Availableonlineatwww.sciencedirect.com SCIENCE PHYSICS ANI MIsTRY F SOLIDS ELSEVIER Journal of Physics and Chemistry of Solids 66(2005)551-554 Fabrication of Sic fiber reinforced Sic composite by chemical vapor infiltration for excellent mechanical properties N. Igawa*, T. Taguchi T. Nozawa, L.L. Snead, T Hinoki, J.C. McLaughlin, Y Katoh, S Jitsukawa, A Kohyama Japan Atomic Energy Research institute, Tokai-mura, Ibaraki-ken 319-1195, Japan Oak Ridge National Laboratory, Oak Ridge, TN 37830-6146, USA Kyoto University, Gokasho, Uji-shi, Kyoto fit 611-0011, Japan Accepted 19 June 2004 The process optimization for the forced-flow/thermal gradient chemical vapor infiltrated Sic based composites with an advanced SiC fiber(Tyranno SA)was carried out. The new SiC/SiC composites had a lower porosity and the uniform distribution of pores compared with conventional CVI. The uniform interphases between SiC fibers and matrix could be obtained by reversing the gas-fiow direction mid-way through the coating process. The tensile strength was slightly increased with the thickness of carbon interphase in the range of 20-250 nm. It was found that the fabric layer orientation and multilayer SiC/C interphase were very effective to improve the mechanical properties C 2004 Elsevier ltd. all rights reserved Keywords: A. Ceramics: A. Interface; B. Vapour deposition 1. Introduction excellent interfacial shear strength and ductility between fiber and matrix which cause high mechanical properties Silicon carbide has high strength and stability at high Among the various fabrication processes of SiC/SiC, the temperature and low induced radioactivity after neutron forced-flow/thermal gradient chemical vapor infiltration irradiation [1]. Though monolithic SiC is extremely brittle (FCVI) method is one of the best techniques to fabricate with low toughness, continuous SiC fiber reinforced Sic the SiC/SiC for fusion applications because this method matrix composites(SiC/SiC) significantly improve these produces a highly pure, stoichiometric, crystalline B-SiC properties and therefore are attractive candidate materials for matrix with low thermal stress which minimizes fiber fusion reactor structural applications [2, 3]. Moreover, high- damage during fabrication [6]. On the other hand, Sic stalline and near -stoichiometric sic fibers including matrix in SiC/SiC fabricated by the CVI method has low Hi-Nicalon Type S and Tyranno SA retain superior density and poor homogeneity, which cause the degradation mechanical properties after oxidation at high temperature of the mechanical and thermal properties. In this study, we and neutron irradiation compared with conventional carried out the optimization of this process to obtain higher advanced fibers are expected to have better mechanical ical properties of SiC/Sic. We also studied the effects of the properties For SiC/SiC composites, the interphase between carbon and multilayer SiC/carbon interphase on the mech fiber and matrix is very important to improve the toughness anical properties. [2]because the optimized interphase brings the deflection of cracks at the interphase, fiber pullout during the fracture, 2. Experimental procedure Corresponding author. Tel: +81 29 282 6099: fax:+8129 284 3813 We adopted two advanced SiC fiber fabrics as preforms E-mail address: igawa@maico. tokai jaeri. go. jp(N. Igawa). 2D-plain weave of Tyranno SA(Ube Industries, Ube, Japan) 0022-3697/S. see front matter 2004 Elsevier Ltd. All rights reserved doi:10.1016/jpcs.200406.030
Fabrication of SiC fiber reinforced SiC composite by chemical vapor infiltration for excellent mechanical properties N. Igawaa,*, T. Taguchia , T. Nozawab , L.L. Sneadb , T. Hinokic , J.C. McLaughlinb , Y. Katohb , S. Jitsukawaa , A. Kohyamac a Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken 319-1195, Japan b Oak Ridge National Laboratory, Oak Ridge, TN 37830-6146, USA c Kyoto University, Gokasho, Uji-shi, Kyoto-fu 611-0011, Japan Accepted 19 June 2004 Abstract The process optimization for the forced-flow/thermal gradient chemical vapor infiltrated SiC based composites with an advanced SiC fiber(Tyranno SA) was carried out. The new SiC/SiC composites had a lower porosity and the uniform distribution of pores compared with conventional CVI. The uniform interphases between SiC fibers and matrix could be obtained by reversing the gas-flow direction mid-way through the coating process. The tensile strength was slightly increased with the thickness of carbon interphase in the range of 20–250 nm. It was found that the fabric layer orientation and multilayer SiC/C interphase were very effective to improve the mechanical properties. q 2004 Elsevier Ltd. All rights reserved. Keywords: A. Ceramics; A. Interface; B. Vapour deposition 1. Introduction Silicon carbide has high strength and stability at high temperature and low induced radioactivity after neutron irradiation [1]. Though monolithic SiC is extremely brittle with low toughness, continuous SiC fiber reinforced SiC matrix composites (SiC/SiC) significantly improve these properties and therefore are attractive candidate materials for fusion reactor structural applications [2,3]. Moreover, highcrystalline and near-stoichiometric SiC fibers including Hi-Nicalon Type S and Tyranno SA retain superior mechanical properties after oxidation at high temperature and neutron irradiation compared with conventional SiC-based fibers [4,5], therefore the composites with these advanced fibers are expected to have better mechanical properties. For SiC/SiC composites, the interphase between fiber and matrix is very important to improve the toughness [2] because the optimized interphase brings the deflection of cracks at the interphase, fiber pullout during the fracture, excellent interfacial shear strength and ductility between fiber and matrix which cause high mechanical properties. Among the various fabrication processes of SiC/SiC, the forced-flow/thermal gradient chemical vapor infiltration (FCVI) method is one of the best techniques to fabricate the SiC/SiC for fusion applications because this method produces a highly pure, stoichiometric, crystalline b-SiC matrix with low thermal stress which minimizes fiber damage during fabrication [6]. On the other hand, SiC matrix in SiC/SiC fabricated by the CVI method has low density and poor homogeneity, which cause the degradation of the mechanical and thermal properties. In this study, we carried out the optimization of this process to obtain higher and more homogeneity composite for the excellent mechanical properties of SiC/SiC. We also studied the effects of the carbon and multilayer SiC/carbon interphase on the mechanical properties. 2. Experimental procedure We adopted two advanced SiC fiber fabrics as preforms: 2D-plain weave of Tyranno SA (Ube Industries, Ube, Japan). 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.06.030 Journal of Physics and Chemistry of Solids 66 (2005) 551–554 www.elsevier.com/locate/jpcs * Corresponding author. Tel.: C81 29 282 6099; fax: C81 29 284 3813. E-mail address: igawa@maico.tokai.jaeri.go.jp (N. Igawa)
552 N igawa et al. /Journal of Physics and Chemistry of Solids 66(2005)551-554 20.0 center middle outer bottom center middle outer Fig. I. Typical distribution of the(a) porosity and(b)thickness of carbon interphase of the FCVl-SiC/SiC. The precursors were 99% purity propylene(Matheson, decreasing the porosity; the average porosity was decreased Morrow, GA, USA)for carbon deposition and the technical to 15% with increasing the fiber volume fraction to 44%.In grade methyltrichlorosilane(MrS, Gelest Inc, Tullytown, the typical cross-section of SiC/SiC composite shown in PA,USA)for SiC deposition and infiltration. The SiC fabric Fig 3, two types of pores were observed; one is a large pore with a fabric layer orientation of [-30/0/301 around fiber bundles, and the other is a small pore in a [0/90] were retrained in a graphite fixture. The fiber bundle. The size of this inter-bundle pores was decreased perform was 75 mm in diameter and 12.5 mm thick and the with increasing the fiber fraction, though that of intra average fiber volume fraction was 35.2 vol%. The carbon bundle pores was nearly constant as a function of fiber interphase was deposited on the fiber surface by decompo- fraction. The distance between fiber bundles is shorter when on of propyle 5×10-2dm3/ min and the fiber fraction is higher, so the inter-bundle pore became I dm /min Ar at 5 Pa, 1100C. The Sic interphase was smaller in SiC/Sic with the higher fiber fraction by the end deposited by the decomposition of MTS with the flow rate of of FCVI process. 0.15g/min and 0.25 dm/min H2 at 5 Pa, 1"C. The Although the carbon interphase of the bottom region was average coating rates of carbon and SiC layer were 1. 2 and thicker than that of the top region by the conventional 5 nm/min, respectively. The multilayer SiC/C interphase infiltration, the distribution of that thickness was signifi consists of 6-set of SiC/C layers: the thickness of carbon layers was fixed at 50 nm and those of Sic were 50 nm(1 cantly improved by flipping midway through the interphase layer), 100 nm(2nd-4th layers)and 500 nm(5th-6th layers) infiltration(Fig. 1(b). Fig. 3(c)shows the typical SEM After interphase deposition, the matrix was formed by image of multilayer SiC/C interphase. It was found that high FCVI process. Details of FCVI process were described uniform layers were formed in this method elsewhere [71 Fig. 4 shows the tensile strength of SiC/Sic with the To estimate the distribution of porosity and interpha carbon interphase as a function of the carbon interphase ickness in a specimen, the specimen was cut into nine thickness. In the present study, the tensile stress-strain sections: the porosity was calculated from the dimensions curve in the results of tensile test exhibited pseude-ductile and the mass of the cut specimen. The microstructure and fracture mode, which means that carbon interphase was interphase thickness was measured by scanning electron effective of the fiber pullout and excellent tensile properti microscope The tensile strength was slightly increased with increasing Tensile tests were carried out at room temperature in air and at 1300C in Ar. The schematic illustration dimension of tensile specimen and details of tensile testing were described in Ref. 81 3. Results and discussion 815 The bottom region, which is the upstream side of cursor in the composite. This tendency was improved by decreas SiC/C ing the precursor and carrier gas flow rates at the latter part of the FCVI process and a much better uniform porosity in the composite was obtained(Fig. 1(a)). The average Fiber fraction(vol % porosity as a function of fiber volume was shown in Fig. 2. It was found that the higher fiber fraction is very effective in Fig. 2. Effect of the fiber volume fraction on the reduction of the porosity
The precursors were 99% purity propylene (Matheson, Morrow, GA, USA) for carbon deposition and the technical grade methyltrichlorosilane (MTS, Gelest Inc., Tullytown, PA, USA) for SiC deposition and infiltration. The SiC fabric layers with a fabric layer orientation of [K308/08/30] and [08/908] were retrained in a graphite fixture. The fiber perform was 75 mm in diameter and 12.5 mm thick and the average fiber volume fraction was 35.2 vol.%. The carbon interphase was deposited on the fiber surface by decomposition of propylene with flow rate of 5!10K2 dm3 /min and 1 dm3 /min Ar at 5 Pa, 1100 8C. The SiC interphase was deposited by the decomposition of MTS with the flow rate of 0.15 g/min and 0.25 dm3 /min H2 at 5 Pa, 1100 8C. The average coating rates of carbon and SiC layer were 1.2 and 9.5 nm/min, respectively. The multilayer SiC/C interphase consists of 6-set of SiC/C layers: the thickness of carbon layers was fixed at 50 nm and those of SiC were 50 nm (1st layer), 100 nm (2nd–4th layers) and 500 nm (5th–6th layers). After interphase deposition, the matrix was formed by FCVI process. Details of FCVI process were described elsewhere [7]. To estimate the distribution of porosity and interphase thickness in a specimen, the specimen was cut into nine sections: the porosity was calculated from the dimensions and the mass of the cut specimen. The microstructure and interphase thickness was measured by scanning electron microscope. Tensile tests were carried out at room temperature in air and at 1300 8C in Ar. The schematic illustration, dimension of tensile specimen and details of tensile testing were described in Ref. [8]. 3. Results and discussion The bottom region, which is the upstream side of precursor gas and lower temperature, had higher porosity in the composite. This tendency was improved by decreasing the precursor and carrier gas flow rates at the latter part of the FCVI process and a much better uniform porosity in the composite was obtained (Fig. 1(a)). The average porosity as a function of fiber volume was shown in Fig. 2. It was found that the higher fiber fraction is very effective in decreasing the porosity; the average porosity was decreased to 15% with increasing the fiber volume fraction to 44%. In the typical cross-section of SiC/SiC composite shown in Fig. 3, two types of pores were observed; one is a large pore around fiber bundles, and the other is a small pore in a bundle. The size of this inter-bundle pores was decreased with increasing the fiber fraction, though that of intrabundle pores was nearly constant as a function of fiber fraction. The distance between fiber bundles is shorter when the fiber fraction is higher, so the inter-bundle pore became smaller in SiC/SiC with the higher fiber fraction by the end of FCVI process. Although the carbon interphase of the bottom region was thicker than that of the top region by the conventional infiltration, the distribution of that thickness was signifi- cantly improved by flipping midway through the interphase infiltration(Fig. 1(b)). Fig. 3(c) shows the typical SEM image of multilayer SiC/C interphase. It was found that high uniform layers were formed in this method. Fig. 4 shows the tensile strength of SiC/SiC with the carbon interphase as a function of the carbon interphase thickness. In the present study, the tensile stress–strain curve in the results of tensile test exhibited pseude-ductile fracture mode, which means that carbon interphase was effective of the fiber pullout and excellent tensile properties. The tensile strength was slightly increased with increasing Fig. 1. Typical distribution of the (a) porosity and (b) thickness of carbon interphase of the FCVI-SiC/SiC. Fig. 2. Effect of the fiber volume fraction on the reduction of the porosity. 552 N. Igawa et al. / Journal of Physics and Chemistry of Solids 66 (2005) 551–554
N Igawa et al. Journal of Physics and Chemistry of Solids 66(2005)551-554 553 7 Fig 3. Cross-sections of FCvI-SiC/SiC. the carbon thickness in the thickness range from 20 to (see Fig. 6)and it seems that multilayer was very effective 250 nm. This result shows that the thicker carbon interphase for fracture behavior. However, the tensile strength was was coated well on the surface of fibers to prevent from slightly degraded at 1300C. This degradation of Sic/Sic adhering between fiber and matrix. The fabric layer with multilayer was slightly larger than that with carbon orientation was very effective to improve the tensile interphase. No significant degradation of tensile strength in strength as shown in Fig. 5. Moreover, the tensile strength Tyranno SA fiber after annealing at 1300C in Ar was of SiC/Sic with multilayer SiC/C interphase at room reported [5]. Therefore the degradation of tensile strength of temperature was by ca. 25% larger than that with the SiC/Sic was probably caused by the partial burn-out of carbon interphase. This increment was caused by the carbon interphase near the fiber surface; this interphase multiple fracture of interphase along the carbon layers disintegration brought the less deflection of cracks at the interphase and decreased the fiber pullout during the fracture. Total thickness of carbon layers in SiC/SiC with multilayer SiC/C interphase was much greater than that with carbon interphase, therefore the greater degradation could be observed in the Sic/Sic with multilayer SiC/C interphase. More optimization of thickness of multilayer 50 Thickness of carbon(nm) Fig. 4. Effect of the thickness of carbon interphase on the tensile strength. □25°ciar 嚣1300C 30°0°30”] carbon 10 um Fig. 5. High temperature tensile strength of FCVI-SiC/SiC. Fig. 6. Fracture surface of FCVl-SiC/SiC with multilayer SiC/C interphase
the carbon thickness in the thickness range from 20 to 250 nm. This result shows that the thicker carbon interphase was coated well on the surface of fibers to prevent from adhering between fiber and matrix. The fabric layer orientation was very effective to improve the tensile strength as shown in Fig. 5. Moreover, the tensile strength of SiC/SiC with multilayer SiC/C interphase at room temperature was by ca. 25% larger than that with the carbon interphase. This increment was caused by the multiple fracture of interphase along the carbon layers (see Fig. 6) and it seems that multilayer was very effective for fracture behavior. However, the tensile strength was slightly degraded at 1300 8C. This degradation of SiC/SiC with multilayer was slightly larger than that with carbon interphase. No significant degradation of tensile strength in Tyranno SA fiber after annealing at 1300 8C in Ar was reported [5]. Therefore the degradation of tensile strength of SiC/SiC was probably caused by the partial burn-out of carbon interphase near the fiber surface; this interphase disintegration brought the less deflection of cracks at the interphase and decreased the fiber pullout during the fracture. Total thickness of carbon layers in SiC/SiC with multilayer SiC/C interphase was much greater than that with carbon interphase, therefore the greater degradation could be observed in the SiC/SiC with multilayer SiC/C interphase. More optimization of thickness of multilayer Fig. 3. Cross-sections of FCVI-SiC/SiC. Fig. 5. High temperature tensile strength of FCVI-SiC/SiC. Fig. 4. Effect of the thickness of carbon interphase on the tensile strength. Fig. 6. Fracture surface of FCVI-SiC/SiC with multilayer SiC/C interphase. N. Igawa et al. / Journal of Physics and Chemistry of Solids 66 (2005) 551–554 553
N. Igawa et al. Journal of Physics and Chemistry of Solids 66(2005)551-554 SiC/C interphase is to be expected to improve the high- and Technology/Advanced Material Systems for Energy temperature mechanical properties Conversion Program sponsored by Japan Science and 4. Concl The process optimization for FCVI-SiC based tes with advanced SiC fibers such as Tyranno SA was [1]R H. Jones, L.L. Seand, A. Kohyama, P. Fenici, Recent advances in the carried out. The new SiC/SiC composites exhibited development of SiC/SiC as a fusion significant improvement in porosity reduction and th Ds.41(1998)15-24 uniform distribution of pores by decreasing the flow rate [2] A Hasegawa, A Kohyama, R.H. Jones, L.L. Snead,, P Fenici, Critical of mrs and h es at the latter part of the FCVI 6beress. The uniform interphases between advanced SiC Mater.283-287(2000)128-137 [3] R.H. Jones, L. Giancarli, A. Hasegawa, Y. Katoh, A. Kohyama and FCVI-Sic matrix could be obtained by B. Riccardi, L.L. Snead, w.J. Weber, Promise and challenges of reversing the gas-flow direction mid-way through the SiC/SiC composites for fusion energy applications. J. Nucl. Mater. coating process. The tensile strength was slightly 307-311(2002)1057-1072 increased with thickness of carbon interphase in the daA. Ur J. Sakamoto. Y. Imai. Microstructure and range of 20-250 nm. We confirmed the fabric layer oxidative degradation behavior of silicon carbide fiber Hi-Nicalon type S,J.Nucl. Mater.258-263(1998)1594-1599 orientation and multilayer Sic/C interphase were very [5 T Ishikawa, Y. Kohtoku, K Kumagawa, T Yamamura. T Nagasawa, ffective to improve the tensile strength High-strength alkali-resistant sintered SiC fibre stable to 2.200C. [6 T.M. Besmann, J.C. McLaughlin, H-T. Lin, Fabrication of ceramic Acknowledgements omposites: forced CVI, J Nucl. Mater. 219(1995)31-35 [7 N. Igawa, T. Taguchi, L L. Snead, Y. Katoh, S. Jitsukawa. A. Kohyama, J.C. McLaughlin, Optimizing the fabrication process The authors would like to thank dr t.m. besmann at Oak for superior mechanical properties in the FCVI SiC matrix/stoichio- Ridge National Laboratory for useful discussion. This study metric SiC fiber composites system, J. Nucl. Mater. 307-311(2002) has been carried out under the US-DOEJAERI Collabora- tive program on fwb structural materials in mixed- [8 onaka, Y. Katoh, A. Kohyama, T. Taguchi of chemical vapor infiltrated Spectrum Fission Reactors, Phase IV. This study was also advanced SiC fiber composites at elevated temperatures, Ceram. Trans. supported by the Core Research for Evolutional Science 144(2002)245-252
SiC/C interphase is to be expected to improve the hightemperature mechanical properties. 4. Conclusion The process optimization for FCVI-SiC based composites with advanced SiC fibers such as Tyranno SA was carried out. The new SiC/SiC composites exhibited significant improvement in porosity reduction and the uniform distribution of pores by decreasing the flow rate of MTS and H2 gases at the latter part of the FCVI process. The uniform interphases between advanced SiC fibers and FCVI-SiC matrix could be obtained by reversing the gas-flow direction mid-way through the coating process. The tensile strength was slightly increased with thickness of carbon interphase in the range of 20–250 nm. We confirmed the fabric layer orientation and multilayer SiC/C interphase were very effective to improve the tensile strength. Acknowledgements The authors would like to thank Dr T.M. Besmann at Oak Ridge National Laboratory for useful discussion. This study has been carried out under the US-DOE/JAERI Collaborative Program on FWB Structural materials in MixedSpectrum Fission Reactors, Phase IV. This study was also supported by the Core Research for Evolutional Science and Technology/Advanced Material Systems for Energy Conversion Program sponsored by Japan Science and Technology Corporation. References [1] R.H. Jones, L.L. Seand, A. Kohyama, P. Fenici, Recent advances in the development of SiC/SiC as a fusion structural material, Fusion Eng. Des. 41 (1998) 15–24. [2] A. Hasegawa, A. Kohyama, R.H. Jones, L.L. Snead,, P. Fenici, Critical issues and current status of SiC/SiC composites for fusion, J. Nucl. Mater. 283–287 (2000) 128–137. [3] R.H. Jones, L. Giancarli, A. Hasegawa, Y. Katoh, A. Kohyama, B. Riccardi, L.L. Snead, W.J. Weber, Promise and challenges of SiC/SiC composites for fusion energy applications, J. Nucl. Mater. 307–311 (2002) 1057–1072. [4] M. Takeda, A. Urano, J. Sakamoto, Y. Imai, Microstructure and oxidative degradation behavior of silicon carbide fiber Hi-Nicalon type S, J. Nucl. Mater. 258–263 (1998) 1594–1599. [5] T. Ishikawa, Y. Kohtoku, K. Kumagawa, T. Yamamura, T. Nagasawa, High-strength alkali-resistant sintered SiC fibre stable to 2,200 8C, Nature 391 (1998) 773–775. [6] T.M. Besmann, J.C. McLaughlin, H.-T. Lin, Fabrication of ceramic composites: forced CVI, J. Nucl. Mater. 219 (1995) 31–35. [7] N. Igawa, T. Taguchi, L.L. Snead, Y. Katoh, S. Jitsukawa, A. Kohyama, J.C. McLaughlin, Optimizing the fabrication process for superior mechanical properties in the FCVI SiC matrix/stoichiometric SiC fiber composites system, J. Nucl. Mater. 307–311 (2002) 1205–1209. [8] T. Nozawa, K. Hironaka, Y. Katoh, A. Kohyama, T. Taguchi, S. Jitsukawa, L.L. Snead, Tensile strength of chemical vapor infiltrated advanced SiC fiber composites at elevated temperatures, Ceram. Trans. 144 (2002) 245–252. 554 N. Igawa et al. / Journal of Physics and Chemistry of Solids 66 (2005) 551–554
