Interfaces in Carbon Fiber/PyrolyticCarbon Matrix Composites

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Abstract
Carbon fiber/pyrolytic-carbon matrix composites contain both fiber-matrix and matrixe matrix interfaces and, thus, do not fit the usual concept of fiber-reinforced compositmaterials. This is a consequence of the chemical vapor deposition (CVD) process in which the matrix forms uniform pyrocarbon sheaths around each fiber and ultimately results in matrix-matrix interfaces with various geometries and also produces large oriented voids. The role of the fiber-matrix and matrix-matrix interfaces on the mechanical composite properties has been examined by mechanical testing and optical and scanning electron microscopy. Results will be presented for increasingly complex fibrous substrate geometries with a pyrolytic carbon matrix: single fiber, strand or yarn bundle and circumferential and angle-ply filament-wound cylinders. Data for coated single fibers demonstrate the adequacy of the fiber-matrix interface and the applicability of the rule-of-mixtures to this simple composite. The degree of utilization of these coated fiber units in composite structures, however, is determined by the geometry and strength of the matrix-matrix interface, which in turn, is influenced by the microstructure and properties of the pyrolytic carbon. Observed strengths of unidirectional, infiltrated strands are significantly lower than the strengths of the coated single fiber units of which they are composed. This is because the brittle matrix-matrix interfaces are unable to distribute the load equally between the fiber-matrix units or to redistribute the load around individual unit failures. Attempts to predict strengths for complex angle-ply composites from these unidirectional constituent properties have been only partially successful, as the important role of the matrix-matrix interface has not been adequately quantified. However, optical examination of failure surfaces as a function of fiber modulus and matrix microscructure allows a qualitative interpretation to be made.

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