《复合材料 Composites》课程教学资源（学习资料）第二章 增强体_nicalon-2 Tensile Creep Behavior of SiC-Based Fibers With a Low Oxygen Content

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 114

Tensile 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

April 2007 Sic-Based Fibers and Low Oxygen Conten 1147 Table I. Properties of Sic Fibers Investigated in the Present Study ippon Carbon Co., Japan Ube Industries Ltd, Japan Hi-Nicalon Hi-Nicalon S 43(2) Batch no 225103 320203 M-010071 Diameter (um) Density(g/cm 3.0 3.l Tensile strength(GPa) 2.516 Tensile modulus(GPa) Grains size(nm) X-ray diffraction 5-10 60-70 60-70 Transmission electron microscopy 50400 Chemical composition(wt%/at. % 621/41.3 84/48.l 666/46.0(edge) 69.1/49(edge) 0.3/395(core) 66.1/45.6(core) 37.7/58.5 0.5/50.7(edge) 39.2/60.l(core) 33.5/54.l(core) 1. 16(edg 1.03(edge) .52(c 1.19(core) Fig 1. Schematic diagram of the high-temperature fiber-testing apparatus. circulating through the fiber, under secondary vacuum(residual The fiber was first kept stress-free at the test temperature for pressure <*Pa). In such an environment, active oxidation is 30 min. Then, the stress was applied. This loading step took less infinitively slow. The temperature of the fiber was measured than 10 s. The diameter of each fiber was measured in situ using using a bichromatic pyrometer (IrCON. Niles, IL). The tem- a laser mounted on the testing apparatus. It is given by the av- perature profiles showed that the temperature is uniform over erage of several measurements along the gauge length t To en- more than 95% of the gauge length. Furthermore, it appeared sure a good reproducibility of the results, only those specimens hat grips remained at a temperature close to room temperature with quite uniform diameters along the gauge were tested For during the tests(<50.C). Thus, the loading frame compliance these specimens, the diameters measured along the fiber differed was not affected during the tests fiber deformations can be from the average by <3% derived from grip displacement. Loading frame compliance was Fiber deformations were derived from grip displac taken to be equal to that estimated at room temperature. Com- Data were corrected to account for deformation of the putations of temperature distributions for various thermal con- frame. The loading frame compliance was estimated us ductivities showed that the temperature gradient from the core to the surface of the fiber is<2°at1000°C13 'The cross sections of fibers are circular. The diameter is variable along the gauge

circulating through the fiber, under secondary vacuum (residual pressure B10
4 Pa). In such an environment, active oxidation is infinitively slow.14 The temperature of the fiber was measured using a bichromatic pyrometer (IRCON, Niles, IL). The tem￾perature profiles showed that the temperature is uniform over more than 95% of the gauge length.13 Furthermore, it appeared that grips remained at a temperature close to room temperature during the tests (o501C). Thus, the loading frame compliance was not affected during the tests. Fiber deformations can be derived from grip displacement. Loading frame compliance was taken to be equal to that estimated at room temperature. Com￾putations of temperature distributions for various thermal con￾ductivities showed that the temperature gradient from the core to the surface of the fiber is o21C at 10001C.13 The fiber was first kept stress-free at the test temperature for 30 min. Then, the stress was applied. This loading step took less than 10 s. The diameter of each fiber was measured in situ using a laser mounted on the testing apparatus. It is given by the av￾erage of several measurements along the gauge length.z To en￾sure a good reproducibility of the results, only those specimens with quite uniform diameters along the gauge were tested. For these specimens, the diameters measured along the fiber differed from the average by o3%. Fiber deformations were derived from grip displacement. Data were corrected to account for deformation of the loading frame. The loading frame compliance was estimated using the Quartz chamber x y Camera z Mirror Light Laser y z Mirror Mirror Vacuum captor displacement table Pyrometer Fig. 1. Schematic diagram of the high-temperature fiber-testing apparatus. Table I. Properties of SiC Fibers Investigated in the Present Study Suppliers Nippon Carbon Co., Japan Ube Industries Ltd., Japan Type of fiber Hi-Nicalon Hi-Nicalon S Tyranno SA3 (1) Tyranno SA3 (2) Batch no. 225103 320203 M-0110071 M-0304041 Diameter (mm) 1416 1316 7.5 7.2 Density (g/cm3 ) 2.7416 3.016 3.0 3.1 Tensile strength (GPa) 2.816 2.516 2.816 2.8 Tensile modulus (GPa) 290 375 325 380 Grains size (nm) X-ray diffraction 5–10 20 60–70 60–70 Transmission electron microscopy 5–10 10–50 50–400 50–400 Chemical composition (wt%/at.%) Si 62.1/41.3 68.4/48.1 66.6/46.0 (edge) 69.1/49 (edge) 60.3/39.5 (core) 66.1/45.6 (core) C 37.7/58.5 31.3/51.5 33/53.6 (edge) 30.5/50.7 (edge) 39.2/60.1 (core) 33.5/54.1 (core) O 0.2/0.2 0.3/0.3 0.2/0.2 0.1/0.1 Al — — 0.3/0.2 0.3/0.2 C/Si (at.%) 1.41 1.07 1.16 (edge) 1.03 (edge) 1.52 (core) 1.19 (core) z The cross sections of fibers are circular. The diameter is variable along the gauge. April 2007 SiC-Based Fibers and Low Oxygen Content 1147

1148 Journal of the American Ceramic Society--Sauder and Lamon Vol. 90. No. 4 G-Nicalon Hi-Nicalon S 1.4 1.4 。t。↑。:0.8 Cs-o 0.6 04 42 7=65-4-3=2-101234567 76=5-4-3=2-101 (c) 040+- 08茜 0.6a 0.6a Fig. 2. Si, C, O, and Al atomic concentrations along the diameter of(a) Hi-Nicalon(b), Hi-Nicalon S(c), Tyranno SA3(1)and (d), Tyranno SA3(2) fiber as measured by electron probe microanalyst conventional calibration technique based on tensile tests on fi- Hi-Nicalon S is made up of clusters of Sic grains(Fig bers having various gauge lengths. As indicated above, as the Grain size averages 20 nm (Table D). The largest grains were 50 grips remained at a temperature close to room temperature dur m in size. Carbon is located between the SiC grains(Fig 4) ing the tests, the loading frame compliance estimated at room Grain boundaries do not appear clearly(Fig 4) The concentration in C and Si was not found to be uniform in Most of the creep tests were range Is the sa3 fibers(Fig. 2). There is a larger amount of free carbon for analysis of crept fibers. Stresses in the 50-850MPa resent in the core. The sa3(1)fiber contained a larger amount were applied, whereas the temperatures hel150° of free carbon when compared with more recent SA3(2)(Table 1700C range. Tests were performed for as long as 350 h in D). Elemental composition in SA3 (2) is closer to stoichiomet order to identify the different creep stages suggesting that fibers of this second batch have been improved the sa3()fibers and 100 nm in the sA3(2)ones. Aluminum II. Results and discussion was identified (Table D). According to Ishikawa, Al aggregates (1) Structure and Composition of Fibers The grain size is much larger when compared with Hi-Nica Table i summarizes the results of microstructural lon and Hi-Nicalon S fibers. a difference in the grain size can be All the fibers contain a small amount of oxygen (0.2%) noted from the micrographs shown in Fig. 5. The size of B-sic Higher oxygen contents were reported for Hi-Nicalon fibers rains averaged 200 nm (table D). The largest grains were 400 (0.6-0.9w/o) nm in size. The grains displayed stack faults(Fig. 3). This ex- There is a larger amount of free carbon in the hi-nicalo lains the discrepancy in grain sizes estimated using XRD and fiber when compared with Hi-Nicalon S. Hi-Nicalon S is stoi TEM (Table I). Grain size was larger from the core to the sur- chiometric, but the results indicate an excess of carbon. Figures face of the fibers. As opposed to Hi-Nicalon S fibers, grain 2(a)and(b) show that the element concentration is uniform in boundaries are clearly marked(Fig 4). Carbon shows a turbos- both fibers. However, a carbon-rich phase was detected using tratic structure. It is located between B-Sic grains(Fig 4) EPMA. It was located in the surface. over a thickness 100 nm. ed,6., Hi-Nicalon fiber microstructure is well document Thus, data from the literature can be reported here. (2) Creep behavior faulted.Grain size averaged 5 nm(Table 1). The largest grains Steady-state creep was observed after a more or less long a 9 Hi-Nicalon fibers consist of fine p-SiC grains, which may be The typical creep curves that were obtained are shown in Fig. were 20 nm in size (Table D). The carbon phase(turbostratic mary creep stage, depending on the fiber: after about 140 h for carbon) consists of distorted stacks of 5-10 graphitic planes, 2-5 Hi-Nicalon fibers at 1200C, about 72 h for SA3 (1)fiber at 200C, about 8 h for SA3(2)fiber at 1250C, and about h for

conventional calibration technique based on tensile tests on fi- bers having various gauge lengths.15 As indicated above, as the grips remained at a temperature close to room temperature dur￾ing the tests, the loading frame compliance estimated at room temperature was pertinent. Most of the creep tests were interrupted before fiber failure, for analysis of crept fibers. Stresses in the range 150–850 MPa were applied, whereas the temperatures were in the 11501– 17001C range. Tests were performed for as long as 350 h in order to identify the different creep stages. III. Results and Discussion (1) Structure and Composition of Fibers Table I summarizes the results of microstructural analyses. All the fibers contain a small amount of oxygen (0.2%). Higher oxygen contents were reported for Hi-Nicalon fibers6,16 (0.6–0.9 w/o). There is a larger amount of free carbon in the Hi-Nicalon fiber when compared with Hi-Nicalon S. Hi-Nicalon S is stoi￾chiometric, but the results indicate an excess of carbon. Figures 2(a) and (b) show that the element concentration is uniform in both fibers. However, a carbon-rich phase was detected using EPMA. It was located in the surface, over a thickness o100 nm. The Hi-Nicalon fiber microstructure is well document￾ed.6,17,18 Thus, data from the literature can be reported here. Hi-Nicalon fibers consist of fine b-SiC grains, which may be faulted.18 Grain size averaged 5 nm (Table I). The largest grains were 20 nm in size (Table I). The carbon phase (turbostratic carbon) consists of distorted stacks of 5–10 graphitic planes, 2–5 nm long. Hi-Nicalon S is made up of clusters of SiC grains (Fig. 3). Grain size averages 20 nm (Table I). The largest grains were 50 nm in size. Carbon is located between the SiC grains (Fig. 4). Grain boundaries do not appear clearly (Fig. 4). The concentration in C and Si was not found to be uniform in the SA3 fibers (Fig. 2). There is a larger amount of free carbon present in the core. The SA3 (1) fiber contained a larger amount of free carbon when compared with more recent SA3 (2) (Table I). Elemental composition in SA3 (2) is closer to stoichiometry, suggesting that fibers of this second batch have been improved. A carbon-rich phase was detected on the surface, over 300 nm in the SA3 (1) fibers and 100 nm in the SA3 (2) ones. Aluminum was identified (Table I). According to Ishikawa,19 Al aggregates at grain boundaries. The grain size is much larger when compared with Hi-Nica￾lon and Hi-Nicalon S fibers. A difference in the grain size can be noted from the micrographs shown in Fig. 5. The size of b-SiC grains averaged 200 nm (Table I). The largest grains were 400 nm in size. The grains displayed stack faults (Fig. 3). This ex￾plains the discrepancy in grain sizes estimated using XRD and TEM (Table I). Grain size was larger from the core to the sur￾face of the fibers. As opposed to Hi-Nicalon S fibers, grain boundaries are clearly marked (Fig. 4). Carbon shows a turbos￾tratic structure. It is located between b-SiC grains (Fig. 4). (2) Creep Behavior The typical creep curves that were obtained are shown in Fig. 6. Steady-state creep was observed after a more or less long pri￾mary creep stage, depending on the fiber: after about 140 h for Hi-Nicalon fibers at 12001C, about 72 h for SA3 (1) fiber at 12001C, about 8 h for SA3(2) fiber at 12501C, and about 8 h for 0 10 20 30 40 50 60 70 –7 –6 –5 –4 –3 –2 –1 x (µm) –7 –6 –5 –4 –4 –3 –3 –2 –2 –1 –1 0 0 1 1 2 2 3 3 4 4 0 12 34 5 6 7 567 x (µm) x (µm) –4 –3 –2 –1 0 1 2 3 4 x (µm) at. % (C et Si) 0 10 20 30 40 50 60 70 at. % (C et Si) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 at. % (O) at. % (C, Si) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 at. % (O et AI) at. % (O et AI) C Si O 0 0.2 0.4 0.6 0.8 1 1.2 1.4 at. % (O) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 C Si O 0 10 20 30 40 50 60 70 at. % (C, Si) C Si O Al 0 10 20 30 40 50 60 70 C Si O Al Hi-Nicalon Hi-Nicalon S (a) (b) (c) (d) SA3(1) SA3(2) Fig. 2. Si, C, O, and Al atomic concentrations along the diameter of (a) Hi-Nicalon (b), Hi-Nicalon S (c), Tyranno SA3(1) and (d), Tyranno SA3 (2) fiber as measured by electron probe microanalysis. 1148 Journal of the American Ceramic Society—Sauder and Lamon Vol. 90, No. 4

April 2007 Sic-Based Fibers and Low Oxygen Conten 1149 (b) Hi-Nicalon S Hi-Nicalon S at 1350C. The creep results reported by most au- thors were obtained during much shorter tests(<48 h). Thus, it nay be anticipated that their tests were not sufficiently long, so that true secondary creep stage was probably not reached Figure 7 shows a typical creep curve obtained at incremental temperature steps. It can be noted that creep accelerated at tem- peratures>1600.C(tertiary creep). Creep curves were fitted by the following well-accepted equations of deformations in the primary and in the secondary stages. Tertiary creep is examined in a subsequent section (1) Ep=oA[1-exp(pr) Es= Bot where subscripts e, p, and s refer, respectively, to elastic regime primary, and secondary creep. o is the stress on the fiber, Ec the initial fiber Youngs modulus, I is time, and A, B, n, and p re constants Figure 6 shows that an excellent agreement was obtained for all the fibers. Note that Hi-Nicalon s and sa3 fibers are less 500nm 50nm Based on microstructure analysis, the fibers can be considered SC(251A) to be a mixture of wrinkled carbon-layer packets and Sic grains a possible controlling creep mechanism may involve grain- boundary sliding, carbon diffusion, dewrinkling, deform and sliding of carbon crystallites.6 (3) Creep Mechanisms-Primary Creep Primary creep can be attributed to viscoelastic deformation of carbon at grain boundaries. The viscoelasticity of carbon has SiCu(131A) been discussed by Kelly20 and it has been observed by Sauder et al. on various carbon fibers at high temperatures. Because Fig 3. Microstructure and electron diffraction pattern of (a) Tyranno of the very weak interaction between layer planes, each basal SA3 (2)and(b) Hi-Nicalon S fiber(effective beam size =2. 15 um). plane can deform as a separate unit in two dimensions, which Hi-Nicalon S sA3(2) ig. 4. Lattice fringe images showing the presence of turbostratic carbon at the Sic grain boundary for(a) Hi-Nicalon S and(b) Tyranno SA3(2)fiber

Hi-Nicalon S at 13501C. The creep results reported by most au￾thors were obtained during much shorter tests (o48 h). Thus, it may be anticipated that their tests were not sufficiently long, so that true secondary creep stage was probably not reached. Figure 7 shows a typical creep curve obtained at incremental temperature steps. It can be noted that creep accelerated at tem￾peratures 416001C (tertiary creep). Creep curves were fitted by the following well-accepted equations of deformations in the primary and in the secondary stages. Tertiary creep is examined in a subsequent section: ee ¼ s Eo (1) ep ¼ sA½1
exp ð
ptÞ (2) es ¼ Bsn t (3) e ¼ ee þ ep þ es (4) where subscripts e, p, and s refer, respectively, to elastic regime, primary, and secondary creep. s is the stress on the fiber, Eo is the initial fiber Young’s modulus, t is time, and A, B, n, and p are constants. Figure 6 shows that an excellent agreement was obtained for all the fibers. Note that Hi-Nicalon S and SA3 fibers are less sensitive to creep than Hi-Nicalon. Based on microstructure analysis, the fibers can be considered to be a mixture of wrinkled carbon-layer packets and SiC grains. A possible controlling creep mechanism may involve grain￾boundary sliding, carbon diffusion, dewrinkling, deformation, and sliding of carbon crystallites.6 (3) Creep Mechanisms—Primary Creep Primary creep can be attributed to viscoelastic deformation of carbon at grain boundaries. The viscoelasticity of carbon has been discussed by Kelly20 and it has been observed by Sauder et al. 21 on various carbon fibers at high temperatures. Because of the very weak interaction between layer planes, each basal plane can deform as a separate unit in two dimensions, which carbon carbon SiC 10 nm 7 nm SiC SiC SiC SiC SA3(2) (a) (b) Hi-Nicalon S Fig. 4. Lattice fringe images showing the presence of turbostratic carbon at the SiC grain boundary for (a) Hi-Nicalon S and (b) Tyranno SA3 (2) fiber. Fig. 3. Microstructure and electron diffraction pattern of (a) Tyranno SA3 (2) and (b) Hi-Nicalon S fiber (effective beam size 5 2.15 mm). April 2007 SiC-Based Fibers and Low Oxygen Content 1149

l150 Journal of the American Ceramic Society--Sauder and Lamon Vol. 90. No. 4 sA3(1) (2) Fig. 5. Scanning electron microscope micrographs of the cross sections of (a) Hi-Nicalon. (b)Hi-Nicalon S(c) Tyranno SA3(1), and(d) Tyranno SA3 produces substantial basal plane shear. Furthermore, the mag- It is worth mentioning that primary creep was more signifi nitude of the viscoelastic response of carbon fibers under tension cant in those fibers that contained a large amount of carbon(Hi- depends on the orientation of the graphitic planes. It increases Nicalon). This supports the above carbon deformation-driven with the fraction of graphitic planes with a large angle to loading mechanism direction(isotropic carbon). By contrast, it is limited in the ani Although the Hi-Nicalon and SA3 (1) fibers possessed the sotropic fibers, in which most of the graphitic planes are orient same fraction of carbon, Hi-Nicalon fiber showed a larger sen- ed parallel to the loading direction. In SiC fibers, the orientation sitivity to creep. This discrepancy can be attributed to grain size of graphitic planes is influenced by grain-boundary distribution. which is much smaller in the Hi- Nicalon fiber. It could also Thus, graphitic planes can take on all orientations. Further- be related to the structure of carbon, which displayed a better more, it was indicated earlier that the carbon present in these rganization in the SA3(1) fiber (smaller distance between iC fibers consists of stacks of a few graphitic planes. As a con- two successive layers: doo), as a result of a higher-processing sequence, primary creep may involve deformation of carbon at temperature. A low doog implies a larger stiffness and smaller grain boundaries and grain sliding due to basal plane shear deformations

produces substantial basal plane shear.20 Furthermore, the mag￾nitude of the viscoelastic response of carbon fibers under tension depends on the orientation of the graphitic planes.21 It increases with the fraction of graphitic planes with a large angle to loading direction (isotropic carbon). By contrast, it is limited in the ani￾sotropic fibers, in which most of the graphitic planes are orient￾ed parallel to the loading direction. In SiC fibers, the orientation of graphitic planes is influenced by grain-boundary distribution. Thus, graphitic planes can take on all orientations. Further￾more, it was indicated earlier that the carbon present in these SiC fibers consists of stacks of a few graphitic planes. As a con￾sequence, primary creep may involve deformation of carbon at grain boundaries and grain sliding due to basal plane shear. It is worth mentioning that primary creep was more signifi- cant in those fibers that contained a large amount of carbon (Hi￾Nicalon). This supports the above carbon deformation-driven mechanism. Although the Hi-Nicalon and SA3 (1) fibers possessed the same fraction of carbon, Hi-Nicalon fiber showed a larger sen￾sitivity to creep. This discrepancy can be attributed to grain size, which is much smaller in the Hi-Nicalon fiber. It could also be related to the structure of carbon, which displayed a better organization in the SA3 (1) fiber (smaller distance between two successive layers: d002), as a result of a higher-processing temperature. A low d002 implies a larger stiffness and smaller deformations. (a) Hi-Nicalon (b) Hi-Nicalon S SA3 (1) SA3 (2) (c) (d) Zoom Fig. 5. Scanning electron microscope micrographs of the cross sections of (a) Hi-Nicalon, (b) Hi-Nicalon S, (c) Tyranno SA3 (1), and (d) Tyranno SA3 (2) fibers. 1150 Journal of the American Ceramic Society—Sauder and Lamon Vol. 90, No. 4