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Origin of shrinkage anisotropy during sintering for uniaxially pressed alumina compacts
Powder Technology 127 (2002) 9 – 18 www.elsevier.com/locate/powtec
Origin of shrinkage anisotropy during sintering for uniaxially pressed alumina compacts Anze Shui *, Nozomu Uchida, Keizo Uematsu Department of Chemistry, Ceramics Science Laboratory, Nagaoka University of Technology, Kamitomioka, Nagaoka, Niigata 940-2188, Japan Received 3 August 2001; received in revised form 29 October 2001; accepted 12 December 2001
Abstract Origin of shrinkage anisotropy during sintering is reported on alumina powder compacts made by uniaxial pressing followed by cold isostatic pressing (CIP). Two types of alumina particles were used in this study; one was slightly elongated shape, the other spherical shape. An anisotropic shrinkage occurred during sintering in the compacts made of the elongated shape of particles, even after the CIP. However, no the anisotropic shrinkage happened in the compacts made of the spherical shape of particles, even at the same processing condition to the former. Detailed structure analysis involving the novel immersion liquid technique showed that the particle orientation was found to be an important origin of the shrinkage anisotropy. The particle orientation was developed during uniaxial pressing in the compacts of the elongated shape of particles. Subsequent CIP did not eliminate the shrinkage anisotropy. No particle orientation happened in the compact of the spherical shape of particles. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Particle Shape; Orientation; Sintering Shrinkage; Anisotropy; Alumina
1. Introduction Anisotropic shrinkage of compact during sintering, i.e., sintering deformation is a common and serious problem in ceramics production [1– 13]. The anisotropic shrinkage may cause large defects and cracks to develop in a complex shape of compact during sintering. Otherwise, it requires a timeconsuming machining involving diamond tools for reshaping, and markedly increases the production cost of ceramics. One of origins of the sintering deformation is the nonuniformity in compacts [3,11,14]. The most common nonuniformity is the local difference in packing density of particles. Other nonuniformity, i.e., particle orientation, also causes the anisotropic shrinkage in injection molding, tape casting, slip casting and casting molding systems [2,4 – 7], where strong shear stress is present during forming. Nonisometric particles are oriented by the shear stress, and cause the anisotropic sintering shrinkage of the compacts. In compacts made by die pressing, particle orientation has attracted only little attention except for the systems of extremely elongated particle shape, such as clay-base materials [15]. When the * Corresponding author. Tel.: +81-258-47-9337; fax: +81-258-479300. E-mail address: [email protected] (A. Shui).
particles have near equiaxed shape, they are usually believed to randomly orient in the compacts. However, even the particle shape is slightly elongated, particle orientation may also occur during uniaxial pressing in the uniaxially pressed compacts, and similarly cause the shrinkage anisotropy. Extremely sensitive analytical tool is needed to examine the relevance between the particle orientation and the anisotropic shrinkage in sintering for the compacts. A conventional X-ray diffraction method cannot identify the orientation in the system of slightly elongated shape of particles. A new method of our development will be applied in this study to examine the microstructure of compact and sintered body [16 – 21]. In this technique, the compact is made transparent with an immersion liquid, and its internal structure is observed with a polarized light microscope in the transmission mode. This method has been successfully applied to solve several difficult problems in ceramics, such as the formation mechanism of large flaws and interior stress occurring in ceramics processing [1,4,19 –27]. The characterization method may also reveal the minor particle orientation in the pressed compacts of present study. The aim of this work is to clarify the effect of particle orientation on the shrinkage anisotropy in the uniaxially pressed alumina compact. Two types of commercial particles are used as raw powders, one is of slightly elongated
0032-5910/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 ( 0 2 ) 0 0 0 0 4 - 9
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and the other of spherical particle shapes. The former represents the conventional low soda alumina, which is widely applied for industrial production of alumina ceramics. The latter is high purity alumina. Accordingly, the versatility of this study is expected. The elongated shape of particles should be aligned with their long axis normal to the applied stress of uniaxial compaction in pressing, and randomly oriented within the plane normal to the pressing direction. No specific orientation should happen in the system of spherical particles. These structures will be observed with our novel characterization technique to explain effect of particle orientation on the shrinkage anisotropy for uniaxially pressed alumina compact. In the examination, fabrication of specimens with uniform packing density is essential. To achieve this, the compacts were CIPed after the uniaxial pressing.
2. Experiment Two kinds of commercial powders were used in this study, 160SG-1 (powder of elongated particle shape, Show-
adenko, Japan; described as SG-1 hereafter) and AA05 (powder of spherical shape, Sumitomo Chemical, Japan). The slurry containing 50 mass% alumina was prepared by mixing it with distilled water and 2 mass% PVA in a ball mill. Spray-dried alumina granules were prepared with a spray drier (Model SD13, Mitsui Kozan, Japan, inner diameter 1.3 m) with the inlet air temperature 200 jC. Compacts of diameter 15 mm and thickness 4– 5 mm were prepared by uniaxial pressing (10 –100 MPa) followed by CIP (100 MPa). The shrinkage during sintering was measured under static and continuous measurement conditions. In the static condition, the compact was heated to 1550 jC or 1600 jC (heating rate 10 jC/min) in an electric furnace in air and kept for 2 h for densification. The sintering shrinkages in height and diameter directions (hereafter described as H and D directions, respectively) were determined from the dimensional changes before and after the sintering. For the examination in continuous measurement condition, specimens of the length 2– 2.5 mm were cut from various places in these compacts. The change of specimen length was followed with a dilatometer (TMA-50, Shimazu, Japan) at
Fig. 1. The SEM micrographs of fracture surfaces for the compacts made with elongated shape of alumina particles in the H (a) and D (b) directions; and spherical shape of alumina particles in the H (c) and D (d) directions.
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the heating rate 10 jC /min until 1400 jC. To ensure the accuracy of the measurement, blank tests were executed at least five times before the measurement. The blank tests show that the error was less than 1 Am. The measurements of sintering shrinkage were repeated at least three times with fresh specimens in various compaction conditions, and the maximum error in the shrinkage is less than 0.1% for all the compaction conditions. The green densities of compacts were calculated from their weight and dimensions. The sintered density was determined with the Archimedean method using distilled water. The liquid immersion technique, SEM and X-ray diffraction methods were used to examine the internal structure of the compacts and sintered bodies. In the liquid immersion method, samples were thinned to about 0.1-mm thick with sandpaper, and methylene iodide (refractive index 1.74) was applied to make it transparent. In the SEM observation, samples were treated with gold sputtering.
3. Results Fig. 1 shows the SEM micrographs of fracture surfaces for the compacts made with the alumina of elongated particle shape and the alumina of spherical particle shape in H and D directions. The packing structure of the primary alumina particles appears same in both planes for each compact. Clearly, this conventional characterization tool has not enough power to identify the particle orientation anisotropy in the specimens. Information obtained with this
Fig. 2. Histograms of aspect ratio distributions for the two types of alumina primary particles used in this study.
Fig. 3. The change of green density with uniaxial pressure on the two types of compacts before and after CIPing.
tool is that the low-soda alumina has a slightly elongated particle shape, which is typical for this type of alumina. The nominal particle size, 0.4 Am, is consistent with the micrographs. The alumina of spherical particle shape is noted in the high-purity alumina. The particle size determined by inspection is also consistent with the value provided by the manufacturer, 0.5 Am. Fig. 2 shows histograms of aspect ratio distributions for the two types of alumina primary particles used in this study. The ratio was determined by measuring the maximum length and the length normal to the direction of the maximum length on over five hundreds particles from their SEM micrographs. In alumina of elongated particle shape, the value of aspect ratio varied widely from 1 to 3.4, with the mean value 1.6. For the spherical particles, the aspect ratio has the mean value of 1.1, and its distribution is very narrow compared with that of the elongated shape particles. These particles can be considered as spherical shape in this work. Fig. 3 shows the change of green density with uniaxial pressure on the two types of compacts before and after CIP. The green density markedly increases with uniaxial pressure for both types of the compacts before CIP. It increases slightly with the uniaxial pressure after CIP. Both types of compacts showed similar gradient in the pressure-green density curves after CIP, but the gradient almost was different at various pressures before CIP. The density after CIP always is higher for the compact of spherical particle than that of elongated particle. These results imply that particle packing behavior significantly depend on shape of raw powder, even though they have the similar particle size.
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Fig. 4 shows the change of shrinkage ratio during CIP with uniaxial pressure for the two types of compacts. The shrinkage ratio is defined as the ratio of shrinkage during CIP in H direction relative to that in D direction. Small value less than 1 in the shrinkage ratio corresponds to large shrinkage anisotropy during CIP. The shrinkage ratio was found to be always less than 1 for the both types of compacts, i.e., the D direction shrinks more than the H direction during CIP in all uniaxial pressure conditions. The ratio decreases as the pressure in uniaxial pressing increases for both types of the compacts. The compact of elongated shape of particles always showed smaller the ratio than that of the spherical shape of particles at same pressure. The shrinkage anisotropy during CIP is more significant for the compact made with the elongated particles than that with the spherical particles. These results indicate that the particle transfer such as slip, rotate and realign is more predominant during CIP for the former than the later. Fig. 5 shows the change of sintering shrinkage with temperature for the compacts of elongated particles (uniaxial pressing at 100 MPa and CIP at 100 MPa). The shrinkage always is larger in the H direction than the D direction at all sintering temperatures in both static and continuous measurement conditions. The difference of the shrinkages in the two directions increases as the sintering temperature increases. The same results were obtained for all specimens cut from various locations in this type of compact, with the maximum error less than 0.1%. Clearly, the compacts show anisotropic shrinkage during sintering. Fig. 6 shows the change of the sintering shrinkage with temperature for the compacts made with the spherical shape
Fig. 4. The change of the shrinkage ratio during CIP with uniaxial pressure for the two types of compacts.
Fig. 5. The change of the sintering shrinkage with temperature for the elongated particle compact made at uniaxial pressure 100 MPa and CIP 100 MPa conditions.
of particles at the uniaxial pressure 100 MPa and CIP 100 MPa. An important difference from the above results is that the shrinkage essentially is same in the two directions at all sintering temperatures. The same results also were obtained
Fig. 6. The change of the sintering shrinkage with temperature for the spherical particle compact made at uniaxial pressure 100 MPa and CIP 100 MPa conditions.
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Fig. 7. The change of shrinkage ratio in sintering with the pressure of uniaxial pressing for two types of compacts sintered statically at 1600 jC for 2 h.
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Fig. 7 shows the change of shrinkage ratio in sintering with the pressure of uniaxial pressing for the two types of compacts sintered in an electron furnace in air at 1600 jC for 2 h. The shrinkage ratio in sintering is defined as the ratio of shrinkage during the sintering in the H direction relative to that in the D direction. The value 1 shows isotropic sintering shrinkage in both directions. The system of spherical particles always shows the ratio nearly 1 at all uniaxial pressures, even with and without CIP. Namely, the compacts of spherical particles show the isotropic sintering shrinkage at the compaction conditions. In contrast, the system of elongated particles shows the sintering shrinkage ratio larger than 1 at the compaction conditions. The anisotropy of sintering shrinkage increases markedly with increasing uniaxial pressure. This implies that the anisotropy of particle packing change with the uniaxial pressure. Surprisingly, the CIP treatment significantly increases the shrinkage anisotropy in sintering for this system. Fig. 8 shows the change of the sintering shrinkage ratio with sintering temperature for uniaxially pressed and CIPed compacts made of elongated shape of alumina particles. Data for temperature under 1400 jC were taken in continuous measurement condition and those for 1550 and 1600
for all specimens cut from various locations in the compacts too, with the maximum error less than 0.1%. These results indicate that the anisotropic sintering shrinkage is absent in the compacts made of spherical particles.
Fig. 8. The change of the sintering shrinkage ratio with sintering temperature for uniaxially pressed and CIPed (100 MPa) compacts made of elongated shape of alumina particles.
Fig. 9. The normal optical micrographs in the D (a) and H (b) planes for compact made with alumina particles of elongated shape at uniaxial pressure 100 MPa and CIP 100 MPa.
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jC in static measurement condition. The anisotropy of sintering shrinkage decreases with increasing sintering temperature for all uniaxial pressure. It increases with increasing pressure of uniaxial pressing for all sintering temperatures. Even with the minimum pressure 10 MPa in uniaxial pressing, a marked anisotropy was noted in the compacts. Interestingly, the anisotropy changes with sintering temperature and pressure in uniaxial pressing. Fig. 9 shows the normal optical micrographs in the D plane (a) and the H plane (b) for compact made with alumina particles of elongated shape at uniaxial pressure 100 MPa and CIP 100 MPa. Spray-dried granules of deformed shape are distinctly seen in these micrographs. The micrograph of the D plane shows that the granules retain mostly their original round shape. Granules are deformed into elliptical shape in the micrograph of the H plane. Clearly, the granules have been deformed anisotropically in uniaxial pressing, even after CIP. The subsequent CIP did not cause the deformed granules to recover to their original spherical shape. On the other hand, the similar results were observed for the compacts of spherical shape of particles.
Fig. 10 shows the crossed polarized light micrographs of the two planes for the compact made with the elongated particles at uniaxial pressure 100 MPa and CIP 100 MPa. Micrographs of the H plane show the change of brightness in matrix with the rotation of specimen at every 45j. The specimen has anisotropic optical property in this plane. Micrographs of the D plane showed only dark image for all angles of the rotation. The specimen has optically isotropic property in this plane. The similar results were noted for other specimens cut from various locations in these compacts. Clearly, the elongated particles aligned in the H plane with their long axis (normal c-axis of alumina hexagonal crystal) normal to the direction of uniaxial pressing, and randomly aligned in the D plane in the compacts [18 – 21]. Fig. 11 shows the crossed polarized light micrographs of the two planes for the compacts made with the spherical particle at uniaxial pressure 100 MPa and CIP 100 MPa. All micrographs show only dark image for all angles of the rotation. The similar results were noted for all other specimens cut from various locations in the compacts. Clearly, there is no preferred orientation of particles in the com-
Fig. 10. The crossed polarized light micrographs of the H planes (a: 0j, b: 45j) and D plane (c: 0j, d: 45j) for compact made with the elongated particles at uniaxial pressure 100 MPa and CIP 100 MPa.
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Fig. 11. The crossed polarized light micrographs of the H plane (a: 0j, b: 45j) and D plane (c: 0j, d: 45j) for compacts made with the spherical particle at uniaxial pressure 100 MPa and CIP 100 MPa.
pacts made with the spherical shape of alumina particles [18 – 21]. Fig. 12 is the crossed polarized light micrographs of the two planes for sintered body made of elongated particles at uniaxial pressure 100 MPa and CIP 100 MPa and in sintering at 1600 jC for 2h. The micrograph of the H plane shows the dark – bright change in the matrix with the rotation at every 45j. Micrographs of the D plane showed no change with the rotation. The similar results also were noted for all other specimens cut from various locations in the sintered bodies. Clearly, the particle orientation produced by uniaxial pressing still is retained in the sintered body. The sintering did not eliminate the particle orientation in the compacts.
4. Discussions Alumina particles of industrial grade of raw material have a slightly elongated particle shape, and are oriented in uniaxially pressed compact even after CIP treatment. The particle orientation caused shrinkage anisotropy of the compacts during sintering. On the other hand, the compacts
of spherical particles showed no particle orientation and isotropic sintering shrinkage, even at the same processing condition to the former. Clearly, the sintering shrinkage anisotropy depends on the particle orientation. The anisotropic shrinkage due to the particle orientation may cause large defect and crack to develop in a complex shape of compact during sintering. It is important to control particle orientation for producing advanced ceramics. This successful finding owes to the new analytical tool used in this study. The particle orientation can be predicted in the uniaxially pressed system of the elongated shape of particles, but this study clearly showed its presence in the system. The particles were raw industrial material, which widely used in industrial production. This study is expected to have a wide reference value. No other tools available could identify the presence of particle orientation, since the degree of orientation is so light. We tried to characterize the particle orientation with the SEM and powder X-ray diffraction including the Rocking curve of many times [28,29]. However, the efforts were unsuccessful. The other origin of shrinkage anisotropy of compact during sintering may be considered to particle packing density. However, no significant density gradient was found
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Fig. 12. The crossed polarized light micrographs of the H plane (a: 0j, b: 45j) and D plane (c: 0j, d: 45j) for sintered body made of elongated particles at uniaxial pressure 100 MPa and CIP 100 MPa and in sintering at 1600 jC for 2h.
in the compacts in a separate paper. In the separate paper, the density distribution was measured directly with the 3D X-ray tomography on the uniaxially pressed alumina compacts. The maximum difference of density was under 0.7%, but inside the margin of error was 0.8% for the 3D X-ray tomography method. Accordingly, the compacts can be considered to homogeneity in particle packing density. Clearly, the sintering shrinkage anisotropy should be ascribed to anisotropy of the particle orientation in this study. First, an important finding is that the shrinkage behavior noted in this study is opposite to what is expected from the nonuniformity of density. Larger shrinkage was noted in sintering in height direction rather than the diametrical direction. An opposite result should be obtained if the shrinkage anisotropy is caused by the difference in packing density of powder particles in the two directions. Packing density of particles should be lower in diametrical direction rather than the height direction. The stress during uniaxial pressing was applied indirectly and directly in uniaxial pressing in the former and latter direction, respectively. Larger shrinkage is expected in the direction of lower packing density of powder particles.
The second important finding is that the effect of CIP on sintering shrinkage anisotropy is also opposite to what results from the nonuniform density. The anisotropy increases after CIP in Fig. 7 in the system of elongated particles. If the nonuniform density is responsible, the anisotropy should decrease after CIP. It should be added that no significant shrinkage anisotropy occurred in sintering in the system of spherical particles in both with and without CIP. The shrinkage anisotropy is consistent to what is expected by the packing structure proposed in this study. In a system where particles of elongated shape are aligned in one direction, the shrinkage in sintering is large in the direction normal to the alignment. A good example is alumina ceramics made with the injection molding. The particles are aligned with their long axis along the flow direction. Larger shrinkage was noted in the direction normal to flow. The similar results are noted in injection molding, tape casting, slip casting and casting molding systems [2,4 –7]. It should be emphasized that this study does not exclude importance of anisotropic particle packing density on the shrinkage anisotropy. This should be an important cause of
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the shrinkage anisotropy in a system with large anisotropy of packing density. In a real system, both the anisotropic packing density and the particle orientation cause the sintering shrinkage anisotropy. It is difficult to explain the sintering shrinkage anisotropy caused by particle orientation in compact with existing theory of sintering. Kaplan et al. [30] suggested that different twin boundary structures result in anisotropic growth of alumina, which is related to the lack of centro-symmetry in the trigonal corundum structure. Seabaugh et al. [31] indicated that modification of diffusion in platelet-shaped alumina particle system to continuously provide material to the anisotropically growing template grains. It is also difficult to identify which is responsible for the shrinkage anisotropy; the change of transport property of alumina with crystal axes or the particle shape. In general, the mass transport property varies with crystalline axis in anisotropic material such as alumina. The shortest axis of alumina particle corresponds to c-axis in this study, and cplane vertical the c-axis. The diffusion coefficient along the c-plane may be larger than its normal plane. Unfortunately, no individual diffusion datum is available for the two directions of alumina single crystal.
[5]
[6]
[7]
[8] [9]
[10] [11]
[12] [13]
[14] [15]
5. Conclusions [16]
The particle orientation is one of origins of sintering shrinkage anisotropy in addition to the anisotropic particle packing density in uniaxially pressed alumina compacts. In the compacts of elongated particles, uniaxial pressing produces the particle orientation. It cannot be eliminated by the subsequent CIP treatment, and causes the sintering shrinkage anisotropy. There is no particle orientation in the compacts made with spherical particles, and the compacts show isotropic sintering shrinkage.
Acknowledgements This study was supported by the Grant-in-Aid for Scientific Research in the Ministry of Education, Culture, Sport, Science and Technology.
[17]
[18]
[19]
[20]
[21]
[22]
References [1] K. Uematsu, H. Ito, S. Ohsaka, H. Takahashi, N. Shinohara, M. Okumiya, Characterization of particle packing in an injection molded green body. J. Am. Ceram. Soc. 78 (11) (1995) 3107 – 3109. [2] T. Zhang, S. Blackburn, J. Bridgwater, Debinding and sintering defects from particle orientation in ceramic injection moulding. J. Mater. Sci. 31 (1996) 5891 – 5896. [3] J.S. Sung, K.D. Koo, J.H. Park, Lamination and sintering shrinkage behavior in multilayered ceramics. J. Am. Ceram. Soc. 82 (3) (1999) 537 – 544. [4] K. Uematsu, S. Ohsaka, N. Shinohara, M. Okumiya, Grain-oriented
[23]
[24]
[25]
[26]
17
microstructure of alumina ceramics made through the injection molding process. J. Am. Ceram. Soc. 80 1997, pp. 1313 – 1315. M. Takahashi, T. Fujiwara, H. Hayashi, O. Nishizato, M. Yamamoto, Mechanical and electromechanical properties of tetragonal ZrO2 fiber/ PZT composites. J. Ceram. Soc. Jpn. 102 (10) (1994) 944 – 949. A.R. Boccaccini, P.A. Trusty, D.M.R. Taplin, Anisotropic shrinkage of barium magnesium aluminosilicate glass powder compacts during sintering. Mater. Lett. 24 (4) (1995) 199 – 205. P.M. Raj, W.R. Cannon, Anisotropic shrinkage in tape-cast alumina: role of processing parameters and particle shape. J. Am. Ceram. Soc. 82 (10) 1999, pp. 2619 – 2625. D.S. Park, C.W. Kim, A modification of tape casting for aligning the whiskers. J. Mater. Sci. 34 (1999) 5827 – 5832. S.J. Stedman, J.R.G. Evans, R.J. Brook, M.J. Hoffmann, Anisotropic sintering shrinkage in injection-moulded composite ceramics. J. Eur. Ceram. Soc. 11 (6) (1993) 523 – 532. H. Michael, J.N. Alwin, G. Peter, P. Guenter, Slip casting of SiC – whisker-reinforced Si3N4. J. Am. Ceram. Soc. 72 (1989) 765 – 769. N. Hirose, J. Asami, Prediction on deformation of fine powder green compact after sintering. J. Jpn. Soc. Powder Powder Metall. 41 (3) (1994) 248 – 252. T.N. Tiegs, P.F. Becher, Sintered Al2O3 – SiC – whisker composites. Am. Ceram. Soc. Bull. 66 (1987) 339 – 342. W.S. Cho, K. Hayashi, Effect of uni-directional orientation of SiC whisker on pressureless-sintering densification of SiC whisker/Si3N4 ceramics composite. J. Jpn. Soc. Powder Powder Metall. 40 (1) (1993) 16 – 19. F.V. Dimarcello, P.L. Key, J.C. Williams, Preferred orientation in Al2O3 substrates. J. Am. Ceram. Soc. 55 (10) (1972) 509 – 514. W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to Ceramics. 2nd edn., Wiley, New York, 1976, pp. 507 – 513. K. Uematsu, J. Kim, Z. Kato, N. Uchida, K. Saito, Direct observation method for internal structure of ceramic green body. J. Ceram. Soc. Jpn. 98 (1990) 515 – 516. K. Uematsu, M. Miyashita, J. Kim, N. Uchida, Direct study on the behavior of flaw-forming defect in sintering. J. Am. Ceram. Soc. 75 (4) (1992) 1016 – 1018. K. Uematsu, H. Ito, Y. Zhang, N. Uchida, Novel characterization method for the processing ceramics by polarized light microscope with liquid immersion technique. Ceram. Trans., Am. Ceram. Soc. 54 1995, pp. 83 – 89. K. Uematsu, N. Uchida, Effect of packing structure of powder particles in ceramic green body on microstructure development in densification process, in: A.P. Tomsia, A. Gleaser (Eds.), Ceramic Microstrcture: Control at the Atomic Level. Plenum, New York, 1998, pp. 239 – 246. K. Uematsu, Immersion microscopy for detailed characterization of defects in ceramic powders and green bodies. Powder Technol. 88 (1996) 291 – 298. K. Uematsu, Y. Zhang, N. Uchida, Characteristics and formation mechanisms of defects in ceramic powder compacts. Ceram. Trans. 51 (1995) 263 – 270. A. Shui, Y. Zhang, N. Uchida, K. Uematsu, Origin of shape deformation during sintering alumina compacts. J. Ceram. Soc. Jpn. 106 (9) (1998) 873 – 876. A. Shui, Z. Kato, S. Tanaka, N. Uchida, K. Uematsu, Isotropic sintering shrinkage in pressed compact of near-spherical alumina particles, Am. Ceram. Soc. Bull. 80 (9) (2001) 29 – 52. Y. Takao, T. Hotta, K. Nakahira, M. Naito, N. Shinohara, M. Uematsu, K. Uematsu, Processing defects and their relevance to strength in alumina ceramics made by slip casting. J. Eur. Ceram. Soc. 20 (2) 2000, pp. 389 – 395. N. Shinohara, M. Okumiya, T. Hotta, K. Nakahira, M. Naito, K. Uematsu, Seasonal variation of microstructure and sintered strength of dry-pressed alumina. J. Am. Ceram. Soc. 82 (1999) 3441 – 3446. N. Shinohara, M. Okumiya, T. Hotta, K. Nakahira, M. Naito, K. Uematsu, Morphological changes in process-related large pores of
18
A. Shui et al. / Powder Technology 127 (2002) 9–18
granular compacted and sintered alumina. J. Am. Ceram. Soc. 83 2000, pp. 1633 – 1640. [27] N. Shinohara, M. Okumiya, T. Hotta, K. Nakahira, M. Naito, K. Uematsu, Variation of the microstructure and fracture strength of cold isostatically pressed alumina ceramics with the alteration of dewaxing procedures. J. Eur. Ceram. Soc. 20 2000, pp. 843 – 849. [28] C.S. Park, J.G. Kim, J.S. Chun, Crystallographic orientation and surface morphology of chemical vapor deposited Al2O3. J. Electrochem. Soc. 130 (7) (1983) 1607 – 1611.
[29] I. Yagi, Y. Hagiwara, K. Murakami, S. Kaneko, Growth of highly oriented SnO2 thin films on glass substrate from tetra-n-butyltin by the spray pyrolysis technique. J. Mater. Res. 8 (7) (1993) 1481 – 1483. [30] W.D. Kaplan, P.R. Kenway, D.G. Brandon, Polymorphic basal twin boundaries and anisotropic growth in alpha-Al2O3. Acta Metall. Mater. 43 (2) (1995) 835 – 848. [31] M.M. Seabaugh, I.H. Kerscht, G.L. Messing, Texture development by templated grain growth in liquid-phase-sintered alpha-alumina. J. Am. Ceram. Soc. 80 (5) (1997) 1181 – 1188.
Origin of shrinkage anisotropy during sintering for uniaxially pressed alumina compacts Anze Shui *, Nozomu Uchida, Keizo Uematsu Department of Chemistry, Ceramics Science Laboratory, Nagaoka University of Technology, Kamitomioka, Nagaoka, Niigata 940-2188, Japan Received 3 August 2001; received in revised form 29 October 2001; accepted 12 December 2001
Abstract Origin of shrinkage anisotropy during sintering is reported on alumina powder compacts made by uniaxial pressing followed by cold isostatic pressing (CIP). Two types of alumina particles were used in this study; one was slightly elongated shape, the other spherical shape. An anisotropic shrinkage occurred during sintering in the compacts made of the elongated shape of particles, even after the CIP. However, no the anisotropic shrinkage happened in the compacts made of the spherical shape of particles, even at the same processing condition to the former. Detailed structure analysis involving the novel immersion liquid technique showed that the particle orientation was found to be an important origin of the shrinkage anisotropy. The particle orientation was developed during uniaxial pressing in the compacts of the elongated shape of particles. Subsequent CIP did not eliminate the shrinkage anisotropy. No particle orientation happened in the compact of the spherical shape of particles. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Particle Shape; Orientation; Sintering Shrinkage; Anisotropy; Alumina
1. Introduction Anisotropic shrinkage of compact during sintering, i.e., sintering deformation is a common and serious problem in ceramics production [1– 13]. The anisotropic shrinkage may cause large defects and cracks to develop in a complex shape of compact during sintering. Otherwise, it requires a timeconsuming machining involving diamond tools for reshaping, and markedly increases the production cost of ceramics. One of origins of the sintering deformation is the nonuniformity in compacts [3,11,14]. The most common nonuniformity is the local difference in packing density of particles. Other nonuniformity, i.e., particle orientation, also causes the anisotropic shrinkage in injection molding, tape casting, slip casting and casting molding systems [2,4 – 7], where strong shear stress is present during forming. Nonisometric particles are oriented by the shear stress, and cause the anisotropic sintering shrinkage of the compacts. In compacts made by die pressing, particle orientation has attracted only little attention except for the systems of extremely elongated particle shape, such as clay-base materials [15]. When the * Corresponding author. Tel.: +81-258-47-9337; fax: +81-258-479300. E-mail address: [email protected] (A. Shui).
particles have near equiaxed shape, they are usually believed to randomly orient in the compacts. However, even the particle shape is slightly elongated, particle orientation may also occur during uniaxial pressing in the uniaxially pressed compacts, and similarly cause the shrinkage anisotropy. Extremely sensitive analytical tool is needed to examine the relevance between the particle orientation and the anisotropic shrinkage in sintering for the compacts. A conventional X-ray diffraction method cannot identify the orientation in the system of slightly elongated shape of particles. A new method of our development will be applied in this study to examine the microstructure of compact and sintered body [16 – 21]. In this technique, the compact is made transparent with an immersion liquid, and its internal structure is observed with a polarized light microscope in the transmission mode. This method has been successfully applied to solve several difficult problems in ceramics, such as the formation mechanism of large flaws and interior stress occurring in ceramics processing [1,4,19 –27]. The characterization method may also reveal the minor particle orientation in the pressed compacts of present study. The aim of this work is to clarify the effect of particle orientation on the shrinkage anisotropy in the uniaxially pressed alumina compact. Two types of commercial particles are used as raw powders, one is of slightly elongated
0032-5910/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 ( 0 2 ) 0 0 0 0 4 - 9
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and the other of spherical particle shapes. The former represents the conventional low soda alumina, which is widely applied for industrial production of alumina ceramics. The latter is high purity alumina. Accordingly, the versatility of this study is expected. The elongated shape of particles should be aligned with their long axis normal to the applied stress of uniaxial compaction in pressing, and randomly oriented within the plane normal to the pressing direction. No specific orientation should happen in the system of spherical particles. These structures will be observed with our novel characterization technique to explain effect of particle orientation on the shrinkage anisotropy for uniaxially pressed alumina compact. In the examination, fabrication of specimens with uniform packing density is essential. To achieve this, the compacts were CIPed after the uniaxial pressing.
2. Experiment Two kinds of commercial powders were used in this study, 160SG-1 (powder of elongated particle shape, Show-
adenko, Japan; described as SG-1 hereafter) and AA05 (powder of spherical shape, Sumitomo Chemical, Japan). The slurry containing 50 mass% alumina was prepared by mixing it with distilled water and 2 mass% PVA in a ball mill. Spray-dried alumina granules were prepared with a spray drier (Model SD13, Mitsui Kozan, Japan, inner diameter 1.3 m) with the inlet air temperature 200 jC. Compacts of diameter 15 mm and thickness 4– 5 mm were prepared by uniaxial pressing (10 –100 MPa) followed by CIP (100 MPa). The shrinkage during sintering was measured under static and continuous measurement conditions. In the static condition, the compact was heated to 1550 jC or 1600 jC (heating rate 10 jC/min) in an electric furnace in air and kept for 2 h for densification. The sintering shrinkages in height and diameter directions (hereafter described as H and D directions, respectively) were determined from the dimensional changes before and after the sintering. For the examination in continuous measurement condition, specimens of the length 2– 2.5 mm were cut from various places in these compacts. The change of specimen length was followed with a dilatometer (TMA-50, Shimazu, Japan) at
Fig. 1. The SEM micrographs of fracture surfaces for the compacts made with elongated shape of alumina particles in the H (a) and D (b) directions; and spherical shape of alumina particles in the H (c) and D (d) directions.
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the heating rate 10 jC /min until 1400 jC. To ensure the accuracy of the measurement, blank tests were executed at least five times before the measurement. The blank tests show that the error was less than 1 Am. The measurements of sintering shrinkage were repeated at least three times with fresh specimens in various compaction conditions, and the maximum error in the shrinkage is less than 0.1% for all the compaction conditions. The green densities of compacts were calculated from their weight and dimensions. The sintered density was determined with the Archimedean method using distilled water. The liquid immersion technique, SEM and X-ray diffraction methods were used to examine the internal structure of the compacts and sintered bodies. In the liquid immersion method, samples were thinned to about 0.1-mm thick with sandpaper, and methylene iodide (refractive index 1.74) was applied to make it transparent. In the SEM observation, samples were treated with gold sputtering.
3. Results Fig. 1 shows the SEM micrographs of fracture surfaces for the compacts made with the alumina of elongated particle shape and the alumina of spherical particle shape in H and D directions. The packing structure of the primary alumina particles appears same in both planes for each compact. Clearly, this conventional characterization tool has not enough power to identify the particle orientation anisotropy in the specimens. Information obtained with this
Fig. 2. Histograms of aspect ratio distributions for the two types of alumina primary particles used in this study.
Fig. 3. The change of green density with uniaxial pressure on the two types of compacts before and after CIPing.
tool is that the low-soda alumina has a slightly elongated particle shape, which is typical for this type of alumina. The nominal particle size, 0.4 Am, is consistent with the micrographs. The alumina of spherical particle shape is noted in the high-purity alumina. The particle size determined by inspection is also consistent with the value provided by the manufacturer, 0.5 Am. Fig. 2 shows histograms of aspect ratio distributions for the two types of alumina primary particles used in this study. The ratio was determined by measuring the maximum length and the length normal to the direction of the maximum length on over five hundreds particles from their SEM micrographs. In alumina of elongated particle shape, the value of aspect ratio varied widely from 1 to 3.4, with the mean value 1.6. For the spherical particles, the aspect ratio has the mean value of 1.1, and its distribution is very narrow compared with that of the elongated shape particles. These particles can be considered as spherical shape in this work. Fig. 3 shows the change of green density with uniaxial pressure on the two types of compacts before and after CIP. The green density markedly increases with uniaxial pressure for both types of the compacts before CIP. It increases slightly with the uniaxial pressure after CIP. Both types of compacts showed similar gradient in the pressure-green density curves after CIP, but the gradient almost was different at various pressures before CIP. The density after CIP always is higher for the compact of spherical particle than that of elongated particle. These results imply that particle packing behavior significantly depend on shape of raw powder, even though they have the similar particle size.
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Fig. 4 shows the change of shrinkage ratio during CIP with uniaxial pressure for the two types of compacts. The shrinkage ratio is defined as the ratio of shrinkage during CIP in H direction relative to that in D direction. Small value less than 1 in the shrinkage ratio corresponds to large shrinkage anisotropy during CIP. The shrinkage ratio was found to be always less than 1 for the both types of compacts, i.e., the D direction shrinks more than the H direction during CIP in all uniaxial pressure conditions. The ratio decreases as the pressure in uniaxial pressing increases for both types of the compacts. The compact of elongated shape of particles always showed smaller the ratio than that of the spherical shape of particles at same pressure. The shrinkage anisotropy during CIP is more significant for the compact made with the elongated particles than that with the spherical particles. These results indicate that the particle transfer such as slip, rotate and realign is more predominant during CIP for the former than the later. Fig. 5 shows the change of sintering shrinkage with temperature for the compacts of elongated particles (uniaxial pressing at 100 MPa and CIP at 100 MPa). The shrinkage always is larger in the H direction than the D direction at all sintering temperatures in both static and continuous measurement conditions. The difference of the shrinkages in the two directions increases as the sintering temperature increases. The same results were obtained for all specimens cut from various locations in this type of compact, with the maximum error less than 0.1%. Clearly, the compacts show anisotropic shrinkage during sintering. Fig. 6 shows the change of the sintering shrinkage with temperature for the compacts made with the spherical shape
Fig. 4. The change of the shrinkage ratio during CIP with uniaxial pressure for the two types of compacts.
Fig. 5. The change of the sintering shrinkage with temperature for the elongated particle compact made at uniaxial pressure 100 MPa and CIP 100 MPa conditions.
of particles at the uniaxial pressure 100 MPa and CIP 100 MPa. An important difference from the above results is that the shrinkage essentially is same in the two directions at all sintering temperatures. The same results also were obtained
Fig. 6. The change of the sintering shrinkage with temperature for the spherical particle compact made at uniaxial pressure 100 MPa and CIP 100 MPa conditions.
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Fig. 7. The change of shrinkage ratio in sintering with the pressure of uniaxial pressing for two types of compacts sintered statically at 1600 jC for 2 h.
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Fig. 7 shows the change of shrinkage ratio in sintering with the pressure of uniaxial pressing for the two types of compacts sintered in an electron furnace in air at 1600 jC for 2 h. The shrinkage ratio in sintering is defined as the ratio of shrinkage during the sintering in the H direction relative to that in the D direction. The value 1 shows isotropic sintering shrinkage in both directions. The system of spherical particles always shows the ratio nearly 1 at all uniaxial pressures, even with and without CIP. Namely, the compacts of spherical particles show the isotropic sintering shrinkage at the compaction conditions. In contrast, the system of elongated particles shows the sintering shrinkage ratio larger than 1 at the compaction conditions. The anisotropy of sintering shrinkage increases markedly with increasing uniaxial pressure. This implies that the anisotropy of particle packing change with the uniaxial pressure. Surprisingly, the CIP treatment significantly increases the shrinkage anisotropy in sintering for this system. Fig. 8 shows the change of the sintering shrinkage ratio with sintering temperature for uniaxially pressed and CIPed compacts made of elongated shape of alumina particles. Data for temperature under 1400 jC were taken in continuous measurement condition and those for 1550 and 1600
for all specimens cut from various locations in the compacts too, with the maximum error less than 0.1%. These results indicate that the anisotropic sintering shrinkage is absent in the compacts made of spherical particles.
Fig. 8. The change of the sintering shrinkage ratio with sintering temperature for uniaxially pressed and CIPed (100 MPa) compacts made of elongated shape of alumina particles.
Fig. 9. The normal optical micrographs in the D (a) and H (b) planes for compact made with alumina particles of elongated shape at uniaxial pressure 100 MPa and CIP 100 MPa.
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jC in static measurement condition. The anisotropy of sintering shrinkage decreases with increasing sintering temperature for all uniaxial pressure. It increases with increasing pressure of uniaxial pressing for all sintering temperatures. Even with the minimum pressure 10 MPa in uniaxial pressing, a marked anisotropy was noted in the compacts. Interestingly, the anisotropy changes with sintering temperature and pressure in uniaxial pressing. Fig. 9 shows the normal optical micrographs in the D plane (a) and the H plane (b) for compact made with alumina particles of elongated shape at uniaxial pressure 100 MPa and CIP 100 MPa. Spray-dried granules of deformed shape are distinctly seen in these micrographs. The micrograph of the D plane shows that the granules retain mostly their original round shape. Granules are deformed into elliptical shape in the micrograph of the H plane. Clearly, the granules have been deformed anisotropically in uniaxial pressing, even after CIP. The subsequent CIP did not cause the deformed granules to recover to their original spherical shape. On the other hand, the similar results were observed for the compacts of spherical shape of particles.
Fig. 10 shows the crossed polarized light micrographs of the two planes for the compact made with the elongated particles at uniaxial pressure 100 MPa and CIP 100 MPa. Micrographs of the H plane show the change of brightness in matrix with the rotation of specimen at every 45j. The specimen has anisotropic optical property in this plane. Micrographs of the D plane showed only dark image for all angles of the rotation. The specimen has optically isotropic property in this plane. The similar results were noted for other specimens cut from various locations in these compacts. Clearly, the elongated particles aligned in the H plane with their long axis (normal c-axis of alumina hexagonal crystal) normal to the direction of uniaxial pressing, and randomly aligned in the D plane in the compacts [18 – 21]. Fig. 11 shows the crossed polarized light micrographs of the two planes for the compacts made with the spherical particle at uniaxial pressure 100 MPa and CIP 100 MPa. All micrographs show only dark image for all angles of the rotation. The similar results were noted for all other specimens cut from various locations in the compacts. Clearly, there is no preferred orientation of particles in the com-
Fig. 10. The crossed polarized light micrographs of the H planes (a: 0j, b: 45j) and D plane (c: 0j, d: 45j) for compact made with the elongated particles at uniaxial pressure 100 MPa and CIP 100 MPa.
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Fig. 11. The crossed polarized light micrographs of the H plane (a: 0j, b: 45j) and D plane (c: 0j, d: 45j) for compacts made with the spherical particle at uniaxial pressure 100 MPa and CIP 100 MPa.
pacts made with the spherical shape of alumina particles [18 – 21]. Fig. 12 is the crossed polarized light micrographs of the two planes for sintered body made of elongated particles at uniaxial pressure 100 MPa and CIP 100 MPa and in sintering at 1600 jC for 2h. The micrograph of the H plane shows the dark – bright change in the matrix with the rotation at every 45j. Micrographs of the D plane showed no change with the rotation. The similar results also were noted for all other specimens cut from various locations in the sintered bodies. Clearly, the particle orientation produced by uniaxial pressing still is retained in the sintered body. The sintering did not eliminate the particle orientation in the compacts.
4. Discussions Alumina particles of industrial grade of raw material have a slightly elongated particle shape, and are oriented in uniaxially pressed compact even after CIP treatment. The particle orientation caused shrinkage anisotropy of the compacts during sintering. On the other hand, the compacts
of spherical particles showed no particle orientation and isotropic sintering shrinkage, even at the same processing condition to the former. Clearly, the sintering shrinkage anisotropy depends on the particle orientation. The anisotropic shrinkage due to the particle orientation may cause large defect and crack to develop in a complex shape of compact during sintering. It is important to control particle orientation for producing advanced ceramics. This successful finding owes to the new analytical tool used in this study. The particle orientation can be predicted in the uniaxially pressed system of the elongated shape of particles, but this study clearly showed its presence in the system. The particles were raw industrial material, which widely used in industrial production. This study is expected to have a wide reference value. No other tools available could identify the presence of particle orientation, since the degree of orientation is so light. We tried to characterize the particle orientation with the SEM and powder X-ray diffraction including the Rocking curve of many times [28,29]. However, the efforts were unsuccessful. The other origin of shrinkage anisotropy of compact during sintering may be considered to particle packing density. However, no significant density gradient was found
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Fig. 12. The crossed polarized light micrographs of the H plane (a: 0j, b: 45j) and D plane (c: 0j, d: 45j) for sintered body made of elongated particles at uniaxial pressure 100 MPa and CIP 100 MPa and in sintering at 1600 jC for 2h.
in the compacts in a separate paper. In the separate paper, the density distribution was measured directly with the 3D X-ray tomography on the uniaxially pressed alumina compacts. The maximum difference of density was under 0.7%, but inside the margin of error was 0.8% for the 3D X-ray tomography method. Accordingly, the compacts can be considered to homogeneity in particle packing density. Clearly, the sintering shrinkage anisotropy should be ascribed to anisotropy of the particle orientation in this study. First, an important finding is that the shrinkage behavior noted in this study is opposite to what is expected from the nonuniformity of density. Larger shrinkage was noted in sintering in height direction rather than the diametrical direction. An opposite result should be obtained if the shrinkage anisotropy is caused by the difference in packing density of powder particles in the two directions. Packing density of particles should be lower in diametrical direction rather than the height direction. The stress during uniaxial pressing was applied indirectly and directly in uniaxial pressing in the former and latter direction, respectively. Larger shrinkage is expected in the direction of lower packing density of powder particles.
The second important finding is that the effect of CIP on sintering shrinkage anisotropy is also opposite to what results from the nonuniform density. The anisotropy increases after CIP in Fig. 7 in the system of elongated particles. If the nonuniform density is responsible, the anisotropy should decrease after CIP. It should be added that no significant shrinkage anisotropy occurred in sintering in the system of spherical particles in both with and without CIP. The shrinkage anisotropy is consistent to what is expected by the packing structure proposed in this study. In a system where particles of elongated shape are aligned in one direction, the shrinkage in sintering is large in the direction normal to the alignment. A good example is alumina ceramics made with the injection molding. The particles are aligned with their long axis along the flow direction. Larger shrinkage was noted in the direction normal to flow. The similar results are noted in injection molding, tape casting, slip casting and casting molding systems [2,4 –7]. It should be emphasized that this study does not exclude importance of anisotropic particle packing density on the shrinkage anisotropy. This should be an important cause of
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the shrinkage anisotropy in a system with large anisotropy of packing density. In a real system, both the anisotropic packing density and the particle orientation cause the sintering shrinkage anisotropy. It is difficult to explain the sintering shrinkage anisotropy caused by particle orientation in compact with existing theory of sintering. Kaplan et al. [30] suggested that different twin boundary structures result in anisotropic growth of alumina, which is related to the lack of centro-symmetry in the trigonal corundum structure. Seabaugh et al. [31] indicated that modification of diffusion in platelet-shaped alumina particle system to continuously provide material to the anisotropically growing template grains. It is also difficult to identify which is responsible for the shrinkage anisotropy; the change of transport property of alumina with crystal axes or the particle shape. In general, the mass transport property varies with crystalline axis in anisotropic material such as alumina. The shortest axis of alumina particle corresponds to c-axis in this study, and cplane vertical the c-axis. The diffusion coefficient along the c-plane may be larger than its normal plane. Unfortunately, no individual diffusion datum is available for the two directions of alumina single crystal.
[5]
[6]
[7]
[8] [9]
[10] [11]
[12] [13]
[14] [15]
5. Conclusions [16]
The particle orientation is one of origins of sintering shrinkage anisotropy in addition to the anisotropic particle packing density in uniaxially pressed alumina compacts. In the compacts of elongated particles, uniaxial pressing produces the particle orientation. It cannot be eliminated by the subsequent CIP treatment, and causes the sintering shrinkage anisotropy. There is no particle orientation in the compacts made with spherical particles, and the compacts show isotropic sintering shrinkage.
Acknowledgements This study was supported by the Grant-in-Aid for Scientific Research in the Ministry of Education, Culture, Sport, Science and Technology.
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[18]
[19]
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[21]
[22]
References [1] K. Uematsu, H. Ito, S. Ohsaka, H. Takahashi, N. Shinohara, M. Okumiya, Characterization of particle packing in an injection molded green body. J. Am. Ceram. Soc. 78 (11) (1995) 3107 – 3109. [2] T. Zhang, S. Blackburn, J. Bridgwater, Debinding and sintering defects from particle orientation in ceramic injection moulding. J. Mater. Sci. 31 (1996) 5891 – 5896. [3] J.S. Sung, K.D. Koo, J.H. Park, Lamination and sintering shrinkage behavior in multilayered ceramics. J. Am. Ceram. Soc. 82 (3) (1999) 537 – 544. [4] K. Uematsu, S. Ohsaka, N. Shinohara, M. Okumiya, Grain-oriented
[23]
[24]
[25]
[26]
17
microstructure of alumina ceramics made through the injection molding process. J. Am. Ceram. Soc. 80 1997, pp. 1313 – 1315. M. Takahashi, T. Fujiwara, H. Hayashi, O. Nishizato, M. Yamamoto, Mechanical and electromechanical properties of tetragonal ZrO2 fiber/ PZT composites. J. Ceram. Soc. Jpn. 102 (10) (1994) 944 – 949. A.R. Boccaccini, P.A. Trusty, D.M.R. Taplin, Anisotropic shrinkage of barium magnesium aluminosilicate glass powder compacts during sintering. Mater. Lett. 24 (4) (1995) 199 – 205. P.M. Raj, W.R. Cannon, Anisotropic shrinkage in tape-cast alumina: role of processing parameters and particle shape. J. Am. Ceram. Soc. 82 (10) 1999, pp. 2619 – 2625. D.S. Park, C.W. Kim, A modification of tape casting for aligning the whiskers. J. Mater. Sci. 34 (1999) 5827 – 5832. S.J. Stedman, J.R.G. Evans, R.J. Brook, M.J. Hoffmann, Anisotropic sintering shrinkage in injection-moulded composite ceramics. J. Eur. Ceram. Soc. 11 (6) (1993) 523 – 532. H. Michael, J.N. Alwin, G. Peter, P. Guenter, Slip casting of SiC – whisker-reinforced Si3N4. J. Am. Ceram. Soc. 72 (1989) 765 – 769. N. Hirose, J. Asami, Prediction on deformation of fine powder green compact after sintering. J. Jpn. Soc. Powder Powder Metall. 41 (3) (1994) 248 – 252. T.N. Tiegs, P.F. Becher, Sintered Al2O3 – SiC – whisker composites. Am. Ceram. Soc. Bull. 66 (1987) 339 – 342. W.S. Cho, K. Hayashi, Effect of uni-directional orientation of SiC whisker on pressureless-sintering densification of SiC whisker/Si3N4 ceramics composite. J. Jpn. Soc. Powder Powder Metall. 40 (1) (1993) 16 – 19. F.V. Dimarcello, P.L. Key, J.C. Williams, Preferred orientation in Al2O3 substrates. J. Am. Ceram. Soc. 55 (10) (1972) 509 – 514. W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to Ceramics. 2nd edn., Wiley, New York, 1976, pp. 507 – 513. K. Uematsu, J. Kim, Z. Kato, N. Uchida, K. Saito, Direct observation method for internal structure of ceramic green body. J. Ceram. Soc. Jpn. 98 (1990) 515 – 516. K. Uematsu, M. Miyashita, J. Kim, N. Uchida, Direct study on the behavior of flaw-forming defect in sintering. J. Am. Ceram. Soc. 75 (4) (1992) 1016 – 1018. K. Uematsu, H. Ito, Y. Zhang, N. Uchida, Novel characterization method for the processing ceramics by polarized light microscope with liquid immersion technique. Ceram. Trans., Am. Ceram. Soc. 54 1995, pp. 83 – 89. K. Uematsu, N. Uchida, Effect of packing structure of powder particles in ceramic green body on microstructure development in densification process, in: A.P. Tomsia, A. Gleaser (Eds.), Ceramic Microstrcture: Control at the Atomic Level. Plenum, New York, 1998, pp. 239 – 246. K. Uematsu, Immersion microscopy for detailed characterization of defects in ceramic powders and green bodies. Powder Technol. 88 (1996) 291 – 298. K. Uematsu, Y. Zhang, N. Uchida, Characteristics and formation mechanisms of defects in ceramic powder compacts. Ceram. Trans. 51 (1995) 263 – 270. A. Shui, Y. Zhang, N. Uchida, K. Uematsu, Origin of shape deformation during sintering alumina compacts. J. Ceram. Soc. Jpn. 106 (9) (1998) 873 – 876. A. Shui, Z. Kato, S. Tanaka, N. Uchida, K. Uematsu, Isotropic sintering shrinkage in pressed compact of near-spherical alumina particles, Am. Ceram. Soc. Bull. 80 (9) (2001) 29 – 52. Y. Takao, T. Hotta, K. Nakahira, M. Naito, N. Shinohara, M. Uematsu, K. Uematsu, Processing defects and their relevance to strength in alumina ceramics made by slip casting. J. Eur. Ceram. Soc. 20 (2) 2000, pp. 389 – 395. N. Shinohara, M. Okumiya, T. Hotta, K. Nakahira, M. Naito, K. Uematsu, Seasonal variation of microstructure and sintered strength of dry-pressed alumina. J. Am. Ceram. Soc. 82 (1999) 3441 – 3446. N. Shinohara, M. Okumiya, T. Hotta, K. Nakahira, M. Naito, K. Uematsu, Morphological changes in process-related large pores of
18
A. Shui et al. / Powder Technology 127 (2002) 9–18
granular compacted and sintered alumina. J. Am. Ceram. Soc. 83 2000, pp. 1633 – 1640. [27] N. Shinohara, M. Okumiya, T. Hotta, K. Nakahira, M. Naito, K. Uematsu, Variation of the microstructure and fracture strength of cold isostatically pressed alumina ceramics with the alteration of dewaxing procedures. J. Eur. Ceram. Soc. 20 2000, pp. 843 – 849. [28] C.S. Park, J.G. Kim, J.S. Chun, Crystallographic orientation and surface morphology of chemical vapor deposited Al2O3. J. Electrochem. Soc. 130 (7) (1983) 1607 – 1611.
[29] I. Yagi, Y. Hagiwara, K. Murakami, S. Kaneko, Growth of highly oriented SnO2 thin films on glass substrate from tetra-n-butyltin by the spray pyrolysis technique. J. Mater. Res. 8 (7) (1993) 1481 – 1483. [30] W.D. Kaplan, P.R. Kenway, D.G. Brandon, Polymorphic basal twin boundaries and anisotropic growth in alpha-Al2O3. Acta Metall. Mater. 43 (2) (1995) 835 – 848. [31] M.M. Seabaugh, I.H. Kerscht, G.L. Messing, Texture development by templated grain growth in liquid-phase-sintered alpha-alumina. J. Am. Ceram. Soc. 80 (5) (1997) 1181 – 1188.