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.