S. Ochiai et al./ Composites: Part A 35(2004)33-40 5μm Imm (a)3,6×103s (d) Fig. 3. Appearance of the fractured SiC/SiC composite exposed in vacuuM at1473Kfor(a)3.6×102,(b)3.6×103,(c)3.6×104and(d)3.6×105s. are formed at the fiber surface due to the decomposition of the fiber and matrix through the generation of Sio and co gases [6,7]. The result mentioned above indicates the (b)3.6×104 (1) The defects shown in Fig. 4 are thought to act to reduce e strength as indicated by Hollon et al. [7]. The rength not only of the fiber but also of the matrix is reduced (2) All specimens showed the fiber-pullout fracture mod accompanied by the interfacial debonding. The increase in fiber-pullout length with increasing exposure time ( Fig 3)means that the interfacial strength between fiber and matrix decreases. The decrease in interfacial strength could be attributed to the consumption of the carbon at the interface and to the thinning of fiber and 5 matrix due to the decomposition. The amount of the carbon at the interface is however small. It is deduced (c)3.6×10 that the carbon is consumed in the initial stage. The large amount of the continuing weight loss in the late stage Fig 4. Appearance of the matrix and fiber in the composite specimens (Fig. 1)could be attributed to the decomposition of fiber posed at the same temperature of 1473 K in vacuum.(a)-(c) show the and matrix defects in the matrix, on the side surface of the pulled-out fiber and in the racture surface of the fiber, respectively. (3)As shown in our recent analysis and simulation of debonding in the composite [12]. the fracture mode determined by the relative ratio of interfacial strength to bonding strength decreased far more than the fiber component strength, and the fracture mode changes strength with progress of the decomposition. gradually from fiber-pullout to nonfiber-pullout typ (4) Some portions of the matrix had flown away upon with increasing relative ratio. If we assume that the fracture of the specimens, as shown in Fig 3(c)and(d) interfacial bonding strength is kept during the vacuum This phenomenon could be attributed to the reduced exposure, the pullout length of fiber decreases with strength of the matrix and interface. Such a fracture decreasing fiber strength. However, it is not the case behavior suggests that the matrix could not support the since the fiber-pullout length increases with decreasing applied load and most applied load was supported by the fiber strength. Thus it is suggested that the interfacial fiberare formed at the fiber surface due to the decomposition of the fiber and matrix through the generation of SiO and CO gases [6,7]. The result mentioned above indicates the followings. (1) The defects shown in Fig. 4 are thought to act to reduce the strength as indicated by Chollon et al. [7]. The strength not only of the fiber but also of the matrix is reduced. (2) All specimens showed the fiber-pullout fracture mode, accompanied by the interfacial debonding. The increase in fiber-pullout length with increasing exposure time (Fig. 3) means that the interfacial strength between fiber and matrix decreases. The decrease in interfacial strength could be attributed to the consumption of the carbon at the interface and to the thinning of fiber and matrix due to the decomposition. The amount of the carbon at the interface is, however, small. It is deduced that the carbon is consumed in the initial stage. The large amount of the continuing weight loss in the late stage (Fig. 1) could be attributed to the decomposition of fiber and matrix. (3) As shown in our recent analysis and simulation of debonding in the composite [12], the fracture mode is determined by the relative ratio of interfacial strength to component strength, and the fracture mode changes gradually from fiber-pullout to nonfiber-pullout type with increasing relative ratio. If we assume that the interfacial bonding strength is kept during the vacuum exposure, the pullout length of fiber decreases with decreasing fiber strength. However, it is not the case since the fiber-pullout length increases with decreasing fiber strength. Thus it is suggested that the interfacial bonding strength decreased far more than the fiber strength with progress of the decomposition. (4) Some portions of the matrix had flown away upon fracture of the specimens, as shown in Fig. 3(c) and (d). This phenomenon could be attributed to the reduced strength of the matrix and interface. Such a fracture behavior suggests that the matrix could not support the applied load and most applied load was supported by the fiber. Fig. 3. Appearance of the fractured SiC/SiC composite exposed in vacuum at 1473 K for (a) 3.6 £ 102 , (b) 3.6 £ 103 , (c) 3.6 £ 104 and (d) 3.6 £ 105 s. Fig. 4. Appearance of the matrix and fiber in the composite specimens exposed at the same temperature of 1473 K in vacuum. (a)–(c) show the defects in the matrix, on the side surface of the pulled-out fiber and in the fracture surface of the fiber, respectively. 36 S. Ochiai et al. / Composites: Part A 35 (2004) 33–40