Effect of OA-POSS Nano-Modification on the Microstructure of Ultrahigh Modulus CF and the Corresponding Failure Mechanism of CF/EP Composites

CFRP composites face limited application due to weak interfacial bonding, especially for ultrahigh modulus carbon fibers (CFs) with a tensile modulus exceeding 500 GPa. In this study, a novel approach involving the grafting of octaaminopropyl polyhedral oligomeric silsesquioxane (OA-POSS) nanoparticles onto ultrahigh modulus CFs was developed to optimize the interfacial bonding with ultrahigh modulus CF and epoxy resin (EP). Comprehensive characterization demonstrated that OA-POSS modification significantly altered the physicochemical structure of the fiber surface; for example, surface roughness increased from 10.3 to 18.1 nm, and the O/C ratio increased significantly from 0.060 to 0.232, confirming the successful introduction of abundant polar functional groups by OA-POSS. The CF-POSS/EP composites exhibited a peak compressive strength of 282.60 MPa (at 3 wt% POSS) and an interlaminar shear strength of 63.05 MPa (at 2 wt% POSS), corresponding to improvements of 31.50% and 20.70%, respectively, over the untreated samples. The performance improvement stems from OA-POSS nanoparticles forming covalent bonds via reactive amino groups, enhancing mechanical interlocking through increased surface roughness, and acting as a stress-relief interphase to promote crack deflection and energy dissipation. This altered the failure mechanism from interfacial separation to matrix internal cracking, enabling more efficient stress transfer across the interface.