H Liu et al/ Materials Science and Engineering A 525 (2009 )121-127 better understanding of the relationship between the fiber surface characteristics and the interfacial microstructure, TEM and elemen- tal mapping analysis were done in the interphase regions of the Kd Sic/SiC composites. TEM cross-sectional images of the KD-1 SiC/SiC composite shown in Fig 8, indicating that a turbostratic Py interphase around 30 nm thick is found, which is formed by the Py C surface layer of the KD-1 SiC fiber. The interfaces between the fiber and the Pyc interphase and between the Pyc interphase and the matrix can b defined clearly. It can be seen that the highly aligned basal planes of the Pyc appear to be almost parallel to the interfaces, as already observed by Appiah et al. [18] in C fiber reinforced laminated KD.2 C-SiC matrix composites. Generally speaking, the approximately perfect orientation of the PyC interphase is very favorable for the mechanical property improvement of the Sic/SiC composites. In Displacement/mm his orientation, load can be more easily transferred, and the FM Fig. 6. The load/ displacement curves of the KD-1 and KD-2 SiC/SiC composites. debonding easily occurs due to the low van der Waals force between the basal planes of the Py c[18 Furthermore as shown in Fig. 8b.no obvious phenomena of elemental interdiffusion or chemical reac trophically. As shown in Table 1, the tensile strength of the tions are observed within the Pyc interphase, so the Pyc interphase is a diffusion barrier for protecting the Sic fibers from chemical KD-2 Sic fiber is around 85% that of the KD-1 SiC fiber, but the flex- damage during composite processing From the analysis above the KD-1 SiC/SiC composite. In order to make clear the strength dif- appropriate interfacial bonding strength for the Sic/Sic compos- ference between the two composites, the microstructure of the two ites, which gives the KD-1 Sic /SiC composite adequate fracture composites was investigated by SEM, TEM and elemental mapping toughness KD-2 SiCr/SiC composite, as shown in Fig. 9a and b, a silicon-based oxide interphase about 30 nm thick is observed 3.3. Microstructural analysis of the KD Sic/Sic composites which is formed by the surface layer of the KD-2 SiC fiber. The interfaces between the fiber and the interphase and between the The fracture surface of the KD-1 and KD-2 SiC /Sic composites interphase and the matrix cannot be defined clearly In order to is given in Fig. 7. Concerning the KD-1 Sicr/Sic composite the frac- understand the element distributions in the interphase, carbon ture surface shows an evident fiber pullout( Fig. 7a), and the pullout elemental mapping(Fig. 9d) of the interphase region as shown lengths can exceed 20 um. The surface of the pulled out fibers is in Fig. 9c was done. As observed in Fig. 9d, a carbon-poor region rather smooth and free of any matrix. The fm debonding is evi- about 10 nm thick is detected, the thickness of which is less than denced by a higher magnification SEM observation( Fig. 7b). In the the interphase and some carbon is also found dispersed in this of the KD-2 SiC/Sic composite, the fracture surface is very region. Therefore, elemental interdiffusion and or chemical reac- and few pulled out fibers can be found ( Fig. 7c). Fig 7d shows tions between the fiber and the interphase and between the strong interfacial bonding occurs in the KD-2 composite. For interphase and the matrix can be confirmed, which will result in 10 Fig. 7. Fracture surface of the KD-1(a and b)and KD-2(c and d)sic Sic composites124 H. Liu et al. / Materials Science and Engineering A 525 (2009) 121–127 Fig. 6. The load/displacement curves of the KD-1 and KD-2 SiCf/SiC composites. non-catastrophically. As shown inTable 1, the tensile strength of the KD-2 SiC fiber is around 85% that of the KD-1 SiC fiber, but the flexu￾ral strength of the KD-2 SiCf/SiC composite is only about 15% that of the KD-1 SiCf/SiC composite. In order to make clear the strength dif￾ference between the two composites, the microstructure of the two composites was investigated by SEM, TEM and elemental mapping analysis. 3.3. Microstructural analysis of the KD SiCf/SiC composites The fracture surface of the KD-1 and KD-2 SiCf/SiC composites is given in Fig. 7. Concerning the KD-1 SiCf/SiC composite, the frac￾ture surface shows an evident fiber pullout (Fig. 7a), and the pullout lengths can exceed 20 m. The surface of the pulled out fibers is rather smooth and free of any matrix. The FM debonding is evi￾denced by a higher magnification SEM observation (Fig. 7b). In the case of the KD-2 SiCf/SiC composite, the fracture surface is very even, and few pulled out fibers can be found (Fig. 7c). Fig. 7d shows that strong interfacial bonding occurs in the KD-2 composite. For better understanding of the relationship between the fiber surface characteristics and the interfacial microstructure, TEM and elemen￾tal mapping analysis were done in the interphase regions of the KD SiCf/SiC composites. TEM cross-sectional images of the KD-1 SiCf/SiC composite shown in Fig. 8, indicating that a turbostratic PyC interphase around 30 nm thick is found, which is formed by the PyC surface layer of the KD-1 SiC fiber. The interfaces between the fiber and the PyC interphase and between the PyC interphase and the matrix can be defined clearly. It can be seen that the highly aligned basal planes of the PyC appear to be almost parallel to the interfaces, as already observed by Appiah et al. [18] in C fiber reinforced laminated C–SiC matrix composites. Generally speaking, the approximately perfect orientation of the PyC interphase is very favorable for the mechanical property improvement of the SiCf/SiC composites. In this orientation, load can be more easily transferred, and the FM debonding easily occurs due to the low van der Waals force between the basal planes of the PyC [18]. Furthermore, as shown in Fig. 8b, no obvious phenomena of elemental interdiffusion or chemical reac￾tions are observed within the PyC interphase, so the PyC interphase is a diffusion barrier for protecting the SiC fibers from chemical damage during composite processing. From the analysis above, the PyC interphase decreases the fiber damage and provides the appropriate interfacial bonding strength for the SiCf/SiC compos￾ites, which gives the KD-1 SiCf/SiC composite adequate fracture toughness. For the KD-2 SiCf/SiC composite, as shown in Fig. 9a and b, a silicon-based oxide interphase about 30 nm thick is observed, which is formed by the surface layer of the KD-2 SiC fiber. The interfaces between the fiber and the interphase and between the interphase and the matrix cannot be defined clearly. In order to understand the element distributions in the interphase, carbon elemental mapping (Fig. 9d) of the interphase region as shown in Fig. 9c was done. As observed in Fig. 9d, a carbon-poor region about 10 nm thick is detected, the thickness of which is less than the interphase, and some carbon is also found dispersed in this region. Therefore, elemental interdiffusion and/or chemical reac￾tions between the fiber and the interphase and between the interphase and the matrix can be confirmed, which will result in Fig. 7. Fracture surface of the KD-1 (a and b) and KD-2 (c and d) SiCf/SiC composites.