Examining Incoherent UV Light as an Assistant to IR Laser on Texturing of CFRP Composites for Adhesive Joining

Carbon Fibre Reinforced Polymers (CFRPs) are widely used in aerospace and automotive industries for their high strength-to-weight ratio, but joining their structures presents challenges. Adhesive bonding is preferred to maintain lightweight advantages, yet strong bonds require precise surface preparation. Laser surface processing, a contactless method, has widely been applied to surface preparation for various materials and extensively explored for CFRPs. This technique shows promise in improving adhesion by removing contaminants, adding surface texture to enhance mechanical interlocking, activating surface chemistry, or fully removing the matrix outer layer to expose carbon fibres for direct bonding. On the other hand, incoherent UV light treatment has been studied across a range of materials, including polymeric and non-polymeric substrates. It breaks down polymer or hydrocarbon bonds at the surface, activating surface chemistry and enhancing adhesion and bonding performance. This research examines the effect of incoherent Ultraviolet (UV) light on enhancing the effectiveness of Infrared (IR) lasers in improving adhesive bonding in CFRP/CFRP structures. Two laser systems were used: a nanosecond (ns) pulsed Near-Infrared (NIR) fibre laser at 1064 nm and a Continuous-Wave (CW) Mid-Infrared (MIR) Carbon Dioxide (CO2) laser at 10600 nm. The UV light was provided by a 46 W germicidal lamp emitting at 254 nm. The study optimised and evaluated the three surface treatment methods individually, followed by combining each laser with UV light to explore their effectiveness. Adhesive bonding was assessed using Single Lap Shear (SLS) tests, with mechanical abrading included for comparison. Various CFRP materials with woven and Unidirectional (UD) reinforcement were used. In comparison to as-received samples, NIR laser treatment with 200-nanosecond pulses enhanced bonding by over 60%. Shorter pulses posed a higher risk of fibre damage, resulting in significantly lower bonding improvement. Laser processing of woven CFRPs presented challenges due to the highly variable outer matrix thickness. CW CO2 laser treatments resulted in slight bonding enhancement, with a risk of fibre-matrix debonding. UV treatment alone improved bonding by 75%, outperforming NIR laser treatments, with Cohesive Substrate Failure (CSF) mode indicating stronger adhesion than the material's interlaminar strength. Combining UV with laser treatments provided limited additional benefit. Mechanical abrading showed a 35% improvement in bonding, offering an industry-relevant comparison. Incoherent UV light treatment significantly enhanced bonding strength and offered a cost-effective alternative.