Nanoscale Interphase Characterization of Porous CNT Buckypaper Composites in Correlation to Interlaminar Mode I Fracture

Enhanced Carbon Fiber Reinforced Polymer composites (CFRPs) with nanoparticulate have been shown to impact all modes of interlaminar fracture performance. The proposed experimentation in this study employed an advanced Atomic Force Microscopy (AFM) technique to explain this interphase and fracture relationship. The interphase region is of vast importance to researchers as it governs carbon monofilament stress transfer and bulk material properties. In this conference paper, nanoscale material property data and ASTM mode I interlaminar fracture results for three-phase buckypaper CFRP samples will be presented and analyzed.

Vacuum filtration and surfactant-free methods were used to manufacture buckypaper membranes with a thickness of 50 to 300 microns. The epoxy-infused buckypaper membranes were placed in front of the crack tip in a stitch-bonded carbon fiber polymer matrix composite using a hand layup technique. Peak Force Quantitative Nanomechanical Mapping (PFQNM) is a highly advanced AFM technique that allows non-destructive and simultaneous capture of imaging and mechanical property data with the nanometer resolution. Probes with a nominal tip radius of 5 nm to 8 nm were used. Three-phase MWCNT samples were cut from different segments of the nanocomposite samples and were polished up to 0.1 microns. PFQNM was used to fully characterize the interphase region between a three-phase sample of carbon monofilament, epoxy resin, and multi-walled carbon nanotube (MWCNT) buckypaper. This experiment captured reproducible nanoscale morphological, viscoelastic, elastic, and energy properties of porous MWCNT buckypaper samples. 

An enlarged interphase region surrounding the CNT buckypaper was found, explaining the improved fracture performance. The buckypaper and epoxy interphase thickness were found to be 40nm to 60nm, higher than the 10-40nm reported for epoxy and carbon monofilaments. Additionally, a distinct interwoven MWCNT structure was observed at the surface of the buckypaper. The observed MWCNT structure explains the increased surface roughness compared to the smooth carbon monofilaments. The increased surface roughness likely improves mechanical interlocking with the epoxy of an adjacent lamina. The interphase and subsurface characterization data at the nanoscale level explain a change in crack propagation toughness. The observed buckypaper surface exhibited inhomogeneous properties within a region of a few square micrometers. The nano-hardness of the BP can reach up to 400% of the averaged hardness of the BP over a few micrometers square area.  An enhancement in the fracture toughness was observed while there is no substantial change in the initiation energy release rate. The improvement in crack propagation energy is due to mechanical interlocking, crack path diversion, and the large interphase zone surrounding the buckypaper.