38 J Nie et al Materials Science and Engineering A 497(2008) 235-238 Fig. 7. Tensile fracture surface of the specimen quenched in the simulated air. (a) Macro-structure and (b) the gaps between fibers and matrix due to the oxidation of Pyc nterface. carbon fiber and sic matrix after quenched in the pure Ar gas. The long fibers pullout can be with increasing thermal cycles. The degradation in modulus, tensile observed, but the failure strain is much lower than that of the as- strength and failure strain caused by the combination of thermo- received composite (as listed in Table 1). This indicates that the mismatch and oxidation in the simulated air was much more severe terfacial bonding was weakened by the thermal cycling and then than that caused by only thermo-mismatch in the pure Ar gas And reduced the load transfer ability between the fiber and the matrix. the changes in resonant frequency can well reveal the degradatior Combining effects both of internal thermo-mismatch stress and in modulus of the needled C/SiC composite stress concentration caused by matrix cracking lead to the decrease of the modulus and tensile strength of the needled C/SiC composite. Acknowledgement 3.3. Effect of the combination of thermal cycling and oxidation in the simulated air ural Science Foundation of kthe financial support of the Nat- 90405015)and the National Young Elitists Foundation(Contract Fig. 7 shows the tensile fracture surface of the specimen after No. 50425208 hed in the simulated air. Within a bundle, the fiber pullout at the centre (as shown in Fig. 7a) and the gaps between fibers Reference and matrix at the edge(as shown in Fig. 7b)can be observed. The fiber pullout indicates that the bonding of the PyC interface was [11 R. Naslain, Int J AppL. Ceram. TechnoL. 2(2005)75-84. weakened by the thermal cycling. And the gaps indicate that the b E christin. dw En. Mater. 4 012)2002)90 F. Cheng Coat. Technol. 200(2006) mal cycling tests. Therefore, when specimen thermal cycled in 5] G. Camus, L Guillaumat, S Baste, Compos. Sci. TechnoL. 56(1996)1363-1372. unavoidably occurred. At high temperatures(1000-1200oC), the 7 Mo M l. xu dr Zha.. L E Cheng, Se N. .0.ng. Scripta Mater. 54(2006) microcracks were closed and the kinetics of formation of silica is and the condensed oxide scale is protective and tends to seal [8]A. Dalmaz, D. D日Cu9m.题ac or/and fill the residual pores and microcracks, stopping(or at least 910.Siron, j. Pailhes, ). Lamon, Compos. Sci. Technol. 59( wing down the in-depth diffusion of oxygen [2]. Conversely, at [10] Y.D. Xu, LF. Cheng. LT Zhang, H.F. Yin, xW. Yin, Mater Sci Eng. A 300(2001 w temperatures, 700<T<900 C, microcracks were opened and this provided the channel for O2 to aggress the Pyc interface and [11 Y.D. Xu, LT Zhang, LF. Cheng. D T Yan, Carbon 36(1998)1051-1056. 12 F.A. Christin then the carbon fibers And the kinetics of oxidation of the PyCinter- [13] Y D Xu, Y.N. Zhang LF. Cheng. LT. Zhang, J]. Lou, JZ. Zhang Ceram.Int33 phase and carbon fibers in the composite is fast whereas that of Sic (2007)439-445. is almost negligible [2]. As shown in Fig. 7b, PyC interphase, cal [14 S.W. Fan, LT. Zhang, Y.D. Xu, LF Cheng, J. Lou, J-Z. Zhang, L Yu, Compos. Sci. echnol67(20072390-2398 bon fiber and Sic matrix were oxidized, leading to gaps between [15].H. Diazvaldes, C. Soutis, ]. Sound Vibrat. 228(1999)1-9 ber and matrix. The oxidation of carbon fibers resulted in the [16 vK srivastava, K Maile, K Bothe, A Udoh, Mater. Sci Eng. A 354(2003 soor load transfer between the fiber and the matrix. Therefore, the n bas sd tec nl 1b2 01 19pm g3 B. Patelb, C. cCofhinb, J. Eldridge. damage caused by the combination of thermo-mismatch and oxi- [18] J.J. Nie Y.D. Xu, LT Zhang. LE Cheng J.Q. Ma. ]. Mater. Proc. Technol, in press. dation in the simulated air was more severe than that in the pure[19] R. Naslain, Comp Ar environment. A29(1998)1145-1155. 4. Conclusions 22 h 3D needled C/SiC composite fabricated by CVI process subjected [23 A. Eckel, R.C. Brad, J Am Ceramic. Soc. 73(1990)1334-1 to thermal cycling from 700 to 1200C in a pure Ar gas and a L Bobet, ]. Lamon, Acta Metall. Mater. 43(1995)2241-22 simulated air. In both environments, the modulus was decreased 12515R Qiao, D Han, G.Q. Luo, Key Eng Mater. 297-300(2005)435-439238 J. Nie et al. / Materials Science and Engineering A 497 (2008) 235–238 Fig. 7. Tensile fracture surface of the specimen quenched in the simulated air. (a) Macro-structure and (b) the gaps between fibers and matrix due to the oxidation of PyC interface, carbon fiber and SiC matrix. after quenched in the pure Ar gas. The long fibers pullout can be observed, but the failure strain is much lower than that of the as￾received composite (as listed in Table 1). This indicates that the interfacial bonding was weakened by the thermal cycling and then reduced the load transfer ability between the fiber and the matrix. Combining effects both of internal thermo-mismatch stress and stress concentration caused by matrix cracking lead to the decrease of the modulus and tensile strength of the needled C/SiC composite. 3.3. Effect of the combination of thermal cycling and oxidation in the simulated air Fig. 7 shows the tensile fracture surface of the specimen after quenched in the simulated air. Within a bundle, the fiber pullout at the centre (as shown in Fig. 7a) and the gaps between fibers and matrix at the edge (as shown in Fig. 7b) can be observed. The fiber pullout indicates that the bonding of the PyC interface was weakened by the thermal cycling. And the gaps indicate that the oxidation of PyC interface and fibers occurred during the ther￾mal cycling tests. Therefore, when specimen thermal cycled in the simulated air, in addition to thermo-mismatch, oxidation was unavoidably occurred. At high temperatures (1000–1200 ◦C), the microcracks were closed and the kinetics of formation of silica is fast and the condensed oxide scale is protective and tends to seal or/and fill the residual pores and microcracks, stopping (or at least slowing down) the in-depth diffusion of oxygen [2]. Conversely, at low temperatures, 700 < T < 900 ◦C, microcracks were opened and this provided the channel for O2 to aggress the PyC interface and then the carbon fibers. And the kinetics of oxidation of the PyC inter￾phase and carbon fibers in the composite is fast whereas that of SiC is almost negligible [2]. As shown in Fig. 7b, PyC interphase, car￾bon fiber and SiC matrix were oxidized, leading to gaps between fiber and matrix. The oxidation of carbon fibers resulted in the loss of fiber strength, and the oxidation of PyC interface led to the poor load transfer between the fiber and the matrix. Therefore, the damage caused by the combination of thermo-mismatch and oxi￾dation in the simulated air was more severe than that in the pure Ar environment. 4. Conclusions 3D needled C/SiC composite fabricated by CVI process subjected to thermal cycling from 700 to 1200 ◦C in a pure Ar gas and a simulated air. In both environments, the modulus was decreased with increasing thermal cycles. The degradation in modulus, tensile strength and failure strain caused by the combination of thermo￾mismatch and oxidation in the simulated air was much more severe than that caused by only thermo-mismatch in the pure Ar gas. And the changes in resonant frequency can well reveal the degradation in modulus of the needled C/SiC composite. Acknowledgements The authors acknowledge the financial support of the Nat￾ural Science Foundation of China (Contract Nos. 50672076 and 90405015) and the National Young Elitists Foundation (Contract No. 50425208). References [1] R. Naslain, Int. J. Appl. Ceram. Technol. 2 (2005) 75–84. [2] R. Naslain, Compos. Sci. Technol. 64 (2004) 155–170. [3] F. Christin, Adv. Eng. Mater. 4 (12) (2002) 903–912. [4] S.J. Wu, L.F. Cheng, L.T. Zhang, Y.D. Xu, Surf. Coat. Technol. 200 (2006) 4489–4492. [5] G. Camus, L. Guillaumat, S. Baste, Compos. Sci. Technol. 56 (1996) 1363–1372. [6] M. Wang, C. Laird, Acta Mater. 44 (1996) 1371–1387. [7] J.Q. Ma, Y.D. Xu, L.T. Zhang, L.F. Cheng, J.J. Nie, N. Dong, Scripta Mater. 54 (2006) 1967–1971. [8] A. Dalmaz, D. Ducret, R. El Guerjouma, P. Reynaud, P. Franciosi, D. Rouby, G. Fantozzi, J.C. Baboux, Compos. Sci. Technol. 60 (2000) 913–925. [9] O. Siron, J. Pailhes, J. Lamon, Compos. Sci. Technol. 59 (1999) 1–12. [10] Y.D. Xu, L.F. Cheng, L.T. Zhang, H.F. Yin, X.W. Yin, Mater. Sci. Eng. A 300 (2001) 196–202. [11] Y.D. Xu, L.T. Zhang, L.F. Cheng, D.T. Yan, Carbon 36 (1998) 1051–1056. [12] F.A. Christin, Int. J. Appl. Ceram. Technol. 2 (2005) 97–104. [13] Y.D. Xu, Y.N. Zhang, L.F. Cheng, L.T. Zhang, J.J. Lou, J.Z. Zhang, Ceram. Int. 33 (2007) 439–445. [14] S.W. Fan, L.T. Zhang, Y.D. Xu, L.F. Cheng, J.J. Lou, J.Z. Zhang, L. Yu, Compos. Sci. Technol. 67 (2007) 2390–2398. [15] S.H. Diazvaldes, C. Soutis, J. Sound Vibrat. 228 (1999) 1–9. [16] V.K. Srivastava, K. Maile, K. Bothe, A. Udoh, Mater. Sci. Eng. A 354 (2003) 292–297. [17] N. Chawlaa, K.K. Chawlab, M. Koopmanb, B. Patelb, C. Coffinb, J.I. 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