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Al2O3 nanocomposites
Materials Science and Engineering A356 (2003) 443 /446 www.elsevier.com/locate/msea
Spark-plasma-sintered BaTiO3/Al2O3 nanocomposites Guo-Dong Zhan, Joshua Kuntz, Julin Wan, Javier Garay, Amiya K. Mukherjee * Department of Chemical Engineering and Materials Science, University of California at Davis, One Shields Avenue, Davis, CA 95616-5294, USA Received 18 March 2002; received in revised form 12 September 2002
Abstract Using spark plasma sintering (SPS), BaTiO3/Al2O3 nanocomposites were successfully consolidated to more than 99% of theoretical density at a sintering temperature as low as 1150 8C in only 3 min. The processing methods for these dense nanocomposites where the retained grain size of alumina matrix was in the nanometer level were developed. The maximum volume content of BaTiO3 in the nanocrystalline matrix for toughening was around 15 vol.%. A significant increase in fracture toughness up to 5.36 MPa1/2 has been achieved in the 7.5 vol.% BaTiO3/Al2O3 nanocomposite. The toughening mechanism might be related to ferroelastic domain switching of ferroelectric phase in these nanocomposites. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Alumina nanocomposite; Spark plasma sintering; Toughening
1. Introduction The sintering of nanocrystalline ceramics is an exciting theme in materials research because such bulk nanocrystalline ceramics exhibit novel properties and functions. Much progress has been made in consolidating nanocrystalline powders by a number of consolidation methods during the past several years. However, these studies have highlighted the problem of consolidating these nanopowders into full dense ceramics without excessive grain growth [1 /6]. Therefore, searching for a new processing technique that requires shorter duration could be the ideal choice. Spark plasma sintering (SPS), a fast consolidation technique that can enhance sintering kinetics and reduce the time available for grain growth, has been used in the present study. It is a pressure-assisted sintering method based on the shortlived generation of high-temperature spark plasma at the interfaces between powder particles. The basic configuration of an SPS system consists of a sintering die with a uniaxial pressurization mechanism, specially designed punch electrodes, vacuum chamber with vacuum atmosphere control, a DC-pulse generator, and control units. During SPS processing, the powder * Corresponding author. Fax: /1-530-752-9554. E-mail address: [email protected] (A.K. Mukherjee).
surfaces are cleaned and activated, and the material is transferred at both the micro and macro levels. Thus, a high quality sintered compact is obtained at a lower temperature and in a shorter time than conventional sintering [7]. On the other hand, nanocrystalline ceramics do not appear to possess high fracture toughness, as was anticipated [8]. Therefore, research on processing fully dense bulk nanocomposites that retain nanocrystalline grain size in matrix and possess moderate fracture toughness as well, is still a challenging problem. Domain switching as a toughening mechanism has been recognized in ferroelectric materials where either an applied compressive stress or electrical field led to domain switching [9]. This behavior has been demonstrated by the facts that anisotropic fracture toughness was observed in these poled materials and the fracture toughness depends on the volume fraction of domains that are aligned favorably in front of the crack tip [10 /13]. R curve behavior due to stress-induced ferroelastic domain switching was also found in BaTiO3 [14]. Moreover, the contribution to toughening due to domain switching in zirconia was almost three times that of the intrinsic toughness [15]. Recently, a new approach for toughening of ceramics has been proposed and investigated where piezoelectric and ferroelectric second phases were incorporated into the ceramic matrix as toughening
0921-5093/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5093(02)00812-2
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phase and energy dissipation by the piezoelectric effect was suggested as a new toughening mechanism [16,17]. In the present study, BaTiO3 was selected as a model ferroelectric toughening second phase for the present study because it has been extensively characterized with regards to its ferroelectric and ferroelastic properties. This paper will report the microstructure, mechanical properties, and toughening mechanisms in BaTiO3/ Al2O3 nanocomposites consolidated by this novel processing technique.
magnification over 600 k /. Grain sizes were estimated from high-resolution SEM of fractured surfaces. Fracture toughness (KIC) was measured by indentation techniques. Indentation tests were performed on a Wilson Tukon hardness tester with a diamond Vickers indenter. The indentation parameters for fracture toughness (KIC) were a 1.5 kg load with a dwell of 15 s. The following equation, proposed by Antis et al., [18] was used for the calculation: 1=2 E P KIC 0:016 (1) Hv c3=2
2. Experimental procedures
where E , Hv, P and c represent Young’s modulus, Vickers hardness, the applied indentation load, and the half-length of the radial crack, respectively.
Cubic BaTiO3 nanopowders were provided by Cabot Corporation, which were prepared by hydrothermal reaction of barium hydroxide (Ba(OH)2) with titanium hydroxide (Ti(OH)4). The average particle size is 60 nm. The pure a-Al2O3 nanopowder used in the present study had an average particle size of /50 nm (obtained from Baikowski International, Charlotte, NC) and surface area of 30 m2 g 1. The gas condensation synthesized gAl2O3 with an average particle size of 32 nm was obtained from Nanophase Technologies Corporation (Darien, IL 60651). The BaTiO3 powders at different volume contents were mixed with the a-Al2O3 nanopowder for 24 h in ethanol using zirconia ball media. A high-energy ball-milling method was used to prepare the starting g-Al2O3 nanopowders. This is that the g-Al2O3 and BaTiO3 powders are high-energy ball-milled with a WC ball and vial set for 24 h. In order to prevent severe powder agglomeration one weight percent polyvinyl alcohol (PVA), a dry milling agent, was added. The PVA is removed after ball-milling through a 350 8C heat treatment in vacuum. SPS was carried out under vacuum in a Dr. Sinter 1050 SPS apparatus (Sumitomo Coal Mining Co., Japan). The powder mixtures were placed into a graphite die (20 mm in inner diameter) and cold-pressed at 200 MPa to green-body with /57% of theoretical density. The SPS processing parameters used in the present study were as follows: (1) an applied pressure of 63 MPa, (2) the heating rate of 200 8C min 1 from 600 8C to the desired temperatures, (3) the pulse duration time of 12 ms and the interval between pulse of 2 ms, and (4) the pulse current of /2000 A and a voltage of 10 V. The temperature was monitored with an optical pyrometer that was focused on the ‘nonthrough’ hole (0.5 mm in diameter and 2 mm in depth) of the graphite die. The final densities of the sintered compacts were determined by the Archimedes’ method with deionized water as the immersion medium. The theoretical densities of the specimens were calculated according to the rule of mixtures. The microstructural observation and microanalysis were carried out using an FEI XL30-SFEG high-resolution scanning electron microscopy with a resolution better than 2 nm and
3. Results and discussion The relative densities for all the SPS materials are given in Table 1. It can be noted that all the materials could be consolidated by SPS at 1150 8C only for 3 min to get almost full density. This is quite different from the pressureless-sintered BaTiO3/Al2O3 composites where the sintering temperatures were higher than 1450 8C but the maximum bulk density obtained was just 92% of the theoretical density of alumina [16]. The microstructures of the fractured surface in the pure alumina and BaTiO3/a-Al2O3 nanocomposites in the present study are shown in Fig. 1. It is very interesting to note that the pure a-Al2O3 consolidated by SPS exhibited a mixture of fracture modes (Fig. 1(a)). This is different from the conventionally sintered monolithic alumina exhibiting intergranular fracture. However, the fracture modes are mainly intergranular in these BaTiO3/Al2O3 nanocomposites, as shown in Fig. 1(b) to (e) for 5, 7.5, 10, and 15 vol.% BaTiO3/Al2O3 nanocomposites, respectively. It is obvious that the microstructures consisted of nanoscale grain sizes in these sintered nanocomposites. It can also be noted that a dramatic grain growth occurred for the pure BaTiO3 with grain size up to 15 mm (Fig. 1(f)). These results demonstrate the effectiveness of SPS over conventional method in obtaining nanocrystalline alumina matrix nanocomposites at quite lower temperatures and shorter sintering duration resulting in high density and nanosized grain size. Moreover, it is very interesting to note that the grain size for the 7.5 vol.%BaTiO3/g-Al2O3 nanocomposite through high-energy ball-milling was as small as 190 nm. It is much finer than 7.5 vol.%BaTiO3/a-Al2O3 nanocomposite without high-energy ball-milling, suggesting that high-energy ball-milling procedure can lead to more refined structure. Table 1 also summarizes the fracture toughness for the present materials. In comparison to other alumina
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445
Table 1 Physical and fracture toughness of BaTiO3/Al2O3 nanocomposites consolidated by SPS at 1150 8C per 3 min Material
Relative density (%)
Mean grain size (nm)
Pure a-Al2O3 5 vol.%BaTiO3/a-Al2O3 7.5 vol.%BaTiO3/a-Al2O3 7.5 vol.%BaTiO3/g-Al2O3 10 vol.%BaTiO3/a-Al2O3 15 vol.%BaTiO3/a-Al2O3 Pure BaTiO3
99.8 99.5 99.6 99.2 99.8 99.9 99.9
3499/10 3689/19 2569/13 1909/15 2819/13 3269/18 15 6359/1969
nanocomposites [8] and pure alumina in the present work, a significant improvement in fracture toughness was observed in the present nanocomposite materials.
Fracture toughness (MPa m1/2) 3.309/0.14 4.749/0.31 5.369/0.38 5.269/0.34 4.989/0.13 4.349/0.39
Fig. 2 shows the relationship between toughness and BaTiO3 volume contents. It can be seen that the fracture toughness increases with increasing BaTiO3 content and
Fig. 1. High-resolution scanning electron micrographs of fractured surfaces for; (a) pure a-Al2O3, (b) 5 vol.%BaTiO3/Al2O3, (c) 7.5 vol.%BaTiO3/ Al2O3 (d) 10 vol.%BaTiO3/Al2O3, (e) 15 vol.%BaTiO3/Al2O3, and (f) pure BaTiO3 nanocomposites consolidated by spark-plasma-sintering at 1150 8C for 3 min.
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contents of BaTiO3 are less than 10 vol.%. A significant increase in fracture toughness up to 5.36 MPa m1/2 was achieved in the 7.5 vol.%BaTiO3/Al2O3 nanocomposite.
Acknowledgements This work was supported by a grant (#G-DAAD 1900-1-0185) from US Army Research Office with Dr William Mullins as the Program Manager. Fig. 2. Relationship between fracture toughness and BaTiO3 contents in spark-plasma-sintered BaTiO3/Al2O3 nanocomposites.
reachs a maximum at 7.5 vol.%. More than 1.6 times increase in fracture toughness over the pure nanocrystalline alumina has been achieved in the 7.5 vol.%BaTiO3/ Al2O3 nanocomposite, suggesting that adding ferroelectric phase into nanocrystalline alumina is effective for toughening. In regard to the particle toughening, it could be ruled out due to the fact that grain size for the second phase is in the nano-region for the present study. Thus, the contributions to toughening by crack bridging and crack deflection due to the second phase is likely to be very small. Therefore, there may be a toughening effect related to stress-induced domain switching toughening of the ferroelectric second phase [19,20].
4. Conclusions SPS to almost theoretical density at a quite low temperature of 1150 8C for only 3 min could successfully consolidate BaTiO3/Al2O3 nanocomposites. The 7.5 vol.%BaTiO3/g-Al2O3 nanocomposite with a mean gain size of alumina matrix as small as 190 nm could be obtained through high-energy ball-milling process. Fracture toughness depends on the contents of BaTiO3 in the nanocrystalline alumina matrix. The optimum
References [1] H. Gleiter, Prog. Mater. Sci. 33 (1989) 223. [2] R.S. Mishra, J.A. Schneider, J.F. Shackelford, A.K. Mukherjee, NanoStruct. Mater. 5 (1995) 525. [3] E.J. Gonzalez, B. Hockey, G.J. Piermarini, Mater. Manufacturing Processes 11 (1996) 951. [4] M.J. Mayo, NanoStruct. Mater. 9 (1997) 717. [5] S. Bhaduri, S.B. Bhaduri, NanoStruct. Mater. 8 (1997) 755. [6] S.-C. Liao, Y.-J. Chen, B.H. Kear, W.E. Mayo, NanoStruct. Mater. 10 (1998) 1063. [7] M. Tokita, J. Soc. Powder Technol. Jpn. 30 (1993) 790 /804. [8] R.S. Mishra, C.E. Lesher, A.K. Mukherjee, in: D.L. Bourell (Ed.), Synthesis and Processing of Nanocrystalline Powder, TMS, 1996, p. 173. [9] K. Mehta, A.V. Virkar, J. Am. Ceram. Soc. 73 (1990) 567. [10] Z. Zhang, R. Raj, J. Am. Ceram. Soc. 78 (1995) 3363. [11] C.S. Lynch, Acta Mater. 46 (1998) 599. [12] R.A. Pferner, G. Thurn, F. Aldinger, Mater. Chem. Phys. 61 (1999) 24. [13] M.J. Busche, K.J. Hsia, Scripta Mater. 44 (2001) 207. [14] F. Meschke, A. Kolleck, G.A. Schneider, J. Eur. Ceram. Soc. 17 (1997) 1143. [15] A.V. Virkar, R.L.K. Matsumoto, J. Am. Ceram. Soc. 69 (1986) c224. [16] B. Yang, X.M. Chen, Mater. Lett. 33 (1997) 237. [17] G.-D. Zhan, J. Kuntz, J. Wan, J. Garay, A.K. Mukherjee, Scripta Mater. (revised, 2001). [18] G.R. Antis, P. Chantikul, B.R. Lawn, D.B. Marshall, J. Am. Ceram. Soc. 64 (1981) 533. [19] G. Winfield, F. Azough, R. Freer, Ferroelectrics 133 (1992) 181. [20] G.H. Haertling, J. Am. Ceram. Soc. 82 (1999) 797.
Spark-plasma-sintered BaTiO3/Al2O3 nanocomposites Guo-Dong Zhan, Joshua Kuntz, Julin Wan, Javier Garay, Amiya K. Mukherjee * Department of Chemical Engineering and Materials Science, University of California at Davis, One Shields Avenue, Davis, CA 95616-5294, USA Received 18 March 2002; received in revised form 12 September 2002
Abstract Using spark plasma sintering (SPS), BaTiO3/Al2O3 nanocomposites were successfully consolidated to more than 99% of theoretical density at a sintering temperature as low as 1150 8C in only 3 min. The processing methods for these dense nanocomposites where the retained grain size of alumina matrix was in the nanometer level were developed. The maximum volume content of BaTiO3 in the nanocrystalline matrix for toughening was around 15 vol.%. A significant increase in fracture toughness up to 5.36 MPa1/2 has been achieved in the 7.5 vol.% BaTiO3/Al2O3 nanocomposite. The toughening mechanism might be related to ferroelastic domain switching of ferroelectric phase in these nanocomposites. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Alumina nanocomposite; Spark plasma sintering; Toughening
1. Introduction The sintering of nanocrystalline ceramics is an exciting theme in materials research because such bulk nanocrystalline ceramics exhibit novel properties and functions. Much progress has been made in consolidating nanocrystalline powders by a number of consolidation methods during the past several years. However, these studies have highlighted the problem of consolidating these nanopowders into full dense ceramics without excessive grain growth [1 /6]. Therefore, searching for a new processing technique that requires shorter duration could be the ideal choice. Spark plasma sintering (SPS), a fast consolidation technique that can enhance sintering kinetics and reduce the time available for grain growth, has been used in the present study. It is a pressure-assisted sintering method based on the shortlived generation of high-temperature spark plasma at the interfaces between powder particles. The basic configuration of an SPS system consists of a sintering die with a uniaxial pressurization mechanism, specially designed punch electrodes, vacuum chamber with vacuum atmosphere control, a DC-pulse generator, and control units. During SPS processing, the powder * Corresponding author. Fax: /1-530-752-9554. E-mail address: [email protected] (A.K. Mukherjee).
surfaces are cleaned and activated, and the material is transferred at both the micro and macro levels. Thus, a high quality sintered compact is obtained at a lower temperature and in a shorter time than conventional sintering [7]. On the other hand, nanocrystalline ceramics do not appear to possess high fracture toughness, as was anticipated [8]. Therefore, research on processing fully dense bulk nanocomposites that retain nanocrystalline grain size in matrix and possess moderate fracture toughness as well, is still a challenging problem. Domain switching as a toughening mechanism has been recognized in ferroelectric materials where either an applied compressive stress or electrical field led to domain switching [9]. This behavior has been demonstrated by the facts that anisotropic fracture toughness was observed in these poled materials and the fracture toughness depends on the volume fraction of domains that are aligned favorably in front of the crack tip [10 /13]. R curve behavior due to stress-induced ferroelastic domain switching was also found in BaTiO3 [14]. Moreover, the contribution to toughening due to domain switching in zirconia was almost three times that of the intrinsic toughness [15]. Recently, a new approach for toughening of ceramics has been proposed and investigated where piezoelectric and ferroelectric second phases were incorporated into the ceramic matrix as toughening
0921-5093/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5093(02)00812-2
444
G.-D. Zhan et al. / Materials Science and Engineering A356 (2003) 443 /446
phase and energy dissipation by the piezoelectric effect was suggested as a new toughening mechanism [16,17]. In the present study, BaTiO3 was selected as a model ferroelectric toughening second phase for the present study because it has been extensively characterized with regards to its ferroelectric and ferroelastic properties. This paper will report the microstructure, mechanical properties, and toughening mechanisms in BaTiO3/ Al2O3 nanocomposites consolidated by this novel processing technique.
magnification over 600 k /. Grain sizes were estimated from high-resolution SEM of fractured surfaces. Fracture toughness (KIC) was measured by indentation techniques. Indentation tests were performed on a Wilson Tukon hardness tester with a diamond Vickers indenter. The indentation parameters for fracture toughness (KIC) were a 1.5 kg load with a dwell of 15 s. The following equation, proposed by Antis et al., [18] was used for the calculation: 1=2 E P KIC 0:016 (1) Hv c3=2
2. Experimental procedures
where E , Hv, P and c represent Young’s modulus, Vickers hardness, the applied indentation load, and the half-length of the radial crack, respectively.
Cubic BaTiO3 nanopowders were provided by Cabot Corporation, which were prepared by hydrothermal reaction of barium hydroxide (Ba(OH)2) with titanium hydroxide (Ti(OH)4). The average particle size is 60 nm. The pure a-Al2O3 nanopowder used in the present study had an average particle size of /50 nm (obtained from Baikowski International, Charlotte, NC) and surface area of 30 m2 g 1. The gas condensation synthesized gAl2O3 with an average particle size of 32 nm was obtained from Nanophase Technologies Corporation (Darien, IL 60651). The BaTiO3 powders at different volume contents were mixed with the a-Al2O3 nanopowder for 24 h in ethanol using zirconia ball media. A high-energy ball-milling method was used to prepare the starting g-Al2O3 nanopowders. This is that the g-Al2O3 and BaTiO3 powders are high-energy ball-milled with a WC ball and vial set for 24 h. In order to prevent severe powder agglomeration one weight percent polyvinyl alcohol (PVA), a dry milling agent, was added. The PVA is removed after ball-milling through a 350 8C heat treatment in vacuum. SPS was carried out under vacuum in a Dr. Sinter 1050 SPS apparatus (Sumitomo Coal Mining Co., Japan). The powder mixtures were placed into a graphite die (20 mm in inner diameter) and cold-pressed at 200 MPa to green-body with /57% of theoretical density. The SPS processing parameters used in the present study were as follows: (1) an applied pressure of 63 MPa, (2) the heating rate of 200 8C min 1 from 600 8C to the desired temperatures, (3) the pulse duration time of 12 ms and the interval between pulse of 2 ms, and (4) the pulse current of /2000 A and a voltage of 10 V. The temperature was monitored with an optical pyrometer that was focused on the ‘nonthrough’ hole (0.5 mm in diameter and 2 mm in depth) of the graphite die. The final densities of the sintered compacts were determined by the Archimedes’ method with deionized water as the immersion medium. The theoretical densities of the specimens were calculated according to the rule of mixtures. The microstructural observation and microanalysis were carried out using an FEI XL30-SFEG high-resolution scanning electron microscopy with a resolution better than 2 nm and
3. Results and discussion The relative densities for all the SPS materials are given in Table 1. It can be noted that all the materials could be consolidated by SPS at 1150 8C only for 3 min to get almost full density. This is quite different from the pressureless-sintered BaTiO3/Al2O3 composites where the sintering temperatures were higher than 1450 8C but the maximum bulk density obtained was just 92% of the theoretical density of alumina [16]. The microstructures of the fractured surface in the pure alumina and BaTiO3/a-Al2O3 nanocomposites in the present study are shown in Fig. 1. It is very interesting to note that the pure a-Al2O3 consolidated by SPS exhibited a mixture of fracture modes (Fig. 1(a)). This is different from the conventionally sintered monolithic alumina exhibiting intergranular fracture. However, the fracture modes are mainly intergranular in these BaTiO3/Al2O3 nanocomposites, as shown in Fig. 1(b) to (e) for 5, 7.5, 10, and 15 vol.% BaTiO3/Al2O3 nanocomposites, respectively. It is obvious that the microstructures consisted of nanoscale grain sizes in these sintered nanocomposites. It can also be noted that a dramatic grain growth occurred for the pure BaTiO3 with grain size up to 15 mm (Fig. 1(f)). These results demonstrate the effectiveness of SPS over conventional method in obtaining nanocrystalline alumina matrix nanocomposites at quite lower temperatures and shorter sintering duration resulting in high density and nanosized grain size. Moreover, it is very interesting to note that the grain size for the 7.5 vol.%BaTiO3/g-Al2O3 nanocomposite through high-energy ball-milling was as small as 190 nm. It is much finer than 7.5 vol.%BaTiO3/a-Al2O3 nanocomposite without high-energy ball-milling, suggesting that high-energy ball-milling procedure can lead to more refined structure. Table 1 also summarizes the fracture toughness for the present materials. In comparison to other alumina
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445
Table 1 Physical and fracture toughness of BaTiO3/Al2O3 nanocomposites consolidated by SPS at 1150 8C per 3 min Material
Relative density (%)
Mean grain size (nm)
Pure a-Al2O3 5 vol.%BaTiO3/a-Al2O3 7.5 vol.%BaTiO3/a-Al2O3 7.5 vol.%BaTiO3/g-Al2O3 10 vol.%BaTiO3/a-Al2O3 15 vol.%BaTiO3/a-Al2O3 Pure BaTiO3
99.8 99.5 99.6 99.2 99.8 99.9 99.9
3499/10 3689/19 2569/13 1909/15 2819/13 3269/18 15 6359/1969
nanocomposites [8] and pure alumina in the present work, a significant improvement in fracture toughness was observed in the present nanocomposite materials.
Fracture toughness (MPa m1/2) 3.309/0.14 4.749/0.31 5.369/0.38 5.269/0.34 4.989/0.13 4.349/0.39
Fig. 2 shows the relationship between toughness and BaTiO3 volume contents. It can be seen that the fracture toughness increases with increasing BaTiO3 content and
Fig. 1. High-resolution scanning electron micrographs of fractured surfaces for; (a) pure a-Al2O3, (b) 5 vol.%BaTiO3/Al2O3, (c) 7.5 vol.%BaTiO3/ Al2O3 (d) 10 vol.%BaTiO3/Al2O3, (e) 15 vol.%BaTiO3/Al2O3, and (f) pure BaTiO3 nanocomposites consolidated by spark-plasma-sintering at 1150 8C for 3 min.
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contents of BaTiO3 are less than 10 vol.%. A significant increase in fracture toughness up to 5.36 MPa m1/2 was achieved in the 7.5 vol.%BaTiO3/Al2O3 nanocomposite.
Acknowledgements This work was supported by a grant (#G-DAAD 1900-1-0185) from US Army Research Office with Dr William Mullins as the Program Manager. Fig. 2. Relationship between fracture toughness and BaTiO3 contents in spark-plasma-sintered BaTiO3/Al2O3 nanocomposites.
reachs a maximum at 7.5 vol.%. More than 1.6 times increase in fracture toughness over the pure nanocrystalline alumina has been achieved in the 7.5 vol.%BaTiO3/ Al2O3 nanocomposite, suggesting that adding ferroelectric phase into nanocrystalline alumina is effective for toughening. In regard to the particle toughening, it could be ruled out due to the fact that grain size for the second phase is in the nano-region for the present study. Thus, the contributions to toughening by crack bridging and crack deflection due to the second phase is likely to be very small. Therefore, there may be a toughening effect related to stress-induced domain switching toughening of the ferroelectric second phase [19,20].
4. Conclusions SPS to almost theoretical density at a quite low temperature of 1150 8C for only 3 min could successfully consolidate BaTiO3/Al2O3 nanocomposites. The 7.5 vol.%BaTiO3/g-Al2O3 nanocomposite with a mean gain size of alumina matrix as small as 190 nm could be obtained through high-energy ball-milling process. Fracture toughness depends on the contents of BaTiO3 in the nanocrystalline alumina matrix. The optimum
References [1] H. Gleiter, Prog. Mater. Sci. 33 (1989) 223. [2] R.S. Mishra, J.A. Schneider, J.F. Shackelford, A.K. Mukherjee, NanoStruct. Mater. 5 (1995) 525. [3] E.J. Gonzalez, B. Hockey, G.J. Piermarini, Mater. Manufacturing Processes 11 (1996) 951. [4] M.J. Mayo, NanoStruct. Mater. 9 (1997) 717. [5] S. Bhaduri, S.B. Bhaduri, NanoStruct. Mater. 8 (1997) 755. [6] S.-C. Liao, Y.-J. Chen, B.H. Kear, W.E. Mayo, NanoStruct. Mater. 10 (1998) 1063. [7] M. Tokita, J. Soc. Powder Technol. Jpn. 30 (1993) 790 /804. [8] R.S. Mishra, C.E. Lesher, A.K. Mukherjee, in: D.L. Bourell (Ed.), Synthesis and Processing of Nanocrystalline Powder, TMS, 1996, p. 173. [9] K. Mehta, A.V. Virkar, J. Am. Ceram. Soc. 73 (1990) 567. [10] Z. Zhang, R. Raj, J. Am. Ceram. Soc. 78 (1995) 3363. [11] C.S. Lynch, Acta Mater. 46 (1998) 599. [12] R.A. Pferner, G. Thurn, F. Aldinger, Mater. Chem. Phys. 61 (1999) 24. [13] M.J. Busche, K.J. Hsia, Scripta Mater. 44 (2001) 207. [14] F. Meschke, A. Kolleck, G.A. Schneider, J. Eur. Ceram. Soc. 17 (1997) 1143. [15] A.V. Virkar, R.L.K. Matsumoto, J. Am. Ceram. Soc. 69 (1986) c224. [16] B. Yang, X.M. Chen, Mater. Lett. 33 (1997) 237. [17] G.-D. Zhan, J. Kuntz, J. Wan, J. Garay, A.K. Mukherjee, Scripta Mater. (revised, 2001). [18] G.R. Antis, P. Chantikul, B.R. Lawn, D.B. Marshall, J. Am. Ceram. Soc. 64 (1981) 533. [19] G. Winfield, F. Azough, R. Freer, Ferroelectrics 133 (1992) 181. [20] G.H. Haertling, J. Am. Ceram. Soc. 82 (1999) 797.