Availableonlineatwww.sciencedirect.com al of DIRECTO nuciear materials ELSEVIER Journal of Nuclear Materials 329-333(2004)572-576 Fabrication of advanced Sic fiber/F-Cvi SiC matrix composites with Sic/C multi-layer interphase T. Taguchi . T Nozawa, N. Igawa, Y. Katoh, S Jitsukawa, A Kohyama b, T. Hinoki b,,LL.Snead Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan Institute of Aduanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Metals and Ceramics Diuision, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831, US.A Abstract Sic/SiC composite with Sic/C multi-layer interphase coated on advanced Sic fibers was fabricated by the forced thermal-gradient chemical vapor infiltration(F-CVI) process. SEM and tEM observations verified that Sic/c multi layer interphase was formed on SiC fibers. Both flexural and tensile strengths of Sic/Sic composite with Sic/C multi yer interphase were approximately l0% higher than composites fabricated with single carbon interphase. The SEM observation of fracture surface for the composite with Sic/C multi-layer interphase revealed cylindrical steps formed around the fiber. Apparently several crack deflections occurred within SiC/C multi-layer interphase. Moreover, the sicl C multi-layer applied in this study operated efficiently to improve the mechanical properties. c 2004 Elsevier B V. All rights reserved the fabrication of Sic/Sic composites because it yield highly crystalline, near-stoichiometoric SiC fiber dam- The continuous silicon carbide fiber reinforced sili- age during processing is minimized. The forced flow date materials for fusion reactors because of their improvement since the r. -CVD process is a further on carbide (SiC/SiC) composites are attractive candi- thermal-gradient CVI (F- ates of producing and high excellent mechanical properties at high temperature and porosity, which are the peculiar problem of CVi, are low induced radioactivity after neutron irradiation [ 1-3]. overcome 9]. Previous work by authors has reported on Recently, highly crystalline near-stoichiometric SiC fi F-CVI process optimization of Sic/Sic composite with bers have been produced including Tyranno SA [4]. carbon( C) interphase including the effect of interphase They have superior mechanical properties under irradi- thickness on tensile strength [10 ation and oxidation compared with their Sic-based It was reported that SiC/C multi-layer interphase predecessors. Various processes have been employed for advanced fiber composite fabrication including chemical mechanical properties compared to single C interphase apor infiltration (CVi), polymer impregnation and [12]. However, the effect of SiC/C multi-layer with the pyrolysis, reaction bonding and nano- infiltration and advanced SiC fiber on the composite tensile strengths transient eutectic-phase process [5-8. Among the vari- has not been reported. In this study, new concept(SiC ous fabrication processes, CVI is attractive process for C)x6 multi-layer interphase was applied with advanced Sic fibers for further improvement in mechanical properties. The first SiC layer is designed to strengthen Corresponding author. Tel. +81-29 282 6099: fax: +81-29 the bond between fiber and interphase. The second to 2825922. orth si are designed to provide multi-stage E-mail address: taguchi@ popsvr tokai jaeri. go. jp(T. Tagu- pull-out of The fifth and sixth Sic layers are de- multi-stage pull-out of fiber bundles 0022-3115/S- see front matter G 2004 Elsevier B.v. All rights reserved. doi: 10.1016/j-jnt
Fabrication of advanced SiC fiber/F-CVI SiC matrix composites with SiC/C multi-layer interphase T. Taguchi a,*, T. Nozawa b , N. Igawa a , Y. Katoh c , S. Jitsukawa a , A. Kohyama b , T. Hinoki b,c, L.L. Snead c a Neutron Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan b Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan c Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831, USA Abstract SiC/SiC composite with SiC/C multi-layer interphase coated on advanced SiC fibers was fabricated by the forced thermal-gradient chemical vapor infiltration (F-CVI) process. SEM and TEM observations verified that SiC/C multilayer interphase was formed on SiC fibers. Both flexural and tensile strengths of SiC/SiC composite with SiC/C multilayer interphase were approximately 10% higher than composites fabricated with single carbon interphase. The SEM observation of fracture surface for the composite with SiC/C multi-layer interphase revealed cylindrical steps formed around the fiber. Apparently several crack deflections occurred within SiC/C multi-layer interphase. Moreover, the SiC/ C multi-layer applied in this study operated efficiently to improve the mechanical properties. 2004 Elsevier B.V. All rights reserved. 1. Introduction The continuous silicon carbide fiber reinforced silicon carbide (SiC/SiC) composites are attractive candidate materials for fusion reactors because of their excellent mechanical properties at high temperature and low induced radioactivity after neutron irradiation [1–3]. Recently, highly crystalline near-stoichiometric SiC fi- bers have been produced includingTyranno SA [4]. They have superior mechanical properties under irradiation and oxidation compared with their SiC-based predecessors. Various processes have been employed for advanced fiber composite fabrication includingchemical vapor infiltration (CVI), polymer impregnation and pyrolysis, reaction bondingand nano-infiltration and transient eutectic-phase process [5–8]. Amongthe various fabrication processes, CVI is attractive process for the fabrication of SiC/SiC composites because it yields highly crystalline, near-stoichiometoric SiC fiber damage during processing is minimized. The forced flow thermal-gradient CVI (F-CVI) process is a further improvement since the low rates of producingand high porosity, which are the peculiar problem of CVI, are overcome [9]. Previous work by authors has reported on F-CVI process optimization of SiC/SiC composite with carbon (C) interphase includingthe effect of interphase thickness on tensile strength [10,11]. It was reported that SiC/C multi-layer interphase composite with conventional fibers achieved superior mechanical properties compared to single C interphase [12]. However, the effect of SiC/C multi-layer with the advanced SiC fiber on the composite tensile strengths has not been reported. In this study, new concept (SiC/ C) · 6 multi-layer interphase was applied with advanced SiC fibers for further improvement in mechanical properties. The first SiC layer is designed to strengthen the bond between fiber and interphase. The second to fourth SiC layers are designed to provide multi-stage pull-out of fibers. The fifth and sixth SiC layers are designed to provide multi-stage pull-out of fiber bundles. * Correspondingauthor. Tel.: +81-29 282 6099; fax: +81-29 282 5922. E-mail address: taguchi@popsvr.tokai.jaeri.go.jp (T. Taguchi). 0022-3115/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2004.04.120 www.elsevier.com/locate/jnucmat Journal of Nuclear Materials 329–333 (2004) 572–576
T. Taguchi et al. I Journal of Nuclear Materi The intermediate C layers are used only to separate each Microstructure was observed by scanning electro SiC layers. It was also reported that thin C layer reduces microscopy (SEM) and transmission electron micros- the sensitivity of oxidation [13] and irradiation [14].a copy (TEM) Composite fracture surface was evaluated thin C layer(50 nm)was therefore chosen in this study. by sEM. Electron probe microanalysis (EPMA)was The characterization of SiC/C multi-layer and the effect carried out to evaluate the chemical composition of the of Sic/C multi-layer on the mechanical properties were fracture surface within the Sic/C multi-layer investigated in this study 3. Results and discussion 2. Experimental procedure The porosity of SiC/SiC composite fabricated in this Tyranno SA fiber SiC fabrics(Ube Industries, Ube, tudy was approximately 20%o with fiber volume fraction Japan)were used as reinforcement for the SiC/Sic of approximately 39%. omposite in this study. The precursor for C deposition A typical cross-sectional SEM microphotograph of was propylene( C3H6). Technical grade methyltrichlo- SiC/SiC composite is shown in Fig. 1. The average rosilane (MTS, CH3,, Gelest Inc, Tullytown, PA, thickness of each layer in SiC/C multi-layer interphase USA)was also used for SiC infiltration SiC fabric layers is given in Table 1. Although the deposition time of with fabric layer orientation of (0%/90%)were restrained each C layer was same, the thickness of outer C layer in a graphite fixture. The Sic and c interphase coating was thicker than that of inner C layer. The reasor on the fiber were sequentially deposited at 1100C and that the deposition area decreased at the outer C layer at 5 Pa. The C deposition condition of flow rate is 50 compared to inner C layer since the space among cm/min C3H6 and 1000 cm/min Ar. The SiC deposition vicinal fibers was filled with the inner SiC and C lay- condition of flow rate is 0. 15 g/min MTS carried by 250 ers m/min of H2. Table I shows the condition parameters Fig 2 shows the cross-sectional TEM micrographs of of Sic/c multi-layer interphase deposition. After the Sic/Sic composite. The Sic/c multi-layer consisted of interphase layer was deposited, the preform was infil- six(SiC/C)layers. Electron diffraction patterns for the trated at 1200C under atmospheric pressure with a corresponding Sic layers and Sic fiber are also shown in MTS flow rate of 0.3 g/min carried by 750 cm/min of Fig. 2. The electron diffraction pattern of second Sic H,. A composite with single C layer as an interphase. layer was similar to that of sic fiber with fine grain size was also fabricated (40 nm)[4]. The TEM observation reveals that the Mechanical properties of the composite were evalu- inner SiC layers (first to fourth SiC layers) consisted of ated by 3-point bending and tensile testing. Three-point finer Sic grains compared to the outer Sic layers(fifth bending was carried out at ambient temperature with and sixth SiC layers)and Sic matrix. The grain sizes of cross-head speed of 0. 1 mm/min and a support span fifth and sixth SiC layers were almost the same as that of length of 20 mm. The test specimen was 2x5x25 mm Sic matrix. A very thin C layer formed between SiC Tensile testing was carried out at ambient temperature fiber and first Sic layer. The Sic fibers were coated by with cross-head speed of 0.5 mm/min. Further details of poly vinyl alcohol(PVA)as sizing material [4]. A very tensile test and specimen is described elsewhere [10, 11]. thin C layer formed by carbonizing the PVa since the The each number of specimens measured by bending fibers were heated at 1100C prior to the initial SiC and tensile testing is three interphase deposition Table I Condition parameters of Sic/C multi-layer interphase deposition Material Designed thickness(nm) Measured thickness(nm) Deposition time(min) (SiC/C×6 000000 03050 00000
The intermediate C layers are used only to separate each SiC layers. It was also reported that thin C layer reduces the sensitivity of oxidation [13] and irradiation [14]. A thin C layer (50 nm) was therefore chosen in this study. The characterization of SiC/C multi-layer and the effect of SiC/C multi-layer on the mechanical properties were investigated in this study. 2. Experimental procedure Tyranno SA fiber SiC fabrics (Ube Industries, Ube, Japan) were used as reinforcement for the SiC/SiC composite in this study. The precursor for C deposition was propylene (C3H6). Technical grade methyltrichlorosilane (MTS, CH3SiCl3, Gelest Inc., Tullytown, PA, USA) was also used for SiC infiltration. SiC fabric layers with fabric layer orientation of (0/90) were restrained in a graphite fixture. The SiC and C interphase coating on the fiber were sequentially deposited at 1100 C and at 5 Pa. The C deposition condition of flow rate is 50 cm3/min C3H6 and 1000 cm3/min Ar. The SiC deposition condition of flow rate is 0.15 g/min MTS carried by 250 cm3/min of H2. Table 1 shows the condition parameters of SiC/C multi-layer interphase deposition. After the interphase layer was deposited, the preform was infiltrated at 1200 C under atmospheric pressure with a MTS flow rate of 0.3 g/min carried by 750 cm3/min of H2. A composite with single C layer as an interphase was also fabricated. Mechanical properties of the composite were evaluated by 3-point bendingand tensile testing. Three-point bendingwas carried out at ambient temperature with cross-head speed of 0.1 mm/min and a support span length of 20 mm. The test specimen was 2 · 5 · 25 mm3. Tensile testingwas carried out at ambient temperature with cross-head speed of 0.5 mm/min. Further details of tensile test and specimen is described elsewhere [10,11]. The each number of specimens measured by bending and tensile testingis three. Microstructure was observed by scanningelectron microscopy (SEM) and transmission electron microscopy (TEM). Composite fracture surface was evaluated by SEM. Electron probe microanalysis (EPMA) was carried out to evaluate the chemical composition of the fracture surface within the SiC/C multi-layer. 3. Results and discussion The porosity of SiC/SiC composite fabricated in this study was approximately 20% with fiber volume fraction of approximately 39%. A typical cross-sectional SEM microphotograph of SiC/SiC composite is shown in Fig. 1. The average thickness of each layer in SiC/C multi-layer interphase is given in Table 1. Although the deposition time of each C layer was same, the thickness of outer C layer was thicker than that of inner C layer. The reason is that the deposition area decreased at the outer C layer compared to inner C layer since the space among vicinal fibers was filled with the inner SiC and C layers. Fig. 2 shows the cross-sectional TEM micrographs of SiC/SiC composite. The SiC/C multi-layer consisted of six (SiC/C) layers. Electron diffraction patterns for the correspondingSiC layers and SiC fiber are also shown in Fig. 2. The electron diffraction pattern of second SiC layer was similar to that of SiC fiber with fine grain size (40 nm) [4]. The TEM observation reveals that the inner SiC layers (first to fourth SiC layers) consisted of finer SiC grains compared to the outer SiC layers (fifth and sixth SiC layers) and SiC matrix. The grain sizes of fifth and sixth SiC layers were almost the same as that of SiC matrix. A very thin C layer formed between SiC fiber and first SiC layer. The SiC fibers were coated by poly vinyl alcohol (PVA) as sizingmaterial [4]. A very thin C layer formed by carbonizingthe PVA since the fibers were heated at 1100 C prior to the initial SiC interphase deposition. Table 1 Condition parameters of SiC/C multi-layer interphase deposition Material Designed thickness (nm) Measured thickness (nm) Deposition time (min) (SiC/C)·6 SiC 50 56 4 C 50 37 30 SiC 200 119 15 C 50 35 30 SiC 200 132 15 C 50 40 30 SiC 200 153 15 C 50 41 30 SiC 500 432 38 C 50 62 30 SiC 500 572 38 C 50 83 30 T. Taguchi et al. / Journal of Nuclear Materials 329–333 (2004) 572–576 573
T. Taguchi et al. I Journal of Nuclear Materials 329-333(2004)572-576 The normalized flexural and tensile strengths of the siC/ Sic composite with SiC/C multi-layer were approxi- mately 10%o higher than those of the composite with single C interphase The SEM micrographs of fracture surfaces of Sic/ Sic composite with SiC/C multi-layer after flexural and tensile tests are shown in Fig. 4. The cylindrical steps around the fiber were observed after flexural and tensile tests and several crack deflections occurred within SiC/C multi-layer interphase. Furthermore, pull-out of fiber bundles occurred. These results indicate that the second to fourth Sic layers and fifth to sixth Sic layers oper 00615K035,的 5 um ated efficiently to improve the fracture behavior. Since Fig. 1. Typical cross-sectional SEM microphotograph of Sic both multiple pull-out of fibers and fiber bundles oc SiC composite with SiC/C multi-layer interphase. curred, higher fracture energy was absorbed compared to single C layer. The grain size of inner Sic layer was The tested composites had different porosities and fiber volume fractions. Araki et al. [5]reported that the 700 flexural strength increased with decrease aComposite with single C layer composite. Since the load is mainly maintained by un a composite with SiC/C multiLayer. fractured fibers and friction between fractured fiber and interphase above the proportional limit stress, the flex- ural and tensile strengths depend on the porosity and a400 tensile strengths were, therefore, normalized by the fol lowing equation c200 Normalized strength original strength Vr 100 where v and vr are fiber volume fraction and the 0 average fiber volume fraction of the composites he normalized flexural and tensile strengths of the Fig. 3. Normalized flexural and tensile strengths of the Sic/Sic SiC/SiC composite with SiC/C multi-layer and the composite with SiC/C multi-layer and the composite with single composite with single C interphase are shown in Fig 3 500nm Fig. 2. Typical cross-sectional TEM microphotographs of Sic/Sic composite with SiC/C multi-layer interphase and the electron diffraction patterns for(A)SiC fiber, (B)second Sic layer and (C)Sic matrix
The tested composites had different porosities and fiber volume fractions. Araki et al. [15] reported that the flexural strength increased with decreased porosity of composite. Since the load is mainly maintained by unfractured fibers and friction between fractured fiber and interphase above the proportional limit stress, the flexural and tensile strengths depend on the porosity and fiber volume fraction of composite. The flexural and tensile strengths were, therefore, normalized by the followingequation: Normalized strength ¼ original strength 1 porosity Vf V f ; where Vf and V f are fiber volume fraction and the average fiber volume fraction of the composites. The normalized flexural and tensile strengths of the SiC/SiC composite with SiC/C multi-layer and the composite with single C interphase are shown in Fig. 3. The normalized flexural and tensile strengths of the SiC/ SiC composite with SiC/C multi-layer were approximately 10% higher than those of the composite with single C interphase. The SEM micrographs of fracture surfaces of SiC/ SiC composite with SiC/C multi-layer after flexural and tensile tests are shown in Fig. 4. The cylindrical steps around the fiber were observed after flexural and tensile tests and several crack deflections occurred within SiC/C multi-layer interphase. Furthermore, pull-out of fiber bundles occurred. These results indicate that the second to fourth SiC layers and fifth to sixth SiC layers operated efficiently to improve the fracture behavior. Since both multiple pull-out of fibers and fiber bundles occurred, higher fracture energy was absorbed compared to single C layer. The grain size of inner SiC layer was Fig. 1. Typical cross-sectional SEM microphotograph of SiC/ SiC composite with SiC/C multi-layer interphase. Fig. 2. Typical cross-sectional TEM microphotographs of SiC/SiC composite with SiC/C multi-layer interphase and the electron diffraction patterns for (A) SiC fiber, (B) second SiC layer and (C) SiC matrix. Fig. 3. Normalized flexural and tensile strengths of the SiC/SiC composite with SiC/C multi-layer and the composite with single C interphase. 574 T. Taguchi et al. / Journal of Nuclear Materials 329–333 (2004) 572–576
T. Taguchi et al I Journal of Nuclear Materials 329-333(2004)572-576 10 Fig. 4. SEM micorophotographs of fracture surface after(a) fexural test and (b) tensile test. much smaller than that of Sic matrix. Cao reported that multi-layer was around 50 nm Table 1). Results indicate the flexural strength of Sic increased with decreasing the that 50 nm-thick C layer was thick enough for the crack grain size [16]. The strength of inner SiC layer might be deflection. The particular SiC/C multi-layer configura higher than that of Sic matrix. By the crack deflection tion in this study operated efficiently to improve the within the outer C layer, the apparent thick fibers, which mechanical properties consisted of Sic fiber and inner Sic/c layer,were formed. These apparent thick fibers were able to retain higher load compared to the Sic fibers. Furthermore 4. Conclusion the pull-out length might be increased because of the apparent thick fibers. From above reasons, the Sic/SiC composite with SiC/C multi-layer interphase mechanical strength of composite with SiC/C multi-layer coated on Tyranno SA SiC fibers was fabricated by F- was higher than that of composite with single C layer CVI process. The characterization of SiC/C multi-layer The chemical composition of fiber surface on the and the effect of Sic/C multi-layer on the mechanical fracture surface was slightly large amount of C com- properties of the composite were investigate paring to the amount of Si by EPMA evaluation. The chemical composition of fracture surface after fexural (1) The sEM and tEM observation verified that SiC/C test evaluated by epma is summarized in Table 2. The multi-layer interphase was formed on Sic fibers in analyzed points of EPMA are also given in Fig 4. This this process result indicates that the crack passed within the C layer (2)Both flexural and tensile strengths of Sic/Sic com- although the very thin C existed between SiC fiber and posite with Sic/c multi-layer interphase were first SiC layer. The reason is that the bonding strength approximately 10%o higher than that with single C between SiC fiber and C layer was high enough since the interphase surface of Tyranno SA fiber was as rough as the Sic (3)Pull-out of fiber bundles as well as pull-out of single layer(Fig. 2). fiber occurred in the composite with SiC/C multi The results of epma reveal that the crack defection occurred at almost every C layer. The SEM observation (4) Crack deflections occurred at almost every 50 nm reveals that the measured thickness of each c in the thick C layer. The particular SiC/C multi-layer con- figuration used in this study operated efficiently to improve the mechanical properties of Sic/Sic com Table posite Chemical composition of the fracture surface within SiC/C multi-layer evaluated by EPMA oints Si(at % C(at % Acknowledgements This study was carried out under the US-DOE JAERI Collaborative Program on FWB Structur Materials in Mixed-Spectrum Fission Reactors, Phase IV. This study was also supported by CREST-ACE 709 ( Core Research for Evolutional Science and Technology/ Advanced Material Systems for Energy Conversion) The analyzed poir program sponsored by the Japan Science and Technol ints were shown in Fig 4 ogy Corporation
much smaller than that of SiC matrix. Cao reported that the flexural strength of SiC increased with decreasing the grain size [16]. The strength of inner SiC layer might be higher than that of SiC matrix. By the crack deflection within the outer C layer, the apparent thick fibers, which consisted of SiC fiber and inner SiC/C layer, were formed. These apparent thick fibers were able to retain higher load compared to the SiC fibers. Furthermore, the pull-out length might be increased because of the apparent thick fibers. From above reasons, the mechanical strength of composite with SiC/C multi-layer was higher than that of composite with single C layer. The chemical composition of fiber surface on the fracture surface was slightly large amount of C comparingto the amount of Si by EPMA evaluation. The chemical composition of fracture surface after flexural test evaluated by EPMA is summarized in Table 2. The analyzed points of EPMA are also given in Fig. 4. This result indicates that the crack passed within the C layer although the very thin C existed between SiC fiber and first SiC layer. The reason is that the bondingstrength between SiC fiber and C layer was high enough since the surface of Tyranno SA fiber was as rough as the SiC layer (Fig. 2). The results of EPMA reveal that the crack deflection occurred at almost every C layer. The SEM observation reveals that the measured thickness of each C in the multi-layer was around 50 nm (Table 1). Results indicate that 50 nm-thick C layer was thick enough for the crack deflection. The particular SiC/C multi-layer configuration in this study operated efficiently to improve the mechanical properties. 4. Conclusions SiC/SiC composite with SiC/C multi-layer interphase coated on Tyranno SA SiC fibers was fabricated by FCVI process. The characterization of SiC/C multi-layer and the effect of SiC/C multi-layer on the mechanical properties of the composite were investigated. (1) The SEM and TEM observation verified that SiC/C multi-layer interphase was formed on SiC fibers in this process. (2) Both flexural and tensile strengths of SiC/SiC composite with SiC/C multi-layer interphase were approximately 10% higher than that with single C interphase. (3) Pull-out of fiber bundles as well as pull-out of single fiber occurred in the composite with SiC/C multilayer. (4) Crack deflections occurred at almost every 50 nm thick C layer. The particular SiC/C multi-layer con- figuration used in this study operated efficiently to improve the mechanical properties of SiC/SiC composite. Acknowledgements This study was carried out under the US-DOE/ JAERI Collaborative Program on FWB Structural Materials in Mixed-Spectrum Fission Reactors, Phase IV. This study was also supported by CREST-ACE (Core Research for Evolutional Science and Technology/ Advanced Material Systems for Energy Conversion) program sponsored by the Japan Science and Technology Corporation. Fig. 4. SEM micorophotographs of fracture surface after (a) flexural test and (b) tensile test. Table 2 Chemical composition of the fracture surface within SiC/C multi-layer evaluated by EPMA Analyzed points Si (at.%) C (at.%) A 51 49 B 33 67 C 47 53 D 32 68 E 41 59 F 27 63 G 30 70 H 39 61 The analyzed points were shown in Fig. 4. T. Taguchi et al. / Journal of Nuclear Materials 329–333 (2004) 572–576 575
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