Phosphorus‐Functionalized Interfacial Architectures Enabling Synergistic Mechanical Reinforcement and Flame Retardancy in Basalt Fiber/Epoxy Composites

ABSTRACT Insufficient interfacial compatibility and inherent flammability of polymer matrices are two critical factors that limit the practical application of basalt fiber–reinforced polymer composites (BFRPs). In this study, an interfacial engineering strategy based on the in situ growth of nano‐SiO ₂ is proposed. DOPO‐functionalized linear and cage‐like interfacial structures are then constructed on the basalt fiber surface using silane coupling agent (KH560) and polyhedral oligomeric silsesquioxane (POSS), respectively. Our results demonstrate that interfacial modification remarkably strengthened fiber–resin interfacial bonding, leading to significant improvement in tensile and flexural properties. Additionally, the modified composites exhibit substantially increased limiting oxygen index values and both achieve a UL‐94 V‐0 rating, accompanied by effective suppression of heat release during the combustion. Mechanistic analysis reveals that the enhanced flame retardancy originates from the synergistic action of radical quenching by DOPO in the gas phase as well as the formation of a compact, insulating char layer promoted by silicon‐based structures in the condensed phase. Notably, the DOPO‐POSS system exhibits more pronounced advantages in interfacial reinforcement and char‐layer stability owing to its three‐dimensional cage architecture. This work offers an effective interfacial engineering strategy for the design of BFRPs with simultaneously enhanced mechanical performance and fire safety.