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Compression-induced reversible phase transformation with a cubic-like structure in 3 mol.% yttria-stabilized zirconia
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Scripta Materialia 61 (2009) 927–930 www.elsevier.com/locate/scriptamat
Compression-induced reversible phase transformation with a cubic-like structure in 3 mol.% yttria-stabilized zirconia Tsung-Her Yeh,a Chen-Chia Choua,* and Hsin-Yi Leeb a
Department of Mechanical Engineering, National Taiwan University of Science and Technology, 43, Keelung Road, Section 4, Taipei-10672, Taiwan, ROC b National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan, ROC Received 19 May 2009; revised 6 July 2009; accepted 21 July 2009 Available online 25 July 2009
A peculiar phase transformation in modified zirconia was investigated and identified using Raman spectroscopy and in situ compression–diffraction experiments with synchrotron radiation. Neither tetragonal-to-monoclinic phase transformation nor ferroelastic domain switching was observed in specimens under uniaxial compression, but formation of an unusual stress-induced phase transformation from the previously reported non-transformable cubic and t0 phases was found. The cubic phase transforming to the t0 phase frequently takes place by applying external stress and returns to the original state as loading is released. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Synchrotron radiation; Phase transformations; Raman spectroscopy; Compression test; Zirconia
It has been reported that 3 mol.% yttria-stabilized zirconia (3YSZ) possesses high fracture toughness, which is mainly attributed to the stress-induced tetragonal-to-monoclinic (t-to-m) phase transformation in the vicinity of the propagating cracks [1]. Moreover, ferroelastic domain switching, another possible toughening mechanism in zirconia, differs from the t-to-m phase transformation mechanism, in which only orientation switching occurs within the process zone without changing the crystal structure [2]. In addition to t-to-m phase transformation and ferroelastic domain switching, the sintered surfaces of zirconia ceramics may undergo different types of phase transformations due to external stimuli and different compositions. Kim et al. [3] reported the formation of a rhombohedral (r) phase on the grinding surface of 3YSZ. The r phase was only generated under such a grinding condition and was not observed in a uniaxial compression experiment, implying that stress conditions may be responsible for different phase transformations. The diffusionless product of cubic (c) to metastable tetragonal phase transition was first introduced as t0 phase by Miller and Smielek [4]. It is observed that a t0 phase in YSZ forms without diffusion from c-ZrO2 during rapid cooling from high temperatures. However, it has been ob* Corresponding author. E-mail: [email protected]
served that the t0 -ZrO2 did not necessarily form by rapid quenching from c-ZrO2, but did so by slow cooling. The transition from c-to-t0 phase can be either diffusional or diffusionless in yttria-containing zirconia, depending upon the cooling condition. The t0 phase formed by rapid quenching with diffusionless transformation often contains a submicron array of anti-phase domains and twins. Lin et al. [5] reported formation of the t0 phase in a soluteredistributed cubic matrix of a slowly cooled Y-partially stabilized zirconia (PSZ) sintered at 1400 °C for 2 h, which showed no deformation accommodation twins. Lanteri et al. [6] reported that a slowly cooled single-crystal two-phase alloy (8 wt.% Y2O3) heated at 1600 °C for 50 h also exhibits t0 phase. The precipitates of t0 phase form a ‘‘colony” structure in the cubic phase by slow cooling with diffusional transformation, which has a different morphology to the quenched one [7,8]. Chan et al. [9] proposed that t0 -zirconia possessing high strength and fracture toughness could be obtained by the ferroelastic transformation, rather than phase transformation, of t0 phase. In addition to t0 phase, an orthorhombic (o) phase formed by t-to-o-to-m phase transformation has also been reported in the YSZ system [10]. A previous investigation indicated that the o phase is a high-pressure phase of zirconia [11,12]. The t-to-o transformation [13] is attributed to a large expansion mismatch stress between the t phase and the surrounding matrix phases.
1359-6462/$ - see front matter Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2009.07.019
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The tetragonal phase of a 3YSZ specimen transformed to cubic and monoclinic phases after thermal annealing at 1600 °C for 10 h, but the specimen still exhibited high toughness around 10 MPa m1/2. Besides, the toughness of a 7.5 mol.% YNbO4-doped 3YSZ specimen is around 8.5 MPa m1/2, and the microstructure shows the existence of 33% c phase and no m phase after thermal annealing at 1600 °C for 10–20 h [14,15]. It is interesting that the amount of the cubic phase increased and the grain boundary constraint was released by annealing for a long time at 1600 °C, and that the toughness was found to be enhanced in 3YSZ and 5 mol.% YNbO4-doped 3YSZ specimens. Hence, the toughness of zirconia seems to have close correlations with the cubic amount/stability and the constrained force in specimens. To clarify the role of the cubic phase, a uniaxial compressive load was applied to simulate the deformation behavior in specimens, and we focused our investigation on the possible phase transformation in a 3YSZ specimen. Moreover, the relationship between the magnitude of the uniaxial compressive load, the variation of the localized phase structure and the in situ variation in phase transformation of 3YSZ material was studied using Raman spectroscopy and an in situ compression–diffraction experiment with synchrotron radiation. The YNbO4 powders were prepared from Y2O3 (99.99% purity, Research chemicals, USA) and Nb2O5 (99.9%, Meldform, UK), and mixtures of 5 mol.% YNbO4 and 3YSZ (99.9% purity, HSY-3, Daiichi, Japan) were sequentially processed by wet centrifugal ball milling and calcination at 1100 °C for 1 h. Commercial 3YSZ (99.9%, HSY-3, Daiichi, Japan) and 5 mol.% YNbO4doped 3YSZ powders were compressed using a cold-isostatic press, under a load of 2000 kgf, to form thin rectangular green pellets (2.0 1.45 0.2 cm). The final specimens were obtained by sintering these green pellets at 1475–1575 °C for 1 h. Phase identification and the localized fine structure of the specimens were inspected using a Raman scattering spectrometer (IR 750/Raman 950, USA) with an He–Ne laser source (k = 632.8 nm). In situ high-resolution X-ray diffraction–compression investigations were carried out with the wiggler beamline BL-17B at the Synchrotron Radiation Research Center (SRRC), Hsinchu, Taiwan. Synchrotron radiation is a powerful tool for the study of diffraction. The high intensity and collimation of synchrotron radiation allows high-resolution studies, both in the small- and high-angle regions. The specimen bonded with a strain gauge was loaded using a custom-designed gripper and compressed uniaxially by a screw wrench. The applied load can be evaluated by a corresponding strain with a unitless value of le from a strain indicator. In situ X-ray diffraction was performed, with the size of the X-ray spotted area being around 0.20 cm2. The experimental set-up is shown in Figure 1. Figure 2(a) and (b) illustrates the X-ray diffraction patterns of 3YSZ sintered at 1475 °C for 1 h and at 1575 °C for 1 h, respectively. The phase structure of 3YSZ sintered at 1475 °C for 1 h displays in a tetragonal form. However, the background of intensity between the 0 0 2t and 2 0 0t peaks at 2h around 35°, which was indicated by an arrow, seems to exhibit higher intensity than the background in the whole range of the X-ray diffraction pattern of 3YSZ. Formation of the monoclinic
Figure 1. Experimental set-up for the in situ high-resolution X-ray diffraction–compression test.
Figure 2. X-ray diffraction patterns of 3YSZ specimens sintered at (a) 1475 °C for 1 h and (b) 1575 °C for 1 h. (c) Raman scattering spectrum of 3 mol.% Y2O3-stabilized zirconia.
phase, as indicated by a star, was contributed by the tto-m phase transformation, and the background of intensity between the 0 0 2t and 2 0 0t peaks at 2h around 35° in the 3YSZ specimen sintered at 1575 °C was much higher than that of the 3YSZ sintered at 1475 °C. This shows that the t phase probably transformed to the c and m phases following an increase in the sintering temperature. To verify the phase structure of 3YSZ sintered at 1475 °C for 1 h, the Raman scattering spectrum was adopted to investigate the localized fine structure. Figure 2(c) displays the dependence of the Raman shift by crystal vibration on the localized fine structure of 3YSZ. The Raman shifts for the m, t and c phases were compared with those in previous works [16,17]. The main structure of 3YSZ was found to contain a tetragonal phase and other phases. The peak at the Raman shift of 380 cm1 and other Raman shift peaks, indicated by rhombic marks, were attributed to a monoclinic phase. The open circles represent the Raman shifts of vibration of the tetragonal symmetry. Raman shift peaks of the cubic symmetry vibration, indicated by square marks, were observed at 461 and 642 cm1. Accordingly, it was observed that small amounts of cubic and monoclinic phases exist in the 3YSZ specimen after sintering at 1475 °C for 1 h. The results of the in situ SRRC compression–diffraction experiment of the 3YSZ specimen with 2h around 28, 35, 60 and 74° under sequentially increasing compression are shown in Figure 2. Different amounts of load were applied with a strain increment of 500 le in each cycle.
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Unusual phenomena were observed in the present study, as shown in Figure 3. First, when the load was applied to the 3YSZ specimen, no variation in the peak intensity of 1 1 1m was found, as shown in Figure 3(a). Increasing the load from 500 to 2000 le caused a significant decrease in the peak intensities of the 0 0 2t and 2 0 0t reflections (Fig. 3(b)) and a significant increase in the intensities of the 0 0 4t and 4 0 0t reflections (Fig. 3(d)), and an increase in the 0 0 2t and 2 0 0t peaks and a decrease in the 0 0 4t and 4 0 0t peaks were also found when the applied load was released. The intensities of the 1 1 3t and 3 1 1t reflections remained the same (Fig. 3(c)). The reversible behavior of the {2 0 0}t and {4 0 0}t tetragonal reflections implies that the orientation relationship was rearranged in the specimen due to internal forces, which are produced by the application of an external force. Accordingly, neither t-to-m phase transformation nor ferroelastic domain switching of the tetragonal phase was observed under such a uniaxial compressive load. The phase structures of the o, r and other phases were not found in the diffraction patterns of 3YSZ in the in situ compression–diffraction experiment because no 0 2 0o, 0 0 2o, 2 0 0o, 0 1 2r, 0 4 0o, 0 0 4o, 4 0 0o and 0 2 4r peaks [3,10–13] were observed in the diffraction patterns with 2h around 35 and 74°. Secondly, an interesting phenomenon was observed when the load was increased to 2000 le. An extra peak that was close to the 0 2 0c peak position of the cubic phase appeared between the 0 0 2t and 2 0 0t reflections. When the applied load was released, the extra peak (indicated by an arrow in Fig. 3(b)) disappeared. In addition, two fine reflections between the 0 0 4t and 4 0 0t peaks, also indicated by arrows, were observed and determined to be t0 phase reflections, as indicated by the tetragonality (c/a) and lattice parameters, which were found to be 1.006 ˚ and c = 5.12 A ˚ , respectively. Sugiyama and a = 5.09 A and Kubo [18] reported that the diffraction pattern of t0 phase heavily overlapped the cubic reflections, which was indicative of a pseudo-cubic (t0 ) phase whose tetragonality (c/a ratio around 1.006–1.010) was almost equal to unity. Therefore, the t0 peaks containing 0 0 4t0 and 4 0 0t0 reflections are indicated by arrows in Figure 3(d). Simulation of the peak variations in the cubic and t0 phases with 2h around 35 and 74° with applied loads of 1500 and 2000 le are shown in Figure 3(e) and (f), which demonstrates the existence of the cubic and t0 phases as well as the phase transformation between them. The 0 0 2t0 and 2 0 0t0 peak positions of the t0 phase were around
Figure 3. X-ray diffraction patterns of 3YSZ sintered at 1475 °C for 1 h using the in situ compression–diffraction experiment at 2h of (a) 28°, (b) 35°, (c) 60° and (d) 74°, and peak simulation of cubic and metastable tetragonal (t0 ) phases in the X-ray diffraction patterns of 3YSZ at 2h of (e) 35° and (f) 74° with increasing load from 1500 to 2000 le.
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2h of 35°, near the 2 0 0c reflection of the cubic phase. A ‘‘decrease” in the peak intensity of the 0 2 0c reflection and an ‘‘increase” in the intensities of the 0 0 2t, and 2 0 0t, reflections were observed due to the increase in load from 1500 to 2000 le; the same results also occurred in the X-ray diffraction simulation of 3YSZ around 2h of 74°. The relationship between the peak variations of the c and t0 phases demonstrated that the transition of t0 -to-c was induced by applying a compressive load to 2000 le and the c-to-t0 phase transformation happened after the external loading was released. Significant variations in the intensities of the c and t0 peaks are found under compressive loading, indicating the dynamic change in the crystal structure of 3YSZ. Similarly, peak variation between the c and t0 phases of 3YSZ sintered at 1475 °C for 1 h was also observed in the 5 mol.% YNbO4-doped 3YSZ specimen, as shown in Figure 4. Unusual peaks in the 5 mol.% YNbO4-doped 3YSZ seem to be induced more easily than in pure 3YSZ as the load applied was only up to 500 le, indicating that phase transformation between the t0 and c phases depends not only on the applied compression, but also on the doped element in the specimen, as suggested in Figure 4(a)–(d). Trivalent yttrium oxide, Y2O3, is a common stabilizer of zirconia which works by suppressing the formation of t-to-m phase transformation during the cooling process. Doping Y2O3 into zirconia produces oxygen vacancies, increases the amount of c phase and reduces the content of t and m phases; however, t-to-m phase transformation was easily induced by adding Nb2O5 into zirconia [19,20]. The co-existence of Y2O3 and Nb2O5 modifies the phase stability of ZrO2-based materials and therefore clarify the phase transformation characteristic of zirconia ceramics. Peak separation and structural simulation demonstrate the co-existence of the cubic and t0 phases, as seen in Figure 4(e) and (f). The result in this work is different from the previous view that t0 phase is stable due to its high yttria content and that it improves the toughness of zirconia materials by its ferroelastic behavior. The t0 phase was often identified as ‘‘a non-transformation phase”, as the results of Sakuma et al. [21] reported, and the t0 phase possesses high resistance to transformation to the m phase. However, Zhu et al. [22] reported that diffusional phase separation took place and that the t0 phase could decompose into the cubic phase at low concentration (4 mol.%) yttria-doped ZrO2 aged for 10–80 h. This means that formation of the t0 phase takes place not only at high yttira concentrations but also with low yttria content. In fact, t0 -to-m phase transformation of arc-melted ZrO2–RO1.5 (R = Sm, Y, Er and Sc) was observed in water at 200 °C and 100 MPa pressure [23]; therefore it is reasonable to assume that phase transformation of t0 phase may occur easily at low yttria concentration, with the application of a compressive load and under hydrothermal conditions. This suggests that t0 phase is not in fact ‘‘a non-transformable phase”, and the phase transformation behavior of the t0 phase seems to depend on the chemical free energy (dopant content) as well as the nonchemical free energy (grain size, strain filed, compressive load, etc.). Since both t0 -to-m and t0 -to-c transformations were observed, the difference between the t0 -to-m reported by
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Figure 4. XRD diffraction patterns of a 5 mol.% YNbO4-doped 3YSZ specimen sintered at 1475 °C for 1 h using the in situ compression– diffraction experiment at 2h around (a) 28°, (b) 35°, (c) 60° and (d) 74°, and peak simulation of the cubic and a metastable tetragonal (t0 ) phases in the X-ray diffraction patterns of 5 mol.% YNbO4-doped 3YSZ at 2h of (e) 35° and (f) 74° with increasing load to from the initial state to 500 le.
Yashima et al. and the t0 -to-c observed in this work was discussed. Yashima et al. studied the t0 -to-m phase transformation using homogeneous t0 -samples prepared by the arc-melting furnace and a rapid quenching technique. Serious lattice deformation and large internal stresses have been shown to exist in the t0 phase fabricated by the rapid quenching technique [24,25]. Therefore t0 -to-m phase transformation was easily induced by external stress to form the monoclinic phase. The monoclinic phase in the arc-melted t0 samples [23] after hydrothermal treatment at 200 °C and 100 MPa pressure is very similar to the results formed by diffusionless martensitic t-to-m phase transformation during quenching of melts and/or during crushing and grinding of the samples. However, the specimens for the in situ high-resolution X-ray diffraction-compression test were prepared by solid-state reactions and were sintered at high temperatures. The solid-state reactions would often yield compositional and microstructural inhomogeneity, because a phase separation occurs in the two-phase (t + c) region due to the solubility limit of yttria in the tetragonal phase at sintering temperatures. When both Y and Nb were introduced into a 3Y–ZrO2 matrix, grain growth occurred, indicating an enhancement of element diffusion. Therefore, the microstructures and internal stresses of these materials exhibit quite different characteristics from those of rapidly quenched specimens. The microstructure/stress distribution and the chemical composition inhomogeneity, as well as the testing environments of the two investigations, were different; therefore, a different mechanism of phase transformation might be induced. Based upon the above results, a reversible t0 -to-c was found in the present study under uniaxial compression load and a t0 -to-m phase transformation was observed under the high-pressure thermal condition of 200 °C and 100 MPa. It is interesting that the t0 phase, which might be an intermediate phase between the cubic phase and the monoclinic phase, could transfer to other kinds of phase structures under different forms of external loading. Further investigations on the phase transformation of the t0 phase might clarify the relations among the transformable t0 phase and the cubic and monoclinic phases. In summary, in situ compression–diffraction experiments with incremental compressive loading on the stress-induced phase transformation of 3YSZ ceramics were investigated. There was no variation in the amount
of the tetragonal and monoclinic phases under uniaxial loading, suggesting that the t-to-m phase transformation and the domain switching in the tetragonal crystals do not contribute to energy absorption before fracture/ crack generation in the 3YSZ specimen. On the other hand, an in situ compression–diffraction experiment shows that a reversible phase transformation occurs between the cubic phase and the transformable t0 phase in the specimens during the application and release of the compressive load, and that this depends not only on the compression applied, but also on the doping element in the specimens. Financial support from the Synchrotron Radiation Research Center and National Science Council of Taiwan, Republic of China through project numbers NSC 97-2221-E-011-006 and NSC 97-2923-M-029-001MY3 is gratefully acknowledged by the authors. [1] R.C. Garvie, R.J.H. Hannink, R.T. Pascoe, Nature (London) 258 (5537) (1975) 703. [2] M.V. Swain, R.J.H. Hannink, R.C. Garvie, Fract. Mech. Ceram. 6 (1983) 339. [3] D.J. Kim, H.J. Jung, H.J. Kim, J. Mater. Sci. 14 (4) (1995) 285. [4] R.A. Miller, J.C. Smielek, in: A.H. Heuer, L.W. Hobbs (Eds.), Science and Technology of Zirconia II, Columbus Press, Columbus, OH, 1981, pp. 241–255. [5] S.J. Lin, H.Y. Lu, P. Shen, J. Mater. Sci. 26 (1991) 112. [6] V. Lanteri, R. Chaim, A.H. Heuer, J. Am. Ceram. Soc. 69 (1986) C-258. [7] A.H. Heuer, R. Chaim, V. Lanteri, Acta Metall. 35 (1987) 661. [8] T. Sakuma, J. Mater. Sci. 22 (1987) 4470. [9] C.J. Chan, F.F. Lange, M. Ruhle, J.F. Jue, A.V. Virkar, J. Am. Ceram. Soc. 74 (1991) 807. [10] D.B. Marshall, M.R. James, J.R. Porter, J. Am. Ceram. Soc. 72 (2) (1989) 218. [11] 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 (7) (1997) 1910. [12] C.A. Bateman, M.R. Notis, J. Mater. Sci. Lett. 9 (1990) 803. [13] C.J. Howard, E.H. Kisi, R.B. Roberts, R.J. Hill, J. Am. Ceram. Soc. 73 (1990) 2828. [14] Y.C. Lai, Master Thesis, National Taiwan University of Science and Technology, Taiwan, 1995. [15] Y.C. Lai, C.C. Chou, J. Ceram. Soc. Taiwan 1 (1995) 26–32. [16] D.J. Kim, J. Am. Ceram. Soc. 73 (1990) 115. [17] G. Gritzner, C. Puchner, J. Eur. Ceram. Soc. 13 (1994) 387. [18] M. Sugiyama, H. Kubo, in: S. Somiya, N. Yamamoto, H. Yanagida (Eds.), Science and Technology of Zirconia III, American Ceramics Society, Westerville, OH, 1988. [19] D. Casellas, F.L. Cumbrera, F.S. Bajo, W. Forsling, L. Llanes, M. Anglada, J. Eur. Ceram. Soc. 21 (2001) 765. [20] M. Yashima, T. Hirose, M. Kakihana, Y. Suzuki, M. Yoshimura, J. Am. Ceram. Soc. 80 (1) (1997) 171. [21] T. Sakuma, Y.I. Yoshizawa, H. Suto, J. Mater. Sci. 20 (1985) 2399. [22] W.Z. Zhu, T.C. Lei, Y. Zhou, Z.S. Ding, J. Mater. Sci. 30 (1995) 6235. [23] M. Yashima, T. Nagatome, T. Noma, N. Ishizawa, Y. Suzuki, M. Yoshimura, J. Am. Ceram. Soc. 78 (8) (1995) 2229. [24] D. Baither, B. Baufeld, U. Messerschmidt, A.H. Foitzik, M. Ruhle, J. Am. Ceram. Soc. 80 (7) (1997) 1691. [25] B. Baufeld, D. Baither, U. Messerschmidt, M. Bartsch, A.H. Foitzik, M. Ruhle, J. Am. Ceram. Soc. 80 (7) (1997) 1699.
Scripta Materialia 61 (2009) 927–930 www.elsevier.com/locate/scriptamat
Compression-induced reversible phase transformation with a cubic-like structure in 3 mol.% yttria-stabilized zirconia Tsung-Her Yeh,a Chen-Chia Choua,* and Hsin-Yi Leeb a
Department of Mechanical Engineering, National Taiwan University of Science and Technology, 43, Keelung Road, Section 4, Taipei-10672, Taiwan, ROC b National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan, ROC Received 19 May 2009; revised 6 July 2009; accepted 21 July 2009 Available online 25 July 2009
A peculiar phase transformation in modified zirconia was investigated and identified using Raman spectroscopy and in situ compression–diffraction experiments with synchrotron radiation. Neither tetragonal-to-monoclinic phase transformation nor ferroelastic domain switching was observed in specimens under uniaxial compression, but formation of an unusual stress-induced phase transformation from the previously reported non-transformable cubic and t0 phases was found. The cubic phase transforming to the t0 phase frequently takes place by applying external stress and returns to the original state as loading is released. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Synchrotron radiation; Phase transformations; Raman spectroscopy; Compression test; Zirconia
It has been reported that 3 mol.% yttria-stabilized zirconia (3YSZ) possesses high fracture toughness, which is mainly attributed to the stress-induced tetragonal-to-monoclinic (t-to-m) phase transformation in the vicinity of the propagating cracks [1]. Moreover, ferroelastic domain switching, another possible toughening mechanism in zirconia, differs from the t-to-m phase transformation mechanism, in which only orientation switching occurs within the process zone without changing the crystal structure [2]. In addition to t-to-m phase transformation and ferroelastic domain switching, the sintered surfaces of zirconia ceramics may undergo different types of phase transformations due to external stimuli and different compositions. Kim et al. [3] reported the formation of a rhombohedral (r) phase on the grinding surface of 3YSZ. The r phase was only generated under such a grinding condition and was not observed in a uniaxial compression experiment, implying that stress conditions may be responsible for different phase transformations. The diffusionless product of cubic (c) to metastable tetragonal phase transition was first introduced as t0 phase by Miller and Smielek [4]. It is observed that a t0 phase in YSZ forms without diffusion from c-ZrO2 during rapid cooling from high temperatures. However, it has been ob* Corresponding author. E-mail: [email protected]
served that the t0 -ZrO2 did not necessarily form by rapid quenching from c-ZrO2, but did so by slow cooling. The transition from c-to-t0 phase can be either diffusional or diffusionless in yttria-containing zirconia, depending upon the cooling condition. The t0 phase formed by rapid quenching with diffusionless transformation often contains a submicron array of anti-phase domains and twins. Lin et al. [5] reported formation of the t0 phase in a soluteredistributed cubic matrix of a slowly cooled Y-partially stabilized zirconia (PSZ) sintered at 1400 °C for 2 h, which showed no deformation accommodation twins. Lanteri et al. [6] reported that a slowly cooled single-crystal two-phase alloy (8 wt.% Y2O3) heated at 1600 °C for 50 h also exhibits t0 phase. The precipitates of t0 phase form a ‘‘colony” structure in the cubic phase by slow cooling with diffusional transformation, which has a different morphology to the quenched one [7,8]. Chan et al. [9] proposed that t0 -zirconia possessing high strength and fracture toughness could be obtained by the ferroelastic transformation, rather than phase transformation, of t0 phase. In addition to t0 phase, an orthorhombic (o) phase formed by t-to-o-to-m phase transformation has also been reported in the YSZ system [10]. A previous investigation indicated that the o phase is a high-pressure phase of zirconia [11,12]. The t-to-o transformation [13] is attributed to a large expansion mismatch stress between the t phase and the surrounding matrix phases.
1359-6462/$ - see front matter Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2009.07.019
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The tetragonal phase of a 3YSZ specimen transformed to cubic and monoclinic phases after thermal annealing at 1600 °C for 10 h, but the specimen still exhibited high toughness around 10 MPa m1/2. Besides, the toughness of a 7.5 mol.% YNbO4-doped 3YSZ specimen is around 8.5 MPa m1/2, and the microstructure shows the existence of 33% c phase and no m phase after thermal annealing at 1600 °C for 10–20 h [14,15]. It is interesting that the amount of the cubic phase increased and the grain boundary constraint was released by annealing for a long time at 1600 °C, and that the toughness was found to be enhanced in 3YSZ and 5 mol.% YNbO4-doped 3YSZ specimens. Hence, the toughness of zirconia seems to have close correlations with the cubic amount/stability and the constrained force in specimens. To clarify the role of the cubic phase, a uniaxial compressive load was applied to simulate the deformation behavior in specimens, and we focused our investigation on the possible phase transformation in a 3YSZ specimen. Moreover, the relationship between the magnitude of the uniaxial compressive load, the variation of the localized phase structure and the in situ variation in phase transformation of 3YSZ material was studied using Raman spectroscopy and an in situ compression–diffraction experiment with synchrotron radiation. The YNbO4 powders were prepared from Y2O3 (99.99% purity, Research chemicals, USA) and Nb2O5 (99.9%, Meldform, UK), and mixtures of 5 mol.% YNbO4 and 3YSZ (99.9% purity, HSY-3, Daiichi, Japan) were sequentially processed by wet centrifugal ball milling and calcination at 1100 °C for 1 h. Commercial 3YSZ (99.9%, HSY-3, Daiichi, Japan) and 5 mol.% YNbO4doped 3YSZ powders were compressed using a cold-isostatic press, under a load of 2000 kgf, to form thin rectangular green pellets (2.0 1.45 0.2 cm). The final specimens were obtained by sintering these green pellets at 1475–1575 °C for 1 h. Phase identification and the localized fine structure of the specimens were inspected using a Raman scattering spectrometer (IR 750/Raman 950, USA) with an He–Ne laser source (k = 632.8 nm). In situ high-resolution X-ray diffraction–compression investigations were carried out with the wiggler beamline BL-17B at the Synchrotron Radiation Research Center (SRRC), Hsinchu, Taiwan. Synchrotron radiation is a powerful tool for the study of diffraction. The high intensity and collimation of synchrotron radiation allows high-resolution studies, both in the small- and high-angle regions. The specimen bonded with a strain gauge was loaded using a custom-designed gripper and compressed uniaxially by a screw wrench. The applied load can be evaluated by a corresponding strain with a unitless value of le from a strain indicator. In situ X-ray diffraction was performed, with the size of the X-ray spotted area being around 0.20 cm2. The experimental set-up is shown in Figure 1. Figure 2(a) and (b) illustrates the X-ray diffraction patterns of 3YSZ sintered at 1475 °C for 1 h and at 1575 °C for 1 h, respectively. The phase structure of 3YSZ sintered at 1475 °C for 1 h displays in a tetragonal form. However, the background of intensity between the 0 0 2t and 2 0 0t peaks at 2h around 35°, which was indicated by an arrow, seems to exhibit higher intensity than the background in the whole range of the X-ray diffraction pattern of 3YSZ. Formation of the monoclinic
Figure 1. Experimental set-up for the in situ high-resolution X-ray diffraction–compression test.
Figure 2. X-ray diffraction patterns of 3YSZ specimens sintered at (a) 1475 °C for 1 h and (b) 1575 °C for 1 h. (c) Raman scattering spectrum of 3 mol.% Y2O3-stabilized zirconia.
phase, as indicated by a star, was contributed by the tto-m phase transformation, and the background of intensity between the 0 0 2t and 2 0 0t peaks at 2h around 35° in the 3YSZ specimen sintered at 1575 °C was much higher than that of the 3YSZ sintered at 1475 °C. This shows that the t phase probably transformed to the c and m phases following an increase in the sintering temperature. To verify the phase structure of 3YSZ sintered at 1475 °C for 1 h, the Raman scattering spectrum was adopted to investigate the localized fine structure. Figure 2(c) displays the dependence of the Raman shift by crystal vibration on the localized fine structure of 3YSZ. The Raman shifts for the m, t and c phases were compared with those in previous works [16,17]. The main structure of 3YSZ was found to contain a tetragonal phase and other phases. The peak at the Raman shift of 380 cm1 and other Raman shift peaks, indicated by rhombic marks, were attributed to a monoclinic phase. The open circles represent the Raman shifts of vibration of the tetragonal symmetry. Raman shift peaks of the cubic symmetry vibration, indicated by square marks, were observed at 461 and 642 cm1. Accordingly, it was observed that small amounts of cubic and monoclinic phases exist in the 3YSZ specimen after sintering at 1475 °C for 1 h. The results of the in situ SRRC compression–diffraction experiment of the 3YSZ specimen with 2h around 28, 35, 60 and 74° under sequentially increasing compression are shown in Figure 2. Different amounts of load were applied with a strain increment of 500 le in each cycle.
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Unusual phenomena were observed in the present study, as shown in Figure 3. First, when the load was applied to the 3YSZ specimen, no variation in the peak intensity of 1 1 1m was found, as shown in Figure 3(a). Increasing the load from 500 to 2000 le caused a significant decrease in the peak intensities of the 0 0 2t and 2 0 0t reflections (Fig. 3(b)) and a significant increase in the intensities of the 0 0 4t and 4 0 0t reflections (Fig. 3(d)), and an increase in the 0 0 2t and 2 0 0t peaks and a decrease in the 0 0 4t and 4 0 0t peaks were also found when the applied load was released. The intensities of the 1 1 3t and 3 1 1t reflections remained the same (Fig. 3(c)). The reversible behavior of the {2 0 0}t and {4 0 0}t tetragonal reflections implies that the orientation relationship was rearranged in the specimen due to internal forces, which are produced by the application of an external force. Accordingly, neither t-to-m phase transformation nor ferroelastic domain switching of the tetragonal phase was observed under such a uniaxial compressive load. The phase structures of the o, r and other phases were not found in the diffraction patterns of 3YSZ in the in situ compression–diffraction experiment because no 0 2 0o, 0 0 2o, 2 0 0o, 0 1 2r, 0 4 0o, 0 0 4o, 4 0 0o and 0 2 4r peaks [3,10–13] were observed in the diffraction patterns with 2h around 35 and 74°. Secondly, an interesting phenomenon was observed when the load was increased to 2000 le. An extra peak that was close to the 0 2 0c peak position of the cubic phase appeared between the 0 0 2t and 2 0 0t reflections. When the applied load was released, the extra peak (indicated by an arrow in Fig. 3(b)) disappeared. In addition, two fine reflections between the 0 0 4t and 4 0 0t peaks, also indicated by arrows, were observed and determined to be t0 phase reflections, as indicated by the tetragonality (c/a) and lattice parameters, which were found to be 1.006 ˚ and c = 5.12 A ˚ , respectively. Sugiyama and a = 5.09 A and Kubo [18] reported that the diffraction pattern of t0 phase heavily overlapped the cubic reflections, which was indicative of a pseudo-cubic (t0 ) phase whose tetragonality (c/a ratio around 1.006–1.010) was almost equal to unity. Therefore, the t0 peaks containing 0 0 4t0 and 4 0 0t0 reflections are indicated by arrows in Figure 3(d). Simulation of the peak variations in the cubic and t0 phases with 2h around 35 and 74° with applied loads of 1500 and 2000 le are shown in Figure 3(e) and (f), which demonstrates the existence of the cubic and t0 phases as well as the phase transformation between them. The 0 0 2t0 and 2 0 0t0 peak positions of the t0 phase were around
Figure 3. X-ray diffraction patterns of 3YSZ sintered at 1475 °C for 1 h using the in situ compression–diffraction experiment at 2h of (a) 28°, (b) 35°, (c) 60° and (d) 74°, and peak simulation of cubic and metastable tetragonal (t0 ) phases in the X-ray diffraction patterns of 3YSZ at 2h of (e) 35° and (f) 74° with increasing load from 1500 to 2000 le.
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2h of 35°, near the 2 0 0c reflection of the cubic phase. A ‘‘decrease” in the peak intensity of the 0 2 0c reflection and an ‘‘increase” in the intensities of the 0 0 2t, and 2 0 0t, reflections were observed due to the increase in load from 1500 to 2000 le; the same results also occurred in the X-ray diffraction simulation of 3YSZ around 2h of 74°. The relationship between the peak variations of the c and t0 phases demonstrated that the transition of t0 -to-c was induced by applying a compressive load to 2000 le and the c-to-t0 phase transformation happened after the external loading was released. Significant variations in the intensities of the c and t0 peaks are found under compressive loading, indicating the dynamic change in the crystal structure of 3YSZ. Similarly, peak variation between the c and t0 phases of 3YSZ sintered at 1475 °C for 1 h was also observed in the 5 mol.% YNbO4-doped 3YSZ specimen, as shown in Figure 4. Unusual peaks in the 5 mol.% YNbO4-doped 3YSZ seem to be induced more easily than in pure 3YSZ as the load applied was only up to 500 le, indicating that phase transformation between the t0 and c phases depends not only on the applied compression, but also on the doped element in the specimen, as suggested in Figure 4(a)–(d). Trivalent yttrium oxide, Y2O3, is a common stabilizer of zirconia which works by suppressing the formation of t-to-m phase transformation during the cooling process. Doping Y2O3 into zirconia produces oxygen vacancies, increases the amount of c phase and reduces the content of t and m phases; however, t-to-m phase transformation was easily induced by adding Nb2O5 into zirconia [19,20]. The co-existence of Y2O3 and Nb2O5 modifies the phase stability of ZrO2-based materials and therefore clarify the phase transformation characteristic of zirconia ceramics. Peak separation and structural simulation demonstrate the co-existence of the cubic and t0 phases, as seen in Figure 4(e) and (f). The result in this work is different from the previous view that t0 phase is stable due to its high yttria content and that it improves the toughness of zirconia materials by its ferroelastic behavior. The t0 phase was often identified as ‘‘a non-transformation phase”, as the results of Sakuma et al. [21] reported, and the t0 phase possesses high resistance to transformation to the m phase. However, Zhu et al. [22] reported that diffusional phase separation took place and that the t0 phase could decompose into the cubic phase at low concentration (4 mol.%) yttria-doped ZrO2 aged for 10–80 h. This means that formation of the t0 phase takes place not only at high yttira concentrations but also with low yttria content. In fact, t0 -to-m phase transformation of arc-melted ZrO2–RO1.5 (R = Sm, Y, Er and Sc) was observed in water at 200 °C and 100 MPa pressure [23]; therefore it is reasonable to assume that phase transformation of t0 phase may occur easily at low yttria concentration, with the application of a compressive load and under hydrothermal conditions. This suggests that t0 phase is not in fact ‘‘a non-transformable phase”, and the phase transformation behavior of the t0 phase seems to depend on the chemical free energy (dopant content) as well as the nonchemical free energy (grain size, strain filed, compressive load, etc.). Since both t0 -to-m and t0 -to-c transformations were observed, the difference between the t0 -to-m reported by
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Figure 4. XRD diffraction patterns of a 5 mol.% YNbO4-doped 3YSZ specimen sintered at 1475 °C for 1 h using the in situ compression– diffraction experiment at 2h around (a) 28°, (b) 35°, (c) 60° and (d) 74°, and peak simulation of the cubic and a metastable tetragonal (t0 ) phases in the X-ray diffraction patterns of 5 mol.% YNbO4-doped 3YSZ at 2h of (e) 35° and (f) 74° with increasing load to from the initial state to 500 le.
Yashima et al. and the t0 -to-c observed in this work was discussed. Yashima et al. studied the t0 -to-m phase transformation using homogeneous t0 -samples prepared by the arc-melting furnace and a rapid quenching technique. Serious lattice deformation and large internal stresses have been shown to exist in the t0 phase fabricated by the rapid quenching technique [24,25]. Therefore t0 -to-m phase transformation was easily induced by external stress to form the monoclinic phase. The monoclinic phase in the arc-melted t0 samples [23] after hydrothermal treatment at 200 °C and 100 MPa pressure is very similar to the results formed by diffusionless martensitic t-to-m phase transformation during quenching of melts and/or during crushing and grinding of the samples. However, the specimens for the in situ high-resolution X-ray diffraction-compression test were prepared by solid-state reactions and were sintered at high temperatures. The solid-state reactions would often yield compositional and microstructural inhomogeneity, because a phase separation occurs in the two-phase (t + c) region due to the solubility limit of yttria in the tetragonal phase at sintering temperatures. When both Y and Nb were introduced into a 3Y–ZrO2 matrix, grain growth occurred, indicating an enhancement of element diffusion. Therefore, the microstructures and internal stresses of these materials exhibit quite different characteristics from those of rapidly quenched specimens. The microstructure/stress distribution and the chemical composition inhomogeneity, as well as the testing environments of the two investigations, were different; therefore, a different mechanism of phase transformation might be induced. Based upon the above results, a reversible t0 -to-c was found in the present study under uniaxial compression load and a t0 -to-m phase transformation was observed under the high-pressure thermal condition of 200 °C and 100 MPa. It is interesting that the t0 phase, which might be an intermediate phase between the cubic phase and the monoclinic phase, could transfer to other kinds of phase structures under different forms of external loading. Further investigations on the phase transformation of the t0 phase might clarify the relations among the transformable t0 phase and the cubic and monoclinic phases. In summary, in situ compression–diffraction experiments with incremental compressive loading on the stress-induced phase transformation of 3YSZ ceramics were investigated. There was no variation in the amount
of the tetragonal and monoclinic phases under uniaxial loading, suggesting that the t-to-m phase transformation and the domain switching in the tetragonal crystals do not contribute to energy absorption before fracture/ crack generation in the 3YSZ specimen. On the other hand, an in situ compression–diffraction experiment shows that a reversible phase transformation occurs between the cubic phase and the transformable t0 phase in the specimens during the application and release of the compressive load, and that this depends not only on the compression applied, but also on the doping element in the specimens. Financial support from the Synchrotron Radiation Research Center and National Science Council of Taiwan, Republic of China through project numbers NSC 97-2221-E-011-006 and NSC 97-2923-M-029-001MY3 is gratefully acknowledged by the authors. [1] R.C. Garvie, R.J.H. Hannink, R.T. Pascoe, Nature (London) 258 (5537) (1975) 703. [2] M.V. Swain, R.J.H. Hannink, R.C. Garvie, Fract. Mech. Ceram. 6 (1983) 339. [3] D.J. Kim, H.J. Jung, H.J. Kim, J. Mater. Sci. 14 (4) (1995) 285. [4] R.A. Miller, J.C. Smielek, in: A.H. Heuer, L.W. Hobbs (Eds.), Science and Technology of Zirconia II, Columbus Press, Columbus, OH, 1981, pp. 241–255. [5] S.J. Lin, H.Y. Lu, P. Shen, J. Mater. Sci. 26 (1991) 112. [6] V. Lanteri, R. Chaim, A.H. Heuer, J. Am. Ceram. Soc. 69 (1986) C-258. [7] A.H. Heuer, R. Chaim, V. Lanteri, Acta Metall. 35 (1987) 661. [8] T. Sakuma, J. Mater. Sci. 22 (1987) 4470. [9] C.J. Chan, F.F. Lange, M. Ruhle, J.F. Jue, A.V. Virkar, J. Am. Ceram. Soc. 74 (1991) 807. [10] D.B. Marshall, M.R. James, J.R. Porter, J. Am. Ceram. Soc. 72 (2) (1989) 218. [11] 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 (7) (1997) 1910. [12] C.A. Bateman, M.R. Notis, J. Mater. Sci. Lett. 9 (1990) 803. [13] C.J. Howard, E.H. Kisi, R.B. Roberts, R.J. Hill, J. Am. Ceram. Soc. 73 (1990) 2828. [14] Y.C. Lai, Master Thesis, National Taiwan University of Science and Technology, Taiwan, 1995. [15] Y.C. Lai, C.C. Chou, J. Ceram. Soc. Taiwan 1 (1995) 26–32. [16] D.J. Kim, J. Am. Ceram. Soc. 73 (1990) 115. [17] G. Gritzner, C. Puchner, J. Eur. Ceram. Soc. 13 (1994) 387. [18] M. Sugiyama, H. Kubo, in: S. Somiya, N. Yamamoto, H. Yanagida (Eds.), Science and Technology of Zirconia III, American Ceramics Society, Westerville, OH, 1988. [19] D. Casellas, F.L. Cumbrera, F.S. Bajo, W. Forsling, L. Llanes, M. Anglada, J. Eur. Ceram. Soc. 21 (2001) 765. [20] M. Yashima, T. Hirose, M. Kakihana, Y. Suzuki, M. Yoshimura, J. Am. Ceram. Soc. 80 (1) (1997) 171. [21] T. Sakuma, Y.I. Yoshizawa, H. Suto, J. Mater. Sci. 20 (1985) 2399. [22] W.Z. Zhu, T.C. Lei, Y. Zhou, Z.S. Ding, J. Mater. Sci. 30 (1995) 6235. [23] M. Yashima, T. Nagatome, T. Noma, N. Ishizawa, Y. Suzuki, M. Yoshimura, J. Am. Ceram. Soc. 78 (8) (1995) 2229. [24] D. Baither, B. Baufeld, U. Messerschmidt, A.H. Foitzik, M. Ruhle, J. Am. Ceram. Soc. 80 (7) (1997) 1691. [25] B. Baufeld, D. Baither, U. Messerschmidt, M. Bartsch, A.H. Foitzik, M. Ruhle, J. Am. Ceram. Soc. 80 (7) (1997) 1699.