Interphase behaviour in graphite-thermoplastic monofilament composites

Abstract

The role of the fibre-matrix interphase in transferring load from the matrix to the fibre in graphite-thermoplastic composites is not well understood. The goal of this work was to alter the interphase of graphite-thermoplastic monofilament composites in a controlled manner by treating the fibre surface, and then correlating fibre surface morphology, fibre surface energy, fibre strength, and matrix properties with interphase behaviour. A monofilament composite system was employed to study the fibre-matrix interphase because fibre-fibre interaction and processing variability are eliminated. A fragmentation method was used to observe the interphase behaviour of the monofilament composites indirectly by measuring the interphase shear stress, a parameter which governs the load transfer from the matrix to the fibre. It was found that the improvement in the ability of the interphase to transfer load from the matrix to the fibre increased with the severity of the treatment and was due primarily to increased micromechanical locking (increased surface roughness). Debonding at the interphase occurred along either the fibre-matrix interphase (in the composites with a tough matrix) or perpendicular to the fibre (in composites with a weaker matrix, and a strong interphase). Thus the matrix properties, by limiting the properties of the composite, strongly influenced the value of improving the interphase properties.