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Spark plasma sintering and mechanical properties of ZrO2(Y2O3)–Al2O3 composites
March 2000
yamoto
Materials Letters 43 Ž2000. 27–31 www.elsevier.comrlocatermatlet
Spark plasma sintering and mechanical properties of ZrO 2 žY2 O 3 / –Al 2 O 3 composites Jinsheng Hong a,) , Lian Gao a , S.D.D.L. Torre b, Hiroki Miyamoto b, Key Miyamoto b a
State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China b Technology Research Institute of Osaka Prefecture, Ayumino 2-7-1, Osaka 594-1157, Japan Received 18 December 1998; received in revised form 4 August 1999; accepted 23 September 1999
Abstract Spark plasma sintering ŽSPS. was conducted on nanocrystalline ZrO 2 ŽY2 O 3 . –20 mol% Al 2 O 3 powder at a heat rate of 6008Crmin with a short holding time. Full density was obtained at sintering temperatures ) 13008C. Considerable grain growth occurred relative to the initial powder particles, but smaller grain size and higher density can be obtained as compared to hot-pressing. High flexural strength and fracture toughness were also achieved for the SPS-resulted composite. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction In many ceramic systems processed by sintering of powder compacts, both full density and finegrained structure in final product are desirable. However, simultaneous attainment of these two objectives appears to be a dilemma, because grain-coarsening always follow the densification during sintering w1x. Therefore, sintering processes that require shorter duration may be an ideal choice. Recently, interest has been growing in the use of spark plasma sintering ŽSPS. to consolidate ceramic compacts w2–4x. The SPS is a new process that provides means by which ceramic powder can be sintered very rapidly to full density. It is similar to hot-pressing, which is carried out in a graphite die, but the heating is accomplished by electric discharge in voids between )
Corresponding author.
particles w5x. The discharges are generated by an instantaneous pulsed direct current applied through electrodes at the top and bottom punches of the graphite. Due to these discharges, the particle surface is instantaneously activated and purified, and concurrently self-heating phenomena are generated among these particles, leading to heat-transfer and masstransfer to be completed in an extremely short time w5x. Therefore, the rapid densification can be accomplished by these microscopic heating processes. In addition, it is shown that SPS allows the maximum density to be obtained at a temperature of about 150–2008C lower than hot-pressing w5,6x. In the present work, an SPS processing with an extremely high heating rate and short sintering duration has been carried out on the ZrO 2 ŽY2 O 3 . –20 mol% Al 2 O 3 composites. The densification and mechanical properties of the sintered composite will be examined.
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 2 2 5 - 6
28
J. Hong et al.r Materials Letters 43 (2000) 27–31
2. Experimental procedures Nanocrystalline ZrO 2 Ž3.0 Y2 O 3 . –20 mol% Al 2 O 3 powder was used in this investigation. The powder was prepared by a chemical precipitation method. An ammonium hydroxide solution was added to a water solution containing zirconium, aluminium and yttrium cations so that zirconium, aluminium and yttrium hydroxides were immediately precipitated. The precipitates were washed and filtered out and then calcined at 5008C for 2 h to produce the composite powder. BET-specific surface area of the calcined powder was measured using an automatic volumetric gas sorption analyser ŽAUTOSORB-1, Quantachrome, USA.. Powder particle size distribution was measured with an ultrafine particle analyser ŽMicrotrac UPA150, Leeds and Northrup, USA. SPS was carried out in a vacuum chamber using an SPS apparatus ŽDr. Sinter 1020, Sumimoto Coal Mining, Japan., the details of which were represented elsewhere w2 3x. The powders were inner placed into a 20-mm-diameter graphite die and then heated to the desired sintering at a heating rate of approximately 6008Crmin. A pressure of 40 MPa was applied from the start and retained to the sintering temperature. The sample was held for 2 min at the sintering temperature. Immediately after holding, the sintering sample was cooled to a temperature below 6008C within 2–3 min. The sintering temperature was measured by means of an optical pyrometer focused on the die surface, which was centred on the sintering sample. The sintered samples were approximately 20 mm in diameter and 5 mm thick. Densities of sintered samples were measured using the Archimedes’ method with distilled water as the immersion medium. For bending strength measurement, sintered samples were cut and ground into bars with a dimension of 4 = 3 = 18 mm3, and then each bar was polished with 600-grit SiC on the side, which would experience tension stress during testing, and was levelled along its length to eliminate edge flaws. The strength measurement was conducted with mechanical tester ŽShimadza AG-20KNG. using a three-point bending method with a span length of 10 mm and a crosshead speed of 0.5 mmrmin. Fracture toughness and Vickers hardness measurements were made on the polished surface by a Vickers tester ŽAKASHI AVK-C2. with a load of 20 kg and a
holding time of 25 s. Microstructures of fracture surfaces were observed using a scanning electron microscope ŽXL 20, Philips.. The grain size of the sintered composite samples was measured by mean linear intercept method using photographs taken with a Philips XL 20 SEM. Phase identification was conducted by an X-ray diffraction ŽRINT 2500 VHF, Rigaku, Japan., using Cu K a radiation at a scan speed of 48 Ž2 u . of min.
3. Results and discussion The ZrO 2 ŽY2 O 3 . –20 mol% Al 2 O 3 powder was calcined at 5008C for 2 h and X-ray analysis revealed this powder consisted of tetragonal ZrO 2 only. No sign of Al 2 O 3 crystallization was found even if the calcination temperature increased up to 13008C. It appears that there is high solid solubility of Al 2 O 3 in ZrO 2 ŽY2 O 3 .. This solubility has also been seen in the ZrO 2 –Al 2 O 3 system w7x. In the present SPS study, the powder calcined at 5008C was used as the starting powder. BET analysis revealed that the powder had an extraordinarily high specific surface area of 280 m2rg. Assuming the powder particles are spherical, the calculated particle size was about 15 nm. Fig. 1 shows the particle size distribution of the powder. It can been seen that the powder had a broad particle size distribution up to 10 mm and an average particle size of 0.7 mm. Thus, it can be concluded that the powder was composed of a large number of aggregates, and the aggregates, in turn, consisted of nanosized particles. Fig. 2 shows the density Žrelative to theoretical density. and the average grain size as a function of sintering temperature for the ZrO 2 ŽY2 O 3 . –Al 2 O 3
Fig. 1. Particle size distribution of the calcined ZrO 2 ŽY2 O 3 . –20 mol% Al 2 O 3 powder.
J. Hong et al.r Materials Letters 43 (2000) 27–31
Fig. 2. Relative density and average grain size vs. sintering temperature for the ZrO 2 ŽY2 O 3 . –20 mol% Al 2 O 3 composites sintered by SPS.
composites sintered by SPS. The density of the sample sintered at 13008C exceeded 99% of theoretical, and at temperatures G 13508C, all samples approached the theoretical density. Fig. 3a–d show representative SEM micrographs of fracture surfaces of the samples described in Fig. 2. The grain size is
29
small Žapproximately 60 nm. for the 12508C sample. At 13008C, the microstructure became coarser and the grain size increased to approximately 180 nm. At temperatures from 13008C to 14508C, there is a sluggish grain growth and the grain size increased from 170 to 250 nm only. At temperatures ) 14508C, the grain size rapidly increased with increasing temperature to the value of 450 nm for the 15008C sample. Even if X-ray analysis revealed that all samples sintered at G 12508C consisted of tetragonal ZrO 2 , a-Al 2 O 3 and a few monoclinic ZrO 2 Žas shown in Fig. 4., the microstructural distinction between ZrO 2 grains and Al 2 O 3 grains is not clear. Therefore, it is not possible to differentiate one from the other, as indicated in Figs. 3a–d, and the grain size described in Fig. 2 is the average value of both ZrO 2 and Al 2 O 3 . One can see that the grain size at all sintering temperatures is much greater than the starting powder particles, indicating that both rapid heating rate and short sintering duration do not lead to a drastic decrease of the grain growth.
Fig. 3. SEM micrographs of fracture surfaces of the ZrO 2 ŽY2 O 3 . –20 mol% Al 2 O 3 composites sintered by SPS at Ža. 12508C, Žb. 13008C, Žc. 14008C, and Žd. 15008C.
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J. Hong et al.r Materials Letters 43 (2000) 27–31
Fig. 4. X-ray diffraction patterns of the ZrO 2 ŽY2 O 3 . –20 mol% Al 2 O 3 composites sintered by SPS at various sintering temperatures.
For a comparison, the calcined powder was sintered by hot-pressing at 14008C for 45 min at a heating rate of 508Crmin under a load of 30 MPa. The sample sintered to 98% of the theoretical density, and the grains coarsened to 400–500 nm. It is obvious from the present result that SPS resulted in relatively higher density and a clear decrease in grain size as compared with hot-pressing. Fig. 5 shows bending strength and fracture toughness of the ZrO 2 ŽY2 O 3 . –Al 2 O 3 composites sintered by SPS as a function of sintering temperature. The flexural strength increased with increasing temperature, but leveled off to the value of 1100 MPa at 14508C, whereas the fracture toughness did not al-
Fig. 6. Vickers hardness vs. sintering temperature for the ZrO 2 ŽY2 O 3 . –20 mol% Al 2 O 3 composites sintered by SPS.
most vary with the sintering temperature, being on the same level of approximately 6.6 MPa P m1r2 . Fig. 6 shows the relationship between Vickers hardness and sintering temperature for the sintered composite. It is clear that the Vickers hardness almost remained constant in the dense samples from 13008C to 14008C, while at temperatures ) 14008C, hardness slightly decreased with increasing temperature. This is probably caused by the influence of grain size on the hardness of the sintered composite, that is, coarser grain sizes lead to lower hardness w8x.
4. Conclusions Nanocrystalline ZrO 2 ŽY2 O 3 . –20 mol% Al 2 O 3 powder was densified by SPS at extremely high heating rate with short sintering duration. Full density and high mechanical properties were obtained for the sintered composite samples. Such fast SPS resulted in a high densification and a reduced grain growth as compared to hot-pressing, even if drastic grain growth is still present during sintering.
References
Fig. 5. Flexural strength and fracture toughness vs. sintering temperature for the ZrO 2 ŽY2 O 3 . –20 mol% Al 2 O 3 composites sintered by SPS.
w1x D.-J. Chen, M.J. Mayo, J. Am. Ceram. Soc. 79 Ž1996. 906– 912. w2x L. Gao, J.S. Hong, H. Miyamoto, S.D.D.L. Torre, J. Inorg. Mater. 12 Ž1998. 18–22. w3x L. Gao, H. Miyamoto, J. Inorg. Mater. 12 Ž1997. 129–133.
J. Hong et al.r Materials Letters 43 (2000) 27–31 w4x S.D.D.L. Torre, H. Miyamoto, K. Miyamoto, J.S. Hong, L. Gao, L. Tonoco-D, E. Rocha-R, H. Balmori-R, Proceedings in 6th International Symposium on Ceramic Materials and Components for Engines, Arita, Saga, Japan, Oct. 19–22, 1997. w5x M. Torida, J. Soc. Powder Technol., Jpn. 30 Ž1993. 790–804. w6x N. Tamari, T. Tanaka, K. Tanaka, I. Kondoh, J. Ceram. Soc. Jpn. 103 Ž1995. 740–742.
31
w7x L. Gao, Q. Liu, J.S. Hong, H. Miyamoto, S.D.D.L. Torre, A. Kakitsuji, K. Liddell, D.P. Thompson, J. Mater. Sci. 33 Ž1998. 1399–1403. w8x R.S. Mishra, C.E. Lesher, A.K. Mukherjee, J. Am. Ceram. Soc. 79 Ž1996. 2989–2992.
yamoto
Materials Letters 43 Ž2000. 27–31 www.elsevier.comrlocatermatlet
Spark plasma sintering and mechanical properties of ZrO 2 žY2 O 3 / –Al 2 O 3 composites Jinsheng Hong a,) , Lian Gao a , S.D.D.L. Torre b, Hiroki Miyamoto b, Key Miyamoto b a
State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China b Technology Research Institute of Osaka Prefecture, Ayumino 2-7-1, Osaka 594-1157, Japan Received 18 December 1998; received in revised form 4 August 1999; accepted 23 September 1999
Abstract Spark plasma sintering ŽSPS. was conducted on nanocrystalline ZrO 2 ŽY2 O 3 . –20 mol% Al 2 O 3 powder at a heat rate of 6008Crmin with a short holding time. Full density was obtained at sintering temperatures ) 13008C. Considerable grain growth occurred relative to the initial powder particles, but smaller grain size and higher density can be obtained as compared to hot-pressing. High flexural strength and fracture toughness were also achieved for the SPS-resulted composite. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction In many ceramic systems processed by sintering of powder compacts, both full density and finegrained structure in final product are desirable. However, simultaneous attainment of these two objectives appears to be a dilemma, because grain-coarsening always follow the densification during sintering w1x. Therefore, sintering processes that require shorter duration may be an ideal choice. Recently, interest has been growing in the use of spark plasma sintering ŽSPS. to consolidate ceramic compacts w2–4x. The SPS is a new process that provides means by which ceramic powder can be sintered very rapidly to full density. It is similar to hot-pressing, which is carried out in a graphite die, but the heating is accomplished by electric discharge in voids between )
Corresponding author.
particles w5x. The discharges are generated by an instantaneous pulsed direct current applied through electrodes at the top and bottom punches of the graphite. Due to these discharges, the particle surface is instantaneously activated and purified, and concurrently self-heating phenomena are generated among these particles, leading to heat-transfer and masstransfer to be completed in an extremely short time w5x. Therefore, the rapid densification can be accomplished by these microscopic heating processes. In addition, it is shown that SPS allows the maximum density to be obtained at a temperature of about 150–2008C lower than hot-pressing w5,6x. In the present work, an SPS processing with an extremely high heating rate and short sintering duration has been carried out on the ZrO 2 ŽY2 O 3 . –20 mol% Al 2 O 3 composites. The densification and mechanical properties of the sintered composite will be examined.
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 2 2 5 - 6
28
J. Hong et al.r Materials Letters 43 (2000) 27–31
2. Experimental procedures Nanocrystalline ZrO 2 Ž3.0 Y2 O 3 . –20 mol% Al 2 O 3 powder was used in this investigation. The powder was prepared by a chemical precipitation method. An ammonium hydroxide solution was added to a water solution containing zirconium, aluminium and yttrium cations so that zirconium, aluminium and yttrium hydroxides were immediately precipitated. The precipitates were washed and filtered out and then calcined at 5008C for 2 h to produce the composite powder. BET-specific surface area of the calcined powder was measured using an automatic volumetric gas sorption analyser ŽAUTOSORB-1, Quantachrome, USA.. Powder particle size distribution was measured with an ultrafine particle analyser ŽMicrotrac UPA150, Leeds and Northrup, USA. SPS was carried out in a vacuum chamber using an SPS apparatus ŽDr. Sinter 1020, Sumimoto Coal Mining, Japan., the details of which were represented elsewhere w2 3x. The powders were inner placed into a 20-mm-diameter graphite die and then heated to the desired sintering at a heating rate of approximately 6008Crmin. A pressure of 40 MPa was applied from the start and retained to the sintering temperature. The sample was held for 2 min at the sintering temperature. Immediately after holding, the sintering sample was cooled to a temperature below 6008C within 2–3 min. The sintering temperature was measured by means of an optical pyrometer focused on the die surface, which was centred on the sintering sample. The sintered samples were approximately 20 mm in diameter and 5 mm thick. Densities of sintered samples were measured using the Archimedes’ method with distilled water as the immersion medium. For bending strength measurement, sintered samples were cut and ground into bars with a dimension of 4 = 3 = 18 mm3, and then each bar was polished with 600-grit SiC on the side, which would experience tension stress during testing, and was levelled along its length to eliminate edge flaws. The strength measurement was conducted with mechanical tester ŽShimadza AG-20KNG. using a three-point bending method with a span length of 10 mm and a crosshead speed of 0.5 mmrmin. Fracture toughness and Vickers hardness measurements were made on the polished surface by a Vickers tester ŽAKASHI AVK-C2. with a load of 20 kg and a
holding time of 25 s. Microstructures of fracture surfaces were observed using a scanning electron microscope ŽXL 20, Philips.. The grain size of the sintered composite samples was measured by mean linear intercept method using photographs taken with a Philips XL 20 SEM. Phase identification was conducted by an X-ray diffraction ŽRINT 2500 VHF, Rigaku, Japan., using Cu K a radiation at a scan speed of 48 Ž2 u . of min.
3. Results and discussion The ZrO 2 ŽY2 O 3 . –20 mol% Al 2 O 3 powder was calcined at 5008C for 2 h and X-ray analysis revealed this powder consisted of tetragonal ZrO 2 only. No sign of Al 2 O 3 crystallization was found even if the calcination temperature increased up to 13008C. It appears that there is high solid solubility of Al 2 O 3 in ZrO 2 ŽY2 O 3 .. This solubility has also been seen in the ZrO 2 –Al 2 O 3 system w7x. In the present SPS study, the powder calcined at 5008C was used as the starting powder. BET analysis revealed that the powder had an extraordinarily high specific surface area of 280 m2rg. Assuming the powder particles are spherical, the calculated particle size was about 15 nm. Fig. 1 shows the particle size distribution of the powder. It can been seen that the powder had a broad particle size distribution up to 10 mm and an average particle size of 0.7 mm. Thus, it can be concluded that the powder was composed of a large number of aggregates, and the aggregates, in turn, consisted of nanosized particles. Fig. 2 shows the density Žrelative to theoretical density. and the average grain size as a function of sintering temperature for the ZrO 2 ŽY2 O 3 . –Al 2 O 3
Fig. 1. Particle size distribution of the calcined ZrO 2 ŽY2 O 3 . –20 mol% Al 2 O 3 powder.
J. Hong et al.r Materials Letters 43 (2000) 27–31
Fig. 2. Relative density and average grain size vs. sintering temperature for the ZrO 2 ŽY2 O 3 . –20 mol% Al 2 O 3 composites sintered by SPS.
composites sintered by SPS. The density of the sample sintered at 13008C exceeded 99% of theoretical, and at temperatures G 13508C, all samples approached the theoretical density. Fig. 3a–d show representative SEM micrographs of fracture surfaces of the samples described in Fig. 2. The grain size is
29
small Žapproximately 60 nm. for the 12508C sample. At 13008C, the microstructure became coarser and the grain size increased to approximately 180 nm. At temperatures from 13008C to 14508C, there is a sluggish grain growth and the grain size increased from 170 to 250 nm only. At temperatures ) 14508C, the grain size rapidly increased with increasing temperature to the value of 450 nm for the 15008C sample. Even if X-ray analysis revealed that all samples sintered at G 12508C consisted of tetragonal ZrO 2 , a-Al 2 O 3 and a few monoclinic ZrO 2 Žas shown in Fig. 4., the microstructural distinction between ZrO 2 grains and Al 2 O 3 grains is not clear. Therefore, it is not possible to differentiate one from the other, as indicated in Figs. 3a–d, and the grain size described in Fig. 2 is the average value of both ZrO 2 and Al 2 O 3 . One can see that the grain size at all sintering temperatures is much greater than the starting powder particles, indicating that both rapid heating rate and short sintering duration do not lead to a drastic decrease of the grain growth.
Fig. 3. SEM micrographs of fracture surfaces of the ZrO 2 ŽY2 O 3 . –20 mol% Al 2 O 3 composites sintered by SPS at Ža. 12508C, Žb. 13008C, Žc. 14008C, and Žd. 15008C.
30
J. Hong et al.r Materials Letters 43 (2000) 27–31
Fig. 4. X-ray diffraction patterns of the ZrO 2 ŽY2 O 3 . –20 mol% Al 2 O 3 composites sintered by SPS at various sintering temperatures.
For a comparison, the calcined powder was sintered by hot-pressing at 14008C for 45 min at a heating rate of 508Crmin under a load of 30 MPa. The sample sintered to 98% of the theoretical density, and the grains coarsened to 400–500 nm. It is obvious from the present result that SPS resulted in relatively higher density and a clear decrease in grain size as compared with hot-pressing. Fig. 5 shows bending strength and fracture toughness of the ZrO 2 ŽY2 O 3 . –Al 2 O 3 composites sintered by SPS as a function of sintering temperature. The flexural strength increased with increasing temperature, but leveled off to the value of 1100 MPa at 14508C, whereas the fracture toughness did not al-
Fig. 6. Vickers hardness vs. sintering temperature for the ZrO 2 ŽY2 O 3 . –20 mol% Al 2 O 3 composites sintered by SPS.
most vary with the sintering temperature, being on the same level of approximately 6.6 MPa P m1r2 . Fig. 6 shows the relationship between Vickers hardness and sintering temperature for the sintered composite. It is clear that the Vickers hardness almost remained constant in the dense samples from 13008C to 14008C, while at temperatures ) 14008C, hardness slightly decreased with increasing temperature. This is probably caused by the influence of grain size on the hardness of the sintered composite, that is, coarser grain sizes lead to lower hardness w8x.
4. Conclusions Nanocrystalline ZrO 2 ŽY2 O 3 . –20 mol% Al 2 O 3 powder was densified by SPS at extremely high heating rate with short sintering duration. Full density and high mechanical properties were obtained for the sintered composite samples. Such fast SPS resulted in a high densification and a reduced grain growth as compared to hot-pressing, even if drastic grain growth is still present during sintering.
References
Fig. 5. Flexural strength and fracture toughness vs. sintering temperature for the ZrO 2 ŽY2 O 3 . –20 mol% Al 2 O 3 composites sintered by SPS.
w1x D.-J. Chen, M.J. Mayo, J. Am. Ceram. Soc. 79 Ž1996. 906– 912. w2x L. Gao, J.S. Hong, H. Miyamoto, S.D.D.L. Torre, J. Inorg. Mater. 12 Ž1998. 18–22. w3x L. Gao, H. Miyamoto, J. Inorg. Mater. 12 Ž1997. 129–133.
J. Hong et al.r Materials Letters 43 (2000) 27–31 w4x S.D.D.L. Torre, H. Miyamoto, K. Miyamoto, J.S. Hong, L. Gao, L. Tonoco-D, E. Rocha-R, H. Balmori-R, Proceedings in 6th International Symposium on Ceramic Materials and Components for Engines, Arita, Saga, Japan, Oct. 19–22, 1997. w5x M. Torida, J. Soc. Powder Technol., Jpn. 30 Ž1993. 790–804. w6x N. Tamari, T. Tanaka, K. Tanaka, I. Kondoh, J. Ceram. Soc. Jpn. 103 Ž1995. 740–742.
31
w7x L. Gao, Q. Liu, J.S. Hong, H. Miyamoto, S.D.D.L. Torre, A. Kakitsuji, K. Liddell, D.P. Thompson, J. Mater. Sci. 33 Ž1998. 1399–1403. w8x R.S. Mishra, C.E. Lesher, A.K. Mukherjee, J. Am. Ceram. Soc. 79 Ž1996. 2989–2992.