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High-temperature compressive deformation of β-SiAlON polycrystals containing minimum amount of intergranular glass phase
Materials Science and Engineering B 148 (2008) 203–206
High-temperature compressive deformation of -SiAlON polycrystals containing minimum amount of intergranular glass phase Kentarou Chihara a,∗ , Daisuke Hiratsuka b , Yutaka Shinoda a , Takashi Akatsu a , Fumihiro Wakai a , Junichi Tatami b , Katsutoshi Komeya b a
Secure Materials Center, Materials and Structures Laboratory, Tokyo Institute of Technology, R3-23 4259 Nagatsuta, Midori, Yokohama 226-8503, Japan b Graduate School of Environment and Information, Yokohama National University, 79-9 Tokiwadai, Hodogaya, Yokohama 240-8501, Japan Received 27 May 2007; received in revised form 21 July 2007; accepted 3 September 2007
Abstract The deformation of equiaxed dense -SiAlON polycrystals with an average grain size of 0.83 m was investigated by compression tests at temperatures ranging from 1823 K to 1973 K. The -SiAlON ceramics were fabricated from -SiAlON nano-powder (Si3 Al3 O3 N5 ) using the Spark-Plasma Sintering technique without additives. The sintered body contained almost no intergranular phase. The deformation was characterized by stress exponent n ∼ 1 in the higher stress region and n ∼ 2 in the lower stress region. The observed values of stress exponent and activation energy, Q ∼ 940 kJ/mol, in the higher stress region were close to those of a SiAlON, which was fully crystallized by annealing for a long time. The possible deformation mechanism of -SiAlON was attributed to diffusion-related mechanisms, such as grain boundary sliding accommodated by diffusional creep. © 2007 Elsevier B.V. All rights reserved. Keywords: Ceramics; Silicon nitride; High-temperature deformation; Compression test
1. Introduction Silicon nitride (Si3 N4 ) and SiAlON, which is the solid solution of Si3 N4 [1,2], are one of the candidates for high-temperature structural materials due to their excellent mechanical properties and thermal stability [3,4]. Si3 N4 based ceramics are usually sintered with the oxide additives, which provide a liquid phase during sintering. Intergranular glass phase, which is formed by reaction of Si3 N4 with additives and impurities, often remains as pockets at triple junctions and films with the equilibrium thickness of a few nanometers at two-grain junctions [5]. The high-temperature deformation, creep and superplasticity of Si3 N4 -based ceramics are strongly affected by volume fraction and chemistry of the intergranular glass phase [6,7]. Creep due to viscous glass phase [8], solution-precipitation creep [9,10] and shear-thickening creep [11] were proposed for the mecha-
∗
Corresponding author. Tel.: +81 45 924 5335; fax: +81 45 924 5339. E-mail address: [email protected] (K. Chihara).
0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.09.008
nisms of high-temperature deformation of glass containing ceramics. The -SiAlON corresponding to the solid solution Si6−z Alz Oz N8−z allows the dissolution of proper amount of aluminum and oxygen into its structure. Glass-reduced SiAlON can be obtained by compensating the composition for the oxidation of raw powder, i.e., by adding aluminum nitride [12]. HR-TEM (high resolution-transmission electron microscopy) observation of high purity -SiAlON polycrystals revealed the lack of intergranular glass at triple junctions and two-grain junctions [13,14]. Lewis and Karunaratne [15] and Pezzotti et al. [16] reported that the strain rate of fully crystallized -SiAlON with the grain size of 1–2 m was significantly decreased in comparison with -SiAlON containing small amount of glass phase. Recently, we synthesized the -SiAlON nano-powder from the SiO2 –Al2 O3 mixture by using the carbothermal reduction and nitridation method (CRN) [17], and obtained glass-reduced -SiAlON polycrystals. In this article, the high-temperature deformation of fine-grained glass-reduced -SiAlON polycrystals, with nominal substitution level z ∼ 3, was studied to understand how the deformation mechanisms of Si3 N4 -based
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materials were affected by decreasing the intergranular glass content. 2. Experimental procedure Ultrafine SiO2 powder, bohemite and ultrafine carbon powder were used for raw material. The details of materials preparation were described elsewhere [18]. The mixture of raw powder was fired at 1723 K for 2 h in N2 atmosphere. After the completion of CRN process, the unreacted carbon powder was removed by heating in air at 973 K for 2 h, then, fine -SiAlON powder was obtained. The -SiAlON powder was sintered at 2073 K for 5 min using SPS (spark-plasma sintering) under a compressive stress of 30 MPa. No additive was used for sintering the samples. The sintered body reached full density (>97% theoretical density). The deformation of the material was assessed by the uniaxial compression test, during which the compression was applied perpendicular to the direction of SPS. Compression tests were conducted at a constant crosshead speed by using an Instron type machine in N2 atmosphere. The sintered body was cut into rectangular bars with dimension of 2 mm × 2 mm × 3 mm height. Each specimen was heated up to the test temperature at a rate of 50 K/min and maintained at the temperature for 1 h to remove the thermal expansion of the test machine system. XRD (X-ray diffraction), FE-SEM (field emission-scanning electron microscopy) and HR-TEM were used for characterizations of sintered bodies and the deformed samples. 3. Results and discussion The sintered body investigated by using the XRD was composed of the -SiAlON as a major phase and Si2 N2 O (or O–SiAlON) as a minor phase. Fig. 1 shows the polished and chemically etched surface of the as-sintered body. The material was composed of equiaxed grains. The average grain size calculated from area of 500 grains was equivalent to 830 nm in
Fig. 2. HR-TEM micrograph of two-grain junction. The intergranular glass film was not found at the grain boundary.
diameter. Fig. 2 shows the HR-TEM micrograph of an interface between two grains. The TEM investigation revealed that almost all grain boundaries were free from glass phase. Considering the stoichiometric reaction during sintering without additives, we expected that the intergranular glass phase was minimized in -SiAlON polycrystals. The compression tests were performed at temperatures from 1823 K to 1973 K. Fig. 3 illustrates the true stress–true strain curves for -SiAlON polycrystals at 1973 K. The specimens could be deformed to true strain of 0.5 with slight or no strain hardening. FE-SEM observation revealed that the significant grain growth and elongation of grain did not occur during deformation. The flow stress was defined as the intersection of two lines extrapolated to the elastic and the plastic strain regions in stress–strain curves. The strain rate, ε˙ , in steady state is often expressed by the following semi-empirical equation: Q ε˙ = Aσ n d −p exp − (1) RT where A is the temperature-dependent constant, σ the flow stress, d the grain size, n the stress exponent, p the grain-size exponent, Q the activation energy for deformation, and RT is the gas constant times the absolute temperature. Fig. 4 shows the logarithmic plot of strain rate versus flow stress for -
Fig. 1. FE-SEM micrograph of the as-sintered body. The polished surface was etched by an alkaline solution. The average grain size was calculated to be 830 nm.
Fig. 3. Typical stress–strain curves for -SiAlON polycrystals in compression at 1973 K.
K. Chihara et al. / Materials Science and Engineering B 148 (2008) 203–206
Fig. 4. Logarithmic plot of strain rate vs. flow stress for -SiAlON polycrystals at various temperatures in compression. The transitions of stress exponents were observed at all temperatures.
SiAlON polycrystals. The stress exponent, the slope of the curve, increased from ∼1 to ∼2 with decreasing stress at all temperatures. The transition stress of n value from ∼1 to ∼2 decreased with increasing temperature. The stress exponent n close to unity was observed in a fully crystallized -SiAlON tested by four-point bending method [15]. The stress exponent n = 1, Newtonian flow, is often attributed to diffusion-related mechanisms, e.g., Nabarro–Herring creep [19,20] and Coble creep [21]. Ashby and Verrall [22] proposed a model of grain boundary sliding (GBS) accommodated by diffusion, which could explain enhanced ductility. The strain rate, ε˙ D , by the diffusion accommodated process is given by 0.72Γ 3.3δ DGB Ω σ − 1 + (2) ε˙ D = 100 D L kTd 2 d d DL where Ω is the atomic volume, Γ the grain-boundary free energy, δ the thickness of the grain boundary, DL the lattice diffusion coefficient and DGB is the grain-boundary diffusion. Eq. (2) predicts that the value of stress exponent gradually increases when the applied stress becomes close to the threshold stress, 0.72Γ d−1 . By assuming Γ = 1 J/m2 and d = 830 nm, the calculated threshold stress was 0.87 MPa. The calculated value is much lower than the applied stress. Furthermore, the temperature dependence of transition stress for stress exponents cannot be explained by this model. When the diffusion is controlled by interface reaction, the stress exponent becomes 2. The strain rate, ε˙ I , for interface reaction controlled diffusion creep is given by [22]: ε˙ I = B
σ2 d
205
ZrO2 (Y-TZP) [24]. It is interesting that the stress exponent of SiAlON with decreased intergranular glass content resembles fine-grained ionic polycrystals. The accommodation process of GBS might be diffusional flow or some dislocation processes. Under the high stress and temperature conditions, the local stress in the -Si3 N4 grains might be high enough to create dislocations [25]. Quantitative models of GBS accommodated by dislocation process are characterized by n = 2 [26]. Generally speaking, polycrystalline material mainly deforms by diffusional process, n = 1, at lower stress region and deformed by dislocation, n = 2, at higher stress region [27]. But, the observed stress exponents in -SiAlON polycrystals decreased from ∼2 to ∼1 with increasing stress. Therefore, we conclude that dislocation does not play an important role in the deformation of the SiAlON. Submicron-sized SiAlON containing the large amount of glass phase exhibits the transition of stress exponents, Newtonian flow (n = 1) could be observed at lower stresses, and the deformation mechanism changed to shear-thickening creep (n < 1) at higher stresses [11,28]. Chen and Hwang [11] suggested that the shear thickening occurred due to the existence of intergranular liquid phase. Shear-thickening behavior was not observed in -SiAlON polycrystals, which contain almost no intergranular glass phase. Then, shear-thickening creep is not responsible to the high-temperature deformation of glass free SiAlONs. Fig. 5 illustrates the Arrhenius plots of the strain rate in the region with n ∼ 2 (stress of 30 MPa) and region with n ∼ 1 (stress of 150 MPa). The activation energy Q was determined from a slope of the plot. The calculated activation energies of -SiAlON polycrystals were 1200 kJ/mol at lower stress region, and 940 kJ/mol at higher stress region. At the lower flow stress region, activation energy was somewhat higher than that of observed at higher flow stress region. The observed activation energies were much higher than the activation energies, around 300–600 kJ/mol [6], of glass-containing Si3 N4 -based ceramics. Low activation energy of glass-containing Si3 N4 -based ceramics is often attributed to viscosity of glass melts.
(3)
where B is the coefficient. It is expected that the stress exponent will change from 1 to 2 with decreasing stress, if the diffusion and the interface reaction are sequential processes. This model was applied to explain the high-temperature deformation of ionic polycrystals. Similar transition behavior, the stress exponent increased from ∼1 to ∼2 with decreasing stress, was observed among high-temperature deformation of submicronsized monolithic Al2 O3 [23] and Y2 O3 stabilized-tetragonal
Fig. 5. Temperature dependence of strain rate for -SiAlON polycrystals. The activation energy was 1200 kJ/mol for the n ∼ 2 region at a stress of 30 MPa and 940 kJ/mol for the n ∼ 1 region at a stress of 150 MPa.
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The high activation energy was also found in fully crystallized SiAlON. Lewis and Karunaratne [15] reported the value of around 830 kJ/mol for the crystallized -SiAlON, which has almost no glassy phase, at temperatures from 1523 K to 1700 K. They suggested that the high activation energy was related to grain-boundary diffusion of Si. 4. Summary The high-temperature deformation of -SiAlON polycrystals, which had almost no intergranular glass phase, was studied in compression. -SiAlON polycrystals could be deformed up to 1973 K without rupture. The deformation was characterized by stress exponent n ∼ 1 in the higher stress region and n ∼ 2 in the lower stress region. This transition behavior of the stress exponents for -SiAlON polycrystals was similar to the deformation of fine-grained monolithic Al2 O3 and Y-TZP. The activation energy was much higher than that of glass-containing Si3 N4 based ceramics. Shear thickening (n < 1), which occurred due to the existence of intergranular liquid phase was not observed in -SiAlON polycrystal ceramics, which had almost no glassy phase. The observed values of stress exponent and the activation energy in the higher stress region were close to those of fully crystallized -SiAlON. Acknowledgment We thank Mr. Akira Genseki (Center for Advanced Materials Analysis, Tokyo Institute of Technology) for the TEM observation. References [1] K.H. Jack, W.I. Wilson, Nature 238 (1972) 28–29.
[2] V.A. Izhevskiy, L.A. Genova, J.C. Bressiani, F. Aldinger, J. Eur. Ceram. Soc. 20 (2000) 2275–2295. [3] F.F. Lange, J. Am. Ceram. Soc. 61 (1978) 53–56. [4] T. Ekstrom, M. Nygren, J. Am. Ceram. Soc. 75 (1992) 259– 276. [5] D.R. Clarke, J. Am. Ceram. Soc. 70 (1987) 15–22. [6] J.J.M. Mart´ınez, A.D. Rodr´ıguez, Prog. Mater. Sci. 49 (2004) 19– 107. [7] E. Narimatsu, Y. Shinoda, T. Akatsu, F. Wakai, J. Eur. Ceram. Soc. 26 (2006) 1069–1074. [8] J.R. Dryden, D. Kucerovsky, D.S. Wilkinson, D.F. Watt, Acta Metall. 37 (1989) 2007–2015. [9] R. Raj, C.K. Chyung, Acta Metall. 29 (1981) 159–166. [10] F. Wakai, Acta Metall. Mater. 4 (1994) 1163–1172. [11] I.-W. Chen, S.L. Hwang, J. Am. Ceram. Soc. 75 (1992) 1073–1079. [12] A. Rosenflanz, J. Am. Ceram. Soc. 85 (2002) 2379–2381. [13] M.H. Lewis, B.D. Powell, P. Drew, R.J. Lumby, B. North, A.J. Taylor, J. Mater. Sci. 12 (1977) 61–74. [14] G. Pezzotti, H.J. Kleebe, K. Ota, K. Okamoto, J. Am. Ceram. Soc. 80 (1997) 2349–2354. [15] B.S.B. Karunaratne, M.H. Lewis, J. Mater. Sci. 15 (1980) 1781– 1789. [16] G. Pezzotti, K. Okamoto, K. Ota, J. Am. Ceram. Soc. 84 (2001) 598– 602. [17] L.J. Gauckler, H.L. Lukas, G. Petzow, J. Am. Ceram. Soc. 58 (1975) 346–347. [18] D. Hiratsuka, J. Tatami, T. Meguro, K. Komeya, I. Hayashi, J.F. Yang, M. Omori, Key Eng. Mater. 317–318 (2006) 633–636. [19] F.R.N. Nabarro, Phil. Mag. 16 (1967) 231–237. [20] C. Herring, J. Appl. Phys. 21 (1950) 437–445. [21] R.L. Coble, J. Appl. Phys. 34 (1963) 1679–1682. [22] M.F. Ashby, R.A. Verrall, Acta Metall. 21 (1973) 149–163. [23] R.M. Cannon, W.H. Rhodes, A.H. Heuer, J. Am. Ceram. Soc. 63 (1980) 46–53. [24] F. Wakai, T. Nagano, J. Mater. Sci. 26 (1991) 241–247. [25] C.-W. Li, F. Reidinger, Acta Mater. 45 (1997) 407–421. [26] A. Ball, M.M. Hutchinson, Met. Sci. J. 3 (1969) 1–7. [27] T.G. Nieh, J. Wadsworth, O.D. Sherby, Superplasticity in Metals and Ceramics, 1st ed., Cambridge University Press, Cambridge, 1997. [28] A. Rosenflanz, I.-W. Chen, J. Am. Ceram. Soc. 80 (1997) 1341–1352.
High-temperature compressive deformation of -SiAlON polycrystals containing minimum amount of intergranular glass phase Kentarou Chihara a,∗ , Daisuke Hiratsuka b , Yutaka Shinoda a , Takashi Akatsu a , Fumihiro Wakai a , Junichi Tatami b , Katsutoshi Komeya b a
Secure Materials Center, Materials and Structures Laboratory, Tokyo Institute of Technology, R3-23 4259 Nagatsuta, Midori, Yokohama 226-8503, Japan b Graduate School of Environment and Information, Yokohama National University, 79-9 Tokiwadai, Hodogaya, Yokohama 240-8501, Japan Received 27 May 2007; received in revised form 21 July 2007; accepted 3 September 2007
Abstract The deformation of equiaxed dense -SiAlON polycrystals with an average grain size of 0.83 m was investigated by compression tests at temperatures ranging from 1823 K to 1973 K. The -SiAlON ceramics were fabricated from -SiAlON nano-powder (Si3 Al3 O3 N5 ) using the Spark-Plasma Sintering technique without additives. The sintered body contained almost no intergranular phase. The deformation was characterized by stress exponent n ∼ 1 in the higher stress region and n ∼ 2 in the lower stress region. The observed values of stress exponent and activation energy, Q ∼ 940 kJ/mol, in the higher stress region were close to those of a SiAlON, which was fully crystallized by annealing for a long time. The possible deformation mechanism of -SiAlON was attributed to diffusion-related mechanisms, such as grain boundary sliding accommodated by diffusional creep. © 2007 Elsevier B.V. All rights reserved. Keywords: Ceramics; Silicon nitride; High-temperature deformation; Compression test
1. Introduction Silicon nitride (Si3 N4 ) and SiAlON, which is the solid solution of Si3 N4 [1,2], are one of the candidates for high-temperature structural materials due to their excellent mechanical properties and thermal stability [3,4]. Si3 N4 based ceramics are usually sintered with the oxide additives, which provide a liquid phase during sintering. Intergranular glass phase, which is formed by reaction of Si3 N4 with additives and impurities, often remains as pockets at triple junctions and films with the equilibrium thickness of a few nanometers at two-grain junctions [5]. The high-temperature deformation, creep and superplasticity of Si3 N4 -based ceramics are strongly affected by volume fraction and chemistry of the intergranular glass phase [6,7]. Creep due to viscous glass phase [8], solution-precipitation creep [9,10] and shear-thickening creep [11] were proposed for the mecha-
∗
Corresponding author. Tel.: +81 45 924 5335; fax: +81 45 924 5339. E-mail address: [email protected] (K. Chihara).
0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.09.008
nisms of high-temperature deformation of glass containing ceramics. The -SiAlON corresponding to the solid solution Si6−z Alz Oz N8−z allows the dissolution of proper amount of aluminum and oxygen into its structure. Glass-reduced SiAlON can be obtained by compensating the composition for the oxidation of raw powder, i.e., by adding aluminum nitride [12]. HR-TEM (high resolution-transmission electron microscopy) observation of high purity -SiAlON polycrystals revealed the lack of intergranular glass at triple junctions and two-grain junctions [13,14]. Lewis and Karunaratne [15] and Pezzotti et al. [16] reported that the strain rate of fully crystallized -SiAlON with the grain size of 1–2 m was significantly decreased in comparison with -SiAlON containing small amount of glass phase. Recently, we synthesized the -SiAlON nano-powder from the SiO2 –Al2 O3 mixture by using the carbothermal reduction and nitridation method (CRN) [17], and obtained glass-reduced -SiAlON polycrystals. In this article, the high-temperature deformation of fine-grained glass-reduced -SiAlON polycrystals, with nominal substitution level z ∼ 3, was studied to understand how the deformation mechanisms of Si3 N4 -based
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materials were affected by decreasing the intergranular glass content. 2. Experimental procedure Ultrafine SiO2 powder, bohemite and ultrafine carbon powder were used for raw material. The details of materials preparation were described elsewhere [18]. The mixture of raw powder was fired at 1723 K for 2 h in N2 atmosphere. After the completion of CRN process, the unreacted carbon powder was removed by heating in air at 973 K for 2 h, then, fine -SiAlON powder was obtained. The -SiAlON powder was sintered at 2073 K for 5 min using SPS (spark-plasma sintering) under a compressive stress of 30 MPa. No additive was used for sintering the samples. The sintered body reached full density (>97% theoretical density). The deformation of the material was assessed by the uniaxial compression test, during which the compression was applied perpendicular to the direction of SPS. Compression tests were conducted at a constant crosshead speed by using an Instron type machine in N2 atmosphere. The sintered body was cut into rectangular bars with dimension of 2 mm × 2 mm × 3 mm height. Each specimen was heated up to the test temperature at a rate of 50 K/min and maintained at the temperature for 1 h to remove the thermal expansion of the test machine system. XRD (X-ray diffraction), FE-SEM (field emission-scanning electron microscopy) and HR-TEM were used for characterizations of sintered bodies and the deformed samples. 3. Results and discussion The sintered body investigated by using the XRD was composed of the -SiAlON as a major phase and Si2 N2 O (or O–SiAlON) as a minor phase. Fig. 1 shows the polished and chemically etched surface of the as-sintered body. The material was composed of equiaxed grains. The average grain size calculated from area of 500 grains was equivalent to 830 nm in
Fig. 2. HR-TEM micrograph of two-grain junction. The intergranular glass film was not found at the grain boundary.
diameter. Fig. 2 shows the HR-TEM micrograph of an interface between two grains. The TEM investigation revealed that almost all grain boundaries were free from glass phase. Considering the stoichiometric reaction during sintering without additives, we expected that the intergranular glass phase was minimized in -SiAlON polycrystals. The compression tests were performed at temperatures from 1823 K to 1973 K. Fig. 3 illustrates the true stress–true strain curves for -SiAlON polycrystals at 1973 K. The specimens could be deformed to true strain of 0.5 with slight or no strain hardening. FE-SEM observation revealed that the significant grain growth and elongation of grain did not occur during deformation. The flow stress was defined as the intersection of two lines extrapolated to the elastic and the plastic strain regions in stress–strain curves. The strain rate, ε˙ , in steady state is often expressed by the following semi-empirical equation: Q ε˙ = Aσ n d −p exp − (1) RT where A is the temperature-dependent constant, σ the flow stress, d the grain size, n the stress exponent, p the grain-size exponent, Q the activation energy for deformation, and RT is the gas constant times the absolute temperature. Fig. 4 shows the logarithmic plot of strain rate versus flow stress for -
Fig. 1. FE-SEM micrograph of the as-sintered body. The polished surface was etched by an alkaline solution. The average grain size was calculated to be 830 nm.
Fig. 3. Typical stress–strain curves for -SiAlON polycrystals in compression at 1973 K.
K. Chihara et al. / Materials Science and Engineering B 148 (2008) 203–206
Fig. 4. Logarithmic plot of strain rate vs. flow stress for -SiAlON polycrystals at various temperatures in compression. The transitions of stress exponents were observed at all temperatures.
SiAlON polycrystals. The stress exponent, the slope of the curve, increased from ∼1 to ∼2 with decreasing stress at all temperatures. The transition stress of n value from ∼1 to ∼2 decreased with increasing temperature. The stress exponent n close to unity was observed in a fully crystallized -SiAlON tested by four-point bending method [15]. The stress exponent n = 1, Newtonian flow, is often attributed to diffusion-related mechanisms, e.g., Nabarro–Herring creep [19,20] and Coble creep [21]. Ashby and Verrall [22] proposed a model of grain boundary sliding (GBS) accommodated by diffusion, which could explain enhanced ductility. The strain rate, ε˙ D , by the diffusion accommodated process is given by 0.72Γ 3.3δ DGB Ω σ − 1 + (2) ε˙ D = 100 D L kTd 2 d d DL where Ω is the atomic volume, Γ the grain-boundary free energy, δ the thickness of the grain boundary, DL the lattice diffusion coefficient and DGB is the grain-boundary diffusion. Eq. (2) predicts that the value of stress exponent gradually increases when the applied stress becomes close to the threshold stress, 0.72Γ d−1 . By assuming Γ = 1 J/m2 and d = 830 nm, the calculated threshold stress was 0.87 MPa. The calculated value is much lower than the applied stress. Furthermore, the temperature dependence of transition stress for stress exponents cannot be explained by this model. When the diffusion is controlled by interface reaction, the stress exponent becomes 2. The strain rate, ε˙ I , for interface reaction controlled diffusion creep is given by [22]: ε˙ I = B
σ2 d
205
ZrO2 (Y-TZP) [24]. It is interesting that the stress exponent of SiAlON with decreased intergranular glass content resembles fine-grained ionic polycrystals. The accommodation process of GBS might be diffusional flow or some dislocation processes. Under the high stress and temperature conditions, the local stress in the -Si3 N4 grains might be high enough to create dislocations [25]. Quantitative models of GBS accommodated by dislocation process are characterized by n = 2 [26]. Generally speaking, polycrystalline material mainly deforms by diffusional process, n = 1, at lower stress region and deformed by dislocation, n = 2, at higher stress region [27]. But, the observed stress exponents in -SiAlON polycrystals decreased from ∼2 to ∼1 with increasing stress. Therefore, we conclude that dislocation does not play an important role in the deformation of the SiAlON. Submicron-sized SiAlON containing the large amount of glass phase exhibits the transition of stress exponents, Newtonian flow (n = 1) could be observed at lower stresses, and the deformation mechanism changed to shear-thickening creep (n < 1) at higher stresses [11,28]. Chen and Hwang [11] suggested that the shear thickening occurred due to the existence of intergranular liquid phase. Shear-thickening behavior was not observed in -SiAlON polycrystals, which contain almost no intergranular glass phase. Then, shear-thickening creep is not responsible to the high-temperature deformation of glass free SiAlONs. Fig. 5 illustrates the Arrhenius plots of the strain rate in the region with n ∼ 2 (stress of 30 MPa) and region with n ∼ 1 (stress of 150 MPa). The activation energy Q was determined from a slope of the plot. The calculated activation energies of -SiAlON polycrystals were 1200 kJ/mol at lower stress region, and 940 kJ/mol at higher stress region. At the lower flow stress region, activation energy was somewhat higher than that of observed at higher flow stress region. The observed activation energies were much higher than the activation energies, around 300–600 kJ/mol [6], of glass-containing Si3 N4 -based ceramics. Low activation energy of glass-containing Si3 N4 -based ceramics is often attributed to viscosity of glass melts.
(3)
where B is the coefficient. It is expected that the stress exponent will change from 1 to 2 with decreasing stress, if the diffusion and the interface reaction are sequential processes. This model was applied to explain the high-temperature deformation of ionic polycrystals. Similar transition behavior, the stress exponent increased from ∼1 to ∼2 with decreasing stress, was observed among high-temperature deformation of submicronsized monolithic Al2 O3 [23] and Y2 O3 stabilized-tetragonal
Fig. 5. Temperature dependence of strain rate for -SiAlON polycrystals. The activation energy was 1200 kJ/mol for the n ∼ 2 region at a stress of 30 MPa and 940 kJ/mol for the n ∼ 1 region at a stress of 150 MPa.
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The high activation energy was also found in fully crystallized SiAlON. Lewis and Karunaratne [15] reported the value of around 830 kJ/mol for the crystallized -SiAlON, which has almost no glassy phase, at temperatures from 1523 K to 1700 K. They suggested that the high activation energy was related to grain-boundary diffusion of Si. 4. Summary The high-temperature deformation of -SiAlON polycrystals, which had almost no intergranular glass phase, was studied in compression. -SiAlON polycrystals could be deformed up to 1973 K without rupture. The deformation was characterized by stress exponent n ∼ 1 in the higher stress region and n ∼ 2 in the lower stress region. This transition behavior of the stress exponents for -SiAlON polycrystals was similar to the deformation of fine-grained monolithic Al2 O3 and Y-TZP. The activation energy was much higher than that of glass-containing Si3 N4 based ceramics. Shear thickening (n < 1), which occurred due to the existence of intergranular liquid phase was not observed in -SiAlON polycrystal ceramics, which had almost no glassy phase. The observed values of stress exponent and the activation energy in the higher stress region were close to those of fully crystallized -SiAlON. Acknowledgment We thank Mr. Akira Genseki (Center for Advanced Materials Analysis, Tokyo Institute of Technology) for the TEM observation. References [1] K.H. Jack, W.I. Wilson, Nature 238 (1972) 28–29.
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