S Ochiai et aL./Composites: Part A 35 (2004)33-40 the estimated values in the preceding work [1] for the ai Based on the results mentioned above in combination exposure For both air-and vacuum exposures, the models with the kinetics of growth of the defects and fracture describe well the experimental results. In this way, when the mechanics, a simple model was presented to describe the all data are superimposed in one graph, the validity of the variation of composite strength as a function of exposure estimated values for both exposures is clearly shown. temperature and time for the vacuum exposure, which Only Stages I and II were found in the case of vacuum could describe the experimental results exposure On the other hand, in the case of air exposure, Stage Ill, where the strength goes down rather slowly with increasing time in comparison with that in Stage I wasAcknowledgements found in addition to Stages l and l. Such a difference arises from the difference in fracture mechanism. The fiber is The present work was supported by the New Energy and fractured directly from the defect in the case of vacuum xposure. Thus, the strength is reduced monotonically with Japan ndustrial Technology Development Organization(NEDO), increasing time. On the other hand, in the case of air exposure, the fiber is fractured by the extension of the crack made by fracture of the brittle Sio2 layer. In this process, the crack does not extend upon formation but it extends after References increment of applied stress when the Sio2 is thin( Stage l), [1 Ochiai S, Kimura S, Tanaka M, Tanaka H, Hojo M, Morishita K. while it extends upon formation when the Sio2 layer is thick Okuda H, Nakayama H, Shibata K, Sato M. Residual strength of SiC/ (Stage I[1]. Thus the reduction in strength is described by SiC composite exposed at high temperatures in air as a function of three stages in the air exposure exposure temperature and time. Compos Part A, in press. The behavior mentioned above might be, however, [2] Bunsell AR, Berger MH. Fine ceramic fibres. New York: Marcel limited to short time exposure. If the saturation of reaction Dekker: 1999 [3] Bunsell AR, Berger MH. Fine diameter ceramic fibres. J Eur Ceramic occurs or the decomposition speed becomes very low for very Soc2000:20:2249-60 long exposure time, the effective defect size would not obey [4] Shimo T, Chen H, Okamura K. High-temperature stability of Eq (1)but would tend to grow very slowly. In such a case, an icalon under Ar or O2 atmosphere. J Mater Sci 1994: 29: 456-63 additional stage where the reduction in strength is not 5] Kakimoto K, Shimo T, Okamura K, Seguchi T, Sato M, Kumagawa strongly dependent on time could be expected to arise. On K, Yamamura T. High-temperature crystallization behavior of si this point, further study under long-term exposure is needed Ti-C-O fiber cured by electron beam irradiation. J Jpn Inst Metals 199458(2):229-34 [6] Shimo T, Okamura K, Hayatsu T Effect of atmosphere on pyrolysis of Nicalon J Mater Sci 1996: 31: 4407-13 4. Conclusions [71 Hollon G, Pailler R, Naslain R, Laanani F, Monthioux M, Olry P. Thermal stability of PCS-derived SiC fibre with a low oxygen content The room temperature residual strength of SiC(ZMI fiber (Hi-Nicalon). J Mater Sci 1997: 32: 327-47 [8] Hollon C, Pailler R, Naslain R, Olry P. bricated by Ube Companies )/SiC composite exposed at microstructure and mechanical behavior at high high temperatures(823-1673 K) in vacuum was studied fibre with a low oxygen content (Hi-Nicalon) and compared with that exposed in air. Main results are 1133-47 summarized as follows [9] Jia N, Bodet R, Tressler RE. Effects of microsturute instability on the reep behavior of Si-C-O(Nicalon)fibers in argon. J Am Ceram Soc 376(12):3051-60 1. The residual strength decreased with increasing exposure [10] Ultra high temperature materials research institute. Study on emperature and time both for vacuum and air exposures durability and life prediction of continuous fiber reinforced The variation of the fracture mode was, however, quite ceramic matrix composite. Research report NEDO-lTK-9911 different. In case of vacuum exposure, only the fiber 2000p.6-9 pullout type occurred and also the pullout length [11] Tanaka Y, Inoue Y, Miyamoto N, Sato M, Yamamura T Properties of Si-Zr-C-O Fiber/SizrC composites dispersed increased with increasing temperature and time, while ZrsiO4 particles in the matrix. Int J Mater Prod Technol 2001 in the case of air exposure, the fracture mode changed 16(1-3):197-205 from fiber-pullout to nonfiber-pullout one with increas- [12] Ochiai S, Hojo M, Schulte K, Fiedler B Nondimensional simulation ing temperature and time of influence of toughness of interface on tensile stress-strain 2. The degradation of the composite due to the exposure at behavior of unidirectional microcomposite. Compos Part A 2001 32(6:749-61 high temperatures could be attributed to the reduction in [13] Metcalfe AG, Klein KJ Interface in metal matrix composites.Effects fiber strength for both environments, but the mechanism of the interface on longitudinal tensile properties. New York: was different: the extension of the decomposition Academic Press: 1974. P. 125-68 induced surface defects into fiber in the case of vacuum [14] Shorshorov MK, Ustinov LM, Zirlin AM, Olefilebko VL, Vonogra- exposure and the extension of the crack made by dov Lv. Brittle interface layers and the tensile strength of metal matrix-fibre composites. J Mater Sci 1979: 14: 1850-61 premature fracture of the SiOz layer into fiber in [15] Ochiai A, Murakami Y. Tensile strength of composites with brittle case of air reaction zones at interface. J Mater Sci 1979: 14: 831-40the estimated values in the preceding work [1] for the air exposure. For both air- and vacuum exposures, the models describe well the experimental results. In this way, when the all data are superimposed in one graph, the validity of the estimated values for both exposures is clearly shown. Only Stages I and II were found in the case of vacuum exposure. On the other hand, in the case of air exposure, Stage III, where the strength goes down rather slowly with increasing time in comparison with that in Stage II was found in addition to Stages II and I. Such a difference arises from the difference in fracture mechanism. The fiber is fractured directly from the defect in the case of vacuum exposure. Thus, the strength is reduced monotonically with increasing time. On the other hand, in the case of air exposure, the fiber is fractured by the extension of the crack made by fracture of the brittle SiO2 layer. In this process, the crack does not extend upon formation but it extends after increment of applied stress when the SiO2 is thin (Stage II), while it extends upon formation when the SiO2 layer is thick (Stage III) [1]. Thus the reduction in strength is described by three stages in the air exposure. The behavior mentioned above might be, however, limited to short time exposure. If the saturation of reaction occurs or the decomposition speed becomes very low for very long exposure time, the effective defect size would not obey Eq. (1) but would tend to grow very slowly. In such a case, an additional stage where the reduction in strength is not strongly dependent on time could be expected to arise. On this point, further study under long-term exposure is needed. 4. Conclusions The room temperature residual strength of SiC(ZMI fiber fabricated by Ube Companies)/SiC composite exposed at high temperatures (823–1673 K) in vacuum was studied and compared with that exposed in air. Main results are summarized as follows 1. The residual strength decreased with increasing exposure temperature and time both for vacuum and air exposures. The variation of the fracture mode was, however, quite different. In case of vacuum exposure, only the fiber￾pullout type occurred and also the pullout length increased with increasing temperature and time, while in the case of air exposure, the fracture mode changed from fiber-pullout to nonfiber-pullout one with increas￾ing temperature and time. 2. The degradation of the composite due to the exposure at high temperatures could be attributed to the reduction in fiber strength for both environments, but the mechanism was different: the extension of the decomposition￾induced surface defects into fiber in the case of vacuum exposure and the extension of the crack made by premature fracture of the SiO2 layer into fiber in the case of air exposure. 3. Based on the results mentioned above in combination with the kinetics of growth of the defects and fracture mechanics, a simple model 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. Acknowledgements The present work was supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan. References [1] Ochiai S, Kimura S, Tanaka M, Tanaka H, Hojo M, Morishita K, Okuda H, Nakayama H, Shibata K, Sato M. Residual strength of SiC/ SiC composite exposed at high temperatures in air as a function of exposure temperature and time. Compos Part A, in press. [2] Bunsell AR, Berger MH. Fine ceramic fibres. New York: Marcel Dekker; 1999. [3] Bunsell AR, Berger MH. Fine diameter ceramic fibres. J Eur Ceramic Soc 2000;20:2249–60. [4] Shimoo T, Chen H, Okamura K. High-temperature stability of Nicalon under Ar or O2 atmosphere. J Mater Sci 1994;29:456–63. [5] Kakimoto K, Shimoo T, Okamura K, Seguchi T, Sato M, Kumagawa K, Yamamura T. High-temperature crystallization behavior of Si– Ti–C–O fiber cured by electron beam irradiation. J Jpn Inst Metals 1994;58(2):229–34. [6] Shimoo T, Okamura K, Hayatsu T. Effect of atmosphere on pyrolysis of Nicalon. J Mater Sci 1996;31:4407–13. [7] Chollon G, Pailler R, Naslain R, Laanani F, Monthioux M, Olry P. Thermal stability of PCS-derived SiC fibre with a low oxygen content (Hi-Nicalon). J Mater Sci 1997;32:327–47. [8] Chollon C, Pailler R, Naslain R, Olry P. Correlation between microstructure and mechanical behavior at high temperatures of a SiC fibre with a low oxygen content (Hi-Nicalon). J Mater Sci 1997;32: 1133–47. [9] Jia N, Bodet R, Tressler RE. Effects of microsturute instability on the creep behavior of Si–C–O (Nicalon) fibers in argon. J Am Ceram Soc 1993;76(12):3051–60. [10] Ultra high temperature materials research institute. Study on durability and life prediction of continuous fiber reinforced ceramic matrix composite. Research report NEDO-ITK-9911. 2000; p. 6–9. [11] Tanaka Y, Inoue Y, Miyamoto N, Sato M, Yamamura T. Properties of Si–Zr–C–O Fiber/SiZrC composites dispersed ZrSiO4 particles in the matrix. Int J Mater Prod Technol 2001; 16(1–3):197–205. [12] Ochiai S, Hojo M, Schulte K, Fiedler B. Nondimensional simulation of influence of toughness of interface on tensile stress–strain behavior of unidirectional microcomposite. Compos Part A 2001; 32(6):749–61. [13] Metcalfe AG, Klein KJ. Interface in metal matrix composites. Effects of the interface on longitudinal tensile properties. New York: Academic Press; 1974. p. 125–68. [14] Shorshorov MK, Ustinov LM, Zirlin AM, Olefilebko VL, Vonogra￾dov LV. Brittle interface layers and the tensile strength of metal matrix–fibre composites. J Mater Sci 1979;14:1850–61. [15] Ochiai A, Murakami Y. Tensile strength of composites with brittle reaction zones at interface. J Mater Sci 1979;14:831–40. S. Ochiai et al. / Composites: Part A 35 (2004) 33–40 39