TEM studies of resin-based matrix microstructure in carbon/carbon composites

Abstract

The microstructure of thermoset-resin-derived matrices in unidrectional carbon/carbon (C/C) composites has been investigated by transmission electron microscopy (TEM). A wide range of carbon fibers—derived from PAN, pitch and rayon—were used with two thermoset-resin-matrix precursors: polyarylacetylene (PAA) and phenolic resins. Specimens were examined in both transverse and longitudinal sections using bright-field imaging and selected-area diffraction (SAD). The current studies confirm in much finer detail earlier observations from optical microscopy and scanning electron microscopy (SEM) that matrix orientation is clearly evident at heat treatment temperatures (HTTs) as low as 1100°C and that graphitized lamellar matrix forms at HTTs > ~2400°C and is concentrated in those interfilament regions in which we expect large components of biaxial tensile stress resulting from restraint of pyrolysis shrinkage. The TEM micrographs also establish that most of the fibers and matrices studied are capable of forming intimate fusion bonds following graphitization heat treatment. It is difficult, however, to estimate the extent to which such bonding exists at any particular HTT. We have also made observations of transversely oriented matrix immediately at the fiber surface in some composites. It is suggested that such a microstructure may form as a consequence of movement or deformation of the matrix during the critical carbon-chain-forming process that accompanies matrix pyrolysis. Microporosity at the fiber surface may play a role in this process by providing the opportunity for development of a network of interpenetrating fibrils that effectively bond fiber to matrix. Observations were also made of ripples or striations (produced by ion-milling) that run transverse to the direction of preferred orientation in both fibers and matrices. Possible mechanisms for their formation are discussed. Although two thermosetting resins employed in this study—PAA and phenolic—differ widely in chemical functionalities and structure, their matrix microstructures in the C Cs are largely indistinguishable by the TEM technique employed here, as well as by earlier X-ray diffraction (XRD) results. These observations emphasize the critical role of polymer pyrolysis, in which thermomechanical processes, along with bond breakage and reforming, effectively overcome the initial large differences in polymer structure, leading to the formation of well-graphitized carbons from otherwise nongraphitizing precursors.