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Synthesis of nanograined ZrO2-based composites by chemical processing and pulse electric current sintering
January 1999
Materials Letters 38 Ž1999. 18–21
Synthesis of nanograined ZrO 2-based composites by chemical processing and pulse electric current sintering Masashi Yoshimura
a,)
, Tatsuki Ohji b, Mutsuo Sando b, Yong-Ho Choa c , Tohru Sekino c , Koichi Niihara c
a
Fine Ceramics Research Association, 1-1 Hirate-cho, Kita-ku, Nagoya 462-8510, Japan National Industrial Research Institute of Nagoya, 1-1 Hirate-cho, Kita-ku, Nagoya 462-8510, Japan The Institute of Scientific and Industrial Research, Osaka UniÕersity, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan b
c
Received 6 April 1998; revised 10 June 1998; accepted 15 June 1998
Abstract Nanograined yttrium-stabilized ZrO 2rAl 2 O 3 composites were fabricated by a pulse electric current sintering ŽPECS. method. The starting powders with sizes less than 10 nm were successfully synthesized by the chemical process. The composites sintered at temperatures above 13008C showed the relative density above 98%. Examination by transmission electron microscopy ŽTEM. showed that the yttrium-stabilized ZrO 2r10 mol% Al 2 O 3 composite consisted of the homogeneous and very fine matrix grains with sizes less than 100 nm. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Chemical processing; Pulse electric current sintering; Nanograined material; ZrO 2 rAl 2 O 3 composites
1. Introduction Nanograined or nanophase materials which are polycrystals mainly consisting of structural elements with crystallites of a few nanometers have recently been the subject of research, motivated by their unusual physical and mechanical properties w1–3x. The distinctive properties of those materials are high self- and solute-diffusivity w4x, enhanced solute-solubility w2x and low-temperature plasticity w5x. For nanocrystalline ceramics, one notable system show-
)
Corresponding author
ing this plasticity, for example, is porous ZrO 2 containing 3 mol% Y2O3 consolidated by pressureless sintering w6x. The material with a grain size of 80 nm exhibited a strain rate 34 times faster than that with a grain size of 300 nm at 11508C under the same stress. In spite of these unique and enhanced properties, the problem is the difficulty to retain the initial nanostructural features during the consolidation stage w3,7x. In order to fabricate the dense specimen with nanosized grains, generally there are two methods used; one is to incorporate second-phase dispersoids to matrix. The grain boundary pinning effect of the second phase particles can suppress coarsening of
00167-577Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 8 . 0 0 1 2 5 - 6
M. Yoshimura et al.r Materials Letters 38 (1999) 18–21
19
grains. Examples are additions of SiC to Al 2 O 3 w8x and nanograined AlN to Cu of the same size w9x. The other approach is the use of pressure sintering with a fast heating rate w10x; by this way, one can pass quickly through the surface diffusion regime where coarsening is active and proceed to the boundary or lattice diffusion regime where densification mechanisms operate. Control of processing based on these approaches therefore can offer an unique route for fabrication of dense composites with nanosized grains. We used pulse electric current sintering ŽPECS. method for fast heating rate. In the present study, the optimum condition for synthesis of nanograined yttrium-stabilized ZrO 2r Al 2 O 3 composites is described. Also, the effects of nanograined Al 2 O 3 dispersoid on the microstructure and density are discussed.
2. Experimental procedure Fig. 1. XRD profiles for calcined powder mixtures at different temperatures for 1 h. Ža. 3Y-ZrO 2 and Žb. 3Y-ZrO 2 r10 mol% Al 2 O 3 .
ZrO 2r10 mol% Al 2 O 3 nanocomposite powders were prepared using a sol–gel synthesis technique with isopropoxide precursors. A total of 3 mol%
Fig. 2. Microstructural analysis by TEM of 3Y-ZrO 2r10 mol% Al 2 O 3 powder mixture after calcination at 9008C for 1 h. Ža. TEM image and Žb. EDS spectrum for powder mixure using the spot size of 5 nm.
20
M. Yoshimura et al.r Materials Letters 38 (1999) 18–21
Fig. 3. Effect of sintering temperature and Al 2 O 3 content on relative density of 3Y-ZrO 2 rAl 2 O 3 and 3Y-ZrO 2 powder compacts.
Y2O3 was used to stabilize ZrO 2 . ZrŽOC 3 H 7 .4 , YŽOC 3 H 7 .4 and AlŽOC 3 H 7 . 3 as precursors were dissolved into isopropanol and boiled for 24 h. The mixed solution was hydrolyzed by ammonium solution, and then filtered and dried. The composite powder was calcined and then deagglomerated by ball milling. The calcined powder mixture was sintered by a PECS method using a spark plasma sintering system ŽModel; SPS-2080, Sumitomo Coals Mining. at 11008 to 15008C for 10 min in vacuum under a pressure of 50 MPa. For comparison, commercial 3Y-ZrO 2 ŽTosoh. was sintered under the same conditions. The heating rate of 1008Crmin was used from room temperature to sintering temperatures. Bulk density was measured using Archimedes’ principle in toluene. Phase identification was carried out by X-ray diffraction ŽXRD. analysis. Microstructure was observed by transmission electron microscopy ŽTEM..
Fig. 4. TEM photographs of the 3Y-ZrO 2 and 3Y-ZrO 2 rAl 2 O 3 composites prepared by chemical process. Ža. 3Y-ZrO 2 sintered at 11008C, Žb. 3Y-ZrO 2 r10 mol% Al 2 O 3 at 11008C, Žc. 3Y-ZrO 2 at 13008C and Žd. 3Y-ZrO 2 r10 mol% Al 2 O 3 at 13008C.
M. Yoshimura et al.r Materials Letters 38 (1999) 18–21
3. Results and discussion Fig. 1 shows the XRD profiles of the 3Y-ZrO 2 and 3Y-ZrO 2r10 mol% Al 2 O 3 powder mixtures. In both cases, the XRD patterns before calcination exhibited the presence of amorphous phases. The peaks of the crystallized t-ZrO 2 in 3Y-ZrO 2 mixture was typically first identified around 6008C and became more obvious at higher temperatures ŽFig. 1a.. However, in the case of 10 mol% Al 2 O 3 powder-added mixture, the crystallization temperature was shifted to higher temperature than 7008C, as shown in Fig. 1b. Also, it showed that there was no evidence of crystalline a-Al 2 O 3 up to 13008C. This result suggests that Al 2 O 3 which dissolved into ZrO 2 w11x might retard the crystallization of ZrO 2 phase and then ZrO 2 existed in a polycrystalline form and after crystallization of ZrO 2 might be presented in the form of poorly crystalline or very small domain size up to 13008C. TEM photograph and EDX analysis using the spot size of 5 nm for 3Y-ZrO 2r10 mol% Al 2 O 3 powder mixture after calcination at 9008C for 1 h are shown in Fig. 2a and b, respectively. The TEM image shows that most of the particles have a uniform size and are smaller than 10 nm. The EDX analysis exhibited clear peaks of Al, Y and Zr from any detected site. The intensities of Al, Y and Zr in the EDX analysis are obtained from detected sites. These results indicate that the homogeneous powder mixture with nanosized particles could be obtained by this process. Fig. 3 shows the effects of sintering temperatures and Al 2 O 3 contents on the relative density of 3YZrO 2 powder mixtures. At a sintering temperature of 11008C, the relative density of the composites showed lager values than that of commercial 3Y-ZrO 2 . Also, increase in Al 2 O 3 content resulted in decrease of the relative density in composites prepared by chemical process. However, all composites sintered at this temperature showed the relative density below 90%. Fully dense Ž) 98%. composites were prepared at temperatures above 13008C in which the relative density of composites prepared by chemical process showed almost the same values. Fig. 4 shows the TEM microstructures for 3YZrO 2 and 3Y-ZrO 2r10 mol% Al 2 O 3 composites
21
sintered at 1100 and 13008C. As expected from the results of Fig. 3, increasing the sintering temperature from 11008 to 13008C resulted in more dense microstructures. One interesting result is that the fully dense composites with Al 2 O 3 as second phase exhibited extremely fine matrix grain size less than 100 nm, while the monolithic 3Y-ZrO 2 showed the matrix grain size larger than 130 nm, as shown in Fig. 4c and d, respectively. The finer matrix grain size in the composite is most likely due to the effective role of Al 2 O 3 inclusions as a grain growth inhibitor for the matrix grains w12x. This result strongly suggests that the fully dense composites having nanograin matrix can be obtained by the addition of second phase using chemical processing and the sintering with rapid heating rate by PECS.
Acknowledgements This work has been carried out as part of the Synergy Ceramics Project under the Industrial Science and Technology Frontier ŽISTF. Program promoted by AIST, MITI, Japan. Under this program, part of the work has been funded through NEDO. The authors are members of the Joint Research Consortium of Synergy Ceramics.
References w1x K. Niihara, J. Ceram. Soc. Jpn. 99 Ž1991. 974–982. w2x H. Gleiter, Prog. Mater. Sci. 33 Ž1989. 223–315. w3x M.Y. Mayo, International Materials Reviews 41 Ž3. Ž1996. 85–115. w4x R. Birringer, H. Hahn, H. Hofier, J. Karch, H. Gleiter, Defect Diffus. Forum 59 Ž1988. 17–32. w5x J. Karch, R. Birringer, Ceram. Int. 16 Ž1990. 291–294. w6x M.J. Mayo, Material & Design 14 Ž6. Ž1993. 323–329. w7x G. Skandan, H. Hahn, M. Roddy, W.R. Canon, J. Am. Ceram. Soc. 77 Ž7. Ž1994. 1706–1710. w8x A. Nakahira, K. Niihara, J. Ceram. Soc. Jpn. 100 Ž4. Ž1992. 448–453. w9x M. Udaka, K. Kawasaki, T. Yamazaki, M. Umemoto, J. Jpn. Inst. Metals 60 Ž6. Ž1996. 607–615. w10x T. Nishimura, M. Mitomo, H. Hirotsuru, M. Kawahara, J. Mater. Sci. Lett. 14 Ž1995. 1046–1047. w11x O. Yamaguchi, M. Shirai, M. Yoshinaka, J. Am. Ceram. Soc. 71 Ž12. Ž1988. C510–C512. w12x C.S. Smith, Trans. Metal. Soc. AIME 175 Ž1948. 15–51.
Materials Letters 38 Ž1999. 18–21
Synthesis of nanograined ZrO 2-based composites by chemical processing and pulse electric current sintering Masashi Yoshimura
a,)
, Tatsuki Ohji b, Mutsuo Sando b, Yong-Ho Choa c , Tohru Sekino c , Koichi Niihara c
a
Fine Ceramics Research Association, 1-1 Hirate-cho, Kita-ku, Nagoya 462-8510, Japan National Industrial Research Institute of Nagoya, 1-1 Hirate-cho, Kita-ku, Nagoya 462-8510, Japan The Institute of Scientific and Industrial Research, Osaka UniÕersity, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan b
c
Received 6 April 1998; revised 10 June 1998; accepted 15 June 1998
Abstract Nanograined yttrium-stabilized ZrO 2rAl 2 O 3 composites were fabricated by a pulse electric current sintering ŽPECS. method. The starting powders with sizes less than 10 nm were successfully synthesized by the chemical process. The composites sintered at temperatures above 13008C showed the relative density above 98%. Examination by transmission electron microscopy ŽTEM. showed that the yttrium-stabilized ZrO 2r10 mol% Al 2 O 3 composite consisted of the homogeneous and very fine matrix grains with sizes less than 100 nm. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Chemical processing; Pulse electric current sintering; Nanograined material; ZrO 2 rAl 2 O 3 composites
1. Introduction Nanograined or nanophase materials which are polycrystals mainly consisting of structural elements with crystallites of a few nanometers have recently been the subject of research, motivated by their unusual physical and mechanical properties w1–3x. The distinctive properties of those materials are high self- and solute-diffusivity w4x, enhanced solute-solubility w2x and low-temperature plasticity w5x. For nanocrystalline ceramics, one notable system show-
)
Corresponding author
ing this plasticity, for example, is porous ZrO 2 containing 3 mol% Y2O3 consolidated by pressureless sintering w6x. The material with a grain size of 80 nm exhibited a strain rate 34 times faster than that with a grain size of 300 nm at 11508C under the same stress. In spite of these unique and enhanced properties, the problem is the difficulty to retain the initial nanostructural features during the consolidation stage w3,7x. In order to fabricate the dense specimen with nanosized grains, generally there are two methods used; one is to incorporate second-phase dispersoids to matrix. The grain boundary pinning effect of the second phase particles can suppress coarsening of
00167-577Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 8 . 0 0 1 2 5 - 6
M. Yoshimura et al.r Materials Letters 38 (1999) 18–21
19
grains. Examples are additions of SiC to Al 2 O 3 w8x and nanograined AlN to Cu of the same size w9x. The other approach is the use of pressure sintering with a fast heating rate w10x; by this way, one can pass quickly through the surface diffusion regime where coarsening is active and proceed to the boundary or lattice diffusion regime where densification mechanisms operate. Control of processing based on these approaches therefore can offer an unique route for fabrication of dense composites with nanosized grains. We used pulse electric current sintering ŽPECS. method for fast heating rate. In the present study, the optimum condition for synthesis of nanograined yttrium-stabilized ZrO 2r Al 2 O 3 composites is described. Also, the effects of nanograined Al 2 O 3 dispersoid on the microstructure and density are discussed.
2. Experimental procedure Fig. 1. XRD profiles for calcined powder mixtures at different temperatures for 1 h. Ža. 3Y-ZrO 2 and Žb. 3Y-ZrO 2 r10 mol% Al 2 O 3 .
ZrO 2r10 mol% Al 2 O 3 nanocomposite powders were prepared using a sol–gel synthesis technique with isopropoxide precursors. A total of 3 mol%
Fig. 2. Microstructural analysis by TEM of 3Y-ZrO 2r10 mol% Al 2 O 3 powder mixture after calcination at 9008C for 1 h. Ža. TEM image and Žb. EDS spectrum for powder mixure using the spot size of 5 nm.
20
M. Yoshimura et al.r Materials Letters 38 (1999) 18–21
Fig. 3. Effect of sintering temperature and Al 2 O 3 content on relative density of 3Y-ZrO 2 rAl 2 O 3 and 3Y-ZrO 2 powder compacts.
Y2O3 was used to stabilize ZrO 2 . ZrŽOC 3 H 7 .4 , YŽOC 3 H 7 .4 and AlŽOC 3 H 7 . 3 as precursors were dissolved into isopropanol and boiled for 24 h. The mixed solution was hydrolyzed by ammonium solution, and then filtered and dried. The composite powder was calcined and then deagglomerated by ball milling. The calcined powder mixture was sintered by a PECS method using a spark plasma sintering system ŽModel; SPS-2080, Sumitomo Coals Mining. at 11008 to 15008C for 10 min in vacuum under a pressure of 50 MPa. For comparison, commercial 3Y-ZrO 2 ŽTosoh. was sintered under the same conditions. The heating rate of 1008Crmin was used from room temperature to sintering temperatures. Bulk density was measured using Archimedes’ principle in toluene. Phase identification was carried out by X-ray diffraction ŽXRD. analysis. Microstructure was observed by transmission electron microscopy ŽTEM..
Fig. 4. TEM photographs of the 3Y-ZrO 2 and 3Y-ZrO 2 rAl 2 O 3 composites prepared by chemical process. Ža. 3Y-ZrO 2 sintered at 11008C, Žb. 3Y-ZrO 2 r10 mol% Al 2 O 3 at 11008C, Žc. 3Y-ZrO 2 at 13008C and Žd. 3Y-ZrO 2 r10 mol% Al 2 O 3 at 13008C.
M. Yoshimura et al.r Materials Letters 38 (1999) 18–21
3. Results and discussion Fig. 1 shows the XRD profiles of the 3Y-ZrO 2 and 3Y-ZrO 2r10 mol% Al 2 O 3 powder mixtures. In both cases, the XRD patterns before calcination exhibited the presence of amorphous phases. The peaks of the crystallized t-ZrO 2 in 3Y-ZrO 2 mixture was typically first identified around 6008C and became more obvious at higher temperatures ŽFig. 1a.. However, in the case of 10 mol% Al 2 O 3 powder-added mixture, the crystallization temperature was shifted to higher temperature than 7008C, as shown in Fig. 1b. Also, it showed that there was no evidence of crystalline a-Al 2 O 3 up to 13008C. This result suggests that Al 2 O 3 which dissolved into ZrO 2 w11x might retard the crystallization of ZrO 2 phase and then ZrO 2 existed in a polycrystalline form and after crystallization of ZrO 2 might be presented in the form of poorly crystalline or very small domain size up to 13008C. TEM photograph and EDX analysis using the spot size of 5 nm for 3Y-ZrO 2r10 mol% Al 2 O 3 powder mixture after calcination at 9008C for 1 h are shown in Fig. 2a and b, respectively. The TEM image shows that most of the particles have a uniform size and are smaller than 10 nm. The EDX analysis exhibited clear peaks of Al, Y and Zr from any detected site. The intensities of Al, Y and Zr in the EDX analysis are obtained from detected sites. These results indicate that the homogeneous powder mixture with nanosized particles could be obtained by this process. Fig. 3 shows the effects of sintering temperatures and Al 2 O 3 contents on the relative density of 3YZrO 2 powder mixtures. At a sintering temperature of 11008C, the relative density of the composites showed lager values than that of commercial 3Y-ZrO 2 . Also, increase in Al 2 O 3 content resulted in decrease of the relative density in composites prepared by chemical process. However, all composites sintered at this temperature showed the relative density below 90%. Fully dense Ž) 98%. composites were prepared at temperatures above 13008C in which the relative density of composites prepared by chemical process showed almost the same values. Fig. 4 shows the TEM microstructures for 3YZrO 2 and 3Y-ZrO 2r10 mol% Al 2 O 3 composites
21
sintered at 1100 and 13008C. As expected from the results of Fig. 3, increasing the sintering temperature from 11008 to 13008C resulted in more dense microstructures. One interesting result is that the fully dense composites with Al 2 O 3 as second phase exhibited extremely fine matrix grain size less than 100 nm, while the monolithic 3Y-ZrO 2 showed the matrix grain size larger than 130 nm, as shown in Fig. 4c and d, respectively. The finer matrix grain size in the composite is most likely due to the effective role of Al 2 O 3 inclusions as a grain growth inhibitor for the matrix grains w12x. This result strongly suggests that the fully dense composites having nanograin matrix can be obtained by the addition of second phase using chemical processing and the sintering with rapid heating rate by PECS.
Acknowledgements This work has been carried out as part of the Synergy Ceramics Project under the Industrial Science and Technology Frontier ŽISTF. Program promoted by AIST, MITI, Japan. Under this program, part of the work has been funded through NEDO. The authors are members of the Joint Research Consortium of Synergy Ceramics.
References w1x K. Niihara, J. Ceram. Soc. Jpn. 99 Ž1991. 974–982. w2x H. Gleiter, Prog. Mater. Sci. 33 Ž1989. 223–315. w3x M.Y. Mayo, International Materials Reviews 41 Ž3. Ž1996. 85–115. w4x R. Birringer, H. Hahn, H. Hofier, J. Karch, H. Gleiter, Defect Diffus. Forum 59 Ž1988. 17–32. w5x J. Karch, R. Birringer, Ceram. Int. 16 Ž1990. 291–294. w6x M.J. Mayo, Material & Design 14 Ž6. Ž1993. 323–329. w7x G. Skandan, H. Hahn, M. Roddy, W.R. Canon, J. Am. Ceram. Soc. 77 Ž7. Ž1994. 1706–1710. w8x A. Nakahira, K. Niihara, J. Ceram. Soc. Jpn. 100 Ž4. Ž1992. 448–453. w9x M. Udaka, K. Kawasaki, T. Yamazaki, M. Umemoto, J. Jpn. Inst. Metals 60 Ž6. Ž1996. 607–615. w10x T. Nishimura, M. Mitomo, H. Hirotsuru, M. Kawahara, J. Mater. Sci. Lett. 14 Ž1995. 1046–1047. w11x O. Yamaguchi, M. Shirai, M. Yoshinaka, J. Am. Ceram. Soc. 71 Ž12. Ž1988. C510–C512. w12x C.S. Smith, Trans. Metal. Soc. AIME 175 Ž1948. 15–51.