Fiber-Matrix Interphase Development in Carbon/Carbon Composites
AEROSPACE CORP EL SEGUNDO CA TECHNOLOGY OPERATIONS
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In carboncarbon CC composites-i.e., a composite in which a carbon matrix is reinforced with carbon fiber when the matrix is derived from a thermosetting resin, we always observe a distinct, highly graphitizable, and well-oriented matrix interphase structure adjacent to the fibers. Qualitatively, the orientation of the interphase is the same as the fiber. It is important to note that thermosetting resins are non-graphitizing when heated in bulk they form isotropic, amorphous glassy carbon. The structure of this interphase becomes more prominent, i.e., more graphitic, as the heat treatment exceeds about 2200 deg C. We have postulated elsewhere that the basis for this graphite interphase development is molecular orientation induced in the degradation of the polymer matrix to carbon as a consequence of restraint of pyrolysis shrinkage at the fibermatrix interface. More specifically, we have hypothesized that the critical factor for development of lamellar graphite by subsequent high temperature heat treatment in this interphase, rather than amorphous glassy carbon, is a state of multiaxial tensile deformation during pyrolysis. We have studied the structural details of the interphase region in more detail using transmission electron microscopy in conjunction with selected area electron diffraction SAD. We have also examined the influence of matrix microstructure on fracture behavior by using an in situ SEM flexure stage that allowed us to follow the crack tip as it advanced through the different microstructural regions. The development of a graphitized carbon interphase can have major effects on properties. Two are discussed in this publication. The first is the effect on matrix density and associated matrix shrinkage. The other is the effect on mechanical properties.
- Laminates and Composite Materials