavior. It is found that appropriate content of SmzOl and A1203 additives can improve the densification considerably.
2. Experimental A commercially available P-Sic powder (Betarundum, Ibiden Co., Ltd. Japan) was used for this study. The powder had an average diameter of 2 pm. The
016%577x/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
27
Volume
17, number
I,2
MATERIALS
Smz03 (Nippon Yttrium Co., Ltd., Japan) and A1203 (Asahi Chemical Ind. Co., Ltd., Japan) powders were used as additives. The total mole percentage of additives is 6 mol%, but varies in mole ratio of Sm203/ AllO (see table 1). The composition of additives in sample 3 corresponds to the eutectic composition between Smz03 and A1203. The theoretical densities are calculated according to the rule of mixtures. Mixing was performed by wet milling in ethanol using nylon-coated steel balls for 24 h. After drying in air, a water solution of 5 wt% polyethanol glycol was added as binder, and the mixtures were dried and milled again. Then the powders were compacted into bars by uniaxial pressing at a pressure of 100 MPa followed by isostatic pressing at 600 MPa to obtain green bodies with uniform high density. The bars were heated to 500’ C at a rate of 2’ C/min and kept for 2 h in order to remove the polyethanol glycol. The samples were sintered in a graphite resistance furnace in an atmosphere of flowing argon at a heating rate of 35”C/min to 1000°C and at a rate of lO’C/ min between 1000’ C and 2000’ C. After holding at 2000°C for 30 min, the samples were furnace cooled. Density was measured by means of the Archimedes law using distilled water. Microstructure was observed by SEM with both fractured and polished surfaces. The polished surface was thermally etched at 1200°C for 10 min. The hardness was determined by Vickers indentation. Elastic modulus was measured by using the resonance method. Strength measurement was conducted on a material test machine with the three-point bending method. Fracture toughness was calculated from the equation
LETTERS
July 1993
3. Results and discussion Fig. 2 shows the change of density with the change of additive composition. The highest density of 92.6% of the theoretical density is obtained with sample 3. As can be seen from fig. 2, samples 2-4 have relatively high densities. This is probably a result of the liquid phase which is formed between A1203 and Sm,03 in this range of composition above the temperature of 1755°C [ 61. The fact that sample 3 has the largest density supports this conclusion, because additives with eutectic composition produce the largest amount of liquid phase in this experiment. In the case of sample 1, it is shown that Al2O3 alone is not as effective in improving densification as the combination of A1203+SmzOj; in this case, no liquid phase occurs in a pure system temperature of 2071 “C. Despite the unavoidable existence of SiOZ in Sic powder, the amount of liquid phase between SiOZ and A1203 is quite moderate, hence the effect of liquid phase sintering is limited. In the case of sample 5, however, the composition of additives corresponds to the compound SmA103 which does not melt until 2 104°C [ 61. This temperature is even higher than that of A&O3 alone and far greater than the sintering temperature employed in this research; thus this also leads to a less densilied sintered body. The SEM micrographs are presented in fig. 3. Figs. 3A and 3E show the neck growth and large pore size. They indicate that the sintered bodies are still in their early stages of sintering. While figs. 3B, 3C and 3D show an increase in average grain size and a decrease
[51:
Table 1 Compositions
and theoretical
densities
of various
mixtures
Sample no.
Sm2Q (mol I)
Al203 (mol%)
Theoretical
1
0
2 3 4 5
1
6 5 4.56 4 3
3.3043 3.4296 3.4760 3.5505 3.6634
1.44 2 3
density
(g/cm’)
0.00
0.20
0.40
SmzOdAb03
0.60
(mole ratio)
Fig. 2. Change of density with additive
28
0.80
composition.
1 .oo
Volume 17, number 1,2
MATERIALS LETTERS
July 1993
Fig. 3. SEM micrographs: (A) 0 mol% Sm,03, 6 mol% Al,03; (B) I mol% Sm*O,, 5 mol% A1,03; (C) and (F) 1.44 mol% Sm201. 4.56 mol% AlzOs; (D) 2 molI Smz03, 4 mol% Al,O,; (E) 3 mol% Sm20,, 3 mol% A&Ox.
and size of pores, which result in densification. They also show that most of the grains are uniform and that no exaggerated grain growth was observed. Fig. JE shows the polished surface of a in the amount
sample with additive of eutectic composition. The bright contrast is the secondary phase because of its higher average atomic number. The secondary phase was observed primarily at multiple grain junctions 29
Volume 17, number I,2
MATERIALS LETTERS
Table 2 Mechanical properties of Sic with eutectic composition additive Hardness (GPa)
Modulus (GPa)
Strength (MPa)
Toughness ( MPa ml/*)
17.1
373.5
344.6
4.6
and between the grains. The majority of Sic grains had rounded edges. This micrograph clearly reveals that the secondary phase becomes liquid at high temperature and wets the surface of the Sic grains, thus resulting in the liquid phase sintering. The microstructure examination supported the result of fig. 2 and the existence of a liquid phase sintering mechanism. Some mechanical properties of the samples with eutectic composition are shown in table 2. The hardness and strength are not very high due to the relatively low density, which can be expected to be improved by further optimizing the composition and sintering conditions. The fracture toughness however is higher than the reported 3.09-3.44 MPa rn1j2 for pressureless sintering of Sic with B and C as additives [ 71. The same result had been reported in a similar system of Sic with Y203 and A1203 as additives [4]. It was concluded that the mismatch in thermal expansion built up residual strain fields around the secondary phase, which improved fracture toughness by a crack deflection process [ 8 1.
4. Conclusions The simultaneous
30
addition
of A1203 and Sm203 is
July 1993
effective in promoting the densification of pressureless sintered Sic. The samples could be sintered to 92.6% of the theoretical density at a temperature of 2000°C and with a holding time of 30 min. The reason for this is suggested to be that the simultaneous addition of Sm20, and A1203 decreases the temperature at which the liquid phase occurs, thus promoting the densification of Sic by a liquid phase sintering mechanism. The sintered body has a fracture toughness of 4.6 MPa m ‘I2 . This method can be expected to be an alternative way of economically producing complex shaped Sic components.
Acknowledgement The author thanks Mr. A. Okubo preparing the samples.
for helping
in
References [ 1] S. Prochazka, in: Proceedings of the Conference on Ceramics for High-Performance Applications, eds. J.J. Burke, A.E. Gorum and R.M. Katz (BrookHill, 1975) p. 239. [2] M. Omori and H. Takei, J. Am. Ceram. Sot. 65 (1982) C92. [3] L. Cordrey, D.E. Niesz and D.J. Shanetield, in: Sintering of advanced ceramics, eds. CA. Handerker, J.E. Blendell and W.A. Kaysser (American Ceramic Society, Columbus, OH, 1990) p. 618. [4] D.H.KimandC.H.Kim,J.Am.Ceram.Soc.73 (1990) 1431. [ 51G.R. Anstis, P. Chantikul, B.R. Lawn and D.B. Marshall, J. Am. Ceram. Sot. 64 ( 198 1) 533. [ 61 M. Mizuno, T. Yamada and T. Noguchi, Yogyo Kyokai Shi (J. Japan. Ceram. Sot.) 85 (1985) 374. [ 71 G. Orange, H. Tanaka and G. Fantozzi, Ceramics Intern. 13 (1987) 159. [8] K.T. Faber and A.G. Evans, Acta Metall. 31 (1983) 565.