Comparative aqueous sizing architectures regulate interfacial failure and load transfer in basalt fiber/epoxy composites

Aqueous sizing provides a scalable route to tailor the fiber/matrix interphase in basalt fiber/epoxy composites, but the influence of coating architecture on failure behavior and load transfer remains insufficiently understood. In this study, three aqueous sizing systems were comparatively applied to ethanol-cleaned basalt fibers under identical fiber, matrix and processing conditions, producing a silane/epoxy-compatible coating, a nano-SiO 2 /silane/polyurethane dispersion (PUD) rigid–flexible coating and a graphene oxide (GO)/carboxylated multi-walled carbon nanotube (MWCNT)-containing coating. Surface morphology and chemistry, bundle-scale apparent wetting, fracture behavior, apparent interlaminar shear strength and flexural properties were evaluated. All sizing treatments altered the fiber surface chemistry and promoted rapid water spreading or penetration. Among them, the nano-SiO 2 /silane/PUD system produced a comparatively continuous and uniformly roughened coating, accompanied by greater resin retention and reduced clean fiber pull-out after failure. Its apparent interlaminar shear strength increased from 19.35 ± 0.66 to 32.46 ± 1.46 MPa, while the flexural strength reached 425.02 ± 10.84 MPa, corresponding to improvements of 67.74% and 142.64%, respectively. Although the GO/MWCNT system exhibited a slightly higher mean flexural modulus, the difference from the nano-SiO 2 /silane/PUD system was not statistically significant, and its apparent interlaminar shear and flexural strengths remained lower. These results suggest that, within the investigated sizing systems, coating continuity, morphological uniformity, interfacial compatibility and mechanical interlocking are associated with improved composite mechanical response, whereas stiffness enhancement alone was insufficient to achieve the highest apparent ILSS and flexural strength.