Investigating the fiber-resin interface of carbon-fiber-reinforced polymer composites with nanoscale analysis
Authors:Puikei Cheng, Lucas Meza, Siddhartha Pathak, Karthik Ramachandran
- Julia Greer, Assistant Professor of Materials Science and Mechanics, California Institute of Technology
- Lucas Meza, Gr, California Institute of Technology
Composites are materials composed of two or more distinct substances, designed to combine properties to maximize performance. This can only occur when the components behave as one bulk material. If the components aren’t properly bonded, adhesive failure can occur, leading to greatly reduced strength. Hence, characterizing and improving interface adhesion is crucial. Previous work suggests that interface adhesion can be estimated by determining the transition of material properties, such as modulus and hardness, across the interface, and that gradual transitions correlate to stronger bonds. Nanoindentation, a method of calculating material properties from small indentations, can detect those property changes across nanometer length scales. In this study, we tested carbon-fiber-reinforced polymers (CFRPs) with the polymers epoxy and PDCPD (polydicyclopentadiene, a polymer developed by Materia Inc.) as the resin. Pure polymer (neat resin) samples were used as controls to determine the optimum holding time and loading force for minimizing viscoelastic effects, like creep. Experiments have shown that the epoxy CFRP is stronger than comparable PDCPD CFRP, suggesting that epoxy CFRP has stronger interface adherence. After CFRP samples were cut and polished to a local RMS roughness of <20nm, AFM (atomic force microscopy) was used to locate isolated fibers with circular cross-sections visible from surface. Nanoindents, using the Hysitron TI-950 TriboIndenter, were done in grid-shaped patterns over the fibers’ cross-section at 500uN max load. Furthermore, SEM images were used to distinguish between adhesive and cohesive failure in bulk samples fractured in tension. From the nanoindentation results, a zone of polymer stiffening was found to extend roughly 1um from the fiber boundary in both samples, although that lies within the zone of indentation for a Berkovich tip. While it was difficult to draw conclusions from the nanoindentation results, SEM images did indicate that interface bonding was stronger in the epoxy CFRP than in PDCPD.