MATERAL CHARACEERLATION ELSEVIER Materials Characterization 57(2006)6-11 Heat treatment effects on creep behavior of polycrystalline Sic fibers JJSha,, J.S. Park, T Hinoki, A Kohyama Graduate School of Energy Science, Kyoto University Gokasho, Uji, Kyoto 611-0011, Japan Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-001L, Japan Received 31 August 2005: received in revised form 26 November 2005: accepted 28 November 2005 Polycrystalline SiC fibers are being considered as potential reinforcement for ceramic matrix composites(CMCs). For these fibers with fine grain size, basic issues arise conceming thermo-mechanical properties and microstructural instability during fabrication and application of CMCs. To examine these issues, three commercially available Sic fibers were heat treated at elevated temperatures for I h in an Ar atmosphere. The creep resistance of Sic fibers was evaluated by the bend stress relaxation(BSr) method, and it was found that the creep resistance could be improved by heat treatment. Combining the results of the Bsr tests with the results of X-ray diffraction examinations indicated that the creep resistance of Sic fibers is mainly related to the B-s grain size and the composition at or adjacent to the grain boundaries. Also, the apparent activation energy of creep for both hi- Nicalon TM and Hi-Nicalon M type S fiber increased with increasing heat treatment temperature. In the case of Tyranno M-SA fiber, the apparent activation energy of creep did not show an obvious dependence on the heat treatment temperature. C 2005 Elsevier Inc. All rights reserved. Keywords: Heat treatment; SiC fibers; Creep; Apparent activation energy 1. Introduction tion. These fibers experience a pyrolysis/sintering process during fabrication and their microstructure and Ceramic matrix composites(CMCs) have been mechanical properties depend on the thermal history. On proposed as potential structural materials for advanced the other hand, the CMCs may be fabricated above the energy systems and propulsion systems [1-3]. Accept- fiber's processing temperature [6, 71, in which case the able performance of high temperature CMCs depends performance of the fibers could be changed by this latter upon judicious selection of ceramic fiber reinforcement thermal exposure. However, the effect of the thermal with the proper chemical, physical and mechanical history on the fibers properties in the CMCs requires further investigation to identify the factors which affect Recently developed SiC-based fibers with high the high temperature properties of these fibers. The crystallite structure and near stoichiometric composition creep resistance of SiC fibers is one of the most critical [4, 5], are promising reinforcement for CMCs fabrica- properties. However, there are significant difficulties in the experimental measurement of tensile creep of advanced small diameter(about 7-14 um) SiC fibers Corresponding author. Present address: Intemational Innovation Center, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan. Tel: +81 In the present study, in order to evaluate the creep 757535194;fax:+81757534841 resistance of these advanced Sic fibers and also to E-mail address: shajianjun(@ iic. kyoto-uLac jp(J. Sha) clarify the creep mechanism to support continuing 044-5803/S-see front matter o 2005 Elsevier Inc. All rights reserved. j. matcha. 2005. 11.019Heat treatment effects on creep behavior of polycrystalline SiC fibers J.J. Sha a,⁎, J.S. Park b , T. Hinoki b , A. Kohyama b a Graduate School of Energy Science, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan b Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Received 31 August 2005; received in revised form 26 November 2005; accepted 28 November 2005 Abstract Polycrystalline SiC fibers are being considered as potential reinforcement for ceramic matrix composites (CMCs). For these fibers with fine grain size, basic issues arise concerning thermo-mechanical properties and microstructural instability during fabrication and application of CMCs. To examine these issues, three commercially available SiC fibers were heat treated at elevated temperatures for 1 h in an Ar atmosphere. The creep resistance of SiC fibers was evaluated by the bend stress relaxation (BSR) method, and it was found that the creep resistance could be improved by heat treatment. Combining the results of the BSR tests with the results of X-ray diffraction examinations indicated that the creep resistance of SiC fibers is mainly related to the β-SiC grain size and the composition at or adjacent to the grain boundaries. Also, the apparent activation energy of creep for both Hi￾Nicalon™ and Hi-Nicalon™ type S fiber increased with increasing heat treatment temperature. In the case of Tyranno™-SA fiber, the apparent activation energy of creep did not show an obvious dependence on the heat treatment temperature. © 2005 Elsevier Inc. All rights reserved. Keywords: Heat treatment; SiC fibers; Creep; Apparent activation energy 1. Introduction Ceramic matrix composites (CMCs) have been proposed as potential structural materials for advanced energy systems and propulsion systems [1–3]. Accept￾able performance of high temperature CMCs depends upon judicious selection of ceramic fiber reinforcement with the proper chemical, physical and mechanical properties. Recently developed SiC-based fibers with high crystallite structure and near stoichiometric composition [4,5], are promising reinforcement for CMCs fabrica￾tion. These fibers experience a pyrolysis/sintering process during fabrication and their microstructure and mechanical properties depend on the thermal history. On the other hand, the CMCs may be fabricated above the fiber's processing temperature [6,7], in which case the performance of the fibers could be changed by this latter thermal exposure. However, the effect of the thermal history on the fiber's properties in the CMCs requires further investigation to identify the factors which affect the high temperature properties of these fibers. The creep resistance of SiC fibers is one of the most critical properties. However, there are significant difficulties in the experimental measurement of tensile creep of advanced small diameter (about 7–14 μm) SiC fibers. In the present study, in order to evaluate the creep resistance of these advanced SiC fibers, and also to clarify the creep mechanism to support continuing Materials Characterization 57 (2006) 6–11 ⁎ Corresponding author. Present address: International Innovation Center, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan. Tel.: +81 75 753 5194; fax: +81 75 753 4841. E-mail address: shajianjun@iic.kyoto-u.ac.jp (J.J. Sha). 1044-5803/$ - see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2005.11.019