Interphase Engineering in Aramid-Fiber-Reinforced Epoxy Composites: From Surface Modification to Mechanical...

Aramid-fiber-reinforced epoxy composites are widely utilized in lightweight structural and protective applications; however, their performance is often limited by intrinsically weak and chemically inert fiber-matrix interfaces. Recent advances have shifted attention from conventional surface activation to deliberate interphase engineering, in which chemical functionality, hierarchical structure, and nanoscale reinforcement are integrated to regulate stress transfer and damage evolution. This review provides a comprehensive overview of interfacial design strategies for aramid/epoxy systems, including reactive surface activation, additive interphase construction, bioinspired polydopamine (PDA)-based coatings, and PDA/poly(ethyleneimine)-mediated hierarchical interphases. Particular emphasis is placed on the role of carbon nanotubes and hybrid nanofillers as interphase-active components rather than simple matrix additives, highlighting the importance of their localization, functionalization, and structural integration. This review further examines how interphase architecture influences mechanical responses under multiple loading conditions, including interlaminar shear, impact, tribological wear, viscoelastic behavior, and ballistic loading. Optimal composite performance depends on achieving an appropriate balance between strong interfacial bonding and damage-tolerant deformation mechanisms. By integrating recent experimental and conceptual developments, this review highlights the transition from "surface modification" to "interphase programming" and provides design guidelines for next-generation aramid fiber composites with improved strength, toughness, and multifunctionality.