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Peculiar stress-induced phase transformation in YNbO4-modified ZrO2(3Y) using in situ compression–diffraction
January 2002
Materials Letters 52 Ž2002. 69–74 www.elsevier.comrlocatermatlet
Peculiar stress-induced phase transformation in YNbO4-modified ZrO 2 ž3Y/ using in situ compression–diffraction Sheng-Dih Yuh, Chen-Chia Chou ) Department of Mechanical Engineering, National Taiwan UniÕersity of Science and Technology, 43 Keelung Road, Section 4, Taipei 10660, Taiwan Received 29 January 2001; accepted 22 March 2001
Abstract The novel stress-induced transformation characteristics of YNbO4-modified ZrO 2 Ž3Y. ceramics were investigated using an in situ compression–diffraction technique employing synchrotron radiation. The observed stress-induced phase transformation of cubic-to-tetragonal andror cubic-to-orthorhombic plays the primary role on energy-absorbing mechanism in the present material system under an elastic cyclic loading process. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Zirconia; Yttrium niobate; Stress-induced phase transformation; X-ray diffraction; Synchrotron radiation
1. Introduction Although the tetragonal-to-monoclinic Žt-to-m. phase transformation toughening has been widely accepted as the dominant mechanism responsible for toughening in several zirconia-based ceramic materials w1,2x, recently, domain switching in the tetragonal phase is one of the other possible toughening mechanisms frequently proposed and discussed for various ceramics w3,4x. In addition to the well-known cubic Žc., tetragonal Žt. and monoclinic Žm. phases for zirconia-based ceramics, the sintered surfaces of these ceramics may undergo different types of transformations due to alloy compositions and external stimuli. For example, Hasegawa et al. w5x found a rhombohedral Žr. phase formed either in ion-implanted layers or on the abraded surfaces of Y-PSZ w6x. Kim et al. ) Corresponding author. Tel.: q886-2-2737-6493; fax: q886-22737-6460. E-mail address: [email protected] ŽC.-C. Chou..
w7x also reported r-phase formation in 3Y-TZP and 12Ce-TZP by grinding. Several authors w8x found a tX-phase in Y2 O 3 –ZrO 2 material systems with high Y2 O 3 contents that formed diffusionlessly from cZrO 2 during rapid cooling from high temperatures. Log et al. w9x fabricated tX-ZrO 2 doped with Y, Yb, Er or Dy using high-temperature annealing in the cubic stability regime and rapid cooling through the tetragonal stability regime. Michel et al. w10x found tX-phase in polydomain ZrO 2 single crystals doped with Y, Gd or Yb. Besides, some other works reported orthorhombic Žo. phases observed in zirconia-based ceramics. Three distinct o-phases were found under certain conditions: the oX-phase Žtermed OI., cotunnite-phase ŽPbCl 2 structure, termed OII. and simple o-phase in space groups of Pbca, Pnam and Pbc2 1 , respectively. The OI- and OII-phases are the very high-pressure forms of pure zirconia w11x. Haines et al. w12x found OII-phase of pure zirconia under high-temperature and high-pressure conditions, and after laser heating at 18 and 26.7 GPa w11x. Liu
00167-577Xr02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 3 6 8 - 8
70
S.-D. Yuh, C.-C. Chou r Materials Letters 52 (2002) 69–74
w13x and Block et al. w14x also found an OII-phase at pressures above 10 and 16.5 GPa, respectively. A simple o-phase was found w15x constantly such as Y, Mg, Ca-modified PSZ, ZTA, ŽMg, Y.-TZP. For Mg-PSZ, the o-phase on fracture surface, in thermally shocked, by quenching below y1008C, after cooling to cryogenic temperature, after tensile testing and after long aging at 11008C and 14008C has been reported w16x and references therein. The o-phase was also observed in TEM w15x. In this paper, we carried out in situ compression–diffraction experiments using synchrotron radiation and reported an
extraordinary phase transformation, nonconventional t-to-m, under cyclic compression in 5 mol% YNbO4-modified ZrO 2 Ž3Y. sintered at 15008C for 1 h. The origin of the energy-absorbing mechanism was also discussed.
2. Experimental procedure YNbO4 powder was prepared from Y2 O 3 Ž99.99%, Research Chemicals, USA. while Nb 2 O5 Ž99.9%, Meldform, UK. was prepared by an oxide-mixing
Fig. 1. Experimental set-up for an in situ X-ray compression–diffraction technique employing synchrotron radiation in the present studies.
S.-D. Yuh, C.-C. Chou r Materials Letters 52 (2002) 69–74
method followed by calcination at 11008C. The composition of the specimens was ZrO 2 Ž3Y. ŽHSY-3, Daiichi, Japan. with 5 mol% YNbO4 . The mixtures of YNbO4 and ZrO 2 Ž3Y. were sequentially processed by wet centrifugal ball milling, vacuum drying at 608C, calcining at 10008C and ball milling again. The resulting powders were then compressed using a cold isostatic press under a loading of 230 MPa to form thin rectangular pellets Ž20.5 = 14.5 = 2 mm.. The final specimens were obtained by sintering these pellets at 15008C in air for 1 h. An in situ uniaxial compression experiment was conducted employing synchrotron radiation because the conventional and even high-resolution X-ray diffraction Že.g., M18XHF, MAC Science, Japan was tried. cannot detect the variation in peak intensity of the specimen under elastic compression. Synchrotron radiation, which has high intensity and collimation, is a powerful tool for diffraction studies. The specimens were loaded using a custom-designed gripper and were bonded with a strain gage and then compressed uniaxially by a screw wrench. In situ highresolution X-ray diffraction–compression measurements were carried out at the wiggler beamline BL-17B at the Synchrotron Radiation Research Center, Hsinchu, Taiwan. The electron storage ring was operated at an energy of 1.5 GeV and at a current of 140–200 mA, which delivered 8.04 KeV X-ray photons with an estimated flux of 10 10 photonsrs for this experiment. The experimental set-up is shown in Fig. 1.
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3. Results and discussion Fig. 2 shows the diffraction patterns of 2 u from 278 to 758 of specimens ZrO 2 Ž3Y. –5 mol% YNbO4 sintered at 15008C. The specimen shows primarily the t- and c-phases accompanying small amounts of the m-phase. Fig. 3 shows the compression–diffraction pattern of the specimen sintered at 15008C at 2 u around Ža. 111 m , Žb. 2004 , Žc. 1314 and Žd. 0044 peaks under cyclic loading conditions with a maximum compression of 400 MPa for three cycles. Two peculiar behaviors were observed in Fig. 3. Firstly, as loading was applied at the first cycle, an extra peak arose at the right-hand side of 111 m and disappeared as loading was released; then, this extra peak arose again as loading was released at the second cycle and disappeared as loading was applied at the third cycle as shown in Fig. 3Ža. Žonly the diffraction patterns of the initial state, first loading step and second released step, are shown here, because other conditions had no significant change in peak intensity.. Secondly, at almost each cycle, for both cases where loading was applied and loading was released, the peak intensity of every c-peak Žthe index of the c-peak was labeled as a combination of c, o and tX phases, which will be discussed later. show significant variation. Meanwhile, scrutinizing the variation of peak intensity for the first loading cycle as loading compressed up to 400 MPa, the intensity of 020 c,o,tX increased, and that of the 131 c,o,tX and
Fig. 2. Diffraction patterns with 2 u from 278 to 758 of ZrO 2 Ž3Y. –5 mol% YNbO4 specimen sintered at 15008C for 1 h.
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S.-D. Yuh, C.-C. Chou r Materials Letters 52 (2002) 69–74
Fig. 3. In situ compression–diffraction patterns of 2 u around Ža. 111 m , Žb. 2004, Žc. 1314 and Žd. 0044 of specimen ZrO 2 Ž3Y. –5 mol% YNbO4 sintered at 15008C for 1 h, showing a peculiar stress-induced phase transformation. The x-axis is 2 u in degrees Ž8. and y-axis is intensity in arbitrary unit.
400 c,tXr014o , decreased. Furthermore, an extra peak arose at the right-hand side of 111 m . After loading was released, the intensity of 020 c,o,tX decreased and that of the 131 c,o,,tX decreased, obviously while 400 c,tXr014o showed nearly no change, and intriguingly, the extra peak disappeared. For the second loading cycle, as loading was applied again, the intensity of 020 c,o,tX increased pronouncedly to a maximum value among all cycles and intensities of the 131 c,o,tX and 400 c,tXr014o increased, and no extra
peak was found. After loading was released, the intensity of 020 c,o,tX deceased and that of both the 131 c,o,tX and 400 c,tXr014o increased significantly to a maximum, and intriguingly, the extra peak appeared with higher intensity than that in the first compressive cycle. At the last loading cycle, no extra peak appeared but 020 c,o,tX , 131 c,o,tX and 400 c,tXr014o still change obviously. The toughening mechanisms of zirconia-based ceramics had been reported to be attributed to a t-to-m
S.-D. Yuh, C.-C. Chou r Materials Letters 52 (2002) 69–74
phase transformation or to a ferroelastic domain switching of the t-phase of the specimens as cited in Section 1. However, the integral intensities of the tand m-peaks show nearly no change, as shown in Fig. 3 Žonly the 111 m and doublets of t-phase are shown here., indicating that the t-phase does not transform to the m-phase in the present material under such a loading condition. Virkar and Matsumoto w17x and Mehta et al. w18x found that the ratio of the peak intensity I Ž002 . t rI Ž200 . t and I Ž113. trI Ž131. t varied from smaller than 1 to greater than 1 after grinding, indicating that a process of domain switching effectively accomplishes through the rotation of the w001x-orientation by 908. However, no switching was observed for the doublets 002 tr200 t , 113tr311 t and 004tr400 t in the present situation as shown in Fig. 3Žb. to Žd. but an extra peak rises near 111 m , as shown in Fig. 3Ža.. Therefore, ferroelastic domain switching of the t-phase does not occur under such a loading condition in the present specimens. It is generally believed that the tX-phase, which is metastable, forms by rapid cooling from the c-phase of suitable solute content and the transformation was a displacive type w19x. However, it has been found that the metastable tetragonal tX-ZrO 2 does not necessarily form by rapid quenching from c-ZrO 2 but can be produced by slowly cooling conditions instead w20,21x. Several authors w22,23x reported the presence of a nonequilibrium tetragonal phase, tX , referred to as Anontransformable,B because of its reluctance to undergo the transformation to the m-phase. Jue and Virkar w24x showed that tX-ZrO 2 did not transform to m-ZrO 2 , and both the t-phase and tX-phase have toughnesses two to three times greater than the cphase, which is neither the t-to-m transformation, nor ferroelastic toughened. Sugiyama and Kubo w23x and Chaim et al. w25x reported that the tX-phase had heavy overlapping diffraction patterns with the cubic reflections, and that the tX-phase was described as a pseudocubic phase whose tetragonality Žcra ratio. is almost unity. Furthermore, Sanchez-Bajo et al. w26x reported a conclusive discrimination between c- and tX-phases by a careful application of the Rietveld method and demonstrated that the t q tX hypothesis is better than the t q c hypothesis in fitting the diffraction pattern at 2 u around 358. In addition to the aforementioned works, Heuer et al. w15x reported that
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most of the o-phases are referred to as a distorted fluorite structures except for the very high-pressure cotunnite phase. Marshall et al. w27x found that the o-ZrO 2 having lots of reflections overlapped with the c-phase. Therefore, we can conclude that the c-phase of the present material may coexist with tXandror o-phases after YNbO4 modification, the peak labeled as 400 c,tXr014o , which was split into two peaks at the initial condition, is good evidence shown in the insert block of Fig. 3Žd., and the tX andror o-phases may possibly be induced under compression as well. As shown in Fig. 3, we are able to see that the c-peaks vary their intensity strongly under different loading conditions, clearly indicating that there exists other energy-absorbing mechanisms in addition to the t-to-m phase transformation and domain switching of the t-phase. The observed stress-induced phase transformation may be attributed to the c-to-tX andror c-to-o phase transformations, as discussed above. The results shown in the present work imply that there is a novel toughening mechanism which absorbs the external loading energy effectively in the YNbO4-modified ZrO 2 Ž3Y. materials. 4. Conclusions The o- andror tX-phases may be formed by doping 5 mol% YNbO4 in ZrO 2 Ž3Y. ceramics and sintered at 15008C for 1 h. In situ compression–diffraction experiments show that the stress-induced phase transformation of c-to-tX andror c-to-o was probably the primary energy-absorbing mechanism in the YNbO4-modified ZrO 2 Ž3Y. material system under elastic cyclic compressions.
References w1x A.G. Evans, R.M. Cannon, Acta Metall. 34 Ž1986. 761, and references therein. w2x D.L. Porter, A.H. Heuer, J. Am. Ceram. Soc. 60 Ž1977. 183, and references therein. w3x M.G. Cain, M.H. Lewis, Mater. Lett. 9 Ž1990. 309, and references therein. w4x G.V. Srinivasan, J.F. Jue, S.Y. Kuo, A.V. Virkar, J. Am. Ceram. Soc. 72 Ž1989. 2098, and references therein. w5x H. Hasegawa, T. Hioki, O. Kamigaito, J. Mater. Sci. Lett. 4 Ž1985. 1092.
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w6x H. Hasegawa, J. Mater. Sci. Lett. 2 Ž1983. 91. w7x D.-J. Kim, H.-J. Jung, H.-J. Kim, J. Mater. Sci. Lett. 14 Ž1995. 285. w8x A.H. Heuer, R. Chaim, V. Lanteri, Acta Metall. 3 Ž1987. 661, and references therein. w9x T. Log, R.A. Cutler, J.F. Jue, A.V. Virkar, J. Mater. Sci. 28 Ž1993. 4503. w10x D. Michel, L. Mazerolles, M. Perez, Y. Jorba, J. Mater. Sci. 18 Ž1983. 2618. w11x J. Haines, L.M. Leger, A. Atouf, J. Am. Ceram. Soc. 78 Ž1995. 445, and references therein. w12x J. Haines, J.M. Leger, S. Hull, J.P. Petitet, A.S. Pereira, C.A. Perottoni, J.A.H. da Jornada, J. Am. Ceram. Soc. 80 Ž1997. 1910. w13x L.G. Liu, J. Phys. Chem. Solids 41 Ž1980. 331. w14x S. Block, J.A.H. Da Jornada, G.J. Piermarini, J. Am. Ceram. Soc. 68 Ž1985. 497. w15x A.H. Heuer, V. Lanter, S.C. Farmer, R. Chaim, R.-R. Lee, B.W. Kibble, R.M. Dickerson, J. Mater. Sci. 24 Ž1989. 124, and references therein. w16x E.H. Kisi, J. Am. Ceram. Soc. 81 Ž1998. 741, and references therein.
w17x A.V. Virkar, R.L.K. Matsumoto, J. Am. Ceram. Soc. 69 Ž1986. C-224. w18x K. Mehta, J.F. Jue, A.V. Virkar, J. Am. Ceram. Soc. 73 Ž1990. 1777. w19x A.H. Heuer, R. Chaim, V. Lanteri, Acta Metall. 35 Ž1987. 661, and references therein. w20x S.J. Lin, H.Y. Lu, P. Shen, J. Mater. Sci. 26 Ž1991. 112. w21x V. Lanteri, R. Chaim, A.H. Heuer, J. Am. Ceram. Soc. 69 Ž1986. C-258. w22x F. Sanchez-Bajo, F.L. Cumbrera, F. Guiberteau, A. Dominguez-Rodriguez, Mater. Lett. 15 Ž1992. 39. w23x M. Sugiyama, H. Kubo, in: S. Somiya, N. Yamamoto, H. Yanagida ŽEds.., Science and Technology of Zirconia III, American Ceramics Society, Westerville, OH, 1988, p. 965. w24x J.F. Jue, A.V. Virkar, J. Am. Ceram. Soc. 73 Ž1990. 3650. w25x R. Chaim, M. Ruhle, A.H. Heuer, J. Am. Ceram. Soc. 68 Ž1985. 427. w26x F. Sanchez-Bajo, I. Cachadina, J.D. de Solier, F. Guiberteau, L. Cumbrera, J. Am. Ceram. Soc. 80 Ž1997. 232. w27x D.B. Marshall, M.R. James, J.R. Porter, J. Am. Ceram. Soc. 72 Ž1989. 218.
Materials Letters 52 Ž2002. 69–74 www.elsevier.comrlocatermatlet
Peculiar stress-induced phase transformation in YNbO4-modified ZrO 2 ž3Y/ using in situ compression–diffraction Sheng-Dih Yuh, Chen-Chia Chou ) Department of Mechanical Engineering, National Taiwan UniÕersity of Science and Technology, 43 Keelung Road, Section 4, Taipei 10660, Taiwan Received 29 January 2001; accepted 22 March 2001
Abstract The novel stress-induced transformation characteristics of YNbO4-modified ZrO 2 Ž3Y. ceramics were investigated using an in situ compression–diffraction technique employing synchrotron radiation. The observed stress-induced phase transformation of cubic-to-tetragonal andror cubic-to-orthorhombic plays the primary role on energy-absorbing mechanism in the present material system under an elastic cyclic loading process. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Zirconia; Yttrium niobate; Stress-induced phase transformation; X-ray diffraction; Synchrotron radiation
1. Introduction Although the tetragonal-to-monoclinic Žt-to-m. phase transformation toughening has been widely accepted as the dominant mechanism responsible for toughening in several zirconia-based ceramic materials w1,2x, recently, domain switching in the tetragonal phase is one of the other possible toughening mechanisms frequently proposed and discussed for various ceramics w3,4x. In addition to the well-known cubic Žc., tetragonal Žt. and monoclinic Žm. phases for zirconia-based ceramics, the sintered surfaces of these ceramics may undergo different types of transformations due to alloy compositions and external stimuli. For example, Hasegawa et al. w5x found a rhombohedral Žr. phase formed either in ion-implanted layers or on the abraded surfaces of Y-PSZ w6x. Kim et al. ) Corresponding author. Tel.: q886-2-2737-6493; fax: q886-22737-6460. E-mail address: [email protected] ŽC.-C. Chou..
w7x also reported r-phase formation in 3Y-TZP and 12Ce-TZP by grinding. Several authors w8x found a tX-phase in Y2 O 3 –ZrO 2 material systems with high Y2 O 3 contents that formed diffusionlessly from cZrO 2 during rapid cooling from high temperatures. Log et al. w9x fabricated tX-ZrO 2 doped with Y, Yb, Er or Dy using high-temperature annealing in the cubic stability regime and rapid cooling through the tetragonal stability regime. Michel et al. w10x found tX-phase in polydomain ZrO 2 single crystals doped with Y, Gd or Yb. Besides, some other works reported orthorhombic Žo. phases observed in zirconia-based ceramics. Three distinct o-phases were found under certain conditions: the oX-phase Žtermed OI., cotunnite-phase ŽPbCl 2 structure, termed OII. and simple o-phase in space groups of Pbca, Pnam and Pbc2 1 , respectively. The OI- and OII-phases are the very high-pressure forms of pure zirconia w11x. Haines et al. w12x found OII-phase of pure zirconia under high-temperature and high-pressure conditions, and after laser heating at 18 and 26.7 GPa w11x. Liu
00167-577Xr02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 3 6 8 - 8
70
S.-D. Yuh, C.-C. Chou r Materials Letters 52 (2002) 69–74
w13x and Block et al. w14x also found an OII-phase at pressures above 10 and 16.5 GPa, respectively. A simple o-phase was found w15x constantly such as Y, Mg, Ca-modified PSZ, ZTA, ŽMg, Y.-TZP. For Mg-PSZ, the o-phase on fracture surface, in thermally shocked, by quenching below y1008C, after cooling to cryogenic temperature, after tensile testing and after long aging at 11008C and 14008C has been reported w16x and references therein. The o-phase was also observed in TEM w15x. In this paper, we carried out in situ compression–diffraction experiments using synchrotron radiation and reported an
extraordinary phase transformation, nonconventional t-to-m, under cyclic compression in 5 mol% YNbO4-modified ZrO 2 Ž3Y. sintered at 15008C for 1 h. The origin of the energy-absorbing mechanism was also discussed.
2. Experimental procedure YNbO4 powder was prepared from Y2 O 3 Ž99.99%, Research Chemicals, USA. while Nb 2 O5 Ž99.9%, Meldform, UK. was prepared by an oxide-mixing
Fig. 1. Experimental set-up for an in situ X-ray compression–diffraction technique employing synchrotron radiation in the present studies.
S.-D. Yuh, C.-C. Chou r Materials Letters 52 (2002) 69–74
method followed by calcination at 11008C. The composition of the specimens was ZrO 2 Ž3Y. ŽHSY-3, Daiichi, Japan. with 5 mol% YNbO4 . The mixtures of YNbO4 and ZrO 2 Ž3Y. were sequentially processed by wet centrifugal ball milling, vacuum drying at 608C, calcining at 10008C and ball milling again. The resulting powders were then compressed using a cold isostatic press under a loading of 230 MPa to form thin rectangular pellets Ž20.5 = 14.5 = 2 mm.. The final specimens were obtained by sintering these pellets at 15008C in air for 1 h. An in situ uniaxial compression experiment was conducted employing synchrotron radiation because the conventional and even high-resolution X-ray diffraction Že.g., M18XHF, MAC Science, Japan was tried. cannot detect the variation in peak intensity of the specimen under elastic compression. Synchrotron radiation, which has high intensity and collimation, is a powerful tool for diffraction studies. The specimens were loaded using a custom-designed gripper and were bonded with a strain gage and then compressed uniaxially by a screw wrench. In situ highresolution X-ray diffraction–compression measurements were carried out at the wiggler beamline BL-17B at the Synchrotron Radiation Research Center, Hsinchu, Taiwan. The electron storage ring was operated at an energy of 1.5 GeV and at a current of 140–200 mA, which delivered 8.04 KeV X-ray photons with an estimated flux of 10 10 photonsrs for this experiment. The experimental set-up is shown in Fig. 1.
71
3. Results and discussion Fig. 2 shows the diffraction patterns of 2 u from 278 to 758 of specimens ZrO 2 Ž3Y. –5 mol% YNbO4 sintered at 15008C. The specimen shows primarily the t- and c-phases accompanying small amounts of the m-phase. Fig. 3 shows the compression–diffraction pattern of the specimen sintered at 15008C at 2 u around Ža. 111 m , Žb. 2004 , Žc. 1314 and Žd. 0044 peaks under cyclic loading conditions with a maximum compression of 400 MPa for three cycles. Two peculiar behaviors were observed in Fig. 3. Firstly, as loading was applied at the first cycle, an extra peak arose at the right-hand side of 111 m and disappeared as loading was released; then, this extra peak arose again as loading was released at the second cycle and disappeared as loading was applied at the third cycle as shown in Fig. 3Ža. Žonly the diffraction patterns of the initial state, first loading step and second released step, are shown here, because other conditions had no significant change in peak intensity.. Secondly, at almost each cycle, for both cases where loading was applied and loading was released, the peak intensity of every c-peak Žthe index of the c-peak was labeled as a combination of c, o and tX phases, which will be discussed later. show significant variation. Meanwhile, scrutinizing the variation of peak intensity for the first loading cycle as loading compressed up to 400 MPa, the intensity of 020 c,o,tX increased, and that of the 131 c,o,tX and
Fig. 2. Diffraction patterns with 2 u from 278 to 758 of ZrO 2 Ž3Y. –5 mol% YNbO4 specimen sintered at 15008C for 1 h.
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S.-D. Yuh, C.-C. Chou r Materials Letters 52 (2002) 69–74
Fig. 3. In situ compression–diffraction patterns of 2 u around Ža. 111 m , Žb. 2004, Žc. 1314 and Žd. 0044 of specimen ZrO 2 Ž3Y. –5 mol% YNbO4 sintered at 15008C for 1 h, showing a peculiar stress-induced phase transformation. The x-axis is 2 u in degrees Ž8. and y-axis is intensity in arbitrary unit.
400 c,tXr014o , decreased. Furthermore, an extra peak arose at the right-hand side of 111 m . After loading was released, the intensity of 020 c,o,tX decreased and that of the 131 c,o,,tX decreased, obviously while 400 c,tXr014o showed nearly no change, and intriguingly, the extra peak disappeared. For the second loading cycle, as loading was applied again, the intensity of 020 c,o,tX increased pronouncedly to a maximum value among all cycles and intensities of the 131 c,o,tX and 400 c,tXr014o increased, and no extra
peak was found. After loading was released, the intensity of 020 c,o,tX deceased and that of both the 131 c,o,tX and 400 c,tXr014o increased significantly to a maximum, and intriguingly, the extra peak appeared with higher intensity than that in the first compressive cycle. At the last loading cycle, no extra peak appeared but 020 c,o,tX , 131 c,o,tX and 400 c,tXr014o still change obviously. The toughening mechanisms of zirconia-based ceramics had been reported to be attributed to a t-to-m
S.-D. Yuh, C.-C. Chou r Materials Letters 52 (2002) 69–74
phase transformation or to a ferroelastic domain switching of the t-phase of the specimens as cited in Section 1. However, the integral intensities of the tand m-peaks show nearly no change, as shown in Fig. 3 Žonly the 111 m and doublets of t-phase are shown here., indicating that the t-phase does not transform to the m-phase in the present material under such a loading condition. Virkar and Matsumoto w17x and Mehta et al. w18x found that the ratio of the peak intensity I Ž002 . t rI Ž200 . t and I Ž113. trI Ž131. t varied from smaller than 1 to greater than 1 after grinding, indicating that a process of domain switching effectively accomplishes through the rotation of the w001x-orientation by 908. However, no switching was observed for the doublets 002 tr200 t , 113tr311 t and 004tr400 t in the present situation as shown in Fig. 3Žb. to Žd. but an extra peak rises near 111 m , as shown in Fig. 3Ža.. Therefore, ferroelastic domain switching of the t-phase does not occur under such a loading condition in the present specimens. It is generally believed that the tX-phase, which is metastable, forms by rapid cooling from the c-phase of suitable solute content and the transformation was a displacive type w19x. However, it has been found that the metastable tetragonal tX-ZrO 2 does not necessarily form by rapid quenching from c-ZrO 2 but can be produced by slowly cooling conditions instead w20,21x. Several authors w22,23x reported the presence of a nonequilibrium tetragonal phase, tX , referred to as Anontransformable,B because of its reluctance to undergo the transformation to the m-phase. Jue and Virkar w24x showed that tX-ZrO 2 did not transform to m-ZrO 2 , and both the t-phase and tX-phase have toughnesses two to three times greater than the cphase, which is neither the t-to-m transformation, nor ferroelastic toughened. Sugiyama and Kubo w23x and Chaim et al. w25x reported that the tX-phase had heavy overlapping diffraction patterns with the cubic reflections, and that the tX-phase was described as a pseudocubic phase whose tetragonality Žcra ratio. is almost unity. Furthermore, Sanchez-Bajo et al. w26x reported a conclusive discrimination between c- and tX-phases by a careful application of the Rietveld method and demonstrated that the t q tX hypothesis is better than the t q c hypothesis in fitting the diffraction pattern at 2 u around 358. In addition to the aforementioned works, Heuer et al. w15x reported that
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most of the o-phases are referred to as a distorted fluorite structures except for the very high-pressure cotunnite phase. Marshall et al. w27x found that the o-ZrO 2 having lots of reflections overlapped with the c-phase. Therefore, we can conclude that the c-phase of the present material may coexist with tXandror o-phases after YNbO4 modification, the peak labeled as 400 c,tXr014o , which was split into two peaks at the initial condition, is good evidence shown in the insert block of Fig. 3Žd., and the tX andror o-phases may possibly be induced under compression as well. As shown in Fig. 3, we are able to see that the c-peaks vary their intensity strongly under different loading conditions, clearly indicating that there exists other energy-absorbing mechanisms in addition to the t-to-m phase transformation and domain switching of the t-phase. The observed stress-induced phase transformation may be attributed to the c-to-tX andror c-to-o phase transformations, as discussed above. The results shown in the present work imply that there is a novel toughening mechanism which absorbs the external loading energy effectively in the YNbO4-modified ZrO 2 Ž3Y. materials. 4. Conclusions The o- andror tX-phases may be formed by doping 5 mol% YNbO4 in ZrO 2 Ž3Y. ceramics and sintered at 15008C for 1 h. In situ compression–diffraction experiments show that the stress-induced phase transformation of c-to-tX andror c-to-o was probably the primary energy-absorbing mechanism in the YNbO4-modified ZrO 2 Ž3Y. material system under elastic cyclic compressions.
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