The effect on the mechanical properties of carbon/epoxy composites of polyamide coatings on the fibers

An experimental investigation has been carried out to study the effect of polyamide coatings on the mechanical properties of unidirectional carbon/epoxy composites. Carbon fibers were coated with polyamide 6,6 by means of interfacial polyamidation and solution dip coating in a laboratory-scale unit developed for this purpose. The amount of coating deposited on the fibers was determined by thermogravimetric analysis (TGA); the morphology and microscale homogeneity were examined by scanning electron microscopy (SEM). Transverse tensile tests and short-beam shear tests were performed in order to assess the effect of the coating on the adhesion of the fiber to the matrix. Composites containing solution-coated fibers exhibited improved tensile and ILSS performance over those of uncoated-fiber composites, although interfacial coating induced deterioration in performance. The double-cantilever beam (DCB) test was used to investigate the effect of the two different polyamide fiber-surface coatings on mode I interlaminar fracture toughness. Initiation fracture toughness (G(ICinit),,) increases for both polyamide coatings. Moreover, interfacial coating causes extensive fiber bridging leading to much increased propagation values. The mechanical results are attributed to the different nature of the two polyamide coatings and to differences in the microstructure of the resulting composites. (C) 1999 Elsevier Science Ltd. All rights reserved.