G.H. Zhou et al./Ceramics international 33(2007)1395-1398 1397 b 20. Oky x300 10um wD11mm 28.kV x310m11m Fig. 2. The morphology of fracture surface: (a) uni-CHSiO2 and(b)uni-C/SiO,+20 wt %o SiC microcracking induced by the mismatch of the thermal composite was higher than that of uni-CrSiOz+ 20 wt %o SiCp expansion coefficients(CTE) between SiCp and the SiOz composite. In the light of interface theory of ceramic matrix atrix [6, 15, 16]. This microcracking also resulted in the composites( CMCs), strong fiber/matrix interfacial bonding decrease of flexural strength parallel to the fiber direction after results in the matrix microcracks propagating through the fiber iCp addition. The second difference is the non-linear region- with the increase of load. Thus, the fibers are easily broken the tail of load-displacement curves. The tail length of uni-C+ Whereas, in the case of weak fiber/matrix interfacial bonding Sio2 composite is longer than that of uni-CHSiO2+ 20 wt %o the matrix microcracks propagate along the weak interface, SiCp composite. These two differences would be further which results in the fiber/matrix interface debonding. Conse discussed in the next part quently, fibers are easily pulled out. According to the fiber pull Typical fractographs of the composite are shown in Fig. 2. out lengths, it can be concluded that the fiber/matrix interfacial Without SiCp addition( Fig 2a), extensive fiber pull-out was bonding of uni-CrSiOz+ 20 wt %o Sicp composite was observed in the fracture surface, and accompanying with very stronger than that of uni-CiSiO2 composite, which might be large pull-out lengths. While, fiber pull-out was short after SiCp related to the fiber/matrix interfacial sliding stress addition(Fig. 2b). Therefore, the different tail lengths in the Thouless et al. [17] reported that the distribution in fiber ad-displacement curves resulted for the two samples, pull-out lengths in unidirectional CMCs should be related respectively. Fiber pull-out lengths can also explain why fiber/matrix interfacial sliding stress, fiber diameter, and flexural strength(parallel to the fiber direction) of uni-C/SiO2 Weibull modulus of the fiber. Presently, fiber/matrix interfacial SiOz Matrix Fig 3. The transmission electron micrographs of composites:(a) uni-C!SiO2 and(b) uni-CHSiO2+ 20 wt. SiCmicrocracking induced by the mismatch of the thermal expansion coefficients (CTE) between SiCp and the SiO2 matrix [6,15,16]. This microcracking also resulted in the decrease of flexural strength parallel to the fiber direction after SiCp addition. The second difference is the non-linear region— the tail of load–displacement curves. The tail length of uni-Cf/ SiO2 composite is longer than that of uni-Cf/SiO2 + 20 wt.% SiCp composite. These two differences would be further discussed in the next part. Typical fractographs of the composite are shown in Fig. 2. Without SiCp addition (Fig. 2a), extensive fiber pull-out was observed in the fracture surface, and accompanying with very large pull-out lengths. While, fiber pull-out was short after SiCp addition (Fig. 2b). Therefore, the different tail lengths in the load–displacement curves resulted for the two samples, respectively. Fiber pull-out lengths can also explain why flexural strength (parallel to the fiber direction) of uni-Cf/SiO2 composite was higher than that of uni-Cf/SiO2 + 20 wt.% SiCp composite. In the light of interface theory of ceramic matrix composites (CMCs), strong fiber/matrix interfacial bonding results in the matrix microcracks propagating through the fiber with the increase of load. Thus, the fibers are easily broken. Whereas, in the case of weak fiber/matrix interfacial bonding, the matrix microcracks propagate along the weak interface, which results in the fiber/matrix interface debonding. Conse￾quently, fibers are easily pulled out. According to the fiber pull￾out lengths, it can be concluded that the fiber/matrix interfacial bonding of uni-Cf/SiO2 + 20 wt.% SiCp composite was stronger than that of uni-Cf/SiO2 composite, which might be related to the fiber/matrix interfacial sliding stress. Thouless et al. [17] reported that the distribution in fiber pull-out lengths in unidirectional CMCs should be related to the fiber/matrix interfacial sliding stress, fiber diameter, and Weibull modulus of the fiber. Presently, fiber/matrix interfacial G.H. Zhou et al. / Ceramics International 33 (2007) 1395–1398 1397 Fig. 2. The morphology of fracture surface: (a) uni-Cf/SiO2 and (b) uni-Cf/SiO2 + 20 wt.% SiCp. Fig. 3. The transmission electron micrographs of composites: (a) uni-Cf/SiO2 and (b) uni-Cf/SiO2 + 20 wt.% SiCp