NEW CARBON MATERIALS Availableonlineatwww.sciencedirect.com Volume 24 Issue 2 June 2009 Online English edition of the Chinese language journal Sciencedirect ite this article as: New carbon materials 2009. 24(2): 173-177 RESEARCH The influence of high temperature exposure to air on the damage to 3D-C/Sic composites HOU Jun-tao, QIAO Sheng- ru*, ZHANG Cheng-yu, ZHANG Yue-bingt Uitra-High-Temperature Structural Composite Laboratory, Northwestem Polytechnical University, Xi an 710072, China; National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, China Abstract: 3D-C/SiC composites, exposed in air at 600, 900, and 1 300C for 0 to 15 h, were investigated by three point bend tests at oom temperature, SEM, and energy dispersive spectroscopy. The results show that the damage curves, expressed as a relative change of elastic modulus, of the composites for a 15 h exposure, could be divided into a sharply increasing stage(stage I)and a steady increasing stage(stage II). Stage I may be caused by a direct oxidation of the carbon fibers and interface carbon layers by the oxygen in air, and stage Il may be caused by a diffuse controlled oxidation of the inner part of the composites. The matrix micro-cracks, induced by a dif- ference of coefficients of thermal expansion between matrix and carbon fibers in the cooling process after composite preparation act as oxygen diffuse paths and are where the oxidation takes place. The fact that the damage decreases with temperature for the same exposure time may be caused by the crack shrinking at high temperature, which decreases the oxidizable surface area and inhibits the diffusion of oxygen into the composites. Key Words: 3D-C/SiC, Thermo-exposure; Damage; Flexural behaviors 1 Introductlon ume fraction of carbon fibers of 40-45%, a porosity of about 17% volume ratio, a density of about 2.0 g/cm3, a pyrolytic Ceramic matrix composites(CMCs), including ca carbon layer thickness of about 200 nm, and an SiC oxIdation bon/silicon carbide(C/SiC)composites, are promising for use barrier coating layer of about 50 um. The specimens were as high temperature structural materials. This kind of materials machined to have a dimension of 50 mmx5 mmx35mm for have a high strength to density ratio and a high temperature the test performance over conventional superalloys in order that little or no cooling is required-31 2.2 Test procedure During the services of C/SiC composites, mechanical After vacuum fatigue, the residual strength of a damage can be caused by an applied stress, which decreases 3D-C/Sic composite does not decrease; however, it increase their modulus- On the other hand, chemical damage may slightly. Therefore, residual strength is not suitable for take place in the high temperature environment, also causing a decrease of their modulus. Up to now, there are few damage evolution after a thermo-exposure at high temperature In this study, the flexural strength and damage evolution of a three-dimensional C/SiC composite (3D-C/SiC) afte thermo-exposure are investigated preliminarily. 2 Materlals and procedure 2.1 Materials The preforms for the 3D-C/SiC composites were woven by a three-dimensional braiding technique, in which T300 carbon fiber bundles were used as weaving yarns and a braid ng angle was 22 as shown in Fig. 1. Pyrolytic carbon layer and SiC matrix were deposited by chemical vapor infiltration Fig 1 Schematic of the structural cell for a 3D-C/SiC composite (CVI)at 900-1 000C. The 3D-C/SiC composite had a vol orm Copyrighto2009, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DO:Io.lol61872-5805(0860046-3
NEW CARBON MATERIALS Volume 24, Issue 2, June 2009 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2009, 24(2):173–177. Received date: 8 April 2008; Revised date: 2 April 2009 *Corresponding author. E-mail: blao@nwpu.edu.cn Copyright©2009, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-5805(08)60046-3 RESEARCH PAPER The influence of high temperature exposure to air on the damage to 3D-C/SiC composites HOU Jun-tao1 , QIAO Sheng-ru1 *, ZHANG Cheng-yu1 , ZHANG Yue-bing2 1 Ultra-High-Temperature Structural Composite Laboratory, Northwestern Polytechnical University, Xi’an 710072,China; 2 National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, China Abstract: 3D-C/SiC composites, exposed in air at 600, 900, and 1 300 °C for 0 to 15 h, were investigated by three point bend tests at room temperature, SEM, and energy dispersive spectroscopy. The results show that the damage curves, expressed as a relative change of elastic modulus, of the composites for a 15 h exposure, could be divided into a sharply increasing stage (stage I) and a steady increasing stage (stage II). Stage I may be caused by a direct oxidation of the carbon fibers and interface carbon layers by the oxygen in air, and stage II may be caused by a diffuse controlled oxidation of the inner part of the composites. The matrix micro-cracks, induced by a difference of coefficients of thermal expansion between matrix and carbon fibers in the cooling process after composite preparation act as oxygen diffuse paths and are where the oxidation takes place. The fact that the damage decreases with temperature for the same exposure time may be caused by the crack shrinking at high temperature, which decreases the oxidizable surface area and inhibits the diffusion of oxygen into the composites. Key Words: 3D-C/SiC; Thermo-exposure; Damage; Flexural behaviors 1 Introduction Ceramic matrix composites (CMCs), including carbon/silicon carbide (C/SiC) composites, are promising for use as high temperature structural materials. This kind of materials have a high strength to density ratio and a high temperature performance over conventional superalloys in order that little or no cooling is required[1-3]. During the services of C/SiC composites, mechanical damage can be caused by an applied stress, which decreases their modulus[4-7]. On the other hand, chemical damage may take place in the high temperature environment, also causing a decrease of their modulus. Up to now, there are few reports on damage evolution after a thermo-exposure at high temperature. In this study, the flexural strength and damage evolution of a three-dimensional C/SiC composite (3D-C/SiC) after thermo-exposure are investigated preliminarily. 2 Materials and procedure 2.1 Materials The preforms for the 3D-C/SiC composites were woven by a three-dimensional braiding technique, in which T300 carbon fiber bundles were used as weaving yarns and a braiding angle was 22º as shown in Fig.1. Pyrolytic carbon layer and SiC matrix were deposited by chemical vapor infiltration (CVI) at 900-1 000 °C. The 3D-C/SiC composite had a volume fraction of carbon fibers of 40-45%, a porosity of about 17 % volume ratio， a density of about 2.0 g/cm3, a pyrolytic carbon layer thickness of about 200 nm, and an SiC oxidation barrier coating layer of about 50 µm. The specimens were machined to have a dimension of 50 mm×5 mm×3.5mm for the test. 2.2 Test procedure After vacuum fatigue ， the residual strength of a 3D-C/SiC composite does not decrease; however, it increases slightly. Therefore, residual strength is not suitable for Fig.1 Schematic of the structural cell for a 3D-C/SiC composite perform
HOU Jun-tao et al. /New Carbon Materials, 2009, 24(2): 173-17 stage Il. The time of stage I becomes shorter and the De value in stage I becomes smaller with exposure temperature. The DE result of a direct oxidation of carbon fibers and interface car- 0.6 bon layers by the oxygen in air while stage lI corresponds to a diffuse controlled oxidation of the inner part of the compos- The coefficients of thermal expansion(CTE) of matrix and carbon fibers in the 3D-C/SiC composites are different. The longitudinal Cte of T300 carbon fibers is about 1×107 roC whereas that of the CVI SiC layers is4.8×10C图 Time t/h As the manufacturing temperature of the 3D-C/SiC compos- ites is about 900-1 000C, the inner stress induced by the Fig2 DE VS time after the thermo-exposure at 600C, 900C and 1 CtE difference between the matrix and carbon fibers leads to 300° In air the formation of matrix micro-cracks in the composites when the composites are cooled down to room temperature. Simila determining the damage degree of a 3D-C/SiC composite. The observation has been found by Lamouroux and Cheng damage value may be above 1 if a relative change of electric resistance is used to evaluate the damage of a 3D-C/SiC com- can be oxidized above 400C. At a temperature between 400 posite. The damage evaluated by a relative change of elastic and 600 SiC oxidation barrier coating layers cannot be oxidized by oxygen. Therefore, the matrix micro-cracks are The 3D-C/SiC specimens were exposed at 600, 900, and the path of entrance for oxygen into the composites as well as 300C in air for different times(0, 0.5, 1, 1.5, 2, 2.5, 3.5, 5, the path of exit for oxide gases out of the composites. The 6.5, 8.5, 10, 11.5, 13.5, and 5 h). The elastic modulus was matrix micro-cracks begin to be clogged as a result of thermal investigated using a three-point bending method under a small expansion of the matrix at 900C, which makes the diffusion ad. The test was carried out on a specimen at the same load of oxygen into the composites difficult. Therefore, the oxida- ing direction, the same supporting point, and the same loading tion reactions take place mainly at the gaps in the fine fiber point to obtain accurate data. The bending span was 40 mm bundles and the damage is smaller at 900C than at 600C rate was 0.3 mm/min The specimens exposed under the same thermal exposure times. The matrix mi- at 1 300C for 15 h in air were bend to cause fracture at 1 300 cro-cracks are completely clogged at 1 300C and the Sic C in air matrix begins to be oxidized to generate SiO2, which has flu- In addition, the fractured surface morphologies of the idity similar to glass at this temperature and can seal the 3D-C/SiC specimens after exposure at 600, 900, and 1300C clogged matrix cracks. Thus, the oxidation damage at I 300 for 6 h were observed under a scanning electron microscope C is the lowest (SEM) 3.2 Fractured surface morphologies 3 Results and discusslon Fig 3 shows the SEM images and energy dispersive 3.1 Damage evaluated by an elastic modulus change spectra of the specimens after thermo-exposure at different temperatures for 6 h. After thermo-exposure for 6 h, the gaps The damage DE of a 3D-C/SiC composite after a appear obvious between carbon fibers and matrix. At 1 300oC thermo-exposure can be determined by elastic modulus as the color of matrix around the fibers seems to be brighter than that of the matrix elsewhere. It can be found in the energy D=1 E spectra that the silica contents vary slightly and the carbon contents drop clearly with temperature. However, the oxygen contents increase sharply only at 1 300C where,Eis the elastic modulus after thermo-exposure and Eo is the initial elastic modulus The appearance of the gaps between fibers and matrix indicates that the oxidation of carbon interface layer and parts Fig 2 shows the damage curves of De versus time afte of fibers occurs. The oxidation activation energy of the carbon the thermo-exposure at 600, 900, and 1300C in air for th fibers is above 30 kcal /mol and that of the carbon interface 3D-C/SiC composites. It can be found that the sample exposed layer is 26 kcal/mol -121. In addition, the bonding of Sic ma- at 600C for 15 h has the largest de of about 80%. The trix and pyrolytic carbon interface layer is weaker than that of ples exposed at 900 and 1 300C have the DE value of about carbon fibers and pyrolytic carbon interface layer. As a result, 38% and 16%, respectively. It should be noted that the dam the matrix micro-cracks induced by residual stress exist age curves can be divided into a sharp increasing stage(stage mainly in the area between the Sic matrix and the pyrolytic I)and a steady increasing stage( stage ID) stage I precedes carbon interface layer. Thus, the oxidation of the 3D-C/SiC
HOU Jun-tao et al. / New Carbon Materials, 2009, 24(2): 173–177 Fig.2 DE vs. time after the thermo-exposure at 600 °C, 900 °C and 1 300 °C in air determining the damage degree of a 3D-C/SiC composite. The damage value may be above 1 if a relative change of electric resistance is used to evaluate the damage of a 3D-C/SiC composite. The damage evaluated by a relative change of elastic modulus is more reasonable and accurate. The 3D-C/SiC specimens were exposed at 600, 900, and 1 300 °C in air for different times (0, 0.5, 1, 1.5, 2, 2.5, 3.5, 5, 6.5, 8.5, 10, 11.5, 13.5, and 5 h). The elastic modulus was investigated using a three-point bending method under a small load. The test was carried out on a specimen at the same loading direction, the same supporting point, and the same loading point to obtain accurate data. The bending span was 40 mm and the loading rate was 0.3 mm/min. The specimens exposed at 1 300 °C for 15 h in air were bend to cause fracture at 1 300 °C in air. In addition, the fractured surface morphologies of the 3D-C/SiC specimens after exposure at 600, 900, and 1300 °C for 6 h were observed under a scanning electron microscope (SEM). 3 Results and discussion 3.1 Damage evaluated by an elastic modulus change The damage DE of a 3D-C/SiC composite after a thermo-exposure can be determined by elastic modulus as follows. 0 1 E E D E ′ = − （1） where, E′is the elastic modulus after thermo-exposure and E0 is the initial elastic modulus. Fig.2 shows the damage curves of DE versus time after the thermo-exposure at 600, 900, and 1300 °C in air for the 3D-C/SiC composites. It can be found that the sample exposed at 600 °C for 15 h has the largest DE of about 80%. The samples exposed at 900 and 1 300 °C have the DE value of about 38% and 16%, respectively. It should be noted that the damage curves can be divided into a sharp increasing stage (stage I ) and a steady increasing stage (stage II); stage I precedes stage II. The time of stage I becomes shorter and the DE value in stage I becomes smaller with exposure temperature. The DE value decreases with exposure temperature. Stage I may be a result of a direct oxidation of carbon fibers and interface carbon layers by the oxygen in air while stage II corresponds to a diffuse controlled oxidation of the inner part of the composites. The coefficients of thermal expansion (CTE) of matrix and carbon fibers in the 3D-C/SiC composites are different. The longitudinal CTE of T300 carbon fibers is about 1×10-7/°C whereas that of the CVI SiC layers is 4.8×10-6/°C [8]. As the manufacturing temperature of the 3D-C/SiC composites is about 900-1 000 °C, the inner stress induced by the CTE difference between the matrix and carbon fibers leads to the formation of matrix micro-cracks in the composites when the composites are cooled down to room temperature. Similar observation has been found by Lamouroux[9] and Cheng[10]. The carbon fibers and the pyrolytic carbon interface layer can be oxidized above 400 °C. At a temperature between 400 and 600 °C, the SiC oxidation barrier coating layers cannot be oxidized by oxygen. Therefore, the matrix micro-cracks are the path of entrance for oxygen into the composites as well as the path of exit for oxide gases out of the composites. The matrix micro-cracks begin to be clogged as a result of thermal expansion of the matrix at 900 °C, which makes the diffusion of oxygen into the composites difficult. Therefore, the oxidation reactions take place mainly at the gaps in the fine fiber bundles and the damage is smaller at 900 °C than at 600 °C under the same thermal exposure times. The matrix micro-cracks are completely clogged at 1 300 °C and the SiC matrix begins to be oxidized to generate SiO2, which has fluidity similar to glass at this temperature and can seal the clogged matrix cracks. Thus, the oxidation damage at 1 300 °C is the lowest. 3.2 Fractured surface morphologies Fig.3 shows the SEM images and energy dispersive spectra of the specimens after thermo-exposure at different temperatures for 6 h. After thermo-exposure for 6 h, the gaps appear obvious between carbon fibers and matrix. At 1 300 °C, the color of matrix around the fibers seems to be brighter than that of the matrix elsewhere. It can be found in the energy spectra that the silica contents vary slightly and the carbon contents drop clearly with temperature. However, the oxygen contents increase sharply only at 1 300 °C. The appearance of the gaps between fibers and matrix indicates that the oxidation of carbon interface layer and parts of fibers occurs. The oxidation activation energy of the carbon fibers is above 30 kcal /mol and that of the carbon interface layer is 26 kcal/mol[11-12]. In addition, the bonding of SiC matrix and pyrolytic carbon interface layer is weaker than that of carbon fibers and pyrolytic carbon interface layer. As a result, the matrix micro-cracks induced by residual stress exist mainly in the area between the SiC matrix and the pyrolytic carbon interface layer. Thus, the oxidation of the 3D-C/SiC
DU Gui-xiang et al./New Carbon Materials, 2009, 24(1): 331-338 40 3.0 3.0 00-1020030040050600 2.003004005006.00 1002003004005.006.00 E/keV Eke∨ E/kev Fig 3 SEM morphology and energy spectrum after thermo-exposure at(a)600C, b)900C, and(c)1 300C for 6h composites begins first on the interface between carbon fibers Table 1 The change of mechanical strength of the sample after and pyrolytic carbon and then expands into the pyrolytic car- exposure at 1 300C for 15 h in air bon layer instead of carbon fibers Flexural Fracture The carbon contents decrease with temperature as a result strens strain of the oxidation as revealed by the energy dispersive spectra Percentage The high oxygen content at 1 300C demonstrates that Sioz is 72% 44% 2.5% retained formed, which can seal the matrix micro-cracks and prevent oxygen from diffusing into the composites 3.4 Life prediction 3.3 Flexural strength and fracture strain The critical value of D is about 0.29 for 3D-C/SIC com- Table 1 lists the change of mechanical strength of sample posites), above which the composite damage is obvious and after exposure at 1 300oC for 15h in air. It can be found that the bearing capacity decreases sharply. In engineering, the the fracture strain of the 3D-C/Sic composite exposed at 1 relative change of elastic modulus is generally below 10%,i.e 300C for 15 h in air is only about 1/40 of the initial value the value of DE is below 0.1 and the flexural strength drops to about 44% of the initial The damage curve was fitted by a cubic equation for value. The degradation of these two properties is far larger temperature using the multiple regression method. The than that of the elastic modulus equation is shown as follows The volume fractions of carbon fibers and pyrolytic car- 600C, DE--0000047-00003/+0.06331;(2) bon interface layer decrease after thermo-exposure, which 900°C,D=0.0002-000492+00498;(3) indicates that the effective area to load burden is decreased 1 300oC, D=00001 t-0.0025 /+0.0237t. (4) and the porosity is increased. As described in the refer ences3-14), the increase of porosity can lead to a decrease of The life times corresponding to the De value of 0. 1 and 0. 29 were evaluated by the fitted equations at each tempera- the elastic modulus and the flexural strength in brittle materI- ture and are listed in Table 2 als
DU Gui-xiang et al. / New Carbon Materials, 2009, 24(1): 331–338 Fig.3 SEM morphology and energy spectrum after thermo-exposure at (a) 600 °C, ( b) 900 °C, and (c) 1 300 °C for 6h composites begins first on the interface between carbon fibers and pyrolytic carbon and then expands into the pyrolytic carbon layer instead of carbon fibers. The carbon contents decrease with temperature as a result of the oxidation as revealed by the energy dispersive spectra. The high oxygen content at 1 300 °C demonstrates that SiO2 is formed, which can seal the matrix micro-cracks and prevent oxygen from diffusing into the composites. 3.3 Flexural strength and fracture strain Table 1 lists the change of mechanical strength of sample after exposure at 1 300 °C for 15h in air. It can be found that the fracture strain of the 3D-C/SiC composite exposed at 1 300 °C for 15 h in air is only about 1/40 of the initial value and the flexural strength drops to about 44% of the initial value. The degradation of these two properties is far larger than that of the elastic modulus. The volume fractions of carbon fibers and pyrolytic carbon interface layer decrease after thermo-exposure, which indicates that the effective area to load burden is decreased and the porosity is increased. As described in the references[13-14], the increase of porosity can lead to a decrease of the elastic modulus and the flexural strength in brittle materials. Table 1 The change of mechanical strength of the sample after exposure at 1 300 °C for 15 h in air Elastic modulus Flexural strength Fracture strain Percentage retained 72% 44% 2.5% 3.4 Life prediction The critical value of DE is about 0.29 for 3D-C/SiC composites[4], above which the composite damage is obvious and the bearing capacity decreases sharply. In engineering, the relative change of elastic modulus is generally below 10%, i.e., the value of DE is below 0.1. The damage curve was fitted by a cubic equation for each temperature using the multiple regression method. The fitted equation is shown as follows. 600 °C, DE= -0.00004t 3 - 0.0003 t 2 + 0.0633t ; (2) 900 °C, DE= 0.0002 t 3 - 0.0049 t 2 + 0.0498t ; (3) 1 300 °C, DE= 0.0001 t 3 - 0.0025 t 2 + 0.0237t . (4) The life times corresponding to the DE value of 0.1 and 0.29 were evaluated by the fitted equations at each temperature and are listed in Table 2
HOUJun-iao et al/New Carbon Materials, 2009, 24(2): 173-177 Table 2 Life time of the 3D-C/SiC composites exposed at 600C 5 Qiao S R, Yang Zx, Han D, et al. Tensile creep damage and 900oC. and 1 300C in air at DE of 0. 1 and 0.29 creep mechanism of 3D-C/SiC composite[J] Journal of Materi- Damage time 600°C 900°C 1300°C als Engineering, 2004, 4: 34-36 D=0.1 3.3h 10.2h 6 Wu X J, Qiao S R, Hou J T, et al. Tensile creep damage of 2D-C/SiC composites evaluated using the fractal dimension and D=0.29 4.8h 11.3h >15h elastic modulus[ J]. New Carbon Materials 2006, 21(4): 321-325. [7 Wang L S, Xiong x, Xiao P, et al. Eflect of high temperature 4 Concluslons treatment on the fabrication and mechanical properties of The damage curves of the 3D-C/SiC composites exposed C/C-SiC composites[J]. New Carbon Materials, 2005, 20(3) at different temperatures in air can be divided into a sharp increasing stage(stage I)and a steady increasing stage(stage 18 Gerald C, Laurent G Stephane B. Development of damage in a II); stage I precedes stage Il. The damage evaluated by the 2D woven C/SiC composite under mechanical loading: I Me- elastic modulus decreases with temperature under the same chanical characterization[J]. Compos Sci TechnoL, 1996, 56 exposure temperature. The fracture strain and flexural strengt 1363-137 decrease to 2 5% and 44% of the initial value after 19 Lamouroux F, Camus Gi Kinetics and mechanisms of oxidation thermo-exposure at 1 300C for 15 h, whose degradations are of 2D woven C/SiC composites: I, Experimental approach[J].J considerably more obvious than that of the elastic modulus Am Ceram soc,1994,77(8):2049-2057 [10 Cheng L F, Xu Y D, Zhang L T, et al. Effect of heat treatment References on the thermal expansion of 2D and 3D C/SiC composites from room temperature to 1 400C[J]. Carbon, 2002, 41(8): 1666 [1 Krenkel W, Berndt F. C/C-SiC composites for space applica- 1670. tions and advanced friction systemsJ). Materials Science and [I1 Lamouroux F, Bourrat X, Naslain R Structure/oxidation behav Engineering A. 2005. 412 ior relationship in the carbonado 22] Schmidta S, Beyera S, Knabeb H, et al. Advanced ceramic ma- 2D-C/PyC/SiC composites[J]. Carbon, 1993, 31: 1273-1288. trix composite materials for current and future propulsion tech- [123 Cheng LE, Xu Y D, Zhang L T, et al. Effect of carbon interlayer nology applications[J]. Acta Astronautica, 2004, 55: 409-420 on oxidation behavior of C/SiC composites with a coating from 3] Ma J Q, Xu Y D, Zhang L T, et al. Preparation and mechanical room temperature to 1 500 C[]. Mater Sci Eng, A 2001, 300 219225 form[J]. Materials Letters, 2007, 61: 312-315 [ 13] Nagarajan A. Ultrasonic study of elasticity-porosity relationship 14 Qiao R, Han D, Li M, et al. Interaction between fatigue and in polystalline alumina J). J Appl Phys, 1971, 42: 3693-3696 creep and life prediction method of 3D-C/SiC(CV/Sih G C, Tu T, [14] Knudsen F P. Dependence of mechanical strength of brittle Wang Z D, editors. Structural Intergrity and Materials Ag- polycrystalline specimens on porosity and gain size]. J Am Ceram Soc,1959,42(8):376-387 University of Science and Technology Press, 2003: 129
HOU Jun-tao et al. / New Carbon Materials, 2009, 24(2): 173–177 Table 2 Life time of the 3D-C/SiC composites exposed at 600 °C, 900 °C, and 1 300 °C in air at DE of 0.1 and 0.29 Damage time 600 °C 900 °C 1 300 °C DE=0.1 1.6 h 3.3 h 10.2 h DE=0.29 4.8 h 11.3 h >15 h 4 Conclusions The damage curves of the 3D- C/SiC composites exposed at different temperatures in air can be divided into a sharp increasing stage (stage I) and a steady increasing stage (stage II); stage I precedes stage II. The damage evaluated by the elastic modulus decreases with temperature under the same exposure temperature. The fracture strain and flexural strength decrease to 2.5% and 44% of the initial value after thermo-exposure at 1 300 °C for 15 h, whose degradations are considerably more obvious than that of the elastic modulus. References [1] Krenkel W, Berndt F. C/C–SiC composites for space applications and advanced friction systems[J]. Materials Science and Engineering A , 2005, 412 : 177-181. [2] Schmidta S, Beyera S, Knabeb H, et al. Advanced ceramic matrix composite materials for current and future propulsion technology applications[J]. Acta Astronautica, 2004, 55: 409-420. [3] Ma J Q, Xu Y D, Zhang L T, et al. Preparation and mechanical properties of C/SiC composites with carbon fiberwoven perform[J]. Materials Letters, 2007, 61: 312-315. [4] Qiao S R, Han D, Li M, et al. Interaction between fatigue and creep and life prediction method of 3D-C/SiC[C]//Sih G C, Tu T, Wang Z D, editors. Structural Intergrity and Materials Aging-Fracture Mechanics and Applications. Shanghai: East China University of Science and Technology Press, 2003: 129. [5] Qiao S R, Yang Z X, Han D, et al. Tensile creep damage and creep mechanism of 3D-C/SiC composite[J]. Journal of Materials Engineering, 2004, 4: 34-36. [6] Wu X J, Qiao S R, Hou J T, et al. Tensile creep damage of 2D-C/SiC composites evaluated using the fractal dimension and elastic modulus[J]. New Carbon Materials 2006, 21(4): 321-325. [7] Wang L S, Xiong X, Xiao P, et al. Effect of high temperature treatment on the fabrication and mechanical properties of C/C-SiC composites[J]. New Carbon Materials, 2005, 20(3): 245-249. [8] Gérald C, Laurent G, Stéphane B. Development of damage in a 2D woven C/SiC composite under mechanical loading: I. Mechanical characterization[J]. Compos Sci Technol, 1996, 56: 1363-1372. [9] Lamouroux F, Camus G. Kinetics and mechanisms of oxidation of 2D woven C/SiC composites: I, Experimental approach[J]. J Am Ceram Soc, 1994, 77(8): 2049-2057. [10] Cheng L F, Xu Y D, Zhang L T, et al. Effect of heat treatment on the thermal expansion of 2D and 3D C/SiC composites from room temperature to 1 400 °C[J]. Carbon, 2002, 41(8): 1666 -1670. [11] Lamouroux F, Bourrat X, Naslain R. Structure/oxidation behavior relationship in the carbonaceous constituents of 2D-C/PyC/SiC composites[J]. Carbon, 1993, 31: 1273-1288. [12] Cheng L F, Xu Y D, Zhang L T, et al. Effect of carbon interlayer on oxidation behavior of C/SiC composites with a coating from room temperature to 1 500 ℃[J]. Mater Sci Eng , A 2001, 300: 219-225. [13] Nagarajan A. Ultrasonic study of elasticity-porosity relationship in polystalline alumina [J]. J Appl Phys, 1971, 42: 3693-3696. [14] Knudsen F P. Dependence of mechanical strength of brittle polycrystalline specimens on porosity and gain size[J]. J Am Ceram Soc, 1959, 42(8): 376-387