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 nanopowder infiltration technique, and the effects of pyrolytic carbon (PyC) interface thickness between fibers and matrix on density, microstructural 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-performance SiC/SiC composites, using chemical vapor infiltration (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 porosity by incorporating both SiC nano-powder infiltration and the advanced SiC fibers with well-crystallized microstructure and near-stoichiometric composition like TyrannoTMSA 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 consequently 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 interface 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 reinforcement. 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 nanopowder (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 diameter of 0.4 lm and SiO2 (Kojundo Chemical Laboratory Co. Ltd., Japan, 99.9% pure) with a mean particle diameter of 1 lm were used as sintering additives. Fig. 1 shows TEM image of SiC nano-powder as-received. Unidirectional 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 elasticity 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–displacement curve in test, L is outer support span, w is specimen width, t is specimen thickness, D is deflection at beam center 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 bending 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. Surface 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 environFig. 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 density) 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-fibertows, 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 thickness of 1.00 lm maintained about 7% porosity. The formation 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 narrow 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 disappeared 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 additives. From thermodynamics, the following reaction is considered 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