Decoupling the effects of topographical roughness and oxidation on the interfacial properties of carbon fiber-epoxy composites

Carbon fiber composites under mechanical loading conditions must effectively transfer stresses from the relatively weak structural polymer matrix to the load-bearing carbon fiber. Oxidation treatments of the carbon fiber surface are a common strategy for improving the interface between fiber and matrix, and is understood to increase both the fiber surface roughness, as well as modify the fiber surface chemistry for better resin compatibility. However, it is challenging to decouple the effects of oxidation treatments on the fiber–matrix interface by experiment alone. Here, molecular dynamics simulations of topographically rough carbon fiber surfaces, both with and without oxidation, are interfaced with a thermoset epoxy matrix to decouple the impact of surface roughness and chemical interactions on the interfacial interaction between fiber and matrix. Smoother surfaces yield a greater enhancement of interfacial shear stress in fiber displacement simulations after oxidation, with the pristine graphite surface yielding the greatest increase relative to its non-oxidized value. Additionally, the results suggest that nanoscale fiber surface corrugation perpendicular to the fiber axis could be employed as a strategy to enhance the interfacial shear strength of composites. Overall, these simulations provide nanoscale insights regarding the interplay between surface roughness and chemistry of composite interfaces, which may inform future fiber surface treatments.