NANOSCALE INTERPHASE CHARACTERIZATION OF POROUS CNT BUCKYPAPER COMPOSITES IN CORRELATION TO INTERLAMINAR MODE I FRACTURE
In this conference paper, nanoscale material property data and ASTM mode I interlaminar fracture results for three-phase buckypaper samples are presented and analyzed. Vacuum filtration and surfactant-free methods were used to manufacture buckypaper membranes. 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), using probes with nominal tip radius in the range of 5 to 8 nm were used. PFQNM fully characterized 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. The buckypaper and epoxy interphase thickness was found to be 50nm, higher than the 10-40nm reported for epoxy and carbon monofilaments. The observed MWCNT structure provides explanation of the increased surface roughness compared to the smooth carbon monofilaments. The increased surface roughness likely improves mechanical interlocking with the epoxy of adjacent lamina. The interphase and subsurface characterization data at the nanoscale level provide explanation for a change in crack propagation toughness. Nanoscale analysis of the buckypaper surface proved the inhomogeneous properties even at the scale of a few square micrometer. The improvement in crack initiation and propagation energy is due to mechanical interlocking, crack path diversion, and the large interphase zone surrounding the buckypaper.