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Effect of coarse particles on the strength of alumina made by slip casting
Powder Technology 149 (2005) 106 – 111 www.elsevier.com/locate/powtec
Effect of coarse particles on the strength of alumina made by slip casting Tadashi Hottaa,*, Hiroya Abeb, Makio Naitob, Minoru Takahashic, Keizo Uematsud, Zenji Katod a
Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki 567-0047, Japan c Ceramics Research Laboratory, Nagoya Institute of Technology, 10-6-29, Asahigaoka, Tajimi 507-0071, Japan d Department of Chemistry, Nagaoka University of Technology, 1603-1, Kamitomioka-cho, Nagaoka 940-2188, Japan b
Received 10 May 2004; received in revised form 19 October 2004; accepted 2 November 2004
Abstract The effect of coarse particles in alumina powder slurry on the microstructure and strength variation of the sintered alumina bodies made by slip casting was examined. A commercially available low-soda alumina powder was used as the raw material. Coarse agglomerated particles were added in the alumina powder slurry just before slip casting. Alumina ceramics were fabricated through slip casting process. The ceramic strength and fracture toughness were examined. Fracture origins and microstructure were observed with scanning electron microscopy (SEM). The internal structures of sintered bodies were examined by using mid-infrared microscope. As a result, direct observation technique using mid-infrared microscope enabled us to analyze a small amount of coarse agglomerated particles in sintered body. The strength variation of the sintered bodies was correlated with the size of coarse agglomerated particles detected by mid-infrared microscopy. D 2004 Elsevier B.V. All rights reserved. Keywords: Alumina ceramics; Slip casting; Strength; Microstructure; Internal structure; Mid-infrared microscopy
1. Introduction According to fracture mechanics, the strength and, thus, the reliability of ceramics are governed by detrimental structures, one of which behaves as a fracture origin. A wide variety of features is known to behave as detrimental structures, including cracks, pores, inclusions and coarse particles [1–3]. However, very few explicit understandings have been established on the characteristics of these detrimental features and their relevance to strength, leaving difficulties in controlling the quality of ceramics. Recent study shows the relevance between the coarse pore and the strength of ceramics explicitly [4–7]. An excellent correlation is noted between them [8–14]. The effect of coarse particles on the strength of ceramics is, however, much less understood. Clearly, a fundamental study is needed to understand the effect of coarse particles on the strength of ceramics. The aim of this study is to examine the effect of coarse particles on the strength of ceramics. A reasonable approach * Correspondent author. Tel.: +81 52 871 3500; fax: +81 52 871 3599. E-mail address: [email protected] (T. Hotta). 0032-5910/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2004.11.004
is to examine the strength of a model ceramic containing a controlled quantity of known coarse particles. Care should be taken in the characteristics of specimens to obtain a clear result in this approach. First, the matrix of ceramic should not contain detrimental natural defects which behave as uncontrolled fracture origins. Second, the coarse particles added should have very similar characteristics to natural ones. The structure of the model ceramics should be examined accurately. In this study, the slip casting process was used to make the model ceramics. The novel IR microscopy [15] was applied to show the distribution of coarse particles explicitly.
2. Experimental procedure 2.1. Fabrication of test specimens The specimens were prepared through the procedure shown in Fig. 1. Low soda alumina powder (AL-160SG-4, Showa Denko K.K., Japan) was used as raw material. The nominal average particle size was 0.5 Am. The powder was
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apparatus (LB-140, Katomasu Tekkosho, Japan). Finally, each slurry was cast in gypsum molds (1001009 mm) to prepare green compacts with coarse particles. After drying, the compact was heated at 1550 8C for 2 h in an electric furnace to sinter the model ceramics. 2.2. Characterization
Fig. 1. Fabrication process of alumina ceramics.
placed in an alumina pot mill (SSA-999; Nikkato, Japan; volume 2000 ml) with 2 kg of alumina ball (SSA-999; Nikkato; diameter 5 mm), and 400 g of aqueous solution (2 mass%) of dispersant of polyacrylic acid type (CERUNA D305, Chukyo Yushi, Japan) and mixed for 24 h to make a slurry with the solid content 50 vol.%. The slurry was passed through a mesh (opening 2 mm) to separate the balls. Weighed slurry was placed in a container and stirred continuously with a stirrer while a fixed amount of coarse particles was added. The coarse particles for addition were prepared from the unground raw material used in the production of the present fine alumina powder. The coarse particles were classified into three fractions before addition by sieving. Each fraction of coarse particles was added to each slurry; therefore, three kinds of slurries with coarse particles were prepared. The amount of each coarse particles was adjusted to make the number of particles per unit volume of the model ceramics approximately constant as shown in Table 1. Each slurry was kept stirring for 2 h after the coarse particles were added. Air bubbles in the slurry were removed for 10 min with the vacuum de-bubble
The particle size distributions of raw powder and the powder milled for 24 h were examined with a particle size analyzer (SediGraph 5100, Micromeritics, USA). The density of compact was determined from the size and the mass. The density of the ceramics was determined by Archimedes method. The strength and the fracture toughness of ceramics were evaluated with the four-point bending method (JIS R 1601 [16]) and the SEPB method (JIS R 1607 [17]), respectively. The coarse particle inclusions and the fracture origins were examined with scanning electron microscopy (SEM S-800, Hitachi, Japan). The microstructure of ceramics was examined with SEM after polishing and thermal etching. The internal structure of ceramics was also examined with a mid-IR microscope. This microscope allows examination of much thicker specimens (1.5 mm) than the conventional optical microscope [8–14,18–22]. Many micrographs taken at various positions were analyzed to measure the size and number of coarse particles.
3. Results Fig. 2 shows the particle size distributions of raw powder and milled powder. They are very similar except for particle size larger than 3 Am. Careful analysis shows that the coarse particles were preferentially removed in the raw powder by milling. The particle size ranged 0.1–8 Am for the raw powder and 0.1–5 Am for the milled powder. The average particle size was 0.5 Am in both powders and equal to the nominal value of the powder.
Table 1 Coarse particles size and their contents Sieve apertures/Am
Contents/mass (%)
38–45 53–63 75–90
0.0125 0.035 0.1
Fig. 2. Particle size distributions of raw and ground powders.
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Fig. 3. SEM micrographs of coarse alumina particles prepared from sieving (75–90 Am).
Fig. 3 shows SEM micrographs for coarse particles of the size range 75–90 Am, which were prepared by sieving. In the micrograph at high magnification, platelet-shape particles form aggregates of porous structure with sizes of 10–20 Am. Their unique morphology should correspond to that of the mother salt, i.e., aluminum hydroxide, in this material [23,24]. In the micrograph at low magnification, these aggregates form the coarse aggregates of large scale. In this paper, such coarse aggregates are referred to as coarse particles. Table 2 shows the measured densities of green compact and ceramics. The densities were approximately the same for all compacts. The densities of all ceramics were again the same. Clearly, addition of a small amount of coarse particle has no effect on the densities of ceramics. Fig. 4 shows the Weibull plots and the fracture toughness for all specimens. The data of the specimen without any coarse particle addition were also shown as references. The strength decreased with increasing size of coarse particles added. The Weibull moduli were similar and over 20 for all specimens. All ceramics have basically the same fracture toughness. Fig. 5 shows SEM micrographs of representative fracture origins noted in this study. The specimen contains coarse particles of the size range 75–90 Am. The fracture origin (a) was noted in the specimens of the lowest strength (370 MPa) and (b) in that of the average strength (406 MPa). They were both coarse particles. Low strength was noted in
the specimen containing large coarse particles. Similar results were noted in all specimens examined in this study. Fig. 6 shows an SEM micrograph of the ceramics containing the coarse particles of the size range 75–90 Am. The microstructure appears fairly uniform. The maximum particle size is 10–15 Am. Pores are located both at grain boundaries and in the particles. Many pores have sizes under 3 Am and exceptionally large pores have sizes under 10 Am. Essentially the same structure was noted in all other specimens. Clearly, the amount of coarse particles added was too small to be found readily on the polished/ etched surface. Fig. 7 shows the IR photomicrographs for the internal structures at various depths of ceramics containing the coarse particles of the size range 75–90 Am. These micrographs were taken by changing the specimen stage in the depth direction. The depth from the top surface of the specimen increases in the order (a), (b) and (c). The coarse
Table 2 Densities of green bodies and sintered bodies Density/103 kg m3
Coarse particles size, sieve apertures/Am
Green body
Sintered body
38–45 53–63 75–90
2.31 2.32 2.31
3.95 3.95 3.96
Fig. 4. Mechanical properties of sintered bodies.
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Fig. 5. Examples of fracture origins in alumina ceramics having 75–90 Am coarse particles: (a) 370 MPa, (b) 406 MPa.
particles appear round and dark in the micrographs. All the coarse particles can be observed clearly over a thickness of the specimen, 1.5 mm. The size of the coarse particle appeared essentially the same as that added in the sample preparation. Fig. 8 shows the internal structure of ceramics containing coarse particles of various sizes. The size of coarse particles in the ceramic matrix increased with increasing size of coarse particles added. Again, the sizes of coarse particles are the same as those of the coarse particles added in the preparation of specimens. Fig. 9 shows the size distribution of coarse particles in the three types of specimens, which were determined
through the analysis of the IR micrographs. In the measurement, the size of coarse particle was represented by the size of equivalent sphere. The number of particles was counted for the size interval of 5 Am. The number was then divided by the volume of the sample subjected for examination (area multiplied by the thickness) and by the size range. The result obtained corresponds to the number of coarse particles per unit volume per unit size range. All three distribution curves have maxima near the nominal size of coarse particles added. In the regions exceeding the maxima, the curves are approximated by the straight line in the logarithmic scale. All three straight lines appear parallel to each other.
4. Discussion The results showed clearly the effect of coarse particles on the strength. The fracture was always initiated at the coarse particles in the matrix. No fracture origin other than the coarse particles was found in this study. Clearly, the present IR microscopy can characterize coarse particles accurately. The size and shape of coarse particles in the IR micrographs (Fig. 8) are consistent with those noted in the SEM micrograph (Fig. 3) and on the fracture surface in Fig. 5. No significant grain growth should have taken place in sintering within the coarse particles. This can be explained by the highly porous structure with large particles (Fig. 3) of the coarse particle added. The driving force for grain growth is low and the pores inhibit the grain growth also. It is interesting to discuss the change of strength with the size of coarse particles. The fracture strength r is related to the size of fracture origin c as follows, Fig. 6. SEM micrograph of sintered body having coarse particles of 75–90 Am.
r ¼ KIC =Y c1=2
ð1Þ
110
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Fig. 7. Internal structures of sintered body having 75–90 Am coarse particles with observed mid-infrared microscope of transmission mode in the same field of vision; (a) 0.1 mm, (b) 0.7 mm, (c) 1.4 mm in depth from the surface of sample.
where K IC is fracture toughness and Y is the shape factor [25]. In the three kinds of ceramics prepared in this study, the shape factor of the fracture origin (coarse particle) Y and the fracture toughness can be assumed the same. The difference of strength should have been caused mainly by the size of fracture origin. In this case, it is important to note that the size distributions of coarse particles have the same shape in Fig. 9. The strength should be inversely proportional to the one-half power of defect size at the same defect density. Here we focus on the case where one defect is present in the effective volume (one defect/3.1 mm3). Because the number of defects is counted for a 5 Am interval, the defect number density for this case becomes 0.065 mm3 Am1. Comparing defect number density, 0.065 mm3 Am1, in the linear portion in Fig. 9, the relative sizes of defect are given as 1, 0.76 and 0.56 for the specimens prepared with the coarse particles of 75–90, 53–63 and 32–45 Am, respectively. The relative strengths estimated from Eq. (1) with these values are 1, 1.14 and 1.33,
Fig. 8. Mid-infrared micrographs of sintered bodies having coarse particles of (a) 38–45 Am, (b) 53–63 Am, and (c) 75–90 Am.
Fig. 9. Relation between defect number density and defect size for the samples.
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respectively, for above specimens. These estimated relative values of strength agree very well with the measured relative strengths of 1, 1.15 and 1.28. The good agreement shows that the strength is governed by the coarse particle in the present system.
5. Conclusion The effects of coarse particles on the microstructure and strength variation were examined on alumina ceramics containing coarse particles of controlled characteristics. The following conclusions were obtained: (1)
(2) (3)
Direct observation technique using a mid-infrared microscope of transmission mode enabled us to analyze a small amount of coarse agglomerated particles in sintered body. A small amount of coarse particles governed the fracture strength of sintered body. The strength variation of the sintered bodies was correlated with the size distribution of coarse particles detected by mid-infrared microscopy.
Acknowledgement The authors would like to thank Showa Denko K.K. for the alumina raw materials.
References [1] A.G. Evans, Structural reliability: A processing-dependent phenomenon, J. Am. Ceram. Soc. 65 (3) (1982) 127 – 137. [2] F.F. Lange, Powder processing science and technology for increased reliability, J. Am. Ceram. Soc. 72 (1) (1989) 3 – 15. [3] J. Seidel, N. Claussen, J. Rodel, Reliability of alumina ceramics 2: Effect of processing, J. Eur. Ceram. Soc. 17 (1997) 727 – 733. [4] N. Shinohara, M. Okumiya, T. Hotta, K. Nakahira, M. Naito, K. Uematsu, Formation mechanisms of processing defects and their relevance to the strength in alumina ceramics made by powder compaction process, J. Mater. Sci. 34 (1999) 4271 – 4277. [5] N. Shinohara, M. Okumiya, T. Hotta, K. Nakahira, M. Naito, K. Uematsu, Morphological changes in process-related large pores of granular compacted and sintered alumina, J. Am. Ceram. Soc. 83 (7) (2000) 1633 – 1640. [6] N. Shinohara, S. Katori, M. Okumiya, T. Hotta, K. Nakahira, M. Naito, Yong-Ick Cho, K. Uematsu, Effect of heat treatment of alumina granules on the compaction behavior and properties of green and sintered bodies, J. Eur. Ceram. Soc. 22 (2002) 2841 – 2848. [7] Y. Takao, T. Hotta, K. Nakahira, M. Naito, N. Shinohara, M. Okumiya, K. Uematsu, Processing defects and their relevance to strength in alumina ceramics made by slip casting, J. Eur. Ceram. Soc. 20 (2000) 389 – 395.
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[8] N. Shinohara, M. Okumiya, T. Hotta, K. Nakahira, M. Naito, K. Uemastu, Seasonal variation of microstructure and sintered strength of dry-pressed alumina, J. Am. Ceram. Soc. 82 (12) (1999) 3441 – 3446. [9] T. Hotta, K. Nakahira, M. Naito, N. Shinohara, M. Okumiya, K. Uemastu, Origin of strength change in ceramics associated with the alteration of spray dryer, J. Mater. Res. 14 (7) (1999) 2974 – 2979. [10] N. Shinohara, M. Okumiya, T. Hotta, K. Nakahira, M. Naito, K. Uematsu, Variation of microstructure and fracture strength of cold isostatically pressed alumina ceramics with alteration of dewaxing procedures, J. Eur. Ceram. Soc. 20 (2000) 843 – 849. [11] T. Hotta, K. Nakahira, M. Naito, N. Shinohara, M. Okumiya, K. Uemastu, Origin of the strength change of silicon nitride ceramics with the alteration of spray drying conditions, J. Eur. Ceram. Soc. 21 (2001) 603 – 610. [12] H. Abe, T. Hotta, M. Naito, N. Shinohara, M. Okumiya, H. Kamiya, K. Uematsu, Origin of strength variation of silicon nitride ceramics with CIP condition in a powder compaction process, Powder Technol. 119 (2001) 194 – 200. [13] H. Abe, T. Hotta, M. Naito, N. Shinohara, K. Uemastu, Direct observation of detrimental defects in ceramics, Am. Ceram. Soc. Bull. 81 (1) (2002) 31 – 34. [14] H. Abe, M. Naito, T. Hotta, N. Shinohara, K. Uemastu, Flaw size distribution in high-quality alumina, J. Am. Ceram. Soc. 86 (6) (2003) 1019 – 1021. [15] Z. Kato, T. Sato, S. Tanaka, N. Uchida, K. Uematsu, Mid-infrared microscopy, Am. Ceram. Soc. Bull. 81 (12) (2002) 42 – 44. [16] Japanese Industrial Standard, R 1601, Testing Method for Flexural Strength (Modulus of Rupture) of Fine Ceramics, Japan Standard Association, Tokyo, Japan, 1995. [17] Japanese Industrial Standard, R 1607, Testing Method for Fracture toughness of Fine Ceramics, Japan Standard Association, Tokyo, Japan, 1995. [18] D.J. Clinton, R. Morrell, M. McNamee, Br. Ceram., Trans. J. 85 (1986) 175 – 179. [19] J.-Y. Kim, N. Uchida, K. Uematsu, Effect of HIPing on defect and strength characteristics in Y-TZP, J. Ceram. Soc. Jpn. 100 (3) (1992) 311 – 314. [20] N. Miyashita, J.-Y. Kim, Z. Kato, N. Uchida, K. Uematsu, Effect of compaction pressure on internal structure of sintered alumina, J. Ceram. Soc. Jpn. 100 (11) (1992) 1357 – 1360. [21] K. Uematsu, M. Sekiguchi, J.-Y. Kim, K. Saito, Y. Mutoh, M. Inoue, Y. Fujino, A. Miyamoto, Effect of processing condition on the characteristics of pores in hot isostatically pressed alumina, J. Mater. Sci. 28 (1993) 1788 – 1792. [22] M. Naito, T. Hotta, O. Hayakawa, N. Shinohara, M. Okumiya, K. Uematsu, New characterization techniques to control the manufacturing process of fine ceramics, in: A. Jillavenkatesa, G.Y. Onoda (Eds.), Advances in Process Measurements for Ceramic Industry, American Ceramic Society, Ohio, USA, 1999, pp. 87 – 105. [23] T.J. Carbone, Production process, properties, and applications for calcined and high-purity aluminas, in: L.D. Hart (Ed.), Alumina Chemicals, The American Ceramic Society, Westerville, Ohio, 1990, pp. 99 – 108. [24] I.B. Cutler, Active powders, in: G.Y. Onoda Jr., L.L. Hench (Eds.), Ceramic Processing Before Firing, Wiley, New York, 1978, pp. 21 – 30. [25] G.K. Bansal, Effect of flaw shape on strength of ceramics, J. Am. Ceram. Soc. 59 (1976) 87 – 88.
Effect of coarse particles on the strength of alumina made by slip casting Tadashi Hottaa,*, Hiroya Abeb, Makio Naitob, Minoru Takahashic, Keizo Uematsud, Zenji Katod a
Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki 567-0047, Japan c Ceramics Research Laboratory, Nagoya Institute of Technology, 10-6-29, Asahigaoka, Tajimi 507-0071, Japan d Department of Chemistry, Nagaoka University of Technology, 1603-1, Kamitomioka-cho, Nagaoka 940-2188, Japan b
Received 10 May 2004; received in revised form 19 October 2004; accepted 2 November 2004
Abstract The effect of coarse particles in alumina powder slurry on the microstructure and strength variation of the sintered alumina bodies made by slip casting was examined. A commercially available low-soda alumina powder was used as the raw material. Coarse agglomerated particles were added in the alumina powder slurry just before slip casting. Alumina ceramics were fabricated through slip casting process. The ceramic strength and fracture toughness were examined. Fracture origins and microstructure were observed with scanning electron microscopy (SEM). The internal structures of sintered bodies were examined by using mid-infrared microscope. As a result, direct observation technique using mid-infrared microscope enabled us to analyze a small amount of coarse agglomerated particles in sintered body. The strength variation of the sintered bodies was correlated with the size of coarse agglomerated particles detected by mid-infrared microscopy. D 2004 Elsevier B.V. All rights reserved. Keywords: Alumina ceramics; Slip casting; Strength; Microstructure; Internal structure; Mid-infrared microscopy
1. Introduction According to fracture mechanics, the strength and, thus, the reliability of ceramics are governed by detrimental structures, one of which behaves as a fracture origin. A wide variety of features is known to behave as detrimental structures, including cracks, pores, inclusions and coarse particles [1–3]. However, very few explicit understandings have been established on the characteristics of these detrimental features and their relevance to strength, leaving difficulties in controlling the quality of ceramics. Recent study shows the relevance between the coarse pore and the strength of ceramics explicitly [4–7]. An excellent correlation is noted between them [8–14]. The effect of coarse particles on the strength of ceramics is, however, much less understood. Clearly, a fundamental study is needed to understand the effect of coarse particles on the strength of ceramics. The aim of this study is to examine the effect of coarse particles on the strength of ceramics. A reasonable approach * Correspondent author. Tel.: +81 52 871 3500; fax: +81 52 871 3599. E-mail address: [email protected] (T. Hotta). 0032-5910/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2004.11.004
is to examine the strength of a model ceramic containing a controlled quantity of known coarse particles. Care should be taken in the characteristics of specimens to obtain a clear result in this approach. First, the matrix of ceramic should not contain detrimental natural defects which behave as uncontrolled fracture origins. Second, the coarse particles added should have very similar characteristics to natural ones. The structure of the model ceramics should be examined accurately. In this study, the slip casting process was used to make the model ceramics. The novel IR microscopy [15] was applied to show the distribution of coarse particles explicitly.
2. Experimental procedure 2.1. Fabrication of test specimens The specimens were prepared through the procedure shown in Fig. 1. Low soda alumina powder (AL-160SG-4, Showa Denko K.K., Japan) was used as raw material. The nominal average particle size was 0.5 Am. The powder was
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apparatus (LB-140, Katomasu Tekkosho, Japan). Finally, each slurry was cast in gypsum molds (1001009 mm) to prepare green compacts with coarse particles. After drying, the compact was heated at 1550 8C for 2 h in an electric furnace to sinter the model ceramics. 2.2. Characterization
Fig. 1. Fabrication process of alumina ceramics.
placed in an alumina pot mill (SSA-999; Nikkato, Japan; volume 2000 ml) with 2 kg of alumina ball (SSA-999; Nikkato; diameter 5 mm), and 400 g of aqueous solution (2 mass%) of dispersant of polyacrylic acid type (CERUNA D305, Chukyo Yushi, Japan) and mixed for 24 h to make a slurry with the solid content 50 vol.%. The slurry was passed through a mesh (opening 2 mm) to separate the balls. Weighed slurry was placed in a container and stirred continuously with a stirrer while a fixed amount of coarse particles was added. The coarse particles for addition were prepared from the unground raw material used in the production of the present fine alumina powder. The coarse particles were classified into three fractions before addition by sieving. Each fraction of coarse particles was added to each slurry; therefore, three kinds of slurries with coarse particles were prepared. The amount of each coarse particles was adjusted to make the number of particles per unit volume of the model ceramics approximately constant as shown in Table 1. Each slurry was kept stirring for 2 h after the coarse particles were added. Air bubbles in the slurry were removed for 10 min with the vacuum de-bubble
The particle size distributions of raw powder and the powder milled for 24 h were examined with a particle size analyzer (SediGraph 5100, Micromeritics, USA). The density of compact was determined from the size and the mass. The density of the ceramics was determined by Archimedes method. The strength and the fracture toughness of ceramics were evaluated with the four-point bending method (JIS R 1601 [16]) and the SEPB method (JIS R 1607 [17]), respectively. The coarse particle inclusions and the fracture origins were examined with scanning electron microscopy (SEM S-800, Hitachi, Japan). The microstructure of ceramics was examined with SEM after polishing and thermal etching. The internal structure of ceramics was also examined with a mid-IR microscope. This microscope allows examination of much thicker specimens (1.5 mm) than the conventional optical microscope [8–14,18–22]. Many micrographs taken at various positions were analyzed to measure the size and number of coarse particles.
3. Results Fig. 2 shows the particle size distributions of raw powder and milled powder. They are very similar except for particle size larger than 3 Am. Careful analysis shows that the coarse particles were preferentially removed in the raw powder by milling. The particle size ranged 0.1–8 Am for the raw powder and 0.1–5 Am for the milled powder. The average particle size was 0.5 Am in both powders and equal to the nominal value of the powder.
Table 1 Coarse particles size and their contents Sieve apertures/Am
Contents/mass (%)
38–45 53–63 75–90
0.0125 0.035 0.1
Fig. 2. Particle size distributions of raw and ground powders.
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Fig. 3. SEM micrographs of coarse alumina particles prepared from sieving (75–90 Am).
Fig. 3 shows SEM micrographs for coarse particles of the size range 75–90 Am, which were prepared by sieving. In the micrograph at high magnification, platelet-shape particles form aggregates of porous structure with sizes of 10–20 Am. Their unique morphology should correspond to that of the mother salt, i.e., aluminum hydroxide, in this material [23,24]. In the micrograph at low magnification, these aggregates form the coarse aggregates of large scale. In this paper, such coarse aggregates are referred to as coarse particles. Table 2 shows the measured densities of green compact and ceramics. The densities were approximately the same for all compacts. The densities of all ceramics were again the same. Clearly, addition of a small amount of coarse particle has no effect on the densities of ceramics. Fig. 4 shows the Weibull plots and the fracture toughness for all specimens. The data of the specimen without any coarse particle addition were also shown as references. The strength decreased with increasing size of coarse particles added. The Weibull moduli were similar and over 20 for all specimens. All ceramics have basically the same fracture toughness. Fig. 5 shows SEM micrographs of representative fracture origins noted in this study. The specimen contains coarse particles of the size range 75–90 Am. The fracture origin (a) was noted in the specimens of the lowest strength (370 MPa) and (b) in that of the average strength (406 MPa). They were both coarse particles. Low strength was noted in
the specimen containing large coarse particles. Similar results were noted in all specimens examined in this study. Fig. 6 shows an SEM micrograph of the ceramics containing the coarse particles of the size range 75–90 Am. The microstructure appears fairly uniform. The maximum particle size is 10–15 Am. Pores are located both at grain boundaries and in the particles. Many pores have sizes under 3 Am and exceptionally large pores have sizes under 10 Am. Essentially the same structure was noted in all other specimens. Clearly, the amount of coarse particles added was too small to be found readily on the polished/ etched surface. Fig. 7 shows the IR photomicrographs for the internal structures at various depths of ceramics containing the coarse particles of the size range 75–90 Am. These micrographs were taken by changing the specimen stage in the depth direction. The depth from the top surface of the specimen increases in the order (a), (b) and (c). The coarse
Table 2 Densities of green bodies and sintered bodies Density/103 kg m3
Coarse particles size, sieve apertures/Am
Green body
Sintered body
38–45 53–63 75–90
2.31 2.32 2.31
3.95 3.95 3.96
Fig. 4. Mechanical properties of sintered bodies.
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Fig. 5. Examples of fracture origins in alumina ceramics having 75–90 Am coarse particles: (a) 370 MPa, (b) 406 MPa.
particles appear round and dark in the micrographs. All the coarse particles can be observed clearly over a thickness of the specimen, 1.5 mm. The size of the coarse particle appeared essentially the same as that added in the sample preparation. Fig. 8 shows the internal structure of ceramics containing coarse particles of various sizes. The size of coarse particles in the ceramic matrix increased with increasing size of coarse particles added. Again, the sizes of coarse particles are the same as those of the coarse particles added in the preparation of specimens. Fig. 9 shows the size distribution of coarse particles in the three types of specimens, which were determined
through the analysis of the IR micrographs. In the measurement, the size of coarse particle was represented by the size of equivalent sphere. The number of particles was counted for the size interval of 5 Am. The number was then divided by the volume of the sample subjected for examination (area multiplied by the thickness) and by the size range. The result obtained corresponds to the number of coarse particles per unit volume per unit size range. All three distribution curves have maxima near the nominal size of coarse particles added. In the regions exceeding the maxima, the curves are approximated by the straight line in the logarithmic scale. All three straight lines appear parallel to each other.
4. Discussion The results showed clearly the effect of coarse particles on the strength. The fracture was always initiated at the coarse particles in the matrix. No fracture origin other than the coarse particles was found in this study. Clearly, the present IR microscopy can characterize coarse particles accurately. The size and shape of coarse particles in the IR micrographs (Fig. 8) are consistent with those noted in the SEM micrograph (Fig. 3) and on the fracture surface in Fig. 5. No significant grain growth should have taken place in sintering within the coarse particles. This can be explained by the highly porous structure with large particles (Fig. 3) of the coarse particle added. The driving force for grain growth is low and the pores inhibit the grain growth also. It is interesting to discuss the change of strength with the size of coarse particles. The fracture strength r is related to the size of fracture origin c as follows, Fig. 6. SEM micrograph of sintered body having coarse particles of 75–90 Am.
r ¼ KIC =Y c1=2
ð1Þ
110
T. Hotta et al. / Powder Technology 149 (2005) 106–111
Fig. 7. Internal structures of sintered body having 75–90 Am coarse particles with observed mid-infrared microscope of transmission mode in the same field of vision; (a) 0.1 mm, (b) 0.7 mm, (c) 1.4 mm in depth from the surface of sample.
where K IC is fracture toughness and Y is the shape factor [25]. In the three kinds of ceramics prepared in this study, the shape factor of the fracture origin (coarse particle) Y and the fracture toughness can be assumed the same. The difference of strength should have been caused mainly by the size of fracture origin. In this case, it is important to note that the size distributions of coarse particles have the same shape in Fig. 9. The strength should be inversely proportional to the one-half power of defect size at the same defect density. Here we focus on the case where one defect is present in the effective volume (one defect/3.1 mm3). Because the number of defects is counted for a 5 Am interval, the defect number density for this case becomes 0.065 mm3 Am1. Comparing defect number density, 0.065 mm3 Am1, in the linear portion in Fig. 9, the relative sizes of defect are given as 1, 0.76 and 0.56 for the specimens prepared with the coarse particles of 75–90, 53–63 and 32–45 Am, respectively. The relative strengths estimated from Eq. (1) with these values are 1, 1.14 and 1.33,
Fig. 8. Mid-infrared micrographs of sintered bodies having coarse particles of (a) 38–45 Am, (b) 53–63 Am, and (c) 75–90 Am.
Fig. 9. Relation between defect number density and defect size for the samples.
T. Hotta et al. / Powder Technology 149 (2005) 106–111
respectively, for above specimens. These estimated relative values of strength agree very well with the measured relative strengths of 1, 1.15 and 1.28. The good agreement shows that the strength is governed by the coarse particle in the present system.
5. Conclusion The effects of coarse particles on the microstructure and strength variation were examined on alumina ceramics containing coarse particles of controlled characteristics. The following conclusions were obtained: (1)
(2) (3)
Direct observation technique using a mid-infrared microscope of transmission mode enabled us to analyze a small amount of coarse agglomerated particles in sintered body. A small amount of coarse particles governed the fracture strength of sintered body. The strength variation of the sintered bodies was correlated with the size distribution of coarse particles detected by mid-infrared microscopy.
Acknowledgement The authors would like to thank Showa Denko K.K. for the alumina raw materials.
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