Effect of fiber surface modification on mechanical and thermal properties of milled carbon fiber reinforced PEEK composite filaments and their FDM-3D printed composites

Fused deposition modeling (FDM) additively manufactured milled carbon fiber reinforced polyether ether ketone (MCF/PEEK) composites have received widespread attention in aerospace and other high-performance fields. However, the prevalent problems of poor adhesion and wettability between the fibers and the matrix have resulted in insufficient interfacial bonding, which severely limits the further improvement of composite properties. Therefore, this study systematically investigated the effects of two fiber surface modification strategies, polyetherimide (PEI) sizing and PEI combined with nanofillers sizing, on the physical and chemical structure of fiber surfaces and the properties of composites. The results demonstrated that MCF, PEI sizing resin, and hybrid fillers were tightly bonded through electrostatic and hydrogen bonding interactions, forming a strong interface. Surface modification reduced the crystallization performance of composites to varying degrees, but significantly enhanced the mechanical properties of the filament. The PEI modified composites achieved the highest tensile strength (170.8 MPa), which was 16.3 % higher than that of the unmodified composites. Compared with the PEI modified composites, the introduction of hybrid fillers for synergistic modification slightly reduced the tensile strength due to the enrichment of thicker interfacial transition layers, but increased the elongation at break by 20.20%. The effects of surface modification on FDM-3D printed composites were similar to those of filaments, and its density and elongation at break were further improved compared to filament. The FDM-3D printed composites modified by PEI showed the highest tensile and bending strengths, at 140.7 MPa and 273.3 MPa, respectively, which were 7.49 % and 17.20 % higher than those of the unmodified composites.