Interphase brittle

Fig. 15. The interfacial shear strength for coated fibers (C) versus uncoated carbon fibers in an epoxy matrix. The finish layer increases the shear strength by creation of a brittle interphase. From Drzal et al. 7S)...

Despite the emphasis on favorable interactions between the matrix and reinforcement and compound formation between them, it may be beneficial in certain circumstances for the interaction between the two primary constituents to be relatively weak. This is especially true in ceramic-ceramic composites, where both constituents are brittle, and the only way to impart some ductility on the composite is for the interphase to fail gracefully —that is, for the fibers to actually pull out of the matrix in a controlled manner. Optimization of the interphase properties in advanced composites is currently the focus of much research. 

Exposure to 70 °C gives similar results for the surface treated fiber (Fig. 24). That is, a complete reversibility in noted. The finished fiber (i.e. the fiber with the interphase consisting of the amine deficient brittle interlayer) experiences a nonrecovery of interfacial shear strength after moisture exposure and dehydration. Parallel surface spectroscopic investigation of the fiber surfaces show that under these conditions the fiber surface chemistry is not permanently altered by this exposure. Model studies of epoxies with the amine deficient composition of the interphase show that, the wet Tb of this material is about 70 °C. Therefore, the interphase is at or above its wet Tg and therefore because of the compliant nature of this material, stresses cannot be transfered efficiently and the interface is permanently distorted. 

In particular for the mechanical responses across the interphase, it would be obvious that long distance forces let higher strain levels across the interphase than those from short distances, giving rise to brittle materials. 

The bending stress/ displacement curve from three-point-bending test plotted in Fig.6 reveals that the composite shows non-brittle fiacture behavior and the bending stress reaches 493.7 36.7MPa. Consistent with the non-brittle fracture behavior, some fiber pull-out can be observed from the fiacture surface of C /SiC-ZrC composite and the pulled-out fiber surface is smooth as a result of the weak bonding between the fiber and the matrix due to the existence of an interphase. 

The polymer chain-ends, oligomers, and other low molecular weight components migrate to the interphase. Thus, in solid state the immiscible blends are brittle, requiring compatibilization. 

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