- Email: [email protected]
SiC composites
Materials Science & Engineering A 559 (2013) 808–811
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of heat treatment on microstructure and mechanical properties of PIP-SiC/SiC composites Shuang Zhao a,b,n, Xingui Zhou a, Jinshan Yu a, Paul Mummery b a Key Laboratory of Advanced Ceramic Fibres and Composites, College of Aerospace and Materials Engineering, National University of Defense Technology, Changsha 410073, PR China b School of Mechanical, Aerospace, and Civil Engineering, University of Manchester, Manchester M13 9PL, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 July 2012 Received in revised form 4 September 2012 Accepted 7 September 2012 Available online 13 September 2012
Continuous SiC fibre reinforced SiC matrix composites (SiC/SiC) have been studied as materials for heat resistant and nuclear applications. Thermal stability is one of the key issues for SiC/SiC composites. In this study, 3D SiC/SiC composites are fabricated via the polymer impregnation and pyrolysis (PIP) process, and then heat treated at 1400 1C, 1600 1C and 1800 1C in an inert atmosphere for 1 h, respectively. The effect of heat treatment on microstructure and mechanical properties of the composites is investigated. The results indicate that the mechanical properties of the SiC/SiC composites are significantly improved after heat treatment at 1400 1C mainly because the mechanical properties of the matrix are greatly improved due to crystallisation. With the increasing of heat treatment temperature, the properties of the composites are conversely decreased because of severe damage of the fibres and the matrix. & 2012 Elsevier B.V. All rights reserved.
Keywords: SiC/SiC composites Polymer impregnation and pyrolysis Heat treatment Mechanical properties
1. Introduction Continuous SiC fibre reinforced SiC matrix composites (SiC/SiC) have been developed for high temperature applications such as gas turbines because of their high strength at elevated temperatures, general chemical inertness, low specific mass, and low coefficient of thermal expansion [1–5]. SiC/SiC composites are also considered promising candidate materials for fusion and advanced fission energy applications due to their pseudo-ductile fracture mode, excellent irradiation performance, and low tritium permeability while taking advantage of most of the inherent merits as a heat-resistant material [6–10]. Since their target applications required in various environments under elevated temperature conditions, thermal stability of the microstructure and mechanical performances of SiC/SiC composites has been studied. SiC/SiC composites were prepared via chemical vapour infiltration (CVI) process by Araki [11], where decrease of strength and fracture toughness after high temperature heat treatment in vacuum were observed. Udayakumar [12] investigated the improvement of mechanical properties of SiC/SiC composites with BN-interphase prepared by CVI process after intermediate heat treatment. The study of the effects of heat
n Corresponding author at: School of Mechanical, Aerospace and Civil Engineering, University of Manchester, Manchester M13 9PL, UK. Tel.: þ44 07774242674; fax: þ 44 01613063686. E-mail address: [email protected] (S. Zhao).
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.09.027
treatment on Tyranno-SA/SiC composite demonstrated that with advanced fibres, CVI is an appropriate process to produce SiC/SiC composites with excellent thermal stability [13]. Polymer impregnation and pyrolysis (PIP) is a conventional technique for fabricating SiC/SiC composites. The major advantages of PIP lie in the technical applicability of making large-scale components with complicated shape, and the drastic reduction in fabrication cost. However, thermal stability becomes a key issue for PIP-SiC/SiC composites for their potential high temperature applications owing to their amorphous structure [14–17]. In this study, three-dimensional SiC/SiC composites are fabricated via PIP process, and the effect of heat treatment (1400 1C–1800 1C) on the microstructure and mechanical properties of the composites are investigated.
2. Experimental procedure 2.1. Fabrication of the composites KD-I SiC fibre bundles provided by National University of Defense Technology, China were used as the reinforcement. General characteristics of the fibre were described elsewhere [18]. Polycarbosilane (PCS) was used as the polymer precursor of the SiC matrix for the PIP process, and xylene was used as a solvent for PCS. SiC fibre bundles were woven into 3D (3-dimensional, 4-directional) preforms and coated with a 1 mm thick pyrolytic carbon
S. Zhao et al. / Materials Science & Engineering A 559 (2013) 808–811
(PyC) layer as interface to adjust bonding between the matrix and fibres. The coated preforms were then impregnated with PCS solution by a vacuum infiltration method and pyrolyzed at 1100 1C in an inert Argon atmosphere. The impregnation and pyrolysis process were repeated for more than 10 cycles until weight increase was less than 1%. Table 1 Mechanical properties of SiC/SiC composites. Heat treatment temperature (1C)
Flexural strength (MPa)
As-fabricated at 1100 230.2 As-fabricated at 1200 354.5 1400 514.1 1600 55.3 1800 43.1
Flexural modulus (GPa)
Fracture toughness (MPa m1/2)
74.2 – 82.2 21.1 33.7
15.6 21.2 31.9 9.0 1.7
809
Heat treatments in an inert atmosphere for 1 h at temperature ranging from 1400 1C to 1800 1C were carried out in a Hi-Multi 5000 sinter stove.
2.2. Characterization evaluation Flexural strength and modulus of the composites were characterized by four-point bending. Fracture toughness was measured by single edge notch beam (SENB) method, using an Instron 5569 Dual Column Testing Systems. Nano-indentation tests were performed using MTS nano-indenter XP at the Stress and Damage Characterization Unit in the University of Manchester. Morphology of the specimens was analyzed by scanning electron microscopy (SEM) using Philips XL30 FEG SEM. The structural changes were examined with X-ray diffraction analysis using a Philips X’pert MPD diffractometer.
Fig. 1. Load–displacement curves of SiC/SiC composites at various heat treatment temperatures.
Fig. 2. Fracture morphology of SiC/SiC composites at various heat treatment temperatures: (a) as-fabricated, (b) 1400 1C, (c) 1600 1C and (d) 1800 1C.
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S. Zhao et al. / Materials Science & Engineering A 559 (2013) 808–811
3. Results and discussion The mechanical properties of SiC/SiC composites after various temperature heat treatments are listed in Table 1. The conventional pyrolysis temperature of PIP process was 1200 1C and SiC/SiC composites with the same fibre, precursor and interface as in this work were fabricated [19,20], their properties are also shown in Table 1. The flexural strength of as-fabricated SiC/SiC composites which is pyrolyzed at 1100 1C is only 230.2 MPa, less than those fabricated at 1200 1C. However, both flexural strength and fracture toughness of the composites increase significantly to 514.1 MPa and 31.9 MPa m1/2 after heat treatment at 1400 1C, beyond which the strength and toughness decrease to a rather low level at temperatures of 1600 1C and 1800 1C. The load/displacement curves recorded during the four-point bending tests are shown in Fig. 1. SiC/SiC composites of the asfabricated and heat treatment at 1400 1C and 1600 1C exhibit nonbrittle fracture behaviour, where extended regions can be seen after the initial failure. However, the composites heat treatment at 1800 1C turn out to be brittle, resulting in the sudden fall of load. Fracture behaviour of the SiC/SiC composites are also indicated by SEM micrographs in Fig. 2. Large quantities of long pulled-out fibres can be seen on the fracture surfaces of the composites in the as-fabricated and 1400 1C and 1600 1C heat treatment conditions. This is typical of crack arresting, deflecting and branching behaviour which leads to the pseudo-ductile fracture mode of the composites. The fracture surface of the composites heat treatment at 1800 1C is flat with little gibbous planes, illustrating the fast propagation of cracks through the specimens with no crack deflection or arrest at the fibres.
Table 2 Modulus and hardness of fibres and matrices of SiC/SiC composites. Heat treatment temperature (1C)
Modulus (GPa)
Hardness (GPa)
Fibre As-fabricated 1400 1600 1800
106.40 115.06 83.71 45.21
Modulus (GPa)
Hardness (GPa)
Matrix 12.03 14.12 5.50 1.31
104.38 187.65 108.71 104.56
11.37 20.03 6.15 7.58
The modulus and microhardness of the fibres and matrices of the composites measured by nano-indentation tests are listed in Table 2. The changes in properties of the fibres and the matrices show different trends with the increase of heat treatment temperature. The modulus and hardness of the matrices are increased remarkably after heat treatment at 1400 1C, and then reduced to a low level after heat treatment at 1600 1C and 1800 1C. The SiC matrix pyrolyzed at 1100 1C is mostly amorphous with a small amount of microcrystalline b-SiC and residual carbon. The matrix gets more crystalline with elevation of heat treatment temperature, whereas its mechanical properties are degraded due to the decomposition reactions and carbothermic reductions when the temperature exceeds 1500 1C, the following reactions occur [21]: SiC1 þ x-SiC þxC
(1)
SiC1 þ x þ trace O-SiCþCO
(2)
The modulus and hardness of the fibres change slightly after heat treatment at 1400 1C, but decrease greatly with increasing temperature above 1600 1C. The KD-1 SiC fibres used for this study are multiphase fibres consisted of crystalline b-SiC, amorphous Si–C–O and free carbon phase [22]. Deoxidation after heat treatment above 1600 1C results in the degradation of properties of the SiC composites. In addition to reactions (1) and (2) above, following reactions are proposed to take place: SiCxOy-SiCþSiO(g)
(3)
SiCxOy-SiCþCO
(4)
The microstructural changes of the matrices are confirmed by X-ray diffraction analysis (shown in Fig. 3). The as-fabricated SiC matrix is almost amorphous, and the crystallisation process begins when the heat treatment temperature is above 1400 1C. Phase transformation from b-SiC to a-SiC takes place when the temperature reaches 1800 1C. The effect of heat treatment on microstructure and mechanical properties of the SiC/SiC composites has been determined. The modulus of the matrix is close to that of the fibres of as-fabricated composites, so the matrix carries equivalent load to the fibres on straining. When the load reaches point A in Fig. 1, cracks initiate in the matrix leading to the decrease in the modulus of the composites, meanwhile the fibres bridge the crack and the load continues to ascend until fracture of the fibres. After heat treatment at 1400 1C, the mechanical properties of the fibres remain at a similar level while those of the matrix are greatly
Fig. 3. XRD patterns of the matrices with various heat treatment temperatures.
S. Zhao et al. / Materials Science & Engineering A 559 (2013) 808–811
improved due to crystallisation. Thus the cracks initiate at point B in Fig. 1, and the strength and toughness of the composites become much higher. After heat treatment at 1600 1C, both the matrix and the fibres are severely damaged, leading to the degradation of the mechanical properties of the composites. After heat treatment above 1800 1C, the composites exhibit a brittle fracture mode because the fibres totally fail as reinforcements.
4. Conclusions 3D SiC/SiC composites are fabricated by the PIP process, and then heat treated to various temperatures. The effect of heat treatment on microstructures and mechanical properties is investigated. SiC/SiC composites fabricated at 1100 1C are mostly amorphous. They get more crystallised with elevation of heat treatment temperature. After heat treatment at 1400 1C, the mechanical properties of the composites are greatly improved; flexural strength and fracture toughness are 514.1 MPa and 31.9 MPa m1/2, respectively. When the heat treatment temperature exceeds 1600 1C, the mechanical properties of the composites are degraded drastically with increasing temperature. The SiC/SiC composites all exhibit pseudo-ductile fracture behaviour except those heat treated at 1800 1C.
Acknowledgements Financial supports from the Ministry of Education of China under the New Century Excellent Talent in University (NCET-07– 0228) and from the China Scholarship Council (CSC) are gratefully acknowledged.
811
References [1] H. Ohnabe, S. Masaki, M. Onozuka, K. Miyahara, T. Sasa, Compos. Part A 30 (1999) 489–496. [2] J. Kimmel, N. Miriyala, J. Price, K. More, P. Tortorelli, H. Eaton, G. Linsey, E. Sun, J. Eur. Ceram. Soc. 22 (2002) 2769–2775. [3] R. Naslain, Compos. Sci. Technol. 64 (2004) 155–170. [4] R.R. Naslain, Int. J. Appl. Ceram. Technol. 2 (2005) 75–84. [5] Y. Katoh, L.L. Snead, C.H. Henager, A. Hasegawa, A. Kohyama, B. Riccardi, H. Hegeman, J. Nucl. Mater. 367–370 (2007) 659–671. [6] A. Hasegawa, A. Kohyama, R.H. Jones, L.L. Snead, B. Riccardi, P. Fenici, J. Nucl. Mater. 283 (2000) 128–137. [7] R.H. Jones, L. Giancarli, A. Hasegawa, Y. Katoh, J. Nucl. Mater. 307–311 (2002) 1057–1072. [8] B. Riccardi, L. Giancarli, A. Hasegawa, Y. Katoh, A. Kohyama, R.H. Jones, L.L. Snead, J. Nucl. Mater. 329 (2004) 56–65. [9] T. Nozawa, T. Hinoki, A. Hasegawa, A. Kohyama, Y. Katoh, L.L. Snead, C.H. Henager, J.B.J. Hegeman, J. Nucl. Mater. 386–88 (2009) 622–627. [10] L.L. Snead, T. Nozawa, M. Ferraris, Y. Katoh, R. Shinavski, M. Sawan, J. Nucl. Mater. 417 (2011) 330–339. [11] H. Araki, H. Suzuki, W. Yang, S. Sato, T. Noda, J. Nucl. Mater. 258–263 (Part 2) (1998) 1540–1545. [12] A. Udayakumar, A. Sri Ganesh, S. Raja, M. Balasubramanian, J. Eur. Ceram. Soc. 31 (2011) 1145–1153. [13] W. Yang, H. Araki, A. Kohyama, Q. Yang, T. Noda, Ceram. Int. 33 (2007) 141–146. [14] A. Kohyama, M. Kotani, Y. Katoh, T. Nakayasu, M. Sato, T. Yamamura, K. Okamura, J. Nucl. Mater. 283–287 (2000) 565–569. [15] Y. Katoh, M. Kotani, H. Kishimoto, W. Yang, A. Kohyama, J. Nucl. Mater. 289 (2001) 42–47. [16] M. Kotani, A. Kohyama, Y. Katoh, J. Nucl. Mater. 289 (2001) 37–41. [17] C.A. Nannetti, B. Riccardi, A. Ortona, A. La Barbera, E. Scafe , G. Vekinis, J. Nucl. Mater. 307–311 (Part 2) (2002) 1196–1199. [18] Z.Y. Chu, C.X. Feng, Y.C. Song, J. Wang, Y.D. Wang, X.D. Li, Sci. Eng. Compos. Mater. 10 (2002) 131–139. [19] S. Zhao, X.G. Zhou, H.J. Yu, H.L. Wang, Y.C. Wu, Q.Y. Huang, Z.Q. Zhu, Z.L. Huang, Fusion Eng. Des. 85 (2010) 1624–1626. [20] H.J. Yu, X.G. Zhou, W. Zhang, H.X. Peng, C.R. Zhang, Z.L. Huang, Compos. Sci. Technol. 71 (2011) 699–704. [21] Y. Ma, S. Wang, Z.H. Chen, Mater. Sci. Eng. A. 528 (2011) 3069–3072. [22] Z. Chunman, Z. Bing, L. Xiaodong, W. Yifei, Acta Polym. Sinica. 1 (2004) 246–250.
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of heat treatment on microstructure and mechanical properties of PIP-SiC/SiC composites Shuang Zhao a,b,n, Xingui Zhou a, Jinshan Yu a, Paul Mummery b a Key Laboratory of Advanced Ceramic Fibres and Composites, College of Aerospace and Materials Engineering, National University of Defense Technology, Changsha 410073, PR China b School of Mechanical, Aerospace, and Civil Engineering, University of Manchester, Manchester M13 9PL, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 July 2012 Received in revised form 4 September 2012 Accepted 7 September 2012 Available online 13 September 2012
Continuous SiC fibre reinforced SiC matrix composites (SiC/SiC) have been studied as materials for heat resistant and nuclear applications. Thermal stability is one of the key issues for SiC/SiC composites. In this study, 3D SiC/SiC composites are fabricated via the polymer impregnation and pyrolysis (PIP) process, and then heat treated at 1400 1C, 1600 1C and 1800 1C in an inert atmosphere for 1 h, respectively. The effect of heat treatment on microstructure and mechanical properties of the composites is investigated. The results indicate that the mechanical properties of the SiC/SiC composites are significantly improved after heat treatment at 1400 1C mainly because the mechanical properties of the matrix are greatly improved due to crystallisation. With the increasing of heat treatment temperature, the properties of the composites are conversely decreased because of severe damage of the fibres and the matrix. & 2012 Elsevier B.V. All rights reserved.
Keywords: SiC/SiC composites Polymer impregnation and pyrolysis Heat treatment Mechanical properties
1. Introduction Continuous SiC fibre reinforced SiC matrix composites (SiC/SiC) have been developed for high temperature applications such as gas turbines because of their high strength at elevated temperatures, general chemical inertness, low specific mass, and low coefficient of thermal expansion [1–5]. SiC/SiC composites are also considered promising candidate materials for fusion and advanced fission energy applications due to their pseudo-ductile fracture mode, excellent irradiation performance, and low tritium permeability while taking advantage of most of the inherent merits as a heat-resistant material [6–10]. Since their target applications required in various environments under elevated temperature conditions, thermal stability of the microstructure and mechanical performances of SiC/SiC composites has been studied. SiC/SiC composites were prepared via chemical vapour infiltration (CVI) process by Araki [11], where decrease of strength and fracture toughness after high temperature heat treatment in vacuum were observed. Udayakumar [12] investigated the improvement of mechanical properties of SiC/SiC composites with BN-interphase prepared by CVI process after intermediate heat treatment. The study of the effects of heat
n Corresponding author at: School of Mechanical, Aerospace and Civil Engineering, University of Manchester, Manchester M13 9PL, UK. Tel.: þ44 07774242674; fax: þ 44 01613063686. E-mail address: [email protected] (S. Zhao).
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.09.027
treatment on Tyranno-SA/SiC composite demonstrated that with advanced fibres, CVI is an appropriate process to produce SiC/SiC composites with excellent thermal stability [13]. Polymer impregnation and pyrolysis (PIP) is a conventional technique for fabricating SiC/SiC composites. The major advantages of PIP lie in the technical applicability of making large-scale components with complicated shape, and the drastic reduction in fabrication cost. However, thermal stability becomes a key issue for PIP-SiC/SiC composites for their potential high temperature applications owing to their amorphous structure [14–17]. In this study, three-dimensional SiC/SiC composites are fabricated via PIP process, and the effect of heat treatment (1400 1C–1800 1C) on the microstructure and mechanical properties of the composites are investigated.
2. Experimental procedure 2.1. Fabrication of the composites KD-I SiC fibre bundles provided by National University of Defense Technology, China were used as the reinforcement. General characteristics of the fibre were described elsewhere [18]. Polycarbosilane (PCS) was used as the polymer precursor of the SiC matrix for the PIP process, and xylene was used as a solvent for PCS. SiC fibre bundles were woven into 3D (3-dimensional, 4-directional) preforms and coated with a 1 mm thick pyrolytic carbon
S. Zhao et al. / Materials Science & Engineering A 559 (2013) 808–811
(PyC) layer as interface to adjust bonding between the matrix and fibres. The coated preforms were then impregnated with PCS solution by a vacuum infiltration method and pyrolyzed at 1100 1C in an inert Argon atmosphere. The impregnation and pyrolysis process were repeated for more than 10 cycles until weight increase was less than 1%. Table 1 Mechanical properties of SiC/SiC composites. Heat treatment temperature (1C)
Flexural strength (MPa)
As-fabricated at 1100 230.2 As-fabricated at 1200 354.5 1400 514.1 1600 55.3 1800 43.1
Flexural modulus (GPa)
Fracture toughness (MPa m1/2)
74.2 – 82.2 21.1 33.7
15.6 21.2 31.9 9.0 1.7
809
Heat treatments in an inert atmosphere for 1 h at temperature ranging from 1400 1C to 1800 1C were carried out in a Hi-Multi 5000 sinter stove.
2.2. Characterization evaluation Flexural strength and modulus of the composites were characterized by four-point bending. Fracture toughness was measured by single edge notch beam (SENB) method, using an Instron 5569 Dual Column Testing Systems. Nano-indentation tests were performed using MTS nano-indenter XP at the Stress and Damage Characterization Unit in the University of Manchester. Morphology of the specimens was analyzed by scanning electron microscopy (SEM) using Philips XL30 FEG SEM. The structural changes were examined with X-ray diffraction analysis using a Philips X’pert MPD diffractometer.
Fig. 1. Load–displacement curves of SiC/SiC composites at various heat treatment temperatures.
Fig. 2. Fracture morphology of SiC/SiC composites at various heat treatment temperatures: (a) as-fabricated, (b) 1400 1C, (c) 1600 1C and (d) 1800 1C.
810
S. Zhao et al. / Materials Science & Engineering A 559 (2013) 808–811
3. Results and discussion The mechanical properties of SiC/SiC composites after various temperature heat treatments are listed in Table 1. The conventional pyrolysis temperature of PIP process was 1200 1C and SiC/SiC composites with the same fibre, precursor and interface as in this work were fabricated [19,20], their properties are also shown in Table 1. The flexural strength of as-fabricated SiC/SiC composites which is pyrolyzed at 1100 1C is only 230.2 MPa, less than those fabricated at 1200 1C. However, both flexural strength and fracture toughness of the composites increase significantly to 514.1 MPa and 31.9 MPa m1/2 after heat treatment at 1400 1C, beyond which the strength and toughness decrease to a rather low level at temperatures of 1600 1C and 1800 1C. The load/displacement curves recorded during the four-point bending tests are shown in Fig. 1. SiC/SiC composites of the asfabricated and heat treatment at 1400 1C and 1600 1C exhibit nonbrittle fracture behaviour, where extended regions can be seen after the initial failure. However, the composites heat treatment at 1800 1C turn out to be brittle, resulting in the sudden fall of load. Fracture behaviour of the SiC/SiC composites are also indicated by SEM micrographs in Fig. 2. Large quantities of long pulled-out fibres can be seen on the fracture surfaces of the composites in the as-fabricated and 1400 1C and 1600 1C heat treatment conditions. This is typical of crack arresting, deflecting and branching behaviour which leads to the pseudo-ductile fracture mode of the composites. The fracture surface of the composites heat treatment at 1800 1C is flat with little gibbous planes, illustrating the fast propagation of cracks through the specimens with no crack deflection or arrest at the fibres.
Table 2 Modulus and hardness of fibres and matrices of SiC/SiC composites. Heat treatment temperature (1C)
Modulus (GPa)
Hardness (GPa)
Fibre As-fabricated 1400 1600 1800
106.40 115.06 83.71 45.21
Modulus (GPa)
Hardness (GPa)
Matrix 12.03 14.12 5.50 1.31
104.38 187.65 108.71 104.56
11.37 20.03 6.15 7.58
The modulus and microhardness of the fibres and matrices of the composites measured by nano-indentation tests are listed in Table 2. The changes in properties of the fibres and the matrices show different trends with the increase of heat treatment temperature. The modulus and hardness of the matrices are increased remarkably after heat treatment at 1400 1C, and then reduced to a low level after heat treatment at 1600 1C and 1800 1C. The SiC matrix pyrolyzed at 1100 1C is mostly amorphous with a small amount of microcrystalline b-SiC and residual carbon. The matrix gets more crystalline with elevation of heat treatment temperature, whereas its mechanical properties are degraded due to the decomposition reactions and carbothermic reductions when the temperature exceeds 1500 1C, the following reactions occur [21]: SiC1 þ x-SiC þxC
(1)
SiC1 þ x þ trace O-SiCþCO
(2)
The modulus and hardness of the fibres change slightly after heat treatment at 1400 1C, but decrease greatly with increasing temperature above 1600 1C. The KD-1 SiC fibres used for this study are multiphase fibres consisted of crystalline b-SiC, amorphous Si–C–O and free carbon phase [22]. Deoxidation after heat treatment above 1600 1C results in the degradation of properties of the SiC composites. In addition to reactions (1) and (2) above, following reactions are proposed to take place: SiCxOy-SiCþSiO(g)
(3)
SiCxOy-SiCþCO
(4)
The microstructural changes of the matrices are confirmed by X-ray diffraction analysis (shown in Fig. 3). The as-fabricated SiC matrix is almost amorphous, and the crystallisation process begins when the heat treatment temperature is above 1400 1C. Phase transformation from b-SiC to a-SiC takes place when the temperature reaches 1800 1C. The effect of heat treatment on microstructure and mechanical properties of the SiC/SiC composites has been determined. The modulus of the matrix is close to that of the fibres of as-fabricated composites, so the matrix carries equivalent load to the fibres on straining. When the load reaches point A in Fig. 1, cracks initiate in the matrix leading to the decrease in the modulus of the composites, meanwhile the fibres bridge the crack and the load continues to ascend until fracture of the fibres. After heat treatment at 1400 1C, the mechanical properties of the fibres remain at a similar level while those of the matrix are greatly
Fig. 3. XRD patterns of the matrices with various heat treatment temperatures.
S. Zhao et al. / Materials Science & Engineering A 559 (2013) 808–811
improved due to crystallisation. Thus the cracks initiate at point B in Fig. 1, and the strength and toughness of the composites become much higher. After heat treatment at 1600 1C, both the matrix and the fibres are severely damaged, leading to the degradation of the mechanical properties of the composites. After heat treatment above 1800 1C, the composites exhibit a brittle fracture mode because the fibres totally fail as reinforcements.
4. Conclusions 3D SiC/SiC composites are fabricated by the PIP process, and then heat treated to various temperatures. The effect of heat treatment on microstructures and mechanical properties is investigated. SiC/SiC composites fabricated at 1100 1C are mostly amorphous. They get more crystallised with elevation of heat treatment temperature. After heat treatment at 1400 1C, the mechanical properties of the composites are greatly improved; flexural strength and fracture toughness are 514.1 MPa and 31.9 MPa m1/2, respectively. When the heat treatment temperature exceeds 1600 1C, the mechanical properties of the composites are degraded drastically with increasing temperature. The SiC/SiC composites all exhibit pseudo-ductile fracture behaviour except those heat treated at 1800 1C.
Acknowledgements Financial supports from the Ministry of Education of China under the New Century Excellent Talent in University (NCET-07– 0228) and from the China Scholarship Council (CSC) are gratefully acknowledged.
811
References [1] H. Ohnabe, S. Masaki, M. Onozuka, K. Miyahara, T. Sasa, Compos. Part A 30 (1999) 489–496. [2] J. Kimmel, N. Miriyala, J. Price, K. More, P. Tortorelli, H. Eaton, G. Linsey, E. Sun, J. Eur. Ceram. Soc. 22 (2002) 2769–2775. [3] R. Naslain, Compos. Sci. Technol. 64 (2004) 155–170. [4] R.R. Naslain, Int. J. Appl. Ceram. Technol. 2 (2005) 75–84. [5] Y. Katoh, L.L. Snead, C.H. Henager, A. Hasegawa, A. Kohyama, B. Riccardi, H. Hegeman, J. Nucl. Mater. 367–370 (2007) 659–671. [6] A. Hasegawa, A. Kohyama, R.H. Jones, L.L. Snead, B. Riccardi, P. Fenici, J. Nucl. Mater. 283 (2000) 128–137. [7] R.H. Jones, L. Giancarli, A. Hasegawa, Y. Katoh, J. Nucl. Mater. 307–311 (2002) 1057–1072. [8] B. Riccardi, L. Giancarli, A. Hasegawa, Y. Katoh, A. Kohyama, R.H. Jones, L.L. Snead, J. Nucl. Mater. 329 (2004) 56–65. [9] T. Nozawa, T. Hinoki, A. Hasegawa, A. Kohyama, Y. Katoh, L.L. Snead, C.H. Henager, J.B.J. Hegeman, J. Nucl. Mater. 386–88 (2009) 622–627. [10] L.L. Snead, T. Nozawa, M. Ferraris, Y. Katoh, R. Shinavski, M. Sawan, J. Nucl. Mater. 417 (2011) 330–339. [11] H. Araki, H. Suzuki, W. Yang, S. Sato, T. Noda, J. Nucl. Mater. 258–263 (Part 2) (1998) 1540–1545. [12] A. Udayakumar, A. Sri Ganesh, S. Raja, M. Balasubramanian, J. Eur. Ceram. Soc. 31 (2011) 1145–1153. [13] W. Yang, H. Araki, A. Kohyama, Q. Yang, T. Noda, Ceram. Int. 33 (2007) 141–146. [14] A. Kohyama, M. Kotani, Y. Katoh, T. Nakayasu, M. Sato, T. Yamamura, K. Okamura, J. Nucl. Mater. 283–287 (2000) 565–569. [15] Y. Katoh, M. Kotani, H. Kishimoto, W. Yang, A. Kohyama, J. Nucl. Mater. 289 (2001) 42–47. [16] M. Kotani, A. Kohyama, Y. Katoh, J. Nucl. Mater. 289 (2001) 37–41. [17] C.A. Nannetti, B. Riccardi, A. Ortona, A. La Barbera, E. Scafe , G. Vekinis, J. Nucl. Mater. 307–311 (Part 2) (2002) 1196–1199. [18] Z.Y. Chu, C.X. Feng, Y.C. Song, J. Wang, Y.D. Wang, X.D. Li, Sci. Eng. Compos. Mater. 10 (2002) 131–139. [19] S. Zhao, X.G. Zhou, H.J. Yu, H.L. Wang, Y.C. Wu, Q.Y. Huang, Z.Q. Zhu, Z.L. Huang, Fusion Eng. Des. 85 (2010) 1624–1626. [20] H.J. Yu, X.G. Zhou, W. Zhang, H.X. Peng, C.R. Zhang, Z.L. Huang, Compos. Sci. Technol. 71 (2011) 699–704. [21] Y. Ma, S. Wang, Z.H. Chen, Mater. Sci. Eng. A. 528 (2011) 3069–3072. [22] Z. Chunman, Z. Bing, L. Xiaodong, W. Yifei, Acta Polym. Sinica. 1 (2004) 246–250.