Hybrid epoxy-LMPAEK composite co-cured joints: A study on UVO pretreatment and its effect on interlaminar toughness

Manufacturing complex composite structures often requires joining dissimilar materials, which is traditionally achieved using intermediates such as rivets or adhesives. When joining thermoplastic composites to thermoset composites, co-curing is an interesting alternative because it offers a direct connection without intermediates, potentially yielding improved mechanical properties in demanding applications at lower cost and weight. In this work, thermoplastic LMPAEK was bonded to carbon/Epoxy using an autoclave curing process based on two bonding configurations: (I) a thermoplastic LMPAEK film between thermoset C/Epoxy laminates; (II) thermoplastic C/LMPAEK directly bonded to thermoset carbon/Epoxy. For both configurations, an initial state of low-crystalline and high-crystalline LMPAEK was used. UV-ozone (UVO) treatment prior to the bonding process proved crucial for improving the surface energy of the thermoplastic LMPAEK, as confirmed by various surface analysis techniques. UVO exposure was shown to improve the fracture toughness of the interface between the thermoplastic and thermoset composite material (configuration I), as demonstrated by cohesive fracture in the C/Epoxy peel arm during mandrel peel testing. Results for configuration II showed better bonding performance for the original low crystalline C/LMPAEK tape than for the highly crystalline and smooth, press-consolidated specimens. The effect of the initial state of the thermoplastic composite surface suggests that other mechanisms also play a role alongside physical and chemical bonding, such as mechanical interlocking and interdiffusion.