Characterization of the Fiber-Matrix Interphase and its Influence on Mechanical Properties of Unidirectional Composites

Abstract

The fiber-matrix interphase is commonly altered in graphite fiber rein-forced composites by changing the level of surface treatment imparted to the fibers and by using different fiber sizings. In this study, the influence of both surface treatment and fiber sizing on the mechanical properties of unidirectional composites was studied. Three material systems having the same high modulus Apollo carbon fibers (manufactured by Courtoulds Research) and HC 9106-3 toughened epoxy matrix, but with different fiber sizing and surface treatment, were used to study the effects of the interphase. For convenience, these are designated 810 A, 820 A and 8100 systems. The fibers used in the 810 A and 8100 systems received 100% industry standard surface treatment, while the fibers used in the 820 A system received 200% industry standard surface treatment. The "A" and "O" represent unreacted bisphenol-A epoxy and polyvinylpyrrolidone (PVP) sizing respectively. The formation of distinctly different interphase in these three material systems was confirmed using a pernanganic etching technique. The etching study also revealed a highly nonuniform distribution of PVP in the 8100 system. Unidirectional tensile test results indicated significant reduction in the longitudinal stiffness of the PVP sized 8100 laminates. However, the tensile strength and failure strain of this system were vastly greater than those of the epoxy sized 810 A laminates. The systems with 10% and 200% levels of fiber surface treatment had similar longitudinal tensile properties. Transverse tensile test results indicated that the interracial bond strength was greatest in the 820 A system and lowest in the PVP sized 8100 system. The transverse flexural strength of the 8100 and 820 A systems were however found to be significantly greater than that of the 810 A system. It is postulated that the differences in unidirectional mechanical properties in these material systems are essentially due to differences in the interfacial bond strength and efficiency of load transfer from the matrix to the fiber.