J.Am. Ceran.Soe,9l46-115602007) Dol:10.111.1551-29162007.01535x C 2007 The American Ceramic Society urna Tensile Creep Behavior of SiC-Based Fibers With a Low Oxygen Content Cedric Sauder'and Jacques Lamon Laboratoire des Composites Thermostructuraux, UMR 5801: CNRS-Snecma-CEA-UBl, 33600 Pessac, France Commissariat a Energie atomique, DEC/SPUA/LMPC, 13108 Saint Paul les Durance, france The creep behavior of Hi-Nicalon, Hi-Nicalon S, and Tyranno During most of the tensile creep tests reported in the litera- SA3 fibers is investigated at temperatures up to 1700.C. Tensile ture, the fiber was not at a uniform temperature( cold-gripping tests were carried out on a high-capability fiber testing appara- method). Furthermore, the test duration rarely exceeds 48 h, for tus in which the fiber is heated uniformly under vacuum. Anal- practical reasons associated with the design of the experimental sis of initial microstructure and composition of fibers wa performed using various techniques. All the fibers experienced The cold grip-based technique presents a few important draw- a steady-state creep. Primary creep was found to be more or less backs. Fiber specimens are generally quite long (length averages ignificant depending on fiber microstructure Steady-state creep 100 mm) and there is a significant temperature gradient. Owing was shown to result from grain-boundary sliding. Activation en- to the temperature gradient, determination of creep strain from ergy and stress exponents were determined. Creep mechanisms fiber deformation is not straightforward. Heavy and tedious re discussed on the basis of activation energy and stress expo- calibration operations are required. Then, the use of long spec- nent data. Finally, tertiary creep was observed at very high imens is not recommended because of fiber diameter variation temperatures. Tertiary creep was related to volatilization of along the gauge. Specimen length must be selected with respect Sic. Results are discussed with respect to fiber microstructure. to characteristic diameter wave length 2.0 is about 160 mm for Tyranno SA and Hi-Nicalon fibers. Specimens with a uni- form diameter along the length can be obtained when the fiber gauge is significantly shorter than 2. L. Introduction In the hot grip-based technique, short specimens are used and the entire fiber can be at a uniform temperature. Some authors composites are designed to be used at high tempera- claim that the fiber may degrade due to the cement used for ures in various systems, including aerojet engines and gripping fiber ends. This difficulty can be overcome with recent stationary gas turbines for electrical power/steam cogeneration products. Furthermore, the results obtained g the hot grip- Furthermore, owing to the stability of SiC under neutron based technique appeared to be consistent with other available irradiation and to recent progress in the fabrication of stoichic data. 3 tes are candidates for nuclear Thus, in the present paper, some effort was directed toward applications, such as the structural component facing the the testing method, in order to overcome the difficulties associ- on fiber reinforcement, and more particularly on the ss depi plasma in fusion reactor blankets, or the control rod in tI ated with the cold grip-based technique, and to produce valu- Generation IV nuclear power plants able creep data on the last generation of Sic-based fibers. The structural performances of SiC/SiC composite of mechanical properties of fibers to temperature and environ IL. Fibers and Experimental Procedure ment. The present paper focuses on the creep behavior of Sic based fibers with a low oxygen content in an inert atmosphere. Description ofFibers These fibers are potential candidates for reinforcement of Sic Hi-Nicalon and Hi-Nicalon S(Nippon Carbon Co., Tokyo Sic composites for nuclear applications. Japan) and Tyranno SA3 (Ube Industry Ltd, Tokyo Several papers on the creep behavior of ceramic fibers apan) Sic-based fibers were investigated ( Table I). Two are available in the literature. Data have been produced different batches of Tyranno SA3 fibers were tested (they are on SiC-based fibers(see for instance Yu and colleagues). referred to as SA3(I)and SA3(2). Sylramic fibers were not The authors mainly used uniaxial tensile loading condition considered in this study because they contain boron, which or a qualitative technique such as bending stress relaxation. But only of these paper terested in the irradiation II mechanisms.4.6. itative of fibe d structure Testing tiny objects such as small-diameter ceramic fibers(the were performed using X-ray diffraction(XRD), Raman diameter may be as small as 10 um) at high temperatures for oscopy, transmission electron microscopy (TEM), electron long times is not straightforward. The results and analyses may for TEM were prepared following the method proposed by robe microanalysis(EPMA), and fractography. Specimens be biased as a result of the testing conditions. The testing meth- od thus warrants consideration in order to produce valuable Berger and Bunsel (2) Creep Tests The fiber samples were taken from tows(gauge length 25 mm) Graphite grips were affixed to sample ends using carbon-based cement C34 (from UCAR Co., Graftech International Ltd No. 22022. Received July 17, 2006: approved November 27, 2006. Parma, OH) rk was supported by CNRS and CEA and was accomplished as part of the CPR The creep tests were performed on a tensile device arch program. correspondence should be addressed. e-mail: lamon(@ Icts. designed for testing carbon fibers at temperature 3000C.Heating is generated by an 114Tensile Creep Behavior of SiC-Based Fibers With a Low Oxygen Content Ce´dric Sauder* and Jacques Lamonw Laboratoire des Composites Thermostructuraux, UMR 5801: CNRS–Snecma–CEA–UB1, 33600 Pessac, France *Commissariat a` l’e´nergie atomique, DEC/SPUA/LMPC, 13108 Saint Paul les Durance, France The creep behavior of Hi-Nicalon, Hi-Nicalon S, and Tyranno SA3 fibers is investigated at temperatures up to 17001C. Tensile tests were carried out on a high-capability fiber testing appara￾tus in which the fiber is heated uniformly under vacuum. Anal￾ysis of initial microstructure and composition of fibers was performed using various techniques. All the fibers experienced a steady-state creep. Primary creep was found to be more or less significant depending on fiber microstructure. Steady-state creep was shown to result from grain-boundary sliding. Activation en￾ergy and stress exponents were determined. Creep mechanisms are discussed on the basis of activation energy and stress expo￾nent data. Finally, tertiary creep was observed at very high temperatures. Tertiary creep was related to volatilization of SiC. Results are discussed with respect to fiber microstructure. I. Introduction SIC/SIC composites are designed to be used at high tempera￾tures in various systems, including aerojet engines and stationary gas turbines for electrical power/steam cogeneration. Furthermore, owing to the stability of SiC under neutron irradiation and to recent progress in the fabrication of stoichio￾metric fibers, SiC/SiC composites are candidates for nuclear applications, such as the structural component facing the plasma in fusion reactor blankets,1,2 or the control rod in the Generation IV nuclear power plants. The structural performances of SiC/SiC composites depend on fiber reinforcement, and more particularly on the sensitivity of mechanical properties of fibers to temperature and environ￾ment. The present paper focuses on the creep behavior of SiC￾based fibers with a low oxygen content in an inert atmosphere. These fibers are potential candidates for reinforcement of SiC/ SiC composites for nuclear applications. Several papers on the creep behavior of ceramic fibers are available in the literature. Data have been produced on SiC-based fibers (see for instance Yu and colleagues3–8). The authors mainly used uniaxial tensile loading conditions, or a qualitative technique such as bending stress relaxation. But only some of these papers were interested in the creep mechanisms.4,6,8 Testing tiny objects such as small-diameter ceramic fibers (the diameter may be as small as 10 mm) at high temperatures for long times is not straightforward. The results and analyses may be biased as a result of the testing conditions. The testing meth￾od thus warrants consideration in order to produce valuable results. During most of the tensile creep tests reported in the litera￾ture, the fiber was not at a uniform temperature (cold-gripping method). Furthermore, the test duration rarely exceeds 48 h, for practical reasons associated with the design of the experimental setup. The cold grip-based technique presents a few important draw￾backs. Fiber specimens are generally quite long (length averages 100 mm) and there is a significant temperature gradient. Owing to the temperature gradient, determination of creep strain from fiber deformation is not straightforward.9 Heavy and tedious calibration operations are required. Then, the use of long spec￾imens is not recommended because of fiber diameter variation along the gauge. Specimen length must be selected with respect to characteristic diameter wave length l. 10 l is about 160 mm for Tyranno SA and Hi-Nicalon fibers.10 Specimens with a uni￾form diameter along the length can be obtained when the fiber gauge is significantly shorter than l. In the hot grip-based technique, short specimens are used and the entire fiber can be at a uniform temperature. Some authors claim that the fiber may degrade due to the cement used for gripping fiber ends. This difficulty can be overcome with recent products. Furthermore, the results obtained using the hot grip￾based technique appeared to be consistent with other available data.3 Thus, in the present paper, some effort was directed toward the testing method, in order to overcome the difficulties associ￾ated with the cold grip-based technique, and to produce valu￾able creep data on the last generation of SiC-based fibers. II. Fibers and Experimental Procedure (1) Description of Fibers Hi-Nicalon and Hi-Nicalon S (Nippon Carbon Co., Tokyo, Japan) and Tyranno SA3 (Ube Industry Ltd., Tokyo, Japan) SiC-based fibers were investigated (Table I). Two different batches of Tyranno SA3 fibers were tested (they are referred to as SA3 (1) and SA3 (2)). Sylramic fibers were not considered in this study because they contain boron, which makes them sensitive to significant degradation under neutron irradiation.11 Quantitative analyses of fibers composition and structure were performed using X-ray diffraction (XRD), Raman spec￾troscopy, transmission electron microscopy (TEM), electron probe microanalysis (EPMA), and fractography. Specimens for TEM were prepared following the method proposed by Berger and Bunsell.12 (2) Creep Tests The fiber samples were taken from tows (gauge length 25 mm). Graphite grips were affixed to sample ends using carbon-based cement C34 (from UCAR Co., Graftech International Ltd., Parma, OH). The creep tests were performed on a tensile device (Fig. 1) designed for testing carbon fibers at temperatures up to 30001C.13 Heating is generated by an electric current F. Wakai—contributing editor This work was supported by CNRS and CEA and was accomplished as part of the CPR ISMIR research program. w Author to whom correspondence should be addressed. e-mail: lamon@lcts. u-bordeaux1.fr Manuscript No. 22022. Received July 17, 2006; approved November 27, 2006. Journal J. Am. Ceram. Soc., 90 [4] 1146–1156 (2007) DOI: 10.1111/j.1551-2916.2007.01535.x r 2007 The American Ceramic Society 1146