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Toughening mechanisms of ZTA ceramics at cryogenic temperature (77 K)
Author’s Accepted Manuscript Toughening mechanisms of ZTA ceramics at cryogenic temperature (77K) Juan Chen, Zhipeng Xie, Weining Zeng, Weiwei Wu www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(16)32078-8 http://dx.doi.org/10.1016/j.ceramint.2016.11.072 CERI14160
To appear in: Ceramics International Received date: 1 October 2016 Revised date: 4 November 2016 Accepted date: 11 November 2016 Cite this article as: Juan Chen, Zhipeng Xie, Weining Zeng and Weiwei Wu, Toughening mechanisms of ZTA ceramics at cryogenic temperature (77K), Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.11.072 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Toughening mechanisms of ZTA ceramics at cryogenic temperature (77 K)
Juan Chen, Zhipeng Xie*, Weining Zeng, Weiwei Wu
State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China
*
Corresponding author. [email protected] (Z. Xie)
Abstract ZTA ceramics containing 20 wt.% ZrO2 were fabricated at different sintering temperatures (1450, 1500 and 1550 °C) by SPS and HP processes, respectively. The influence of sintering process on the mechanical properties of ZTA ceramics at 298 K and 77 K was investigated. It can be seen that the bending strength and fracture toughness of samples prepared by the two processes both improved at cryogenic temperature. The stress-induced martensitic transformation toughening mechanism was confirmed by the in-situ Raman technique. The tetragonal ZrO2 would be even more easy to transform because of the residual stress generated when temperature decreased from 298 K to 77 K. Therefore, the transformation toughening effect would become stronger, result in the increase of mechanical properties.
Keywords: ZTA; cryogenic temperature; toughening mechanism; stress-induced martensitic transformation.
1. Introduction Zirconia toughened alumina (ZTA), in which alumina is the primary phase (70% to 95%) and zirconia is the secondary phase (30% to 5%), is a material that combines the advantageous properties of monolithic alumina and zirconia [1]. The addition of zirconia to alumina results in higher strength and fracture toughness with little reduction in hardness and elastic modulus compared with monolithic Al2O3 ceramic [2]. Due to their excellent mechanical properties at room temperature, ZTA ceramics are one of the extremely important structural materials. Major research efforts have focused on the applications at room and high temperatures [3]. Currently, there is an urgent demand of study on the mechanical properties of structural ceramics at cryogenic temperatures as the materials used in cryogenic field raising considerable attention owing to the high-speed development of cryogenic engineering and technology. However, few researches have studied the cryogenic mechanical properties of ZTA ceramics and their potential applications in the cryogenic environment [4]. In this paper, a series of ZTA ceramics were fabricated at different sintering temperatures and processes. The bending strength and fracture toughness were measured at room (298 K) and cryogenic temperatures (77 K) to study the influence of temperature on the mechanical properties. The toughening mechanisms of ZTA ceramics at cryogenic temperature were also discussed in detail.
2. Experimental procedures Commercially available ZTA powders with 20 wt.% ZrO2 (MS-LZTA-BS, Farmeiya, China) produced by coprecipitation process were used as raw materials. In order to investigate the effects of sintering temperature on the mechanical properties of ZTA ceramics, the powders were calcined at 1450, 1500 and 1550 °C. The ZTA powders were weighed and filled into a graphite die with an inside diameter of 30 mm, and sintered by two kinds of methods: spark plasma sintering (SPS) and hot pressing (HP) processes. Under a vacuum and applied pressure of 30 MPa (before 1000 °C) or 50 MPa (after 1000 °C), the powders were sintered at firing temperature with holding time five min for SPS and HP processes. The applied pressure of HP process is 30 MPa, with holding time one hour. The phase compositions of ZTA powders and sintered samples were identified by X-ray diffraction technology (XRD, D8 advance, Bruker, Germany). The bulk densities of the consolidated samples were measured by Archimede’s method with deionized water as the immersion medium, and the theoretical density (TD) of the samples were calculated according to the rule of mixture. The mechanical properties at 298 K and 77 K were tested in a universal testing machine (SUNS UTM4000, China). Bending strength was measured by three-point bending technique with specimen dimension 2.0 mm ×1.5 mm ×25 mm and support span 20 mm, according to the standard procedure ASTM C1161-02c. Fracture toughness was tested using single-edge notched beam (SENB) and three-point bending technique. The specimen size for fracture toughness was 3 mm × 4 mm × 23 mm with support span 16 mm, the
width of notch was less than 0.2 mm, and the depth of notch was 2 mm (ASTM C1421-10). The microstructure of fracture surface was confirmed by scanning electron microscope (SEM, Leo 1530, Zeiss, Germany) under secondary electron imaging and backscattered electron (BSE) signal. The average grain size was determined by calculating the average diameter value of more than 100 random grains from SEM pictures. In-situ Raman spectroscopy (in-situ Raman，LabRAM HR Evolution，France) was used to evaluate the martensitic transformation of tetragonal ZrO2 in ZTA ceramics. Vickers indentation [5] was performed on polished surface of ZTA samples under a load of 100 N at 298 K and 77 K. After indentation, Raman data of the cracks were collected quickly at the same temperature. 3. Results and discussion 3.1 XRD and microstructure analysis XRD patterns for ZTA powders are shown in Fig. 1. There are three crystalline phases in ZTA powders: α-Al2O3, tetragonal zirconia (t-ZrO2) and monoclinic zirconia (m-ZrO2). After sintering process, m-ZrO2 phase disappeared, only α-Al2O3 and t-ZrO2 phases existed (Fig. 2). It can be illustrated that m-ZrO2 transformed into t-ZrO2 when temperature increased, and was not produced again during the cooling process. Extensive literatures have reported that t-ZrO2 phase in ZTA ceramics could have stress-induced martensitic transformation as a toughening mechanism, resulting in the substantial increase of bending strength and fracture toughness for ZTA ceramics [6-8]. Therefore, the existence of t-ZrO2 phase offered the possibility of stress-induced phase transformation toughening mechanism in ZTA samples.
The densities of ZTA samples were shown in Table 1. The relative densities of all the samples exceeded 97%. The densities of ZTA samples obtained by SPS process decreased as sintering temperature increased. But the densities of ZTA samples sintered at 1450 °C and 1500 °C by HP process were the same, when the sintering temperature increased to 1550 °C, the density decreased a little. Compared with the HP sintering process, the densities of ZTA samples prepared by SPS process were slightly higher at the same sintering temperature. This result can be explained by the different pressure exerted during the sintering process. For a certain initial particle size, a threshold pressure exists and above which the pressure is an effective driving force for densification [9]. High applied pressure is beneficial both at the first and intermediate stages of sintering [10]. At firing temperature, the applied pressures were 50 MPa and 30 MPa for SPS and HP processes, respectively. Therefore, the densities of ZTA ceramics sintered by SPS were a little higher than those sintered by HP process. The greatest densities were both obtained at 1450 °C for SPS and HP processes. However, the densities of the samples prepared by the two processes have not changed greatly with the increase of sintering temperature.
Fig. 1 XRD patterns for ZTA powders
Fig. 2 sintered surface XRD patterns of ZTA. (1) SPS; (2) HP
Table 1 The densities of ZTA samples at various sintering temperatures (unit: g/cm3) Sintering Temperature / °C
Sintering Process
1450
1500
1550
SPS
4.41
4.39
4.36
HP
4.40
4.39
4.33
SEM pictures of fracture surfaces are displayed in Fig. 3. In order to distinguish the two different phases in ZTA ceramics, the pictures were taken by black scattered electron (BSE) signals. It is obvious that the t-ZrO2 grains (white) uniformly dispersed in the Al2O3 matrix (black) at the grain boundaries. The grain size distribution of Al2O3 phase was uniform, and no abnormal growth phenomenon was observed in Al2O3 grains. Therefore, the presence of t-ZrO2 grains would inhibit the abnormal growth of Al2O3 grains. Due to the six pictures in Fig. 3 were collected at the same magnification, it can be intuitively seen that the grain sizes of t-ZrO2 and Al2O3 particles both increased significantly when the sintering temperature increased for SPS and HP processes.
Fig. 3 SEM pictures for fracture surfaces by BSE signals of ZTA samples. (1-1) SPS-1450 °C; (1-2) SPS-1500 °C; (1-3) SPS-1550 °C; (2-1) HP-1450 °C; (2-2) HP-1500 °C; (2-3) HP-1550 °C
Table 2 The average grain size of ZTA ceramics Grain Size / μm
Sintering Sintering Temperature Process
ZrO2
Al2O3
1450
0.71
1.07
1500
1.01
1.53
1550
1.75
2.93
1450
1.12
1.8
1500
1.28
2.04
1550
1.91
3.67
/ °C
SPS
HP
The grain sizes of ZTA samples sintered at various temperatures by SPS and HP processes were measured and shown in Table 2. To attain a highly accurate value of the average grain size, more than 100 grains were calculated for ZrO2 and Al2O3 phases. The results showed that the average grain sizes of ZrO2 and Al2O3 phases increased as the sintering temperature improved for the same sintering process, indicating that the increase of sintering temperature would contribute to the growth of ZrO2 and Al2O3 grains. Meanwhile, compared with the samples sintered by SPS process, the grain sizes of the samples sintered by HP process were higher. 3.2 Mechanical properties at different temperatures The bending strengths of ZTA samples sintered by SPS and HP processes were
tested using the three-point bending method at 298 K and 77 K. The results are shown in Fig. 4, the difference of bending strengths between the samples sintered by SPS and HP processes was small when tested at 298 K. For the samples prepared by HP process, the bending strength decreased when sintering temperature increased at 298 K. While a higher bending strength was obtained at 1500 °C for samples sintered by SPS process. However, the bending strengths both improved significantly for the samples sintered by SPS and HP processes at 77 K. The increase magnitude of bending strength for samples sintered by HP process was larger. For example, the bending strength obtained at 77 K increased twice as large as the datum obtained at 298 K for the samples sintered at 1500 °C.
Fig. 4 Bending strengths of ZTA samples sintered by SPS and HP processes at 298 K (RT) and 77 K (LN)
Fig. 5 Fracture toughnesses of ZTA samples sintered by SPS and HP processes at 298 K (RT) and 77 K (LN) The fracture toughnesses of ZTA samples tested at various temperatures are plotted in Fig. 5. For the samples prepared by the same sintering process, the fracture toughness increased basically when temperature decreased from 298 K to 77 K, except for the samples sintered at 1550 °C by HP process, which remained almost unchanged (increased less than 3%). It can be concluded that the fracture toughness of ZTA ceramics would increase at cryogenic temperatures. This results were accordance with the studies of ZTA ceramics by Lange [11]. At 298 K, the largest fracture toughness was obtained when samples sintered at 1450 °C for both sintering processes. For the samples sintered by HP process, at the same test temperature, the fracture toughness decreased basically with sintering temperature increased. The fracture toughness of samples prepared by SPS process decreased when the sintering temperature increased at 298 K. While the relationship between the fracture toughness and sintering temperature was opposite at 77 K.
It is well known that the transformation toughening effect of ZrO2 is affected by the martensitic temperature (Ms) [12, 13]. One of the most important factors that influence the Ms is the grain size of tetragonal ZrO2. The Ms raises as the grain size of tetragonal ZrO2 increases [14], leading to the reduction of the difference ( T ) between the test temperature (T) and the Ms, which is expressed as T =T -M s . And the smaller the T is, the higher the contribution of the martensitic transformation toughening [15]. Therefore, the contribution of martensitic transformation toughening effect becomes larger when the grain size of tetragonal ZrO2 increases. However, the fracture toughness of ZrO2 will decrease after reaching a critical grain size because of the premature phase transformation [16], and the critical grain sizes of 2Y-TZP are 733 nm and 301 nm when test temperatures are 293 K and 77 K, respectively [17]. The grain sizes of ZrO2 in ZTA ceramics increased as the sintering temperature increasing (shown in Table 2). Nevertheless, the fracture toughness of ZTA samples did not show distinct correlation with the grain size of ZrO2, the fracture toughness increased as the grain size of ZrO2 increased only observed in the samples sintered by SPS process and tested at 77 K. The data of strength/toughness trade-off for Y-TZP and Ce-TZP have been clearly shown that Al2O3 additions increase strength but decrease toughness [18, 19]. Therefore, the existence of Al2O3 has a great influence on the fracture toughness of ZTA ceramics, which will discuss later. 3.3 Factors affecting the mechanical properties at various temperatures The toughening mechanism of ZTA ceramics is related to the volume expansion and shear strain produced by the t-ZrO2 to m-ZrO2 transformation. In-situ Raman has
been used to figure out whether the martensitic transformation occurred or not in ZTA ceramics at 298 K and 77 K. The advantage of this technique is that the Raman signals from both monoclinic and tetragonal phases are strong [20] and its excellent capability for micro-area analysis. The characteristic band for monoclinic phase is monoclinic doublet (bands at 181 cm-1 and 192 cm-1), while the tetragonal bands are at 148 cm-1 and 264 cm-1 [21, 22]. Meanwhile, the Raman spectrum for cubic phase becomes wider in the mixed phase samples, so its characteristic band is not readily discerned [14, 21]. Two areas are chosen to collect the Raman spectra: ZrO2 particles far away from the indentation (matrix) and in the crack propagation path (crack). As shown in Fig. 6, the martensitic transformation was occurred when indentation crack propagated at 298 K and 77 K, for the monoclinic doublet measured in the crack propagation path was much stronger than that of matrix (noted by encircled dotted line). Because the transformation could only be observed in ZrO2 particles along the crack propagation path, it is very difficult to compare the content of t-ZrO2 phase occurring stress-induced transformation for samples tested at 298 K and 77 K. Therefore, the content of stress-induced martensitic transformation at different temperature can hardly be quantitatively measured. The in-situ Raman analysis of indentation cracks indicated that the t-ZrO2 grains in ZTA samples were able to transform under stress. Therefore, it can be speculated that the stress-induced martensitic transformation was the toughening mechanism in ZTA samples. Besides the fracture toughness of ZTA samples sintered by HP process at 1550 °C，the bending strength and fracture toughness of ZTA samples both increased
more than 40% at 77 K compared with those at 298 K. While the bending strength of Al2O3
did not change obviously when temperature decreased from 293 K to 77 K
[23]. When test temperature decreased from 298 K to 77 K, the bending strength and fracture toughness of 99%, 92% and translucent alumina increased less than 20% [24, 25]. On the other hand, many researches have shown that the mechanical properties of ZrO2 stabilized with CeO2 [12, 19, 26], MgO [27, 28] and Y2O3 [29-32]increase significantly at cryogenic temperatures because of the stress-induced martensitic transformation. In this work, the mechanical properties of ZTA ceramics enhanced remarkably when temperature decreased from 298 K to 77 K. The reason was that the free energy difference between t-ZrO2 and m-ZrO2 increased linearly at 77 K, resulting in a stronger stress-induced martensitic transformation toughening effect [29]. Therefore, the stress-induced martensitic transformation toughening of t-ZrO2 phase is the main reason why the mechanical properties of ZTA ceramics increased at 77 K.
Fig. 6 In-situ Raman spectra of ZTA samples. (a) 298 K; (b) 77 K. The monoclinic doublet was encircled by the dotted line.
The residual stress generated when the temperature decreased from sintering temperature down to room temperature. In two-phase materials, the differences between the thermal expansion coefficients and elastic moduli of the particles and matrix would result in residual stress. The residual stresses of particle (σp) and matrix (σm) are expressed as [33]: 1 2p 1 f p m 1 4 f p p p m T 2 Em 1 f p Ep -1
(1)
and,
m
fp 1 fp
p
(2)
where subscript p and m indicate ZrO2 particle and Al2O3 matrix, respectively, σ is the residual stress, α is the thermal expansion coefficient, E is the elastic modulus, v is the Poisson’s ratio, f is the volume fraction, T is the difference in temperature over which the stresses are locked in. According to Eq. (1) and (2), the residual stress will increase as the test temperature decreases. As the Al2O3 matrix and ZrO2 inclusion having different thermal expansion coefficient (CTE of 8.5 × 10-6 °C-1 for Al2O3 and 10.2 × 10-6 °C-1 for ZrO2) [5] and elastic modulus (EAlumina=380 GPa and EZirconia=210 GPa) [34], the thermal expansion effect pre-loads the ZrO2 particles in tension when temperature decreases from 298 K to 77 K, whereas the t to m transformation puts it in compression. The fact that the ZrO2 is in residual tension makes it even more easy to transform [35]. Therefore, the stress-induced martensitic transformation toughening effect became stronger in ZTA samples at cryogenic temperatures.
4. Conclusions The effect of sintering parameters (sintering temperature and sintering process) on the densification, microstructure and mechanical properties of ZTA ceramics has been investigated in the present work. The XRD results showed that the crystalline phases in ZTA samples were t-ZrO2 and α-Al2O3. The ZrO2 particles dispersed in the grain boundaries of Al2O3 matrix, and no abnormal grain growth existed in Al2O3 phase. The grain sizes of ZrO2 and Al2O3 phases increased with the sintering temperature increased from 1450 °C to 1550 °C. The bending strength and fracture toughness for the samples prepared by SPS and HP processes both increased significantly when test temperature decreased from 298 K to 77 K. In-situ Raman analysis revealed that the t-ZrO2 phase in ZTA samples could have stress-induced martensitic transformation toughening effect. The toughening effect became stronger at 77 K, resulting in the increase of cryogenic mechanical properties.
Acknowledgment This work was financially supported by the National Natural Science Foundation of China (Grand no. 51232004).
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PII: DOI: Reference:
S0272-8842(16)32078-8 http://dx.doi.org/10.1016/j.ceramint.2016.11.072 CERI14160
To appear in: Ceramics International Received date: 1 October 2016 Revised date: 4 November 2016 Accepted date: 11 November 2016 Cite this article as: Juan Chen, Zhipeng Xie, Weining Zeng and Weiwei Wu, Toughening mechanisms of ZTA ceramics at cryogenic temperature (77K), Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.11.072 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Toughening mechanisms of ZTA ceramics at cryogenic temperature (77 K)
Juan Chen, Zhipeng Xie*, Weining Zeng, Weiwei Wu
State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China
*
Corresponding author. [email protected] (Z. Xie)
Abstract ZTA ceramics containing 20 wt.% ZrO2 were fabricated at different sintering temperatures (1450, 1500 and 1550 °C) by SPS and HP processes, respectively. The influence of sintering process on the mechanical properties of ZTA ceramics at 298 K and 77 K was investigated. It can be seen that the bending strength and fracture toughness of samples prepared by the two processes both improved at cryogenic temperature. The stress-induced martensitic transformation toughening mechanism was confirmed by the in-situ Raman technique. The tetragonal ZrO2 would be even more easy to transform because of the residual stress generated when temperature decreased from 298 K to 77 K. Therefore, the transformation toughening effect would become stronger, result in the increase of mechanical properties.
Keywords: ZTA; cryogenic temperature; toughening mechanism; stress-induced martensitic transformation.
1. Introduction Zirconia toughened alumina (ZTA), in which alumina is the primary phase (70% to 95%) and zirconia is the secondary phase (30% to 5%), is a material that combines the advantageous properties of monolithic alumina and zirconia [1]. The addition of zirconia to alumina results in higher strength and fracture toughness with little reduction in hardness and elastic modulus compared with monolithic Al2O3 ceramic [2]. Due to their excellent mechanical properties at room temperature, ZTA ceramics are one of the extremely important structural materials. Major research efforts have focused on the applications at room and high temperatures [3]. Currently, there is an urgent demand of study on the mechanical properties of structural ceramics at cryogenic temperatures as the materials used in cryogenic field raising considerable attention owing to the high-speed development of cryogenic engineering and technology. However, few researches have studied the cryogenic mechanical properties of ZTA ceramics and their potential applications in the cryogenic environment [4]. In this paper, a series of ZTA ceramics were fabricated at different sintering temperatures and processes. The bending strength and fracture toughness were measured at room (298 K) and cryogenic temperatures (77 K) to study the influence of temperature on the mechanical properties. The toughening mechanisms of ZTA ceramics at cryogenic temperature were also discussed in detail.
2. Experimental procedures Commercially available ZTA powders with 20 wt.% ZrO2 (MS-LZTA-BS, Farmeiya, China) produced by coprecipitation process were used as raw materials. In order to investigate the effects of sintering temperature on the mechanical properties of ZTA ceramics, the powders were calcined at 1450, 1500 and 1550 °C. The ZTA powders were weighed and filled into a graphite die with an inside diameter of 30 mm, and sintered by two kinds of methods: spark plasma sintering (SPS) and hot pressing (HP) processes. Under a vacuum and applied pressure of 30 MPa (before 1000 °C) or 50 MPa (after 1000 °C), the powders were sintered at firing temperature with holding time five min for SPS and HP processes. The applied pressure of HP process is 30 MPa, with holding time one hour. The phase compositions of ZTA powders and sintered samples were identified by X-ray diffraction technology (XRD, D8 advance, Bruker, Germany). The bulk densities of the consolidated samples were measured by Archimede’s method with deionized water as the immersion medium, and the theoretical density (TD) of the samples were calculated according to the rule of mixture. The mechanical properties at 298 K and 77 K were tested in a universal testing machine (SUNS UTM4000, China). Bending strength was measured by three-point bending technique with specimen dimension 2.0 mm ×1.5 mm ×25 mm and support span 20 mm, according to the standard procedure ASTM C1161-02c. Fracture toughness was tested using single-edge notched beam (SENB) and three-point bending technique. The specimen size for fracture toughness was 3 mm × 4 mm × 23 mm with support span 16 mm, the
width of notch was less than 0.2 mm, and the depth of notch was 2 mm (ASTM C1421-10). The microstructure of fracture surface was confirmed by scanning electron microscope (SEM, Leo 1530, Zeiss, Germany) under secondary electron imaging and backscattered electron (BSE) signal. The average grain size was determined by calculating the average diameter value of more than 100 random grains from SEM pictures. In-situ Raman spectroscopy (in-situ Raman，LabRAM HR Evolution，France) was used to evaluate the martensitic transformation of tetragonal ZrO2 in ZTA ceramics. Vickers indentation [5] was performed on polished surface of ZTA samples under a load of 100 N at 298 K and 77 K. After indentation, Raman data of the cracks were collected quickly at the same temperature. 3. Results and discussion 3.1 XRD and microstructure analysis XRD patterns for ZTA powders are shown in Fig. 1. There are three crystalline phases in ZTA powders: α-Al2O3, tetragonal zirconia (t-ZrO2) and monoclinic zirconia (m-ZrO2). After sintering process, m-ZrO2 phase disappeared, only α-Al2O3 and t-ZrO2 phases existed (Fig. 2). It can be illustrated that m-ZrO2 transformed into t-ZrO2 when temperature increased, and was not produced again during the cooling process. Extensive literatures have reported that t-ZrO2 phase in ZTA ceramics could have stress-induced martensitic transformation as a toughening mechanism, resulting in the substantial increase of bending strength and fracture toughness for ZTA ceramics [6-8]. Therefore, the existence of t-ZrO2 phase offered the possibility of stress-induced phase transformation toughening mechanism in ZTA samples.
The densities of ZTA samples were shown in Table 1. The relative densities of all the samples exceeded 97%. The densities of ZTA samples obtained by SPS process decreased as sintering temperature increased. But the densities of ZTA samples sintered at 1450 °C and 1500 °C by HP process were the same, when the sintering temperature increased to 1550 °C, the density decreased a little. Compared with the HP sintering process, the densities of ZTA samples prepared by SPS process were slightly higher at the same sintering temperature. This result can be explained by the different pressure exerted during the sintering process. For a certain initial particle size, a threshold pressure exists and above which the pressure is an effective driving force for densification [9]. High applied pressure is beneficial both at the first and intermediate stages of sintering [10]. At firing temperature, the applied pressures were 50 MPa and 30 MPa for SPS and HP processes, respectively. Therefore, the densities of ZTA ceramics sintered by SPS were a little higher than those sintered by HP process. The greatest densities were both obtained at 1450 °C for SPS and HP processes. However, the densities of the samples prepared by the two processes have not changed greatly with the increase of sintering temperature.
Fig. 1 XRD patterns for ZTA powders
Fig. 2 sintered surface XRD patterns of ZTA. (1) SPS; (2) HP
Table 1 The densities of ZTA samples at various sintering temperatures (unit: g/cm3) Sintering Temperature / °C
Sintering Process
1450
1500
1550
SPS
4.41
4.39
4.36
HP
4.40
4.39
4.33
SEM pictures of fracture surfaces are displayed in Fig. 3. In order to distinguish the two different phases in ZTA ceramics, the pictures were taken by black scattered electron (BSE) signals. It is obvious that the t-ZrO2 grains (white) uniformly dispersed in the Al2O3 matrix (black) at the grain boundaries. The grain size distribution of Al2O3 phase was uniform, and no abnormal growth phenomenon was observed in Al2O3 grains. Therefore, the presence of t-ZrO2 grains would inhibit the abnormal growth of Al2O3 grains. Due to the six pictures in Fig. 3 were collected at the same magnification, it can be intuitively seen that the grain sizes of t-ZrO2 and Al2O3 particles both increased significantly when the sintering temperature increased for SPS and HP processes.
Fig. 3 SEM pictures for fracture surfaces by BSE signals of ZTA samples. (1-1) SPS-1450 °C; (1-2) SPS-1500 °C; (1-3) SPS-1550 °C; (2-1) HP-1450 °C; (2-2) HP-1500 °C; (2-3) HP-1550 °C
Table 2 The average grain size of ZTA ceramics Grain Size / μm
Sintering Sintering Temperature Process
ZrO2
Al2O3
1450
0.71
1.07
1500
1.01
1.53
1550
1.75
2.93
1450
1.12
1.8
1500
1.28
2.04
1550
1.91
3.67
/ °C
SPS
HP
The grain sizes of ZTA samples sintered at various temperatures by SPS and HP processes were measured and shown in Table 2. To attain a highly accurate value of the average grain size, more than 100 grains were calculated for ZrO2 and Al2O3 phases. The results showed that the average grain sizes of ZrO2 and Al2O3 phases increased as the sintering temperature improved for the same sintering process, indicating that the increase of sintering temperature would contribute to the growth of ZrO2 and Al2O3 grains. Meanwhile, compared with the samples sintered by SPS process, the grain sizes of the samples sintered by HP process were higher. 3.2 Mechanical properties at different temperatures The bending strengths of ZTA samples sintered by SPS and HP processes were
tested using the three-point bending method at 298 K and 77 K. The results are shown in Fig. 4, the difference of bending strengths between the samples sintered by SPS and HP processes was small when tested at 298 K. For the samples prepared by HP process, the bending strength decreased when sintering temperature increased at 298 K. While a higher bending strength was obtained at 1500 °C for samples sintered by SPS process. However, the bending strengths both improved significantly for the samples sintered by SPS and HP processes at 77 K. The increase magnitude of bending strength for samples sintered by HP process was larger. For example, the bending strength obtained at 77 K increased twice as large as the datum obtained at 298 K for the samples sintered at 1500 °C.
Fig. 4 Bending strengths of ZTA samples sintered by SPS and HP processes at 298 K (RT) and 77 K (LN)
Fig. 5 Fracture toughnesses of ZTA samples sintered by SPS and HP processes at 298 K (RT) and 77 K (LN) The fracture toughnesses of ZTA samples tested at various temperatures are plotted in Fig. 5. For the samples prepared by the same sintering process, the fracture toughness increased basically when temperature decreased from 298 K to 77 K, except for the samples sintered at 1550 °C by HP process, which remained almost unchanged (increased less than 3%). It can be concluded that the fracture toughness of ZTA ceramics would increase at cryogenic temperatures. This results were accordance with the studies of ZTA ceramics by Lange [11]. At 298 K, the largest fracture toughness was obtained when samples sintered at 1450 °C for both sintering processes. For the samples sintered by HP process, at the same test temperature, the fracture toughness decreased basically with sintering temperature increased. The fracture toughness of samples prepared by SPS process decreased when the sintering temperature increased at 298 K. While the relationship between the fracture toughness and sintering temperature was opposite at 77 K.
It is well known that the transformation toughening effect of ZrO2 is affected by the martensitic temperature (Ms) [12, 13]. One of the most important factors that influence the Ms is the grain size of tetragonal ZrO2. The Ms raises as the grain size of tetragonal ZrO2 increases [14], leading to the reduction of the difference ( T ) between the test temperature (T) and the Ms, which is expressed as T =T -M s . And the smaller the T is, the higher the contribution of the martensitic transformation toughening [15]. Therefore, the contribution of martensitic transformation toughening effect becomes larger when the grain size of tetragonal ZrO2 increases. However, the fracture toughness of ZrO2 will decrease after reaching a critical grain size because of the premature phase transformation [16], and the critical grain sizes of 2Y-TZP are 733 nm and 301 nm when test temperatures are 293 K and 77 K, respectively [17]. The grain sizes of ZrO2 in ZTA ceramics increased as the sintering temperature increasing (shown in Table 2). Nevertheless, the fracture toughness of ZTA samples did not show distinct correlation with the grain size of ZrO2, the fracture toughness increased as the grain size of ZrO2 increased only observed in the samples sintered by SPS process and tested at 77 K. The data of strength/toughness trade-off for Y-TZP and Ce-TZP have been clearly shown that Al2O3 additions increase strength but decrease toughness [18, 19]. Therefore, the existence of Al2O3 has a great influence on the fracture toughness of ZTA ceramics, which will discuss later. 3.3 Factors affecting the mechanical properties at various temperatures The toughening mechanism of ZTA ceramics is related to the volume expansion and shear strain produced by the t-ZrO2 to m-ZrO2 transformation. In-situ Raman has
been used to figure out whether the martensitic transformation occurred or not in ZTA ceramics at 298 K and 77 K. The advantage of this technique is that the Raman signals from both monoclinic and tetragonal phases are strong [20] and its excellent capability for micro-area analysis. The characteristic band for monoclinic phase is monoclinic doublet (bands at 181 cm-1 and 192 cm-1), while the tetragonal bands are at 148 cm-1 and 264 cm-1 [21, 22]. Meanwhile, the Raman spectrum for cubic phase becomes wider in the mixed phase samples, so its characteristic band is not readily discerned [14, 21]. Two areas are chosen to collect the Raman spectra: ZrO2 particles far away from the indentation (matrix) and in the crack propagation path (crack). As shown in Fig. 6, the martensitic transformation was occurred when indentation crack propagated at 298 K and 77 K, for the monoclinic doublet measured in the crack propagation path was much stronger than that of matrix (noted by encircled dotted line). Because the transformation could only be observed in ZrO2 particles along the crack propagation path, it is very difficult to compare the content of t-ZrO2 phase occurring stress-induced transformation for samples tested at 298 K and 77 K. Therefore, the content of stress-induced martensitic transformation at different temperature can hardly be quantitatively measured. The in-situ Raman analysis of indentation cracks indicated that the t-ZrO2 grains in ZTA samples were able to transform under stress. Therefore, it can be speculated that the stress-induced martensitic transformation was the toughening mechanism in ZTA samples. Besides the fracture toughness of ZTA samples sintered by HP process at 1550 °C，the bending strength and fracture toughness of ZTA samples both increased
more than 40% at 77 K compared with those at 298 K. While the bending strength of Al2O3
did not change obviously when temperature decreased from 293 K to 77 K
[23]. When test temperature decreased from 298 K to 77 K, the bending strength and fracture toughness of 99%, 92% and translucent alumina increased less than 20% [24, 25]. On the other hand, many researches have shown that the mechanical properties of ZrO2 stabilized with CeO2 [12, 19, 26], MgO [27, 28] and Y2O3 [29-32]increase significantly at cryogenic temperatures because of the stress-induced martensitic transformation. In this work, the mechanical properties of ZTA ceramics enhanced remarkably when temperature decreased from 298 K to 77 K. The reason was that the free energy difference between t-ZrO2 and m-ZrO2 increased linearly at 77 K, resulting in a stronger stress-induced martensitic transformation toughening effect [29]. Therefore, the stress-induced martensitic transformation toughening of t-ZrO2 phase is the main reason why the mechanical properties of ZTA ceramics increased at 77 K.
Fig. 6 In-situ Raman spectra of ZTA samples. (a) 298 K; (b) 77 K. The monoclinic doublet was encircled by the dotted line.
The residual stress generated when the temperature decreased from sintering temperature down to room temperature. In two-phase materials, the differences between the thermal expansion coefficients and elastic moduli of the particles and matrix would result in residual stress. The residual stresses of particle (σp) and matrix (σm) are expressed as [33]: 1 2p 1 f p m 1 4 f p p p m T 2 Em 1 f p Ep -1
(1)
and,
m
fp 1 fp
p
(2)
where subscript p and m indicate ZrO2 particle and Al2O3 matrix, respectively, σ is the residual stress, α is the thermal expansion coefficient, E is the elastic modulus, v is the Poisson’s ratio, f is the volume fraction, T is the difference in temperature over which the stresses are locked in. According to Eq. (1) and (2), the residual stress will increase as the test temperature decreases. As the Al2O3 matrix and ZrO2 inclusion having different thermal expansion coefficient (CTE of 8.5 × 10-6 °C-1 for Al2O3 and 10.2 × 10-6 °C-1 for ZrO2) [5] and elastic modulus (EAlumina=380 GPa and EZirconia=210 GPa) [34], the thermal expansion effect pre-loads the ZrO2 particles in tension when temperature decreases from 298 K to 77 K, whereas the t to m transformation puts it in compression. The fact that the ZrO2 is in residual tension makes it even more easy to transform [35]. Therefore, the stress-induced martensitic transformation toughening effect became stronger in ZTA samples at cryogenic temperatures.
4. Conclusions The effect of sintering parameters (sintering temperature and sintering process) on the densification, microstructure and mechanical properties of ZTA ceramics has been investigated in the present work. The XRD results showed that the crystalline phases in ZTA samples were t-ZrO2 and α-Al2O3. The ZrO2 particles dispersed in the grain boundaries of Al2O3 matrix, and no abnormal grain growth existed in Al2O3 phase. The grain sizes of ZrO2 and Al2O3 phases increased with the sintering temperature increased from 1450 °C to 1550 °C. The bending strength and fracture toughness for the samples prepared by SPS and HP processes both increased significantly when test temperature decreased from 298 K to 77 K. In-situ Raman analysis revealed that the t-ZrO2 phase in ZTA samples could have stress-induced martensitic transformation toughening effect. The toughening effect became stronger at 77 K, resulting in the increase of cryogenic mechanical properties.
Acknowledgment This work was financially supported by the National Natural Science Foundation of China (Grand no. 51232004).
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