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SiC whisker composites and their mechanical properties
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Materials Letters 62 (2008) 2810 – 2813 www.elsevier.com/locate/matlet
Fabrication of toughened Cf/SiC whisker composites and their mechanical properties Y.L. Zhang, Y.M. Zhang, J.C. Han ⁎, Y.Y. Han, W. Yao Center for Composite Materials, Harbin Institute of Technology, Harbin 150080, PR China Received 22 November 2007; accepted 17 January 2008 Available online 5 February 2008
Abstract In this paper, dense short carbon fiber reinforced silicon carbide matrix composites had been fabricated by hot-pressed (HP) sintering using Al2O3 and La2O3 as sintering additives. The results showed that the combination of Al2O3 and La2O3 system was effective to promote densification of short cut carbon fiber reinforced silicon carbide composites (Cf/SiC). The whisker structure of silicon carbide was formed during the annealed treatment at 2023 K for 1 h. However, it was noted that this structure was not observed in the as-received HP material. The mechanism of forming whisker structure was not clear, but this kind of whisker structure was helpful to improve mechanical properties. The combination of grain bridging, crack deflection and whisker debonding would improve the fracture toughness of the Cf/SiC composites. © 2008 Elsevier B.V. All rights reserved. Keywords: Composite materials; Microstructure; Mechanical properties
1. Introduction Silicon carbide matrix composite is one of the main candidates for applications under severe conditions because of its outstanding combination of mechanical, thermal and physical properties. Recently, liquid-phase-sintered SiC (LPS-SiC) has attracted the increasing interest for its ability to form an in-situ toughened material and the potential superior mechanical properties relative to solid state sintered SiC [1–3]. Most studies on LPS-SiC mainly used mixtures of Al2O3 and Y2O3 as sintering additives [4]. LPS-SiC are also sintered with other rare-earth oxide, such as the lanthanum series element et al. At present, lots of reports focus on long carbon fiber reinforced silicon carbide composites. Continuous carbon fiber reinforced silicon carbide matrix composites have been receiving considerable attention for high temperature structural applications, due to their properties such as attractive strength, fracture toughness et al. [5–8].However, the reports about silicon carbide matrix composites reinforced with short carbon fibers are scarce. In the ⁎ Corresponding author. PO Box 3310#. Tel./fax: +86 451 86412236. E-mail address: [email protected] (Y.L. Zhang). 0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.01.090
present work, short carbon fiber reinforced silicon carbide matrix composites have been fabricated by hot-pressing (HP) sintering with Al2O3 and La2O3 as sintering additives. The first purpose of present work is to examine the effectiveness of the Al2O3–La2O3 oxides system as densification sintering aids for Cf/SiC composites. Another more important purpose is to investigate how the formation of in-situ whisker influences the mechanical properties of the Cf/SiC composites. 2. Experimental procedure The mixture of α-SiC powder (Kaihua Co. Ltd., Shandong, China, average size 0.5 μm, purity 99.5%) and carbon fibers 3 mm long (the volume content ratio of SiC to carbon fiber is 7:3) was utilized to prepare the Cf/SiC composite. Al2O3 (ShangHai, China, average size 1.5 μm) and La2O3 (ShangHai, China, average size 0.5 μm) were chosen (the molar ratio was 1:1), the volume content ratio of Additives amount to silicon carbide was 5:95.The mixing process was performed by attrition milling with silicon carbide ceramic balls in an alcoholic medium with attrition milling speed 160 rpm for 8 h. The mixtures were dried in a rotary evaporator, crushed and screened through a 60 mesh sieve. The graphite
Y.L. Zhang et al. / Materials Letters 62 (2008) 2810–2813
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Table 1 The mechanical property of as-received material (HPM) and annealed material (ATM)
Fig. 1. X-ray diffraction patterns of as-received HP material and AT material.
crucible was covered with BN powders to prevent the contact between mixture and the crucible. Sintering was performed under a flowing nitrogen atmosphere and in a graphite crucible in a graphite-heated uniaxial hot-pressing furnace, using heating rates of 10 K/min from room temperature to 1773 K and 5 K/min from 1773 K to 2173 K. The 30 MPa pressure was applied after 1973 K. Hot-pressing process was carried out in vacuum at 2173 K for 1 h under 30 MPa pressure and cooled to room temperature. The sample was annealed at 2023 K for 1 h. The samples were cut and ground into bars (3 mm width, 4 mm height and 36 mm length) with the tensile surface normal to the hot-pressing direction and with the grinding direction parallel to the length of the specimen. The samples for fracture were cut and ground into bars of (2 mm width, 4 mm height and 22 mm length). A notch with 2 mm depth and 0.2 mm width were cut on the 2 mm width surface. Flexural strength was determined by a three-point bending test with a distance of 30 mm and a cross-head speed of 0.5 mm/min. The tensile surfaces are polished and the edges are chamfered. Fracture toughness is measured by three-point technique (SENB) with a 16-mm span at a cross-head speed of 0.05 mm/min. Density was measured by water immersion method. At least five bars were tested for each test. Some measure technologies (such as XRD, SEM, EDS) were applied to test the components and microstructures.
Sample
Flexure strength (MPa)
Fracture toughness (MPa•m1/2)
Relative density
HPM ATM
235 ± 6 297 ± 4
5.21 ± 0.32 6.43 ± 0.19
99.1 98.6
the amorphous phase La3Al5O12. During the cooling process, it was difficult to form the amorphous phase because of low cooling rate. The mechanical properties of as-received HPM and ATM samples were illustrated in Table 1. The relative densities of HPM and ATM samples were higher than 98% theory density. It was proved that the La2O3–Al2O3 sintering additive system was effective for Cf/SiC composites. Perhaps, the silica was present in the form of a thin surface layer on silicon carbide particles, which can make the temperatures for liquid-phase formation lower. The lower annealed temperature and shorter annealed time could improve the mechanical properties. Annealing treatment had a strong influence on the fracture toughness and the flexural strength. Compared with HPM, the ATM had more excellent mechanical properties. The Flexural strength of ATM varied from 235 MPa to 297 MPa. Moreover, the higher fractural toughness increased to about 6.43 MPa•m1/2 after annealed treatment. In the HPM, the second phase of lanthanum-free betaAl2O3 (hexagonal LaAl11O18) was formed during the HP process, which was consistent with the analysis of XRD results (as shown in Fig. 1). Residual La2O3 would distribute on the interfaces of silicon carbide matrix grains and/or carbon fiber, and this kind of structure would improve the fractural toughness. SEM observations (figure omitted) showed that matrix material consisted of finer grains in the ATM. Further researches revealed that HPM and ATM samples exhibited similar grain-size distributions. Fig. 2 showed the fracture surface of the ATM material in test. The obvious trace of the carbon fiber pull-out can be observed in Fig. 2 (a). The obvious grain growth did not occur in the ATM sample. The
3. Results and discussions X-ray diffraction patterns of as-received hot-pressed sample (HPM) and the annealed sample (ATM) were shown in Fig. 1. From these data, for all the samples, there was silicon carbide in the composites as a major phase. For the HPM, there was also the amorphous carbon. The TEM researches (figures omitted) showed that the amorphous phase carbon was kept in pan-based carbon fiber. It was possible that some fraction of carbon fiber did not transform into the graphite phase during the sintering process. La2O3 phase was observed in the ATM and HPM samples. After annealed treatment, the amount of the amorphous carbon fiber decreased. The hexagonal beta-alumina (LaAl11O18) phase was found as a minor phase. It was difficult to detect the existence of low eutectic phase La3Al5O12. Lanthanum garnet phase was in consistent with garnet structures being stable only for rare-earth ions smaller than 0.106 nm (Gd3+). While the La3+ ion was larger than the Gd3+, so it was very difficult to form the lanthanum garnet phase (La3Al5O12) [9]. In this work, sintered temperature was higher than low eutectic temperature of
Fig. 2. The fracture surface of the ATM material.
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Y.L. Zhang et al. / Materials Letters 62 (2008) 2810–2813
Fig. 3. SEM observation of the in-situ silicon carbide whisker in the ATM sample was shown. Fig. 3 (b) and (c) were the magnified images of the black rectangle areas in Fig. 3 (a) and (b), respectively. Fig. 3 (d) was the energy spectrum distribution of the circle area in Fig. 3 (c).
whisker structure was detected during test (in Fig. 2 (b)). But the distribution of the whisker structure was not uniform in the fracture surface, perhaps which was relative to the annealed condition and additives distribution. The most interesting result of SEM observations was the presence of whisker structure in the ATM sample (in Fig. 3). The results of the EDS analysis showed that the composition was silicon carbide. However, it was noted that this structure was not observed in the HPM material. It implied that silicon carbide whisker structure formed during the annealed process. Undoubtedly, the sintering additives and proper annealed condition may facilitate the growth of finer whisker structure. This kind of whisker structure would promote crack deflection during the fracture process, which enhanced fracture toughness. The formation mechanics of
in-situ silicon carbide whisker structure was beyond the research scope of this paper, which will be investigated in more details in the future. The carbon fiber pull-out was considered as the important reason for improving fracture toughness (Fig. 2 (a)). During the fracture process, carbon fiber pull-out would absorb more energy and enhance the fracture toughness. For silicon carbide matrix composite, grain size had more impact on the flexural strength, the uniform grain distributions would be favorable to increase the flexural strength. The crack deflection and grain bridging were observed in the ATM material (in Fig. 4), which would play an important role in reinforcing fracture toughness of the Cf/SiC composites. It was noted that silicon carbide whisker structure is the significant reason for improving mechanical properties (in Fig. 3).
4. Conclusion
Fig. 4. SEM micrograph of the indentation-induced crack propagation in the polished surface was shown.
In this paper, dense short composites had been prepared by HP sintering with Al2O3 and La2O3 additives. The result showed that the additives system was effective on promoting densification of the HP materials. After the annealed treatment at 2023 K for 1 h, the mechanical properties can be improved obviously. It was the most interesting phenomenon in which silicon carbide whisker structure was present during the annealed treatment process. This kind of whisker structure was helpful to improve mechanical properties. It was the possible reasons for improving mechanical properties that there are sliding and rearranging of silicon carbide matrix grains and proper amount of second phase in the silicon carbide matrix grains. The cooperation of grain bridging, fiber pull-out and crack deflection resulted in much higher mechanical properties of the ATM sample.
Y.L. Zhang et al. / Materials Letters 62 (2008) 2810–2813
Acknowledgements The author would like to thank Prof. Y.M. Zhang and Dr. Ping.Hu for the helpful discussions. References [1] N.P. Padture, J. Am. Ceram. Soc. 77 (1994) 519–523. [2] Y. Zhou, H. Tanaka, S. Otani, et al., Am. Ceram. Soc. Bull. 82 (1999) 1959–1964.
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[3] G.D. Zhan, M. Mitomo, H. Tanaka, J. Am. Ceram. Soc. 83 (2000) 1369–1374. [4] E.Y. Sun, P.F. Becher, K.P. Plucknett, J. Am. Ceram. Soc. 81 (1998) 2831–2840. [5] R. Naslain, Design Compos. Sci. Technol. 64 (2004) 155–170. [6] K. Hiroshi, Comput. Sci. Technol. 59 (1999) 861–872. [7] F. Lamouroux, X. Bourrat, R. Naslain, Carbon 33 (1995) 525–535. [8] S.M. Dong, Y. Katoh, A. Kohyama, Ceram. Int. 28 (2002) 899–905. [9] P. Wu, A.D. Pelton, J. Alloys Compd. 179 (1992) 259–287.
Materials Letters 62 (2008) 2810 – 2813 www.elsevier.com/locate/matlet
Fabrication of toughened Cf/SiC whisker composites and their mechanical properties Y.L. Zhang, Y.M. Zhang, J.C. Han ⁎, Y.Y. Han, W. Yao Center for Composite Materials, Harbin Institute of Technology, Harbin 150080, PR China Received 22 November 2007; accepted 17 January 2008 Available online 5 February 2008
Abstract In this paper, dense short carbon fiber reinforced silicon carbide matrix composites had been fabricated by hot-pressed (HP) sintering using Al2O3 and La2O3 as sintering additives. The results showed that the combination of Al2O3 and La2O3 system was effective to promote densification of short cut carbon fiber reinforced silicon carbide composites (Cf/SiC). The whisker structure of silicon carbide was formed during the annealed treatment at 2023 K for 1 h. However, it was noted that this structure was not observed in the as-received HP material. The mechanism of forming whisker structure was not clear, but this kind of whisker structure was helpful to improve mechanical properties. The combination of grain bridging, crack deflection and whisker debonding would improve the fracture toughness of the Cf/SiC composites. © 2008 Elsevier B.V. All rights reserved. Keywords: Composite materials; Microstructure; Mechanical properties
1. Introduction Silicon carbide matrix composite is one of the main candidates for applications under severe conditions because of its outstanding combination of mechanical, thermal and physical properties. Recently, liquid-phase-sintered SiC (LPS-SiC) has attracted the increasing interest for its ability to form an in-situ toughened material and the potential superior mechanical properties relative to solid state sintered SiC [1–3]. Most studies on LPS-SiC mainly used mixtures of Al2O3 and Y2O3 as sintering additives [4]. LPS-SiC are also sintered with other rare-earth oxide, such as the lanthanum series element et al. At present, lots of reports focus on long carbon fiber reinforced silicon carbide composites. Continuous carbon fiber reinforced silicon carbide matrix composites have been receiving considerable attention for high temperature structural applications, due to their properties such as attractive strength, fracture toughness et al. [5–8].However, the reports about silicon carbide matrix composites reinforced with short carbon fibers are scarce. In the ⁎ Corresponding author. PO Box 3310#. Tel./fax: +86 451 86412236. E-mail address: [email protected] (Y.L. Zhang). 0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.01.090
present work, short carbon fiber reinforced silicon carbide matrix composites have been fabricated by hot-pressing (HP) sintering with Al2O3 and La2O3 as sintering additives. The first purpose of present work is to examine the effectiveness of the Al2O3–La2O3 oxides system as densification sintering aids for Cf/SiC composites. Another more important purpose is to investigate how the formation of in-situ whisker influences the mechanical properties of the Cf/SiC composites. 2. Experimental procedure The mixture of α-SiC powder (Kaihua Co. Ltd., Shandong, China, average size 0.5 μm, purity 99.5%) and carbon fibers 3 mm long (the volume content ratio of SiC to carbon fiber is 7:3) was utilized to prepare the Cf/SiC composite. Al2O3 (ShangHai, China, average size 1.5 μm) and La2O3 (ShangHai, China, average size 0.5 μm) were chosen (the molar ratio was 1:1), the volume content ratio of Additives amount to silicon carbide was 5:95.The mixing process was performed by attrition milling with silicon carbide ceramic balls in an alcoholic medium with attrition milling speed 160 rpm for 8 h. The mixtures were dried in a rotary evaporator, crushed and screened through a 60 mesh sieve. The graphite
Y.L. Zhang et al. / Materials Letters 62 (2008) 2810–2813
2811
Table 1 The mechanical property of as-received material (HPM) and annealed material (ATM)
Fig. 1. X-ray diffraction patterns of as-received HP material and AT material.
crucible was covered with BN powders to prevent the contact between mixture and the crucible. Sintering was performed under a flowing nitrogen atmosphere and in a graphite crucible in a graphite-heated uniaxial hot-pressing furnace, using heating rates of 10 K/min from room temperature to 1773 K and 5 K/min from 1773 K to 2173 K. The 30 MPa pressure was applied after 1973 K. Hot-pressing process was carried out in vacuum at 2173 K for 1 h under 30 MPa pressure and cooled to room temperature. The sample was annealed at 2023 K for 1 h. The samples were cut and ground into bars (3 mm width, 4 mm height and 36 mm length) with the tensile surface normal to the hot-pressing direction and with the grinding direction parallel to the length of the specimen. The samples for fracture were cut and ground into bars of (2 mm width, 4 mm height and 22 mm length). A notch with 2 mm depth and 0.2 mm width were cut on the 2 mm width surface. Flexural strength was determined by a three-point bending test with a distance of 30 mm and a cross-head speed of 0.5 mm/min. The tensile surfaces are polished and the edges are chamfered. Fracture toughness is measured by three-point technique (SENB) with a 16-mm span at a cross-head speed of 0.05 mm/min. Density was measured by water immersion method. At least five bars were tested for each test. Some measure technologies (such as XRD, SEM, EDS) were applied to test the components and microstructures.
Sample
Flexure strength (MPa)
Fracture toughness (MPa•m1/2)
Relative density
HPM ATM
235 ± 6 297 ± 4
5.21 ± 0.32 6.43 ± 0.19
99.1 98.6
the amorphous phase La3Al5O12. During the cooling process, it was difficult to form the amorphous phase because of low cooling rate. The mechanical properties of as-received HPM and ATM samples were illustrated in Table 1. The relative densities of HPM and ATM samples were higher than 98% theory density. It was proved that the La2O3–Al2O3 sintering additive system was effective for Cf/SiC composites. Perhaps, the silica was present in the form of a thin surface layer on silicon carbide particles, which can make the temperatures for liquid-phase formation lower. The lower annealed temperature and shorter annealed time could improve the mechanical properties. Annealing treatment had a strong influence on the fracture toughness and the flexural strength. Compared with HPM, the ATM had more excellent mechanical properties. The Flexural strength of ATM varied from 235 MPa to 297 MPa. Moreover, the higher fractural toughness increased to about 6.43 MPa•m1/2 after annealed treatment. In the HPM, the second phase of lanthanum-free betaAl2O3 (hexagonal LaAl11O18) was formed during the HP process, which was consistent with the analysis of XRD results (as shown in Fig. 1). Residual La2O3 would distribute on the interfaces of silicon carbide matrix grains and/or carbon fiber, and this kind of structure would improve the fractural toughness. SEM observations (figure omitted) showed that matrix material consisted of finer grains in the ATM. Further researches revealed that HPM and ATM samples exhibited similar grain-size distributions. Fig. 2 showed the fracture surface of the ATM material in test. The obvious trace of the carbon fiber pull-out can be observed in Fig. 2 (a). The obvious grain growth did not occur in the ATM sample. The
3. Results and discussions X-ray diffraction patterns of as-received hot-pressed sample (HPM) and the annealed sample (ATM) were shown in Fig. 1. From these data, for all the samples, there was silicon carbide in the composites as a major phase. For the HPM, there was also the amorphous carbon. The TEM researches (figures omitted) showed that the amorphous phase carbon was kept in pan-based carbon fiber. It was possible that some fraction of carbon fiber did not transform into the graphite phase during the sintering process. La2O3 phase was observed in the ATM and HPM samples. After annealed treatment, the amount of the amorphous carbon fiber decreased. The hexagonal beta-alumina (LaAl11O18) phase was found as a minor phase. It was difficult to detect the existence of low eutectic phase La3Al5O12. Lanthanum garnet phase was in consistent with garnet structures being stable only for rare-earth ions smaller than 0.106 nm (Gd3+). While the La3+ ion was larger than the Gd3+, so it was very difficult to form the lanthanum garnet phase (La3Al5O12) [9]. In this work, sintered temperature was higher than low eutectic temperature of
Fig. 2. The fracture surface of the ATM material.
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Y.L. Zhang et al. / Materials Letters 62 (2008) 2810–2813
Fig. 3. SEM observation of the in-situ silicon carbide whisker in the ATM sample was shown. Fig. 3 (b) and (c) were the magnified images of the black rectangle areas in Fig. 3 (a) and (b), respectively. Fig. 3 (d) was the energy spectrum distribution of the circle area in Fig. 3 (c).
whisker structure was detected during test (in Fig. 2 (b)). But the distribution of the whisker structure was not uniform in the fracture surface, perhaps which was relative to the annealed condition and additives distribution. The most interesting result of SEM observations was the presence of whisker structure in the ATM sample (in Fig. 3). The results of the EDS analysis showed that the composition was silicon carbide. However, it was noted that this structure was not observed in the HPM material. It implied that silicon carbide whisker structure formed during the annealed process. Undoubtedly, the sintering additives and proper annealed condition may facilitate the growth of finer whisker structure. This kind of whisker structure would promote crack deflection during the fracture process, which enhanced fracture toughness. The formation mechanics of
in-situ silicon carbide whisker structure was beyond the research scope of this paper, which will be investigated in more details in the future. The carbon fiber pull-out was considered as the important reason for improving fracture toughness (Fig. 2 (a)). During the fracture process, carbon fiber pull-out would absorb more energy and enhance the fracture toughness. For silicon carbide matrix composite, grain size had more impact on the flexural strength, the uniform grain distributions would be favorable to increase the flexural strength. The crack deflection and grain bridging were observed in the ATM material (in Fig. 4), which would play an important role in reinforcing fracture toughness of the Cf/SiC composites. It was noted that silicon carbide whisker structure is the significant reason for improving mechanical properties (in Fig. 3).
4. Conclusion
Fig. 4. SEM micrograph of the indentation-induced crack propagation in the polished surface was shown.
In this paper, dense short composites had been prepared by HP sintering with Al2O3 and La2O3 additives. The result showed that the additives system was effective on promoting densification of the HP materials. After the annealed treatment at 2023 K for 1 h, the mechanical properties can be improved obviously. It was the most interesting phenomenon in which silicon carbide whisker structure was present during the annealed treatment process. This kind of whisker structure was helpful to improve mechanical properties. It was the possible reasons for improving mechanical properties that there are sliding and rearranging of silicon carbide matrix grains and proper amount of second phase in the silicon carbide matrix grains. The cooperation of grain bridging, fiber pull-out and crack deflection resulted in much higher mechanical properties of the ATM sample.
Y.L. Zhang et al. / Materials Letters 62 (2008) 2810–2813
Acknowledgements The author would like to thank Prof. Y.M. Zhang and Dr. Ping.Hu for the helpful discussions. References [1] N.P. Padture, J. Am. Ceram. Soc. 77 (1994) 519–523. [2] Y. Zhou, H. Tanaka, S. Otani, et al., Am. Ceram. Soc. Bull. 82 (1999) 1959–1964.
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[3] G.D. Zhan, M. Mitomo, H. Tanaka, J. Am. Ceram. Soc. 83 (2000) 1369–1374. [4] E.Y. Sun, P.F. Becher, K.P. Plucknett, J. Am. Ceram. Soc. 81 (1998) 2831–2840. [5] R. Naslain, Design Compos. Sci. Technol. 64 (2004) 155–170. [6] K. Hiroshi, Comput. Sci. Technol. 59 (1999) 861–872. [7] F. Lamouroux, X. Bourrat, R. Naslain, Carbon 33 (1995) 525–535. [8] S.M. Dong, Y. Katoh, A. Kohyama, Ceram. Int. 28 (2002) 899–905. [9] P. Wu, A.D. Pelton, J. Alloys Compd. 179 (1992) 259–287.