Development of Superplastic Ceramics

Chapter 9.9

Development of Superplastic Ceramics Fumihiro Wakai Secure Materials Center, Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori, Yokohama, Kanagawa 226-8503, Japan

Chapter Outline 1. Introduction 765 2. Mechanical Properties and Mechanism of Superplasticity 765 3. Superplastic Ceramics 766

1. INTRODUCTION Superplasticity is phenomenologically defined as an ability of a polycrystalline material to exhibit extraordinarily large elongations in tension at elevated temperatures. The fine structure superplasticity is a property commonly found in many metals, alloys, intermetallics, and ceramics at temperatures >2/3Tm (where Tm is the absolute melting point), when the grain size is very small (less than several micrometers for metals and less than 1 mm for ceramics) and stable during deformation [1,2]. Micrograins apparently move past one another like sand particles flowing, as long as the fracture is suppressed by accommodation processes such as diffusion and dislocation motion. The tensile specimens can be elongated uniformly without necking at relatively low stresses (Figure 1). This property has been applied to manufacture net-shaped components of some superplastic alloys. While ceramics are brittle materials that show almost no plastic deformation at ambient temperatures, a very wide range of superplastic ceramics had been developed since the superplasticity of zirconia (ZrO2) and its composite was

4. Application of Superplasticity References

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found in the mid-1980s [3,4]. The ceramic superplasticity is common to fine-grained ceramics that are produced by controlling the microstructures to sub-micrometer scales, for example, oxides (ZrO2, alumina, and their composites), nonoxides (silicon nitride [5], SiC), bioceramics, and superconductors, as has been reviewed extensively [6e8]. The ceramic superplasticity is of industrial interest, as it forms the basis of a fabrication method that can be used to produce components having complex shapes from materials that are hard to machine. The superplasticity of ZrO2 has been applied to superplastic forging, sinter forging, sheet forming, gas-pressure forming, extrusion, deep drawing, stretch forming, and superplastic joining. The superplastic sinter forging is also used to produce a textured silicon nitride with improved strength and fracture toughness. The use of superplastic forming may become even more widespread if large deformation can be achieved at higher strain rates, lower flow stresses, and lower temperatures. The better superplastic ceramics have been developed by improving processing methods and by controlling the grain boundary structure and its chemistry by adding dopant atoms [9]. Especially, high-strain-rate superplasticity in ZrO2-based composite is promising for an efficient shapeforming technology [10,11].

2. MECHANICAL PROPERTIES AND MECHANISM OF SUPERPLASTICITY FIGURE 1 Superplastic elongation of an SiAlON [57]. Undeformed (top) and deformed (bottom) specimens. The elongation of 470% has been achieved.

Handbook of Advanced Ceramics. http://dx.doi.org/10.1016/B978-0-12-385469-8.00041-1 Copyright Ó 2013 Elsevier Inc. All rights reserved.

In superplastic alloys, the flow stress is particularly sensitive to the rate of deformation s ¼ K ε_ m

(1)

765

766

where s is the stress, ε_ is the strain rate, m is the strain-rate sensitivity index, and K is a constant. The very large elongation occurs at m  0:3, because large m is a necessary condition for the stability of uniform elongation without local necking. Alternatively, the constitutive equation is described as [12]     ADGb b p s  s0 n (2) ε_ ¼ G kT d where D is the diffusion coefficient ( ¼ D0 expðQ=kTÞ,Q is the activation energy), G is the shear modulus, b is the Burgers vector, k is the Boltzmann constant, T is the absolute temperature, n is the stress exponent, p is an exponent of the inverse grain size, s0 is the threshold stress [13], and A is a dimensionless constant. Equation (2) is equivalent to Eqn (1) at s0 ¼ 0, and n is the inverse of m. The grain structure remains almost equiaxed after the elongation of hundred percent in superplasticity. This behavior is quite different from that observed in diffusional creep (Figure 2(a)) where grains distort and elongate. Ashby and Verral [14] proposed the grain switching model for superplasticity (Figure 2(b)) from the observation of deformation of oil grains or soap froth. The four grains exchange their neighbors at the final state so that the equiaxed shape is restored. Gifkins [15] considered that the boundary “mantle” of the grain behaved differently from the central “core” of the grain. Grains change their shape by deformation of the “mantle” in his core-mantle model (Figure 2(c)). The shift in the marker line shows grain boundary sliding. The grain compatibility during grain boundary sliding is maintained by a concurrent accommodation process, which involves diffusion, dislocation motion, and grain boundary migration. The grain switching event in the two-dimensional model consists of two

Handbook of Advanced Ceramics

independent processes, formation and disappearance of a grain boundary, in three dimensions [16]. When grain boundary sliding is accommodated by grain boundary diffusion and bulk diffusion, the exponents of inverse grain size are p ¼ 3 and p ¼ 2, respectively. With reference to Eqn (2), at constant temperature, decreasing the value of the grain size d will increase the strain rate. But if the grain size is very small and the grain boundary diffusion is very fast, the strain rate is controlled by interface reaction, which is the creation and annihilation of vacancy, rather than by the kinetics of long-range diffusion [17]. At elevated temperatures, failure of ceramics commonly occurs intergranularly by the cavities growing to coalesce to form cracks [18]. Thus, the suppression of cavity formation is important to allow superplastic elongation. A fast grain boundary diffusion, a small grain size, and a small ratio of grain boundary energy to surface energy are necessary for suppressing the nucleation of cavities [19]. To ensure the reliability of superplastically formed components, it is necessary to characterize the cavitation damage induced by the superplastic deformation. The cavity size increases exponentially with strain in superplasticity of ZrO2 [20]. The cavity formation and the maximum elongation to failure are influenced by processing-dependent microstructural factors such as the size distribution of matrix grains, second-phase particles, and residual defects [21]. Superplastic deformation is often accompanied by grain growth, the rate of which depends on either strain [22] or stress [23] and is usually well in excess of that found in the absence of deformation (static grain growth). The enhanced grain growth in superplasticity is termed the dynamic grain growth.

3. SUPERPLASTIC CERAMICS 3.1. Zirconia

FIGURE 2 Shear deformation in a regular array of grains [18]. (a) Diffusional creep; (b) Soap froth model; (c) Core-mantle model. (The gray circles show the “core”.)

The fracture toughness of ZrO2-based ceramics is enhanced by the stress-activated tetragonal to monoclinic transformation [24,25], in other words, by the transformation toughening. The fine grain size, which was required to retain the metastable tetragonal phase, facilitated also the superplasticity of Y2O3-stabilized tetragonal ZrO2 polycrystals (Y-TZP) [3]. The extensive studies on superplasticity of Y-TZP have been summarized in a review [7]. The relation between the strain rate and stress of high purity Y-TZP is schematically plotted as a solid line in Figure 3 on logarithmic scales. The apparent stress exponent n, which is given by the slope of the curve, is approximately n ¼ 2 at the high stress region, but it increases to n > 3 at the intermediate stress region. Several mechanisms have been proposed to explain this transition in stress exponent: 1)

Chapter | 9.9

Development of Superplastic Ceramics

767

3.2. Composites

FIGURE 3 Relation between strain rate ð_εÞ and stress ðsÞ in arbitrary units. The solid line schematically shows the experimental result for high purity Y-TZP [29], and the broken line shows this for low purity Y-TZP.

Transition of the accommodation process of grain boundary sliding from diffusion controlled one at high stresses to interface controlled one at the intermediate stresses [17,27,28], 2) Threshold stress [7,13]. It has been also reported that the curve shows n ¼ 1 at stresses lower than the threshold stress [29]. The deformation of Y-TZP is significantly affected by small amounts of impurities (broken line in Figure 3) [30]. The strain rate at the low stress region is enhanced by the addition of 0.12 wt% Al2O3, and n z 2 is observed in a wide range of stresses [31]. A similar effect is also observed in a material containing <0.3 wt% SiO2 [32]. The small amount of impurities segregate at grain boundaries, and in some cases, impurity atoms dissolve into ZrO2 grains. The addition of impurity atoms and solute atoms has a wide variety of effects on the grain boundary diffusivities and the lattice diffusivities. It is believed that the grain boundary diffusion and/or bulk diffusion are promoted by these impurity atoms. The large ductility of Y-TZP was achieved by the intentional addition of a large amount of SiO2 [32e35]. The superplasticity of Y-TZP could be improved by doping transition metals [3], alkaline earths, and rare earths [36] also. Especially, the codoping of TiO2 and GeO2 is most effective in decreasing flow stress and improving the ductility [37], because the codoping enhances grain boundary diffusion of Zr cations and decreases the grain boundary energy [38].

The fracture toughness of ceramic composite increases with the dispersion of fine tetragonal ZrO2 particles. In twophase composites, the dispersed second-phase particles often impede the grain boundary migration so that the grain growth of the matrix phase is suppressed. Because the fine microstructure of ZrO2-toughened ceramics is stable at elevated temperature, many fine-grained two-phase and multiphase composites exhibit superplasticity, for example, ZrO2eAl2O3 [4,39e41] and ZrO2-mullite [42]. The addition of the second-phase affects the superplasticity and creep of composites in two ways: First, it modifies the continuum deformation mechanics. The effect of volume fraction of the second phase on strain rate can be predicted by a rheology model [4,42] or by a composite theory [39]. Second, the constituent atoms of the second phase affect interface related deformation characteristics, for example, grain boundary diffusion. When superplasticity occurs at strain rates considerably >102 s1, it is conveniently called high-strain-rate superplasticity [1]. High-strain-rate superplasticity of the ZrO2eAl2O3espinel composite [10,11] was achieved at a temperature (1650  C) 200  C higher than the deformation temperature for Y-TZP (1450  C), mainly because the grain growth was suppressed by dispersion of the secondand the third-phase particles. The grain boundary sliding was partly accommodated by dislocation motion in spinel grains [43], and the threshold stress was observed in the deformation of the composite [44].

3.3. Silicon Nitride Silicon nitride (Si3N4) is a light, hard, and strong engineering ceramic that has been developed mainly as a structural material for high-temperature applications. Although creep resistance and superplasticity are incompatible functions, superplastic forming of silicon nitride can be applied to make wear-resistant components that are used at intermediate temperatures. The liquid-phase sintered Si3N4 has residual glass phase pockets and a thin glass film with a thickness of approximately 1 nm at grain boundaries. The superplasticity and creep of Si3N4 are affected by properties of the intergranular glass phase: chemical composition, quantity, glass transition temperature, viscosity, and solubility of Si3N4 to the liquid. A wide variety of superplastic silicon nitrides have been developed during 1982e2012: (1) Si3N4/SiC nanocomposite, (2) b-silicon nitride, (3) b-SiAlON, (4) equiaxed a/b-silicon nitride, (5) equiaxed a/b-SiAlON, and (6) equiaxed b-silicon nitride. The superplasticity of silicon nitride was found for the first time in the deformation of fine-grained Si3N4-based composite (material (1)), which was developed by sintering

768

amorphous SieCeN powder [5,45]. Self-reinforced b-silicon nitrides are analogs to whisker-reinforced ceramics relying on the formation of elongated rodlike b-grains in sintering of a-powder. When rodlike grains are very small, silicon nitride (material 2) can still exhibit superplastically large elongations despite this peculiar microstructure [46,47]. SiAlON is a solid solution of Si3N4, in its structure, some Si and N atoms are replaced by Al and O atoms. The strain rate of b-SiAlON (material 3) is enhanced significantly by the liquid phase transiently formed during sintering [48,49]. Because equiaxed grains are preferable for grain boundary sliding and thus, for superplasticity, materials consisting of equiaxed a/b-grains have been developed (materials 4 [50], 5 [51,52]). However, the microstructure of these materials is unstable due to the a- to b-phase transition and grain growth during deformation. This problem was solved by using b-grains as the starting powder in sintering, to get stable fine microstructures with equiaxed grains (material 6) [53e55]. When superplastic forming is conducted on silicon nitride with an equiaxed microstructure, the fracture toughness and the strength at elevated temperatures can be improved by a heat treatment to form elongated rodlike grains for the self-reinforcement. On the other hand, in the superplastic deformation of silicon nitride with rodlike grains, the grains align parallelly to the tensile direction in tension tests [54], and in a plane vertical to the compression direction in compression tests [47]. The formation of anisotropic texture can improve the fracture toughness and strength of components in a specific direction [47]. The superplastic forging of silicon nitride has been achieved at high-strain rates by using a spark plasma sintering (SPS) furnace [56]. The superplasticity of silicon nitride takes place by grain boundary sliding accommodated by viscous flow of intergranular glass phase and solution-precipitation creep [57]. In the superplasticity of equiaxed silicon nitride [52], the solution-precipitation process is controlled by diffusion at high stress region [58], and by interface reaction at low stress region [59]. A peculiar phenomenon of shear thickening is also observed in the compressive deformation of SiAlON [51].

3.4. Silicon Carbide Silicon carbide is a covalent material that adopts network structures featuring vertex sharing of [SiC4]. Although the diffusion coefficient in SiC is very slow, the superplasticity can be achieved by doping with a small amount of boron [60]. The doped boron segregates at grain boundaries and takes the place of silicon, forming bonds in a local environment that is similar to that in the B4C structure [61]. It is supposed that the segregated boron enhanced the grain

Handbook of Advanced Ceramics

boundary diffusion, because the diffusion of Si is faster by three orders of magnitude in B4C than in SiC. The superplasticity of SiC is also affected by oxygen impurity atoms [62] and the doping of Al atoms [63].

3.5. Functional Materials Hydroxyapatite (Ca10(PO4)6(OH)2) is biocompatible with bone and can be used for orthopedic and dental implants. When hydroxyapatite is used as a bone replacement, nearnet-shape forming is required to fit for each patients. Hydroxyapatite, with a grain size of sub-micrometer, is translucent, and it can be superplastically deformed at a relatively low temperature of 1000  C due to its fast diffusivity [64e66]. Carbonate apatite (CAP) is resorbed by osteoclasts and is biocompatible similar to hydroxyapatite. Fine-grained CAP can also be superplastically formed [67]. Oxide superconductor is brittle, and it is usually hard to make wires and ribbons by plastic deformation. A finegrained YBa2Cu3O7x could undergo very large deformation at elevated temperatures, and superplastic elongation was demonstrated recently [68].

4. APPLICATION OF SUPERPLASTICITY In the ceramic industry, complex-shaped components with accurate dimensions have been usually fabricated by sintering the shaped powder compact. Therefore, in comparison to the sintering, two factors are important for the practical application of superplastic forming: 1) reliability and 2) efficiency. First, it is necessary to ensure the reliability of the products. The source of variability in strength is related to flaws, particularly for ceramics due to their inherent brittleness. The cavitation must be suppressed by selecting appropriate forming conditions. From the viewpoint of efficiency, the combination of sintering and forming, for example, sinter forging [69], is most promising, because both densification and net shaping are achieved simultaneously. Low temperature and high-strainrate superplastic sinter forging of alumina/spinel composites is actually successfully demonstrated. Porous preforms of nanoceramic composites that were partially densified at low temperatures were superplastically deformed by SPS at 1000e1050  C at 102 s1 [70]. Spark plasma sintering is similar to conventional hot pressing, and the powder compact (green ceramics) is heated by a direct-pulsed DC current in the graphite die. The compressive deformation of SiAlON also exceeded 102 s1 at 1500  C in the SPS apparatus [56]. This result suggests a favorable relationship between high-strain-rate superplasticity and ultrafast sintering. Superplastic forming of ZrO2/alumina/spinel

Chapter | 9.9

Development of Superplastic Ceramics

composite could be performed at 1,150  C by using SPS [71]. Superplastic forming has been applied mainly to ZrO2, for example, sheet forming [72], gas-pressure forming [73], extrusion [74], deep drawing [75], stretch forming [76]. The superplastic forming concurrent with the diffusion bonding process, which is used in the production of titanium alloys in the aerospace industry, is also applicable to ceramics. Superplasticity has been used for the bonding or joining of dissimilar ceramic materials, for example, ZrO2/alumina composites [77e79], ZrO2/hydroxyapatite composites [80], and functionally gradient materials [81]. The superplastic joining of ceramics and ceramic composites has been summarized in a review [82].

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[20]

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[22]

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Handbook of Advanced Ceramics

[57] Mele´ndez-Martinez JJ, Domı´nguez-Rodrı´guez A. Creep of silicon nitride. Prog Mater Sci 2004;49:19e107. [58] Raj R, Chyung CK. Solution-precipitation creep in glass ceramics. Acta Mater 1981;29:159e66. [59] Wakai F. Step model of solution-precipitation creep. Acta Metall Mater 1994;42:1163e72. [60] Shinoda Y, Nagano T, Gu H, Wakai F. Superplasticity of silicon carbide. J Am Ceram Soc 1999;82:2916e8. [61] Gu H, Shinoda Y, Wakai F. Detection of boron segregation to grain boundaries in silicon carbide by spatially resolved electron energyloss spectroscopy. J Am Ceram Soc 1999;82:469e72. [62] Ohtsuka S, Shinoda Y, Akatsu T, Wakai F. Effect of oxygen segregation at grain boundaries on deformation of B, C- doped silicon carbides at elevated temperatures. J Am Ceram Soc 2005;88:1558e63. [63] Tokiyama T, Shinoda Y, Akatsu T, Wakai F. Enhancement of hightemperature deformation in fine-graine silicon carbide with Al doping. Mater Sci Eng B 2008;148:261e4. [64] Wakai F, Kodama Y, Sakaguchi S, Nonami T. Superplasticity of hot isostatically pressed hydroxyapatite. J Am Ceram Soc 1990;73:457e60. [65] Singh D, De La Cinta Lorenzo-Martin M, Routbort JL, Gutie´rrezMora F, Case ED. Plastic deformation of hydroxyapatites and its application to joining. Appl Ceram Tech 2005;2:247e55. [66] Tago K, Itatani K, Suzuki TS, Sakka Y, Koda S. Densification and superplasticity of hydroxy apatite ceramics. J Ceram Soc Jpn 2005;113:669e73. [67] Adachi M, Wakamatsu N, Doi Y. Superplastic deformation in carbonate apatite ceramics under constant compressive loading for near-net-shape production of bioresorbable bone substitutes. Dent Mater J 2008;27:93e8. [68] Albuquerque JM, Harmer MP, Chou YT. Tensile superplastic deformation of YBa2Cu3O7x high-Tc superconductor. Acta Mater 2001;49:2277e84. [69] Panda PC, Wang J, Raj R. Sinter-forging characteristics of finegrained zirconia. J Am Ceram Soc 1988;71:C507e9. [70] G-Zhan D, Garay JE, Mukherjee AK. Ultralow-temperature superplasticity in nanoceramic composites. Nano Lett 2005;5: 2593e7. [71] Hulbert DM, Jiang D, Kuntz JD, Kodera Y, Mukherjee AK. A lowtemperature high-strain-rate formable nanocrystalline superplastic ceramic. Scripta Mater 2007;56:1103e6. [72] Lesuer DR, Wadsworth J, Nieh TG. Forming of superplastic ceramics. Ceram Int 1996;22:381e8. [73] Wittenauer J, Nieh TG, Wadsworth J. Superplastic gas-pressure deformation of YTZ sheet. J Am Ceram Soc 1993;76:1665e72. [74] Wu X, Chen I-W. Hot extrusion of ceramics. J Am Ceram Soc 1992;75:1846e53. [75] Winnubst AJA, Boutz MMR. Superplastic deep drawing of tetragonal ceramics at 1160 C. J Eur Ceram Soc 1998;18:2101e6. [76] Wu X, Chen I-W. Superplastic bulging of fine-grained zirconia. J Am Ceram Soc 1990;73:746e9. [77] Nagano T, Kato H, Wakai F. Diffusion bonding of zirconia/alumina composites. J Am Ceram Soc 1990;73:3476e80. [78] Gutie´rrez-Mora F, Goretta KC, Majumdar S, Routbort JL, Grimdisch M, Domı´nguez-Rodrı´guez A. Influence of internal stresses in superplastic joining of irconia-toughened alumina. Acta Mater 2002;50:3475e85.

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Development of Superplastic Ceramics

[79] Boniecki M, Kalinski D, Librant Z, Wesolowski W. Superplastic joining of alumina and zirconia ceramics. J Eur Ceram Soc 2007;27:1351e5. [80] Singh D, de la Cinta Lorenzo-Martin M, Gutie´rrez-Mora F, Routbort JL, Casa ED. Self-joining of zirconia/hydroxyapatite composites using plastic deformation process. Acta Biomater 2006;2:669e75.

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