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Critical grain size and fracture toughness of 2 mol.% yttria-stabilized zirconia at ambient and cryogenic temperatures
Available online at www.sciencedirect.com
Scripta Materialia 67 (2012) 963–966 www.elsevier.com/locate/scriptamat
Critical grain size and fracture toughness of 2 mol.% yttria-stabilized zirconia at ambient and cryogenic temperatures Weijiang Xue,a Zhipeng Xie,a,⇑ Jian Yia,b and Juan Chena a
State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China b School of Mechanical Engineering, Taizhou University, Taizhou 318000, People’s Republic of China Received 2 May 2012; accepted 23 August 2012 Available online 29 August 2012
We report, for the first time, the critical grain sizes of 2 mol.% yttria-stabilized zirconia at various cryogenic temperatures determined by in situ phase examination. The grain size dependence of fracture toughness revealed that the maximum toughness value corresponds to the critical grain size at each temperature. On the basis of nucleation and thermodynamic theories, an established linear relationship between inverse critical grain size and temperature is explained and a basic understanding of grain size dependence of toughness at room temperature is also gained. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Toughness; Ceramics; Cryogenic temperatures; Raman spectroscopy; Thermodynamics
The outstanding toughening capability exhibited by yttria-stabilized tetragonal zirconia (Y-TZP) at cryogenic temperatures has attracted considerable attention in these materials as potential candidates for cryogenic structural applications. Recent work has demonstrated that the mechanical properties and reliability of 3YTZP can be improved significantly at cryogenic temperatures [1]. Most previous research has focused only on the relationship between cryogenic temperatures and properties [1–3], though a few attempts have been made to investigate the microstructure–property relationship of Y-TZP ceramics at cryogenic temperatures. It is well established that transformation toughening [4–6], the dominant toughening mechanism in Y-TZP ceramics, is grain size dependent [7–9]; in other words, for Y-TZP ceramics with a certain chemical composition, the tetragonal grain size dictates the phase transformation capability. As a result, there exists a critical grain size for a given working temperature, beyond which tetragonal grain will transform to monoclinic phase [10], giving rise to a deterioration in fracture toughness. Therefore, for Y-TZP ceramics, the critical grain size is of great importance both to fabrication and to the optimization of toughness. Up until now, research work has concentrated on the critical grain size of Y-TZPs at room temperature. For example, Wang et al. [11] observed critical grain sizes
⇑ Corresponding author. E-mail addresses: [email protected]. edu.cn (W. Xue), [email protected] (Z. Xie).
of 0.87, 1.34 and 1.46 lm for yttria dopant concentrations (mol.%) of 2, 2.5 and 3Y-TZP. A smaller critical grain size for 2Y-TZP has also been reported by Lange et al. [12]. However, few studies to determine the critical grain size and grain size dependence of fracture toughness of 2YTZP at cryogenic temperatures have been reported. In the present work, we prepared a series of 2Y-TZP ceramics with different grain sizes ranging from 112 to 888 nm by spark plasma sintering. Critical grain sizes at ambient and cryogenic temperatures were determined by phase composition examination using in situ Raman spectroscopy. To our knowledge, this is the first report on the critical grain sizes of 2Y-TZP at cryogenic temperatures. The analysis is supported by the investigation of the grain size dependence of fracture toughness, revealing that the maximum toughness values at 293, 195 and 77 K correspond to the critical grain size at each temperature. On the basis of nucleation and thermodynamic theories, a basic understanding of the dependence of fracture toughness on grain size at room temperature was obtained and a linear relationship between inverse critical grain size and temperature was also established and explained. It is believed that our work could provide an effective way to optimize the fracture toughness of 2Y-TZP ceramics at cryogenic temperatures. Tetragonal zirconia nanopowders stabilized with 2 mol.% Y2O3 (TZ-2Y, Tosoh Corporation, Japan) were used for the preparation of test specimens. The powder was sintered using an spark plasma sintering (SPS)
1359-6462/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.08.030
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machine (SPS-1050T, Sumitomo Coal Mining Co. Ltd., Japan) and a graphite die with a 20 mm inner diameter. Under a vacuum and a uniaxial pressure of 50 MPa, the powder was heated to the desired temperatures ranging from 1200 to 1600 °C with a dwell time from 2 to 15 min. The temperature was controlled by monitoring the surface temperature Ts of the graphite die using an optical pyrometer. Sintered specimens were first heat treated and then ground flat using a diamond grinding wheel and then polished carefully with successively finer diamond pastes. Fracture toughness and hardness were measured by Vickers indentation on polished surface at 293, 195 and 77 K using a Vickers hardness tester (HV-120; Lai Zhou Hardness Tester Manufactory, PR China) with loads of 196 and 49N, respectively. During the cryogenic tests, specimens were indented in the refrigerants (77 K: liquid nitrogen; 195 K: dry ice and ethanol). After indentation, the lengths of the indentation-induced cracks were immediately measured by optical microscopy (BX50; Olympus, Japan). More than 10 perfect indentations with clearly symmetrical indentation impressions and symmetrical crack patterns were used for measurements at each temperature. Because zirconia cracks in a Palmqvist, rather than a half-penny, mode, the fracture toughness KIC was obtained from the expression given by Niihara et al. [13]: K IC ¼ 9:052 103 H 3=5 E2=5 d c1=2
ð1Þ
where E is the Young’s modulus, d is the diagonal of the indentation, c is the crack length, and H is the hardness calculated from the following expression: H ¼ 1:8544 P d 1=2
ð2Þ
where P is the applied load. In the present calculations a value of E = 210 GPa has been assumed for all the samples [14]. Grain size was determined by scanning electron microscopy (JSM-6460LV, Shimadzu, Japan) examination of polished and thermally etched surfaces and also fracture surfaces. Phase compositions were obtained in situ, using a cooling stage, by Raman spectroscopy of polished surfaces (Lab Ram HR, HORIBA Jobin Yvon, France) with an argon ion laser, with a wavelength of 633 nm for Raman excitation. Raman data from six different areas of polished surfaces were collected to calculate the amount of monoclinic phase based on a Raman calibration process reported elsewhere [1]. In order to obtain dense sintered 2Y-TZP ceramics with a wide range of grain sizes from nearly nanometer (100 nm) to submicrometer (900 nm), SPS with various sintering schedules was used due to its lower sintering temperature [15]. Figure 1 shows the microstructures of 2Y-TZP samples sintered at 1200–1600 °C with a dwell time of 15 min. As can be seen from the figure, there is no porosity in the consolidated ceramics corresponding to a measured 99% relative density. The measured average grain size ranged from 112 nm (sintered at 1200 °C/15 min) to 888 nm (sintered at 1600 °C/15 min). Tetragonal (t) phase in pure zirconia (ZrO2) can transform to monoclinic (m) phase at 950 °C on cooling, which is accompanied by a shear strain of 0.16 and a volume expansion of 4%: this is the so-called martensitic transformation [5]. However, tetragonal phase can be retained as a metastable phase at room temperature both by
the addition of certain solutes and the reduction of zirconia particle/grain size [10]. Therefore, for each working temperature, there exists a critical grain size above which tetragonal grains will transform to monoclinic ones for a given chemical composition. When the grain size reaches the critical size, this transformation can be accomplished over a relatively narrow range of temperatures below the working temperature for TZP materials due to its autocatalytic transformation [5]. Thus, Raman spectra, from which the fraction of monoclinic and tetragonal zirconia at a given temperature could be determined, were collected in situ to monitor the evolution of the monoclinic and tetragonal phases. The advantage of this technique is that the Raman signal from both t- and m-ZrO2 is very strong [16]. The Raman spectra of the materials with different grain sizes at 293, 195 and 77 K are presented in Figure 2a–c. Inspection of the Raman spectra of the materials at 293 K (Fig. 2a) shows that when the grain size reached 784 nm, tetragonal phases have already partially transformed to monoclinic phases (about 11%), revealing that the critical grain size of 2Y-TZP at 293 K is 733 nm. Following the above analysis, we can also note from the in situ phase composition examination shown in Figure 2b and c that the critical grain sizes at 195 and 77 K are 575 and 301 nm, respectively. To our knowledge this is the first report on the critical grain sizes of 2Y-TZP at cryogenic temperatures. It is also noted that our result of 2Y-TZP at room temperature is analogous to that reported by Wang et al. [11] (870 nm) but different from that of Lange [12], whose experimental work showed a smaller value of 250 nm. This could be attributed to different processing procedures resulting in different ceramic microstructural parameters such as yttrium distribution in grains, purity, phase assemblage, etc. [17]. Moreover, in order to obtain the relationship between temperature and transformation, samples with grain sizes of 784 and 595 nm were chosen to obtain quantitative results on the fraction of monoclinic phase during cooling from 293 and 195 K, respectively. The evolution of the Raman spectra is shown in Figure 2d and e. It is apparent that further cooling down from 293 and 195 K resulted in the increase of the characteristic peaks of the monoclinic phase at the expense of the tetragonal ones. The fractions of m-ZrO2 were calculated and are plotted as a function of temperature in Figure 2f. About 60–70% tetragonal phase finally transformed to monoclinic in a narrow temperature range at 150 K. The fraction of monoclinic phase increased nearly exponentially with decreasing temperature. For TZP materials, the transforming region is surrounded by t-ZrO2 that can itself transform. The shear strains resulting from an m martensite plate that impinges on the boundary of its parent t grain can stimulate the formation of another martensite plate in the neighboring grain [5]. This autocatalytic nucleation of the t–m martensitic transformation in Y-TZPs has been reported by Ruhle et al. [18]. It is widely known that the fracture toughness of YTZP materials increases with increasing grain size, although the exact dependencies are still a matter of debate [19]. In order to correlate grain size with fracture toughness at cryogenic temperatures, systematic study has been performed on fracture toughness of samples with different grain sizes at 293, 195 and 77 K (see
W. Xue et al. / Scripta Materialia 67 (2012) 963–966
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Figure 1. SEM images of polished and thermally etched surfaces of 2Y-TZP ceramics sintered at (a) 1200 °C, (b) 1300 °C, (c) 1400 °C, (d) 1500 °C, (e) 1550 °C, (f) 1600 °C (with a dwell time of 15 min).
Fig. 3). The indentation toughness exhibited a maximum at the critical grain size for each temperature, further validating our results on the critical grain size which showed that the fracture toughness decreased after reaching a critical grain size, a behavior that is attributed to premature phase transformation [20]. Therefore, we can optimize the fracture toughness of 2Y-TZP according to the critical grain sizes at cryogenic temperatures. It is also noted that the dependence at room temperature can be divided into two areas. First, the toughness was almost constant (4.5 MPa m1/2) up to a grain size of 400 nm. In the second area for larger grains, the toughness increased up to the critical grain size. Similar dependence has been reported in Refs. [17,21] for Y-TZPs. However, the reason for such a dependence remains elusive. In an effort to address the above issue, we should first establish the relationship between microstructural parameters and fracture toughness. According to Becher et al. [19], internal stress (ri) originating from the large anisotropy in crystallographic thermal expansion coefficients of 2Y-TZP (aa along the a axis is 8.9 106 °C1, and ac along the c axis is 13 106 °C1 [22]) plays an important role in the transformation nucleation. The external applied stress (ra) required to initiate the transformation is [19]: ra ¼ rTc ri ¼ rTc rTEA d=r;
ð3Þ
where rTc is the critical transformation stress in the absence of any internal stress, d is the grain size, r is the distance from the grain corner, and rTEA is the stress from thermal expansion anisotropy (rTEA ¼ ðac aa ÞDT , constant at ambient temperature). In addition, the relationship between the intrinsic toughness K0 and the contribution of transformation-toughening to toughness DKT can be described as follows [1,19]: K0 ¼ 0:9½ra 1:6 B E V T eT 1 DK T
ð4Þ
K IC ¼ K 0 þ DK T
ð5Þ
where KIC is the fracture toughness, B is a constant, E is the Young’s modulus, VT is the volume fraction of the tetragonal phase that transforms (related to ra), and eT is the volumetric transformation strain (4%). Thus, the relationship between fracture toughness and microstructure can be derived by combining Eqs. (3)–(5): 1 þ 1 ; ð6Þ K IC ¼ K 0 A V T ðrTc ra Þ 1 where A is a constant (A = 0.91.6BeT); K0 remains constant according to Refs. [1,19]. Becher et al. [19] reported that the formation of a nuclei which is stable
Figure 2. Raman spectra of the materials with different grain sizes at (a) 293 K, (b) 195 K and (c) 77 K; Raman spectra of samples with grain sizes of (e) 784 nm and (f) 595 nm upon cooling from 293 and 195 K, respectively; (f) the calculated fraction of m-ZrO2 as a function of temperature.
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W. Xue et al. / Scripta Materialia 67 (2012) 963–966
Figure 3. Grain size dependence of fracture toughness at 293, 195 and 77 K.
Figure 4. Inverse critical grain size of 2Y-TZP vs. temperature.
and from which the transformation proceeds requires the nuclei to be of sufficient size, r P rcrit, to overcome the nucleation barrier; the resultant internal stress will then increase as the grain size d increases. Therefore, it might be speculated that when the grain size is below 400 nm, the nuclei size is not sufficient to overcome the nucleation barrier, resulting in an almost unchanged ra. According to Eq. (6), fracture toughness will therefore remain constant when the grain size is below 400 nm. When the grain size increases above 400 nm (below dc), the nuclei size can be sufficient for the resultant internal stress to increase as the grain size d increases, leading to an increment in fracture toughness. Following the above ideas about the toughness maximum at the critical grain size for each temperature, it is worth noting the possibility of optimizing the toughness of 2Y-TZP according to the critical grain size at cryogenic temperatures. In an effort to address this issue, the relationship between temperature and inverse critical grain size was established, as shown in Figure 4. It is obvious that the data in Figure 4 describe a linear relationship, which can be explained from a thermodynamic point of view. According to Lange [23]:
grain size at each temperature. Based on nucleation theory, the relationship between fracture toughness and microstructure has been derived to explain the dependence of fracture toughness on grain size at room temperature. Moreover, a linear relationship between inverse critical grain size and temperatures has been established and explained from the viewpoint of thermodynamic. It is believed that this relationship can provide an effective way to optimize the fracture toughness of 2Y-TZP ceramics at cryogenic temperatures.
1 jDGc j DU se f ; / dc Dc
ð7Þ
where (jDGc j DU se f ) is the work-loss per unit volume during the stress-induced transformation, and Dc is the change in boundary energy associated with the phase transformation. Dc can be neglected because the interfacial energy effect appears to affect the critical grain size by less than an order of magnitude while the solid has additional strain energy effects (due to matrix constraint) and interfacial energy effects (due to twinning; also due to the presence of interfaces vs. free surfaces) compared to the powder case [14]. In addition, the calculated plots from experimental data by Lange [24] showed a linear relationship between (jDGc j DU se f ) and temperature for 2Y-TZP. Therefore, it is reasonable that the relationship between temperature and inverse critical grain size is also linear according to Eq. (7). In summary, critical grain sizes of 2Y-TZP sintered by SPS at ambient and cryogenic temperatures were determined by phase composition examination using in situ Raman spectroscopy. The critical grain sizes at 293, 195 and 77 K were 733, 575 and 301 nm, respectively, determined by investigation of the grain size dependence of fracture toughness, revealing that the maximum toughness values correspond to the critical
The authors acknowledge support by the National Natural Science Foundation of China (Grant No. 51232004). [1] W.J. Xue, Z.P. Xie, G.W. Liu, W. Liu, J. Yi, J. Am. Ceram. Soc. 94 (2011) 2775. [2] S. Veitch, M. Marmach, M.V. Swain, Mater. Res. Soc. 78 (1987) 97. [3] T.R. Lai, C.L. Hogg, M.V. Swain, ISIJ Int. 29 (1989) 240. [4] J. Wang, R. Stevens, J. Mater. Sci. 24 (1989) 3421. [5] R.H. Hannink, P.M. Kelly, B.C. Muddle, J. Am. Ceram. Soc. 83 (2000) 461. [6] R.C. Garvie, R.H. Hannink, R.T. Pascoe, Nature 258 (1975) 703. [7] M.V. Swain, J. Mater. Sci. 5 (1986) 1159. [8] J. Eichler, J. Rodel, J. Am. Ceram. Soc. 90 (2007) 2830. [9] A. Suresh, M.J. Mayo, W.D. Porter, C.J. Rawn, J. Am. Ceram. Soc. 86 (2003) 360. [10] D. Casellas, F.L. Cumbrera, F.S. Bajo, W. Forsling, L. Llanes, M. Anglada, J. Euro, Ceram. Soc. 21 (2001) 765. [11] J. Wang, M. Rainforth, R. Stevens, Br. Ceram. Trans. J. 88 (1989) 1. [12] F.F. Lang, J. Mater. Sci. 17 (1982) 240. [13] K. Niihara, R. Morena, D.P.H. Hasselman, J. Mater. Sci. Lett. 1 (1982) 13. [14] A. Bravo-Leon, Y. Morikawa, M. Kawahara, M.J. Mayo, Acta Mater. 50 (2002) 4555. [15] Z.J. Shen, M. Johosson, Z. Zhao, M. Nygren, J. Am. Ceram. Soc. 85 (2002) 1921. [16] V. Lughi, D.R. Clarke, Acta Mater. 55 (2007) 2049. [17] M. Trunec, Ceram. Silik. 52 (2008) 165. [18] M. Ruhle, N. Claussen, A.H. HeuerAdvances in Ceramics. Science and Technology of Zirconia III, vol. 12, American Ceramic Society, Columbus, OH, 1985, pp. 352–370. [19] P.F. Becher, M.V. Swain, J. Am. Ceram. Soc. 75 (1992) 493. [20] J. Eichler, M. Hoffman, U. Eisele, J. Rodel, J. Euro, Ceram. Soc. 26 (2006) 3575. [21] B.V.M. Kumar, W.S. Kim, S.H. Hong, H.T. Bae, D.S. Lim, Mater. Sci. Eng. A 527 (2010) 474. [22] S. Schmauder, H. Schubert, J. Am. Ceram. Soc. 69 (1986) 534. [23] F.F. Lang, J. Mater. Sci. 17 (1982) 225. [24] F.F. Lang, J. Mater. Sci. 17 (1982) 255.
Scripta Materialia 67 (2012) 963–966 www.elsevier.com/locate/scriptamat
Critical grain size and fracture toughness of 2 mol.% yttria-stabilized zirconia at ambient and cryogenic temperatures Weijiang Xue,a Zhipeng Xie,a,⇑ Jian Yia,b and Juan Chena a
State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China b School of Mechanical Engineering, Taizhou University, Taizhou 318000, People’s Republic of China Received 2 May 2012; accepted 23 August 2012 Available online 29 August 2012
We report, for the first time, the critical grain sizes of 2 mol.% yttria-stabilized zirconia at various cryogenic temperatures determined by in situ phase examination. The grain size dependence of fracture toughness revealed that the maximum toughness value corresponds to the critical grain size at each temperature. On the basis of nucleation and thermodynamic theories, an established linear relationship between inverse critical grain size and temperature is explained and a basic understanding of grain size dependence of toughness at room temperature is also gained. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Toughness; Ceramics; Cryogenic temperatures; Raman spectroscopy; Thermodynamics
The outstanding toughening capability exhibited by yttria-stabilized tetragonal zirconia (Y-TZP) at cryogenic temperatures has attracted considerable attention in these materials as potential candidates for cryogenic structural applications. Recent work has demonstrated that the mechanical properties and reliability of 3YTZP can be improved significantly at cryogenic temperatures [1]. Most previous research has focused only on the relationship between cryogenic temperatures and properties [1–3], though a few attempts have been made to investigate the microstructure–property relationship of Y-TZP ceramics at cryogenic temperatures. It is well established that transformation toughening [4–6], the dominant toughening mechanism in Y-TZP ceramics, is grain size dependent [7–9]; in other words, for Y-TZP ceramics with a certain chemical composition, the tetragonal grain size dictates the phase transformation capability. As a result, there exists a critical grain size for a given working temperature, beyond which tetragonal grain will transform to monoclinic phase [10], giving rise to a deterioration in fracture toughness. Therefore, for Y-TZP ceramics, the critical grain size is of great importance both to fabrication and to the optimization of toughness. Up until now, research work has concentrated on the critical grain size of Y-TZPs at room temperature. For example, Wang et al. [11] observed critical grain sizes
⇑ Corresponding author. E-mail addresses: [email protected]. edu.cn (W. Xue), [email protected] (Z. Xie).
of 0.87, 1.34 and 1.46 lm for yttria dopant concentrations (mol.%) of 2, 2.5 and 3Y-TZP. A smaller critical grain size for 2Y-TZP has also been reported by Lange et al. [12]. However, few studies to determine the critical grain size and grain size dependence of fracture toughness of 2YTZP at cryogenic temperatures have been reported. In the present work, we prepared a series of 2Y-TZP ceramics with different grain sizes ranging from 112 to 888 nm by spark plasma sintering. Critical grain sizes at ambient and cryogenic temperatures were determined by phase composition examination using in situ Raman spectroscopy. To our knowledge, this is the first report on the critical grain sizes of 2Y-TZP at cryogenic temperatures. The analysis is supported by the investigation of the grain size dependence of fracture toughness, revealing that the maximum toughness values at 293, 195 and 77 K correspond to the critical grain size at each temperature. On the basis of nucleation and thermodynamic theories, a basic understanding of the dependence of fracture toughness on grain size at room temperature was obtained and a linear relationship between inverse critical grain size and temperature was also established and explained. It is believed that our work could provide an effective way to optimize the fracture toughness of 2Y-TZP ceramics at cryogenic temperatures. Tetragonal zirconia nanopowders stabilized with 2 mol.% Y2O3 (TZ-2Y, Tosoh Corporation, Japan) were used for the preparation of test specimens. The powder was sintered using an spark plasma sintering (SPS)
1359-6462/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.08.030
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machine (SPS-1050T, Sumitomo Coal Mining Co. Ltd., Japan) and a graphite die with a 20 mm inner diameter. Under a vacuum and a uniaxial pressure of 50 MPa, the powder was heated to the desired temperatures ranging from 1200 to 1600 °C with a dwell time from 2 to 15 min. The temperature was controlled by monitoring the surface temperature Ts of the graphite die using an optical pyrometer. Sintered specimens were first heat treated and then ground flat using a diamond grinding wheel and then polished carefully with successively finer diamond pastes. Fracture toughness and hardness were measured by Vickers indentation on polished surface at 293, 195 and 77 K using a Vickers hardness tester (HV-120; Lai Zhou Hardness Tester Manufactory, PR China) with loads of 196 and 49N, respectively. During the cryogenic tests, specimens were indented in the refrigerants (77 K: liquid nitrogen; 195 K: dry ice and ethanol). After indentation, the lengths of the indentation-induced cracks were immediately measured by optical microscopy (BX50; Olympus, Japan). More than 10 perfect indentations with clearly symmetrical indentation impressions and symmetrical crack patterns were used for measurements at each temperature. Because zirconia cracks in a Palmqvist, rather than a half-penny, mode, the fracture toughness KIC was obtained from the expression given by Niihara et al. [13]: K IC ¼ 9:052 103 H 3=5 E2=5 d c1=2
ð1Þ
where E is the Young’s modulus, d is the diagonal of the indentation, c is the crack length, and H is the hardness calculated from the following expression: H ¼ 1:8544 P d 1=2
ð2Þ
where P is the applied load. In the present calculations a value of E = 210 GPa has been assumed for all the samples [14]. Grain size was determined by scanning electron microscopy (JSM-6460LV, Shimadzu, Japan) examination of polished and thermally etched surfaces and also fracture surfaces. Phase compositions were obtained in situ, using a cooling stage, by Raman spectroscopy of polished surfaces (Lab Ram HR, HORIBA Jobin Yvon, France) with an argon ion laser, with a wavelength of 633 nm for Raman excitation. Raman data from six different areas of polished surfaces were collected to calculate the amount of monoclinic phase based on a Raman calibration process reported elsewhere [1]. In order to obtain dense sintered 2Y-TZP ceramics with a wide range of grain sizes from nearly nanometer (100 nm) to submicrometer (900 nm), SPS with various sintering schedules was used due to its lower sintering temperature [15]. Figure 1 shows the microstructures of 2Y-TZP samples sintered at 1200–1600 °C with a dwell time of 15 min. As can be seen from the figure, there is no porosity in the consolidated ceramics corresponding to a measured 99% relative density. The measured average grain size ranged from 112 nm (sintered at 1200 °C/15 min) to 888 nm (sintered at 1600 °C/15 min). Tetragonal (t) phase in pure zirconia (ZrO2) can transform to monoclinic (m) phase at 950 °C on cooling, which is accompanied by a shear strain of 0.16 and a volume expansion of 4%: this is the so-called martensitic transformation [5]. However, tetragonal phase can be retained as a metastable phase at room temperature both by
the addition of certain solutes and the reduction of zirconia particle/grain size [10]. Therefore, for each working temperature, there exists a critical grain size above which tetragonal grains will transform to monoclinic ones for a given chemical composition. When the grain size reaches the critical size, this transformation can be accomplished over a relatively narrow range of temperatures below the working temperature for TZP materials due to its autocatalytic transformation [5]. Thus, Raman spectra, from which the fraction of monoclinic and tetragonal zirconia at a given temperature could be determined, were collected in situ to monitor the evolution of the monoclinic and tetragonal phases. The advantage of this technique is that the Raman signal from both t- and m-ZrO2 is very strong [16]. The Raman spectra of the materials with different grain sizes at 293, 195 and 77 K are presented in Figure 2a–c. Inspection of the Raman spectra of the materials at 293 K (Fig. 2a) shows that when the grain size reached 784 nm, tetragonal phases have already partially transformed to monoclinic phases (about 11%), revealing that the critical grain size of 2Y-TZP at 293 K is 733 nm. Following the above analysis, we can also note from the in situ phase composition examination shown in Figure 2b and c that the critical grain sizes at 195 and 77 K are 575 and 301 nm, respectively. To our knowledge this is the first report on the critical grain sizes of 2Y-TZP at cryogenic temperatures. It is also noted that our result of 2Y-TZP at room temperature is analogous to that reported by Wang et al. [11] (870 nm) but different from that of Lange [12], whose experimental work showed a smaller value of 250 nm. This could be attributed to different processing procedures resulting in different ceramic microstructural parameters such as yttrium distribution in grains, purity, phase assemblage, etc. [17]. Moreover, in order to obtain the relationship between temperature and transformation, samples with grain sizes of 784 and 595 nm were chosen to obtain quantitative results on the fraction of monoclinic phase during cooling from 293 and 195 K, respectively. The evolution of the Raman spectra is shown in Figure 2d and e. It is apparent that further cooling down from 293 and 195 K resulted in the increase of the characteristic peaks of the monoclinic phase at the expense of the tetragonal ones. The fractions of m-ZrO2 were calculated and are plotted as a function of temperature in Figure 2f. About 60–70% tetragonal phase finally transformed to monoclinic in a narrow temperature range at 150 K. The fraction of monoclinic phase increased nearly exponentially with decreasing temperature. For TZP materials, the transforming region is surrounded by t-ZrO2 that can itself transform. The shear strains resulting from an m martensite plate that impinges on the boundary of its parent t grain can stimulate the formation of another martensite plate in the neighboring grain [5]. This autocatalytic nucleation of the t–m martensitic transformation in Y-TZPs has been reported by Ruhle et al. [18]. It is widely known that the fracture toughness of YTZP materials increases with increasing grain size, although the exact dependencies are still a matter of debate [19]. In order to correlate grain size with fracture toughness at cryogenic temperatures, systematic study has been performed on fracture toughness of samples with different grain sizes at 293, 195 and 77 K (see
W. Xue et al. / Scripta Materialia 67 (2012) 963–966
965
Figure 1. SEM images of polished and thermally etched surfaces of 2Y-TZP ceramics sintered at (a) 1200 °C, (b) 1300 °C, (c) 1400 °C, (d) 1500 °C, (e) 1550 °C, (f) 1600 °C (with a dwell time of 15 min).
Fig. 3). The indentation toughness exhibited a maximum at the critical grain size for each temperature, further validating our results on the critical grain size which showed that the fracture toughness decreased after reaching a critical grain size, a behavior that is attributed to premature phase transformation [20]. Therefore, we can optimize the fracture toughness of 2Y-TZP according to the critical grain sizes at cryogenic temperatures. It is also noted that the dependence at room temperature can be divided into two areas. First, the toughness was almost constant (4.5 MPa m1/2) up to a grain size of 400 nm. In the second area for larger grains, the toughness increased up to the critical grain size. Similar dependence has been reported in Refs. [17,21] for Y-TZPs. However, the reason for such a dependence remains elusive. In an effort to address the above issue, we should first establish the relationship between microstructural parameters and fracture toughness. According to Becher et al. [19], internal stress (ri) originating from the large anisotropy in crystallographic thermal expansion coefficients of 2Y-TZP (aa along the a axis is 8.9 106 °C1, and ac along the c axis is 13 106 °C1 [22]) plays an important role in the transformation nucleation. The external applied stress (ra) required to initiate the transformation is [19]: ra ¼ rTc ri ¼ rTc rTEA d=r;
ð3Þ
where rTc is the critical transformation stress in the absence of any internal stress, d is the grain size, r is the distance from the grain corner, and rTEA is the stress from thermal expansion anisotropy (rTEA ¼ ðac aa ÞDT , constant at ambient temperature). In addition, the relationship between the intrinsic toughness K0 and the contribution of transformation-toughening to toughness DKT can be described as follows [1,19]: K0 ¼ 0:9½ra 1:6 B E V T eT 1 DK T
ð4Þ
K IC ¼ K 0 þ DK T
ð5Þ
where KIC is the fracture toughness, B is a constant, E is the Young’s modulus, VT is the volume fraction of the tetragonal phase that transforms (related to ra), and eT is the volumetric transformation strain (4%). Thus, the relationship between fracture toughness and microstructure can be derived by combining Eqs. (3)–(5): 1 þ 1 ; ð6Þ K IC ¼ K 0 A V T ðrTc ra Þ 1 where A is a constant (A = 0.91.6BeT); K0 remains constant according to Refs. [1,19]. Becher et al. [19] reported that the formation of a nuclei which is stable
Figure 2. Raman spectra of the materials with different grain sizes at (a) 293 K, (b) 195 K and (c) 77 K; Raman spectra of samples with grain sizes of (e) 784 nm and (f) 595 nm upon cooling from 293 and 195 K, respectively; (f) the calculated fraction of m-ZrO2 as a function of temperature.
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Figure 3. Grain size dependence of fracture toughness at 293, 195 and 77 K.
Figure 4. Inverse critical grain size of 2Y-TZP vs. temperature.
and from which the transformation proceeds requires the nuclei to be of sufficient size, r P rcrit, to overcome the nucleation barrier; the resultant internal stress will then increase as the grain size d increases. Therefore, it might be speculated that when the grain size is below 400 nm, the nuclei size is not sufficient to overcome the nucleation barrier, resulting in an almost unchanged ra. According to Eq. (6), fracture toughness will therefore remain constant when the grain size is below 400 nm. When the grain size increases above 400 nm (below dc), the nuclei size can be sufficient for the resultant internal stress to increase as the grain size d increases, leading to an increment in fracture toughness. Following the above ideas about the toughness maximum at the critical grain size for each temperature, it is worth noting the possibility of optimizing the toughness of 2Y-TZP according to the critical grain size at cryogenic temperatures. In an effort to address this issue, the relationship between temperature and inverse critical grain size was established, as shown in Figure 4. It is obvious that the data in Figure 4 describe a linear relationship, which can be explained from a thermodynamic point of view. According to Lange [23]:
grain size at each temperature. Based on nucleation theory, the relationship between fracture toughness and microstructure has been derived to explain the dependence of fracture toughness on grain size at room temperature. Moreover, a linear relationship between inverse critical grain size and temperatures has been established and explained from the viewpoint of thermodynamic. It is believed that this relationship can provide an effective way to optimize the fracture toughness of 2Y-TZP ceramics at cryogenic temperatures.
1 jDGc j DU se f ; / dc Dc
ð7Þ
where (jDGc j DU se f ) is the work-loss per unit volume during the stress-induced transformation, and Dc is the change in boundary energy associated with the phase transformation. Dc can be neglected because the interfacial energy effect appears to affect the critical grain size by less than an order of magnitude while the solid has additional strain energy effects (due to matrix constraint) and interfacial energy effects (due to twinning; also due to the presence of interfaces vs. free surfaces) compared to the powder case [14]. In addition, the calculated plots from experimental data by Lange [24] showed a linear relationship between (jDGc j DU se f ) and temperature for 2Y-TZP. Therefore, it is reasonable that the relationship between temperature and inverse critical grain size is also linear according to Eq. (7). In summary, critical grain sizes of 2Y-TZP sintered by SPS at ambient and cryogenic temperatures were determined by phase composition examination using in situ Raman spectroscopy. The critical grain sizes at 293, 195 and 77 K were 733, 575 and 301 nm, respectively, determined by investigation of the grain size dependence of fracture toughness, revealing that the maximum toughness values correspond to the critical
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