Tensile properties, void contents, dispersion and fracture behaviour of 3D printed carbon nanofiber reinforced composites

In this paper, additive manufacturing technology has been used in processing carbon nanofiber (CNF) reinforced thermoplastic composites through fused deposition modeling (FDM). The effects of nanofiber concentrations, nozzle geometries on void contents, and tensile properties of FDM printed nanocomposites have been studied. The contact angles and void geometries have been characterized as a function of bead spreading orientations. Such measurements were carried out using micrographs of optical and scanning electron microscopy (SEM) examinations. The dispersion and orientation of nanofiber in polylactic acid (PLA) matrix have been studied using transmission electron microscopy observations. Finally, the fracture surfaces of both neat PLA and CNF/PLA nanocomposites have been investigated in order to understand the failure mechanism. The results show improved modulus, strength, and strain up to elastic limit in CNF/PLA nanocomposites in comparison to neat PLA matrix. The square nozzle geometry shows increased tensile strength and reduced void geometry in comparison to circular shaped nozzle. Three types of voids such as inter-bead, intra-bead, and interfacial bead were observed with total voids ranging from 24% to 30% with circular nozzle. The level of void content is reduced to about 7% for square nozzle. Contact angles and bead orientation play an important role in forming void contents. CNF/PLA nanocomposites exhibit decreased contact angle and higher void contents in comparison to neat PLA resin. Transmission electron microscopy study of CNF/PLA nanocomposites show uniform dispersion of carbon nanofibers with aligned orientation in both composite filament and three-dimensional printed specimen. Fractured surfaces of CNF/PLA nanocomposites show fiber pullout mechanism and comparatively coarse surface topography indicating higher fractured energy than neat polylactic acid resin.