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Improvement of the mechanical properties of hot-pressed silicon-carbide-fiber-reinforced silicon carbide composites by polycarbosilane impregnation
Composites Science and Technology 61 (2001) 1323–1329 www.elsevier.com/locate/compscitech
Improvement of the mechanical properties of hot-pressed silicon-carbide-fiber-reinforced silicon carbide composites by polycarbosilane impregnation Katsumi Yoshida1,*, Masamitsu Imai, Toyohiko Yano Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8550, Japan Received 16 February 2001; accepted 18 February 2001
Abstract Green sheets of SiC with Al2O3–Y2O3–CaO sintering additives prepared by the doctor-blade method and polycarbosilane (PCS)impregnated Hi-Nicalon cloth with a BN coating were used for the fabrication of SiC-fiber-reinforced SiC (SiC/SiCf) composites by hot-pressing. Two kinds of SiC/SiCf composites with different fiber volume fractions were fabricated and their room-temperature mechanical properties were investigated. These composites showed non-brittle fracture behavior. The maximum strength of a composite with 52 vol.% of fibers was about twice as high as that of a composite with 40 vol.% of fibers, and the composite hotpressed at 1700 C showed the highest maximum strength. In this fabrication process, PCS-impregnation into Hi-Nicalon cloth was an effective way of forming the matrix between fibers. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Ceramic-matrix composites; A. Preceramic polymer; B. Mechanical properties; Hot-pressing
1. Introduction A composite consisting of silicon carbide reinforced with continuous SiC fibers (SiC/SiCf) is one of the candidate ceramic materials for high-temperature structural applications since SiC shows excellent high-temperature mechanical properties, high thermal conductivity and good oxidation, corrosion and wear resistance [1,2]. Furthermore, SiC shows low activation on account of its low atomic number and good resistance to high-energy neutron irradiation and it is expected to be used as structural material in future fusion reactors [3–14]. SiC/SiCf composite is mainly fabricated by chemical vapor infiltration (CVI) and polymer infiltration and pyrolysis (PIP) methods [15–17]. These processes have some advantages such as high purity and low damage to fibers as a consequence of the relatively low processing temperature. There is a variety of CVI processes and some of them can form the matrix with relatively high * Corresponding author. Tel.: +81-3-5734-3082; fax: +81-3-57342959. E-mail address: [email protected] (K. Yoshida). 1 Research Fellow of the Japan Society for the Promotion of Science.
speed, for example, in times of the order of a day by the forced CVI method [16]. In general, however, these processes require long manufacturing times, resulting in high processing cost. Furthermore, the composites fabricated by these processes usually contain about 10–20 vol.% of extended large voids, resulting in low mechanical and thermal properties. In order to simplify the fabrication process and to obtain dense SiC/SiCf composites with high mechanical and thermal properties, the authors have studied a fabrication process using hot-pressing, which offers the ability to fabricate dense composites [18–20]. The present authors have reported that SiC/SiCf composite was fabricated by using a green sheet of SiC with Al2O3–Y2O3–CaO sintering additives and SiC slurry-impregnated two-dimensional (2D) plain-weave Hi-Nicalon cloth with and without a BN coating by hot-pressing at 1750 C [19,20]. Although the composites fabricated by this process achieved nearly full density, they fractured in a brittle manner. It was considered that the interfacial bonding between fiber and matrix was too strong as a result of the reaction between the BN coating on the fiber and matrix components such as the sintering additives, and the fiber was degraded severely by exposure at a temperature as high as 1750 C. Moreover,
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Hi-Nicalon fibers were deformed since fibers were in direct contact with each other because of insufficient impregnation by the SiC slurry between each fiber, resulting in low mechanical properties [19]. In order to form the matrix between the fibers and to improve the mechanical properties of the SiC/SiCf composite, polycarbosilane (PCS)-impregnated Hi-Nicalon cloths were used for the reinforcement instead of SiC slurry-impregnated Hi-Nicalon cloths. It is expected that the SiC matrix can be formed between fibers by the use of a liquid precursor. In addition, increase of fiber volume fraction should also be effective in improving the mechanical properties of the SiC/SiCf composite. PCS impregnation into SiC filaments was already employed by Nakano et al. [21], but they fabricated only onedimensional SiC/SiCf composites by a filament winding method. In this study, two kinds of composites with a different volume fraction of fibers were fabricated by hot-pressing from green SiC sheet with sintering additives and PCSimpregnated Hi-Nicalon cloth. The effects of fiber volume fraction and sintering temperature on mechanical properties of the SiC/SiCf composite at room temperature were evaluated.
2. Experimental procedure 2.1. Fabrication of green sheet In this study, an Al2O3–Y2O3–CaO system was chosen for the fabrication of the SiC/SiCf composite as sintering additives because of their low liquidus temperatures [22]. Submicron b-SiC powders (ultrafine, average particle size: 0.28 mm, Ibiden, Japan), sintering additives (20 mass% in total) using Al2O3 (average particle size: 0.18
mm, AKP-50, Sumitomo Chemical, Japan), Y2O3 (average particle size: 2–3 mm, 99.9%, High Purity Chemical, Japan) and CaO (99.9%, Kanto Chemical, Japan), and some organics were used for the fabrication of the SiC green sheet. The green sheet was prepared using laboratory-scale doctor-blade equipment (DP150, Tsugawa Seiki, Japan). Two kinds of green sheets with different thickness were prepared by adjusting the blade height to 0.5–0.7 mm and at a carrier film speed of 10 cm/min. These sheets were dried at room temperature. The thicknesses of the green sheets were 105–125 and 220–285 mm, respectively. Details of the composition, organics in the green sheet, and the fabrication process were described elsewhere [19]. The green sheet was cut to 35 mm35 mm. 2.2. Fabrication of the SiC/SiCf composite Schematic illustration of the fabrication process of the SiC/SiCf composite is shown in Fig. 1. In this study, two-dimensionally (0 /90 ) plain-woven BN-coated HiNicalon (Nippon Carbon, Japan) fiber cloth was used as the reinforcement. The thickness of the BN-coating was 0.4 mm. The cloth was cut to 35 mm35 mm. Polycarbosilane (PCS, NIPUSI-Type S, Nippon Carbon, Japan) was used for the impregnation into HiNicalon fiber cloth. PCS is a mixture of two molecular components, –CH3SiHCH2– and –(CH3)2SiCH2–. Density, melting point and average molecular weight of PCS powder used in this study were 1.10 g/cm3, 242–249 C and 1470–2440 C, respectively. PCS powder was dissolved in toluene at 80 C and the cloths were impregnated with PCS-toluene solution under reduced pressure. The sizing agent was removed prior to polycarbosilaneimpregnation. The PCS-impregnated cloths were dried at 130 C. The green sheets and the PCS-impregnated cloths were stacked alternately and heat-treated at 300 C for 24
Fig. 1. Schematic illustration of the fabrication process of the SiC/SiCf composite.
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h in air under a pressure of 20 kPa. In this process, the heating rate was 10 C/h from 150 to 300 C in order to prevent the combustion of PCS due to rapid oxidation [23]. The introduction of oxygen into the PCS structure during the heat-treatment in air, i.e. thermal oxidationcuring, was promoted. Thermal oxidation curing was performed in order to prevent the impregnated-PCS into Hi-Nicalon cloths from flowing out due to lowering viscosity of PCS during hot-pressing. The stacked green body was hot-pressed at 1650, 1700 and 1750 C for 1 h in Ar atmosphere under a uniaxial pressure of 40 MPa. It was reported that the weight residue of the oxidation-cured PCS after pyrolysis at 1300 C and subsequent heat-treatment at 1500–1700 C in Ar was 43–66% [24]. In this study, hot-pressing was performed at 1650–1750 C in Ar atmosphere, and the weight residue of PCS was considered to be similar to these values. Two kinds of composites with different volume fractions of fibers were fabricated by use of the green sheets with different thicknesses. Fiber volume fraction of the composites with thicker or thinner sheets was about 40 and 52 vol.%, respectively. 2.3. Mechanical properties Hot-pressed specimens were cut into rectangular bars (width: 3.5 mm, thickness: 2.3–3.2 mm, length: 34 mm).
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Bulk density was measured by Archimedes’ method. Theoretical density of the SiC/SiCf composite was calculated as that of a mixture of SiC matrix with sintering additives and Hi-Nicalon fiber. Three-point bending strength was measured at room temperature in air with a cross-head speed of 0.1 mm/ min and a lower span of 30 mm. Bending strength measurement was performed using a universal testing machine (Instron 1185, USA). Fracture energy was calculated from the area of load-displacement curve in bending strength measurement divided by twice the fracture surface area. Fracture surface were observed by a scanning electron microscope (SEM).
3. Results and discussion 3.1. Microstructure and bulk density Fig. 2 shows the difference in microstructure of the SiC/SiCf composites using slurry-impregnated and PCSimpregnated Hi-Nicalon cloths as the reinforcement. In the case of composite with slurry-impregnated Hi-Nicalon cloth [Fig. 2(a)], the SiC matrix did not form between fibers sufficiently. The fibers contacted directly each other, and then they deformed into polyhedral prism as seen in Tyrannohex composites [25]. In contrast, in the
Fig. 2. Microstructure of the SiC/SiCf composites fabricated by hot-pressing at 1750 C. (a) SiC slurry-impregnated and (b) PCS-impregnated HiNicalon cloths with BN-coating were used as the reinforcement.
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case of PCS-impregnated Hi-Nicalon cloths, sufficient formation of the SiC matrix between fibers could be achieved and round shape of the fiber was maintained after hot-pressing. In this fabrication process, PCSimpregnation into Hi-Nicalon cloths is an effective way to form SiC matrix between the fibers. Therefore, all of the following results were obtained for the composites with PCS-impregnated cloths. Fig. 3 shows the change in bulk density of the SiC/ SiCf composites with sintering temperature. Bulk density of the composites with 40 vol.% of fibers decreased with lowering sintering temperature and the relative density was about 89–97%. The composites with 52 vol.% of fibers did not show much difference in bulk density regardless of sintering temperature and the relative density was about 93–94%.
fibers hot-pressed at 1650, 1700 and 1750 C was about 2.3, 1.2 and 0.5 kJ/m2, respectively. Fig. 6 shows the maximum bending strength of the SiC/SiCf composites measured at room temperature. In the case of the composites with slurry-impregnated HiNicalon cloths, maximum strength decreased with lowering sintering temperature, and the values were 130– 220 MPa. Maximum strength of the composites with 52
3.2. Mechanical properties The typical load-displacement curves of the SiC/SiCf composites with 40 and 52 vol.% of fibers in three-point bending test at room temperature are shown in Fig. 4. For comparison, the load-displacement curves of the composites with slurry-impregnated Hi-Nicalon cloths fabricated by hot-pressing at 1650 and 1750 C are shown in Fig. 5 [20]. In the case of the composite with slurry-impregnated Hi-Nicalon cloths, the composites hot-pressed at 1700 C (not shown in Fig. 4) or 1750 C displayed completely brittle fracture behavior, whereas the composites obtained in this study showed non-brittle fracture behavior. The load-displacement curves spread more widely with lowering sintering temperature independent of fiber volume fraction. Fracture energy increased with lowering sintering temperature in both cases of fiber volume fraction, and larger values were measured for the composite with higher fiber content. Fracture energy of the composites with 52 vol.% of
Fig. 3. Bulk density of the SiC/SiCf composites fabricated by hotpressing at various sintering temperature.
Fig. 4. Typical load-displacement curves of the SiC/SiCf composites with about (a) 40) vol.% and (b) 52 vol.% of fibers fabricated by hotpressing at various sintering temperature.
Fig. 5. Typical load-displacement curves of the SiC/SiCf composites with slurry-impregnated Hi-Nicalon cloths fabricated by hot-pressing at various sintering temperature. Fiber volume fraction of the composite is about 40 vol.%.
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Fig. 6. Maximum strength of the SiC/SiCf composites with about 40 and 52 vol.% of fibers fabricated by hot-pressing at various sintering temperature.
vol.% of fibers was about twice as high as that of the composites with 40 vol.% of fibers, and the SiC/SiCf composites hot-pressed at 1700 C showed higher maximum strength than the composites hot-pressed at 1650 or 1750 C. In the case of the hot-pressing temperature of 1700 C, the composites with 40 and 52 vol.% of fibers showed maximum strength of 120 and 240 MPa, respectively. The present authors evaluated the mechanical properties of 2D-SiC/SiCf composites fabricated by the CVI and PIP methods, which were supplied as the round robin test materials [26]. The SiC/SiCf composites fabricated by the CVI method (Vf=30%) had a maximum strength of 180–460 MPa, and the composites fabricated by the PIP method (Vf=40%) showed a maximum strength of 20–50 MPa at room temperature. The maximum strength of the composites fabricated in this study was lower than the highest value of the composites fabricated by the CVI method since the fiber strength of the composite fabricated by the CVI method should be higher than that of the composite fabricated by hotpressing mainly due to the difference in the processing temperature, i.e., processing temperature of hot-pressing is much higher than that of CVI. It has been reported that the tensile strength of Hi-Nicalon fiber after the thermal exposure in Ar atmosphere is maintained at around the original strength up to 1400 C, however, it decreased gradually above 1400 C. The Hi-Nicalon fiber heat-treated at 1600 C retains approximately half the original strength [27–29]. SEM micrographs of the fracture surface of the SiC/ SiCf composites with 40 and 52 vol.% of fibers are shown in Figs. 7 and 8, respectively. In the composite with 40 vol.% of fibers, the length of fiber pull-out was very short, whereas the composites with 52 vol.% of fibers showed large fiber pull-out.
Fig. 7. SEM micrographs of the fracture surface of the SiC/SiCf composites hot-pressed at (a) 1650 C, (b) 1700 C and (c) 1750 C after a three-point bending test at room temperature. Fiber volume fraction was about 40 vol.%.
The interfacial strength between fiber and matrix significantly affects the mechanical properties of fiberreinforced composites, and sintering temperature is
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Fig. 8. SEM micrographs of the fracture surface of the SiC/SiCf composites hot-pressed at (a) 1650 C, (b) 1700 C and (c) 1750 C after a three-point bending test at room temperature. Fiber volume fraction was about 52 vol.%.
considered to be one of the most important factors influencing the characteristics of interfacial strength [20]. As sintering temperature is increased, the reactions between fiber/fiber coating/matrix will progress, and
internal compressive stress from the matrix due to the difference of thermal expansion coefficient between matrix and fiber, and then interfacial strength between fiber and matrix may increase. In the case of the composite obtained in this study, the thermal expansion coefficient of the matrix was considered to be higher than that of the fiber, because the SiC matrix contained components with higher thermal expansion coefficient, such as Al2O3 (8.810 6 K 1) and yttrium aluminum garnet (YAG: Y3Al5O12, 5.110 6 K 1) compared with Hi-Nicalon fiber (3.510 6 K 1). Then, the tensile residual stress on the matrix and the compressive residual stress on the fibers would have taken place and the tight interface between fiber and matrix was formed. As a result, the composite would show a reduction in strength for matrix cracking and have difficulty in fiber pull-out at R.T. The present authors [30] investigated the microstructure of the composites hot-pressed with Al2O3– Y2O3–CaO additives using slurry-impregnated HiNicalon cloth by transmission electron microscopy. It was shown that some crystals grew from the matrix into a BN layer and the thickness of the layer was not homogeneous. Furthermore, growth of SiC crystals in the fiber, which induced degradation of fiber strength, was accelerated by the diffusion of sintering additives from the matrix. Therefore, it was confirmed that some reactions between matrix/fiber coating/fiber occurred in the case of slurry-impregnated cloths, but these interactions between matrix/fiber coating/fiber were effectively suppressed in the case of PCS-impregnated cloths. In the case of the hot-pressing temperature of 1650 C, the strengthening of the matrix is not enough due to higher viscosity of the glassy phase at sintering temperature. Higher viscosity causes insufficient matrix impregnation between fibers, therefore, fiber/matrix interfacial strength would be weak, resulting in decreasing maximum strength and large fiber pull-out. In contrast, in the case of the hot-pressing temperature of 1750 C, the strengthening of the matrix progressed due to lower viscosity and higher diffusion rate of the glassy phase at sintering temperature. Then the interfacial strength would be relatively high, but the degradation of fiber due to the exposure to high-temperature simultaneously progressed, resulting in low maximum strength, short fiber pull-out and then low fracture energy. In this study, it was concluded that the temperature of 1700 C would be a better condition for hot-pressing to obtain the SiC/SiCf composite with good mechanical properties. The difference in maximum strength, and then fracture energy depended greatly on the fiber volume fraction. From these results, it is concluded that the use of PCS-impregnated Hi-Nicalon cloths as the reinforcement and an increase in fiber volume fraction are effective ways to improve the mechanical properties of SiC/ SiCf composites.
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4. Conclusions Green sheets of SiC with Al2O3–Y2O3–CaO sintering additives prepared by the doctor-blade method and polycarbosilane (PCS)-impregnated 2D woven Hi-Nicalon cloth with BN-coating were used for the fabrication of SiC/SiCf composite by hot-pressing at 1650–1750 C. Two kinds of SiC/SiCf composites with different volume fractions of fibers were fabricated and their room temperature mechanical properties were evaluated. PCS-impregnation into Hi-Nicalon cloth was an effective way to form the matrix between fibers. The composites fabricated in this study showed non-brittle fracture behavior. Maximum strength of the composite with 52 vol.% of fibers was about twice as high as that of the composite with 40 vol.% of fibers, and the composite hot-pressed at 1700 C showed higher maximum strength than the composites hot-pressed at 1650 and 1750 C. Fracture energy increased with lowering sintering temperature. These results indicate sintering temperature affects the characteristics of interfacial bonding between fiber and matrix in the SiC/SiCf composite of the present study.
Acknowledgements This work was partly supported by the Research for the Future Program (RFTF97R12101) from JSPS and a Grant-in-Aid for JSPS fellows from the Ministry of Education, Science, Sports and Culture of Japan.
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Improvement of the mechanical properties of hot-pressed silicon-carbide-fiber-reinforced silicon carbide composites by polycarbosilane impregnation Katsumi Yoshida1,*, Masamitsu Imai, Toyohiko Yano Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8550, Japan Received 16 February 2001; accepted 18 February 2001
Abstract Green sheets of SiC with Al2O3–Y2O3–CaO sintering additives prepared by the doctor-blade method and polycarbosilane (PCS)impregnated Hi-Nicalon cloth with a BN coating were used for the fabrication of SiC-fiber-reinforced SiC (SiC/SiCf) composites by hot-pressing. Two kinds of SiC/SiCf composites with different fiber volume fractions were fabricated and their room-temperature mechanical properties were investigated. These composites showed non-brittle fracture behavior. The maximum strength of a composite with 52 vol.% of fibers was about twice as high as that of a composite with 40 vol.% of fibers, and the composite hotpressed at 1700 C showed the highest maximum strength. In this fabrication process, PCS-impregnation into Hi-Nicalon cloth was an effective way of forming the matrix between fibers. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Ceramic-matrix composites; A. Preceramic polymer; B. Mechanical properties; Hot-pressing
1. Introduction A composite consisting of silicon carbide reinforced with continuous SiC fibers (SiC/SiCf) is one of the candidate ceramic materials for high-temperature structural applications since SiC shows excellent high-temperature mechanical properties, high thermal conductivity and good oxidation, corrosion and wear resistance [1,2]. Furthermore, SiC shows low activation on account of its low atomic number and good resistance to high-energy neutron irradiation and it is expected to be used as structural material in future fusion reactors [3–14]. SiC/SiCf composite is mainly fabricated by chemical vapor infiltration (CVI) and polymer infiltration and pyrolysis (PIP) methods [15–17]. These processes have some advantages such as high purity and low damage to fibers as a consequence of the relatively low processing temperature. There is a variety of CVI processes and some of them can form the matrix with relatively high * Corresponding author. Tel.: +81-3-5734-3082; fax: +81-3-57342959. E-mail address: [email protected] (K. Yoshida). 1 Research Fellow of the Japan Society for the Promotion of Science.
speed, for example, in times of the order of a day by the forced CVI method [16]. In general, however, these processes require long manufacturing times, resulting in high processing cost. Furthermore, the composites fabricated by these processes usually contain about 10–20 vol.% of extended large voids, resulting in low mechanical and thermal properties. In order to simplify the fabrication process and to obtain dense SiC/SiCf composites with high mechanical and thermal properties, the authors have studied a fabrication process using hot-pressing, which offers the ability to fabricate dense composites [18–20]. The present authors have reported that SiC/SiCf composite was fabricated by using a green sheet of SiC with Al2O3–Y2O3–CaO sintering additives and SiC slurry-impregnated two-dimensional (2D) plain-weave Hi-Nicalon cloth with and without a BN coating by hot-pressing at 1750 C [19,20]. Although the composites fabricated by this process achieved nearly full density, they fractured in a brittle manner. It was considered that the interfacial bonding between fiber and matrix was too strong as a result of the reaction between the BN coating on the fiber and matrix components such as the sintering additives, and the fiber was degraded severely by exposure at a temperature as high as 1750 C. Moreover,
0266-3538/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(01)00031-8
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Hi-Nicalon fibers were deformed since fibers were in direct contact with each other because of insufficient impregnation by the SiC slurry between each fiber, resulting in low mechanical properties [19]. In order to form the matrix between the fibers and to improve the mechanical properties of the SiC/SiCf composite, polycarbosilane (PCS)-impregnated Hi-Nicalon cloths were used for the reinforcement instead of SiC slurry-impregnated Hi-Nicalon cloths. It is expected that the SiC matrix can be formed between fibers by the use of a liquid precursor. In addition, increase of fiber volume fraction should also be effective in improving the mechanical properties of the SiC/SiCf composite. PCS impregnation into SiC filaments was already employed by Nakano et al. [21], but they fabricated only onedimensional SiC/SiCf composites by a filament winding method. In this study, two kinds of composites with a different volume fraction of fibers were fabricated by hot-pressing from green SiC sheet with sintering additives and PCSimpregnated Hi-Nicalon cloth. The effects of fiber volume fraction and sintering temperature on mechanical properties of the SiC/SiCf composite at room temperature were evaluated.
2. Experimental procedure 2.1. Fabrication of green sheet In this study, an Al2O3–Y2O3–CaO system was chosen for the fabrication of the SiC/SiCf composite as sintering additives because of their low liquidus temperatures [22]. Submicron b-SiC powders (ultrafine, average particle size: 0.28 mm, Ibiden, Japan), sintering additives (20 mass% in total) using Al2O3 (average particle size: 0.18
mm, AKP-50, Sumitomo Chemical, Japan), Y2O3 (average particle size: 2–3 mm, 99.9%, High Purity Chemical, Japan) and CaO (99.9%, Kanto Chemical, Japan), and some organics were used for the fabrication of the SiC green sheet. The green sheet was prepared using laboratory-scale doctor-blade equipment (DP150, Tsugawa Seiki, Japan). Two kinds of green sheets with different thickness were prepared by adjusting the blade height to 0.5–0.7 mm and at a carrier film speed of 10 cm/min. These sheets were dried at room temperature. The thicknesses of the green sheets were 105–125 and 220–285 mm, respectively. Details of the composition, organics in the green sheet, and the fabrication process were described elsewhere [19]. The green sheet was cut to 35 mm35 mm. 2.2. Fabrication of the SiC/SiCf composite Schematic illustration of the fabrication process of the SiC/SiCf composite is shown in Fig. 1. In this study, two-dimensionally (0 /90 ) plain-woven BN-coated HiNicalon (Nippon Carbon, Japan) fiber cloth was used as the reinforcement. The thickness of the BN-coating was 0.4 mm. The cloth was cut to 35 mm35 mm. Polycarbosilane (PCS, NIPUSI-Type S, Nippon Carbon, Japan) was used for the impregnation into HiNicalon fiber cloth. PCS is a mixture of two molecular components, –CH3SiHCH2– and –(CH3)2SiCH2–. Density, melting point and average molecular weight of PCS powder used in this study were 1.10 g/cm3, 242–249 C and 1470–2440 C, respectively. PCS powder was dissolved in toluene at 80 C and the cloths were impregnated with PCS-toluene solution under reduced pressure. The sizing agent was removed prior to polycarbosilaneimpregnation. The PCS-impregnated cloths were dried at 130 C. The green sheets and the PCS-impregnated cloths were stacked alternately and heat-treated at 300 C for 24
Fig. 1. Schematic illustration of the fabrication process of the SiC/SiCf composite.
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h in air under a pressure of 20 kPa. In this process, the heating rate was 10 C/h from 150 to 300 C in order to prevent the combustion of PCS due to rapid oxidation [23]. The introduction of oxygen into the PCS structure during the heat-treatment in air, i.e. thermal oxidationcuring, was promoted. Thermal oxidation curing was performed in order to prevent the impregnated-PCS into Hi-Nicalon cloths from flowing out due to lowering viscosity of PCS during hot-pressing. The stacked green body was hot-pressed at 1650, 1700 and 1750 C for 1 h in Ar atmosphere under a uniaxial pressure of 40 MPa. It was reported that the weight residue of the oxidation-cured PCS after pyrolysis at 1300 C and subsequent heat-treatment at 1500–1700 C in Ar was 43–66% [24]. In this study, hot-pressing was performed at 1650–1750 C in Ar atmosphere, and the weight residue of PCS was considered to be similar to these values. Two kinds of composites with different volume fractions of fibers were fabricated by use of the green sheets with different thicknesses. Fiber volume fraction of the composites with thicker or thinner sheets was about 40 and 52 vol.%, respectively. 2.3. Mechanical properties Hot-pressed specimens were cut into rectangular bars (width: 3.5 mm, thickness: 2.3–3.2 mm, length: 34 mm).
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Bulk density was measured by Archimedes’ method. Theoretical density of the SiC/SiCf composite was calculated as that of a mixture of SiC matrix with sintering additives and Hi-Nicalon fiber. Three-point bending strength was measured at room temperature in air with a cross-head speed of 0.1 mm/ min and a lower span of 30 mm. Bending strength measurement was performed using a universal testing machine (Instron 1185, USA). Fracture energy was calculated from the area of load-displacement curve in bending strength measurement divided by twice the fracture surface area. Fracture surface were observed by a scanning electron microscope (SEM).
3. Results and discussion 3.1. Microstructure and bulk density Fig. 2 shows the difference in microstructure of the SiC/SiCf composites using slurry-impregnated and PCSimpregnated Hi-Nicalon cloths as the reinforcement. In the case of composite with slurry-impregnated Hi-Nicalon cloth [Fig. 2(a)], the SiC matrix did not form between fibers sufficiently. The fibers contacted directly each other, and then they deformed into polyhedral prism as seen in Tyrannohex composites [25]. In contrast, in the
Fig. 2. Microstructure of the SiC/SiCf composites fabricated by hot-pressing at 1750 C. (a) SiC slurry-impregnated and (b) PCS-impregnated HiNicalon cloths with BN-coating were used as the reinforcement.
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case of PCS-impregnated Hi-Nicalon cloths, sufficient formation of the SiC matrix between fibers could be achieved and round shape of the fiber was maintained after hot-pressing. In this fabrication process, PCSimpregnation into Hi-Nicalon cloths is an effective way to form SiC matrix between the fibers. Therefore, all of the following results were obtained for the composites with PCS-impregnated cloths. Fig. 3 shows the change in bulk density of the SiC/ SiCf composites with sintering temperature. Bulk density of the composites with 40 vol.% of fibers decreased with lowering sintering temperature and the relative density was about 89–97%. The composites with 52 vol.% of fibers did not show much difference in bulk density regardless of sintering temperature and the relative density was about 93–94%.
fibers hot-pressed at 1650, 1700 and 1750 C was about 2.3, 1.2 and 0.5 kJ/m2, respectively. Fig. 6 shows the maximum bending strength of the SiC/SiCf composites measured at room temperature. In the case of the composites with slurry-impregnated HiNicalon cloths, maximum strength decreased with lowering sintering temperature, and the values were 130– 220 MPa. Maximum strength of the composites with 52
3.2. Mechanical properties The typical load-displacement curves of the SiC/SiCf composites with 40 and 52 vol.% of fibers in three-point bending test at room temperature are shown in Fig. 4. For comparison, the load-displacement curves of the composites with slurry-impregnated Hi-Nicalon cloths fabricated by hot-pressing at 1650 and 1750 C are shown in Fig. 5 [20]. In the case of the composite with slurry-impregnated Hi-Nicalon cloths, the composites hot-pressed at 1700 C (not shown in Fig. 4) or 1750 C displayed completely brittle fracture behavior, whereas the composites obtained in this study showed non-brittle fracture behavior. The load-displacement curves spread more widely with lowering sintering temperature independent of fiber volume fraction. Fracture energy increased with lowering sintering temperature in both cases of fiber volume fraction, and larger values were measured for the composite with higher fiber content. Fracture energy of the composites with 52 vol.% of
Fig. 3. Bulk density of the SiC/SiCf composites fabricated by hotpressing at various sintering temperature.
Fig. 4. Typical load-displacement curves of the SiC/SiCf composites with about (a) 40) vol.% and (b) 52 vol.% of fibers fabricated by hotpressing at various sintering temperature.
Fig. 5. Typical load-displacement curves of the SiC/SiCf composites with slurry-impregnated Hi-Nicalon cloths fabricated by hot-pressing at various sintering temperature. Fiber volume fraction of the composite is about 40 vol.%.
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Fig. 6. Maximum strength of the SiC/SiCf composites with about 40 and 52 vol.% of fibers fabricated by hot-pressing at various sintering temperature.
vol.% of fibers was about twice as high as that of the composites with 40 vol.% of fibers, and the SiC/SiCf composites hot-pressed at 1700 C showed higher maximum strength than the composites hot-pressed at 1650 or 1750 C. In the case of the hot-pressing temperature of 1700 C, the composites with 40 and 52 vol.% of fibers showed maximum strength of 120 and 240 MPa, respectively. The present authors evaluated the mechanical properties of 2D-SiC/SiCf composites fabricated by the CVI and PIP methods, which were supplied as the round robin test materials [26]. The SiC/SiCf composites fabricated by the CVI method (Vf=30%) had a maximum strength of 180–460 MPa, and the composites fabricated by the PIP method (Vf=40%) showed a maximum strength of 20–50 MPa at room temperature. The maximum strength of the composites fabricated in this study was lower than the highest value of the composites fabricated by the CVI method since the fiber strength of the composite fabricated by the CVI method should be higher than that of the composite fabricated by hotpressing mainly due to the difference in the processing temperature, i.e., processing temperature of hot-pressing is much higher than that of CVI. It has been reported that the tensile strength of Hi-Nicalon fiber after the thermal exposure in Ar atmosphere is maintained at around the original strength up to 1400 C, however, it decreased gradually above 1400 C. The Hi-Nicalon fiber heat-treated at 1600 C retains approximately half the original strength [27–29]. SEM micrographs of the fracture surface of the SiC/ SiCf composites with 40 and 52 vol.% of fibers are shown in Figs. 7 and 8, respectively. In the composite with 40 vol.% of fibers, the length of fiber pull-out was very short, whereas the composites with 52 vol.% of fibers showed large fiber pull-out.
Fig. 7. SEM micrographs of the fracture surface of the SiC/SiCf composites hot-pressed at (a) 1650 C, (b) 1700 C and (c) 1750 C after a three-point bending test at room temperature. Fiber volume fraction was about 40 vol.%.
The interfacial strength between fiber and matrix significantly affects the mechanical properties of fiberreinforced composites, and sintering temperature is
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Fig. 8. SEM micrographs of the fracture surface of the SiC/SiCf composites hot-pressed at (a) 1650 C, (b) 1700 C and (c) 1750 C after a three-point bending test at room temperature. Fiber volume fraction was about 52 vol.%.
considered to be one of the most important factors influencing the characteristics of interfacial strength [20]. As sintering temperature is increased, the reactions between fiber/fiber coating/matrix will progress, and
internal compressive stress from the matrix due to the difference of thermal expansion coefficient between matrix and fiber, and then interfacial strength between fiber and matrix may increase. In the case of the composite obtained in this study, the thermal expansion coefficient of the matrix was considered to be higher than that of the fiber, because the SiC matrix contained components with higher thermal expansion coefficient, such as Al2O3 (8.810 6 K 1) and yttrium aluminum garnet (YAG: Y3Al5O12, 5.110 6 K 1) compared with Hi-Nicalon fiber (3.510 6 K 1). Then, the tensile residual stress on the matrix and the compressive residual stress on the fibers would have taken place and the tight interface between fiber and matrix was formed. As a result, the composite would show a reduction in strength for matrix cracking and have difficulty in fiber pull-out at R.T. The present authors [30] investigated the microstructure of the composites hot-pressed with Al2O3– Y2O3–CaO additives using slurry-impregnated HiNicalon cloth by transmission electron microscopy. It was shown that some crystals grew from the matrix into a BN layer and the thickness of the layer was not homogeneous. Furthermore, growth of SiC crystals in the fiber, which induced degradation of fiber strength, was accelerated by the diffusion of sintering additives from the matrix. Therefore, it was confirmed that some reactions between matrix/fiber coating/fiber occurred in the case of slurry-impregnated cloths, but these interactions between matrix/fiber coating/fiber were effectively suppressed in the case of PCS-impregnated cloths. In the case of the hot-pressing temperature of 1650 C, the strengthening of the matrix is not enough due to higher viscosity of the glassy phase at sintering temperature. Higher viscosity causes insufficient matrix impregnation between fibers, therefore, fiber/matrix interfacial strength would be weak, resulting in decreasing maximum strength and large fiber pull-out. In contrast, in the case of the hot-pressing temperature of 1750 C, the strengthening of the matrix progressed due to lower viscosity and higher diffusion rate of the glassy phase at sintering temperature. Then the interfacial strength would be relatively high, but the degradation of fiber due to the exposure to high-temperature simultaneously progressed, resulting in low maximum strength, short fiber pull-out and then low fracture energy. In this study, it was concluded that the temperature of 1700 C would be a better condition for hot-pressing to obtain the SiC/SiCf composite with good mechanical properties. The difference in maximum strength, and then fracture energy depended greatly on the fiber volume fraction. From these results, it is concluded that the use of PCS-impregnated Hi-Nicalon cloths as the reinforcement and an increase in fiber volume fraction are effective ways to improve the mechanical properties of SiC/ SiCf composites.
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4. Conclusions Green sheets of SiC with Al2O3–Y2O3–CaO sintering additives prepared by the doctor-blade method and polycarbosilane (PCS)-impregnated 2D woven Hi-Nicalon cloth with BN-coating were used for the fabrication of SiC/SiCf composite by hot-pressing at 1650–1750 C. Two kinds of SiC/SiCf composites with different volume fractions of fibers were fabricated and their room temperature mechanical properties were evaluated. PCS-impregnation into Hi-Nicalon cloth was an effective way to form the matrix between fibers. The composites fabricated in this study showed non-brittle fracture behavior. Maximum strength of the composite with 52 vol.% of fibers was about twice as high as that of the composite with 40 vol.% of fibers, and the composite hot-pressed at 1700 C showed higher maximum strength than the composites hot-pressed at 1650 and 1750 C. Fracture energy increased with lowering sintering temperature. These results indicate sintering temperature affects the characteristics of interfacial bonding between fiber and matrix in the SiC/SiCf composite of the present study.
Acknowledgements This work was partly supported by the Research for the Future Program (RFTF97R12101) from JSPS and a Grant-in-Aid for JSPS fellows from the Ministry of Education, Science, Sports and Culture of Japan.
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