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Colloidal processing and superplastic properties of zirconia- and alumina-based nanocomposites
Scripta mater. 44 (2001) 2075–2078 www.elsevier.com/locate/scriptamat
COLLOIDAL PROCESSING AND SUPERPLASTIC PROPERTIES OF ZIRCONIA- AND ALUMINA-BASED NANOCOMPOSITES Y. Sakka, T.S. Suzuki, K. Morita, K. Nakano and K. Hiraga National Research Institute for Metals, 1-2-1 Sengen, Tsukuba 305-0047, Japan (Received August 21, 2000) (Accepted December 6, 2000) Keywords: Powder processing; Ceramics; Mechanical properties— high temperature; Superplasticity
Introduction Superplasticity provides the possibility of high-temperature deformation processing of dense ceramics and has the advantages of greater shape formability and better dimensional accuracy. Preliminary studies of fine-grained zirconia, alumina, and their composites have shown that certain requirements must be met to achieve superplasticity: fine grain size, reduction of residual defects, homogeneous microstructure, and the inhibition of grain growth during high-temperature deformation [1,2]. Colloidal processing of fine particles is a useful technique to achieve the above requirements [3]. In this study, colloidal processing was applied to obtain dense and homogeneous zirconia-dispersed alumina and alumina-dispersed zirconia nanocomposites, and excellent superplastic properties were achieved. The effects of the microstructure such as grain size, second-phase size, and cavity density on the superplastic properties are discussed in a comparison with conventionally prepared samples.
Experimental Procedure The materials used were 3 mol% Y2O3 doped tetragonal ZrO2 powder (TZ3Y; average particle size: 60 nm; Tosoh Corp.) and ␣-Al2O3 powder (TM-DAR; 0.15 m; Taimei Chem.). To obtain well-dispersed suspensions of Al2O3-ZrO2 systems, an appropriate amount of ammonium polycarboxylate (ALON A-6114; Toagosei Chem.) was added to improve colloidal stability by electrosteric repulsion [4]. Here, redispersion is necessary because fine particles tend to agglomerate due to van der Waals attraction. We have shown that ultrasonication effectively disperses fine Al2O3 and ZrO2 particles [5]. To avoid long-range segregation during slip casting, the solids content of each suspension was adjusted to exceed 30 vol%. The suspensions of Al2O3-ZrO2 were consolidated by slip casting and treated by cold isostatic pressing (CIP) at 400 MPa to improve packing density. In conventional dry processing, the same powders are mixed in a ball mill with a high-purity alumina ball and ethanol [2,6]. After drying, they were pressed at 20 MPa and finally subjected to CIP at 400 MPa. Sintering was performed in air at fixed temperatures. The densities of the green and sintered bodies were measured by Archimedes’ method using kerosene or distilled water. Bone-shaped flat tensile specimens with gauge lengths of 5 to 10 mm were machined and tensile tested at fixed temperatures, at an initial strain rate of 1.7 ⫻ 10⫺4 to 1.2 ⫻ 10⫺2 s⫺1. Quantitative 1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00889-2
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Figure 1. Comparison of tensile flow curves among three samples.
microstructural examinations were conducted on the polished surfaces of both as-sintered and deformed samples using scanning electron microscopy (SEM) and an optical microscope equipped with an image analyzer [6]. Results and Discussion Zirconia-Dispersed Alumina The relative densities of the sintered bodies of 10 vol% TZ-dispersed Al2O3 sintered at 1673 K for 2 h prepared by colloidal processing (sample C) and by dry processing (sample D) were 99.6% and 99.5%, respectively. The lower sintering temperature resulted in fine-grained microstructures with an average Al2O3 grain size of 0.45 m in both samples. The sintering temperature was about 100 K lower and the grain size about half that in earlier studies [7,8]. The aging of sample D at 1773 K resulted in an alumina grain size of 0.94 m and relative densities higher than 99.7% (sample D⬘). The creep parameters defined by the following semi-empirical equation were evaluated at a true strain rate of 0.1 [9,10]: Strain rate ⫽ And⫺p, where A is a material constant, and d are the stress and the average alumina grain size, n is the stress exponent, and p is the grain size exponent. No difference was seen in the creep parameters of samples C and D, where n ⫽ 2.0, p ⫽ 2.0, and the activation energy was 750 kJ/mol. Previous studies have shown that the reduction of alumina grain size from 1.5 to about 1.0 m decreases the level of flow stress and results in the enhancement of elongation independently of the processing used [10]. Fig. 1 compares the tensile flow curves of the three samples. In the dry processed samples (D and D⬘), the reduction of grain size from 0.94 to 0.45 m decreases the flow stress, but does not lead to a further increase in the tensile ductility. In the colloidally processed sample (C), however, the tensile ductility reaches 550% when the initial grain size is reduced to 0.45 m. The grain growth, density, and creep parameters of C and D are almost identical. Hence, other microstructural factors must be involved in the difference in elongation. Table 1 summarizes TZ particle dispersion in the sintered samples [10]. For the same volume fraction of TZ particles, the average second-phase diameter dZ in sample C is smaller than that in sample D, and the numerical density of the particles NvZ is about two times higher in C than in D. The ratio of the particle to grain junction densities is defined as fP ⫽ NgZ/Ng, where Ng is the number of four grain junctions per unit volume. The fP value is a measure of the pinning efficiency against the
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TABLE 1 Evaluation of Zirconia Particle Dispersion of the Three Samples Samples
Density (%)
dA (m)
dZ (m)
NvZ (m3)
fP
C D D’
99.6 99.5 99.7
0.45 0.45 0.94
0.07 0.12 0.20
142 61 13
1.00 0.45 0.83
grain growth. The fP value predicts that grain growth in D will tend to be more active than that in C until the grain size reaches that in sample D⬘. Another difference was observed in the size distribution and fraction of residual cavities. The numerical densities of cavities in C (area fraction: 0.15%) were less than 40% of those in D (area fraction: 0.29%). The total area fraction of residual cavities differs by only 0.14%, which closely agrees with the data of density measurement, although higher numerical densities of cavities should lead to the acceleration of damage accumulation [2]. The uniform dispersion of fine TZ particles and reduced numerical density of residual cavities are additional prerequisties for large tensile ductility in TZ-dispersed alumina nanocomposite. Alumina-Dispersed TZ The green density after slip casting (about 50%) of well-dispersed Al2O3-TZ suspensions was increased by CIP treatment at 400 MPa (about 55%). The green compact obtained showed narrow pore size distribution, resulting in low-temperature sintering and high density without large pores. When the Al2O3 addition was lower than 7.7 vol%, densification started at a lower temperature compared to the TZ without Al2O3 addition [3]. This indicates that a small amount Al2O3 addition can decrease the sintering temperature and time required. In fact, sintering at 1573 K for 2 h was sufficient to obtain dense polycrystals having a theoretical density exceeding 99%. This temperature is about 100 K lower than the temperature used in conventional dry processing. The grain size of 0.3 vol% Al2O3-dispersed TZ was 0.23 m, and no amorphous phases were observed along grain boundaries nor at multiple junctions. Fig. 2 shows the effect of Al2O3 addition on the tensile elongation at an initial strain rate of 1.3 ⫻ 10⫺4 (undoped) or 1.9 ⫻ 10⫺4 (alumina added) s⫺1, and flow stress at a true strain rate of 0.1 at 1573 K. A significant decrease in the flow stress and increase in the tensile ductility of the TZ samples were confirmed with a small amount of Al2O3 addition. When the temperature was increased to 1723 K, the
Figure 2. Effect of alumina additions on tensile flow stress and engineering strain.
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tensile ductility of 0.3 vol% Al2O3-dispersed TZ reached about 1000% along the gauge portion. With regard to the high strain-rate superplasticity, tensile ductility exceeding 300% was observed even at an initial strain rate of 1.2 ⫻ 10⫺2 s⫺1 at 1723 K. The stress exponent of 0.3 vol% Al2O3-dispersed TZ was evaluated as 2.2 at 1673 K and the apparent activation energy as 519 kJ/mol [11]. These values are consistent with previous data of n ⫽ 2 and Q ⫽ 480 kJ/mol for conventionally processed samples in which the initial grain size was 0.4 m [12]. The grain size refinement to 0.23 m by colloidal processing did not affect the creep parameters. Some factors can be assumed to contribute to the enhanced tensile ductility. One is enhanced cation diffusion in the Al2O3-dispersed TZ. A second factor is the refinement of grain size to 0.23 m, since the reduced grain size resulted in a decrease in the level of flow stress. A third factor is the decrease in initial defects in the case of samples prepared by the colloidal processing as described in TZdispersed Al2O3. Conclusion TZ-dispersed Al2O3 and Al2O3-dispersed TZ nanocomposites were prepared by colloidal processing and low-temperature sintering. Large tensile elongation exceeding 550% can be obtained for 10 vol% TZ-dispersed Al2O3 and 1000% for 0.3 vol% Al2O3-dispersed TZ. Their creep parameters were similar to those of samples prepared by conventional dry processing. The excellent superplasticities achieved are due to the reduction of residual defects, and the fine-grained and homogeneous microstructures. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
I-W. Chen and L. A. Xue, J. Am. Ceram. Soc. 44, 2585 (1990). K. Hiraga and K. Nakano, Mater. Sci. Forum. 234 –235, 387 (1997). Y. Sakka and K. Hiraga, Nippon Kagaku Kaishi. 1999, 497 (1999) (in Japanese). J. Cesaramo III, I. A. Aksay, and A. Bleier, J. Am. Ceram. Soc. 71, 250 (1988). T. S. Suzuki, Y. Sakka, K. Nakano, and K. Hiraga, Mater. Trans. JIM. 39, 689 (1998). K. Hiraga, K. Nakano, T. S. Suzuki, and Y. Sakka, Scripta Mater. 39, 1273 (1998). K. Okada and T. Sakuma, J. Am. Ceram. Soc. 79, 499 (1990). K. Nakano, T. S. Suzuki, K. Hiraga, and Y. Sakka, Scripta Mater. 38, 33 (1998). T. S. Suzuki, Y. Sakka, K. Nakano, and K. Hiraga, Mater. Sci. Forum, 304 –306, 489 (1999). K. Hiraga, T. S. Suzuki, Y. Sakka, and K. Nakano, ICM8, vol. III, p. 981 (1999). T. S. Suzuki, Y. Sakka, K. Nakano, and K. Hiraga, Scripta Mater. in press. E. Sato, H. Morioka, K. Kuribayashi, and D. Sundaraman, J. Mater. Sci. 34, 4511 (1999).
COLLOIDAL PROCESSING AND SUPERPLASTIC PROPERTIES OF ZIRCONIA- AND ALUMINA-BASED NANOCOMPOSITES Y. Sakka, T.S. Suzuki, K. Morita, K. Nakano and K. Hiraga National Research Institute for Metals, 1-2-1 Sengen, Tsukuba 305-0047, Japan (Received August 21, 2000) (Accepted December 6, 2000) Keywords: Powder processing; Ceramics; Mechanical properties— high temperature; Superplasticity
Introduction Superplasticity provides the possibility of high-temperature deformation processing of dense ceramics and has the advantages of greater shape formability and better dimensional accuracy. Preliminary studies of fine-grained zirconia, alumina, and their composites have shown that certain requirements must be met to achieve superplasticity: fine grain size, reduction of residual defects, homogeneous microstructure, and the inhibition of grain growth during high-temperature deformation [1,2]. Colloidal processing of fine particles is a useful technique to achieve the above requirements [3]. In this study, colloidal processing was applied to obtain dense and homogeneous zirconia-dispersed alumina and alumina-dispersed zirconia nanocomposites, and excellent superplastic properties were achieved. The effects of the microstructure such as grain size, second-phase size, and cavity density on the superplastic properties are discussed in a comparison with conventionally prepared samples.
Experimental Procedure The materials used were 3 mol% Y2O3 doped tetragonal ZrO2 powder (TZ3Y; average particle size: 60 nm; Tosoh Corp.) and ␣-Al2O3 powder (TM-DAR; 0.15 m; Taimei Chem.). To obtain well-dispersed suspensions of Al2O3-ZrO2 systems, an appropriate amount of ammonium polycarboxylate (ALON A-6114; Toagosei Chem.) was added to improve colloidal stability by electrosteric repulsion [4]. Here, redispersion is necessary because fine particles tend to agglomerate due to van der Waals attraction. We have shown that ultrasonication effectively disperses fine Al2O3 and ZrO2 particles [5]. To avoid long-range segregation during slip casting, the solids content of each suspension was adjusted to exceed 30 vol%. The suspensions of Al2O3-ZrO2 were consolidated by slip casting and treated by cold isostatic pressing (CIP) at 400 MPa to improve packing density. In conventional dry processing, the same powders are mixed in a ball mill with a high-purity alumina ball and ethanol [2,6]. After drying, they were pressed at 20 MPa and finally subjected to CIP at 400 MPa. Sintering was performed in air at fixed temperatures. The densities of the green and sintered bodies were measured by Archimedes’ method using kerosene or distilled water. Bone-shaped flat tensile specimens with gauge lengths of 5 to 10 mm were machined and tensile tested at fixed temperatures, at an initial strain rate of 1.7 ⫻ 10⫺4 to 1.2 ⫻ 10⫺2 s⫺1. Quantitative 1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00889-2
2076
Zr- AND Al-BASED NANOCOMPOSITES
Vol. 44, Nos. 8/9
Figure 1. Comparison of tensile flow curves among three samples.
microstructural examinations were conducted on the polished surfaces of both as-sintered and deformed samples using scanning electron microscopy (SEM) and an optical microscope equipped with an image analyzer [6]. Results and Discussion Zirconia-Dispersed Alumina The relative densities of the sintered bodies of 10 vol% TZ-dispersed Al2O3 sintered at 1673 K for 2 h prepared by colloidal processing (sample C) and by dry processing (sample D) were 99.6% and 99.5%, respectively. The lower sintering temperature resulted in fine-grained microstructures with an average Al2O3 grain size of 0.45 m in both samples. The sintering temperature was about 100 K lower and the grain size about half that in earlier studies [7,8]. The aging of sample D at 1773 K resulted in an alumina grain size of 0.94 m and relative densities higher than 99.7% (sample D⬘). The creep parameters defined by the following semi-empirical equation were evaluated at a true strain rate of 0.1 [9,10]: Strain rate ⫽ And⫺p, where A is a material constant, and d are the stress and the average alumina grain size, n is the stress exponent, and p is the grain size exponent. No difference was seen in the creep parameters of samples C and D, where n ⫽ 2.0, p ⫽ 2.0, and the activation energy was 750 kJ/mol. Previous studies have shown that the reduction of alumina grain size from 1.5 to about 1.0 m decreases the level of flow stress and results in the enhancement of elongation independently of the processing used [10]. Fig. 1 compares the tensile flow curves of the three samples. In the dry processed samples (D and D⬘), the reduction of grain size from 0.94 to 0.45 m decreases the flow stress, but does not lead to a further increase in the tensile ductility. In the colloidally processed sample (C), however, the tensile ductility reaches 550% when the initial grain size is reduced to 0.45 m. The grain growth, density, and creep parameters of C and D are almost identical. Hence, other microstructural factors must be involved in the difference in elongation. Table 1 summarizes TZ particle dispersion in the sintered samples [10]. For the same volume fraction of TZ particles, the average second-phase diameter dZ in sample C is smaller than that in sample D, and the numerical density of the particles NvZ is about two times higher in C than in D. The ratio of the particle to grain junction densities is defined as fP ⫽ NgZ/Ng, where Ng is the number of four grain junctions per unit volume. The fP value is a measure of the pinning efficiency against the
Vol. 44, Nos. 8/9
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TABLE 1 Evaluation of Zirconia Particle Dispersion of the Three Samples Samples
Density (%)
dA (m)
dZ (m)
NvZ (m3)
fP
C D D’
99.6 99.5 99.7
0.45 0.45 0.94
0.07 0.12 0.20
142 61 13
1.00 0.45 0.83
grain growth. The fP value predicts that grain growth in D will tend to be more active than that in C until the grain size reaches that in sample D⬘. Another difference was observed in the size distribution and fraction of residual cavities. The numerical densities of cavities in C (area fraction: 0.15%) were less than 40% of those in D (area fraction: 0.29%). The total area fraction of residual cavities differs by only 0.14%, which closely agrees with the data of density measurement, although higher numerical densities of cavities should lead to the acceleration of damage accumulation [2]. The uniform dispersion of fine TZ particles and reduced numerical density of residual cavities are additional prerequisties for large tensile ductility in TZ-dispersed alumina nanocomposite. Alumina-Dispersed TZ The green density after slip casting (about 50%) of well-dispersed Al2O3-TZ suspensions was increased by CIP treatment at 400 MPa (about 55%). The green compact obtained showed narrow pore size distribution, resulting in low-temperature sintering and high density without large pores. When the Al2O3 addition was lower than 7.7 vol%, densification started at a lower temperature compared to the TZ without Al2O3 addition [3]. This indicates that a small amount Al2O3 addition can decrease the sintering temperature and time required. In fact, sintering at 1573 K for 2 h was sufficient to obtain dense polycrystals having a theoretical density exceeding 99%. This temperature is about 100 K lower than the temperature used in conventional dry processing. The grain size of 0.3 vol% Al2O3-dispersed TZ was 0.23 m, and no amorphous phases were observed along grain boundaries nor at multiple junctions. Fig. 2 shows the effect of Al2O3 addition on the tensile elongation at an initial strain rate of 1.3 ⫻ 10⫺4 (undoped) or 1.9 ⫻ 10⫺4 (alumina added) s⫺1, and flow stress at a true strain rate of 0.1 at 1573 K. A significant decrease in the flow stress and increase in the tensile ductility of the TZ samples were confirmed with a small amount of Al2O3 addition. When the temperature was increased to 1723 K, the
Figure 2. Effect of alumina additions on tensile flow stress and engineering strain.
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tensile ductility of 0.3 vol% Al2O3-dispersed TZ reached about 1000% along the gauge portion. With regard to the high strain-rate superplasticity, tensile ductility exceeding 300% was observed even at an initial strain rate of 1.2 ⫻ 10⫺2 s⫺1 at 1723 K. The stress exponent of 0.3 vol% Al2O3-dispersed TZ was evaluated as 2.2 at 1673 K and the apparent activation energy as 519 kJ/mol [11]. These values are consistent with previous data of n ⫽ 2 and Q ⫽ 480 kJ/mol for conventionally processed samples in which the initial grain size was 0.4 m [12]. The grain size refinement to 0.23 m by colloidal processing did not affect the creep parameters. Some factors can be assumed to contribute to the enhanced tensile ductility. One is enhanced cation diffusion in the Al2O3-dispersed TZ. A second factor is the refinement of grain size to 0.23 m, since the reduced grain size resulted in a decrease in the level of flow stress. A third factor is the decrease in initial defects in the case of samples prepared by the colloidal processing as described in TZdispersed Al2O3. Conclusion TZ-dispersed Al2O3 and Al2O3-dispersed TZ nanocomposites were prepared by colloidal processing and low-temperature sintering. Large tensile elongation exceeding 550% can be obtained for 10 vol% TZ-dispersed Al2O3 and 1000% for 0.3 vol% Al2O3-dispersed TZ. Their creep parameters were similar to those of samples prepared by conventional dry processing. The excellent superplasticities achieved are due to the reduction of residual defects, and the fine-grained and homogeneous microstructures. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
I-W. Chen and L. A. Xue, J. Am. Ceram. Soc. 44, 2585 (1990). K. Hiraga and K. Nakano, Mater. Sci. Forum. 234 –235, 387 (1997). Y. Sakka and K. Hiraga, Nippon Kagaku Kaishi. 1999, 497 (1999) (in Japanese). J. Cesaramo III, I. A. Aksay, and A. Bleier, J. Am. Ceram. Soc. 71, 250 (1988). T. S. Suzuki, Y. Sakka, K. Nakano, and K. Hiraga, Mater. Trans. JIM. 39, 689 (1998). K. Hiraga, K. Nakano, T. S. Suzuki, and Y. Sakka, Scripta Mater. 39, 1273 (1998). K. Okada and T. Sakuma, J. Am. Ceram. Soc. 79, 499 (1990). K. Nakano, T. S. Suzuki, K. Hiraga, and Y. Sakka, Scripta Mater. 38, 33 (1998). T. S. Suzuki, Y. Sakka, K. Nakano, and K. Hiraga, Mater. Sci. Forum, 304 –306, 489 (1999). K. Hiraga, T. S. Suzuki, Y. Sakka, and K. Nakano, ICM8, vol. III, p. 981 (1999). T. S. Suzuki, Y. Sakka, K. Nakano, and K. Hiraga, Scripta Mater. in press. E. Sato, H. Morioka, K. Kuribayashi, and D. Sundaraman, J. Mater. Sci. 34, 4511 (1999).