312 H Mei et aL. Materials Science and Engineering A 460-461(2007)306-313 posites. The greater the extents of the thermal cycling damage the more the matrix cracks acting as the oxygen tunnels and the severer the fiber oxidation of the composites. Therefore, the 2D architecture has a poorer thermal shock resistance than the 3D braided architecture. which in turn results in the lower oxidation resistances of 2D composites It is interesting to discuss the influence of the fiber architec 90 tures in composites on the crack propagation resistance, which Cracking is also believed to be a contributing factor for the different oxidation regimes. According to the oxidation kinetics mech developed by Eckel et al. [16], the reaction-controlled and diffusion-controlled kinetics can be used to interpret the crAcking different oxidation regimes from Figs. 7a and &a. On the fractured sections of the 2D C/SiC composite specimens,a great number of fibers became thinner and thinner, indicat- ing that the oxidizing atmosphere diffuses inwards along the opening and propagating cracks largely and easily, leading Fig 10. A propagating crack penetrates easily through the transverse%fiber to a uniform oxidation governed by the reaction-controlled bundles vertical to the tensile axis, and eventually is deflected and hindered by kinetic as the high strength longitudinal 90. fibers. PeRT / Ko ex 2) mainly deflect longitudinally or arrested beneath the coatings to form a superficial oxidation governed by diffusion-controlled where x is termed the recession distance of carbon phase from kinetics the surface into the center. Under the same environmental condi- tions, a typical superficial oxidation governed by the diffusion- 4. Conclusions ontrolled kinetics occurs in the braided 3D C/SiC composites owing to the self-closure of the highly oriented transverse During thermal cycling between 900 and 1200.C and fatigue cracks, as stress of 60+ 20 MPa, the braided 3D C/SiC composites exhibit (1+x)Dk/D)+1 larger strain increment, better oxidation resistance and thermal (3) shock resistance than the 2D composites. As indicated by the D/D+1 residual strength measurements, the thermally cycled braided the parabolic rate constant, respectively. x is the oxidant par- 2D composites. Differences in the fiber architectures are taken tial pressure, P the total pressure(Pa), Pe the molar density into account to be response for the results with respect to the of carbon(mol/m ), R the gas constant(J/mol K), T the abso- following lute temperature (K), ko a constant(m/s), e the activation energy (/mol), t the duration of the test, and Dk and D are (1)Thermal cycling and fatigue stress resulted in the irregu- the Knudsen diffusion coefficient and Fick diffusion coefficient. lar netty cracks and regular wavy cracks on the top surface respectively. coatings of the 2D architecture and 3d braided architecture The crack initiation and propagation depend significantly on composites, respectively. Simultaneously, both side surface the arrangements of the fibers in composites. Generally, the coatings always generated the highly oriented transverse external stress-induced cracks initiate on the brittle ceramic cracks. Differences in the self-closure capacity of these coatings disregarding the fiber architectures in the preforms cracks during heating lead to the different oxidation resis- Subsequently, the substantial crack propagation pattern is tance of the two architectures strongly relevant to the fiber architectures and has a key influence (2) The constraints between longitudinal(90) and transverse on oxidation regimes. As shown in Fig. 10, a propagating crack (0)fiber bundles in 2D architecture are likely to enhance penetrates easily through the transverse 0o fiber bundles vertical physical destruction during thermal cycling, through which to the tensile axis, and eventually is deflected and hindered by the fiber oxidation can be aggravated. In contrast, the the high strength longitudinal 90 fibers. Thus, only one half of 3d braided architecture exhibits excellent thermal shock the fibers in 2D architecture (i.e, longitudinal fibers )can effec resistance by the collective and highly oriented transverse tively hinder the transverse crack propagation. As a result, the cracking. cracks in 2D composites can propagate rapidly from the outer (3) Only one half of the fibers in the 2D architecture can effec- coatings into the cores through which the fibers are oxidized uni- tively hinder the transverse crack propagation from the formly according to the reaction-controlled kinetics mechanism. surface to the center whereas almost all the fibers in the By contrast, almost all the fibers in the 3d braided architecture 3D braided architecture can deflect longitudinally the trans can resist the crack growth transversely. The transverse cracks verse cracks, which result in the uniform oxidation governed hardly propagate across so many longitudinal fibers, and are by a linear reaction-controlled kinetics mechanism for the312 H. Mei et al. / Materials Science and Engineering A 460–461 (2007) 306–313 posites. The greater the extents of the thermal cycling damage, the more the matrix cracks acting as the oxygen tunnels and the severer the fiber oxidation of the composites. Therefore, the 2D architecture has a poorer thermal shock resistance than the 3D braided architecture, which in turn results in the lower oxidation resistances of 2D composites. It is interesting to discuss the influence of the fiber architec￾tures in composites on the crack propagation resistance, which is also believed to be a contributing factor for the different oxidation regimes. According to the oxidation kinetics mech￾anisms developed by Eckel et al. [16], the reaction-controlled and diffusion-controlled kinetics can be used to interpret the different oxidation regimes from Figs. 7a and 8a. On the fractured sections of the 2D C/SiC composite specimens, a great number of fibers became thinner and thinner, indicat￾ing that the oxidizing atmosphere diffuses inwards along the opening and propagating cracks largely and easily, leading to a uniform oxidation governed by the reaction-controlled kinetic as x = Klt = χP ρcRT k0 exp − Q RT t (2) where x is termed the recession distance of carbon phase from the surface into the center. Under the same environmental condi￾tions, a typical superficial oxidation governed by the diffusion￾controlled kinetics occurs in the braided 3D C/SiC composites owing to the self-closure of the highly oriented transverse cracks, as x2 = Kpt = 4D P ρcRT ln (1 + χ)(Dk/D) + 1 Dk/D + 1 t (3) where Kl, Kp are referred to as the linear rate constant and the parabolic rate constant, respectively. χ is the oxidant par￾tial pressure, P the total pressure (Pa), ρc the molar density of carbon (mol/m3), R the gas constant (J/mol K), T the abso￾lute temperature (K), k0 a constant (m/s), Q the activation energy (J/mol), t the duration of the test, and Dk and D are the Knudsen diffusion coefficient and Fick diffusion coefficient, respectively. The crack initiation and propagation depend significantly on the arrangements of the fibers in composites. Generally, the external stress-induced cracks initiate on the brittle ceramic coatings disregarding the fiber architectures in the preforms. Subsequently, the substantial crack propagation pattern is strongly relevant to the fiber architectures and has a key influence on oxidation regimes. As shown in Fig. 10, a propagating crack penetrates easily through the transverse 0◦ fiber bundles vertical to the tensile axis, and eventually is deflected and hindered by the high strength longitudinal 90◦ fibers. Thus, only one half of the fibers in 2D architecture (i.e., longitudinal fibers) can effec￾tively hinder the transverse crack propagation. As a result, the cracks in 2D composites can propagate rapidly from the outer coatings into the cores, through which the fibers are oxidized uni￾formly according to the reaction-controlled kinetics mechanism. By contrast, almost all the fibers in the 3D braided architecture can resist the crack growth transversely. The transverse cracks hardly propagate across so many longitudinal fibers, and are Fig. 10. A propagating crack penetrates easily through the transverse 0◦ fiber bundles vertical to the tensile axis, and eventually is deflected and hindered by the high strength longitudinal 90◦ fibers. mainly deflect longitudinally or arrested beneath the coatings to form a superficial oxidation governed by diffusion-controlled kinetics. 4. Conclusions During thermal cycling between 900 and 1200 ◦C and fatigue stress of 60 ± 20 MPa, the braided 3D C/SiC composites exhibit larger strain increment, better oxidation resistance and thermal shock resistance than the 2D composites. As indicated by the residual strength measurements, the thermally cycled braided 3D C/SiC composites suffer less loss in the strength than the 2D composites. Differences in the fiber architectures are taken into account to be response for the results with respect to the following: (1) Thermal cycling and fatigue stress resulted in the irregu￾lar netty cracks and regular wavy cracks on the top surface coatings of the 2D architecture and 3D braided architecture composites, respectively. Simultaneously, both side surface coatings always generated the highly oriented transverse cracks. Differences in the self-closure capacity of these cracks during heating lead to the different oxidation resis￾tance of the two architectures. (2) The constraints between longitudinal (90◦) and transverse (0◦) fiber bundles in 2D architecture are likely to enhance physical destruction during thermal cycling, through which the fiber oxidation can be aggravated. In contrast, the 3D braided architecture exhibits excellent thermal shock resistance by the collective and highly oriented transverse cracking. (3) Only one half of the fibers in the 2D architecture can effec￾tively hinder the transverse crack propagation from the surface to the center whereas almost all the fibers in the 3D braided architecture can deflect longitudinally the trans￾verse cracks, which result in the uniform oxidation governed by a linear reaction-controlled kinetics mechanism for the