Availableonlineatwww.sciencedirect.com SCIENCE DIRECTo composites Part A: applied science ELSEVIER Composites: Part A 35(2004)33-40 www.elsevier.com/locate/composites Degradation of Sic/Sic composite due to exposure at high temperatures in vacuum in comparison with that in air S. Ochiai,, S. Kimura, H. Tanaka, M. Tanaka, M. Hojo, K. Morishita, H Okuda H Nakayama, M. Tamura, K Shibata, M. Sato "International Innovation Center, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Graduate School of Engineering, Kyoto University, Sakyo kae 60-8501, Japan Japan Ultra-high Temperature Materials Research institute, 3-1-8 Higashimachi, Tajimi City, Gifit 507-0801 dapan Ultra-high Temperature Materials Research Institute, 573-3 Okiube, Ube ciry, Yamaguchi 755-000 Received 25 April 2003: revised 1 August 2003: accepted 5 September 2003 Abstract Room temperature residual strength of the SiC/Sic composite exposed in vacuum at high temperatures(823-1673 K) was studied and compared with that exposed in air. The vacuum-exposed composite showed only the fiber-pullout type fracture, and the pullout length increased with increasing exposure temperature and time, while the fractured mode of the air-exposed one changed with progressing oxidation; from the fiber-pullout type to the nonfiber-pullout one characterized by the overall fracture perpendicular to the tensile axi without fiber-pullout. The reduction in residual strength in the case of vacuum exposure was attributed mainly to the extension of the decomposition-induced defects on the fiber surface into fiber, while that in the case of air exposure mainly to the extension of the crack made by premature fracture of the Sio2 layer into the fiber. A simple model based on the kinetics of the growth of the defects and fracture exposure, which could describe the experimental resule posite strength as a function of exposure temperature and time for the vacuum mechanics was presented to describe the variation of co C 2004 Elsevier ltd. all rights reserved Keywords: A Ceramic-matrix composites(CMCs); B Environmental degradation; B Strength; C Damage mechanics 1. Introduction strength of the composite could be attributed to the reduction in fiber strength due to the extension In the preceding paper [1], the degradation due to the crack made by the premature fracture of the SiOz layer into exposure in air at high temperatures of the Sic/Sic fiber. (3)A simple model based on the kinetics of the growth composed of Si-Zr-C-O fiber(ZMI fiber with a describe the variation of composite strength as a function of composition of Sizr<ooI C14400.32, produced by Ube exposure temperature and time, which could describe the Companies) and ZrSiO4 particles (30 mass%)-dispersec experimental results The main results are summarized In the present work, the degradation behavior due to the follows.(1)With increasing exposure temperature and time, high temperature exposure in vacuum was studied by using (a)the residual strength of the exposed composite decreased the same specimens in order to compare the result for air and(b)the fracture mode changed from the fiber-pullout exposure and to try to describe the reduction as a function of type to the nonfiber-pullout one characterized by the overall exposure temperature and time by modeling also for the fracture perpendicular to the tensile axis without fiber- vacuum environment. Concerning the thermal stability of the pullout. (2)The main reason for the reduction in residual SiC fibers made from the precursors such as polycarbosilane and polytititanocarbosilane, extensive studies have been Corresponding author. Tel : +81-75-753-4834; fax:+81-75-753-4841 carried out [2-9]. Among them, Simao et al. [6 demon E-imail address: ochiai @iic. kyoto-u ac jp(S Ochiai) strated that the decomposition leads to the crystallization of 1359-835X/S- see front matter 2004 Elsevier Ltd. All rights reserved. doi: 10. 1016/j-compositesa. 2003.09.006Degradation of SiC/SiC composite due to exposure at high temperatures in vacuum in comparison with that in air S. Ochiaia,*, S. Kimurab , H. Tanakab , M. Tanakab , M. Hojob , K. Morishitab , H. Okudaa , H. Nakayamac , M. Tamurad , K. Shibatad , M. Satoe a International Innovation Center, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan b Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan c Japan Ultra-high Temperature Materials Research Institute, 3-1-8 Higashimachi, Tajimi City, Gifu 507-0801, Japan d Japan Ultra-high Temperature Materials Research Institute, 573-3 Okiube, Ube city, Yamaguchi 755-0001, Japan e Ube Research Laboratory, Ube Industries Ltd, 1978-5 Kogushi, Ube city, Yamaguchi 755-8633, Japan Received 25 April 2003; revised 1 August 2003; accepted 5 September 2003 Abstract Room temperature residual strength of the SiC/SiC composite exposed in vacuum at high temperatures (823–1673 K) was studied and compared with that exposed in air. The vacuum-exposed composite showed only the fiber-pullout type fracture, and the pullout length increased with increasing exposure temperature and time, while the fractured mode of the air-exposed one changed with progressing oxidation; from the fiber-pullout type to the nonfiber-pullout one characterized by the overall fracture perpendicular to the tensile axis without fiber-pullout. The reduction in residual strength in the case of vacuum exposure was attributed mainly to the extension of the decomposition-induced defects on the fiber surface into fiber, while that in the case of air exposure mainly to the extension of the crack made by premature fracture of the SiO2 layer into the fiber. A simple model based on the kinetics of the growth of the defects and fracture mechanics was presented to describe the variation of composite strength as a function of exposure temperature and time for the vacuum exposure, which could describe the experimental results. q 2004 Elsevier Ltd. All rights reserved. Keywords: A. Ceramic–matrix composites (CMCs); B. Environmental degradation; B. Strength; C. Damage mechanics 1. Introduction In the preceding paper [1], the degradation due to the exposure in air at high temperatures of the SiC/SiC composite was studied using the composite specimens composed of Si–Zr–C–O fiber (ZMI fiber with a composition of SiZr,0.01C1.44O0.32, produced by Ube Companies) and ZrSiO4 particles (30 mass%)–dispersed Si–Zr–C matrix. The main results are summarized as follows. (1) With increasing exposure temperature and time, (a) the residual strength of the exposed composite decreased and (b) the fracture mode changed from the fiber-pullout type to the nonfiber-pullout one characterized by the overall fracture perpendicular to the tensile axis without fiber￾pullout. (2) The main reason for the reduction in residual strength of the composite could be attributed to the reduction in fiber strength due to the extension of the crack made by the premature fracture of the SiO2 layer into fiber. (3) A simple model based on the kinetics of the growth of the SiO2 layer and fracture mechanics was presented to describe the variation of composite strength as a function of exposure temperature and time, which could describe the experimental results. In the present work, the degradation behavior due to the high temperature exposure in vacuum was studied by using the same specimens in order to compare the result for air exposure and to try to describe the reduction as a function of exposure temperature and time by modeling also for the vacuum environment. Concerning the thermal stability of the SiC fibers made from the precursors such as polycarbosilane and polytititanocarbosilane, extensive studies have been carried out [2–9]. Among them, Simoo et al. [6] demon￾strated that the decomposition leads to the crystallization of 1359-835X/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2003.09.006 Composites: Part A 35 (2004) 33–40 www.elsevier.com/locate/compositesa * Corresponding author. Tel.: þ81-75-753-4834; fax: þ81-75-753-4841. E-mail address: ochiai@iic.kyoto-u.ac.jp (S. Ochiai)