《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_Influence of pyrolytic carbon interface thickness on microstructure

Availableonlineatwww.sciencedirect.com COMPOSITES . ScienceDirect SCIENCE AND TECHNOLOGY ELSEVIER Composites Science and Technology 68(2008)98-105 w.elsevier. com/locate/compscitech Influence of pyrolytic carbon interface thickness on microstructure and mechanical properties of SiC/SiC composites by Nite process Kazuya Shimoda , Joon-Soo Park , Tatsuya Hinoki, Akira Kohyama Graduate School of Energy Science, Kyoto University, Institute of Energy Science and Technology Co. Ltd. Tokyo 101-0041 Japan Institute of Adranced Energy, Kyoto Unicersity, Gok Kyoto 611-0011 Received 25 October 2006: received in revised form 9 May 2007: accepted 16 May 2007 Available online 2 June 2007 Abstract Unidirectional SiC/SiC composites were prepared by Nano-Infiltration and Transient Eutectic-phase(NItE)process using Sic nano- powder infiltration technique, and the effects of pyrolytic carbon(Pyc) interface thickness between fibers and matrix on density, micro- structural evolution and mechanical properties were characterized. SiC fibers both with and without Pyc interface were employed as reinforcement and Sic nano-powder was employed for matrix formation with 12 mass% sintering additives of the total powder. The thickness of PyC layer deposited by chemical vapor deposition(CVD)process was highly-accurately controlled at about 0.25, 0.50 and 1.00 um. Nearly full-dense SiC/SiC composites with uncoated fibers caused strong interaction between fibers and matrix, resulting in a brittle fracture behavior without fiber pull-out. Higher strength with a pseudo-ductile fracture behavior could be obtained using 0.50 um of PyC interface thickness, where a lot of deflects and branches of the propagating cracks and fiber pull-out were observed. Induced PyC interface conditions strongly affect the density, microstructural evolution, and therefore dominate mechanical properties and fracture behaviors c 2007 Elsevier Ltd. All rights reserved Keywords: A. Ceramic-matrix composites( CMCs): B. Interface: B Stress/strain curves; E. Powder processing 1. Introduction high-crystallinity and near-stoichiometric composition in matrix have been recognized as key requirements for Continuous SiC fiber reinforced SiC matrix(SiC/SiC) high-temperature and neutron irradiation application [9 composites are promising structural candidates for 11]. From these aspects, a newly innovative process called advanced nuclear energy systems, such as gas cooled fast Nano-Infiltration and Transient Eutectic-phase (NITE reactor (GFR), very high temperature reactor (VhTR) process has been extensively developed in our group at and fusion reactor due to their potentiality for providing Kyoto University. NITE process, which is first successful excellent mechanical properties at high-temperature and application of liquid phase sintering(LPS) process to low induced radioactivity [1-4] matrix densification for SiC/SiC composites, enabled There have been many efforts to develop high-perfor- production of well-crystallized SiC matrix with low poros- mance SiC/SiC composites, using chemical vapor infiltra- ity by incorporating both SiC nano-powder infiltration and I), polymer infiltration and pyrolysis(PIP), the advanced SiC fibers with well-crystallized microstruc- reaction sintering/melting infiltration(RS/MI) and their ture and near-stoichiometric composition like TyrannoTM. combined processes [5-8]. However, the low porosity, SA fibers [12-16]. General properties advanced SiC/Sic are summarized in Table I Corresponding author. Tel +81 774 383465: fax: +81 774 383467 The role of interface/interphase in ceramic composites E-mail address: k-simd @iae. kyoto-uLac jp(K. Shimoda) is extremely important for structure applications. The 0266-3538/- see front matter e 2007 Elsevier Ltd. All rights reserved doi: 10.1016j. compscitech. 2007.05.037

Influence of pyrolytic carbon interface thickness on microstructure and mechanical properties of SiC/SiC composites by NITE process Kazuya Shimoda a,*, Joon-Soo Park b,c, Tatsuya Hinoki c , Akira Kohyama c a Graduate School of Energy Science, Kyoto University, Gokasho Uji, Kyoto 611-0011, Japan b Institute of Energy Science and Technology Co. Ltd., Kanda, Tokyo 101-0041, Japan c Institute of Advanced Energy, Kyoto University, Gokasho Uji, Kyoto 611-0011, Japan Received 25 October 2006; received in revised form 9 May 2007; accepted 16 May 2007 Available online 2 June 2007 Abstract Unidirectional SiC/SiC composites were prepared by Nano-Infiltration and Transient Eutectic-phase (NITE) process using SiC nano￾powder infiltration technique, and the effects of pyrolytic carbon (PyC) interface thickness between fibers and matrix on density, micro￾structural evolution and mechanical properties were characterized. SiC fibers both with and without PyC interface were employed as reinforcement and SiC nano-powder was employed for matrix formation with 12 mass% sintering additives of the total powder. The thickness of PyC layer deposited by chemical vapor deposition (CVD) process was highly-accurately controlled at about 0.25, 0.50 and 1.00 lm. Nearly full-dense SiC/SiC composites with uncoated fibers caused strong interaction between fibers and matrix, resulting in a brittle fracture behavior without fiber pull-out. Higher strength with a pseudo-ductile fracture behavior could be obtained using 0.50 lm of PyC interface thickness, where a lot of deflects and branches of the propagating cracks and fiber pull-out were observed. Induced PyC interface conditions strongly affect the density, microstructural evolution, and therefore dominate mechanical properties and fracture behaviors. 2007 Elsevier Ltd. All rights reserved. Keywords: A. Ceramic-matrix composites (CMCs); B. Interface; B. Stress/strain curves; E. Powder processing 1. Introduction Continuous SiC fiber reinforced SiC matrix (SiC/SiC) composites are promising structural candidates for advanced nuclear energy systems, such as gas cooled fast reactor (GFR), very high temperature reactor (VHTR) and fusion reactor due to their potentiality for providing excellent mechanical properties at high-temperature and low induced radioactivity [1–4]. There have been many efforts to develop high-perfor￾mance SiC/SiC composites, using chemical vapor infiltra￾tion (CVI), polymer infiltration and pyrolysis (PIP), reaction sintering/melting infiltration (RS/MI) and their combined processes [5–8]. However, the low porosity, high-crystallinity and near-stoichiometric composition in matrix have been recognized as key requirements for high-temperature and neutron irradiation application [9– 11]. From these aspects, a newly innovative process called Nano-Infiltration and Transient Eutectic-phase (NITE) process has been extensively developed in our group at Kyoto University. NITE process, which is first successful application of liquid phase sintering (LPS) process to matrix densification for SiC/SiC composites, enabled a production of well-crystallized SiC matrix with low poros￾ity by incorporating both SiC nano-powder infiltration and the advanced SiC fibers with well-crystallized microstruc￾ture and near-stoichiometric composition like TyrannoTM￾SA fibers [12–16]. General properties and issues for advanced SiC/SiC are summarized in Table 1. The role of interface/interphase in ceramic composites is extremely important for structure applications. The 0266-3538/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2007.05.037 * Corresponding author. Tel.: +81 774 383465; fax: +81 774 383467. E-mail address: k-simd@iae.kyoto-u.ac.jp (K. Shimoda). www.elsevier.com/locate/compscitech Available online at www.sciencedirect.com Composites Science and Technology 68 (2008) 98–105 COMPOSITES SCIENCE AND TECHNOLOGY

K. Shimoda et al./ Composites Science and Technology 68(2008)98-105 Table l al Sic/Sic composites for structural materials and corresponding issues Category Advanced fiber CvI Advanced fiber MI Crystallized PIP NITE Fiber Tyranno-SA/Hi-Nicalon Type-S Interphase PyC/SiC Matrix 3C-SIC 3C-SiC 3C-SiC Excess Si l06~ None Excess C Occationally Other phase(s) None Oxides. <I0% Moderate Low Moderate-high Thermal conductivity High Low-moderate Moderate-high Good interfaces arres st,deflect and branch the propagating crack that have initiated at outer or pore surfaces of the matrix The propagating of cracks deflected along the interface enables energy dissipation through friction, and conse- quently allows a pseudo-ductile macroscopic fracture behavior in ceramic composites [17-19]. pyrolytic carbon (PyC) has conventionally been employed as interfaces in SiC/SiC composites. Therefore, mechanical properties and fracture behavior might be highly improved by the processing control of interface. This paper focused on the influences of forming process and thickness of Pyc inter face on the density, process induced damage, mechanical 100nm properties and fracture behavior of unidirectional SiC/ Sic composites by NItE process ig. I. TEM micrograph of as-received SiC nano-powd 2. Experimental procedure TyrannoTM-SA grade-3 polycrystalline SiC fiber tows Y203=9 wt%(Al2O3: Y203=60: 40) and SiO2=3 wt%) / be Industries Ltd, Ube, Japan)were used as reinforce- Those prepared green sheets were unidirectionally stacked ent. General characteristics of the fiber are listed in th the fiber volume fraction of about 42-55% in a Table 2. The thickness of Py C layer deposited by chemical graphite fixture, and then hot-pressed at 1800C for 2 h vapor deposition(CVD) was highly-accurately controlled in Ar atmosphere under a pressure of 20 MPa after at about 0.25, 0.50 and 1.00 um. Ultra-fine B-SiC nano- drying powder (Sumitomo Osaka Cement Co. Ltd, Japan, T-1 Density and porosity of hot-pressed composites were grade)with a mean particle diameter of 30 nm was used determined by the Archimedes principle, using distilled or matrix formation, and Al2O3(Sumitomo Chemical water as the immersion medium. Theoretical density was Industries Ltd, Japan, 99.99% pure) with a mean particle calculated by following the ratio of a mixture of Sic diameter of 0.3 um, Y,O3(Kojundo Chemical Laboratory nano-powder, sintering additives and fiber volume fraction Co Ltd, Japan, 99.99% pure)with a mean particle diam- Three-point bending test(test bars 4.0 Wx25x20mm' eter of 0.4 um and SiO,(Kojundo Chemical Laboratory was carried out at room-temperature, with crosshead speed Co.Ltd, Japan,99.9%pure)with a mean particle diame- of 0.5 mm/min and outer support span of 18 mm in an ter of I um were used as sintering additives. Fig. I shows INSTRON 5581 test machine, using the number of 3-5 TEM image of SiC nano-powder as-received. Unidirec- specimens. Bending bars were cut parallel to the fiber axis tional uncoated and PyC coated fiber tows were Flexural stress(o), flexural strain(e) and modulus of elas impregnated in SiC "nano-slurry, which is mixture of ticity in bending(E)were calculated by the following Eqs Sic nano-powder and sintering additives (Al,O3+ (1H3), respectively Table 2 Properties of Tyranno M-SA fiber(grade-3 SiC fiber Atomic ratio(C/Si) Diameter(um) Density(mg/m,) Filaments/yarn Tensile strength(GPa) Elastic modulus(GPa) TyrannoM-SA 1.08 3.10

interfaces arrest, deflect and branch the propagating cracks that have initiated at outer or pore surfaces of the matrix. The propagating of cracks deflected along the interface enables energy dissipation through friction, and conse￾quently allows a pseudo-ductile macroscopic fracture behavior in ceramic composites [17–19]. pyrolytic carbon (PyC) has conventionally been employed as interfaces in SiC/SiC composites. Therefore, mechanical properties and fracture behavior might be highly improved by the processing control of interface. This paper focused on the influences of forming process and thickness of PyC inter￾face on the density, process induced damage, mechanical properties and fracture behavior of unidirectional SiC/ SiC composites by NITE process. 2. Experimental procedure TyrannoTM-SA grade-3 polycrystalline SiC fiber tows (Ube Industries Ltd., Ube, Japan) were used as reinforce￾ment. General characteristics of the fiber are listed in Table 2. The thickness of PyC layer deposited by chemical vapor deposition (CVD) was highly-accurately controlled at about 0.25, 0.50 and 1.00 lm. Ultra-fine b-SiC nano￾powder (Sumitomo Osaka Cement Co. Ltd., Japan, T-1 grade) with a mean particle diameter of 30 nm was used for matrix formation, and Al2O3 (Sumitomo Chemical Industries Ltd., Japan, 99.99% pure) with a mean particle diameter of 0.3 lm, Y2O3 (Kojundo Chemical Laboratory Co. Ltd., Japan, 99.99% pure) with a mean particle diam￾eter of 0.4 lm and SiO2 (Kojundo Chemical Laboratory Co. Ltd., Japan, 99.9% pure) with a mean particle diame￾ter of 1 lm were used as sintering additives. Fig. 1 shows TEM image of SiC nano-powder as-received. Unidirec￾tional uncoated and PyC coated fiber tows were impregnated in SiC ‘‘nano’’-slurry, which is mixture of SiC nano-powder and sintering additives (Al2O3 + Y2O3 = 9 wt% (Al2O3:Y2O3 = 60:40) and SiO2 = 3 wt%). Those prepared green sheets were unidirectionally stacked with the fiber volume fraction of about 42–55% in a graphite fixture, and then hot-pressed at 1800 C for 2 h in Ar atmosphere under a pressure of 20 MPa after drying. Density and porosity of hot-pressed composites were determined by the Archimedes principle, using distilled water as the immersion medium. Theoretical density was calculated by following the ratio of a mixture of SiC nano-powder, sintering additives and fiber volume fraction. Three-point bending test (test bars 4.0w · 25L · 2.0T mm3 ) was carried out at room-temperature, with crosshead speed of 0.5 mm/min and outer support span of 18 mm in an INSTRON 5581 test machine, using the number of 3–5 specimens. Bending bars were cut parallel to the fiber axis. Flexural stress (r), flexural strain (e) and modulus of elas￾ticity in bending (E) were calculated by the following Eqs. (1)–(3), respectively: Table 1 List of developmental SiC/SiC composites for structural materials and corresponding issues Category Advanced fiber CVI Advanced fiber MI Crystallized PIP NITE Fiber Tyranno-SA/Hi-Nicalon Type-S Interphase PyC PyC/SiC Varied PyC Matrix Base phase 3C–SiC 3C–SiC 3C–SiC 3C–SiC Porosity 10% 0% 20% 0% Excess Si Occationally 10% None None Excess C None Occationally 10% None Other phase (s) None None None Oxides, <10% General issues Strength Moderate Moderate Low Moderate-high Thermal conductivity Moderate High Low-moderate Moderate-high Hermeticity Poor Poor Poor Good Table 2 Properties of TyrannoTM-SA fiber (grade-3) SiC fiber Atomic ratio (C/Si) Diameter (lm) Density (mg/m3 ) Filaments/yarn Tensile strength (GPa) Elastic modulus (GPa) TyrannoTM-SA 1.08 7.5 3.10 1600 2.51 409 Fig. 1. TEM micrograph of as-received SiC nano-powder. K. Shimoda et al. / Composites Science and Technology 68 (2008) 98–105 99

K Shimoda et al. Composites Science and Technology 68(2008)98-105 3um ig. 2. Cross-sections of uncoated and PyC coated SA fibers formed with CVD technique: (a)uncoated, (b)CVD-0 25, (c)CVD-050 and(d)CVD-100. Table 3 Density and porosity of hot-pressed composites with various interface conditions Uncoated CVD-0.50 CVD-1.00 ED: experimental density, TD: theoretical density. a=3PL/2wt (1) (2) E=0.25Lm/wt3, ▲yAlO1YAG where P is load at a point of deflection of a load-displace ment curve in test, L is outer support span, w is specimen width, t is specimen thickness, D is deflection at beam cen- ter at a given point in the test and m is slop of tangent to the initial straight-line portion of the load-deflection curve Polished cross sections and fracture surface after bend ing test of hot-pressed composites were observed by field emission scanning electron microscopy constitution phases of the matrix were identified by X-ray 607080 diffractometry(XRD), using monolithic SiC fabricated by e/degree(Cu Ko) he same processing condition for SiC/SiC composites in Fig 3.x-ray diffraction profile of the monolithic Sic fabricated by the this study

r ¼ 3PL=2wt2 ð1Þ e ¼ 6Dt=L2 ð2Þ E ¼ 0:25 L3 m=wt3 ; ð3Þ where P is load at a point of deflection of a load–displace￾ment curve in test, L is outer support span, w is specimen width, t is specimen thickness, D is deflection at beam cen￾ter at a given point in the test and m is slop of tangent to the initial straight-line portion of the load–deflection curve. Polished cross sections and fracture surface after bend￾ing test of hot-pressed composites were observed by field emission scanning electron microscopy (FE-SEM). The constitution phases of the matrix were identified by X-ray diffractometry (XRD), using monolithic SiC fabricated by the same processing condition for SiC/SiC composites in this study. Fig. 2. Cross-sections of uncoated and PyC coated SA fibers formed with CVD technique: (a) uncoated, (b) CVD-0.25, (c) CVD-0.50 and (d) CVD-1.00. Table 3 Density and porosity of hot-pressed SiC/SiC composites with various interface conditions Density (mg/m3 ) Open porosity (%) ED/TDa (%) Fiber volume (%) Uncoated 3.15 1.6 99 55 CVD-0.25 3.10 1.5 95 42 CVD-0.50 3.01 3.3 92 48 CVD-1.00 2.84 6.9 87 46 a ED: experimental density, TD: theoretical density. Fig. 3. X-ray diffraction profile of the monolithic SiC fabricated by the same condition for SiC/SiC composites. 100 K. Shimoda et al. / Composites Science and Technology 68 (2008) 98–105

K. Shimoda et al./ Composites Science and Technology 68(2008)98-105 Micro 500um Fig. 4. Typical cross-section of SiC/SiC composites fabricated by (a)CVI process and(b)NITE process. 3. Results and discussions 3.2. Density and microstructural evolution 3.. Coating Density and porosity of hot-pressed SiC/SiC composites dre summar ed in Table 3. The matrix in NITE-SiC/Sic is Fig. 2 shows FE-SEM images of uncoated and Pyc consisted of well-crystallized B-Sic grains with small coated fibers formed with CVD coating technique. Most amount of oxide remnants including aluminum and filaments in the tows through CVD were well coated, with yttrium (mostly YAG: yttrium aluminum garnet), from the thickness of appropriately 0. 25, 0.50 and 1.00 um Sur- XRD analysis as shown in Fig. 3. The presence of YAG face of PyC coated fiber was smooth without relation to should improve sinterability of the matrix in composites PyC layer thickness [20]. Such well-crystallized Sic matrix suggests environ- (a) ntra fiber-tows (c) (d) e 50um Fig. 5. SEM images of polished cross-section of SiC/SiC composites with various PyC interface thickness: (a)uncoated, (b)CVD-0 25, (c)CVD-050 and (d)CVD

3. Results and discussions 3.1. Coating on fiber surface Fig. 2 shows FE-SEM images of uncoated and PyC coated fibers formed with CVD coating technique. Most filaments in the tows through CVD were well coated, with the thickness of appropriately 0.25, 0.50 and 1.00 lm. Sur￾face of PyC coated fiber was smooth without relation to PyC layer thickness. 3.2. Density and microstructural evolution Density and porosity of hot-pressed SiC/SiC composites are summarized in Table 3. The matrix in NITE-SiC/SiC is consisted of well-crystallized b-SiC grains with small amount of oxide remnants including aluminum and yttrium (mostly YAG: yttrium aluminum garnet), from XRD analysis as shown in Fig. 3. The presence of YAG should improve sinterability of the matrix in composites [20]. Such well-crystallized SiC matrix suggests environ￾Fig. 4. Typical cross-section of SiC/SiC composites fabricated by (a) CVI process and (b) NITE process. Fig. 5. SEM images of polished cross-section of SiC/SiC composites with various PyC interface thickness: (a) uncoated, (b) CVD-0.25, (c) CVD-0.50 and (d) CVD-1.00. K. Shimoda et al. / Composites Science and Technology 68 (2008) 98–105 101

K Shimoda et al. Composites Science and Technology 68(2008)98-105 Fibers Mat c) Matri> Fiber Pyc m Fig. 6. Backscattered electron images of polished cross-section at intra-fibers of SiC/SiC composites with various PyC interface thickness: (a)uncoated, (b) CVD-025,(c)CVD-050 and(d) CVD.1.00 mental stability, compared to CVI-SiC/SiC. The differences The increase of PyC thickness decreased the bulk density in properties between NITE and CVI composites come No big pores could be identified at inter-fiber-tows, as from the difference in matrix porosity, which is less than shown in Fig. 5b-d. Pores are mainly distributed in the a few percent in the former. As shown in Fig. 4, This intra-fiber-tows regions. The composites with PyC thick may be supposed that macro-pores(hundreds microns ness of 1.00 um maintained about 7% porosity The forma in size) at inter-fiber-tows and closed micro-pores (oa tion of porous intra-fiber-tows matrix might be due to the few microns)at intra-fiber-tows are mainly observed in poor SiC nano-powder infiltration. As shown in Fig. 6d CVI based composites, on the other hand distributed increasing PyC coating thickness caused the difficulty of micro-pores(below 10 um)at inter-fiber-tows are only SiC nano-powder infiltration due to the promotion of nar- observed in NITE composites. Figs. 5 and 6 show SEM row aisle and/or the attachment between PyC layers For images of cross section and backscattered electron images all that, its density is comparatively higher than that of at intra-fiber-tows of cross section of SiC/SiC composites conventional CVI-, PIP-SiC/SiC composites. Only when with various PyC interface thickness after polishing. In this the thickness of Pyc is about 0. 25 um, PyC layer was dis- study, nearly fully densified(almost 99% of theoretical den- appeared and degraded severely, as shown in Fig. 6b ity) SiC/SiC composites without PyC coating fiber tows Moreover, the thickness of PyC was decreased in the cases could be obtained with the sufficient infiltration of Sic of CVD-050 and CVD-1.00. These can explain through nano-powder into the narrow region of the intra-fiber- the reactions between PyC layer and additional oxide addi- wS,as shown in Fig. 6a. However, uncoated fibers tives. From thermodynamics, the following reaction is con- showed attachment to matrix and irregular fiber edges. sidered as main [21, 22: Table 4 Mechanical properties of hot-pressed SiC/SiC composites with various interface conditions Pyc thickness(nm) Flexural strength(MPa Uncoated 0 l140±50 CVD-0. 25 780±50 247±13 CvD-0.50 l00-250 600±26 185±12 CVD.L.00 409±60 164±8

mental stability, compared to CVI-SiC/SiC. The differences in properties between NITE and CVI composites come from the difference in matrix porosity, which is less than a few percent in the former. As shown in Fig. 4, This may be supposed that macro-pores (hundreds microns in size) at inter-fiber-tows and closed micro-pores (a few microns) at intra-fiber-tows are mainly observed in CVI based composites, on the other hand distributed micro-pores (below 10 lm) at inter-fiber-tows are only observed in NITE composites. Figs. 5 and 6 show SEM images of cross section and backscattered electron images at intra-fiber-tows of cross section of SiC/SiC composites with various PyC interface thickness after polishing. In this study, nearly fully densified (almost 99% of theoretical den￾sity) SiC/SiC composites without PyC coating fiber tows could be obtained with the sufficient infiltration of SiC nano-powder into the narrow region of the intra-fiber￾tows, as shown in Fig. 6a. However, uncoated fibers showed attachment to matrix and irregular fiber edges. The increase of PyC thickness decreased the bulk density. No big pores could be identified at inter-fiber-tows, as shown in Fig. 5b–d. Pores are mainly distributed in the intra-fiber-tows regions. The composites with PyC thick￾ness of 1.00 lm maintained about 7% porosity. The forma￾tion of porous intra-fiber-tows matrix might be due to the poor SiC nano-powder infiltration. As shown in Fig. 6d, increasing PyC coating thickness caused the difficulty of SiC nano-powder infiltration due to the promotion of nar￾row aisle and/or the attachment between PyC layers. For all that, its density is comparatively higher than that of conventional CVI-, PIP-SiC/SiC composites. Only when the thickness of PyC is about 0.25 lm, PyC layer was dis￾appeared and degraded severely, as shown in Fig. 6b. Moreover, the thickness of PyC was decreased in the cases of CVD-0.50 and CVD-1.00. These can explain through the reactions between PyC layer and additional oxide addi￾tives. From thermodynamics, the following reaction is con￾sidered as main [21,22]: Fig. 6. Backscattered electron images of polished cross-section at intra-fibers of SiC/SiC composites with various PyC interface thickness: (a) uncoated, (b) CVD-0.25, (c) CVD-0.50 and (d) CVD-1.00. Table 4 Mechanical properties of hot-pressed SiC/SiC composites with various interface conditions PyC thickness (nm) Flexural strength (MPa) Elastic modulus (GPa) Uncoated 0 1140 ± 50 250 ± 7 CVD-0.25 0 780 ± 50 247 ± 13 CVD-0.50 100–250 600 ± 26 185 ± 12 CVD-1.00 450–600 409 ± 60 164 ± 8 102 K. Shimoda et al. / Composites Science and Technology 68 (2008) 98–105

K. Shimoda et al./ Composites Science and Technology 68(2008)98-105 1200 uncoated 1000 CD0.25 CD-0.50 CⅤD-1.00 詞600 400 0.05 0.15 0.2 isplacement /mm Fig. 7. Typical flexural stress-displacement curves of SiC/SiC composites with various PyC interface thickness. C(s)+ SiO2(s, I)=SiC(s)+2Co(g) (4) difficult leading to porous matrix. Increasing applied pres It is reported that the densification process highly depen- sure(to 20 MPa) will enhance the sliding of Sic nano-pow- dent on hot-pressing conditions, such as sintering tempera- der. While increasing sintering temperature ( to 1800C ture and applied pressure [15, 16]. At lower temperature, will promote the liquid phase formation so that less liquid phase formation will inhibit the sliding of Sic fication is also improved. However, extra temperatu nano-powder. In this case matrix densification becomes pressure might exerted on fibers through the highly (d Fiber pull-out Fiber d-out Fig 8. Fracture surface of SiC/SiC composites with various PyC interface thickness after three-point bending test: (a)uncoated, (b) CVD-0 25 (c)CVD. 0.50and(d)CvD-1.00

3CðsÞ þ SiO2ðs; lÞ ¼ SiCðsÞ þ 2COðgÞ ð4Þ It is reported that the densification process highly depen￾dent on hot-pressing conditions, such as sintering tempera￾ture and applied pressure [15,16]. At lower temperature, less liquid phase formation will inhibit the sliding of SiC nano-powder. In this case, matrix densification becomes difficult leading to porous matrix. Increasing applied pres￾sure (to 20 MPa) will enhance the sliding of SiC nano-pow￾der. While increasing sintering temperature (to 1800 C) will promote the liquid phase formation so that the densi- fication is also improved. However, extra temperature and pressure might exerted on fibers through the highly densifi- Fig. 7. Typical flexural stress–displacement curves of SiC/SiC composites with various PyC interface thickness. Fig. 8. Fracture surface of SiC/SiC composites with various PyC interface thickness after three-point bending test: (a) uncoated, (b) CVD-0.25, (c) CVD- 0.50 and (d) CVD-1.00. K. Shimoda et al. / Composites Science and Technology 68 (2008) 98–105 103

K Shimoda et al. Composites Science and Technology 68(2008)98-105 cation matrix with the severe fiber deformation. In this layer. PyC coating used on fibers is very reactive with study, the thickness of PyC coating is indicated to be one sintering additives, and about 0.30-0.40 um is sacrifi- of the essential issues for matrix densification cially consumed during hot-pressing. (3) Higher strength with a pseudo-ductile fracture behav- 3.3. Mechanical properties and fracture behavior or could be obtained using 0.50 um of PyC coating hickness. This is supposed that suficient Sic nano- Mechanical properties of hot-pressed SiC/SiC compos- owder infiltration at intra-fiber -tows enhanced den ites are listed in Table 4, and typical flexural stress-dis fication process with the existence of PyC coating placement curves and fracture surfaces after bending test layer to protect fibers and PyC interface after are shown in Figs. 7 and 8, respectively. The average flex induced-processing ural strength of the SiC/SiC composites without PyC coat fiber tows was very high, above I GPa. Fracture surface Acknowledgement of the composites was smooth and fiber-tows were not so lear, therefore flexural stress-displacement curve showed This work was supported by Fundamental r&D on Ad- a brittle fracture behavior (without fiber pull-out). The vanced Material system for(High Efficiency Environment SiC/SiC composites with the PyC thickness of 0.25 um also conscious) Very High Temperature Gas-cooled Fast Reac- showed a brittle fracture behavior with smooth fracture tor Core Structures, the program funded by Ministry of surface(without fiber pull-out) due to the absence of Pyc Education, Culture, Sports and Technology of Japan interface to deflect cracks. It is assumed that most PyC interface to deflect cracks was consumed by the reaction References (4)during hot-pressing in the case of the SiC/SiC compos- ites with the PyC thickness of 0. 25 um. The difference of [1] Tressler RE. Recent developments in fibers and interphases for high maximum strength might be due to porosity and fiber vol- ume fraction at intra-fiber-tows. Higher strength with 199930:429-37. pseudo-ductile fracture behavior could be obtained using 2] Naslain R. Design, preparation and properties of non-oxide CMCs 0.50 um of PyC thickness, where a lot of deflects and for application in engines and nuclear reactors: an overview. Compos ci Technol 2004: 64- 155-70 branches of the propagating cracks and fiber pull-out were [3]Kohyama A, Seki M, Abe K, Muroga T, Matsui H, Jitsukawa S, observed (see Fig. 8c). The extra PyC thickness (to et al. Interactions between fusion materials r&d and other technol- 00 um), which enhanced the prevention of Sic nano- ogies. J Nucl Mater 2000: 283-287: 20- owder infiltration at intra-fiber -tows caused the severe 4] Snead LL, Jones R, Kohyama A, Fenici P Status of silicon carbide composites for fusion. J Nucl Mater 1996: 233-237: 26-36. degradation of Pyc interface as well as fibers by induced 5]Araki H, Yang w, Suzuki H, Hu Q, Busabok C, Noda T Fabrication pressure (see Fig. 6d), resulting in poor mechanical and flexural properties of Tyranno-SA/SIC composites with carbon properties. From these results, it is concluded that the infil- interlayer by Cvl. J Nucl Mater 2004: 329: 56 tration efficiency of SiC nano-powder into the narrow [6]Kohyama A, Kotani M,Katoh Y,Nakayasu T,Sato M,Yamamura region of intra-fiber-tows improves the densification T, et al. High-performance SiC/SiC composites by improved PIP processing with new precursor polymers. J Nucl Mater 2000: 283- process and simultaneously protects fibers and PyC 87:565-9 interface during induced-processing [7 Sayano A, Sutoh C, Suyama S, Itoh Y, Nakagawa S Development of a reaction-sintered silicon carbide matrix composite. J Nucl Mate 999;271-272:467-71 4. Conclusions [8] Kotani M, Kohyama A, Katoh Y. Development of SiC/SiC composites by PIP in combination with Rs. J Nucl Mater 2001:289:37-41 Unidirectional SiC/SiC composites were fabricated by [9] Nozawa T, Hinoki T, Katoh Y, Kohyama A. Effects of fibers and NitE process using SiC nano-powder infiltration tech fabrication processes on mechanical properties of neutron irradiated nique with sintering additives for matrix formation. CVD SiC/SiC composites. J Nucl Mater 2002: 307-311: 1173-7 process can coat appropriate PyC thickness with unifor- [) Hinoki T. Spead LL, katoh Y,, Hasegawa A, Nozawa T, Kohyama nity Induced PyC interface conditions strongly affect the fibers and their silicon carbide composites. J Nucl Mater 2002: 283- y, microstructural evolution, and therefore dominate nical properties and fracture behaviors. The results [11] Snead LL, Katoh Y, Kohyama A, Bailey JL, Vaughn NL, Lowden follow RA. Evaluation of neutron irradiated near-stoichiometric silicon carbide fiber composites. J Nucl Mater 2000: 283-287: 551-5 (1) Nearly full-dense SiC/SiC composites with [12] Kohyama A, Dong SM,Katoh Y.Development composites by nano-infiltration and transient eutectic (NITE)pro- uncoated fibers caused strong interaction between cess Ceram Eng Sci Proc 2002: 23: 311-8. fibers and matrix, resulting in a brittle fracture [13]Katoh Y, Dong SM, Kohyama A. A novel processing technique of behavior silicon carbide-based ceramic composites for high temperature (2)Increasing PyC thickness caused the difficulty of sic applications. Ceram Trans 2002: 144: 77-86. nano-powder infiltration with the promotion of nar. l4]Katoh Y Kohyama A, Dong SM, HinokiT, Kai JJ.Microstructure and properties of liquid phase sintered SiC/SiC composites. Ceram row aisle and/or the attachment between PyC coating Eng Sci Proc 2002: 23(3 ): 363

cation matrix with the severe fiber deformation. In this study, the thickness of PyC coating is indicated to be one of the essential issues for matrix densification. 3.3. Mechanical properties and fracture behavior Mechanical properties of hot-pressed SiC/SiC compos￾ites are listed in Table 4, and typical flexural stress–dis￾placement curves and fracture surfaces after bending test are shown in Figs. 7 and 8, respectively. The average flex￾ural strength of the SiC/SiC composites without PyC coat￾ing fiber tows was very high, above 1 GPa. Fracture surface of the composites was smooth and fiber-tows were not so clear, therefore flexural stress–displacement curve showed a brittle fracture behavior (without fiber pull-out). The SiC/SiC composites with the PyC thickness of 0.25 lm also showed a brittle fracture behavior with smooth fracture surface (without fiber pull-out) due to the absence of PyC interface to deflect cracks. It is assumed that most PyC interface to deflect cracks was consumed by the reaction (4) during hot-pressing in the case of the SiC/SiC compos￾ites with the PyC thickness of 0.25 lm. The difference of maximum strength might be due to porosity and fiber vol￾ume fraction at intra-fiber-tows. Higher strength with a pseudo-ductile fracture behavior could be obtained using 0.50 lm of PyC thickness, where a lot of deflects and branches of the propagating cracks and fiber pull-out were observed (see Fig. 8c). The extra PyC thickness (to 1.00 lm), which enhanced the prevention of SiC nano￾powder infiltration at intra-fiber-tows caused the severe degradation of PyC interface as well as fibers by induced￾pressure (see Fig. 6d), resulting in poor mechanical properties. From these results, it is concluded that the infil￾tration efficiency of SiC nano-powder into the narrow region of intra-fiber-tows improves the densification process and simultaneously protects fibers and PyC interface during induced-processing. 4. Conclusions Unidirectional SiC/SiC composites were fabricated by NITE process using SiC nano-powder infiltration tech￾nique with sintering additives for matrix formation. CVD process can coat appropriate PyC thickness with unifor￾mity. Induced PyC interface conditions strongly affect the density, microstructural evolution, and therefore dominate mechanical properties and fracture behaviors. The results are as follows: (1) Nearly full-dense SiC/SiC composites with uncoated fibers caused strong interaction between fibers and matrix, resulting in a brittle fracture behavior. (2) Increasing PyC thickness caused the difficulty of SiC nano-powder infiltration with the promotion of nar￾row aisle and/or the attachment between PyC coating layer. PyC coating used on fibers is very reactive with sintering additives, and about 0.30–0.40 lm is sacrifi- cially consumed during hot-pressing. (3) Higher strength with a pseudo-ductile fracture behav￾ior could be obtained using 0.50 lm of PyC coating thickness. This is supposed that sufficient SiC nano￾powder infiltration at intra-fiber-tows enhanced den￾sification process with the existence of PyC coating layer to protect fibers and PyC interface after induced-processing. Acknowledgements This work was supported by Fundamental R&D on Ad￾vanced Material system for (High Efficiency Environment￾conscious) Very High Temperature Gas-cooled Fast Reac￾tor Core Structures, the program funded by Ministry of Education, Culture, Sports and Technology of Japan. References [1] Tressler RE. Recent developments in fibers and interphases for high temperature ceramic matrix composites. Composites A 1999;30:429–37. [2] Naslain R. Design, preparation and properties of non-oxide CMCs for application in engines and nuclear reactors: an overview. Compos Sci Technol 2004;64:155–70. [3] Kohyama A, Seki M, Abe K, Muroga T, Matsui H, Jitsukawa S, et al. Interactions between fusion materials R&D and other technol￾ogies. J Nucl Mater 2000;283–287:20–7. [4] Snead LL, Jones R, Kohyama A, Fenici P. Status of silicon carbide composites for fusion. J Nucl Mater 1996;233–237:26–36. [5] Araki H, Yang W, Suzuki H, Hu Q, Busabok C, Noda T. Fabrication and flexural properties of Tyranno-SA/SIC composites with carbon interlayer by CVI. J Nucl Mater 2004;329:567–71. [6] Kohyama A, Kotani M, Katoh Y, Nakayasu T, Sato M, Yamamura T, et al. High-performance SiC/SiC composites by improved PIP processing with new precursor polymers. J Nucl Mater 2000;283– 287:565–9. [7] Sayano A, Sutoh C, Suyama S, Itoh Y, Nakagawa S. Development of a reaction-sintered silicon carbide matrix composite. J Nucl Mater 1999;271–272:467–71. [8] Kotani M, Kohyama A, Katoh Y. Development of SiC/SiC composites by PIP in combination with RS. J Nucl Mater 2001;289:37–41. [9] Nozawa T, Hinoki T, Katoh Y, Kohyama A. Effects of fibers and fabrication processes on mechanical properties of neutron irradiated SiC/SiC composites. J Nucl Mater 2002;307–311:1173–7. [10] Hinoki T, Snead LL, Katoh Y, Hasegawa A, Nozawa T, Kohyama A. The effect of high dose/high temperature irradiation on high purity fibers and their silicon carbide composites. J Nucl Mater 2002;283– 287:1157–62. [11] Snead LL, Katoh Y, Kohyama A, Bailey JL, Vaughn NL, Lowden RA. Evaluation of neutron irradiated near-stoichiometric silicon carbide fiber composites. J Nucl Mater 2000;283–287:551–5. [12] Kohyama A, Dong SM, Katoh Y. Development of SiC/SiC composites by nano-infiltration and transient eutectic (NITE) pro￾cess. Ceram Eng Sci Proc 2002;23:311–8. [13] Katoh Y, Dong SM, Kohyama A. 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