In-situ interface characterization of mesophase pitch-based carbon fiber/epoxy composites

Mesophase pitch-based carbon fibers (MPCF) are ideal reinforcement for composite materials used for spacecraft due to their extraordinary thermal conductivity and high stiffness. However, their practical applications are severely limited by a lack of understanding about fiber-matrix interface interactions. Here, through a joint study of fiber push-out testing and molecular dynamics simulations, we investigated the effect of fiber microstructure on the interfacial shear strength of MPCF-reinforced epoxy composites with the microstructure of MPCF being tailored by different spinneret geometries. Our results showed that the surface roughness and interface contact mode are two predominant factors determining the interfacial shear strength (IFSS), which contradicts the common intuition that carbon fibers with higher degree of graphitization generally leads to worse interfacial properties. Owing to the enhanced surface roughness and full edge-plane contact mode, the skin-core structured MPCFs exhibit an IFSS of 17.83 MPa, which is nearly 3 times that of the split-radial structured MPCFs (5.14 MPa). The distinct interface behavior between epoxy and MPCFs with different microstructures, as revealed by molecular dynamics (MD) simulations, stems from more pronounced mechanical interlocking and enhanced intermolecular attraction at the interface. These findings offered significant guidelines for fabricating MPCFs with high thermal conductivity without sacrificing the interfacial load transfer ability through the modulation of surface roughness and interface contact modes. • The interfacial shear strength of composites reinforced by skin-core and split-radial structured MPCFs is characterized by fiber push-out testing. • The mechanism of how spinneret-tailored microstructure affect the interfacial load transfer capability is revealed. • The surface roughness and interface contact mode are two predominant factors in determining the IFSS of composites.