Composites Science and Technology 69(2009)1623-1628 Contents lists available at ScienceDirect Composites Science and Technology ELSEVIER journalhomepagewww.elsevier.com/locate/compscitech High mechanical performance Sic/Sic composites by nite process with tailoring of appropriate fabrication temperature to fiber volume fraction Kazuya Shimoda, Akira Kohyama, Tatsuya Hinoki Institute of Advanced Energy, Kyoto University, Gokasho Uji, Kyoto 611-0011, Japar ARTICLE IN F O ABSTRACT Article history: Received 21 January 2009 (NITE)process, using pyrolytic carbon(PyC)-coated Tyranno-SA SiC fibers as reinforcement and Sic nano- Received in revised form 4 March 2009 powder with sintering additives for matrix formation. The effects of two kinds of fiber volume fraction Available online 24 March 2009 incorporating fabrication temperature were characterized on densification, microstructure and mechan- ical properties. Densification of the composites with low fiber volume fraction(appropriately 30 vol% eywords was developed even at lower fabrication temperature of 1800C, and then saturated at 3rd stage of matrix densification A Ceramic matrix composites(CMCs) esponding to classic liquid phase sintering. Hence, densification of the compo ites with high volume fraction (above 50 vol% became restricted because the many fibers retarded the B Mechanical properties nfiltration of Sic nano-powder at lower fabrication temperature of 1800C. when fabrication tempera B Stress/ strain curves ture increased by 1900C, densification of the composites was effectively enhanced in the intra-fiber E Sintering lltaneously the interaction between Pyc interface and matrix was strengthened. SEM observation on the fracture surface revealed that fiber pull-out length was accordingly changed with fab- rication temperature as well as fiber volume fraction, which dominated tensile fracture behaviors. Through NITE process, SiC/SiC composites with two fracture types were successfully developed by tailor- ing of appropriate fabrication temperature to fiber volume fraction as follows: (1)high ductility type and (2) high strength typ o 2009 Elsevier Ltd. All rights reserved. 1 Introduction cal properties under high temperature and neutron irradiation Advanced nuclear energy systems, such as gas cooled fast impregnation and pyrolysis(Pip)and melt infiltration(Mi)pro- reactor (GFR), very high temperature reactor(VHTR) and fusion cess are mainly adopted for the fabrication of Sic/Sic composites reactor are potential candidates for sustainable energy systems 5-7. However, still total performances of these composites are in the future. In order to realize these attractive energy systems, not satisfied for going of industrial stage. Matrix densification is structural materials must be responsible to keep their perfor- frequently emphasized as a key to improve poor thermo-mechan- mances under very severe environment including high tempera- ical properties in addition to gas tightness[8-10 The hot-press- ture and neutron irradiation. Today a major thrust is by the ing process is effective processing technique to offer the dense development of continuous fiber-reinforced ceramic matri omposites [11. Thermal stability and resistance against creep omposites(CFCCs)in general and continuous silicon carbide fi- of continuous Sic fiber as reinforcement are essential to fabricate ber-reinforced silicon carbide matrix( SiC/Sic)composites in par- SiC/SiC composites by hot-pressing. It has reported that Tyran ticular [1, 2. Because of continuous fiber-reinforcement, SiC/Sic no-SA fibers with high tensile strength and modulus show no composites are more damage tolerant to mechanical and thermal degradation in strength or change in composition on heating to loading( thermal shock) and have the capability for larger and or 1900C in an inert atmosphere and superior bend stress relaxa- complicated components than their SiC monolithic form. Also, in tion creep resistance to previous Sic-based fibers [12]. The devel- comparison to the best high-temperature metallic alloys, SiC/Sic opment of advanced Sic-based fibers with well-crystallized composites are lower specific gravity and thermal expansion, microstructure and near-stoichiometric composition, such as tyr- and have the potential for displaying excellent thermo-mechani- anno-SA fibers, made it possible to fabricate high-performance SiC/SiC composites even under harsh conditions by hot-pressing [13, 14. One of the novel accomplishments is a fabrication pro- uthor. Present address: DEN/DANS/DMN/SRMA, CEA-Sacl Essonne91191, france.Tel.:+33169082754:fax:+33169 cess for SiC/SiC composites, named nano-infiltration and transient 087167 eutectic-phase(NITE) process developed in our group at kyoto Ity 3538/s-see front matter o 2009 Elsevier Ltd. All right erved 016/j-compscitech. 2009.03.011
High mechanical performance SiC/SiC composites by NITE process with tailoring of appropriate fabrication temperature to fiber volume fraction Kazuya Shimoda *, Akira Kohyama, Tatsuya Hinoki Institute of Advanced Energy, Kyoto University, Gokasho Uji, Kyoto 611-0011, Japan article info Article history: Received 21 January 2009 Received in revised form 4 March 2009 Accepted 11 March 2009 Available online 24 March 2009 Keywords: A. Ceramic matrix composites (CMCs) A. Nano particles B. Mechanical properties B. Stress/strain curves E. Sintering abstract Unidirectional SiC/SiC composites are prepared by nano-powder infiltration and transient eutectic-phase (NITE) process, using pyrolytic carbon (PyC)-coated Tyranno-SA SiC fibers as reinforcement and SiC nanopowder with sintering additives for matrix formation. The effects of two kinds of fiber volume fraction incorporating fabrication temperature were characterized on densification, microstructure and mechanical properties. Densification of the composites with low fiber volume fraction (appropriately 30 vol%) was developed even at lower fabrication temperature of 1800 C, and then saturated at 3rd stage of matrix densification corresponding to classic liquid phase sintering. Hence, densification of the composites with high volume fraction (above 50 vol%) became restricted because the many fibers retarded the infiltration of SiC nano-powder at lower fabrication temperature of 1800 C. When fabrication temperature increased by 1900 C, densification of the composites was effectively enhanced in the intra-fiberbundles and simultaneously the interaction between PyC interface and matrix was strengthened. SEM observation on the fracture surface revealed that fiber pull-out length was accordingly changed with fabrication temperature as well as fiber volume fraction, which dominated tensile fracture behaviors. Through NITE process, SiC/SiC composites with two fracture types were successfully developed by tailoring of appropriate fabrication temperature to fiber volume fraction as follows: (1) high ductility type and (2) high strength type. 2009 Elsevier Ltd. All rights reserved. 1. Introduction Advanced nuclear energy systems, such as gas cooled fast reactor (GFR), very high temperature reactor (VHTR) and fusion reactor are potential candidates for sustainable energy systems in the future. In order to realize these attractive energy systems, structural materials must be responsible to keep their performances under very severe environment including high temperature and neutron irradiation. Today a major thrust is by the development of continuous fiber-reinforced ceramic matrix composites (CFCCs) in general and continuous silicon carbide fi- ber-reinforced silicon carbide matrix (SiC/SiC) composites in particular [1,2]. Because of continuous fiber-reinforcement, SiC/SiC composites are more damage tolerant to mechanical and thermal loading (thermal shock) and have the capability for larger and/or complicated components than their SiC monolithic form. Also, in comparison to the best high-temperature metallic alloys, SiC/SiC composites are lower specific gravity and thermal expansion, and have the potential for displaying excellent thermo-mechanical properties under high temperature and neutron irradiation [2–4]. Up to date, chemical vapor infiltration (CVI), polymer impregnation and pyrolysis (PIP) and melt infiltration (MI) process are mainly adopted for the fabrication of SiC/SiC composites [5–7]. However, still total performances of these composites are not satisfied for going of industrial stage. Matrix densification is frequently emphasized as a key to improve poor thermo-mechanical properties in addition to gas tightness [8–10]. The hot-pressing process is effective processing technique to offer the dense composites [11]. Thermal stability and resistance against creep of continuous SiC fiber as reinforcement are essential to fabricate SiC/SiC composites by hot-pressing. It has reported that TyrannoTM-SA fibers with high tensile strength and modulus show no degradation in strength or change in composition on heating to 1900 C in an inert atmosphere and superior bend stress relaxation creep resistance to previous SiC-based fibers [12]. The development of advanced SiC-based fibers with well-crystallized microstructure and near-stoichiometric composition, such as TyrannoTM-SA fibers, made it possible to fabricate high-performance SiC/SiC composites even under harsh conditions by hot-pressing [13,14]. One of the novel accomplishments is a fabrication process for SiC/SiC composites, named nano-infiltration and transient eutectic-phase (NITE) process developed in our group at Kyoto University [8,9,13–17]. 0266-3538/$ - see front matter 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2009.03.011 * Corresponding author. Present address: DEN/DANS/DMN/SRMA, CEA-Saclay, Gif-sur-Yvette Cedex, Essonne 91191, France. Tel.: +33 1 69 08 27 54; fax: +33 1 69 08 71 67. E-mail address: kazuya.shimoda@cea.fr (K. Shimoda). Composites Science and Technology 69 (2009) 1623–1628 Contents lists available at ScienceDirect Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
K. Shimoda et aL/Composites Science and Technology 69(2009)1623-1628 The objective of this article is to provide the results of recent matrix formation. Characterizations of Sic nano-powder employed ctivities in our group at Kyoto University, aimed at developing ad- in this study was described elsewhere [22]. For the preparation of vanced Sic/Sic composites with tailoring of mechanical perfor- prepreg sheets, unidirectional Py C-coated Tyranno-SA fibers were mances by NITE process. Fig. 1 shows two concepts on typical impregnated in Sic 'nano-slurry, which consists of the tensile fracture behavior for CFCCs [1, 18]. A primary need in their of B-Sic na der and sintering additives(Al203+Y as-producted condition is to display as high a proportional limit wt%(Al203: Y203=60: 40)and Sio2=3 wt%)in isopropyl stress(PLS)as possible. The high PLS value will allow the materials The prepreg sheets were dried at room-temperature and cut into to carry high combinations of mechanical and thermal tensile 40 mm x 40 mm square. Prepared prepreg sheets were unidire stress without cracking, and be considered as the useful index of tionally stacked in a graphite die, and then hot-pressed at 1800- laterial/component design base on elastic mechanical behavior 1900C for 1 h in Ar atmosphere under the pressure of 20 MPa a high stress. And thus, the composites, as shown in Fig. 1a, with two kinds of fiber volume fraction. One is low fiber volume are potentially considered as suitable for fusion blankets, heat fraction (low-VA, appropriately 30 vol%. Another is high fiber vol- exchangers and turbine disks. However, during service time, unex- ume fraction(high-V) above 50 vol% pectedly higher stresses may arise that can locally crack the ma- trix, causing immediate material failure if the composites do not 2.2. Characterization of fabricated Sic/sic composites display a large ductile fracture behavior with a high ultimate ten- sile strength(UTS). The composites, as shown in Fig. 1b, are poten The bulk density and open porosity of fabricated composites were measured by the Archimedes'method, using distilled water protection tiles and after burner flaps. Although several approaches as the immersion medium. Theoretical density of fabricated com- have been taken towards two concepts, interface bonding between posites was calculated using the rule of mixtures, which consist fiber and matrix is frequently focused as a technical key because of Sic matrix with sintering additives and Py c-coated Tyranno ad transfer from the matrix to the fibers and vice versa occurs SA fiber. Fabricated composites were subsequently cut into through the interface [19-21. The high strength type requires effi- 3.0 mm x 1.5 mm x 40 mm for monotonic tensile test and cient load transfers from matrix to fibers which are obtained with 3.0 mm x 1.5 mm x 25 mm for three-point bending test, with par strong interface On the other hand, the high ductility type requires allel direction to fiber axis Monotonic tensile test was carried out high fracture toughness contributed by crack bridging and fiber at room-temperature in an INSTRoN 5581 test machine with a pull-out which is obtained with weak interface. For two concepts, crosshead displacement rate of 0.5 mm/min. On both edges of the effects of fiber volume fraction incorporating fabrication tem he tensile bars, the aluminum tabs were affixed to each side using perature were investigated on densification, microstructure and a kind of standard Araldite binder. The gauge length was desig- mechanical properties. Furthermore, tensile fracture behaviors nated to be 20 mm. Tensile strain was recorded from the exten- were discussed based on the interface bonding someter fixed on both sides of the gauge areas. For comparison, three-point bending test was also performed at room-temperature 2. Experimental procedure in the same test machine, with a crosshead displacement rate of 0.5 mm/min and outer support span of 18 mm. Each test number 2.1. NTE process for Si was at least three specimens. Both the polished cross-section and the tensile fracture surface were observed by field emission elec- Pyrolytic carbon(PyC)-coated Tyranno-SArd SiC fibers(Ube tron microscopy (FE-SEM). for SiC/SiC fabrication. Our previous study revealed that the thick- 3. Results and discussion ness of induced-Pyc interface strongly affected the density al microstructure, and therefore dominated fracture behaviors [17]. 3.1. Densification process In the present study, Py C coating was appropriately chosen at the thickness of 0.5 um through chemical vapor deposition(CvD)pro- Before studying the densification process, it is necessary to cessB-SiC nano-powder(Sumitomo Osaka Cement Co Ltd, Japan, investigate densification of the matrix itself without fibers. The T-1 grade) and sintering additives with Al2O3( Kojundo Chemical classic liquid phase sintering densifies in three overlapping stages Laboratory Co. Ltd Japan, mean diameter of 0.3 um. 99.99% pure),(1)rearrangement stage. ( 2)solution-reprecipitation stage and (3) Y203(Kojundo Chemical Laboratory Co Ltd, Japan, mean diameter solid skeleton stage. Fig. 2 shows the effects of the fabrication tem- of 1.0 um, 99.99% pure)and SiO2( Kojundo Chemical Laboratory Co. perature on the densification of monolithic SiC and SiC/Sic com- Ltd, Japan, mean diameter of 1.0 um, 99.9% up pure) were used for posites. For monolithic Sic corresponding to the matrix in NITE b Hah。 via strong fiber bonding Maximum use of 9 bundled fiber strengt High PLS wE。 Larger strain LOw PLS Strain Strain Fig. 1. Concept on tensile fracture behavior for continuous fiber-reinforced ceramic composites: (a) high strength type and(b) high ductility type
The objective of this article is to provide the results of recent activities in our group at Kyoto University, aimed at developing advanced SiC/SiC composites with tailoring of mechanical performances by NITE process. Fig. 1 shows two concepts on typical tensile fracture behavior for CFCCs [1,18]. A primary need in their as-producted condition is to display as high a proportional limit stress (PLS) as possible. The high PLS value will allow the materials to carry high combinations of mechanical and thermal tensile stress without cracking, and be considered as the useful index of material/component design base on elastic mechanical behavior to a high stress. And thus, the composites, as shown in Fig. 1a, are potentially considered as suitable for fusion blankets, heat exchangers and turbine disks. However, during service time, unexpectedly higher stresses may arise that can locally crack the matrix, causing immediate material failure if the composites do not display a large ductile fracture behavior with a high ultimate tensile strength (UTS). The composites, as shown in Fig. 1b, are potentially considered as suitable for combustor liners, thermal protection tiles and after burner flaps. Although several approaches have been taken towards two concepts, interface bonding between fiber and matrix is frequently focused as a technical key because load transfer from the matrix to the fibers and vice versa occurs through the interface [19–21]. The high strength type requires effi- cient load transfers from matrix to fibers which are obtained with strong interface. On the other hand, the high ductility type requires high fracture toughness contributed by crack bridging and fiber pull-out which is obtained with weak interface. For two concepts, the effects of fiber volume fraction incorporating fabrication temperature were investigated on densification, microstructure and mechanical properties. Furthermore, tensile fracture behaviors were discussed based on the interface bonding. 2. Experimental procedure 2.1. NITE process for SiC/SiC composites Pyrolytic carbon (PyC)-coated TyrannoTM-SA3rd SiC fibers (Ube Industrials Ltd., Yamaguchi, Japan) were used as reinforcement for SiC/SiC fabrication. Our previous study revealed that the thickness of induced-PyC interface strongly affected the density and microstructure, and therefore dominated fracture behaviors [17]. In the present study, PyC coating was appropriately chosen at the thickness of 0.5 lm through chemical vapor deposition (CVD) process. b-SiC nano-powder (Sumitomo Osaka Cement Co. Ltd., Japan, T-1 grade) and sintering additives with Al2O3 (Kojundo Chemical Laboratory Co. Ltd. Japan, mean diameter of 0.3 lm, 99.99% pure), Y2O3 (Kojundo Chemical Laboratory Co. Ltd., Japan, mean diameter of 1.0 lm, 99.99% pure) and SiO2 (Kojundo Chemical Laboratory Co. Ltd., Japan, mean diameter of 1.0 lm, 99.9% up pure) were used for matrix formation. Characterizations of SiC nano-powder employed in this study was described elsewhere [22]. For the preparation of prepreg sheets, unidirectional PyC-coated Tyranno-SA fibers were impregnated in SiC ‘nano’-slurry, which consists of the mixture of b-SiC nano-powder and sintering additives (Al2O3 + Y2O3 = 9 wt% (Al2O3:Y2O3 = 60:40) and SiO2 = 3 wt%) in isopropyl alcohol. The prepreg sheets were dried at room-temperature and cut into 40 mm 40 mm square. Prepared prepreg sheets were unidirectionally stacked in a graphite die, and then hot-pressed at 1800– 1900 C for 1 h in Ar atmosphere under the pressure of 20 MPa with two kinds of fiber volume fraction. One is low fiber volume fraction (low-Vf), appropriately 30 vol%. Another is high fiber volume fraction (high-Vf) above 50 vol%. 2.2. Characterization of fabricated SiC/SiC composites The bulk density and open porosity of fabricated composites were measured by the Archimedes’ method, using distilled water as the immersion medium. Theoretical density of fabricated composites was calculated using the rule of mixtures, which consist of SiC matrix with sintering additives and PyC-coated TyrannoTMSA fiber. Fabricated composites were subsequently cut into 3.0 mm 1.5 mm 40 mm for monotonic tensile test and 3.0 mm 1.5 mm 25 mm for three-point bending test, with parallel direction to fiber axis. Monotonic tensile test was carried out at room-temperature in an INSTRON 5581 test machine with a crosshead displacement rate of 0.5 mm/min. On both edges of the tensile bars, the aluminum tabs were affixed to each side using a kind of standard Araldite binder. The gauge length was designated to be 20 mm. Tensile strain was recorded from the extensometer fixed on both sides of the gauge areas. For comparison, three-point bending test was also performed at room-temperature in the same test machine, with a crosshead displacement rate of 0.5 mm/min and outer support span of 18 mm. Each test number was at least three specimens. Both the polished cross-section and the tensile fracture surface were observed by field emission electron microscopy (FE-SEM). 3. Results and discussion 3.1. Densification process Before studying the densification process, it is necessary to investigate densification of the matrix itself without fibers. The classic liquid phase sintering densifies in three overlapping stages: (1) rearrangement stage, (2) solution-reprecipitation stage and (3) solid skeleton stage. Fig. 2 shows the effects of the fabrication temperature on the densification of monolithic SiC and SiC/SiC composites. For monolithic SiC corresponding to the matrix in NITE Fig. 1. Concept on tensile fracture behavior for continuous fiber-reinforced ceramic composites: (a) high strength type and (b) high ductility type. 1624 K. Shimoda et al. / Composites Science and Technology 69 (2009) 1623–1628
3rd stage retard the densification. On the other hand, densification of composites with high-V was not sufficient at lower fabrication 2nd stage could effectively enhance the densification. At 1900C, almost 99% of the theoretical density could be obtained. Densification of the composite with high-V had the difficulty to explain by the clas 3. 2. Microstructural evolutions The matrix in NITE composites consists of well-crystallized Sic grains with small amount of oxide remnants including alum Monolithic Sic um and yttrium(mostly YAG: yttrium aluminum garnet)from b-Composites(High-V XRD analysis[17 The presence of YAG should improve the densi Composites(Low-vy fication of matrix in composites. Fig. 3 shows low-magnification and intra-fiber-bundle microstructures of polished cross-section of Sic/SiC composites. Typically, no inter-fiber-bundle pores were 1700 observed, even in the composites with high-V fabricated at 1800C. Many small pores were observed in the intra-fiber-bun- Fabrication temperature (oc) dles for the composites with high-V fabricated at 1800C. The Fig. 2. Effects of the fabrication temperature on densification of monolithic SiC and composites maintained about 6. 2% open porosity, so that the for- wo SiC/SiC composites. mation of porous intra-fiber-bundle matrix could be identified. With increasing temperature, the pore size and the number of pores decreased, and simultaneously densification in the intra-fi omposites, densification was well reflected to the classic liquid ber-bundles was promoted better. At 1900C, intra-fiber-bundle phase sintering, and thus could be divided into three stages, as pores were strongly eliminated, as shown in Fig. 3e. Even for the shown in Fig. 2. Densification of the composites with low-V was well-densified intra-fiber-bundle matrix, some differences co dramatically promoted at the temperature of 1750-1800C prob- be identified with different fabrication temperature. In the com- ably due to the liquid phase formation of sintering additives lead- posites with low-V fabricated at 1800C, some consolidated parts ing to the rearrangement of SiC nano-powder, and then saturated were loosely formed On the other hand, in the composites with above 1800C In NITE process, Al2O3-Y2O3-SiOz system is used low- and high-V/ fabricated at 1900C, consolidated parts were tightly formed, and matrix and Py c interface also tightly ture for liquid phase between Al203 and Y203 can be lowered to bonded together. In CFCCS, the distance among fibers of intra-ii 1700-1800C by additional SiO2 [23]. The composites with low- ber-bundles is shortened when fiber volume fraction increases V demonstrated similar characteristics of monolithic Sic corre- [24]. Even for Sic nano-powder, infiltration and densification of sponding to the matrix although the introduction of fibers might matrix in the intra-fiber-bundles strongly depended on the fiber 10u 50um 50um um s 50um Ou 50um 50um Fig3. FE-SEM images of polished cross-section of Sic/siC composites with low-V/(a)at 1800C and (b)at 1900.C and with high-V:(c)at 1800C(d)at 1850.C and(e)at
composites, densification was well reflected to the classic liquid phase sintering, and thus could be divided into three stages, as shown in Fig. 2. Densification of the composites with low-Vf was dramatically promoted at the temperature of 1750–1800 C probably due to the liquid phase formation of sintering additives leading to the rearrangement of SiC nano-powder, and then saturated above 1800 C. In NITE process, Al2O3–Y2O3–SiO2 system is used for SiC/SiC composites as sintering additives because the temperature for liquid phase between Al2O3 and Y2O3 can be lowered to 1700–1800 C by additional SiO2 [23]. The composites with lowVf demonstrated similar characteristics of monolithic SiC corresponding to the matrix although the introduction of fibers might retard the densification. On the other hand, densification of the composites with high-Vf was not sufficient at lower fabrication temperature (1800 C). The increasing fabrication temperature could effectively enhance the densification. At 1900 C, almost 99% of the theoretical density could be obtained. Densification of the composite with high-Vf had the difficulty to explain by the classic liquid phase. 3.2. Microstructural evolutions The matrix in NITE composites consists of well-crystallized bSiC grains with small amount of oxide remnants including aluminum and yttrium (mostly YAG: yttrium aluminum garnet) from XRD analysis [17]. The presence of YAG should improve the densi- fication of matrix in composites. Fig. 3 shows low-magnification and intra-fiber-bundle microstructures of polished cross-section of SiC/SiC composites. Typically, no inter-fiber-bundle pores were observed, even in the composites with high-Vf fabricated at 1800 C. Many small pores were observed in the intra-fiber-bundles for the composites with high-Vf fabricated at 1800 C. The composites maintained about 6.2% open porosity, so that the formation of porous intra-fiber-bundle matrix could be identified. With increasing temperature, the pore size and the number of pores decreased, and simultaneously densification in the intra-fi- ber-bundles was promoted better. At 1900 C, intra-fiber-bundle pores were strongly eliminated, as shown in Fig. 3e. Even for the well-densified intra-fiber-bundle matrix, some differences could be identified with different fabrication temperature. In the composites with low-Vf fabricated at 1800 C, some consolidated parts were loosely formed. On the other hand, in the composites with low- and high-Vf fabricated at 1900 C, consolidated parts were tightly formed, and matrix and PyC interface were also tightly bonded together. In CFCCs, the distance among fibers of intra-fi- ber-bundles is shortened when fiber volume fraction increases [24]. Even for SiC nano-powder, infiltration and densification of matrix in the intra-fiber-bundles strongly depended on the fiber Fig. 2. Effects of the fabrication temperature on densification of monolithic SiC and two SiC/SiC composites. Fig. 3. FE-SEM images of polished cross-section of SiC/SiC composites with low-Vf: (a) at 1800 C and (b) at 1900 C and with high-Vf; (c) at 1800 C, (d) at 1850 C and (e) at 1900 C. K. Shimoda et al. / Composites Science and Technology 69 (2009) 1623–1628 1625
K. Shimoda et aL/Composites Science and Technology 69(2009)1623-1628 volume fraction. Therefore infiltration and densification of the m 410 MPa. the pls was 360 MPa and the elastic modulus was trix in the intra-fiber-bundles becomes a challenge for this process. 360 GPa Dong et al. enhanced the intra- fiber-bundle matrix densification by In Fig. 4, the stress-strain curves of typical Sic/SiC composites pregnation during PIP pretreatment using a polymer containing during monotonic tensile tests are presented. High ductility type slurry(polycarbosilane(PCs)mixed with a filler, which was com- in Fig. 1b was obtained at lower fabrication temperature posed of Sic nano-powder and sintering additives)[13, 14 Even (1800C). In addition, the fracture behavior composites were cat- or SiC/SiC composites using Sic submicron-powder, the improve- egorized in two types from the difference on the densification in ment of the intra-fiber-bundle matrix densification by PCS impreg- the intra-fiber-bundle regions, as shown in Fig. 3a and C. the was reported by Yoshida et al [25]. In this present case dense intra-fiber-bundle composites with low-V exhibited very ut PIP treatment, a highly-densified composite with high-V large ductile fracture region after the PLS with the relatively high btained at 1900C under 20 MPa, where near-full densifica- UTS(380 MPa). The porous intra-fiber-bundle composites with temperature increases, liquid phase formation with lower viscosity PLS with the relative low UTS(320 MPa). Typical fractur er the tion could be achieved in the intra-fiber-bundles. As fabrication high-V displayed relatively large ductile fracture region a more effectively promotes grain-boundary sliding, so that fiber faces of those composites with low- and high-Vf are shown in arrangement is continuously enhanced under pressure. That Fig 5a and b, respectively. High strength type in Fig. la was fab- mote the sufficient infiltration and densification of Sic ricated at higher fabrication temperature(1900C)with high-VE powder in the intra-fiber-bundles. As a consequence, densifi This composites displayed the very high mechanical values, such of the composites with high-Vf is contributed mainly to as the UTS (410 MPa), the PLS(360 MPa)and the elastic mod matrix infiltration and densification of Sic nano-powder in the in- ulus (360 GPa). The failure of the composites occurred soon afte tra-fiber-bundles involving fiber rearrangement pressure via liquid the Pls and far from the matrix crack saturation. This behavior phase of sintering additives. However, high external pressure on after the Pls inhibited load transfer from matrix to fibers and re- the surfaces of the fibers with PyC interface might cause potential stricted the longer fiber pull-out, as shown in Fig. 5C. However, damages to the fibers and PyC interface with the increase of fabri- this implies that the enhanced matrix densification of the cation temperature involving pressure omposites with high-V through sufficient infiltration and densi- fication of SiC nano-power via liquid phase is beneficial at least in 3.3. Mechanical properties and fracture behaviors erms of first fracture properties. Cracks were deflected along Pyc matrix interface in spite of fabrication temperature and Some average physical and mechanical properties of the com- ume fraction as shown in Fig 6a. In addition, on posites under two fiber volume fractions incorporating different temperature fabricated composites, some cracks fabrication temperatures are listed in Table 1. The PLS, the UTs PyC/fiber interface were observed as shown in lower and the elastic modulus in monotonic tensile tests were deter- temperature, less-consolidated matrix before 3rd stage might mined following the general guidelines of ASTM C-1275. The Pls form the relatively weak interface bonding between matrix and was the stress corresponding to a 0.0005% offset strain For com- PyC interface required for high ductility type, which exhibits a parison, the three-point bending strength and the elastic modulus wide ductile domain after the Pls with longer fiber pull-out. with in bending were determined by the load-displacement curves longer fiber pull-out, the fibers bridging the matrix cracks can ab according to ASTM C-1341. As shown in Table 1, the bulk densi sorb much more energy to avoid non-catastrophic fracture behav and open porosity had the strong dependence on fiber volume frac- ior due to the friction between matrix and fibers when fibers pull tion of the composites at lower fabrication temperature. Also, the out from matrix, resulting in higher fracture toughness. On the value for three-point bending strength and elastic modulus in other hand, at higher fabrication temperature, well-consolidated bending showed the similar characteristics. When fabrication tem- matrix might form strong interface bonding between matrix and perature increased, the bulk density increased and open porosity Py c interface with efficient load transfer from matrix to fibers re- decreased for the composites with high-V. At 1900C, the open quired for high strength type. The very high PLS with the higl porosity reached the lowest value less than 1%. This could be attri- elastic modulus could be mainly attribution from the strength bution to the intra-fiber-bundle densification, as shown in Fig 3. ened interaction bonds between Py c interface and matrix, as Meanwhile, the values for three-point bending strength and elastic shown in Fig. 6b. Through this study, interface bonding between modulus in bending gradually increased, and the highest value Pyc interface and matrix could be concluded as a significant key vere obtained at 1900C. The bending strength under this fabrica- for tailoring of fracture behaviors. In CFCCs, increasing of volume ion temperature was over 850 MPa Date analysis indicated a sim- fraction of fibers with high tensile strength and modulus as rein- ilar variation trend for both monotonic tensile and three-point forcement should enhance mechanical values of composites in bending test, with increasing fabrication temperature from 1800 strength In NITE process, however, fiber volume fraction strongly to 1900C. Both the Uts and the pls were the highest at affected the porosity and microstructure of the composites, in par 900C. Under this fabrication temperature, the UTs was ticular in the intra-fiber-bundles, and therefore increasing of fiber volume fraction prevented improving the mechanical values (1800C). This study revealed that Effects of fiber volume fraction incorporating fabrication temperature on the average of fabrication temperature was effective way to enhance infiltra- physical and mechanical properties of the composites. tion and densification of Sic nano-powder in the intra-fiber-bun Fabrication temperature(C) 18501900 e interrace bonding. For the composites fabrication by hot-pressing, individual fiber deformation is detect Fiber volume fraction(volg) 33abeinafewcases,indicatedfberdeformatonincudingpote m3) tial fiber creep. For the present study, such a creep formation Ultimate beind ing strength (MPa) 517 375 41 690 860 does not seem to have the significant affected mechanical perfor Elastic modulus in bending(GPa) of the composites. The fibers were well protected by ngth(MPa) 380 50 358 induced-PyC coating and well-consolidation with sufficient-infil- rtional limit stress(MPa tration of SiC nano-powder in the intra- fiber-bundle and could Elastic modulus(GPa) Strain at fracture(%) 02860.1760.1350.1450.127 contribute excellent mechanical performances with non-cata- strophic fracture behavior
volume fraction. Therefore, infiltration and densification of the matrix in the intra-fiber-bundles becomes a challenge for this process. Dong et al. enhanced the intra-fiber-bundle matrix densification by impregnation during PIP pretreatment using a polymer containing slurry (polycarbosilane (PCS) mixed with a filler, which was composed of SiC nano-powder and sintering additives) [13,14]. Even for SiC/SiC composites using SiC submicron-powder, the improvement of the intra-fiber-bundle matrix densification by PCS impregnation was reported by Yoshida et al [25]. In this present case without PIP treatment, a highly-densified composite with high-Vf was obtained at 1900 C under 20 MPa, where near-full densification could be achieved in the intra-fiber-bundles. As fabrication temperature increases, liquid phase formation with lower viscosity more effectively promotes grain-boundary sliding, so that fiber rearrangement is continuously enhanced under pressure. That could promote the sufficient infiltration and densification of SiC nano-powder in the intra-fiber-bundles. As a consequence, densifi- cation of the composites with high-Vf is contributed mainly to matrix infiltration and densification of SiC nano-powder in the intra-fiber-bundles involving fiber rearrangement pressure via liquid phase of sintering additives. However, high external pressure on the surfaces of the fibers with PyC interface might cause potential damages to the fibers and PyC interface with the increase of fabrication temperature involving pressure. 3.3. Mechanical properties and fracture behaviors Some average physical and mechanical properties of the composites under two fiber volume fractions incorporating different fabrication temperatures are listed in Table 1. The PLS, the UTS and the elastic modulus in monotonic tensile tests were determined following the general guidelines of ASTM C-1275. The PLS was the stress corresponding to a 0.0005% offset strain. For comparison, the three-point bending strength and the elastic modulus in bending were determined by the load-displacement curves according to ASTM C-1341. As shown in Table 1, the bulk density and open porosity had the strong dependence on fiber volume fraction of the composites at lower fabrication temperature. Also, the value for three-point bending strength and elastic modulus in bending showed the similar characteristics. When fabrication temperature increased, the bulk density increased and open porosity decreased for the composites with high-Vf. At 1900 C, the open porosity reached the lowest value less than 1%. This could be attribution to the intra-fiber-bundle densification, as shown in Fig 3. Meanwhile, the values for three-point bending strength and elastic modulus in bending gradually increased, and the highest value were obtained at 1900 C. The bending strength under this fabrication temperature was over 850 MPa. Date analysis indicated a similar variation trend for both monotonic tensile and three-point bending test, with increasing fabrication temperature from 1800 to 1900 C. Both the UTS and the PLS were the highest at 1900 C. Under this fabrication temperature, the UTS was 410 MPa, the PLS was 360 MPa and the elastic modulus was 360 GPa. In Fig. 4, the stress–strain curves of typical SiC/SiC composites during monotonic tensile tests are presented. High ductility type in Fig. 1b was obtained at lower fabrication temperature (1800 C). In addition, the fracture behavior composites were categorized in two types from the difference on the densification in the intra-fiber-bundle regions, as shown in Fig. 3a and c. The dense intra-fiber-bundle composites with low-Vf exhibited very large ductile fracture region after the PLS with the relatively high UTS (380 MPa). The porous intra-fiber-bundle composites with high-Vf displayed relatively large ductile fracture region after the PLS with the relative low UTS (320 MPa). Typical fracture surfaces of those composites with low- and high-Vf are shown in Fig. 5a and b, respectively. High strength type in Fig. 1a was fabricated at higher fabrication temperature (1900 C) with high-Vf. This composites displayed the very high mechanical values, such as the UTS (410 MPa), the PLS (360 MPa) and the elastic modulus (360 GPa). The failure of the composites occurred soon after the PLS and far from the matrix crack saturation. This behavior after the PLS inhibited load transfer from matrix to fibers and restricted the longer fiber pull-out, as shown in Fig. 5c. However, this implies that the enhanced matrix densification of the composites with high-Vf through sufficient infiltration and densi- fication of SiC nano-power via liquid phase is beneficial at least in terms of first fracture properties. Cracks were deflected along PyC/ matrix interface in spite of fabrication temperature and fiber volume fraction as shown in Fig. 6a. In addition, only in the higher temperature fabricated composites, some cracks deflected along PyC/fiber interface were observed as shown in Fig. 6b. At lower temperature, less-consolidated matrix before 3rd stage might form the relatively weak interface bonding between matrix and PyC interface required for high ductility type, which exhibits a wide ductile domain after the PLS with longer fiber pull-out. With longer fiber pull-out, the fibers bridging the matrix cracks can absorb much more energy to avoid non-catastrophic fracture behavior due to the friction between matrix and fibers when fibers pullout from matrix, resulting in higher fracture toughness. On the other hand, at higher fabrication temperature, well-consolidated matrix might form strong interface bonding between matrix and PyC interface with efficient load transfer from matrix to fibers required for high strength type. The very high PLS with the high elastic modulus could be mainly attribution from the strengthened interaction bonds between PyC interface and matrix, as shown in Fig. 6b. Through this study, interface bonding between PyC interface and matrix could be concluded as a significant key for tailoring of fracture behaviors. In CFCCs, increasing of volume fraction of fibers with high tensile strength and modulus as reinforcement should enhance mechanical values of composites in strength. In NITE process, however, fiber volume fraction strongly affected the porosity and microstructure of the composites, in particular in the intra-fiber-bundles, and therefore increasing of fiber volume fraction prevented improving the mechanical values at lower temperature (1800 C). This study revealed that increasing of fabrication temperature was effective way to enhance infiltration and densification of SiC nano-powder in the intra-fiber-bundles as well as the interface bonding. For the composites fabrication by hot-pressing, individual fiber deformation is detectable in a few cases, indicated fiber deformation including potential fiber creep. For the present study, such a creep formation does not seem to have the significant affected mechanical performances of the composites. The fibers were well protected by induced-PyC coating and well-consolidation with sufficient-infiltration of SiC nano-powder in the intra-fiber-bundle, and could contribute excellent mechanical performances with non-catastrophic fracture behavior. Table 1 Effects of fiber volume fraction incorporating fabrication temperature on the average physical and mechanical properties of the composites. Fabrication temperature (C) 1800 1850 1900 Fiber volume fraction (vol%) 31 52 55 31 53 Bulk density (g/cm3 ) 2.94 2.81 2.96 3.06 3.11 Open porosity (%) 2.1 6.2 4.4 1.0 0.6 Ultimate bending strength (MPa) 517 375 711 690 860 Elastic modulus in bending (GPa) 160 122 174 184 277 Ultimate tensile strength (MPa) 380 322 356 350 358 Proportional limit stress (MPa) 216 177 209 259 408 Elastic modulus (GPa) 289 277 345 338 354 Strain at fracture (%) 0.286 0.176 0.135 0.145 0.127 1626 K. Shimoda et al. / Composites Science and Technology 69 (2009) 1623–1628
(b) UIS- 380Ma PLs -350MPa PLS 250~20MPa 200 150 000000 0.1 Tensile strain(%) Tensile strain(%) 450S w410MPa UTS UTS R400}~360MPa w 320MPa E 350 PLS 350MP P 3250~180MPa 兰 02 Tensile strain(%) Tensile strain (%) Tensile strain ( %) Fig 4. Typical stress-strain curves of SiC/Sic composites with low-V(a)at 1800C and (b)at 1900C and with high-V(c)at 1800C. (d) at 1850"C and (e)at 1900C Our 10um Fig. 5. Fracture surfaces of the Sic/Sic composites after monotonic tensile tests: (a) with low-V, at 1800C. (b)with high-V/ at 1800C and (c)with high-V, at 1900.C. 4. Conclusions sification became restricted because the fibers retarded infiltration of SiC nano-powders at lower fabrication temperature of 1800C. Densification, microstructure and mechanical properties of Sic/ Increasing of fabrication temperature dramatically enhanced the SiC composites by nano-powder infiltration and transient eutectic- infiltration and densification of Sic nano-powder in the intra-fl phase(NITE) process were highly dependent on fiber volume frac- ber-bundles and simultaneously strengthened the interaction be- tion incorporating fabrication temperature. In the composites with tween Pyc interface and matrix. Even under harsh fabrication low fiber volume fraction, densification was well achieved even at temperature of 1900C, the fibers were well protected by in- lower fabrication temperature of 1800C and then saturated at the duced-PyC coating and well-consolidation with sufficient infiltra- 3rd stage of matrix densification corresponding to classic liquid tion of Sic nano-powder in the intra-fiber-bundle. For the phase sintering In the composites with high volume fraction, den- composite with high fiber volume fraction, that could contribute
4. Conclusions Densification, microstructure and mechanical properties of SiC/ SiC composites by nano-powder infiltration and transient eutecticphase (NITE) process were highly dependent on fiber volume fraction incorporating fabrication temperature. In the composites with low fiber volume fraction, densification was well achieved even at lower fabrication temperature of 1800 C and then saturated at the 3rd stage of matrix densification corresponding to classic liquid phase sintering. In the composites with high volume fraction, densification became restricted because the fibers retarded infiltration of SiC nano-powders at lower fabrication temperature of 1800 C. Increasing of fabrication temperature dramatically enhanced the infiltration and densification of SiC nano-powder in the intra-fi- ber-bundles and simultaneously strengthened the interaction between PyC interface and matrix. Even under harsh fabrication temperature of 1900 C, the fibers were well protected by induced-PyC coating and well-consolidation with sufficient infiltration of SiC nano-powder in the intra-fiber-bundle. For the composite with high fiber volume fraction, that could contribute Fig. 4. Typical stress–strain curves of SiC/SiC composites with low-Vf: (a) at 1800 C and (b) at 1900 C and with high-Vf; (c) at 1800 C, (d) at 1850 C and (e) at 1900 C. Fig. 5. Fracture surfaces of the SiC/SiC composites after monotonic tensile tests: (a) with low-Vf at 1800 C, (b) with high-Vf at 1800 C and (c) with high-Vf at 1900 C. K. Shimoda et al. / Composites Science and Technology 69 (2009) 1623–1628 1627
matri Pyc interfa Fig. 6. Cracks in the SiC/Sic composite with high-Vy at 1900C:(a)deflected along Pyc/matrix interface and(b)deflected along Pyc/fiber interface. the excellent mechanical performances, the UTs (410 MPa), the [11] Park K, Vasilos T Processing, microstructure and mechanical properties of PLS (360 MPa)and the elastic modulus (a360 GPa ) with non-cat sed Sic continuous fbre/Sic composites. J Mater Sci alkali-resistant sintered Sic fiber stable to 2200.C Nature 1998: 391: 773-5 tailoring of appropriate fabrication temperature to fiber volume [131 Dong SM. Katoh Y Kohyama a Preparation of sic/sic composites by Hot fraction as follows: (1) high ductility type and(2) high strength 2003:86(1)26-32. type [14 Dong SM, Katoh Y, Kohyama A Processing optimization and mechanical References [15 Kohyama A, Dong SM, Katoh Y. Development of Sic/siC tes by nano- 1 mposites. In: Proceedings of the 5th international conference mperature ceramic matrix composites(HTCMC5) Ohio(USA): The re on high filtration and transient eutectic (NITE) process. ng Sci Proc 2002:23:311 [16] Katoh Y, Kohyama A, Nozawa T, Sato M. Sic/SiC com ceramics Society: 2004. p. 187-9 ines and nuclear reactors sa overview compos so [171 shimoda K. Park Js, Hinoki T. Kohyama A. Influence of pyrolytic carbon [3I Katoh Y, Snead LL Henager CH, Hasegawa A, Kohyama A, Riccardi B, et al. omposites by Nme process. Compos Technol 2008: 68(1): 98-105. [18] Katoh Y, Kohyal applications. J Nucl Mater 2007: 367-370(1): 659-71 rence on fusion eactor materials(ICFRM-11)prese [19l Rebillat F. Lamon. Guette A The concept of a strong interface to [5] Naslain R Materials design and processing of high temperature ceramic matrix Sic composites with a BN interphase. Acta Mater mposites: state of the art and future trends. Adv Compos Mater [20 Evans AG. Perspective on the development of high-toughness ceramics. J Am 161 ones R, Szweda A Petrak D. Polymer derived ceramic matrix composites. (211 Rebillat F. Lamon ) Naslain R, Curzio EL Feber MK,. Properties of 7 Kotani M, Inoue T, Kohyama A, Okamura K, Katoh Y Consolidation of polymer- erived Sic matrix composites: processing and microstructure. Compos [22] Shimoda K, Park JS, Hinoki T. Kohyama A. Influence of surface structure of Sic echnol2002:62(16)2 [8 Hino T, Hayashishita E, Kohyama A, Yamauchi Y, Hirohata Y. Helium gas roscopy on basic bility of Sic/Sic composite after heat cycles. J Nucl Mater 2007: 36 characteristics. Appl Surf Sci 2007: 253(24): 9450-6. 370(1):736-41 [23] Levin EM, Robbins CR, McMurdie HE Phase diagrams for ceramists. The nposites by the combined fabrication process of ICVi and NITE methods. J [24] Hull D, Clyne TV aterials. 2nd ed. England: Cambring University Press: 1996. [10)Yoshida K, Yano T. Room and high-temperature mechanical and thermal 1251 Yoshida K, imai, Yano ts Improvement of the operties of hot- properties of Sic fiber-reinforced previous Sic composite sintered under composites by polycarbosilane impregnation Compos Sci Technol 2001: 61(9): 1323-9
the excellent mechanical performances, the UTS (410 MPa), the PLS (360 MPa) and the elastic modulus (360 GPa) with non-catastrophic fracture behavior. Through NITE process, SiC/SiC composites with two fracture types were successfully developed by tailoring of appropriate fabrication temperature to fiber volume fraction as follows: (1) high ductility type and (2) high strength type. References [1] DiCarlo JA. Microstructural optimization of high temperature SiC/SiC composites. In: Proceedings of the 5th international conference on high temperature ceramic matrix composites (HTCMC5). Ohio (USA): The American Ceramics Society; 2004. p. 187–92. [2] Naslain R. Design, preparation and properties of non-oxide CMCs for application in engines and nuclear reactors: an overview. Compos Sci Technol 2004;64(2):155–70. [3] Katoh Y, Snead LL, Henager CH, Hasegawa A, Kohyama A, Riccardi B, et al. Current status and critical issues for development of SiC composites for fusion applications. J Nucl Mater 2007;367–370(1):659–71. [4] Low IM, editor. Ceramic matrix composites – microstructure, properties and applications. England: Woodhead Publishing; 2006. [5] Naslain R. Materials design and processing of high temperature ceramic matrix composites: state of the art and future trends. Adv Compos Mater 1996;8(1):3–16. [6] Jones R, Szweda A, Petrak D. Polymer derived ceramic matrix composites. Composites: Part A 1999;30(4):569–75. [7] Kotani M, Inoue T, Kohyama A, Okamura K, Katoh Y. Consolidation of polymerderived SiC matrix composites: processing and microstructure. Compos Technol 2002;62(16):2179–88. [8] Hino T, Hayashishita E, Kohyama A, Yamauchi Y, Hirohata Y. Helium gas permeability of SiC/SiC composite after heat cycles. J Nucl Mater 2007;367– 370(1):736–41. [9] Shimoda K, Hinoki T, Katoh Y, kohyama A. Development of the tailored SiC/SiC composites by the combined fabrication process of ICVI and NITE methods. J Nucl Mater 2009;384(2):103–8. [10] Yoshida K, Yano T. Room and high-temperature mechanical and thermal properties of SiC fiber-reinforced previous SiC composite sintered under pressure. J Nucl Mater 2000;283–287(1):560–4. [11] Park K, Vasilos T. Processing, microstructure and mechanical properties of hot-pressed SiC continuous fibre/SiC composites. J Mater Sci 1997;32(2):295–300. [12] Ishikawa T, Kohtoku Y, Kumagawa K, Yamamura T, Nagasawa T. High-strength alkali-resistant sintered SiC fiber stable to 2200 C. Nature 1998;391:773–5. [13] Dong SM, Katoh Y, Kohyama A. Preparation of SiC/SiC Composites by Hot pressing, using Tyranno-SA fiber as reinforcement. J Am Ceram Soc 2003;86(1):26–32. [14] Dong SM, Katoh Y, Kohyama A. Processing optimization and mechanical evaluation of hot pressed 2D Tyranno-SA/SiC composites. J Eur Ceram Soc 2003;23(8):1223–31. [15] Kohyama A, Dong SM, Katoh Y. Development of SiC/SiC composites by nanoinfiltration and transient eutectic (NITE) process. Ceram Eng Sci Proc 2002;23:311–8. [16] Katoh Y, Kohyama A, Nozawa T, Sato M. SiC/SiC composites through transient eutectic-phase route for fusion applications. J Nucl Mater 2004;329– 333(1):587–91. [17] Shimoda K, Park JS, Hinoki T, Kohyama A. Influence of pyrolytic carbon interface thickness on microstructure and mechanical properties of SiC/SiC composites by NITE process. Compos Technol 2008;68(1):98–105. [18] Katoh Y, Kohyama A, Nozawa T. SiC/SiC composites through transient eutecticphase route for fusion applications. In: 11th international conference on fusion reactor materials (ICFRM-11) presented at Kyoto, Japan; 2003. [19] Rebillat F, Lamon J, Guette A. The concept of a strong interface applied to SiC/ SiC composites with a BN interphase. Acta Mater 2000;48(18- 19):4609–46018. [20] Evans AG. Perspective on the development of high-toughness ceramics. J Am Ceram Soc 1990;73(2):187–206. [21] Rebillat F, Lamon J, Naslain R, Curzio EL, Feber MK, Besmann TM. Properties of multilayered interphases in SiC/SiC chemical-vapor-infiltrated composites with ‘weak’ and ‘strong’ interfaces. J Am Ceram Soc 1998;81(9):2315–26. [22] Shimoda K, Park JS, Hinoki T, Kohyama A. Influence of surface structure of SiC nano-sized powder analyzed by X-ray photoelectron spectroscopy on basic powder characteristics. Appl Surf Sci 2007;253(24):9450–6. [23] Levin EM, Robbins CR, McMurdie HE. Phase diagrams for ceramists. The American Ceramic Society; 1969. p. 165. [24] Hull D, Clyne TW. An introduction to composite materials. 2nd ed. England: Cambring University Press; 1996. [25] Yoshida K, Imai M, Yano T. Improvement of the mechanical properties of hotpressed silicon-carbide-fiber-reinforced silicon carbide composites by polycarbosilane impregnation. Compos Sci Technol 2001;61(9):1323–9. Fig. 6. Cracks in the SiC/SiC composite with high-Vf at 1900 C: (a) deflected along PyC/matrix interface and (b) deflected along PyC/fiber interface. 1628 K. Shimoda et al. / Composites Science and Technology 69 (2009) 1623–1628