Ultra-Thin interphase in Flax/Epoxy composites revealed by AFM PF-QNM nanomechanics

• The existence of a transition zone at the fibre/matrix interface is investigated. • A near-field measurement technique is applied on a flax/epoxy composite. • Nanomechanical AFM mappings finely assess the elastic properties of the interphase. • An ultra-thin (around 80 nm) transition zone is observed and described. The fibre/matrix interface is a critical yet poorly understood feature of natural-fibre-reinforced composites, where load transfer and damage initiation are controlled. At the nanoscale, this boundary may evolve into an interphase, a narrow region with a gradient of mechanical properties, but its occurrence in bio-based systems remains unclear. Here, we use Atomic Force Microscopy in PeakForce Quantitative Nanomechanical Mapping mode (AFM-PF-QNM) to directly probe the nanoscale elastic modulus across flax/epoxy interfaces. Systematic mappings of multiple fibres reveal that, under the present conditions, an interphase is not consistently formed. When present, it is extremely thin (≈ 40–120 nm) and exhibits a distinct modulus gradient. These findings provide the first nanoscale evidence of the ultra-thin and irregular nature of interphase formation in flax/epoxy composites, challenging the assumption of a uniform and thick interphase in plant-fibre systems and offering new insights for interface engineering.