Very little fundamental information is known about the mechanism by which surface treatments and surface finishes influence fiber-matrix adhesion and composite properties. This study is directed at elucidating the molecular mechanisms by which surface treatments affect fiber-matrix adhesion and composite properties.
A polyacrylonitrile based graphite fiber and epoxy matrix were chosen as a model system. Substantial changes were detected in the value of the interfacial shear strength attainable for this fiber-matrix system when only the interphase region was altered. Observation of the interphase under load and high resolution transmission electron microscope (TEM) of ultramicrotomed sections were able to discriminate between various failure mechanisms operating at the fiber-matrix interphase.
An untreated graphite fiber possesses a weak structural layer that can not support high shear loads. When this fiber is stressed in shear the fiber separates from the matrix. Failure analysis shows that the interfacial fracture path involves some fiber fracture in the outer layers as well as along the interface. Surface treatment removes this defect laden outer layer of the fiber and also adds surface chemical groups which increase the interaction with the matrix by a factor of two. The fracture path becomes purely interfacial with no fiber failure. Both mechanisms, that is, surface chemical interactions and morphological changes in the outer layers of the fiber contribute to the increased interfacial shear strength attainable with the surface treated fiber in the same matrix.
Application of a fiber surface finish increases the interfacial shear strength. The mechanism by which this can occur is through the creation of a brittle interphase around the fiber. The original finish layer devoid of crosslinking agent receives some through diffusion from the surrounding resin mixture. This results in a layer more brittle than the matrix. Stress analysis shows that this brittle layer promotes better shear transfer resulting in an increased interfacial shear strength. However, at the fiber breaks, matrix cracks grow perpendicular instead of parallel to the fiber surface under shear loading because of this brittle layer.
Small composite specimens containing the fibers with various surface treatments and possessing the various failure modes were fractured in order to determine if the interphase effects had an influence on composite properties. Results indicate that the single filament behavior is observable in the composite.