Influence of Processing on Morphology, Interface, and Delamination in PEEK/Carbon Composites

Abstract

It is well known that the mechanical properties of thermoplastic composites depend upon processing parameters such as cooling rate, melting temperature, and residence time at the melt temperature. This is due to the change in matrix morphology, fiber/matrix interaction, and interphase structure with processing conditions. In previous works on carbon fiber/PEEK composites, it has been found that an increase in the melting temperature and the residence time improves the short-beam-shear strength of the composite, and increasing the cooling rate results in a decrease in this strength. Crystallinity at the interface appeared to be efficient only if there is a strong physico-chemical interaction between the matrix and the fiber. Moreover, transcrystallinity does not seem to be the primary factor responsible for a good fiber/matrix interaction in thermoplastic composite materials. The purpose of this work is to foster the understanding of the role of morphology in the bulk matrix and at the fiber/matrix interphase in the composite performance. The morphology of the matrix is observed by optical microscopy and compared with the analysis results from differential scanning calorimetry (DSC). The composite performance is investigated by Mode-I and Mode-Il delamination tests. The results show that the transcrystalline region strongly depends on the molding parameters and controls the fiber/matrix interfacial strength in the composite. A better-defined spherulite structure is observed with specific molding temperature and residence time at melt temperature. The cooling rate also strongly affects the fiber/matrix interaction and the transcrystalline phase. The Mode-I delamination performance is controlled mainly by the matrix morphology, whereas the Mode-Il delamination resistance is related to the fiber/matrix interfacial strength and the interphase structure in the composite.