J Mater Sci (2015) 50:5039–5046 DOI 10.1007/s10853-015-9053-6
Effects of composition and sintering temperature on the structure and compressive property of the lamellar Al2O3–ZrO2 scaffolds prepared by freeze casting Rui-Fen Guo1 • Ping Shen1 • Chang Sun1 • Yu-Jie Fu1 • Yu-Hua Liu1 Zhen-An Ren1 • Qi-Chuan Jiang1
•
Received: 10 March 2015 / Accepted: 22 April 2015 / Published online: 29 April 2015 Ó Springer Science+Business Media New York 2015
Abstract Using 3 wt% TiO2 nanopowders as sintering aid, we prepared porous lamellar Al2O3–ZrO2 composites with an initial solid loading of 30 vol% and the Al2O3/ ZrO2 weight ratios of 1:9, 3:7, 5:5, 7:3, and 9:1 by freeze casting. The viscosity of water-based slurries decreased with increasing Al2O3 content while the porosity and the wavelength of the lamellae in the sintered scaffolds showed first an increase and then decrease with maximum values appearing at Al2O3:ZrO2 = 5:5. A relatively large amount of tetragonal (t) ZrO2 transformed to monoclinic (m) ZrO2 with the increase in the Al2O3 content and sintering temperature, thus reducing stress-induced phase transformation effect during loading. The compressive strength of the sintered scaffolds depended on sintering temperature, porosity, and the phase composition in the composites. A
& Ping Shen
[email protected] Rui-Fen Guo
[email protected] Chang Sun
[email protected] Yu-Jie Fu
[email protected] Yu-Hua Liu
[email protected] Zhen-An Ren
[email protected] Qi-Chuan Jiang
[email protected] 1
Key Laboratory of Automobile Materials (Ministry of Education), Department of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun 130025, People’s Republic of China
maximum value of 105 ± 10 MPa was achieved in the scaffolds with a composition of Al2O3:ZrO2 = 3:7 after sintering at 1450 °C for 2 h.
Introduction Freeze casting [1–3], also termed as ice templating, has received wide attention during the past several years owing to its outstanding advantages such as simpleness, cheapness, and environmental friendliness as well as controllable pore structures and sizes in products. It has been applied to the preparation of porous structural ceramics [4], solid oxide fuel cells [5], and biological materials such as bone substitutes [6] with interconnected pore channels and relatively high strength. Alumina (Al2O3) and zirconia (ZrO2) are important structural and functional ceramics, receiving wide applications in many fields. Table 1 summarizes recent progress on the preparation of porous Al2O3, ZrO2, and Al2O3–ZrO2 composites [7–18] using freeze casting under different conditions. A major concern on the mechanical properties of the porous scaffolds is their compressive strength. As seen from the reported data in Table 1, the compressive strength of the porous scaffolds is generally low and in comparison the strength of ZrO2 is larger than that of Al2O3. The relatively high compressive strength of porous Al2O3 scaffolds reported by Yoon et al. [8] was attributed to a high sintering temperature (1600 °C) and a long dwelling time (3 h). It is worth mentioning that the alumina and zirconia powders used in all the studies listed in Table 1 were in nano- or submicron sizes (typically 60–800 nm). If economic micron-size powders (e.g., 5 lm) were used, our previous study [19] showed that the compressive strength of the porous Al2O3 scaffolds with an
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Table 1 Summary of researches on porous Al2O3, ZrO2, and their composite scaffolds fabricated by freeze casting Material
Solvent
Solid loading (vol%)
Al2O3:ZrO2
Sintering temperature/time
Compressive strength (MPa)
Reference
Al2O3
Camphene
–
–
1450 °C/3 h
11.6 ± 1.2
[7]
Al2O3
Camphene
10–25
–
1600 °C/3 h
11-95
[8]
Al2O3
Camphene
5–20
–
1450 °C/3 h
2–16
[9]
ZrO2
Water
45
–
1300–1400 °C/1 h
23.57–63.86
[10]
ZrO2
Tert-butyl alcohol
15
–
1450 °C/2 h
–
[11]
ZrO2
Camphene
10–20
–
1400–1550 °C/2 h
18–59
[12]
ZrO2 ZrO2
Camphene Camphene
10–20 10–20
– –
1450oC/3 h 1500 °C/2 h
19–58 41–63
[13] [14] [15]
Al2O3–ZrO2
Camphene
10–20
30:70
1200–1400 °C/2–4 h
0.83–22
Al2O3–ZrO2
Water
11–30
20:80
1550 °C/2 h
15–81
[16]
Al2O3–ZrO2
Water
15–35
90:10
1600 °C/2 h
–
[17]
Al2O3–ZrO2
Tert-butyl alcohol
10–25
95:5–85:15–95:5
1400–1500 °C/2 h
63–378
[18]
initial solid loading of 25 vol% reached only 2 ± 1 MPa after sintering at 1500 °C for 2 h. Nevertheless, with the addition of 3 wt% MgO–Al2O3–SiO2 nanopowders in a eutectic composition as sintering aid, the compressive strength of the sintered scaffolds with the initial solid loadings of 25 and 30 vol% reached 25 ± 2 and 64 ± 2 MPa, respectively, suggesting that the introduction of sintering aid opened a new and effective way to prepare low-cost and high-strength porous ceramics by freeze casting. It is well known that ZrO2 has a tetragonal-monoclinic phase transformation during cooling, which brings about several toughening and strengthening effects. Zirconiatoughened alumina (ZTA) is a typical example of the application of such effects. However, up to date, only limited work has been performed on the preparation of porous Al2O3–ZrO2 composites using the freeze casting technique [15–18,20]. Zhang [15] prepared porous Al2O3–ZrO2 composites with the initial solid loading of 20 vol% and Al2O3:ZrO2 & 30:70 by weight and reported a maximum compressive strength of 22 MPa for the scaffolds sintered at 1400 °C for 4 h. Liu et al. [16] fabricated the Al2O3– ZrO2 composites with the initial solid loadings of 15–70 wt% in a constant weight ratio of 20:80 (Al2O3: ZrO2). The compressive strength was reported to reach 81 MPa for 70 wt% (*30 vol%) initial solid loading after sintering at 1550 °C for 2 h. Choi et al. [18] prepared the Al2O3–ZrO2 composites with the initial solid loadings of 10–25 vol%. As the porosity decreased from 64 to 32 %, the compressive strength of the sintered composites remarkably increased from 63 to 376 MPa. The achievement of the high compressive strength was attributed to the symmetric three-layer stacks (A–B–A) fabricated by sequential slurry casting in the sequence of 95/5 (layer A) and 85/15 vol% (layer B) Al2O3/ZrO2. In spite of these
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studies, no systematic work has been carried out on the effects of composition and sintering temperature on the structure and compressive property of the porous Al2O3– ZrO2 scaffolds fabricated by freeze casting. In this work, we prepared porous Al2O3–ZrO2 composites with various compositions using freeze casting and then investigated the effects of composition and sintering temperature on their structure and compressive property with a primary purpose to determine the optimal composition for their potential applications as porous structural ceramics and dental materials.
Materials and procedure 3 mol% yttria-stabilized zirconia powders (D50 = 1.2 lm) and alumina powders (D50 = 5 lm, 99.95 %) were used as raw materials to prepare the composites with initial solid loading of 30 vol% and the Al2O3/ZrO2 weight ratios of 1:9, 3:7, 5:5, 7:3, and 9:1. 3 wt% TiO2 nanopowders (20–30 nm, C99.9 %) based on total powder contents were added into water-based slurries as sintering aid [21]. Aqueous suspensions were prepared using 0.5 wt% sodium polymethacrylate and 1.5 wt% ammonium polyacrylate as dispersant. Lamellar porous Al2O3–ZrO2 scaffolds in dimensions of /15 9 25 mm were fabricated by freeze casting using an apparatus similar to that reported in Ref. [1]. The slurries were ball-milled for 12 h using alumina balls and de-aired by stirring in a vacuum desiccator for about 30 min to remove air bubbles. They were then poured into a Teflon mold and kept at -20 °C for directional solidification by heat conduction through a Cu rod, whose end was immersed in liquid N2. After demolding, the frozen samples were placed in a freeze dryer at -50 °C for 48 h under a
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10 Pa vacuum to sublimate the ice. Sintering was carried out in air at a heating rate of 4 °C/min to 500 °C, holding for 30 min to burn out the dispersant, and then followed by 5 °C/min to 1400, 1450, and 1500 °C, respectively, holding for 2 h. The viscosity of the slurry was measured by using a viscometer (DV - 79 ? PRO, NiRun Co. Ltd., Shanghai, China). The porosity of the sintered scaffolds was calculated by measuring their dimensions and weight. The microstructures were observed under a scanning electron microscope (SEM, Evo18, Carl Zeiss, Germany). The phases in the composites were identified by X-ray diffraction (XRD, D/Max 2500PC Rigaku, Japan). The compressive property was tested using an Instron 5689 universal testing machine (Instron Corp., USA) under a strain rate of 0.5 mm s-1.
Results and discussion Viscosity of slurries Figure 1 shows the variation in the viscosity of the slurries with an initial solid loading of 30 vol% and different Al2O3/ZrO2 weight ratios with shear rate using a logarithmetic coordinate. As indicated, the viscosity of all the slurries decreases with increasing shear rate, showing shear-thinning behavior. Moreover, the viscosity of the slurry decreases with the increase in the Al2O3 content, which is consistent with the result reported by Yang et al. [20] and can be explained by the following facts: (i) higher zeta–potential of Al2O3 than that of ZrO2 under saturated absorption condition, giving a larger repulsive force [22]; (ii) larger particle size of Al2O3 than that of ZrO2. According to the Woodcock formula [23],
Fig. 1 Variation in viscosity of water-based Al2O3–ZrO2 slurries with shear rate (A represents for Al2O3 and Z for ZrO2)
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h ¼ d
2 5 þ 3pu 6
12 1;
ð1Þ
where u is the volume fraction of ceramic, d is the particle diameter and h is the intermolecular distance, proportional to the particle diameter under a constant initial solid loading. The average particle sizes (D50) of Al2O3 and ZrO2 used in our experiments were 5 and 1.2 lm, respectively. The smaller intermolecular distance owing to the smaller particles increased the van der Waals interaction and their contact probability. Accordingly, the mobility of the particles in the suspension was weakened and higher viscosity was observed. Microstructures and phase compositions Figure 2 shows the typical microstructures of the Al2O3– ZrO2 preform with Al2O3:ZrO2 = 3:7 after sintering at 1450 °C for 2 h. Measurement of the wavelength between the lamellar layers clearly shows an increase in the wavelength with increasing distance from the cold Cu rod as a result of slowing down in freezing velocity with growing ice front. The freezing velocity at the bottom layer was very fast, and the growing ice front could engulf the ceramic particles, leading to the formation of a relatively uniform and dense layer. The presence of this dense layer changed the subsequent freezing dynamics by decreasing the front freezing velocity since the heat conductivity of ice and ceramic particles is poor. The particles were then rejected by the growth of the ice front due to decreasing freezing velocity, resulting in compositional undercooling and formation of a cellular microstructure. Afterwards, with the further decrease in the freezing velocity, the lamellar structure was developed. The formation mechanism of these structures has been elaborated by Deville et al. [1]. The sintered samples exhibited a certain degree of shrinkage primarily due to the presence of sintering aid. Figure 3 shows the variation in the porosity of the samples after sintering at different temperatures with the Al2O3/ ZrO2 ratio. The curves show a parabolic-like shape with the maximum porosity appearing at Al2O3/ZrO2 = 5:5. Moreover, with the increase in the sintering temperature, the porosity decreases considerably. Figure 4 shows the typical microstructures of the composites sintered at 1450 °C with different Al2O3/ZrO2 weight ratios. It is clear that when ZrO2 was the matrix, i.e., the dominant composition, the wavelength of lamellar structure (k), and the thickness of ceramic wall increased slightly with increasing Al2O3 content. However, when Al2O3 was the matrix phase, both the wavelength and the ceramic wall thickness decreased dramatically with increasing Al2O3 content, which led to the densification in the scaffolds. When
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Fig. 3 Variation in the porosity of sintered composites with the Al2O3:ZrO2 weight ratio
Fig. 2 Microstructures of the freeze-cast scaffold with a composition of Al2O3:ZrO2 = 3:7 after sintering at 1450 °C for 2 h (k is the lamellar structure wavelength and d is the distance away from the Cu cold finger)
Al2O3:ZrO2 = 5:5, the wavelength and the ceramic wall thickness seemed much larger. Such morphologies are generally consistent with the variation in the porosity, as shown in Fig. 3, and would be explained in the following. When ZrO2 was the matrix phase in the composite, the Al2O3 particles, which were homogeneously distributed in
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the ZrO2 matrix, could inhibit the growth of the ZrO2 grains during the sintering process, thus refining the grains and finally promoting the densification [22]. With the increase in the Al2O3 content, the compatibility between the zirconia and alumina particles deteriorated owing to the difference in their thermal expansion coefficients. The particles tended to agglomerate and grow fast during the sintering, which became a large obstacle to achieve densification. This problem became more serious with the increase in the Al2O3 content, giving rise to higher porosity in the sintered scaffolds. On the contrary, when Al2O3 was the primary phase, the fine zirconia particles could fill in the interstitial sites among the alumina particles to promote the powder compactness. Besides, TiO2 has a more remarkable effect on Al2O3 than on ZrO2 in promoting sintering, as will be explained in more detail later. Therefore, the density of the preforms increased and the porosity decreased with further increasing Al2O3 content. When the Al2O3 and ZrO2 contents were identical or close, the particles were separated by the other species to a large extent. Because of mismatch in thermal expansion coefficients, the sintering efficiency was weakened, leading to relatively larger lamellar thickness together with larger spacing between the adjacent lamellae. Therefore, the porosity of the sintered composites showed first an increase and then decrease with increasing Al2O3 content, reaching a maximum value at Al2O3:ZrO2 = 5:5. In addition, the thermal conductivity of Al2O3 is much larger than that of ZrO2 (36 W m-1 K-1 for A12O3 and 2– 3.3 W m-1 K-1 for ZrO2 at room temperature [24]). A larger thermal conductivity of the slurry gives a faster solidification velocity of water during freeze casting. According to the relationship of k ¼ A v1 [3], where k is the lamellar spacing, v is the freezing velocity, and A is a constant depending on solid content and particle size. A
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Fig. 4 Comparison of the microstructures at a constant distance (15 mm) away from the bottom for the composites sintered at 1450 °C for 2 h (from left to right Al2O3:ZrO2 = 1:9, 3:7, 5:5, 7:3, and 9:1; a–e along freezing direction; f–j perpendicular to freezing direction)
faster freezing velocity gives a narrower lamellar spacing and smaller lamellar thickness. Therefore, the lamellar thickness decreased with increasing Al2O3 content. In addition, densification of the preform during sintering may also reduce the lamellar thickness to a certain extent. Accordingly, under the combined effects of the freezing velocity and sintering densification, the lamellar thickness showed first a slight increase and then decrease with increasing Al2O3 content. Because ZrO2 has a tetragonal ? monoclinic phase transformation during cooling after sintering, the final phase constituents and their amount were influenced by the sintering temperature, the composition, and the density of the composites. Figure 5 shows the XRD patterns of the Al2O3–ZrO2 composites with different compositions after sintering at different temperatures. As seen from Fig. 5a, the intensity of the diffraction peaks shows a remarkable difference with increasing Al2O3 content. Despite the fact that there were t-ZrO2 and m-ZrO2 phases in the raw ZrO2 powders, the amount of the m-ZrO2 phase was small. However, after sintering at 1450 °C, the intensity of t-ZrO2 diffraction peak decreased while that of m-ZrO2 increased although their total amount decreased with increasing Al2O3 content in the composites, especially for the case of Al2O3:ZrO2 C 5:5, where the primary diffraction peak of ZrO2 changed from t-ZrO2 to m-ZrO2. According to the Al2O3-ZrO2 [25] and Al2O3-Y2O3 phase diagrams [26], Al2O3 will not react with ZrO2, but it can react with Y2O3 at high temperatures to form yttrium aluminum garnet (YAG). Because the amount of Y2O3 in the ZrO2 powders was very small (3 mol%), the amount of the YAG reaction product should be even smaller and thus this phase was unable to be detected by XRD. Instead, Rana [27] found that the migration of Y3? across the Al2O3–ZrO2 interface at a high sintering temperature (1600 °C) resulted in the formation of nano-YAG at the interface. With increasing Al2O3 content, the chance of its contact with Y2O3
increased, which greatly weakened the solid solubility of Y2O3 in the ZrO2 matrix and thus reduced the stability of t-ZrO2, leading to the phase transformation of t-ZrO2 to m-ZrO2 during cooling. Consequently, the diffraction peak of m-ZrO2 increased while that of t-ZrO2 decreased with increasing Al2O3 content in the composites. On the other hand, as seen from Fig. 5b, more amount of t-ZrO2 transformed to m-ZrO2 with the increase in the sintering temperature, which might be related to the fact that the stability of t-ZrO2 depends on the zirconia particle size [27]. Above a critical value (defined as Dc), spontaneous transformation of t-ZrO2 to m-ZrO2 could occur during cooling. It was reported that for 3 mol% Y2O3 stabilized ZrO2 powders, Dc & 1.0 lm [27], while in our experiments the average particle size of ZrO2 was 1.2 lm. Therefore, the spontaneous t ? m transformation could occur. Increasing sintering temperature could not only promote densification but also enhance grains growth, making the small grains whose sizes were initially below the critical value grow into sufficiently large and further promoting the t ? m phase transformation. Moreover, increasing temperature also promoted the reaction between Al2O3 and Y2O3 and thus reduced the stability of t-ZrO2. Accordingly, the diffraction peaks of m-ZrO2 increased with increasing temperature, as shown in Fig. 5b. Compressive property The mechanical property of the sintered composites is affected by their microstructure, phase composition, and porosity. Figure 6 shows the typical compressive engineering stress–strain curves of the porous composites and Table 2 gives the average compressive strength values. As can be seen, with the increase in the sintering temperature and the Al2O3 content, the compressive strength of the samples shows a large variation with the maximum value of 105 ± 10 MPa appearing for the composites with
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Fig. 5 XRD patterns of a the composites with different ratios of Al2O3:ZrO2 sintered at 1450 °C; b the composites with Al2O3:ZrO2 = 3:7 sintered at different temperatures
Fig. 6 Engineering compressive curves for the porous composites sintered at different temperatures for 2 h
Al2O3:ZrO2 = 3:7 and after sintering at 1450 °C for 2 h. This value is much larger than that reported by Liu et al. [16] (*81 MPa) with similar composition (Al2O3: ZrO2 = 20:80) and initial solid loading. It is worth mentioning that the particle sizes (D50) of Al2O3 and ZrO2 used in Liu et al.’s work were 0.7 and 0.1 lm, respectively, much smaller than those used in our experiments, and the sintering temperature was higher (1550 °C, 2 h). The smaller particle size and the higher sintering temperature favor to produce stronger scaffolds using the same fabricating technique. However, the result is opposite. We suppose that the higher compressive strength obtained in our composite samples should be related not only to the composition of Al2O3/ZrO2 but also to the presence of the TiO2 sintering aid. The mechanism for the promotion of sintering by the addition of TiO2 to Al2O3 could be attributed to the formation of substitutional solid solution,
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increasing the concentration of the Al3? vacancies as generated by Ti4? substituting for Al3? and the atomic diffusivity. Therefore, the sintering of the Al2O3 ceramic was greatly promoted through a solid-state reaction [21]. Although TiO2 might have a certain effect on the sintering of ZrO2, the effect is far less prominent than that on Al2O3 since the ion radii of Ti4? and Zr4? are quite close (7.4 ± 10-2 and 7.2 9 10-2 nm, respectively) and the lattice distortion caused by the formation of substitutional solid solution is quite small [28]. This result qualitatively explains the fact of lower porosity in the composites with higher alumina content than that with higher zirconia content. Furthermore, it is seen from Table 2 that the compressive strength of the composites with Al2O3:ZrO2 = 3:7 reached the maximum values regardless of the sintering temperature. Generally speaking, the compressive strength
J Mater Sci (2015) 50:5039–5046 Table 2 Compressive strength of the sintered Al2O3–ZrO2 composites with different compositions
5045
Sintering temperature (°C)
Compressive strength (MPa) Al2O3:ZrO2 1:9
3:7
5:5
7:3
9:1
1400
47 ± 7
60 ± 9
28 ± 1
14 ± 3
36 ± 5
1450
78 ± 9
105 ± 10
61 ± 1
41 ± 3
66 ± 7
1500
80 ± 5
91 ± 8
86 ± 5
48 ± 5
48 ± 5
strength decreased. Although the composites with Al2O3: ZrO2 = 5:5 had the largest porosity, their compressive strength was medium possibly because the amount of t-ZrO2 was larger than that in the composites with higher Al2O3 contents. With the increase in the sintering temperature, the density of the composites increased and thus the compressive strength generally improved. However, the anomalous behavior of decreasing compressive strength for the composites with Al2O3:ZrO2 = 3:7 sintered at 1500 °C, as compared with those sintered at 1450 °C, might be attributed to the decrease in the amount of t-ZrO2, which weakened the phase transformation strengthening effect. Fig. 7 Cracks in the composite with Al2O3:ZrO2 = 1:9 after sintering at 1450 °C for 2 h
Conclusions (1)
of the composites decreased in the order of Al2O3/ZrO2 of 3:7, 1:9, 9:1, 5:5, and 7:3 for those sintered at the same temperature and increased with increasing temperature for a definite composition. In order to achieve higher compressive strength, relatively high density of the composites with fine lamellar structures and retaining of more metastable t-ZrO2 phase at room temperature are two important factors. The latter provides a stress-induced phase transformation effect [29] during loading. In this sense, it was necessary to add appropriate alumina content to retain more amount of t-ZrO2 in the Al2O3–ZrO2 composites. When Al2O3:ZrO2 B3:7, the amount of the metastable t-ZrO2 phase retained in the composites was relatively large so that the stress-induced phase transformation effect improved the compressive property of the composite under external loading. Besides, the introduction of Al2O3 could also play a role of particle dispersion strengthening. The lower compressive strength of the composites with Al2 O3:ZrO2 = 1:9, even though they had lower porosity and more amount of metastable t-ZrO2 phase than those of Al2O3:ZrO2 = 3:7, should result from the presence of visible cracks in the samples, as shown in Fig. 7. When Al2O3:ZrO2 C 7:3, on the one hand, the amount of the ZrO2 phase (including t-ZrO2) decreased and on the other, the presence of a large amount of Al2O3 promoted the t ? m transformation. Accordingly, the compressive
(2)
(3)
(4)
The viscosity of water-based slurries decreased with increasing Al2O3 content owing to relatively higher zeta potential and larger particle size of Al2O3, which reduced the van der Waals force between particles. The porosity and lamellar thickness in the sintered scaffolds first increased and then decreased with increasing Al2O3 content in the slurry and the maximum values appeared in the composites with the composition of Al2O3:ZrO2 = 5:5 in weight ratio. For the composites sintered at the same temperature, the compressive strength decreased in the overall trend of Al2O3:ZrO2 = 3:7, 1:9, 9:1, 5:5, and 7:3. This sequence was determined by the composition, density, and structure of the composites. With the increase in the Al2O3 content, more amount of t-ZrO2 transformed into m-ZrO2 and thus weakened the phase transformation strengthening effect. With the increase in the sintering temperature, the porosity in the sintered scaffolds greatly decreased and the compressive strength generally increased. However, because of the reduction in the amount of t-ZrO2, the maximum compressive strength (105 ± 10 MPa) was appeared in the composites with Al2O3:ZrO2 = 3:7 after sintering at 1450 °C rather than at 1500 °C.
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5046 Acknowledgements This work is supported by National Basic Research Program of China (973 program) (No. 2012CB619600) and the Fundamental Research Funds for the Central Universities (Jilin University).
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