Analyzing carbon fiber structures observed by helium ion microscopy and their mechanical properties

In this work we present images of carbon fiber fractured surfaces observed for the first time by Helium Ion Microscopy (HIM) at high magnifications, aiming to capture defects ranging in size between 10 nm and 100 nm. Nominal critical defect sizes of various commercial carbon fibers were calculated from the Griffith Equation and their critical defect size distributions were calculated from their Weibull strength distributions. HIM images of the carbon fibers with the standard modulus (~230 GPa), intermediate modulus (~280 GPa), high modulus (>350 GPa) and ultra-high strength (>6.8 GPa) are shown to have contrasting microstructures. Some of the three dimensional (3D) structural features, like crystalline sheets, flakes, and sets of amorphous material, have not been previously captured by Scanning Electron Microscopy (SEM). Furthermore, a representative intermediate modulus carbon fiber was mechanically tested by the single filament test method and its strength characteristics were corelated to structural features observed by HIM. HIM imaging offers a new, reproducible, and feasible technique to analyze the original 3D nanostructures of carbon fibers.