Surface analysis of carbon fibres modified with PVAL coating and the composite interfaces

High-resolution X-ray photoelectron spectroscopy (XPS) has been used to analyse the fibre surface and composite interfaces with and without polyvinyl alcohol (PVAL) coating (both fibres being commercially surface-treated and sized). Major functional groups on the fibre surface are also identified by Gaussian curve-fitting of carbon peaks to study the correlations of surface chemistry with the observed failure mechanisms of the uncoated and coated fibre composites. The main difference in the fracture surface of the fibre composites with and without the coating is that the latter has a significant amount of silicon (about 6 at% concentration) associated with the epoxy matrix, but silicon is almost absent in the PVAL-coated fibre composites. This suggests that the debonding mechanism in the uncoated fibre composite, which has a strong interfacial bonding, is controlled by the combination of cohesive failure of the matrix material and adhesive failure at the interface. In contrast, the PVAL coating promotes adhesive failure due to the weak bonding at the fibre-matrix interface. This observation is consistent with SEM observations in that the uncoated fibre composite consists of significant deformation of matrix material which covers the majority of the fracture surface and tiny epoxy resin particles adhering to the debonded fibre surface, whereas the coated fibre composite shows a lesser amount of matrix deformation with relatively clean fibre surface.