- Email: [email protected]
Ceramics superplasticity
Current Opinion in Solid State and Materials Science 4 (1999) 461–465
Ceramics superplasticity Fumihiro Wakai a , *, Naoki Kondo b , Yutaka Shinoda c a
Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226 -8503, Japan b National Industrial Research Institute of Nagoya, 1 -1 Hirate, Kita-ku, Nagoya 462 -8510, Japan c Ceramics Superplasticity Project, ICORP, Japan Science and Technology Corporation, 2 -4 -1 Mutsuno, Atsuta-ku, Nagoya 456 -8587, Japan
Abstract Ceramics superplasticity has been one of the intensive research fields in the last decade. Although most of the reports are still limited to those of zirconia, new developments have been achieved in superplasticity of Si 3 N 4 and SiC in recent years. It is clearly demonstrated that the superplasticity is one of the common properties of fine-grained ceramics at elevated temperatures. Superplastic forming and strengthening by superplastic forging are applicable to a wide range of ceramics including oxides, non-oxides, and ceramics with / without intergranular glass phase. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Although ceramics are brittle materials which show almost no plastic deformation at ambient temperatures, some fine-grained ceramics can be superplastic at elevated temperatures. Superplasticity refers to an ability of polycrystalline solids to exhibit exceptionally large elongation in tension. The application of superplasticity makes it possible to fabricate ceramic components by superplastic forming (SPF), SPF concurrent with diffusion bonding, and superplastic sinter-forging just like superplastic metals. Furthermore the superplastic deformation plays an important role in stress-assisted densification processes such as hot isostatic pressing (HIP) and hot pressing (HP). The micrograin superplasticity is believed to take place by grain boundary sliding at elevated temperatures. The sliding of rigid grains generates cavities and cracks inevitably. Then the essential mechanism of superplasticity is the accommodation process of grain boundary sliding so that the polycrystalline materials can be stretched exten~ sively without fracture. The relation between strain rate (´) and stress (s ) in deformation of polycrystalline solids is generally expressed as follows,
´~ 5 As n exp(2Q /RT ) /d p
(1)
where d is a grain size, n is a stress exponent, p is an exponent for grain size, Q is an activation energy, T is a temperature, and R is the gas constant. It is often observed *Corresponding author. E-mail address: [email protected] (F. Wakai)
that the parameters (n, p, Q, and A) for one material vary with temperature and stress level. It suggests that multiple processes take part as accommodation mechanisms; for example, diffusion, dislocation motion, and solution–precipitation process. The finding of superplasticity in zirconia triggered extensive investigations on ceramic superplasticity [1]. Numerous papers and reviews have been published since then [2–**7]. The recent review by Jimenez-Melendo [**7] covers wide articles on superplasticity of zirconia. However, there does not exist a consensus on mechanisms of superplasticity in zirconia yet. One of the new trends in the 1990s was the superplasticity of Si 3 N 4 and SiC. They will be not only useful for structural components of engines at very high temperatures but also for wear-resistant components at medium temperatures below 10008C. Furthermore it is interesting that a class of hardest materials can be stretched just like a chewing gum. The structure and the nature of the grain boundary affect accommodation processes in grain boundary sliding significantly. In many metals and ceramics impurity atoms segregate at grain boundaries, but no glass film has been detected by high-resolution transmission electron microscopy (HRTEM). On the other hand, glass phase pockets remain frequently at grain boundaries of liquid-phase sintered ceramics. The glass phase behaves as a viscous fluid (super-cooled liquid) at temperatures higher than those of the glass transition. When the glass wets the crystal completely, a thin glass film with a thickness from 0.5 to a few nm is observed at two-grain junctions [8]. The glass phase acts as a lubricant for grain boundary sliding,
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and also as a path for matter transport through solution– precipitation process. Then the mechanism of superplasticity for glass-containing ceramics will be different from the intrinsic superplasticity of metals and ceramics that do not contain a glass film. This paper describes the recent developments in the superplasticity of Si 3 N 4 which is enhanced by intergranular glass phase, the intrinsic superplasticity of SiC, and that of zirconia.
2. Superplasticity enhanced by intergranular glass phase – Si 3 N 4 The strategy for developing superplastic Si 3 N 4 is to fabricate fine-grained material by liquid phase sintering at relatively low temperatures. Various materials for superplasticity have been developed, for example, Si 3 N 4 / SiC composite [9,10], SiAlON [11–15], and monolithic Si 3 N 4 [16–24]. The grain size of these materials is less than 0.5 mm generally. The Si-Y-Al-O-N glass phase exists at grain boundaries of the materials which were sintered with Y 2 O 3 and Al 2 O 3 as additives. The largest elongation of 470% has been achieved for b-SiAlON as shown in Fig. 1 [*15]. Two approaches were taken to obtain the material consisting of equiaxed fine grains which had been regarded as a ‘classical’ requirement for micrograin superplasticity. One approach was the densification of SiAlON at low temperatures intending to suppress the transformation of equiaxed a-grains into elongated b-grains [14]. The other approach was the fabrication of equiaxed b-Si 3 N 4 [18,19]. However, since the fracture toughness of materials with fine equiaxed grains is low, the heat-treatment for anisotropic grain growth is necessary to improve the fracture toughness after the superplastic forming. The strain rate in superplasticity is greatly affected by the chemical composition of intergranular glass phase. The strain rate is enhanced by reducing the viscosity of glass, for example, the SiAlON with Li-containing glass de-
formed at a strain rate of 10 22 s 21 in compression in the temperature range of 1500 to 16008C [14]. One of the peculiar characteristics in superplasticity of Si 3 N 4 -based ceramics is the asymmetric deformation behavior in compression and in tension [16]. The strain rate in tension is generally higher than that in compression. This asymmetric behavior is known in the creep of Si 3 N 4 also. Two regions were distinguished in stress–strain rate curves of compressive deformation of some SiAlON; stress exponent of 1 (Newtonian flow) at lower stresses and stress exponent of 0.5 (shear thickening creep) at higher stresses [11]. It is supposed that the grains covered with glass films contact with each other directly when the compressive stress exceeds a critical value. The observation by HRTEM revealed that the thickness of intergranular glass film varied with the angle between the interface and the direction of compressive stress [19]. The strain whirls, which suggest the direct contact of grains, are also observed in deformed Si 3 N 4 at high compressive stresses [17]. However, presence of shear thickening creep was not always observed in compressive deformation [20]. The liquid phase promotes the grain boundary sliding so that even the materials which contain rod-shaped b-grains can be superplastically deformed [12,13,**22–24]. In this ‘non-classical’ superplasticity the anisotropic b-grains tend to align with strain, producing a fiber-strengthening effect. These phenomena lead to a strengthening and toughening of Si 3 N 4 by compressive superplastic deformation [**22]. Superplastically extruded silicon nitride exhibited highly anisotropic microstructures, where rod-shaped grains aligned along the extruding direction. The improvement in strength and fracture toughness was mainly due to the effective operation of grain bridging and pull-out by the grain alignment. The superplastic deformation was analyzed by a viscous flow model taking account of the grain alignment [12,24]. The model assumed the viscous flow of intergranular glass phase and rigid grains. The model predicted the strain hardening by grain alignment fairly well. It is suggested
Fig. 1. Undeformed and superplastically deformed Si 3 N 4 specimens. An elongation of over 470% is noted (Reprinted with permission from J. Ceram. Soc. Japan).
F. Wakai et al. / Current Opinion in Solid State and Materials Science 4 (1999) 461 – 465
that the interface-reaction controlled solution–precipitation creep [25] also acts as accommodation process for the material with small volume fraction of glass phase at temperatures where grain growth occurs noticeably [20,24]. The enhancement of superplastic deformation by intergranular glass phase was also applicable to liquid-phase sintered SiC too [26–28].
3. Intrinsic superplasticity – SiC Is the superplasticity of covalent polycrystalline solids possible where no help of intergranular glass phase is expected? The superplastic elongation was achieved for B, C-doped SiC which was fabricated by hot isostatic pressing recently [29,*30]. The intergranular glass film was not detected under TEM observation. This result illustrates that the phenomena of superplasticity can be observed in all fine-grained polycrystalline solids regardless of the difference in atomic bond and structure of grain boundaries. The characterization of grain boundary structure and its chemistry are essential to understand sintering, grain growth and deformation at elevated temperatures. In order to obtain the information on the nature of chemical bond at grain boundary, the method to separate the electron energy loss signal (EELS) from grain boundary has been developed [**31]. The method provides quantitative information on segregation of impurity atoms and ‘chemical width’ of grain boundary. For example, boron segregates at grain boundaries of B, C-doped SiC [*32]. The boron takes the place of silicon but forms bonds in a local environment that are similar to those in B 4 C structure. The presence of B–C and B–B bonds at the grain boundary is beneficial to increase the self-diffusion rate, because the local structure and bonding have more varieties. This is why boron enhanced the sintering and superplasticity of SiC.
4. Superplasticity of zirconia Jimenez-Melendo et al. [**7] reviewed the extensive classical studies on superplasticity of Y 2 O 3 -stabilized tetragonal ZrO 2 polycrystals (Y-TZP). Therefore we will focus our attention on new progress in the last 18 months here. The superplasticity of Y-TZP is significantly affected by addition of small amount of impurities (SiO 2 and Al 2 O 3 ) which segregate at grain boundaries. The materials are classified as high-purity (,0.1 wt.%) or low-purity (.0.1 wt.%) frequently. The apparent stress exponent, n, for the high purity materials varies from 2 (region II) to .3 (region I) as the stress is decreased. The constitutive equation in region II was almost identical to the empirical equation derived from superplastic metals [33]. The threshold stress for grain boundary sliding was proposed to explain the
463
occurrence of region I. The role of interface-reaction controlled diffusional creep in superplasticity of Y-TZP has been suggested by several researchers [34,35]. Berbon [**36] showed that the transition from region II to region I was consistent with the Arzt’s model [37] of interfacereaction controlled diffusional creep. The strain rate in the lower stress region is enhanced on low-purity materials, and then only region II is observed. This change was attributed to an assumption that the impurity formed a thin glass film at the grain boundary [35]. However, recent HRTEM observation could not detect the presence of such a film at ambient temperature [38]. The detailed investigation on effects of SiO 2 and Al 2 O 3 revealed that the drastic change in region I was induced by very small amount of impurity segregation [39,*40]. Unfortunately there is no theoretical model to explain why simple segregation of impurities change the stress exponent so drastically. In spite of the large accumulation of reports on Y-TZP there exists no model which explains all of the experimental data consistently. The intentional addition of transition metals [41–43] and glasses [44,45] into Y-TZP has been conducted to enhance the strain rate and lower the temperature for superplastic forming. It was believed that the intergranular glass film promoted the superplasticity of Y-TZP with SiO 2 glass [44]. However, Ikuhara [46] revealed that it was not true from HRTEM observation. He reported that the enhancement in strain rate was due to the segregation of Si and the presence of glass pockets. Oka [*47] found that the codoping of CaO and TiO 2 was the most effective to promote the superplastic elongation at strain rate higher than 10 22 s 21 . Several attempts have been conducted to predict the elongation to failure of Y-TZP by single parameter, such as, Zener–Hollomon parameter or Needleman–Rice parameter [*48]. The fracture process consists of (1) cavity nucleation, (2) cavity growth, and (3) creep crack growth. Hiraga [49,**50] analyzed the damage accumulation in Y-TZP, and found that the controlling process was changed by the difference in impurity content. His result suggests that several factors must be taken into account in order to predict the elongation to failure of different materials.
5. Conclusions Complicated-shaped components with accurate dimensions have been usually fabricated by sintering the shaped powder compact in the ceramic industry. Therefore the additional advantages besides forming shaped components are desirable for the practical application of superplasticity. From this view the superplastic forging of Si 3 N 4 by utilizing intergranular glass phase is promising for future applications, because strengthening and toughening can be achieved concurrently with forming. Although the maximum elongation to failure has been
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pursued in the literature, such specimens cannot be used because they contain a large amount of cavities. The deformation properties under conditions in which cavities are suppressed are more important to assure the reliability of shaped components. In order to incorporate the technology of superplasticity into the ceramic industry, the combination with the sintering process, for example, superplastic sinter-forging [51] should be pursued in the near future.
[19]
[20]
[21]
[**22]
References Papers of particular interest, published within the annual period of review, have been highlighted as: * of special interest; ** of outstanding interest. [1] Wakai F, Sakaguchi S, Matsuno Y. Superplasticity of yttriastabilized tetragonal ZrO 2 polycrystals. Adv Ceram Mater 1986;1:259–63. [2] Maehara Y, Langdon TG. Review superplasticity in ceramics. J Mater Sci 1990;25:2276–86. [3] Chen IW, Xue LA. Development of superplastic structural ceramics. J Am Ceram Soc 1990;73:2585–609. [4] Wakai F. Superplasticity of ceramics. Ceram Int 1991;17:153–63. [5] Nieh TG, Wadsworth J, Wakai F. Recent advances in superplastic ceramics and ceramic composites. Int Mater Rev 1991;36:146–61. [6] Sakuma T. Superplasticity in alloys. Curr Opin Solid State Mater Sci 1997;2:296–9. [**7] Jimenez-Melendo M, Dominguez-Rodriguez A, Bravo-Leon A. Superplastic flow of fine-grained yttria-stabilized zirconia polycrystals: constitutive equation and deformation mechanisms. J Am Ceram Soc 1998;81:2761–76. An extensive review (with 111 references) which summarizes the experimental results in classical literature on Y-TZP. [8] Kleebe HJ. Structure and chemistry of interfaces in Si 3 N 4 ceramics studied by transmission electron microscopy. J Ceram Soc Japan 1997;105:453–75. [9] Wakai F, Kodama Y, Sakaguchi S, Murayama N, Izaki K, Niihara K. A superplastic covalent crystal composite. Nature 1990;344:421–3. [10] Rouxel T, Wakai F, Izaki K. Tensile ductility of superplastic Al 2 O 3 – Y 2 O 3 –Si 3 N 4 / SiC composite. J Am Ceram Soc 1992;75:2363–72. [11] Chen IW, Hwang SL. Shear thickening creep in superplastic silicon nitride. J Am Ceram Soc 1992;75:1073–9. [12] Wu X, Chen IW. Exaggerated texture and grain growth in a superplastic SiAlON. J Am Ceram Soc 1992;75:2733–41. [13] Hwang SL, Chen IW. Superplastic forming of SiAlON ceramics. J Am Ceram Soc 1994;77:2575–85. [14] Rosenflanz A, Chen IW. ‘Classical’ superplasticity of SiAlON ceramics. J Am Ceram Soc 1997;80:1341–52. [*15] Kondo N, Ohji T, Wakai F. Deformation conditions of b-SiAlON to achieve large superplastic elongation. J Ceram Soc Japan 1998;106:1040–2. The largest elongation has been achieved in SiAlON. [16] Rouxel T, Rossignol F, Besson JL, Goursat P. Superplastic forming of an a-phase rich silicon nitride. J Mater Res 1997;12:480–92. [17] Burger P, Duclos R, Crampon J. Microstructure characterization in superplastically deformed silicon nitride. J Am Ceram Soc 1997;80:879–85. [18] Mitomo M, Hirotsuru H, Suematsu H, Nishimura T. Fine-grained
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silicon nitride ceramics prepared from b-powder. J Am Ceram Soc 1995;78:211–4. Wang CM, Mitomo M, Nishimura T, Bando Y. Grain boundary film thickness in superplastically deformed silicon nitride. J Am Ceram Soc 1997;80:1213–21. Schneider JA, Mukherjee AK. Effects of microstructure on the deformation mechanisms in silicon nitride. J Am Ceram Soc 1999;82:761–4. Sato E, Kondo N, Wakai F. Particle size, shape and orientation distributions—general spheroid problem and application on deformed Si 3 N 4 microstructures. Phil Mag A 1996;74:215–28. Kondo N, Ohji T, Wakai F. Strengthening and toughness of silicon nitride by superplastic deformation. J Am Ceram Soc 1998;81:713–6. The bending strength and fracture toughness were increased by superplastic deformation. Kondo N, Sato E, Wakai F. Geometrical microstructural development in superplastic silicon nitride with rod-shaped grains. J Am Ceram Soc 1998;81:3221–7. Kondo Y, Suzuki Y, Ohji T, Sato E, Wakai F. Change in stress sensitivity and activation energy during superplastic deformation of silicon nitride. Mater Sci Eng 1999;A268:141–6. Wakai F. Step model of solution–precipitation creep. Acta Metall Mater 1994;42:1163–72. Mitomo M, Kim YW, Hirotsuru H. Fabrication of silicon carbide nanoceramics. J Mater Res 1996;11:1601–4. Wang CM, Mitomo M, Emoto H. Microstructure of liquid phase sintered superplastic silicon carbide ceramics. J Mater Res 1997;12:3266–70. Nagano T, Gu H, Shinoda Y, Zhan D, Mitomo M, Wakai F. Tensile ductility of liquid-phase sintered b-silicon carbide at elevated temperatures. Mater Sci Forum 1999;304–306:507–12. Shinoda Y, Nagano T, Wakai F. Fabrication of nanograined silicon carbide by ultrahigh-pressure hot isostatic pressing. J Am Ceram Soc 1999;82:771–3. Shinoda Y, Nagano T, Gu H, Wakai F. Superplasticity of silicon carbide. J Am Ceram Soc 1999;82:2916–8. This is the first report that a covalent ceramic can be superplastically elongated without the help of an intergranular glass phase. Gu H. Quantification of interfacial parameters in spatially resolved analysis involving ELNES separation. Ultramicroscopy 1999; 76:159–85. This paper describes the methods for quantitatively analyzing the grain boundary segregation and intergranular film by electron energy loss spectroscopy. Gu H, Shinoda Y, Wakai F. Detection of boron segregation to grain boundaries in silicon carbide by spatially resolved energy-loss spectroscopy. J Am Ceram Soc 1999;82:469–72. The detection of boron segregation at grain boundary of SiC is important for understanding sintering mechanism and deformation. Jimenez-Melendo M, Dominguez-Rodriguez A. Like-metal superplasticity of fine-grained Y 2 O 3 -stabilized zirconia ceramics. Phil Mag A 1999;79:1591–608. Wakai F, Nagano T. Role of interface-controlled diffusion creep on superplasticity of yttria-stabilized tetragonal ZrO 2 polycrystals. J Mater Sci Lett 1988;7:607–9. Lakki A, Schaller R, Nauer M, Carry C. High temperature superplastic creep and internal friction of yttria-doped zirconia polycrystals. Acta Mater 1993;41:2845–53. Berbon MZ, Langdon TG. An examination of the flow process in superplastic yttria-stabilized tetragonal zirconia. Acta Mater 1999;47:2485–95. New model was proposed to understand the deformation mechanism of Y-TZP. Arzt E, Ashby MF, Verral RA. Interface-controlled diffusional creep. Acta Metall 1983;31:1977–89. Hines JA, Ikuhara Y, Chokshi AH, Sakuma T. The influence of trace
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impurities on the mechanical characteristics of a superplastic 2 mol.% yttria stabilized zirconia. Acta Mater 1998;46:5557–68. Hiraga K, Yasuda H, Sakka Y. The tensile creep behavior of superplastic tetragonal zirconia doped with small amounts of SiO 2 . Mater Sci Eng A 1997;A234–236:1026–9. Sato E, Morioka H, Kuribayashi K, Sundararaman D. Alumina segregation at grain boundaries in superplastic Y-TZP. Mater Sci Forum 1999;304–306:459–64. The segregation of very small amount of alumina affects deformation mechanisms of Y-TZP. Hwang CMJ, Chen IW. Effect of a liquid phase on superplasticity of 2 mol.% Y 2 O 3 -stabilized tetragonal zirconia polycrystals. J Am Ceram Soc 1990;73:1626–32. Seidensticker JR, Mayo MJ. Superplasticity in 3Y-TZP doped with small amounts of copper oxide. Script Metall Mater 1994;31:1749– 54. Takeli S, Davis TI. Influence of a transition metal oxide (CuO) on the superplastic behavior of 8 mol.% yttria-stabilized cubic zirconia polycrystals (8Y-CSZ). J Mater Sci 1998;33:3267–73. Yoshizawa Y, Sakuma T. Role of grain boundary glass phase on the superplastic deformation of tetragonal zirconia polycrystal. J Am Ceram Soc 1990;73:3069–73. Gust M, Goo G, Wolfenstine J, Mecartney ML. Influence of
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amorphous grain boundary phases on the superplastic behavior of 3-mol.%-yttria-stabilized tetragonal zirconia polycrystals (3YTZP). J Am Ceram Soc 1993;76:1681–90. Ikuhara Y, Thavorniti P, Sakuma T. Solute segregation at grain boundaries in superplastic SiO 2 -doped TZP. Acta Mater 1997;45:5275–84. Oka M, Tabuchi N, Takashi T. High strain rate superplasticity in ceramics. Mater Sci Forum 1999;304–306:451–8. The best way to enhance the strain rate of Y-TZP was developed. Kondo T, Takigawa Y, Ikuhara Y, Sakuma T. Critical assessments of tensile ductility in superplastic TZP and TiO 2 -doped TZP. Mater Trans JIM 1998;39:1108–14. An interesting hypothesis was proposed to understand the elongation to fracture in Y-TZP. Hiraga K, Nakano K. Cavitation damage mechanisms in a superplastic zirconia (3Y-TZP). Mater Sci Forum 1997;243–245:387–92. Sakka Y, Hiraga K. Preparation methods and superplastic properties of fine-grained zirconia and alumina based ceramics. Nippon Kagaku Kaishi 1999:497–508 (in Japanese). The systematic investigation on the relationship between cavitation damage and initial microstructures. Venkatachari KR, Raj R. Enhancement of strength through sinter forging. J Am Ceram Soc 1987;70:514–20.
Ceramics superplasticity Fumihiro Wakai a , *, Naoki Kondo b , Yutaka Shinoda c a
Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226 -8503, Japan b National Industrial Research Institute of Nagoya, 1 -1 Hirate, Kita-ku, Nagoya 462 -8510, Japan c Ceramics Superplasticity Project, ICORP, Japan Science and Technology Corporation, 2 -4 -1 Mutsuno, Atsuta-ku, Nagoya 456 -8587, Japan
Abstract Ceramics superplasticity has been one of the intensive research fields in the last decade. Although most of the reports are still limited to those of zirconia, new developments have been achieved in superplasticity of Si 3 N 4 and SiC in recent years. It is clearly demonstrated that the superplasticity is one of the common properties of fine-grained ceramics at elevated temperatures. Superplastic forming and strengthening by superplastic forging are applicable to a wide range of ceramics including oxides, non-oxides, and ceramics with / without intergranular glass phase. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Although ceramics are brittle materials which show almost no plastic deformation at ambient temperatures, some fine-grained ceramics can be superplastic at elevated temperatures. Superplasticity refers to an ability of polycrystalline solids to exhibit exceptionally large elongation in tension. The application of superplasticity makes it possible to fabricate ceramic components by superplastic forming (SPF), SPF concurrent with diffusion bonding, and superplastic sinter-forging just like superplastic metals. Furthermore the superplastic deformation plays an important role in stress-assisted densification processes such as hot isostatic pressing (HIP) and hot pressing (HP). The micrograin superplasticity is believed to take place by grain boundary sliding at elevated temperatures. The sliding of rigid grains generates cavities and cracks inevitably. Then the essential mechanism of superplasticity is the accommodation process of grain boundary sliding so that the polycrystalline materials can be stretched exten~ sively without fracture. The relation between strain rate (´) and stress (s ) in deformation of polycrystalline solids is generally expressed as follows,
´~ 5 As n exp(2Q /RT ) /d p
(1)
where d is a grain size, n is a stress exponent, p is an exponent for grain size, Q is an activation energy, T is a temperature, and R is the gas constant. It is often observed *Corresponding author. E-mail address: [email protected] (F. Wakai)
that the parameters (n, p, Q, and A) for one material vary with temperature and stress level. It suggests that multiple processes take part as accommodation mechanisms; for example, diffusion, dislocation motion, and solution–precipitation process. The finding of superplasticity in zirconia triggered extensive investigations on ceramic superplasticity [1]. Numerous papers and reviews have been published since then [2–**7]. The recent review by Jimenez-Melendo [**7] covers wide articles on superplasticity of zirconia. However, there does not exist a consensus on mechanisms of superplasticity in zirconia yet. One of the new trends in the 1990s was the superplasticity of Si 3 N 4 and SiC. They will be not only useful for structural components of engines at very high temperatures but also for wear-resistant components at medium temperatures below 10008C. Furthermore it is interesting that a class of hardest materials can be stretched just like a chewing gum. The structure and the nature of the grain boundary affect accommodation processes in grain boundary sliding significantly. In many metals and ceramics impurity atoms segregate at grain boundaries, but no glass film has been detected by high-resolution transmission electron microscopy (HRTEM). On the other hand, glass phase pockets remain frequently at grain boundaries of liquid-phase sintered ceramics. The glass phase behaves as a viscous fluid (super-cooled liquid) at temperatures higher than those of the glass transition. When the glass wets the crystal completely, a thin glass film with a thickness from 0.5 to a few nm is observed at two-grain junctions [8]. The glass phase acts as a lubricant for grain boundary sliding,
1359-0286 / 00 / $ – see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S1359-0286( 99 )00053-4
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F. Wakai et al. / Current Opinion in Solid State and Materials Science 4 (1999) 461 – 465
and also as a path for matter transport through solution– precipitation process. Then the mechanism of superplasticity for glass-containing ceramics will be different from the intrinsic superplasticity of metals and ceramics that do not contain a glass film. This paper describes the recent developments in the superplasticity of Si 3 N 4 which is enhanced by intergranular glass phase, the intrinsic superplasticity of SiC, and that of zirconia.
2. Superplasticity enhanced by intergranular glass phase – Si 3 N 4 The strategy for developing superplastic Si 3 N 4 is to fabricate fine-grained material by liquid phase sintering at relatively low temperatures. Various materials for superplasticity have been developed, for example, Si 3 N 4 / SiC composite [9,10], SiAlON [11–15], and monolithic Si 3 N 4 [16–24]. The grain size of these materials is less than 0.5 mm generally. The Si-Y-Al-O-N glass phase exists at grain boundaries of the materials which were sintered with Y 2 O 3 and Al 2 O 3 as additives. The largest elongation of 470% has been achieved for b-SiAlON as shown in Fig. 1 [*15]. Two approaches were taken to obtain the material consisting of equiaxed fine grains which had been regarded as a ‘classical’ requirement for micrograin superplasticity. One approach was the densification of SiAlON at low temperatures intending to suppress the transformation of equiaxed a-grains into elongated b-grains [14]. The other approach was the fabrication of equiaxed b-Si 3 N 4 [18,19]. However, since the fracture toughness of materials with fine equiaxed grains is low, the heat-treatment for anisotropic grain growth is necessary to improve the fracture toughness after the superplastic forming. The strain rate in superplasticity is greatly affected by the chemical composition of intergranular glass phase. The strain rate is enhanced by reducing the viscosity of glass, for example, the SiAlON with Li-containing glass de-
formed at a strain rate of 10 22 s 21 in compression in the temperature range of 1500 to 16008C [14]. One of the peculiar characteristics in superplasticity of Si 3 N 4 -based ceramics is the asymmetric deformation behavior in compression and in tension [16]. The strain rate in tension is generally higher than that in compression. This asymmetric behavior is known in the creep of Si 3 N 4 also. Two regions were distinguished in stress–strain rate curves of compressive deformation of some SiAlON; stress exponent of 1 (Newtonian flow) at lower stresses and stress exponent of 0.5 (shear thickening creep) at higher stresses [11]. It is supposed that the grains covered with glass films contact with each other directly when the compressive stress exceeds a critical value. The observation by HRTEM revealed that the thickness of intergranular glass film varied with the angle between the interface and the direction of compressive stress [19]. The strain whirls, which suggest the direct contact of grains, are also observed in deformed Si 3 N 4 at high compressive stresses [17]. However, presence of shear thickening creep was not always observed in compressive deformation [20]. The liquid phase promotes the grain boundary sliding so that even the materials which contain rod-shaped b-grains can be superplastically deformed [12,13,**22–24]. In this ‘non-classical’ superplasticity the anisotropic b-grains tend to align with strain, producing a fiber-strengthening effect. These phenomena lead to a strengthening and toughening of Si 3 N 4 by compressive superplastic deformation [**22]. Superplastically extruded silicon nitride exhibited highly anisotropic microstructures, where rod-shaped grains aligned along the extruding direction. The improvement in strength and fracture toughness was mainly due to the effective operation of grain bridging and pull-out by the grain alignment. The superplastic deformation was analyzed by a viscous flow model taking account of the grain alignment [12,24]. The model assumed the viscous flow of intergranular glass phase and rigid grains. The model predicted the strain hardening by grain alignment fairly well. It is suggested
Fig. 1. Undeformed and superplastically deformed Si 3 N 4 specimens. An elongation of over 470% is noted (Reprinted with permission from J. Ceram. Soc. Japan).
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that the interface-reaction controlled solution–precipitation creep [25] also acts as accommodation process for the material with small volume fraction of glass phase at temperatures where grain growth occurs noticeably [20,24]. The enhancement of superplastic deformation by intergranular glass phase was also applicable to liquid-phase sintered SiC too [26–28].
3. Intrinsic superplasticity – SiC Is the superplasticity of covalent polycrystalline solids possible where no help of intergranular glass phase is expected? The superplastic elongation was achieved for B, C-doped SiC which was fabricated by hot isostatic pressing recently [29,*30]. The intergranular glass film was not detected under TEM observation. This result illustrates that the phenomena of superplasticity can be observed in all fine-grained polycrystalline solids regardless of the difference in atomic bond and structure of grain boundaries. The characterization of grain boundary structure and its chemistry are essential to understand sintering, grain growth and deformation at elevated temperatures. In order to obtain the information on the nature of chemical bond at grain boundary, the method to separate the electron energy loss signal (EELS) from grain boundary has been developed [**31]. The method provides quantitative information on segregation of impurity atoms and ‘chemical width’ of grain boundary. For example, boron segregates at grain boundaries of B, C-doped SiC [*32]. The boron takes the place of silicon but forms bonds in a local environment that are similar to those in B 4 C structure. The presence of B–C and B–B bonds at the grain boundary is beneficial to increase the self-diffusion rate, because the local structure and bonding have more varieties. This is why boron enhanced the sintering and superplasticity of SiC.
4. Superplasticity of zirconia Jimenez-Melendo et al. [**7] reviewed the extensive classical studies on superplasticity of Y 2 O 3 -stabilized tetragonal ZrO 2 polycrystals (Y-TZP). Therefore we will focus our attention on new progress in the last 18 months here. The superplasticity of Y-TZP is significantly affected by addition of small amount of impurities (SiO 2 and Al 2 O 3 ) which segregate at grain boundaries. The materials are classified as high-purity (,0.1 wt.%) or low-purity (.0.1 wt.%) frequently. The apparent stress exponent, n, for the high purity materials varies from 2 (region II) to .3 (region I) as the stress is decreased. The constitutive equation in region II was almost identical to the empirical equation derived from superplastic metals [33]. The threshold stress for grain boundary sliding was proposed to explain the
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occurrence of region I. The role of interface-reaction controlled diffusional creep in superplasticity of Y-TZP has been suggested by several researchers [34,35]. Berbon [**36] showed that the transition from region II to region I was consistent with the Arzt’s model [37] of interfacereaction controlled diffusional creep. The strain rate in the lower stress region is enhanced on low-purity materials, and then only region II is observed. This change was attributed to an assumption that the impurity formed a thin glass film at the grain boundary [35]. However, recent HRTEM observation could not detect the presence of such a film at ambient temperature [38]. The detailed investigation on effects of SiO 2 and Al 2 O 3 revealed that the drastic change in region I was induced by very small amount of impurity segregation [39,*40]. Unfortunately there is no theoretical model to explain why simple segregation of impurities change the stress exponent so drastically. In spite of the large accumulation of reports on Y-TZP there exists no model which explains all of the experimental data consistently. The intentional addition of transition metals [41–43] and glasses [44,45] into Y-TZP has been conducted to enhance the strain rate and lower the temperature for superplastic forming. It was believed that the intergranular glass film promoted the superplasticity of Y-TZP with SiO 2 glass [44]. However, Ikuhara [46] revealed that it was not true from HRTEM observation. He reported that the enhancement in strain rate was due to the segregation of Si and the presence of glass pockets. Oka [*47] found that the codoping of CaO and TiO 2 was the most effective to promote the superplastic elongation at strain rate higher than 10 22 s 21 . Several attempts have been conducted to predict the elongation to failure of Y-TZP by single parameter, such as, Zener–Hollomon parameter or Needleman–Rice parameter [*48]. The fracture process consists of (1) cavity nucleation, (2) cavity growth, and (3) creep crack growth. Hiraga [49,**50] analyzed the damage accumulation in Y-TZP, and found that the controlling process was changed by the difference in impurity content. His result suggests that several factors must be taken into account in order to predict the elongation to failure of different materials.
5. Conclusions Complicated-shaped components with accurate dimensions have been usually fabricated by sintering the shaped powder compact in the ceramic industry. Therefore the additional advantages besides forming shaped components are desirable for the practical application of superplasticity. From this view the superplastic forging of Si 3 N 4 by utilizing intergranular glass phase is promising for future applications, because strengthening and toughening can be achieved concurrently with forming. Although the maximum elongation to failure has been
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pursued in the literature, such specimens cannot be used because they contain a large amount of cavities. The deformation properties under conditions in which cavities are suppressed are more important to assure the reliability of shaped components. In order to incorporate the technology of superplasticity into the ceramic industry, the combination with the sintering process, for example, superplastic sinter-forging [51] should be pursued in the near future.
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