Engineering the flax/epoxy interface: Impact of iron oxide fibre functionalization revealed by microbond testing

The fibre/matrix interface plays a crucial role in governing the mechanical performance and durability of composite materials. Surface functionalization of fibres can improve interfacial adhesion by tailoring the chemical or physical affinity between constituents. At the microscale, the microbond test, based on the debonding of a matrix microdroplet deposited on a single fibre, is widely used to quantify the interfacial shear strength (IFSS). This study examines the influence of iron-oxide-based functionalization on the interfacial behaviour of flax/epoxy composites using microbond tests. To ensure repeatability and precise control of loading, a dedicated in-house testing device was developed. A total of 50 droplets for native flax fibres and 57 droplets for functionalized fibres were debonded, demonstrating that iron oxide functionalization increases the IFSS by about 16%. Complementary morphological characterizations, including Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray spectroscopy (EDX), and Focused Ion Beam (FIB) analyses, were performed before and after debonding. These observations reveal that the oxide coating is heterogeneous but continuous along the fibre, with a thickness ranging from a few hundred nanometres to several micrometres and clarify the role of the oxide layer in local failure processes. Based on these results, a failure scenario for the functionalized flax/epoxy interface is proposed. Debonding initiates at the fibre/oxide interface, propagates within the oxide layer through microcracking and intergranular fracture, and ends at the matrix/oxide interface during droplet debonding. This multi-stage process highlights the complex role of the oxide layer, enhancing adhesion while introducing specific fracture pathways.