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SiC composites
Fusion Engineering and Design 87 (2012) 1244–1248
Contents lists available at SciVerse ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Thermal shock properties of 2D-SiCf /SiC composites Sang Pill Lee a,∗ , Jin Kyung Lee a , In Soo Son a , Dong Su Bae b , Akira Kohyama c a
Department of Mechanical Engineering, Dongeui University, Busan 614-714, Republic of Korea Department of Materials Science & Engineering, Dongeui University, Busan 614-714, Republic of Korea c OASIS, Muroran Institute of Technology, Muroran, Hokkaido, Japan b
a r t i c l e
i n f o
Article history: Available online 8 April 2012 Keywords: SiCf /SiC composite Liquid phase sintering Thermal shock property Microstructure Flexural strength
a b s t r a c t This paper dealt with the thermal shock properties of SiCf /SiC composites reinforced with two dimensional SiC fabrics. SiCf /SiC composites were fabricated by a liquid phase sintering process, using a commercial nano-size SiC powder and oxide additive materials. An Al2 O3 –Y2 O3 –SiO2 powder mixture was used as a sintering additive for the consolidation of SiC matrix region. In this composite system, Tyranno SA SiC fabrics were also utilized as a reinforcing material. The thermal shock test for SiCf /SiC composites was carried out at the elevated temperature. Both mechanical strength and microstructure of SiCf /SiC composites were investigated by means of optical microscopy, SEM and three point bending test. SiCf /SiC composites represented a dense morphology with a porosity of about 8.2% and a flexural strength of about 160 MPs. The characterization of SiCf /SiC composites was greatly affected by the history of cyclic thermal shock. Especially, SiCf /SiC composites represented a reduction of flexural strength at the thermal shock temperature difference higher than 800 ◦ C. © 2012 Elsevier B.V. All rights reserved.
1. Introduction SiC fiber reinforced SiC matrix composite materials (SiCf /SiC composites) have been considered for advanced blanket module components of fusion power plants, such as first wall, divertor and coolant channel in dual-coolant lead-lithium (Pb–17Li) breeder [1–4]. These composites possessed some favorable properties such as high temperature strength, good thermo-chemical stability, low electrical conductivity and low thermal expansion coefficient. The composite process by various SiC fabric structures was mainly utilized to improve the brittle behavior of SiC materials. With a recent development of high crystalline SiC fiber, the recent strategy for high performance SiCf /SiC composites is driven to the formation of high crystalline SiC matrix in the intra-fiber bundle region of various fabric structures [5–8]. Majority of R&D researches for the property evaluation of SiCf /SiC composites were also devoted to the establishment of baseline properties such as microstructure, bending strength, tensile strength, associated with the optimization of fabricating conditions [9–12]. Among the manufacturing processes of SiCf /SiC composites, liquid phase sintering (LPS) process can be recognized as an attractive method for providing the dense SiC matrix with a high crystalline [13,14]. In addition, the utilization of ultra-fine SiC powder with submicron or nano size for the fabrication of SiCf /SiC composites also was very effective
∗ Corresponding author. Tel.: +82 51 890 1662; fax: +82 51 890 2232. E-mail address: [email protected] (S.P. Lee). 0920-3796/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2012.02.114
for the improvement of their mechanical properties due to the enhancement of SiC matrix region [8,12,15]. In order to extend various applications of SiCf /SiC composites for high temperature structural components, it is still necessary to investigate their thermal resistance properties. SiCf /SiC composites are mainly subjected to the thermal damage at the elevated temperature. Consequently, the mechanical properties of SiCf /SiC composites must be retained under the severe service environments such as high temperature and oxidizing atmosphere. The thermal damages of SiCf /SiC composites must be well understood, prior to the practical applications. Especially, it is very important to examine the thermal shock properties of SiCf /SiC composites for coolant channel components. Unfortunately, there are a few results for the thermal properties of liquid phase sintered SiCf /SiC composites. In the present study, the mechanical properties of SiCf /SiC composites reinforced with two dimensional SiC fabrics were investigated, based on the detailed analysis of their microstructures. Especially, the effect of thermal shock temperature difference and thermal shock cycle number on the flexural strength of SiCf /SiC composites was examined through an observation of their fractured surfaces. 2. Experimental procedure A commercial SiC powder with an average particle size of about 30 nm was used for the fabrication of SiCf /SiC composites. A mixture of commercial Al2 O3 , Y2 O3 and SiO2 particles was selected as a sintering additive material for the consolidation of SiC matrix.
S.P. Lee et al. / Fusion Engineering and Design 87 (2012) 1244–1248
The complex mixture slurry containing SiC, Al2 O3 , Y2 O3 , SiO2 and dispersants was prepared, using a ball milling device. SiCf /SiC composites were fabricated by a liquid phase sintering (LPS) process, in which the sintering additives around the SiC particles were transformed into some secondary eutectoids for the densification of matrix region. In this composite system, the unidirectional Tyranno SA SiC fibers were utilized as a reinforcing material. The unidirectional Tyranno SA SiC fibers were prepared with a prepreg of thin shape, using a conventional filament winding and tape casting routes [16]. The prepreg sheet of unidirectional Tyranno SA SiC fibers was fabricated, after a large number of fiber rovings was pulled from a series of creels into the liquid bath of complex mixture slurry. The fabric preform for the fabrication of SiCf /SiC composites was prepared by the orthogonal stacking of sixteen prepreg sheets. A rectangular compact body of fiber preform was consolidated at the sintering temperature of 1820 ◦ C for the creation of secondary phases around starting SiC particles, using a hot press. The applied pressure and its holding time under vacuum atmosphere were 20 MPa and 1 h, respectively. The dimension of as-pressed SiCf /SiC composites was 2(t) mm × 40 mm × 40 mm. The microstructure of SiCf /SiC composites reinforced with two dimensional SiC fabrics were analyzed by scanning electron microscope (SEM) and optical microscopy, after a mechanical polishing by diamond powders. In order to investigate the mechanical properties of SiCf /SiC composites, the three point bending test was carried out at the room temperatures. Especially, the strength degradation of SiCf /SiC composites suffered from the thermal shock was estimated. The bending load was applied to the upper portion of stacking fabric layers. The dimensions of bending test samples was 2(t) mm × 4 mm × 25 mm. The span length and the crosshead speed for the bending test were 18 mm and 0.1 mm/min, respectively. The thermal shock test for SiCf /SiC composites was carried out at the temperature difference range (T) from 25 ◦ C to 900 ◦ C. In order to examine the cyclic thermal properties of SiCf /SiC composites, the thermal shock test was repeatedly performed up to 7 cycles at the temperature difference of 800 ◦ C. The thermal shock test system is mainly composed of the heating furnace with driving motor and thermocouple and the control box for the measurement of heating and cooling time. The test sample maintained at the selected thermal shock temperature differences for thirty minutes, prior to the dropping into the water. The sintered densities of SiCf /SiC composites were determined by the Archimedes’ method. The pore volume fraction of SiCf /SiC composites associated with the variation of thermal shock test conditions was also calculated from the measured density. The fractured surfaces of SiCf /SiC composites were also observed to examine the variation of flexural strength by the thermal shock.
3. Results and discussion Fig. 1 shows the microstructures for the cross-section of SiCf /SiC composites fabricated by a LPS process. The cross-section of SiCf /SiC composites corresponds to the transverse portion of stacked fabric prepreg sheets. It was found that the utilization of fabric prepreg containing complex slurry mixture for the fabrication of SiCf /SiC composites was effective for the densification of SiC matrix region. In other words, SiCf /SiC composites showed a dense morphology without some sintering defects such as matrix cracking and delamination between fiber bundles. Especially, the dense SiC matrix was created at the inter-fiber bundle region between the orthogonal stacking of fabric prepreg. In addition, the SiC matrix was densely formed inside the vacant narrow spaces of SiC fibers, even if there were some amount of pores in the intra-fiber bundle region. However, the plastic deformation of SiC fiber entirely occurred at the insufficient portion
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Fig. 1. Microstructures for the cross-section of SiCf /SiC composites.
of complex matrix mixture in the intra-fiber bundle region. The detectable cracking of SiC fiber by a hot-pressing process was not observed in the cross section of SiCf /SiC composites. SiCf /SiC composites represented an average density of about 3.0 Mg/m3 and a pore volume fraction of about 8.2%. Fig. 2 shows the surfaces of SiCf /SiC composites depending on the variation of thermal shock temperature differences. The surface of SiCf /SiC composites was observed by an optical microscopy. It was found that SiCf /SiC composites were affected by the differences of thermal shock temperature. SiCf /SiC composites displayed different behaviors of crack initiation and its propagation, according to the increase of thermal shock temperature differences. The thermal shock crack on the sample surface of SiCf /SiC composites was primarily observed at the temperature difference of 600 ◦ C. This crack mainly went cross the fiber bundle region. On the other hand, SiCf /SiC composites represented multiple cracks in the fiber bundle region at the thermal shock temperature difference of 900 ◦ C. Especially, large amount of matrix cracks and their extensive propagation was created at the inter-fiber bundle region between fiber prepregs. This is maybe due to the strengthening of thermal stress by the increase of thermal shock temperature difference. Fig. 3 shows the pore volume fraction of SiCf /SiC composites depending on the variation of thermal shock cycle number. The cyclic thermal shock test for SiCf /SiC composites was performed at the thermal shock temperature difference of 800 ◦ C. It is found that the pore volume fraction of SiCf /SiC composites is affected by the number of thermal shock cycle. As-pressed SiCf /SiC composites
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Flexural strength (MPa)
300
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0 0
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Thermal shock temperature difference ( ) Fig. 4. Effect of thermal shock temperature difference on the flexural strength of SiCf /SiC composites.
Fig. 2. Surfaces of SiCf /SiC composites depending on the variation of thermal shock temperature differences.
without a thermal shock possessed a pore volume fraction of about 8.2%. Such a porosity level tended to increase with the increase of thermal shock cycle number. Especially, SiCf /SiC composites represented a high pore volume fraction of about 14.0% at the thermal shock number of 7 cycles. The increase of pore volume fraction by the cyclic thermal shock is maybe due to the formation of micro cracks and the activation of oxidation during the thermal shock history at the elevated temperature.
Fig. 4 shows the effect of thermal shock temperature difference on the flexural strength of SiCf /SiC composites. SiCf /SiC composites displayed a typical brittle fracture behavior without stable crack propagation beyond the maximum load, due to the absence of interfacial coating layer. SiCf /SiC composites possessed a flexural strength of about 160 MPa at the room temperature. SiCf /SiC composites showed a good resistance for the thermal shock at the high temperature, even if the micro cracks occurred at the surface of sample during the thermal shock history. In other words, the room temperature strength of SiCf /SiC composites entirely maintained up to the thermal shock temperature difference of 800 ◦ C with the increase of thermal shock temperature differences. However, SiCf /SiC composites led to the reduction of flexural strength at the thermal shock temperature difference of 900 ◦ C. This is due to the creation of surface crack and its propagation by the excess increase of thermal shock temperature differences, as shown in Fig. 2. Fig. 5 shows the effect of thermal shock cycle number on the flexural strength of SiCf /SiC composites. The cyclic thermal shock test for SiCf /SiC composites was carried out at the thermal shock temperature difference of 800 ◦ C. It is found that the flexural strength of SiCf /SiC composites by the cyclic thermal shock has a similar tendency with the variation of pore volume fraction. SiCf /SiC composites represented a reduction of flexural strength with the
300
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Flexural strength (MPa)
Pore volume fraction (%)
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Number of thermal shock cycle Fig. 3. Pore volume fraction of SiCf /SiC composites depending on the variation of thermal shock cycle number.
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Number of thermal shock cycle Fig. 5. Effect of thermal shock cycle number on the flexural strength of SiCf /SiC composites.
S.P. Lee et al. / Fusion Engineering and Design 87 (2012) 1244–1248
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increase of thermal shock cycle number. However, SiCf /SiC composites maintained the constant flexural strength after the thermal shock number of 3 cycles. SiCf /SiC composites had a flexural strength of about 110 MPa at the thermal shock number of 3 cycles. Such a decrease of flexural strength is maybe due to the correlation between the increase of pore volume fraction and the creation of surface crack and its propagation by the cyclic thermal shock. Fig. 6 shows the fracture surfaces of SiCf /SiC composites by the variation of thermal shock temperature difference. Such a fracture surface was observed, using a bending test specimen. SiCf /SiC composites entirely exhibited a similar fracture mode, regardless of the temperature difference of thermal shock. In other words, SiCf /SiC composites represented some amount of fiber pull-outs and interfacial delaminations in the intra-fiber bundle region, corresponding to a longitudinal array of fiber bundle. The inter-granular fracture mode was also observed at the inter-fiber bundle region between alternative stacking of fiber prepregs. However, SiCf /SiC composites suffered from the thermal shock temperature difference of 900 ◦ C displayed an extensive laminar delamination at the transverse array of fiber bundles, due to the degradation by the thermal shock history. This can be considered as another factor for the reduction of flexural strength by the increase of thermal shock temperature difference. As shown in Fig. 6(c), large amount of fiber deformation was also observed at the intra-fiber bundle region. Such a plastic deformation of fiber is maybe caused by an insufficient impregnation of matrix complex mixture inside fiber bundles. Therefore, in order to improve the mechanical properties of SiCf /SiC composites, it is necessary to promote the high densification of matrix region between the narrow spaces of fiber bundles through the preparation route variation of fiber prepregs. 4. Conclusions SiCf /SiC composites showed a dense morphology with a density of about 3.0 Mg/m3 , even if there were some amount of pores in the intra-fiber bundle region. The plastic deformation of SiC fiber also occurred at the intra-fiber bundle region, owing to the insufficient impregnation of complex slurry mixture. SiCf /SiC composites possessed a flexural strength of 160 MPa at the room temperature, accompanying some amount of fiber pull-outs and interfacial delaminations insides the intra-fiber bundle. The characterization of SiCf /SiC composites was greatly affected by the cyclic thermal shock. Large amount of cracks and their extensive propagation occurred at the surface of SiCf /SiC composites, according to the increase of thermal shock temperature differences. SiCf /SiC composites showed the reduction of flexural strength at the thermal shock temperature difference higher than 800 ◦ C. The pore volume fraction (about 8.2%) of as-pressed SiCf /SiC composites increased with the increase of thermal shock cycle number, owing to the formation of micro cracks and the activation of oxidation at the elevated temperature. As a result, the flexural strength of SiCf /SiC composites decreased with the increase of thermal shock cycle number. References
Fig. 6. Fracture surfaces of SiCf /SiC composites by the variation of thermal shock temperature difference.
[1] C.P.C. Wong, M. Abdou, M. Dagher, Y. Katoh, R.J. Kurtz, S. Malang, et al., An overview of the US DCLL ITER-TBM program, Fusion Engineering and Design 85 (2010) 1129–1132. [2] Y. Katoh, L.L. Snead, C.H. Henager Jr., A. Hasegawa, A. Kohyama, B. Riccardi, et al., Current status and critical issues for development of SiC composites for fusion applications, Journal of Nuclear Materials 367–370 (2007) 659–671. [3] B. Riccardi, L. Giancarli, A. Hasegawa, Y. Katoh, A. Kohyama, R.H. Jones, et al., Issues and advances in SiCf/SiC composites development for fusion reactors, Journal of Nuclear Materials 329–333 (2004) 56–65. [4] P. Norajitra, L. Bühler, U. Fischer, K. Kleefeldt, S. Malang, G. Reimann, et al., The EU advanced lead lithium blanket concept using SiCf /SiC flow channel inserts as electrical and thermal insulators, Fusion Engineering and Design 58–59 (2001) 629–634.
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[5] Y. Xu, L. Cheng, L. Zhang, X. Yin, H. Yin, High performance 3D textile Hi-Nicalon SiC/SiC composites by chemical vapor infiltration, Ceramic International 27 (2001) 565–570. [6] S.P. Lee, H.K. Yoon, J.S. Park, Y. Katoh, A. Kohyama, D.H. Kim, et al., Reaction sintering process of tyranno SA/SiC composites and their characterization, Fusion Engineering and Design 61–62 (2002) 717–722. [7] S. Dong, Y. Katoh, A. Kohyama, Processing optimization and mechanical evaluation of hot pressed 2D Tyranno-SA/SiC composites, Journal of European Ceramic Society 23 (2003) 1223–1231. [8] K. Shimoda, J.S. Park, T. Hinoki, A. Kohyama, Influence of pyrolytic carbon interface thickness on microstructure and mechanical properties of SiC/SiC composites by NITE process, Composite Science and Technology 68 (2008) 98–105. [9] M. Kotani, A. Kohyama, Y. Katoh, Development of SiC/SiC composites by PIP in combination with RS, Journal of Nuclear Materials 289 (2001) 37–41. [10] S.P. Lee, H.K. Yoon, J.S. Park, Y. Katoh, A. Kohyama, Processing and properties of SiC and SiC/SiC composite materials by melt infiltration process, International Journal of Modern Physics B 17 (2003) 1833–1838.
[11] G.N. Morscher, Tensile creep and rupture of 2D-woven SiC/SiC composites for high temperature applications, Journal of European Ceramic Society 30 (2010) 2209–2221. [12] K. Shimoda, J.S. Park, T. Hinoki, A. Kohyama, Microstructural optimization of high-temperature SiC/SiC composites by NITE process, Journal of Nuclear Materials 386–388 (2009) 634–638. [13] S.P. Lee, Y.S. Shin, D.S. Bae, B.H. Min, J.S. Park, A. Kohyama, Fabrication of liquid phase sintered SiC materials and their characterization, Fusion Engineering and Design 81 (2006) 963–967. [14] S.P. Lee, M.H. Lee, J.K. Lee, A. Kohyama, J.H. Lee, High temperature characterization of LPS-SiC based materials with oxide additives, Journal of Nuclear Materials 388 (2009) 483–486. [15] S.P. Lee, M.H. Lee, J.K. Lee, Properties of liquid phase sintered SiC materials containing Al2 O3 and Y2 O3 Particles, Journal of Ocean Engineering and Technology 22 (2008) 59–64. [16] A. Kohyama, J.S. Park, H.C. Jung, Advanced SiC fibers and SiC/SiC composites toward industrialization, Journal of Nuclear Materials 417 (2011) 340–343.
Contents lists available at SciVerse ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Thermal shock properties of 2D-SiCf /SiC composites Sang Pill Lee a,∗ , Jin Kyung Lee a , In Soo Son a , Dong Su Bae b , Akira Kohyama c a
Department of Mechanical Engineering, Dongeui University, Busan 614-714, Republic of Korea Department of Materials Science & Engineering, Dongeui University, Busan 614-714, Republic of Korea c OASIS, Muroran Institute of Technology, Muroran, Hokkaido, Japan b
a r t i c l e
i n f o
Article history: Available online 8 April 2012 Keywords: SiCf /SiC composite Liquid phase sintering Thermal shock property Microstructure Flexural strength
a b s t r a c t This paper dealt with the thermal shock properties of SiCf /SiC composites reinforced with two dimensional SiC fabrics. SiCf /SiC composites were fabricated by a liquid phase sintering process, using a commercial nano-size SiC powder and oxide additive materials. An Al2 O3 –Y2 O3 –SiO2 powder mixture was used as a sintering additive for the consolidation of SiC matrix region. In this composite system, Tyranno SA SiC fabrics were also utilized as a reinforcing material. The thermal shock test for SiCf /SiC composites was carried out at the elevated temperature. Both mechanical strength and microstructure of SiCf /SiC composites were investigated by means of optical microscopy, SEM and three point bending test. SiCf /SiC composites represented a dense morphology with a porosity of about 8.2% and a flexural strength of about 160 MPs. The characterization of SiCf /SiC composites was greatly affected by the history of cyclic thermal shock. Especially, SiCf /SiC composites represented a reduction of flexural strength at the thermal shock temperature difference higher than 800 ◦ C. © 2012 Elsevier B.V. All rights reserved.
1. Introduction SiC fiber reinforced SiC matrix composite materials (SiCf /SiC composites) have been considered for advanced blanket module components of fusion power plants, such as first wall, divertor and coolant channel in dual-coolant lead-lithium (Pb–17Li) breeder [1–4]. These composites possessed some favorable properties such as high temperature strength, good thermo-chemical stability, low electrical conductivity and low thermal expansion coefficient. The composite process by various SiC fabric structures was mainly utilized to improve the brittle behavior of SiC materials. With a recent development of high crystalline SiC fiber, the recent strategy for high performance SiCf /SiC composites is driven to the formation of high crystalline SiC matrix in the intra-fiber bundle region of various fabric structures [5–8]. Majority of R&D researches for the property evaluation of SiCf /SiC composites were also devoted to the establishment of baseline properties such as microstructure, bending strength, tensile strength, associated with the optimization of fabricating conditions [9–12]. Among the manufacturing processes of SiCf /SiC composites, liquid phase sintering (LPS) process can be recognized as an attractive method for providing the dense SiC matrix with a high crystalline [13,14]. In addition, the utilization of ultra-fine SiC powder with submicron or nano size for the fabrication of SiCf /SiC composites also was very effective
∗ Corresponding author. Tel.: +82 51 890 1662; fax: +82 51 890 2232. E-mail address: [email protected] (S.P. Lee). 0920-3796/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2012.02.114
for the improvement of their mechanical properties due to the enhancement of SiC matrix region [8,12,15]. In order to extend various applications of SiCf /SiC composites for high temperature structural components, it is still necessary to investigate their thermal resistance properties. SiCf /SiC composites are mainly subjected to the thermal damage at the elevated temperature. Consequently, the mechanical properties of SiCf /SiC composites must be retained under the severe service environments such as high temperature and oxidizing atmosphere. The thermal damages of SiCf /SiC composites must be well understood, prior to the practical applications. Especially, it is very important to examine the thermal shock properties of SiCf /SiC composites for coolant channel components. Unfortunately, there are a few results for the thermal properties of liquid phase sintered SiCf /SiC composites. In the present study, the mechanical properties of SiCf /SiC composites reinforced with two dimensional SiC fabrics were investigated, based on the detailed analysis of their microstructures. Especially, the effect of thermal shock temperature difference and thermal shock cycle number on the flexural strength of SiCf /SiC composites was examined through an observation of their fractured surfaces. 2. Experimental procedure A commercial SiC powder with an average particle size of about 30 nm was used for the fabrication of SiCf /SiC composites. A mixture of commercial Al2 O3 , Y2 O3 and SiO2 particles was selected as a sintering additive material for the consolidation of SiC matrix.
S.P. Lee et al. / Fusion Engineering and Design 87 (2012) 1244–1248
The complex mixture slurry containing SiC, Al2 O3 , Y2 O3 , SiO2 and dispersants was prepared, using a ball milling device. SiCf /SiC composites were fabricated by a liquid phase sintering (LPS) process, in which the sintering additives around the SiC particles were transformed into some secondary eutectoids for the densification of matrix region. In this composite system, the unidirectional Tyranno SA SiC fibers were utilized as a reinforcing material. The unidirectional Tyranno SA SiC fibers were prepared with a prepreg of thin shape, using a conventional filament winding and tape casting routes [16]. The prepreg sheet of unidirectional Tyranno SA SiC fibers was fabricated, after a large number of fiber rovings was pulled from a series of creels into the liquid bath of complex mixture slurry. The fabric preform for the fabrication of SiCf /SiC composites was prepared by the orthogonal stacking of sixteen prepreg sheets. A rectangular compact body of fiber preform was consolidated at the sintering temperature of 1820 ◦ C for the creation of secondary phases around starting SiC particles, using a hot press. The applied pressure and its holding time under vacuum atmosphere were 20 MPa and 1 h, respectively. The dimension of as-pressed SiCf /SiC composites was 2(t) mm × 40 mm × 40 mm. The microstructure of SiCf /SiC composites reinforced with two dimensional SiC fabrics were analyzed by scanning electron microscope (SEM) and optical microscopy, after a mechanical polishing by diamond powders. In order to investigate the mechanical properties of SiCf /SiC composites, the three point bending test was carried out at the room temperatures. Especially, the strength degradation of SiCf /SiC composites suffered from the thermal shock was estimated. The bending load was applied to the upper portion of stacking fabric layers. The dimensions of bending test samples was 2(t) mm × 4 mm × 25 mm. The span length and the crosshead speed for the bending test were 18 mm and 0.1 mm/min, respectively. The thermal shock test for SiCf /SiC composites was carried out at the temperature difference range (T) from 25 ◦ C to 900 ◦ C. In order to examine the cyclic thermal properties of SiCf /SiC composites, the thermal shock test was repeatedly performed up to 7 cycles at the temperature difference of 800 ◦ C. The thermal shock test system is mainly composed of the heating furnace with driving motor and thermocouple and the control box for the measurement of heating and cooling time. The test sample maintained at the selected thermal shock temperature differences for thirty minutes, prior to the dropping into the water. The sintered densities of SiCf /SiC composites were determined by the Archimedes’ method. The pore volume fraction of SiCf /SiC composites associated with the variation of thermal shock test conditions was also calculated from the measured density. The fractured surfaces of SiCf /SiC composites were also observed to examine the variation of flexural strength by the thermal shock.
3. Results and discussion Fig. 1 shows the microstructures for the cross-section of SiCf /SiC composites fabricated by a LPS process. The cross-section of SiCf /SiC composites corresponds to the transverse portion of stacked fabric prepreg sheets. It was found that the utilization of fabric prepreg containing complex slurry mixture for the fabrication of SiCf /SiC composites was effective for the densification of SiC matrix region. In other words, SiCf /SiC composites showed a dense morphology without some sintering defects such as matrix cracking and delamination between fiber bundles. Especially, the dense SiC matrix was created at the inter-fiber bundle region between the orthogonal stacking of fabric prepreg. In addition, the SiC matrix was densely formed inside the vacant narrow spaces of SiC fibers, even if there were some amount of pores in the intra-fiber bundle region. However, the plastic deformation of SiC fiber entirely occurred at the insufficient portion
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Fig. 1. Microstructures for the cross-section of SiCf /SiC composites.
of complex matrix mixture in the intra-fiber bundle region. The detectable cracking of SiC fiber by a hot-pressing process was not observed in the cross section of SiCf /SiC composites. SiCf /SiC composites represented an average density of about 3.0 Mg/m3 and a pore volume fraction of about 8.2%. Fig. 2 shows the surfaces of SiCf /SiC composites depending on the variation of thermal shock temperature differences. The surface of SiCf /SiC composites was observed by an optical microscopy. It was found that SiCf /SiC composites were affected by the differences of thermal shock temperature. SiCf /SiC composites displayed different behaviors of crack initiation and its propagation, according to the increase of thermal shock temperature differences. The thermal shock crack on the sample surface of SiCf /SiC composites was primarily observed at the temperature difference of 600 ◦ C. This crack mainly went cross the fiber bundle region. On the other hand, SiCf /SiC composites represented multiple cracks in the fiber bundle region at the thermal shock temperature difference of 900 ◦ C. Especially, large amount of matrix cracks and their extensive propagation was created at the inter-fiber bundle region between fiber prepregs. This is maybe due to the strengthening of thermal stress by the increase of thermal shock temperature difference. Fig. 3 shows the pore volume fraction of SiCf /SiC composites depending on the variation of thermal shock cycle number. The cyclic thermal shock test for SiCf /SiC composites was performed at the thermal shock temperature difference of 800 ◦ C. It is found that the pore volume fraction of SiCf /SiC composites is affected by the number of thermal shock cycle. As-pressed SiCf /SiC composites
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S.P. Lee et al. / Fusion Engineering and Design 87 (2012) 1244–1248
Flexural strength (MPa)
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200
100
0 0
200
600
400
800
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Thermal shock temperature difference ( ) Fig. 4. Effect of thermal shock temperature difference on the flexural strength of SiCf /SiC composites.
Fig. 2. Surfaces of SiCf /SiC composites depending on the variation of thermal shock temperature differences.
without a thermal shock possessed a pore volume fraction of about 8.2%. Such a porosity level tended to increase with the increase of thermal shock cycle number. Especially, SiCf /SiC composites represented a high pore volume fraction of about 14.0% at the thermal shock number of 7 cycles. The increase of pore volume fraction by the cyclic thermal shock is maybe due to the formation of micro cracks and the activation of oxidation during the thermal shock history at the elevated temperature.
Fig. 4 shows the effect of thermal shock temperature difference on the flexural strength of SiCf /SiC composites. SiCf /SiC composites displayed a typical brittle fracture behavior without stable crack propagation beyond the maximum load, due to the absence of interfacial coating layer. SiCf /SiC composites possessed a flexural strength of about 160 MPa at the room temperature. SiCf /SiC composites showed a good resistance for the thermal shock at the high temperature, even if the micro cracks occurred at the surface of sample during the thermal shock history. In other words, the room temperature strength of SiCf /SiC composites entirely maintained up to the thermal shock temperature difference of 800 ◦ C with the increase of thermal shock temperature differences. However, SiCf /SiC composites led to the reduction of flexural strength at the thermal shock temperature difference of 900 ◦ C. This is due to the creation of surface crack and its propagation by the excess increase of thermal shock temperature differences, as shown in Fig. 2. Fig. 5 shows the effect of thermal shock cycle number on the flexural strength of SiCf /SiC composites. The cyclic thermal shock test for SiCf /SiC composites was carried out at the thermal shock temperature difference of 800 ◦ C. It is found that the flexural strength of SiCf /SiC composites by the cyclic thermal shock has a similar tendency with the variation of pore volume fraction. SiCf /SiC composites represented a reduction of flexural strength with the
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Pore volume fraction (%)
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Number of thermal shock cycle Fig. 5. Effect of thermal shock cycle number on the flexural strength of SiCf /SiC composites.
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increase of thermal shock cycle number. However, SiCf /SiC composites maintained the constant flexural strength after the thermal shock number of 3 cycles. SiCf /SiC composites had a flexural strength of about 110 MPa at the thermal shock number of 3 cycles. Such a decrease of flexural strength is maybe due to the correlation between the increase of pore volume fraction and the creation of surface crack and its propagation by the cyclic thermal shock. Fig. 6 shows the fracture surfaces of SiCf /SiC composites by the variation of thermal shock temperature difference. Such a fracture surface was observed, using a bending test specimen. SiCf /SiC composites entirely exhibited a similar fracture mode, regardless of the temperature difference of thermal shock. In other words, SiCf /SiC composites represented some amount of fiber pull-outs and interfacial delaminations in the intra-fiber bundle region, corresponding to a longitudinal array of fiber bundle. The inter-granular fracture mode was also observed at the inter-fiber bundle region between alternative stacking of fiber prepregs. However, SiCf /SiC composites suffered from the thermal shock temperature difference of 900 ◦ C displayed an extensive laminar delamination at the transverse array of fiber bundles, due to the degradation by the thermal shock history. This can be considered as another factor for the reduction of flexural strength by the increase of thermal shock temperature difference. As shown in Fig. 6(c), large amount of fiber deformation was also observed at the intra-fiber bundle region. Such a plastic deformation of fiber is maybe caused by an insufficient impregnation of matrix complex mixture inside fiber bundles. Therefore, in order to improve the mechanical properties of SiCf /SiC composites, it is necessary to promote the high densification of matrix region between the narrow spaces of fiber bundles through the preparation route variation of fiber prepregs. 4. Conclusions SiCf /SiC composites showed a dense morphology with a density of about 3.0 Mg/m3 , even if there were some amount of pores in the intra-fiber bundle region. The plastic deformation of SiC fiber also occurred at the intra-fiber bundle region, owing to the insufficient impregnation of complex slurry mixture. SiCf /SiC composites possessed a flexural strength of 160 MPa at the room temperature, accompanying some amount of fiber pull-outs and interfacial delaminations insides the intra-fiber bundle. The characterization of SiCf /SiC composites was greatly affected by the cyclic thermal shock. Large amount of cracks and their extensive propagation occurred at the surface of SiCf /SiC composites, according to the increase of thermal shock temperature differences. SiCf /SiC composites showed the reduction of flexural strength at the thermal shock temperature difference higher than 800 ◦ C. The pore volume fraction (about 8.2%) of as-pressed SiCf /SiC composites increased with the increase of thermal shock cycle number, owing to the formation of micro cracks and the activation of oxidation at the elevated temperature. As a result, the flexural strength of SiCf /SiC composites decreased with the increase of thermal shock cycle number. References
Fig. 6. Fracture surfaces of SiCf /SiC composites by the variation of thermal shock temperature difference.
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