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Fabrication of gradient porous β-SiAlON ceramics via a camphene-based freeze casting process
Materials Science & Engineering A 558 (2012) 742–746
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Fabrication of gradient porous b-SiAlON ceramics via a camphene-based freeze casting process Zhaoping Hou n, Feng Ye, Limeng Liu, Qiang Liu School of Materials Science and Engineering, Harbin Institute of Technology, Building C3, Room 515, Science Park, Herbin 150001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 May 2012 Received in revised form 20 August 2012 Accepted 21 August 2012 Available online 28 August 2012
Gradient porous b-SiAlON ceramics were prepared by unidirectional freezing of camphene-based suspensions on a cold substrate at 0 1C and subsequent sintering at 1800 1C for 2 h. Effects of solid loading content in the suspensions on porosities, pore structures and strengths of the porous ceramics were investigated. The results showed that a gradient microstructure consisting of aligned channels, dendritic and equiaxed pores from the bottom to the top of the sample was created by the freezing process. Both the microstructure observation and the pore size distribution indicated that with the increase of the solid content from 10 vol% to 30 vol%, the porosity decreased from 71.3% to 36.1% and the pore size decreased from 20 mm to 5 mm. As a result, the compressive strength increased significantly from 49 MPa to 313 MPa. The sample fabricated from the slurry with 10 vol% solid loading content showed a large strain for failure up to 9%. & 2012 Elsevier B.V. All rights reserved.
Keywords: b-Sialon Freeze casting Porosity Microstructure Mechanical properties
1. Introduction Porous ceramics have attracted considerable attention as a new class of materials with a wide range of applications under high temperature and corrosive environments, such as gas filters, catalysis supports, high-temperature thermal insulations, light weight structural components and molten-metal filtrations [1]. Among these porous ceramics, SiAlON ceramics have aroused an increasing interest as promising engineering materials for higher temperature applications owing to their outstanding mechanical properties, superior thermal stability, high creep resistance and good oxidation/corrosion performance [2,3]. Porous SiAlON ceramics are conventionally fabricated based on a straightforward processing route by means of partial sintering of powder mixtures which leads to the pore formation. However, the conventional route suffers from inherent limitations. On one hand, this method results in a relatively low porosity than 60%, higher porosity up to 90% is impossible. On the other hand, it is challenging for it to control pore structures, such as porosity, pore size and shape, because the pores are homogeneously buried in the microstructure as a result of the straightforward processing route. The freeze casting recently emerged as a good candidate for fabrication of porous ceramics with controlled pore microstructure [4]. The typical process consists of freezing material suspension followed by sublimated and subsequent sintering to densify the walls, provides a scaffold with a unique porous microstructure which replicates the shapes of the connected frozen solvent crystal.
As shapes of the frozen solvent crystal depend on the types of solvent, freezing rate and the freezing direction, the microstructure in obtained ceramics is highly tuned by varying process parameters. b-SiAlON is a solid solution of b-Si3N4 with the general formula of Si6 zAlzOzN8 z. b-SiAlON (Si6 zAlzOzN8 z) is commonly produced by reaction sintering of a or b-Si3N4, Al2O3, and AlN as the starting powders. Due to the fast hydrolysis of AlN when in contact with water [5], the starting powders have to be processed in certain organic solvents. Camphene (C10H16) has been shown to be a favorable sublimable vehicle in the preparation of porous ceramic by freezing suspensions [6]. The striking advantage of camphene is the melting point of approximately 45 1C which allows more flexibilities in the process as it can be frozen and easily sublimed at room temperature. Most papers concentrate on the fabrication of homogeneous pore structure [7,8]. In terms of ceramics with graded structures produced by camphene-based freeze casting, only few studies were reported [9]. For special application, such as biocompatible [10], filtration [11]and thermal shock resistant structure [12], ceramics with graded pore structure were desired. The purpose of the present work is to propose unidirectional freeze casting as a route for fabricating porous b-SiAlON with interconnected and graded pore microstructure and to determine the important variables that control the pore microstructure characteristics and mechanical properties of the final product.
2. Experimental procedure n
Corresponding author. Tel/fax: þ 86 451 86413921. E-mail address: [email protected] (Z. Hou).
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.08.094
The composition of the investigation was Si4Al2O2N6, i.e. z ¼2 in the formula of Si6 zAlzOzN8 z for b-SiAlON. a-Si3N4 (5 wt%b,
Z. Hou et al. / Materials Science & Engineering A 558 (2012) 742–746
d50 ¼ 0.50 um, Junyu Ceramic Co., Ltd., Shanghai, China), Al2O3 (Grade A16SG, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) and AlN (grade C, H.C. Starck, Berlin, Germany) were used as the starting materials. Camphene (C10H16) (95% Purity, Guangzhou Huanpu chemical Factory, Guangzhou, China) was selected as the solvent without any further purification. A dispersant (HAO FAST 923, Shanghai haoyang Co., Ltd., Shanghai, China) addition was 1 wt%, based on the total weight of the ceramic powders. The b-SiAlON precursors were mixed in ethanol for 12 h with silicon nitride balls as the mixing media. After drying, different fractions of camphene were added and ball milled at 60 1C for 24 h in sealed bottles to homogenize the slurries. The ceramic particle concentrations in the slurries were 10 vol%, 20 vol%, and 30 vol%. Sealed bottles were necessary to prevent the loss of camphene from evaporation during the warm milling. Because losing of camphene vapor was ever-present, the actual solid loading content in the slurry was calculated with the weight change after the complete sublimation of camphene from the cast bodies. Immediately after the resultant warm slurries were poured into polyethylene molds (Ø 60 mm 20 mm) which was prewarmed at 60 1C, the molds were set on a copper plate which was immersed in ice-water at 0 1C for freezing the green compacts. The mold set-ups should have induced unidirectional solidification of the slurries. The solidified camphene in green compacts was eliminated by evaporating in the open air at room temperature for about 4 days. Sintering was performed in a conventional graphite resistant furnace at 1800 1C for 2 h under a nitrogen-gas pressure of 0.4 MPa. A BN-Si3N4 powder bed (50 wt% BN, 50% Si3N4) was used. The heating and cooling rates were both 10 1C/min. Densities of the sintered samples were calculated by the mass and volume measurements. Pore size distribution was measured by mercury porosimetry (Autopore 9500, Micrometics Co., UAS). Crystalline phases of the produced porous ceramics were characterized by X-ray diffraction (XRD). The microstructures of the sintered samples were observed using scanning electron microscope (SEM). Pore size in the different regions of specimens was also determined by measuring the average size of pores from the SEM micrographs. For the compressive strength measurement, samples with dimensions of 6 6 9 mm were loading at a crosshead speed 0.5 mm/min (Instron 3369, Instron Corp., USA). The compressive loading is parallel to the solidification direction. The test specimens were taken out from the central zone of the sintered samples. During the tests, the stress and strain responses were recorded. Six samples were tested and average values with standard deviations were reported.
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Fig. 1. XRD patterns of sintered b-SiAlON ceramics with different solid loading contents.
Table 1 Porosities and compressive strengths of the porous b-SiAlON ceramics. Solid content
Actual solid content (vol%)
Porositya (%)
Porosityb (%)
Compressive strength (MPa)
10 vol% 20 vol% 30 vol%
10.6 23.9 32.2
71.3 47.0 36.1
63.5 44.8 32.1
49.2 73.2 168.4 78.7 312.7 710.6
a b
Calculated from the sample dimension and weight. Measured by mercury intrusion porosimetry.
walls [13]. This concentrated ceramic powder is of technical importance, as it is possible to sinter dense ceramic walls and formation of b-SiAlON phase, regardless of the initial solid loading content used. 3.2. Porosities The porosities of fabricated porous b-SiAlON ceramics with different solid loading contents are listed in Table 1. The porosity of obtained porous b-SiAlON ceramics decreased proportionately from 71.3% to 36.1% with the increase of the solid loading content of slurry from 10.6% to 32.2%. The result suggests that the porosities in the porous b-SiAlON ceramics can be controlled by manipulating the initial solid loading content of the slurry. In addition, the porosities measured by mercury porosimetry are almost the same as those calculated from the dimensions and weights of the samples, which indicates that most of the pores in the obtained porous materials are open pores. 3.3. Characterization of porous structure
3. Result and discussion 3.1. Phase formation XRD patterns of the porous b-SiAlON ceramics from the different slurries of 10 vol%, 20 vol%, and 30 vol% solid contents are shown in Fig. 1. All the samples showed very similar patterns, which means all the samples consisted of b-SiAlON phase and no other secondary phase was detected, indicating that the reaction among the Si3N4, Al2O3, and AlN was complete to form the desired b-SiAlON phase. The full formation of b-SiAlON was attributed to the unique freeze casting process. In the process, when the camphene forms dendrites under an appropriate temperature gradient, the ceramic particles presented in the slurry are repelled by the growing camphene dendrites, become concentrated between the dendrites and form highly packed ceramic powder
Microstructures of the obtained samples with different solid loading contents are shown in Fig. 2. The pore structures varied with the distance to the bottom of the directionally solidified sample where the solidification initiated. The sample with 10 vol% solid loading content fabricated herein showed three distinctive zones (bottom, middle, top), each having a different pore structure. In the bottom region, the morphologies of the pores are aligned elongated pores which were formed parallel to the direction of solidification. Compared to the bottom region, the middle region of the sample had a distinguished columnar pore structure replicating camphene dendrites with growing direction not parallel to solidification direction. In the top region, the morphologies were notably changed compared with those formed in the bottom and middle regions and were found to be equiaxial pore structure, as shown in Fig. 2 (10 vol%, top).
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Fig. 2. SEM micrographs of obtained porous b-SiAlON ceramics with different initial solid loading contents, showing the pore structures developed parallel to the direction of freezing.
Obviously, the pores in the sintered b-SiAlON samples replicated the camphene crystals, therefore their morphologies were strongly affected by the temperature gradients during freeze casting of the slurries. Owing to the thermal resistance of solidified layer, the temperature distribution in the slurry-mold setup showed a gradual decrease with the increase of distance from bottom of the mold, thereby leading the temperature gradients to decrease from bottom to top. Immediately after the warm slurry prepared at 60 1C reaching the chilled plate at 0 1C, a high heat extraction rate in the chill resulted in a steep thermal gradient. The imposed temperature gradient favors the growth of camphene crystals whose /001S or c-direction coincides with the temperature gradient, which leads to the growth of primary dendrites down the temperature gradient [14]. Therefore, aligned elongated pores were formed in the bottom region. In the middle region, the temperature gradient decreases and the camphene crystals grow in the c-direction with side-branching or secondary dendritic in the orthogonal a- and b-direction, thereby resulting in the dendritic pore. In the top region, it was found that the columnar pore transformed to equiaxed pore. This transition during solidification in this work was also observed in Al 3 pct Cu solidified directionally due to the temperature gradient, in which the hot alloy that is poured in to the chilled mold solidified, so the transition herein can be explained on the basis of the well-known solidification theory of ingots. When the temperature gradient in suspension decreases sufficiently, the existing nuclei ahead of the dendrite tip grows to a sufficiently large size. As a result, the concentration of liquid camphene in between the two dendrite arms decays toward critical value, thus reduced the crystals growth rate and leads to stop of the advancing dendrite growth [15]. Similar morphologies were observed for the pore structures in the three regions of the samples fabricated from 20 vol% solid loading content (Fig. 2, 20 vol%). Meanwhile, it is observed that with the solid loading content increased from 10 vol% to 20 vol%, the pore size of three regions of sample were decreased and the wall thickness in the structure were increased correspondingly. However, when the solid loading content further increased to 30 vol%, the aligned elongated pores in the bottom region and dendritic pore in the middle region disappeared and were completely replaced by the irregular pores
(Fig. 2, 30 vol%). This was because the camphene concentration in the slurry decreased when the solid loading content increased from 10 vol% to 30 vol%. In the solidification process, the camphene content in 30 vol% slurry was insufficient to get camphene crystals elongated and hence have a roundlike shape. 3.4. Pore size distribution Fig. 3 shows cross-section of ceramic specimens with different solid contents at the top regions which is perpendicular to freezing direction. The images demonstrate the influence of the initial solid loading on pore size and wall thickness. With increasing solid loading content, the pore size decreased and the wall thickness increased. The pore size of the samples in the top region was 35 mm, 15 mm, and 6 mm at 10 vol%, 20 vol%, and 30 vol% respectively. As seen in Fig. 2, as the distance to the bottom increased, the pore size in different regions increased. Pore sizes have been reported in the alumina–water system by Koch et al. [16] versus cooling rate. Further away from the cold surface, the camphene growth rate becomes much lower due to the reduction of temperature gradient. When the freezing velocity decreases, the magnitude of supercooling ahead of the solidifying interface is decreased, and as a result the tip radius of the crystals increases [17]. Similarly, the pore size in the top region in this work represented the maximum value throughout the specimens. The effects of different solid loading contents on the pore size distribution were also determined by mercury porosimetry is shown in Fig. 4. With increasing the solid loading content from 10 vol% to 30 vol%, the pore size range decreased. The pore size of samples with 10 vol% solid loading content ranges between 10 mm and 30 mm, those with 20 vol% solid loading had pore size of 6 mm–15 mm and those with 30 vol% solid loading pore size of 3 mm–5 mm. It should be mentioned that the pore size measured by mercury porosimetry are smaller than those estimated from the SEM micrographs shown in Fig. 3, wherein the pore size was 35 mm, 15 mm, and 6 mm at 10 vol%, 20 vol%, and 30 vol% respectively. The results imply that the pore channels were completely interconnected, consequently the relatively narrow pore represented the accurate pore size measured by mercury porosimetry.
Z. Hou et al. / Materials Science & Engineering A 558 (2012) 742–746
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Fig. 3. SEM micrographs of obtained porous b-SiAlON ceramics with different initial solid loading (a and d) 10 vol%, (b and e) 20 vol%, (c and f) 30 vol%. Cross-section perpendicular to the freezing direction at top region.
Fig. 4. Pore size distribution of the obtained porous b-SiAlON ceramics with different solid contents of slurries. (a) 10 vol%; (b) 20 vol%; (c) 30 vol%.
3.5. Compressing strength Solid loading content plays a critical role in determining the porosity and porous structure of sintered samples, consequently affects the compressive strength of samples. The compressive strengths of the porous b-SiAlON materials were listed in Table 1. The compressive strength increased from 49 MPa to 312 MPa when the porosity of porous ceramic decreased from 71% to 36%, corresponding to the increase of solid loading contents in the starting slurries from 10 vol% to 30 vol%. Fig. 5 shows stress–strain curves in compression testing for obtained porous b-SiAlON ceramics with different initial solid loading contents. An interesting feature of the stress–strain response is the high strain tolerance of the sample acquired from 10 vol% slurry. Unlike samples obtained from high solid loading content (20 vol% and 30 vol%) which show the brittle behavior with sudden drops and failure at low strain, the sample fabricated from 10 vol% slurry (with the highest porosity) showed some degree of continuous small breakage before the final failure and thus resulted in a series of slight decreases in the stress with higher deformation of 9%. Fig. 6 shows sintered porous b-SiAlON specimens after compressive loading. The samples from the high solid loading contents (20 vol% and 30 vol%) fractured into several pieces. However, the
Fig. 5. The stress–strain curves for sintered porous b-SiAlON with different solid loading contents.
sample of 10 vol% nearly kept its original shape after compressive testing. The large strain for failure was observed in the 10 vol% b-SiAlON sample, was also presented in the oriented HA and 13–93 bioactive glass scaffolds prepared by unidirectional freezing [18,19]. Although the underlying reasons of this behavior in this work are still not elucidated, it should be related to high porosity. In addition, the strength of a porous ceramic is strongly affected not only by the porosity, but also by the pore size as well as the densification of ceramic wall. The compressive strengths of the porous b-SiAlON ceramics fabricated in the present work were higher than those reported in the literature [20]. The achievement of high compressive strengths is clearly attributed to the dense ceramic walls without any noticeable microstructure defects in walls (in Fig. 7). And the elongated grains of b-SiAlON in ceramic wall would be highly beneficial to the mechanical properties of the porous ceramics.
4. Conclusions Gradient porous b-SiAlON ceramics were fabricated using a method based on unidirectional freezing of camphene-based suspensions. The gradient porous structure of 10 vol% showed
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Z. Hou et al. / Materials Science & Engineering A 558 (2012) 742–746
Fig. 6. Optical images of b-SiAlON samples after compressive loading. (a) 10 vol%; (b) 20 vol%; (c) 30 vol%.
b-SiAlON grains. Additionally, it was interesting to find that the sample fabricated from 10 vol% slurry showed a large strain for failure ( 9%).
References
Fig. 7. SEM image of the porous b-SiAlON ceramics walls with 10 vol% solid loading content.
three distinct regions including aligned channel, dendritic and equiaxed pore respectively, due to the decreasing temperature gradient from the bottom to the top in the freeze casting process. Initial solid loading content played a primary role in the resulting pore structure, porosity and pore size of the obtained porous ceramics. Increasing the solid loading content of the slurries from 10 vol% to 30 vol% would decrease the porosity and pore size, and change the pore structure. As a result, the compressive strength was affected, increasing from 49.22 MPa to 312.7 MPa for the respective increase in solid loading content. The excellent compressive strength was attributed to the good densification of ceramic wall coupled with formation of elongated
[1] A.R. Studart, U.T. Gonzenbach, E. Tervoort, L.J. Gauckler, J. Am. Ceram. Soc. 89 (2006) 1771–1789. [2] X. Xu, R.L. Fu, K.X. Chen, J.M.F. Ferreira, Mater. Lett. 59 (2005) 2601–2604. [3] J.F. Yang, Y. Beppu, G.J. Zhang, T. Ohji, S. Kanzaki, J. Am. Ceram. Soc. 85 (2002) 1879–1881. [4] S. Deville, Adv. Eng. Mater. 10 (2008) 155–169. [5] I. Ganesh, S.M. Olhero, J.M.F. Ferreira, Adv. Appl. Ceram. 108 (2009) 111–117. [6] K. Araki, J.W. Halloran, J. Am. Ceram. Soc. 87 (2004) 1859–1863. [7] B Yoon, C. Park, H. Kim, Y. Koh, J. Am. Ceram. Soc. 90 (2007) 3759–3766. [8] B Yoon, Y. Koh, C. Park, H. Kim, J. Am. Ceram. Soc. 90 (2007) 1744–1752. [9] A. Macchetta, I.G Turner, C.R. Bowen, Acta. Biomater. 5 (2009) 1319–1327. [10] A. Tampieri, G. Celotti, S. Sprio, A. Delcogliano, S. Franzese, Biomaterials 22 (2001) 1365–1370. [11] J. Moon, A.C. Caballero, L. Hozer, Y. Chiang, M.J. Cima, Mater. Sci. Eng. A 298 (2001) 110–119. [12] K. Satyamurthy, J.P. Singh, M.P. Kamat, D.P.H. Hasselman, J. Am. Ceram. Soc 62 (1979) 431–432. [13] K. Araki, J.W. Halloran, J. Am. Ceram. Soc. 88 (2005) 1108–1114. [14] X. Liu, M.N. Rahaman, Q. Fu, Acta Biomater. 7 (2011) 406–416. [15] I. Ziv, F. Weinberg, Metall. Trans. B 20 (1989) 731–734. [16] D. Koch, L. Andresen, T. Schmedders, G. Grathwohl, J. Sol-Gel Sci. Technol. 26 (2003) 149–152. [17] E.S.Sylvain Deville, Ravi K. Nalla, Antoni P. Tomsia, Science 311 (2006) 515–518. [18] Q. Fu, M.N. Rahaman, B.S. Bal, R.F. Brown, J. Biomed. Mater. Res. A 93 (2010) 1380–1390. [19] Q. Fu, M.N. Rahaman, F. Dogan, B.S. Bal, J. Biomed. Mater. Res. B 86 (2008) 514–522. [20] S. Deville, E. Saiz, A.P. Tomsia, Biomaterials 27 (2006) 5480–5489.
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Fabrication of gradient porous b-SiAlON ceramics via a camphene-based freeze casting process Zhaoping Hou n, Feng Ye, Limeng Liu, Qiang Liu School of Materials Science and Engineering, Harbin Institute of Technology, Building C3, Room 515, Science Park, Herbin 150001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 May 2012 Received in revised form 20 August 2012 Accepted 21 August 2012 Available online 28 August 2012
Gradient porous b-SiAlON ceramics were prepared by unidirectional freezing of camphene-based suspensions on a cold substrate at 0 1C and subsequent sintering at 1800 1C for 2 h. Effects of solid loading content in the suspensions on porosities, pore structures and strengths of the porous ceramics were investigated. The results showed that a gradient microstructure consisting of aligned channels, dendritic and equiaxed pores from the bottom to the top of the sample was created by the freezing process. Both the microstructure observation and the pore size distribution indicated that with the increase of the solid content from 10 vol% to 30 vol%, the porosity decreased from 71.3% to 36.1% and the pore size decreased from 20 mm to 5 mm. As a result, the compressive strength increased significantly from 49 MPa to 313 MPa. The sample fabricated from the slurry with 10 vol% solid loading content showed a large strain for failure up to 9%. & 2012 Elsevier B.V. All rights reserved.
Keywords: b-Sialon Freeze casting Porosity Microstructure Mechanical properties
1. Introduction Porous ceramics have attracted considerable attention as a new class of materials with a wide range of applications under high temperature and corrosive environments, such as gas filters, catalysis supports, high-temperature thermal insulations, light weight structural components and molten-metal filtrations [1]. Among these porous ceramics, SiAlON ceramics have aroused an increasing interest as promising engineering materials for higher temperature applications owing to their outstanding mechanical properties, superior thermal stability, high creep resistance and good oxidation/corrosion performance [2,3]. Porous SiAlON ceramics are conventionally fabricated based on a straightforward processing route by means of partial sintering of powder mixtures which leads to the pore formation. However, the conventional route suffers from inherent limitations. On one hand, this method results in a relatively low porosity than 60%, higher porosity up to 90% is impossible. On the other hand, it is challenging for it to control pore structures, such as porosity, pore size and shape, because the pores are homogeneously buried in the microstructure as a result of the straightforward processing route. The freeze casting recently emerged as a good candidate for fabrication of porous ceramics with controlled pore microstructure [4]. The typical process consists of freezing material suspension followed by sublimated and subsequent sintering to densify the walls, provides a scaffold with a unique porous microstructure which replicates the shapes of the connected frozen solvent crystal.
As shapes of the frozen solvent crystal depend on the types of solvent, freezing rate and the freezing direction, the microstructure in obtained ceramics is highly tuned by varying process parameters. b-SiAlON is a solid solution of b-Si3N4 with the general formula of Si6 zAlzOzN8 z. b-SiAlON (Si6 zAlzOzN8 z) is commonly produced by reaction sintering of a or b-Si3N4, Al2O3, and AlN as the starting powders. Due to the fast hydrolysis of AlN when in contact with water [5], the starting powders have to be processed in certain organic solvents. Camphene (C10H16) has been shown to be a favorable sublimable vehicle in the preparation of porous ceramic by freezing suspensions [6]. The striking advantage of camphene is the melting point of approximately 45 1C which allows more flexibilities in the process as it can be frozen and easily sublimed at room temperature. Most papers concentrate on the fabrication of homogeneous pore structure [7,8]. In terms of ceramics with graded structures produced by camphene-based freeze casting, only few studies were reported [9]. For special application, such as biocompatible [10], filtration [11]and thermal shock resistant structure [12], ceramics with graded pore structure were desired. The purpose of the present work is to propose unidirectional freeze casting as a route for fabricating porous b-SiAlON with interconnected and graded pore microstructure and to determine the important variables that control the pore microstructure characteristics and mechanical properties of the final product.
2. Experimental procedure n
Corresponding author. Tel/fax: þ 86 451 86413921. E-mail address: [email protected] (Z. Hou).
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.08.094
The composition of the investigation was Si4Al2O2N6, i.e. z ¼2 in the formula of Si6 zAlzOzN8 z for b-SiAlON. a-Si3N4 (5 wt%b,
Z. Hou et al. / Materials Science & Engineering A 558 (2012) 742–746
d50 ¼ 0.50 um, Junyu Ceramic Co., Ltd., Shanghai, China), Al2O3 (Grade A16SG, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) and AlN (grade C, H.C. Starck, Berlin, Germany) were used as the starting materials. Camphene (C10H16) (95% Purity, Guangzhou Huanpu chemical Factory, Guangzhou, China) was selected as the solvent without any further purification. A dispersant (HAO FAST 923, Shanghai haoyang Co., Ltd., Shanghai, China) addition was 1 wt%, based on the total weight of the ceramic powders. The b-SiAlON precursors were mixed in ethanol for 12 h with silicon nitride balls as the mixing media. After drying, different fractions of camphene were added and ball milled at 60 1C for 24 h in sealed bottles to homogenize the slurries. The ceramic particle concentrations in the slurries were 10 vol%, 20 vol%, and 30 vol%. Sealed bottles were necessary to prevent the loss of camphene from evaporation during the warm milling. Because losing of camphene vapor was ever-present, the actual solid loading content in the slurry was calculated with the weight change after the complete sublimation of camphene from the cast bodies. Immediately after the resultant warm slurries were poured into polyethylene molds (Ø 60 mm 20 mm) which was prewarmed at 60 1C, the molds were set on a copper plate which was immersed in ice-water at 0 1C for freezing the green compacts. The mold set-ups should have induced unidirectional solidification of the slurries. The solidified camphene in green compacts was eliminated by evaporating in the open air at room temperature for about 4 days. Sintering was performed in a conventional graphite resistant furnace at 1800 1C for 2 h under a nitrogen-gas pressure of 0.4 MPa. A BN-Si3N4 powder bed (50 wt% BN, 50% Si3N4) was used. The heating and cooling rates were both 10 1C/min. Densities of the sintered samples were calculated by the mass and volume measurements. Pore size distribution was measured by mercury porosimetry (Autopore 9500, Micrometics Co., UAS). Crystalline phases of the produced porous ceramics were characterized by X-ray diffraction (XRD). The microstructures of the sintered samples were observed using scanning electron microscope (SEM). Pore size in the different regions of specimens was also determined by measuring the average size of pores from the SEM micrographs. For the compressive strength measurement, samples with dimensions of 6 6 9 mm were loading at a crosshead speed 0.5 mm/min (Instron 3369, Instron Corp., USA). The compressive loading is parallel to the solidification direction. The test specimens were taken out from the central zone of the sintered samples. During the tests, the stress and strain responses were recorded. Six samples were tested and average values with standard deviations were reported.
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Fig. 1. XRD patterns of sintered b-SiAlON ceramics with different solid loading contents.
Table 1 Porosities and compressive strengths of the porous b-SiAlON ceramics. Solid content
Actual solid content (vol%)
Porositya (%)
Porosityb (%)
Compressive strength (MPa)
10 vol% 20 vol% 30 vol%
10.6 23.9 32.2
71.3 47.0 36.1
63.5 44.8 32.1
49.2 73.2 168.4 78.7 312.7 710.6
a b
Calculated from the sample dimension and weight. Measured by mercury intrusion porosimetry.
walls [13]. This concentrated ceramic powder is of technical importance, as it is possible to sinter dense ceramic walls and formation of b-SiAlON phase, regardless of the initial solid loading content used. 3.2. Porosities The porosities of fabricated porous b-SiAlON ceramics with different solid loading contents are listed in Table 1. The porosity of obtained porous b-SiAlON ceramics decreased proportionately from 71.3% to 36.1% with the increase of the solid loading content of slurry from 10.6% to 32.2%. The result suggests that the porosities in the porous b-SiAlON ceramics can be controlled by manipulating the initial solid loading content of the slurry. In addition, the porosities measured by mercury porosimetry are almost the same as those calculated from the dimensions and weights of the samples, which indicates that most of the pores in the obtained porous materials are open pores. 3.3. Characterization of porous structure
3. Result and discussion 3.1. Phase formation XRD patterns of the porous b-SiAlON ceramics from the different slurries of 10 vol%, 20 vol%, and 30 vol% solid contents are shown in Fig. 1. All the samples showed very similar patterns, which means all the samples consisted of b-SiAlON phase and no other secondary phase was detected, indicating that the reaction among the Si3N4, Al2O3, and AlN was complete to form the desired b-SiAlON phase. The full formation of b-SiAlON was attributed to the unique freeze casting process. In the process, when the camphene forms dendrites under an appropriate temperature gradient, the ceramic particles presented in the slurry are repelled by the growing camphene dendrites, become concentrated between the dendrites and form highly packed ceramic powder
Microstructures of the obtained samples with different solid loading contents are shown in Fig. 2. The pore structures varied with the distance to the bottom of the directionally solidified sample where the solidification initiated. The sample with 10 vol% solid loading content fabricated herein showed three distinctive zones (bottom, middle, top), each having a different pore structure. In the bottom region, the morphologies of the pores are aligned elongated pores which were formed parallel to the direction of solidification. Compared to the bottom region, the middle region of the sample had a distinguished columnar pore structure replicating camphene dendrites with growing direction not parallel to solidification direction. In the top region, the morphologies were notably changed compared with those formed in the bottom and middle regions and were found to be equiaxial pore structure, as shown in Fig. 2 (10 vol%, top).
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Fig. 2. SEM micrographs of obtained porous b-SiAlON ceramics with different initial solid loading contents, showing the pore structures developed parallel to the direction of freezing.
Obviously, the pores in the sintered b-SiAlON samples replicated the camphene crystals, therefore their morphologies were strongly affected by the temperature gradients during freeze casting of the slurries. Owing to the thermal resistance of solidified layer, the temperature distribution in the slurry-mold setup showed a gradual decrease with the increase of distance from bottom of the mold, thereby leading the temperature gradients to decrease from bottom to top. Immediately after the warm slurry prepared at 60 1C reaching the chilled plate at 0 1C, a high heat extraction rate in the chill resulted in a steep thermal gradient. The imposed temperature gradient favors the growth of camphene crystals whose /001S or c-direction coincides with the temperature gradient, which leads to the growth of primary dendrites down the temperature gradient [14]. Therefore, aligned elongated pores were formed in the bottom region. In the middle region, the temperature gradient decreases and the camphene crystals grow in the c-direction with side-branching or secondary dendritic in the orthogonal a- and b-direction, thereby resulting in the dendritic pore. In the top region, it was found that the columnar pore transformed to equiaxed pore. This transition during solidification in this work was also observed in Al 3 pct Cu solidified directionally due to the temperature gradient, in which the hot alloy that is poured in to the chilled mold solidified, so the transition herein can be explained on the basis of the well-known solidification theory of ingots. When the temperature gradient in suspension decreases sufficiently, the existing nuclei ahead of the dendrite tip grows to a sufficiently large size. As a result, the concentration of liquid camphene in between the two dendrite arms decays toward critical value, thus reduced the crystals growth rate and leads to stop of the advancing dendrite growth [15]. Similar morphologies were observed for the pore structures in the three regions of the samples fabricated from 20 vol% solid loading content (Fig. 2, 20 vol%). Meanwhile, it is observed that with the solid loading content increased from 10 vol% to 20 vol%, the pore size of three regions of sample were decreased and the wall thickness in the structure were increased correspondingly. However, when the solid loading content further increased to 30 vol%, the aligned elongated pores in the bottom region and dendritic pore in the middle region disappeared and were completely replaced by the irregular pores
(Fig. 2, 30 vol%). This was because the camphene concentration in the slurry decreased when the solid loading content increased from 10 vol% to 30 vol%. In the solidification process, the camphene content in 30 vol% slurry was insufficient to get camphene crystals elongated and hence have a roundlike shape. 3.4. Pore size distribution Fig. 3 shows cross-section of ceramic specimens with different solid contents at the top regions which is perpendicular to freezing direction. The images demonstrate the influence of the initial solid loading on pore size and wall thickness. With increasing solid loading content, the pore size decreased and the wall thickness increased. The pore size of the samples in the top region was 35 mm, 15 mm, and 6 mm at 10 vol%, 20 vol%, and 30 vol% respectively. As seen in Fig. 2, as the distance to the bottom increased, the pore size in different regions increased. Pore sizes have been reported in the alumina–water system by Koch et al. [16] versus cooling rate. Further away from the cold surface, the camphene growth rate becomes much lower due to the reduction of temperature gradient. When the freezing velocity decreases, the magnitude of supercooling ahead of the solidifying interface is decreased, and as a result the tip radius of the crystals increases [17]. Similarly, the pore size in the top region in this work represented the maximum value throughout the specimens. The effects of different solid loading contents on the pore size distribution were also determined by mercury porosimetry is shown in Fig. 4. With increasing the solid loading content from 10 vol% to 30 vol%, the pore size range decreased. The pore size of samples with 10 vol% solid loading content ranges between 10 mm and 30 mm, those with 20 vol% solid loading had pore size of 6 mm–15 mm and those with 30 vol% solid loading pore size of 3 mm–5 mm. It should be mentioned that the pore size measured by mercury porosimetry are smaller than those estimated from the SEM micrographs shown in Fig. 3, wherein the pore size was 35 mm, 15 mm, and 6 mm at 10 vol%, 20 vol%, and 30 vol% respectively. The results imply that the pore channels were completely interconnected, consequently the relatively narrow pore represented the accurate pore size measured by mercury porosimetry.
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Fig. 3. SEM micrographs of obtained porous b-SiAlON ceramics with different initial solid loading (a and d) 10 vol%, (b and e) 20 vol%, (c and f) 30 vol%. Cross-section perpendicular to the freezing direction at top region.
Fig. 4. Pore size distribution of the obtained porous b-SiAlON ceramics with different solid contents of slurries. (a) 10 vol%; (b) 20 vol%; (c) 30 vol%.
3.5. Compressing strength Solid loading content plays a critical role in determining the porosity and porous structure of sintered samples, consequently affects the compressive strength of samples. The compressive strengths of the porous b-SiAlON materials were listed in Table 1. The compressive strength increased from 49 MPa to 312 MPa when the porosity of porous ceramic decreased from 71% to 36%, corresponding to the increase of solid loading contents in the starting slurries from 10 vol% to 30 vol%. Fig. 5 shows stress–strain curves in compression testing for obtained porous b-SiAlON ceramics with different initial solid loading contents. An interesting feature of the stress–strain response is the high strain tolerance of the sample acquired from 10 vol% slurry. Unlike samples obtained from high solid loading content (20 vol% and 30 vol%) which show the brittle behavior with sudden drops and failure at low strain, the sample fabricated from 10 vol% slurry (with the highest porosity) showed some degree of continuous small breakage before the final failure and thus resulted in a series of slight decreases in the stress with higher deformation of 9%. Fig. 6 shows sintered porous b-SiAlON specimens after compressive loading. The samples from the high solid loading contents (20 vol% and 30 vol%) fractured into several pieces. However, the
Fig. 5. The stress–strain curves for sintered porous b-SiAlON with different solid loading contents.
sample of 10 vol% nearly kept its original shape after compressive testing. The large strain for failure was observed in the 10 vol% b-SiAlON sample, was also presented in the oriented HA and 13–93 bioactive glass scaffolds prepared by unidirectional freezing [18,19]. Although the underlying reasons of this behavior in this work are still not elucidated, it should be related to high porosity. In addition, the strength of a porous ceramic is strongly affected not only by the porosity, but also by the pore size as well as the densification of ceramic wall. The compressive strengths of the porous b-SiAlON ceramics fabricated in the present work were higher than those reported in the literature [20]. The achievement of high compressive strengths is clearly attributed to the dense ceramic walls without any noticeable microstructure defects in walls (in Fig. 7). And the elongated grains of b-SiAlON in ceramic wall would be highly beneficial to the mechanical properties of the porous ceramics.
4. Conclusions Gradient porous b-SiAlON ceramics were fabricated using a method based on unidirectional freezing of camphene-based suspensions. The gradient porous structure of 10 vol% showed
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Fig. 6. Optical images of b-SiAlON samples after compressive loading. (a) 10 vol%; (b) 20 vol%; (c) 30 vol%.
b-SiAlON grains. Additionally, it was interesting to find that the sample fabricated from 10 vol% slurry showed a large strain for failure ( 9%).
References
Fig. 7. SEM image of the porous b-SiAlON ceramics walls with 10 vol% solid loading content.
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