S.M. Dong et al. Ceramics International 28(2002)899-905 curve of TSA/C/SiC demonstrates a different behavior with respect to the TSA/SiC composite. After reaching TSA/S the maximum value, the load decreases gradually indi- cating a pseudo-ductile fracture behavior. This char acteristic might be ascribed to the relatively loose matrix and weak interface also, providing the lower elastic TSA/C/SIC modulus. Even though the weak interface is benefitial for the crack bridging and fiber pull-out, it is simulta neously detrimental for strength because of the low load transfer ability from matrix to fibers through the weak interface Fig. 6. Stress/displacement curves of the composites. 3.3. Fracture behaviors of the composites Because am af in the composites, a radial tension Fig. 7 shows the fracture surfaces of the composites stress was induced to the fiber/matrix interface by ther- For TSA/SiC, short fiber pull-out is mal mismatch. This might explain the occurrence of Although polymer derived matrix directly bonded with some fiber/matrix debonding as indicated in Fig 3 For fibers, this bonding was weakened by the tensile effect TSA/C/SiC, the carbon coating will simultaneously induced by thermal mismatch so that the debonding affect the interfacial stresses [17]. The radial tension between fibers and matrix became possible. In situ stress can weaken the fiber/matrix interfacial bonding observation during bending test indicated that crack and significantly reduce the applied load during the fiber propagated nearly parallel to the direction of the load push-out and push-back [13]. This effect might be bene- applied. No delamination along fiber layers could be ficial for the composite TSA SiC to prevent a too strong found and the tensile fracture is predominant. TSA/ CI fiber/matrix bonding in the case of no coating applied. Sic composite demonstrates long fiber pull-out, as The roughness induced radial stress will highly con- shown in Fig. 7b. The in situ observation indicated that ribute to the IFS, as analyzed in literature [13, 14, 16]. In the fracture propagated in two directions: one is nearly the present study, Tyranno SA fiber is confirmed to parallel to that of the applied load and another is nearly have a rather rough surface so that the contribution to erpendicular to that of the applied load. It means that the IFS is high. This characteristic will finally affect the both tensile fracture and shear fracture exist in this mechanical properties of the composites. composite Typical stress/displacement curves obtained from The dense matrix in the TSA SiC composite would bending test are shown in Fig. 6. TSA/SiC composite prevent delamination along the fiber layers. The strong displays not only higher strength but also a high mod- bonding was beneficial for load transfer from matrix to ulus of elasticity, as summarized in Table 2. These fibers and simultaneously limited the fibers pull-out. In mechanical properties may be attributed to the densely TSA/C/SiC composite, since the density is relatively low formed matrix and the strong interface between fibers and the matrix was loosely formed during PIMP pro- and matrix. The strong interface is beneficial for the cess, cracks easily propagate along the weak region load transfer from matrix to fibers so that higher Meanwhile, the weak load transfer between matrix and strength could be obtained [20, 21]. Load/ displacement fibers in this composite allows the long fibers pull-out SOum Fig. 7. SEM micrographs of the fracture surface after bending test showing the different behaviors of the PIMP composites: (a) TSA/SiC, (b) TSABecause r m < r f in the composites,a radial tension stress was induced to the fiber/matrix interface by ther￾mal mismatch. This might explain the occurrence of some fiber/matrix debonding as indicated in Fig. 3. For TSA/C/SiC,the carbon coating will simultaneously affect the interfacial stresses [17]. The radial tension stress can weaken the fiber/matrix interfacial bonding and significantly reduce the applied load during the fiber push-out and push-back [13]. This effect might be bene- ficial for the composite TSA/SiC to prevent a too strong fiber/matrix bonding in the case of no coating applied. The roughness induced radial stress will highly con￾tribute to the IFS, as analyzed in literature [13,14,16]. In the present study,Tyranno SA fiber is confirmed to have a rather rough surface so that the contribution to the IFS is high. This characteristic will finally affect the mechanical properties of the composites. Typical stress/displacement curves obtained from bending test are shown in Fig. 6. TSA/SiC composite displays not only higher strength but also a high mod￾ulus of elasticity,as summarized in Table 2. These mechanical properties may be attributed to the densely formed matrix and the strong interface between fibers and matrix. The strong interface is beneficial for the load transfer from matrix to fibers so that higher strength could be obtained [20,21]. Load/displacement curve of TSA/C/SiC demonstrates a different behavior with respect to the TSA/SiC composite. After reaching the maximum value,the load decreases gradually indi￾cating a pseudo-ductile fracture behavior. This char￾acteristic might be ascribed to the relatively loose matrix and weak interface also,providing the lower elastic modulus. Even though the weak interface is benefitial for the crack bridging and fiber pull-out,it is simulta￾neously detrimental for strength because of the low load transfer ability from matrix to fibers through the weak interface. 3.3. Fracture behaviors of the composites Fig. 7 shows the fracture surfaces of the composites. For TSA/SiC,short fiber pull-out is apparent (Fig. 7a). Although polymer derived matrix directly bonded with fibers,this bonding was weakened by the tensile effect induced by thermal mismatch so that the debonding between fibers and matrix became possible. In situ observation during bending test indicated that crack propagated nearly parallel to the direction of the load applied. No delamination along fiber layers could be found,and the tensile fracture is predominant. TSA/C/ SiC composite demonstrates long fiber pull-out,as shown in Fig. 7b. The in situ observation indicated that the fracture propagated in two directions: one is nearly parallel to that of the applied load and another is nearly perpendicular to that of the applied load. It means that both tensile fracture and shear fracture exist in this composite. The dense matrix in the TSA/SiC composite would prevent delamination along the fiber layers. The strong bonding was beneficial for load transfer from matrix to fibers and simultaneously limited the fibers pull-out. In TSA/C/SiC composite,since the density is relatively low and the matrix was loosely formed during PIMP pro￾cess,cracks easily propagate along the weak region. Meanwhile,the weak load transfer between matrix and fibers in this composite allows the long fibers pull-out. Fig. 6. Stress/displacement curves of the composites. Fig. 7. SEM micrographs of the fracture surface after bending test showing the different behaviors of the PIMP composites: (a) TSA/SiC,(b) TSA/ C/SiC. 904 S.M. Dong et al. / Ceramics International 28 (2002) 899–905