Fracture toughness of single pyrolyzed carbon nanofibers

Carbon nanofibers (CNFs) made via pyrolysis of polymeric precursors are attractive for a wide range of load bearing applications. However, by virtue of network of covalent bonds, they are prone to catastrophic failure under complex loading. Fracture toughness provides a reliable parameter for assessing structural integrity. In this study, polyacrylonitrile (PAN) precursor fibers were electrospun and carbonized at 900 °C and 1400 °C to establish a detailed structure–property relationship in carbon nanofibers. For the first time, the fracture toughness of individual carbon nanofibers was directly measured and quantitatively linked to their inherent microstructural defects. Fracture toughness measurements showed a size-dependent toughening, increasing from 0.41 to 1.36 as fiber diameter decreased from ∼550 nm to ∼280 nm. In contrast, carbonization temperature had no measurable influence on fracture toughness. This is despite the fact that the carbon nanofibers made at higher temperatures were measurably more graphitic, indicating that the failure takes place within the amorphous regions of the material. Phase field continuum models provided further evidence in support of this failure mechanism. We showed that the Griffith theory of fracture adequately attributes, rather quantitatively, their fracture and failure to the surface flaws which were observed via high resolution imaging. These findings highlight the importance of diameter-controlled processing for enhancing the toughness of carbon nanofibers, while also providing fundamental insights into their flaw sensitivity and structure–property relationships.