J Nie et al. Materials Science and Engineering A 497(2008)235-238 Table 1 Tensile properties of the needled C/SiC composite before and after cycled for 90 Environments Strain(‰) Tensile strength(MPa) As-received composites Short-cut-fiber web Ar gas 0.38 53.2 90° unidirectional I Table 2 The Youngs modulus(E)and expansion coefficient()[5, 22-24 Materials trength(MPa) GPa) (×10-6K-1) T700cr4900 038 Fig. 3. Microstructure of the as-received needled C/SiC composite. of the needled C/SiC composite, including strength, fracture strain and modulus. After quenched in pure Ar gas for 90 cycles, the retain ing rates of strength and strain are 53.5% and 53.8%, respectively. The lower retaining rates of 25. 4% and 33.5% were found for the specimens cycled in the simulated air. This indicates that a more 08 severe degradation was caused by the combination of the thermal cycling and oxidation. 3. 2. Effect of thermal cycling in pure Ar gas It is well known that, SiC matrix, PyC interface and carbon fiber have different expansion coefficients, as shown in Table 215, 22-24]. The differences between these expansion coefficients are very great and cannot beignored. The needled C/Sic composite was fabricated Thermal cycles at 1100 C. When it was cooled to room temperature, the expan- sion coefficient mismatch between carbon fiber, Pyc interface and Fig. 4. Relative reduction in modulus for the specimens cycled in both environ- ents SiC matrix would induce residual thermal stress During the ther- mal cycling tests, these microcracks were promoted by the thermal 3. 1. Degradations in modulus and tensile properties stress in Sic matrix created by coefficients mismatch at low tem- perature. The thermo-stress in SiC matrix, o th(m), can be estimated The relative changes in modulus calculated from the expression by the following expression[25] (2)for specimens quenched in both environments are plotted in oth(m)=Ec x(am-ai)AT thermal cycles. And the reduction rate in modulus of the specimen where Ec is the Young's lus of the composites, AT is the thermal cycled in the simulated air was much greater than that in temperature difterence ue of Ec and AT are 75 GPa [18 the pure Ar gas and500°C,res (3), the thermo-stress in the sic After thermal cycled for 90 cycles in the pure Ar gas and sim- matrix is estimated to be 67.5 MPa. It will promote the microc ulated air, the residual tensile strength were measured, and the racks extending in the Sic matrix, weaken the interfacial bonding. sults are presented in Fig. 5 and Table 1. It is obvious that the or even lead to matrix cracking at the stress concentration sites such as pores and needling potions within the needled c/sic com- thermal cycling plays a great degradation on the tensile propertes posite. Fig. 6 shows the tensile fracture surface of the specimen as-reseived composite simulated a 0 LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLu 40.5060.7 Strain(%) 100um ig ed oeso ac tesile properties of the needed clsic compost Fig. 6. Tensile fracture surface of the specimen after quenched in pure Ar gas.J. Nie et al. / Materials Science and Engineering A 497 (2008) 235–238 237 Fig. 3. Microstructure of the as-received needled C/SiC composite. Fig. 4. Relative reduction in modulus for the specimens cycled in both environ￾ments. 3.1. Degradations in modulus and tensile properties The relative changes in modulus calculated from the expression (2) for specimens quenched in both environments are plotted in Fig. 4. In both environments, modulus decreased with increasing thermal cycles. And the reduction rate in modulus of the specimen thermal cycled in the simulated air was much greater than that in the pure Ar gas. After thermal cycled for 90 cycles in the pure Ar gas and sim￾ulated air, the residual tensile strength were measured, and the results are presented in Fig. 5 and Table 1. It is obvious that the thermal cycling plays a great degradation on the tensile properties Fig. 5. Residual tensile properties of the needled C/SiC composite after thermal cycled for 90 cycles. Table 1 Tensile properties of the needled C/SiC composite before and after cycled for 90 cycles in different environments Environments Strain (%) Tensile strength (MPa) As-received composites 0.71 158.9 Ar gas 0.38 85.5 Simulated air 0.18 53.2 Table 2 The Young’s modulus (E) and expansion coefficient (˛) [5,22–24] Materials Strength (MPa) E (GPa) ˛l (×10−6 K−1) ˛r (×10−6 K−1) T700 Cf 4900 230 −0.38 7 PyC – 30 3 26 SiC – 430 4.8 4.8 of the needled C/SiC composite, including strength, fracture strain andmodulus. After quenched in pure Ar gas for 90 cycles, the retain￾ing rates of strength and strain are 53.5% and 53.8%, respectively. The lower retaining rates of 25.4% and 33.5% were found for the specimens cycled in the simulated air. This indicates that a more severe degradation was caused by the combination of the thermal cycling and oxidation. 3.2. Effect of thermal cycling in pure Ar gas It is well known that, SiC matrix, PyC interface and carbon fiber have different expansion coefficients, as shown inTable 2 [5,22–24]. The differences between these expansion coefficients are very great and cannot be ignored. The needled C/SiC composite was fabricated at 1100 ◦C. When it was cooled to room temperature, the expan￾sion coefficient mismatch between carbon fiber, PyC interface and SiC matrix would induce residual thermal stress. During the ther￾mal cycling tests, these microcracks were promoted by the thermal stress in SiC matrix created by coefficients mismatch at low tem￾perature. The thermo-stress in SiC matrix, th(m), can be estimated by the following expression [25]: th(m) = Ec × (am − ai) T (3) where Ec is the Young’s modulus of the composites, T is the temperature difference. The value of Ec and T are 75 GPa [18] and 500 ◦C, respectively. From Eq. (3), the thermo-stress in the SiC matrix is estimated to be 67.5 MPa. It will promote the microc￾racks extending in the SiC matrix, weaken the interfacial bonding, or even lead to matrix cracking at the stress concentration sites, such as pores and needling potions within the needled C/SiC com￾posite. Fig. 6 shows the tensile fracture surface of the specimen Fig. 6. Tensile fracture surface of the specimen after quenched in pure Ar gas.