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High-temperature strength and toughness behaviors for reaction-bonded SiC ceramics below 1400 °C
Materials Letters 59 (2005) 1732 – 1735 www.elsevier.com/locate/matlet
High-temperature strength and toughness behaviors for reaction-bonded SiC ceramics below 1400 8C Qing-Wei Huanga,T, Li-Hui Zhub a
State Key Laboratory of High Performance and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China b School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China Received 4 March 2004; received in revised form 17 January 2005; accepted 24 January 2005 Available online 8 March 2005
Abstract High-temperature strength and toughness behaviors of reaction-bonded SiC ceramics with 12 and 26 vol.% of free Si were investigated. The flexural strength and fracture toughness started to increase at 1000 8C and reached a maximum at 13008 and 1330 8C before a sharp drop, respectively. The transition from transgranular to intergranular fracture is considered to lead to the slight increase of strength and toughness from room temperature to 1000 8C, while the plastic deformation of free Si contributes to the great increase above 1000 8C. However, too high a temperature will result in the extreme softening of free Si and therefore decrease strength and toughness. D 2005 Elsevier B.V. All rights reserved. Keywords: Reaction-bonded SiC ceramics; High-temperature strength and toughness
1. Introduction Silicon carbide (SiC) is one of those promising structural and electrical materials for high temperature applications in the aeronautics, energy and transportation industries because of its excellent mechanical properties and resistance to high temperature corrosion [1,2]. Reaction-bonded SiC ceramics have many attractive properties for a number of these applications [3–14]. They have low synthesizing temperature, short synthesizing time and minimal dimension change after the densification process (near-net shaping) in comparison with pressure-less sintered SiC and hot-pressed sintered SiC. Moreover, fine, high purity SiC is not required for the fabrication of reaction-bonded SiC ceramics, which results in a significant reduction in cost. The microstructure and high temperature mechanical behaviors of reaction-bonded SiC ceramics have been studied, but a lot of investigations are concentrated on the
T Corresponding author. E-mail address: [email protected] (Q.-W. Huang). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.01.049
creep behaviors. Results show that the creep rate of reaction-bonded SiC ceramics depends on temperature, free silicon content and stress, and the controlling creep mechanism was determined to be dislocation glide/climb and the interaction between SiC grains [3–5,10]. Limited investigation [7] on high-temperature strength and toughness shows that flexural strength and fracture toughness of reaction-bonded SiC ceramics tend to increase and then decrease with increasing temperature, but the strengthening and toughening mechanisms are not clear. In this work, high-temperature strength and toughness of reaction-bonded SiC ceramics with 12 and 26 vol.% free Si (the remanent was SiC) below 1400 8C were investigated and the strengthening and toughening mechanisms were also proposed.
2. Experimental Commercially available SiC (98.20 wt.% SiC, 0.25 wt.% free C, 0.50 wt.% SiO2, 0.04 wt.% Fe2O3, grain size: 42 Am, Xinhui SiC, P.R. China), petroleum coke (Chemical Reagent,
Q.-W. Huang, L.-H. Zhu / Materials Letters 59 (2005) 1732–1735
3. Results and discussion Fig. 1 shows the effect of temperature on flexural strength of reaction-bonded SiC ceramics with 12 and 26 vol.% free Si. The flexural strength of reaction-bonded SiC ceramics hardly changes from room temperature to 800 8C, then starts to increase at 1000 8C and reaches a maximum at 1300 8C. Compared with room-temperature strength, the samples at 1300 8C with 12 and 26 vol.% free silicon
400
Flexural strength σf / MPa
P.R. China), free silicon (99.41 wt.% Si, 0.22 wt.% Fe, 0.08 wt.% Al, 0.004 wt.% Ca, 0.2 wt.% Mg, 0.09 wt.% Na+K, Chemical Reagent, P.R. China), and phenolic resin (Chemical Reagent, P.R. China) were chosen as the starting materials. SiC, petroleum coke and phenol resin according to the weight ratio of 1:0:0.1 and 1:0.15:0.1 were mixed and then pressed into plates with dimensions of 30306 mm at 150 MPa. After being hardened at 110 8C, debonded and carbonized at 1050 8C, the compacts were in contact with liquid silicon in a graphite resistance furnace at 1600 8C in a vacuum of 0.65 Pa for 30 min. After reaction bonding, the phases of sintered bodies were examined with an X-ray diffractometer (XRD, Rigaku, Japan). Results show that the sintered bodies are composed of Si and SiC, and no carbon remained. The microstructure was examined by optical microscope and the volume fraction of free Si was determined by point count method. Results show that there are 12 and 26 vol.% free Si remaining in the sintered bodies when the weight ratio of SiC/petroleum coke/phenol resin is 1:0.15:0.1 and 1:0:0.1, respectively. The density of sintered body was measured by the Archimedes principle. It is 3.11 g/cm3 and 2.98 g/cm3, respectively, for 12 and 26 vol.% free Si. The sintered bodies were cut into parallelepipeds with dimensions of 3064 mm with a diamond saw, and mechanically polished with diamond pastes after cutting, finishing with a 1 Am. For the toughness test, especially, a notch of 1.5F0.1 mm in the direction of thickness was sectioned in every specimen. Strength and toughness tests were made using four-point bending (Model 402, Instron, USA), inner span of 20 mm and outer span of 40 mm, at 0.5 and 0.05 mm min 1 cross-head speed, respectively. 8 samples at each temperature were tested and the flexural strength and fracture toughness are averaged. In order to study the effect of oxidation on high temperature strength, a group of samples with 12 vol.% free Si was oxidized at 1300 8C for 15 min, 2 h and 6 h before tested at 1100 8C. In addition, flexural strength of samples with 12% free Si was measured in argon atmosphere at 1100, 1200 and 1300 8C for the convenience of comparison. The fractured surfaces of samples failed at room temperature, 800, 1000 and 1300 8C were examined by scanning electron microscope (Model S-570, Hitachi, Tokyo, Japan).
1733
300
12 vol% Si 26 vol% Si
200
100
0
0
300
600
900
1200
1500
Temperature T/ºC Fig. 1. Variation of flexural strength of reaction-bonded SiC ceramics with temperature.
show a 15 and 11% increase, respectively. Then the flexural strength of reaction-bonded SiC ceramics tends to decrease sharply. The flexural strength of sample at 1370 8C is much lower than room temperature. The variation trend of fracture toughness with temperature is similar to flexural strength except that the maximum fracture toughness occurs at 1330 8C, as shown in Fig. 2. For the sample with 12 vol.% free Si, the fracture toughness at 1330 8C is 4.8 MPa m1/2 and increases by 114% compared with room temperature, while for the sample with 26 vol.% free Si, it is 4.9 MPa m1/2 and increases by 158%. The fractured surface of sample with 12 vol.% free Si failed at room temperature is shown in Fig. 3(a), in which gray or black areas are SiC and white areas are free Si. Transgranular cleavage facets have been observed and fracture morphology is smooth and planar. The sample at 800 8C still fails in the form of transgranular fracture. Though the fractured surfaces at 1000 and 1300 8C are covered with oxide film, SiC grains can be clearly observed, as well as the holes due to the pullout of SiC particles (see Fig. 3(b) and (c)). Thus they belong to intergranular failure. The change in fracture mode from transgranular at room temperature to intergranular at elevated temperatures may be due to the change of interface bonding strength between Si and SiC grains. In this paper, commercially available SiC and silicon were used, and there are some impurity elements. The investigations by Ness and Page [15] show that a layer of amorphous phase containing some impurity elements such as K, Ca, Al, Fe et al. forms at the interface of Si/SiC. At high temperatures, this amorphous film may soften and even melt due to the presence of these impurity elements, resulting in the decrease of Si/SiC interface bonding strength. At this time, cracks tend to propagate along the interface of Si/SiC. When test temperature increases from room temperature to 1000 8C, the samples with 12 vol.% free Si show a 4% and 26% increase for strength and toughness while the samples with 26 vol.% free Si show a 1% and 35% increase for strength and toughness. Obviously, the change in fracture mode from
1734
Q.-W. Huang, L.-H. Zhu / Materials Letters 59 (2005) 1732–1735 6 5
Flexural strength σf / MPa
Fracture toughness K IC / MPa.m
1/2
400
12 vol% Si 26 vol% Si
4 3 2 1 0
0
300
600
900
1200
1500
300
200
100
0 0
Temperature T/ºC
(a) SiC cleavage facets
SiC SiC
20µm (b)
SiC
holes
4
6
Time t/h
Fig. 2. Variation of fracture toughness of reaction-bonded SiC ceramics with temperature.
Si
2
20µm (c)
SiC
SiC holes
20µm Fig. 3. Fractured surfaces of samples with 12 vol.% free silicon failed at (a) room temperature, (b) 1000 8C and (c) 1300 8C.
Fig. 4. The flexural strength of samples with 12 vol.% free silicon at 1100 8C as a function of oxidation time (samples are oxidized at 1300 8C).
transgranular to intergranular favors the increase of strength and toughness owing to the increased crack propagation distance. However, because Si/SiC interface bonding energy tends to decrease with increasing temperature due to the excessive softening of amorphous film, the change in fracture mode cannot be responsible for the increase of strength and toughness above 1000 8C. Oxidation at elevated temperatures may also contribute to the increase of flexural strength and fracture toughness of reaction-bonded SiC ceramics. The investigations by Kim et al. [16] and Huang and Jin [17] show that short periods of oxidation slightly increase the room-temperature flexural strength due to the formation of oxide scale on the surface of specimens. Chu et al. [18] found that the cracks in reactionbonded SiC ceramics can be healed by being filled with amorphous silica produced by the oxidation of silicon and silicon carbide. Fig. 4 shows the effect of oxidation time on the flexural strength of sample with 12 vol.% free Si at 1100 8C and is in accordance with the above-mentioned investigations, that is the longer oxidation time is, the higher flexural strength is obtained. For example, when holding time is prolonged from 0.25 h to 6 h, flexural strength increases from 322 to 345 MPa. However in this paper high temperature strength and toughness test are carried out in a short time, therefore oxidation has a minor effect on the increase of strength and toughness. In order to further prove this, flexural strength of samples with 12 vol.% free Si at elevated temperatures in air and argon atmosphere is compared and listed in Table 1. It can be seen that when tested in air and argon atmosphere, almost the same strength was obtained at the same temperature, indicating that oxidation should not contribute to the increase of strength and toughness above 1000 8C. Table 1 Flexural strength of samples with 12 vol.% free Si at elevated temperatures in air and argon atmosphere Temperature/8C Flexural strength/MPa
in air in argon
1100
1200
1300
322 325
338 336
355 357
Q.-W. Huang, L.-H. Zhu / Materials Letters 59 (2005) 1732–1735
The plastic deformation of free Si plays an important role in the increase of flexural strength and fracture toughness above 1000 8C. In general, plastic deformation of SiC does not take place until 1600 8C [19]. But free Si will begin to deform by dislocation glide and climb at high temperatures above 1100 8C [20]. The plastic deformation of free Si hinders crack propagation, leading to the increase of flexural strength and fracture toughness of reaction-bonded SiC ceramics. The investigation by Chakrabarti and Das [21] shows that the ability of free Si deformation increases with temperature. Therefore, flexural strength and fracture toughness of Si/SiC ceramics increase with temperature. However, too high a temperature make it difficult to resist crack propagation due to the softening of free Si, resulting in the decrease of flexural strength and fracture toughness of reaction-bonded SiC (see Fig. 1). In addition, the difference in loading rate between strength and toughness test may lead to different distortion behaviors of free Si and therefore the difference in strength–temperature and toughness–temperature behaviors (see Figs. 1 and 2).
Acknowledgement This research is supported by the National Natural Science Foundation of China (50202015 and 50101004).
References [1] [2] [3] [4] [5] [6] [7] [8]
[9] [10] [11]
4. Conclusion The flexural strength of reaction-bonded SiC ceramics starts to increase at 1000 8C, then reaches a maximum at 1300 8C. The variation trend of fracture toughness with temperature is similar to flexural strength except that the maximum fracture toughness occurs at 1330 8C. The transition from transgranular to intergranular fracture is considered to lead to the slight increase of strength and toughness from room temperature to 1000 8C, while the plastic deformation of free Si contributes to the great increase above 1000 8C. Above 1330 8C, the softening of free Si may result in the sharp decrease of strength and toughness.
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[12] [13]
[14] [15] [16] [17] [18] [19] [20] [21]
S.J. Dapkunas, Am. Ceram. Soc. Bull. 67 (1988) 388. H.B. Strock, Spect. Mater. Manufat. 35 (1992) 1. C.H. Carter, R.F. Davis, J. Bentley, J. Am. Ceram. Soc. 67 (1984) 409. S.M. Wiederhorn, D.E. Robert, T.J. Chuang, L. Chuck, J. Am. Ceram. Soc. 71 (1988) 602. B.J. Hockey, S.M. Wiederhorn, J. Am. Ceram. Soc. 75 (1992) 1822. M. Singh, D.R. Behrendt, J. Mater. Res. 9 (1994) 1701. O.P. Chakranarti, S. Ghosh, J. Mukerji, Ceram. Int. 20 (1994) 283. Y. Chiang, P. Messnar, C.D. Terwilliger, Reaction-formed silicon carbide, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 195 (1995) 131. A. Mun˜oz, J. Martı´nez Ferna´dez, A. Domı´nguez Rodrı´guez, M.J. Singh, J. Eur. Ceram. Soc. 18 (1998) 65. S.M. Wiederhorn, B.J. Hockey, J.D. Freach, J. Eur. Ceram. Soc. 19 (1999) 2273. J. Martı´nez Ferna´dez, F.M. Valera Feria, M. Singh, Scr. Mater. 43 (2000) 813. J. Martı´nez Ferna´dez, F.M. Valera Feria, A.R. de Arellano Lo´pez, M. Singh, J. Eur. Ceram. Soc. 22 (2002) 2719. U. Paik, H.C. Park, S.C. Choi, C.G. Ha, J.W. Kim, Y.G. Jung, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 334 (2002) 267. J. Martı´nez Ferna´dez, F.M. Valera Feria, A.R. de Arellano Lo´pez, F.M. Valera Feria, M. Singh, Acta Mater. 51 (2003) 3259. J.N. Ness, T.F. Page, J. Mater. Sci. 21 (1986) 1377. H.W. Kim, H.E. Kim, H. Song, J. Ha, J. Am. Ceram. Soc. 82 (1999) 1601. Q.W. Huang, Z.H. Jin, J. Mater. Process. Technol. 110 (2001) 142. M.C. Chu, S.J. Cho, H.M. Park, K.J. Yoon, H. Ryu, Mater. Lett. 58 (2004) 1313. C.H. Carter, R.E. Davis, J. Am. Ceram. Soc. 67 (1984) 732. G.L. Pearson, W.T. Read, W.L. Feldman, Acta Metall. 5 (1957) 191. O.P. Chakrabarti, P.K. Das, Ceram. Int. 27 (2001) 559 – 563.
High-temperature strength and toughness behaviors for reaction-bonded SiC ceramics below 1400 8C Qing-Wei Huanga,T, Li-Hui Zhub a
State Key Laboratory of High Performance and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China b School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China Received 4 March 2004; received in revised form 17 January 2005; accepted 24 January 2005 Available online 8 March 2005
Abstract High-temperature strength and toughness behaviors of reaction-bonded SiC ceramics with 12 and 26 vol.% of free Si were investigated. The flexural strength and fracture toughness started to increase at 1000 8C and reached a maximum at 13008 and 1330 8C before a sharp drop, respectively. The transition from transgranular to intergranular fracture is considered to lead to the slight increase of strength and toughness from room temperature to 1000 8C, while the plastic deformation of free Si contributes to the great increase above 1000 8C. However, too high a temperature will result in the extreme softening of free Si and therefore decrease strength and toughness. D 2005 Elsevier B.V. All rights reserved. Keywords: Reaction-bonded SiC ceramics; High-temperature strength and toughness
1. Introduction Silicon carbide (SiC) is one of those promising structural and electrical materials for high temperature applications in the aeronautics, energy and transportation industries because of its excellent mechanical properties and resistance to high temperature corrosion [1,2]. Reaction-bonded SiC ceramics have many attractive properties for a number of these applications [3–14]. They have low synthesizing temperature, short synthesizing time and minimal dimension change after the densification process (near-net shaping) in comparison with pressure-less sintered SiC and hot-pressed sintered SiC. Moreover, fine, high purity SiC is not required for the fabrication of reaction-bonded SiC ceramics, which results in a significant reduction in cost. The microstructure and high temperature mechanical behaviors of reaction-bonded SiC ceramics have been studied, but a lot of investigations are concentrated on the
T Corresponding author. E-mail address: [email protected] (Q.-W. Huang). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.01.049
creep behaviors. Results show that the creep rate of reaction-bonded SiC ceramics depends on temperature, free silicon content and stress, and the controlling creep mechanism was determined to be dislocation glide/climb and the interaction between SiC grains [3–5,10]. Limited investigation [7] on high-temperature strength and toughness shows that flexural strength and fracture toughness of reaction-bonded SiC ceramics tend to increase and then decrease with increasing temperature, but the strengthening and toughening mechanisms are not clear. In this work, high-temperature strength and toughness of reaction-bonded SiC ceramics with 12 and 26 vol.% free Si (the remanent was SiC) below 1400 8C were investigated and the strengthening and toughening mechanisms were also proposed.
2. Experimental Commercially available SiC (98.20 wt.% SiC, 0.25 wt.% free C, 0.50 wt.% SiO2, 0.04 wt.% Fe2O3, grain size: 42 Am, Xinhui SiC, P.R. China), petroleum coke (Chemical Reagent,
Q.-W. Huang, L.-H. Zhu / Materials Letters 59 (2005) 1732–1735
3. Results and discussion Fig. 1 shows the effect of temperature on flexural strength of reaction-bonded SiC ceramics with 12 and 26 vol.% free Si. The flexural strength of reaction-bonded SiC ceramics hardly changes from room temperature to 800 8C, then starts to increase at 1000 8C and reaches a maximum at 1300 8C. Compared with room-temperature strength, the samples at 1300 8C with 12 and 26 vol.% free silicon
400
Flexural strength σf / MPa
P.R. China), free silicon (99.41 wt.% Si, 0.22 wt.% Fe, 0.08 wt.% Al, 0.004 wt.% Ca, 0.2 wt.% Mg, 0.09 wt.% Na+K, Chemical Reagent, P.R. China), and phenolic resin (Chemical Reagent, P.R. China) were chosen as the starting materials. SiC, petroleum coke and phenol resin according to the weight ratio of 1:0:0.1 and 1:0.15:0.1 were mixed and then pressed into plates with dimensions of 30306 mm at 150 MPa. After being hardened at 110 8C, debonded and carbonized at 1050 8C, the compacts were in contact with liquid silicon in a graphite resistance furnace at 1600 8C in a vacuum of 0.65 Pa for 30 min. After reaction bonding, the phases of sintered bodies were examined with an X-ray diffractometer (XRD, Rigaku, Japan). Results show that the sintered bodies are composed of Si and SiC, and no carbon remained. The microstructure was examined by optical microscope and the volume fraction of free Si was determined by point count method. Results show that there are 12 and 26 vol.% free Si remaining in the sintered bodies when the weight ratio of SiC/petroleum coke/phenol resin is 1:0.15:0.1 and 1:0:0.1, respectively. The density of sintered body was measured by the Archimedes principle. It is 3.11 g/cm3 and 2.98 g/cm3, respectively, for 12 and 26 vol.% free Si. The sintered bodies were cut into parallelepipeds with dimensions of 3064 mm with a diamond saw, and mechanically polished with diamond pastes after cutting, finishing with a 1 Am. For the toughness test, especially, a notch of 1.5F0.1 mm in the direction of thickness was sectioned in every specimen. Strength and toughness tests were made using four-point bending (Model 402, Instron, USA), inner span of 20 mm and outer span of 40 mm, at 0.5 and 0.05 mm min 1 cross-head speed, respectively. 8 samples at each temperature were tested and the flexural strength and fracture toughness are averaged. In order to study the effect of oxidation on high temperature strength, a group of samples with 12 vol.% free Si was oxidized at 1300 8C for 15 min, 2 h and 6 h before tested at 1100 8C. In addition, flexural strength of samples with 12% free Si was measured in argon atmosphere at 1100, 1200 and 1300 8C for the convenience of comparison. The fractured surfaces of samples failed at room temperature, 800, 1000 and 1300 8C were examined by scanning electron microscope (Model S-570, Hitachi, Tokyo, Japan).
1733
300
12 vol% Si 26 vol% Si
200
100
0
0
300
600
900
1200
1500
Temperature T/ºC Fig. 1. Variation of flexural strength of reaction-bonded SiC ceramics with temperature.
show a 15 and 11% increase, respectively. Then the flexural strength of reaction-bonded SiC ceramics tends to decrease sharply. The flexural strength of sample at 1370 8C is much lower than room temperature. The variation trend of fracture toughness with temperature is similar to flexural strength except that the maximum fracture toughness occurs at 1330 8C, as shown in Fig. 2. For the sample with 12 vol.% free Si, the fracture toughness at 1330 8C is 4.8 MPa m1/2 and increases by 114% compared with room temperature, while for the sample with 26 vol.% free Si, it is 4.9 MPa m1/2 and increases by 158%. The fractured surface of sample with 12 vol.% free Si failed at room temperature is shown in Fig. 3(a), in which gray or black areas are SiC and white areas are free Si. Transgranular cleavage facets have been observed and fracture morphology is smooth and planar. The sample at 800 8C still fails in the form of transgranular fracture. Though the fractured surfaces at 1000 and 1300 8C are covered with oxide film, SiC grains can be clearly observed, as well as the holes due to the pullout of SiC particles (see Fig. 3(b) and (c)). Thus they belong to intergranular failure. The change in fracture mode from transgranular at room temperature to intergranular at elevated temperatures may be due to the change of interface bonding strength between Si and SiC grains. In this paper, commercially available SiC and silicon were used, and there are some impurity elements. The investigations by Ness and Page [15] show that a layer of amorphous phase containing some impurity elements such as K, Ca, Al, Fe et al. forms at the interface of Si/SiC. At high temperatures, this amorphous film may soften and even melt due to the presence of these impurity elements, resulting in the decrease of Si/SiC interface bonding strength. At this time, cracks tend to propagate along the interface of Si/SiC. When test temperature increases from room temperature to 1000 8C, the samples with 12 vol.% free Si show a 4% and 26% increase for strength and toughness while the samples with 26 vol.% free Si show a 1% and 35% increase for strength and toughness. Obviously, the change in fracture mode from
1734
Q.-W. Huang, L.-H. Zhu / Materials Letters 59 (2005) 1732–1735 6 5
Flexural strength σf / MPa
Fracture toughness K IC / MPa.m
1/2
400
12 vol% Si 26 vol% Si
4 3 2 1 0
0
300
600
900
1200
1500
300
200
100
0 0
Temperature T/ºC
(a) SiC cleavage facets
SiC SiC
20µm (b)
SiC
holes
4
6
Time t/h
Fig. 2. Variation of fracture toughness of reaction-bonded SiC ceramics with temperature.
Si
2
20µm (c)
SiC
SiC holes
20µm Fig. 3. Fractured surfaces of samples with 12 vol.% free silicon failed at (a) room temperature, (b) 1000 8C and (c) 1300 8C.
Fig. 4. The flexural strength of samples with 12 vol.% free silicon at 1100 8C as a function of oxidation time (samples are oxidized at 1300 8C).
transgranular to intergranular favors the increase of strength and toughness owing to the increased crack propagation distance. However, because Si/SiC interface bonding energy tends to decrease with increasing temperature due to the excessive softening of amorphous film, the change in fracture mode cannot be responsible for the increase of strength and toughness above 1000 8C. Oxidation at elevated temperatures may also contribute to the increase of flexural strength and fracture toughness of reaction-bonded SiC ceramics. The investigations by Kim et al. [16] and Huang and Jin [17] show that short periods of oxidation slightly increase the room-temperature flexural strength due to the formation of oxide scale on the surface of specimens. Chu et al. [18] found that the cracks in reactionbonded SiC ceramics can be healed by being filled with amorphous silica produced by the oxidation of silicon and silicon carbide. Fig. 4 shows the effect of oxidation time on the flexural strength of sample with 12 vol.% free Si at 1100 8C and is in accordance with the above-mentioned investigations, that is the longer oxidation time is, the higher flexural strength is obtained. For example, when holding time is prolonged from 0.25 h to 6 h, flexural strength increases from 322 to 345 MPa. However in this paper high temperature strength and toughness test are carried out in a short time, therefore oxidation has a minor effect on the increase of strength and toughness. In order to further prove this, flexural strength of samples with 12 vol.% free Si at elevated temperatures in air and argon atmosphere is compared and listed in Table 1. It can be seen that when tested in air and argon atmosphere, almost the same strength was obtained at the same temperature, indicating that oxidation should not contribute to the increase of strength and toughness above 1000 8C. Table 1 Flexural strength of samples with 12 vol.% free Si at elevated temperatures in air and argon atmosphere Temperature/8C Flexural strength/MPa
in air in argon
1100
1200
1300
322 325
338 336
355 357
Q.-W. Huang, L.-H. Zhu / Materials Letters 59 (2005) 1732–1735
The plastic deformation of free Si plays an important role in the increase of flexural strength and fracture toughness above 1000 8C. In general, plastic deformation of SiC does not take place until 1600 8C [19]. But free Si will begin to deform by dislocation glide and climb at high temperatures above 1100 8C [20]. The plastic deformation of free Si hinders crack propagation, leading to the increase of flexural strength and fracture toughness of reaction-bonded SiC ceramics. The investigation by Chakrabarti and Das [21] shows that the ability of free Si deformation increases with temperature. Therefore, flexural strength and fracture toughness of Si/SiC ceramics increase with temperature. However, too high a temperature make it difficult to resist crack propagation due to the softening of free Si, resulting in the decrease of flexural strength and fracture toughness of reaction-bonded SiC (see Fig. 1). In addition, the difference in loading rate between strength and toughness test may lead to different distortion behaviors of free Si and therefore the difference in strength–temperature and toughness–temperature behaviors (see Figs. 1 and 2).
Acknowledgement This research is supported by the National Natural Science Foundation of China (50202015 and 50101004).
References [1] [2] [3] [4] [5] [6] [7] [8]
[9] [10] [11]
4. Conclusion The flexural strength of reaction-bonded SiC ceramics starts to increase at 1000 8C, then reaches a maximum at 1300 8C. The variation trend of fracture toughness with temperature is similar to flexural strength except that the maximum fracture toughness occurs at 1330 8C. The transition from transgranular to intergranular fracture is considered to lead to the slight increase of strength and toughness from room temperature to 1000 8C, while the plastic deformation of free Si contributes to the great increase above 1000 8C. Above 1330 8C, the softening of free Si may result in the sharp decrease of strength and toughness.
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[12] [13]
[14] [15] [16] [17] [18] [19] [20] [21]
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