COMPOSITES SCIENCE AND TECHNOLOGY ELSEVIER 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 Yoshida,*, 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 Al2OxY2O3-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/SiCr)composites by hot-pressing. Two kinds of SiC/SiCr 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 omposite with 52 vol. of fibers was about twice as high as that of a composite with 40 vol. of fibers, and the composite hot pressed 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. C 2001 Elsevier Science Ltd. All rights reserved Keywords: A Ceramic-matrix composites; A. Preceramic polymer; B Mechanical properties; Hot-pressing 1. Introduction speed, for example, in times of the order of a day by the forced CVI method [16]. In general, however, these pro- A composite consisting of silicon carbide reinforced cesses require long manufacturing times, resulting in high with continuous SiC fibers(SiC/SiCe) is one of the candi- processing cost. Furthermore, the composites fabricated date ceramic materials for high-temperature structural by these processes usually contain about 10-20 vol. of applications since SiC shows excellent high-temperature extended large voids, resulting in low mechanical and mechanical properties, high thermal conductivity and thermal properties. In order to simplify the fabrication good oxidation, corrosion and wear resistance [1, 2]. Fur- process and to obtain dense Sic/SiCr composites with thermore, Sic shows low activation on account of its high mechanical and thermal properties, the authors low atomic number and good resistance to high-energy have studied a fabrication process using hot-pressing. neutron irradiation and it is expected to be used as which offers the ability to fabricate dense composites structural material in future fusion reactors [3-14 [18-20 SiC/SiCr composite is mainly fabricated by chemical The present authors have reported that SiC/SiCr vapor infiltration( CVI) and polymer infiltration and composite was fabricated by using a green sheet of sic pyrolysis(PIP)methods [15-17. These processes have with Al2O3-Y2O3-Cao sintering additives and Sic some advantages such as high purity and low damage to slurry-impregnated two-dimensional(2D) plain-weave fibers as a consequence of the relatively low processing Hi-Nicalon cloth with and without a BN coating by temperature. There is a variety of CVi processes and hot-pressing at 1750C [19, 20]. Although the composites some of them can form the matrix with relatively high fabricated by this process achieved nearly full density they fractured in a brittle manner. It was considered hat the interfacial bonding between fiber and matrix Corresponding author. Tel : +81-3-5734-3082: fax: +81-3-5734- was too strong as a result of the reaction between the BN E-mail address: yoshida(@ nr titech ac jp(K. Yoshida). coating on the fiber and matrix components such as the Research Fellow of the Japan Society for the Promotion of sintering additives, and the fiber was degraded severely by exposure at a temperature as high as 1750C. Moreover, 0266-3538/01/S-see front matter C 2001 Elsevier Science Ltd. All rights reserved. PII:S0266-3538(01)00031-8
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 1700C 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 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 1750C [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 1750C. Moreover, 0266-3538/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(01)00031-8 Composites Science and Technology 61 (2001) 1323–1329 www.elsevier.com/locate/compscitech * Corresponding author. Tel.: +81-3-5734-3082; fax: +81-3-5734- 2959. E-mail address: yoshida@nr.titech.ac.jp (K. Yoshida). 1 Research Fellow of the Japan Society for the Promotion of Science
K. Yoshida et al. / Composites Science and Technology 61(2001)1323-1329 Hi-Nicalon fibers were deformed since fibers were in um, AKP-50, Sumitomo Chemical, Japan), Y2O3(aver direct contact with each other because of insufficient age particle size: 2-3 um, 99.9%, High Purity Chemical impregnation by the Sic slurry between each fiber, Japan) and Cao(99.9%, Kanto Chemical, Japan), and resulting in low mechanical properties [19] some organics were used for the fabrication of the SiC In order to form the matrix between the fibers and to green sheet. The green sheet was prepared using labora- improve the mechanical properties of the SiC/SiCr com- tory-scale doctor-blade equipment (DP150, Tsugawa posite, polycarbosilane (PCS)-impregnated Hi-Nicalon Seiki, Japan). Two kinds of green sheets with different cloths were used for the reinforcement instead of Sic thickness were prepared by adjusting the blade height to slurry-impregnated Hi-Nicalon cloths. It is expected that 0.5-0.7 mm and at a carrier film speed of 10 cm/min the SiC matrix can be formed between fibers by the use of These sheets were dried at room temperature. The thick a liquid precursor. In addition, increase of fiber volume nesses of the green sheets were 105-125 and 220-285 um fraction should also be effective in improving the respectively. Details of the composition, organics in the mechanical properties of the SiC/SiCr composite. PCs green sheet, and the fabrication process were described impregnation into SiC filaments was already employed elsewhere [19]. The green sheet was cut to 35 mmx35 by Nakano et al. [21], but they fabricated only one- mm. dimensional SiC/SiCr composites by a filament winding method 2. 2. Fabrication of the SiC/Sicr composite In this study, two kinds of composites with a different olume fraction of fibers were fabricated by hot-pressing Schematic illustration of the fabrication process of the from green SiC sheet with sintering additives and PCs- Sic/SiCr composite is shown in Fig. 1. In this study, impregnated Hi-Nicalon cloth. The effects of fiber volume two-dimensionally (0/90)plain-woven BN-coated Hi fraction and sintering temperature on mechanical proper- Nicalon(Nippon Carbon, Japan) fiber cloth was used as ties of the SiC/SiCr composite at room temperature were the reinforcement. The thickness of the BN-coating was evaluated 0.4 um. The cloth was cut to 35 mmx 35 mm Polycarbosilane(PCs, NIPUSI-Type S Car. bon, Japan) was used for the impregnation into Hi 2. Experimental procedure Nicalon fiber cloth. pcs is a mixture of two molecular components, -CH3- and-(CH3)2SIiCH2-Den- 2.1. Fabrication of green sheet sity, melting point and average molecular weight of PCS powder used in this study were 1.10 g/cm, 242-249oC In this study, an Al2O3-Y2O3-CaO system was cho- and 1470-24400C, respectively. PCS powder was dissolved n for the fabrication of the SiC/SiCr composite as in toluene at 80C and the cloths were impregnated with sintering additives because of their low liquidus tem- PCS-toluene solution under reduced pressure peratures [22] The sizing agent was removed prior to polycarbosilane Submicron B-Sic powders(ultrafine, average particle impregnation. The PCS-impregnated cloths were dried at size: 0.28 Hm, Ibiden, Japan), sintering additives (20 130 C. The green sheets and the PCs-impregnated cloths mass% in total) using Al,O3(average particle size: 0.18 were stacked alternately and heat-treated at 300 C for 24 20 kPa -Sic green sheet thickness:(a)220-285 u m, (b)105-125 u m I PCS- impregnated Hi-Nicalon cloth Heat-treatment Hot-pressing 1650C,1700Cand1750°C 300C,24h, 1h in Ar, 40MPa In air 2D SiC/SiCt composite Fiber volume fraction: ( a)40 vol% Fig. 1. Schematic illustration of the fabrication process of the SiC/SiCr composite
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 , 242–249C and 1470–2440C, respectively. PCS powder was dissolved in toluene at 80C 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 130C. The green sheets and the PCS-impregnated cloths were stacked alternately and heat-treated at 300C for 24 Fig. 1. Schematic illustration of the fabrication process of the SiC/SiCf composite. 1324 K. Yoshida et al. / Composites Science and Technology 61 (2001) 1323–1329
K. Yoshida et al. Composites Science and Technology 61(2001)1323-1329 h in air under a pressure of 20 kPa. In this process, the Bulk density was measured by Archimedes' method. heating rate was 10oC/h from 150 to 300oC in order to Theoretical density of the SiC/SiCr composite was cal- prevent the combustion of Pcs due to rapid oxidation culated as that of a mixture of Sic matrix with sintering [23]. The introduction of oxygen into the PCS structure additives and Hi- Nicalon fiber during the heat-treatment in air, i.e. thermal oxidation- Three-point bending strength was measured at room curing, was promoted. Thermal oxidation curing was temperature in air with a cross-head speed of 0. 1 mm performed in order to prevent the impregnated-PCs min and a lower span of 30 mm. Bending strength into Hi-Nicalon cloths from flowing out due to lowering measurement was performed using a universal testing viscosity of PCs during hot-pressing. machine (Instron 1185, USA). Fracture energy was cal The stacked green body was hot-pressed at 1650, 1700 culated from the area of load-displacement curve in nd 1750oC for I h in Ar atmosphere under a uniaxial bending strength measurement divided by twice the due of the oxidation-cured PCS after pyrolysis at 1300C a scanning electron microscope (SEM re observed by pressure of 40 MPa. It was reported that the weight resi- fracture surface area. Fracture surface w and subsequent heat-treatment at 1500-1700oc in Ar was 43-66%[24. In this study, hot-pressing was performed at 1650-1750oC in Ar atmosphere, and the weight residue 3. Results and discussion of pcs was considered to be similar to these values Two kinds of composites with different volume fr ac- 3.. Microstructure and bulk density ons of fibers were fabricated by use of the green sheets with different thicknesses. Fiber volume fraction of the Fig. 2 shows the difference in microstructure of the composites with thicker or thinner sheets was about 40 SiC/SiCr composites using slurry-impregnated and PCs and 52 vol %o impregnated Hi-Nicalon cloths as the reinforcement. In the case of composite with slurry-impregnated Hi-Nica 2.3. Mechanical properties lon cloth[Fig. 2(a)], the SiC matrix did not form between fibers sufficiently. The fibers contacted directly each Hot-pressed specimens were cut into rectangular bars other, and then they deformed into polyhedral prism as (width: 3.5 mm, thickness: 2.3-3.2 mm, length: 34 mm). seen in Tyrannohex composites [25]. In contrast, in the (a) Slurry-impregnated (b)PCS-impregnated 50m Fig. 2. Microstructure of the SiC/SiCr composites fabricated by hot-pressing at 1750.C(a) SiC slurry-impregnated and(b)PCSimpregnated Hi- Nicalon cloths with BN-coating were used as the reinforcement
h in air under a pressure of 20 kPa. In this process, the heating rate was 10C/h from 150 to 300C 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 1750C 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 1300C and subsequent heat-treatment at 1500–1700C in Ar was 43–66% [24]. In this study, hot-pressing was performed at 1650–1750C 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). 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 1750C. (a) SiC slurry-impregnated and (b) PCS-impregnated HiNicalon cloths with BN-coating were used as the reinforcement. K. Yoshida et al. / Composites Science and Technology 61 (2001) 1323–1329 1325
1326 K. Yoshida et al. / Composites Science and Technology 61(2001)1323-1329 case of PCS-impregnated Hi-Nicalon cloths, sufficient fibers hot-pressed at 1650, 1700 and 1750C was about formation of the SiC matrix between fibers could be 2.3, 1.2 and 0.5 kJ/m, respectively achieved and round shape of the fiber was maintained Fig. 6 shows the maximum bending strength of the after hot-pressing. In this fabrication process, PCS- SiC/SiCr composites measured at room temperature. In impregnation into Hi-Nicalon cloths is an effective way the case of the composites with slurry-impregnated Hi to form SiC matrix between the fibers. Therefore all of Nicalon cloths, maximum strength decreased with low the following results were obtained for the composites ering sintering temperature, and the values were 130- with PCS-impregnated cloths 220 MPa Maximum strength of the composites with 52 Fig 3 shows the change in bulk density of the SiC SiCr composites with sintering temperature. Bulk den a)40 vol% of fibers sity 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% 1700°C 1650°c 3.2. Mechanical properties (b)52 vol% of fibe ers The typical load-displacement curves of the SiC/SiCe 1700° ites 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 1750Care 1650°C shown in Fig. 5 [20]. In the case of the composite with 100 slurry-impregnated Hi-Nicalon cloths, the composites hot-pressed at 1700 C (not shown in Fig. 4)or 1750C displayed completely brittle fracture behavior, whereas the composites obtained in this study showed non-brit- tle fracture behavior. The load-displacement curves spread more widely with lowering sintering temperature 0.2 independent of fiber volume fraction. Fracture energy Displacement(mm) increased with lowering sintering temperature in both Fig 4. Typical load-displacement curves of the SiC/SiCr composites cases of fiber volume fraction, and larger values were with about(a)40)voL. and(b)52 vol. of fibers fabricated by hot- measured for the composite with higher fiber content pressing at various sintering temperature Fracture energy of the composites with 52 vol. of 200 3.4r 1750 oo-40vol%of fibers H52vol%of fibers 0.2 0.4 0.6 1650 1750 Displacement(mm) Sintering temperature(C) Fig. 5. Typical load-displacement curves of the Sic/SiCr composites vith slurry-impregnated Hi-Nicalon cloths fabricated by hot-pressing Fig. 3. Bulk density of the SiC/SiCr composites fabricated by hot at various sintering temperature. Fiber volume fraction of the com- posite is about 40 voL%
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%. 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 1750C are shown in Fig. 5 [20]. In the case of the composite with slurry-impregnated Hi-Nicalon cloths, the composites hot-pressed at 1700C (not shown in Fig. 4) or 1750C 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 fibers hot-pressed at 1650, 1700 and 1750C 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 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.%. 1326 K. Yoshida et al. / Composites Science and Technology 61 (2001) 1323–1329
K. Yoshida et al. / Composites Science and Technology 61(2001)1323-1329 oo-40vol% of fibers a 052vol% of fibers 三品E 1650 1750 Sintering temperature(C) Fig. 6. Maximum strength of the Sic/SiCr composites with about 40 and 52 vol %o of fibers fabricated by hot-pressing at various sintering (b) vol. of fibers was about twice as high as that of the composites with 40 vol. of fibers, and the SiC/SiCt composites hot-pressed at 1700C showed higher max imum strength than the composites hot-pressed at 1650 or 1750oC. 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 The present authors evaluated the mechanical prop erties of 2D-Sic/SiCr composites fabricated by the CVI and PIP methods, which were supplied as the round robin test materials [26]. The SiC/SiCr composites fab ricated by the CVi method (Vr=30%)had a maximum strength of 180-460 MPa, and the composites fabricated C by the Plp method (Ve=40%) showed a maximum strength of 20-50 MPa at room temperature. The max imum strength of the composites fabricated in this study was lower than the highest value of the composites fab- ricated by the Cvi method since the fiber strength of the composite fabricated by the CvI method should b higher than that of the composite fabricated by hot pressing 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 1400C, however, it 50 um decreased gradually above 1400@C. The Hi-Nicalon fiber heat-treated at 1600 C retains approximately half Fig. 7. SEM micrographs of the fracture surface of the Sic/Sic the original strength [27-29 composites hot- pressed at(a)160°C,(b)1700°andc)1750° after SEM micrographs of the fracture surface of the Sic/ a three-point bending test at room temperature Fiber volume fraction SiCr composites with 40 and 52 vol% of fibers are was about voL% shown in Figs. 7 and 8, respectively. In the composite with 40 vol. of fibers, the length of The interfacial strength between fiber and matrix sig very short, whereas the composites with 52 vol. of nificantly affects the mechanical properties of fiber fibers showed large fiber pull-out. reinforced composites, and sintering temperature is
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 1700C showed higher maximum strength than the composites hot-pressed at 1650 or 1750C. In the case of the hot-pressing temperature of 1700C, 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 1400C, however, it decreased gradually above 1400C. The Hi-Nicalon fiber heat-treated at 1600C 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. The interfacial strength between fiber and matrix significantly affects the mechanical properties of fiberreinforced composites, and sintering temperature is 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. Fig. 7. SEM micrographs of the fracture surface of the SiC/SiCf composites hot-pressed at (a) 1650C, (b) 1700C and (c) 1750C after a three-point bending test at room temperature. Fiber volume fraction was about 40 vol.%. K. Yoshida et al. / Composites Science and Technology 61 (2001) 1323–1329 1327
K. Yoshida et al. / Composites Science and Technology 61(2001)1323-1329 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 com- posite 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 coeficient, such as Al2O3( 8.8x10K- )and yttrium aluminum garnet(YAG: Y3Al5O12, 5.1x10-6K-)compared with Hi-Nicalon fiber (3. 5x10-6K-). Then, the tensile resi- dual 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 redu strength for matrix cracking and have difficulty in fiber (b) pull-out at RT The present authors [30] investigated the mic structure of Y2O3-Cao additives using slurry-impregnated Hi Nicalon 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 interac- tions between matrix/fiber coating/fiber were effectively suppressed in the case of PCS-impregnated cloths In the case of the hot-pressing temperature of 1650oC the strengthening of the matrix is not enough due to higher viscosity of the glassy phase at sintering tempera ture. Higher viscosity causes insufficient matrix impreg nation 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 1750oC, the trengthening of the matrix progressed due to lot of the glassy phase sintering temperature. Then the interfacial strength would be relatively high, but the degradation of fiber due to the exposure to h progressed, resulting in low maximum strength, short fiber pull-out and then low fracture energy. In this study, 0 d that the temperature of 1700C would be a better condition for hot-pressing to obtain the SiC/SiCt Fig. 8. SEM micrographs of the fracture surface of the Sic/SiCt composites hot- pressed at(a)1650°C,(b)1700°Cand(c)1750° after composite with good mechanical properties. The differ a three-point bending test at room ter ure. Fiber volume fraction ence in maximum strength, and then fracture energy was about 52 vol% depended greatly on the fiber volume fraction. From these results it is concluded that the use of considered to be one of the most important factors PCS-impregnated Hi-Nicalon cloths as the reinforce- influencing the characteristics of interfacial strength ment and an increase in fiber volume fraction are effec [20]. As sintering temperature is increased, the reactions tive ways to improve the mechanical properties of SiC/ between fiber/fiber coating/matrix will progress, and SiCr composites
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 (8.8106 K1 ) and yttrium aluminum garnet (YAG: Y3Al5O12, 5.1106 K1 ) compared with Hi-Nicalon fiber (3.5106 K1 ). 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 1650C, 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 1750C, 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 1700C 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. Fig. 8. SEM micrographs of the fracture surface of the SiC/SiCf composites hot-pressed at (a) 1650C, (b) 1700C and (c) 1750C after a three-point bending test at room temperature. Fiber volume fraction was about 52 vol.%. 1328 K. Yoshida et al. / Composites Science and Technology 61 (2001) 1323–1329
K. Yoshida et al. Composites Science and Technology 61(2001)1323-1329 4. Conclusions [11] Corelli CH, Hoole J, Lazzaro J, Lee Cw. Mechanical, thermal, nd microstructural properties of neutron-irradiated SiC. J Am Green sheets of SiC with AlO Y2O3-CaO sintering Ceram Soc1983:66(7):529-37 additives prepared by the doctor-blade method and poly- [12 Price R. Properties o slicon carbide tor nuclear fuel partic carbosilane(PCS)-impregnated 2D woven Hi-Nicalon [13] Wu CH, Bonal JP Kryger B. The effect of high-dose neutron cloth with BN-coating were used for the fabrication of irradiation on the properties of graphite and silicon carbide. J Sic/SiCr composite by hot-pressing at 1650-17500C. Two Nucl Mater 1994: 208(I and 2): 1-7. kinds of SiC/SiCr composites with different volume frac- [14 Suzuki T. Yano T, Mori T, Miyazaki H, Iseki T Neutron irradia- tions of fibers were fabricated and their room tempera- tion damage of silicon carbide. Fusion Technol 1995: 27(3): 314-25. [5 Strife JR, Brennan JJ, Prewo KM. Status of continuous fiber- ture mechanical properties were evaluated reinforced ceramic matrix composite processing technology cPCS-impregnation into Hi-Nicalon cloth was an Ceram Eng Sci Proc 1990: 11(7-8): 871-919 Tective way to form the matrix between fibers. The [16] Geoghegan PJ. Chemical vapor infiltrated composites. In composites fabricated in this study showed non-brittle fracture behavior. Maximum strength of the composite ican Society of Mechanical Engineers, 1992. p. 113-23n SR, editor. Flight-vehicle materials, structures and dynar assessment and future directions. vol. 3. New York. with 52 vol. of fibers was about twice as high as that [7 Hurwitz FI. Polymeric precursors for fibers and matrices. In of the composite with 40 vol. of fibers, and the com- Levine Sr, editor. Flight-vehicle materials, structures and posite hot-pressed at 1700C showed higher maximum dynamics-assessment and future directions, vol. 3. New York: strength than the composites hot-pressed at 1650 and The American Society of Mechanical Engineers, 1992. P 59-77 1750%C. Fracture energy increased with lowering sinter- [18] Yano T, Budiyanto, Yoshida K, Iseki T Fabrication of silicon ing temperature. These results indicate sintering tem- carbide fiber-reinforced silicon carbide composite by hot-press. ing. Fusion Eng design 1998: 41: 157-63. perature affects the characteristics of interfacial bonding [19] Yoshida K, Budiyanto, Imai M, Yano T. Processing and micro- between fiber and matrix in the SiC/SiCr composite of cture of silicon carbide fiber-reinforced silicon carb the present study posite by hot-pressing. J Nucl Mater 1998: 258-263: 1960-5 20 Yoshida K, Imai M, Yano T. Microstructure and mechanical properties of hot-pressed silicon carbide fiber-reinforced silicon carbide composite. Key Eng Mater 1999: 164-165: 217-20 Acknowledgements 21] Nakano K, Sasaki K, Saka H, Fujikawa M, Ichikawa H. SiC- This work was partly supported by the Research for Evans AG. Naslain R. editors. Ceramic transactions. voL 58 the Future Program(RFTF97R12101)from JSPS and a High-temperature ceramic-matrix composites Il: manufacturing and materials development. Westerville(OH): American Ceramic Grant-in-Aid for JSPS fellows from the Ministry of Society,1995.p.215-29 Education, Science, Sports and Culture of Japan 22] Mitomo M, Kim Yw. Hirotsuru H. Fabrication of silicon car- bide nanoceramics. J Mater Sci 1996: 11(7): 1601-4 23 Shimo T, Sugimoto M, Okamura K. Kinetics of curing poly. arbosilane fiber by oxidation treatment. J. Ceram. Soc. Japan References 1991: 99: 514-9(in Japanese) 24 Kakimoto K, Wakai F, Bill J, Aldinger F. Synthesis of Si-C-O [1 Herbell TP Sanders WA. Monolithic ceramics. In: Levine SR, bulk ceramics with various chemical compositions from poly editor. Flight-vehicle materials, structures and dynamics-asses carbosilane. J Am Ceram Soc 1999: 82. 2337-41 nent and future directions, vol 3. New York: The American [25] Ishikawa T, Kajii S, Matsunaga K, Hogami T, Kohtoku Y Society of Mechanical Engineers, 1992. p 19-41 Nagasawa T. A tough, thermally conductive silicon carbide [2 Whalen TJ. Processing and properties of structural silicon car. bide Ceram Eng Sci Proc 1986: 7(9-10): 1135-43. 1998;282:1295-7 3 Rovner JH, Hopkins GR. Ceramic materials for fusion. Nucl [26] Yoshida K, Imai M, Yano T. Room- and elevated-temperature Tech19762903):274302. mechanical properties of SiC fiber-reinforced Sic composites 4 Hopkins GR, Price RJ. Fusion reactor design with ceramics fabricated by CVI and PIP methods. J Ceram Soc Japan Nucl Eng Design/Fusion 1985: 2(I and 2): 111-43 2000108(3):224-9. 5 Jones RH, Henager Jr CH, Hollenberg Gw. Composite materials [27] Ichikawa H, Okamura K, Seguchi T Oxygen-free ceramic fibers for fusion applications. J Nucl Mater 1992: 191-194: 75-83. om organosilicon precursors and e-beam curing. In: Evans AG [6 Fenici P, Scholz Hw. Advanced-low activation materials. Fiber. Naslain R. editors. Ceramic transactions. vol. 58. high-tem inforced ceramic composites. J Nucl Mater 1994: 212-215: 60-8 ture ceramic-matrix composites Il: manufacturing and materials [7 Snead LL, Jones RH, Kohyama A, Fenici P. Status of silicon development. Westerville(OH): American Ceramic Society, 1995 p.6574 [8 Jones RH, Henager Jr CH, Youngblood GE, Heinisch HL SiC/ [28] Chollon G, Pailler R, Naslain R, Laanani F, Monthioux M, Olry Sic composites for structural applications in fusion energy sys- P. Thermal stability of a PCS-derived SiC fibre with a low oxygen tems Fusion Technol 1996: 30(3): 969-76. [9 Donato A, Andreani R. Material requirements and perspectives [29] Shimoo T, Tsukada I, Narisawa M, Seguchi T, Okamura K. for future thermonuclear fusion reactors. Fusion Technol Change in properties of polycarbosilane-derived SiC fibers at high temperatures. J Ceram Soc Japan 1997: 105(7): 559-63 [10 Harrison SD, Corelli JC. Microstructure of neutron irradiation- [30 Yano T, Yamamoto Y, Yoshida K. TEM investigation and nduced defects in sintered and siliconized SiC. J Nucl Mater fracture behavior of SiC/SiC composites fabricated by hot-press. 1984:122and123:833-9 ng. Key Eng Mater 1999: 166: 135-8
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 1650–1750C. 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 1700C showed higher maximum strength than the composites hot-pressed at 1650 and 1750C. 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. References [1] Herbell TP, Sanders WA. Monolithic ceramics. In: Levine SR, editor. Flight-vehicle materials, structures and dynamics-assessment and future directions, vol. 3. New York: The American Society of Mechanical Engineers, 1992. p. 19–41. [2] Whalen TJ. Processing and properties of structural silicon carbide. Ceram Eng Sci Proc 1986;7(9-10):1135–43. [3] Rovner JH, Hopkins GR. Ceramic materials for fusion. Nucl Tech 1976;29(3):274–302. [4] Hopkins GR, Price RJ. Fusion reactor design with ceramics. Nucl Eng Design /Fusion 1985;2(1 and 2):111–43. [5] Jones RH, Henager Jr CH, Hollenberg GW. Composite materials for fusion applications. J Nucl Mater 1992;191-194:75–83. [6] Fenici P, Scholz HW. Advanced-low activation materials. Fiberreinforced ceramic composites. J Nucl Mater 1994;212-215:60–8. [7] Snead LL, Jones RH, Kohyama A, Fenici P. Status of silicon carbide composites for fusion. J Nucl Mater 1996;233-237:26–36. [8] Jones RH, Henager Jr CH, Youngblood GE, Heinisch HL. SiC/ SiC composites for structural applications in fusion energy systems. Fusion Technol 1996;30(3):969–76. [9] Donato A, Andreani R. Material requirements and perspectives for future thermonuclear fusion reactors. Fusion Technol 1996;29(1):58–72. [10] Harrison SD, Corelli JC. Microstructure of neutron irradiationinduced defects in sintered and siliconized SiC. J Nucl Mater 1984;122 and 123:833–9. [11] Corelli CH, Hoole J, Lazzaro J, Lee CW. Mechanical, thermal, and microstructural properties of neutron-irradiated SiC. J Am Ceram Soc 1983;66(7):529–37. [12] Price RJ. Properties of silicon carbide for nuclear fuel particle coatings. Nucl Technol 1977;35(2):320–36. [13] Wu CH, Bonal JP, Kryger B. The effect of high-dose neutron irradiation on the properties of graphite and silicon carbide. J Nucl Mater 1994;208(1 and 2):1–7. [14] Suzuki T, Yano T, Mori T, Miyazaki H, Iseki T. Neutron irradiation damage of silicon carbide. Fusion Technol 1995;27(3):314–25. [15] Strife JR, Brennan JJ, Prewo KM. Status of continuous fiberreinforced ceramic matrix composite processing technology. Ceram Eng Sci Proc 1990;11(7-8):871–919. [16] Geoghegan PJ. Chemical vapor infiltrated composites. In: Levine SR, editor. Flight-vehicle materials, structures and dynamicsassessment and future directions, vol. 3. New York: The American Society of Mechanical Engineers, 1992. p. 113–37. [17] Hurwitz FI. Polymeric precursors for fibers and matrices. In: Levine SR, editor. Flight-vehicle materials, structures and dynamics-assessment and future directions, vol. 3. New York: The American Society of Mechanical Engineers, 1992. p. 59–77. [18] Yano T, Budiyanto, Yoshida K, Iseki T. Fabrication of silicon carbide fiber-reinforced silicon carbide composite by hot-pressing. Fusion Eng Design 1998;41:157–63. [19] Yoshida K, Budiyanto, Imai M, Yano T. Processing and microstructure of silicon carbide fiber-reinforced silicon carbide composite by hot-pressing. J Nucl Mater 1998;258-263:1960–5. [20] Yoshida K, Imai M, Yano T. Microstructure and mechanical properties of hot-pressed silicon carbide fiber-reinforced silicon carbide composite. Key Eng Mater 1999;164-165:217–20. [21] Nakano K, Sasaki K, Saka H, Fujikawa M, Ichikawa H. SiCand Si3N4-matrix composites according to the hot-pressing route. In: Evans AG, Naslain R, editors. Ceramic transactions, vol. 58, High-temperature ceramic-matrix composites II: manufacturing and materials development. Westerville (OH): American Ceramic Society, 1995. p. 215–29. [22] Mitomo M, Kim YW, Hirotsuru H. Fabrication of silicon carbide nanoceramics. J Mater Sci 1996;11(7):1601–4. [23] Shimoo T, Sugimoto M, Okamura K. Kinetics of curing polycarbosilane fiber by oxidation treatment. J. Ceram. Soc. Japan 1991; 99: 514–9 (in Japanese). [24] Kakimoto K, Wakai F, Bill J, Aldinger F. Synthesis of Si-C-O bulk ceramics with various chemical compositions from polycarbosilane. J Am Ceram Soc 1999;82:2337–41. [25] Ishikawa T, Kajii S, Matsunaga K, Hogami T, Kohtoku Y, Nagasawa T. A tough, thermally conductive silicon carbide composite with high strength up to 1600C in air. Science 1998;282:1295–7. [26] Yoshida K, Imai M, Yano T. Room- and elevated-temperature mechanical properties of SiC fiber-reinforced SiC composites fabricated by CVI and PIP methods. J Ceram Soc Japan 2000;108(3):224–9. [27] Ichikawa H, Okamura K, Seguchi T. Oxygen-free ceramic fibers from organosilicon precursors and e-beam curing. In: Evans AG, Naslain R, editors. Ceramic transactions, vol. 58, high-temperature ceramic-matrix composites II: manufacturing and materials development. Westerville (OH): American Ceramic Society, 1995. p. 65–74. [28] Chollon G, Pailler R, Naslain R, Laanani F, Monthioux M, Olry P. Thermal stability of a PCS-derived SiC fibre with a low oxygen content (Hi-Nicalon). J Mater Sci 1997;32:327–47. [29] Shimoo T, Tsukada I, Narisawa M, Seguchi T, Okamura K. Change in properties of polycarbosilane-derived SiC fibers at high temperatures. J Ceram Soc Japan 1997;105(7):559–63. [30] Yano T, Yamamoto Y, Yoshida K. TEM investigation and fracture behavior of SiC/SiC composites fabricated by hot-pressing. Key Eng Mater 1999;166:135–8. K. Yoshida et al. / Composites Science and Technology 61 (2001) 1323–1329 1329