Flat-top picosecond laser texturing of CFRP: Biomimetic hierarchy for tunable wettability and high-strength AA7075/CFRP bonding

Abstract

Carbon-fiber-reinforced polymer (CFRP) composites are widely used in lightweight aerospace, automotive, and wind-energy structures owing to their high specific strength and stiffness and durability. However, dissimilar bonding is highly sensitive to surface conditions: CFRP surfaces exhibit low surface energy and may retain mold-release agents, and conventional pretreatments have limitations in controlling and homogenizing surface topography and chemical activation, which undermines interfacial adhesion and reliability. This work presents a 355 nm flat-top picosecond-laser approach that integrates bioinspired hierarchical texturing with surface activation, enabling programmable construction of tree-frog–inspired “plateau-connected-groove” patterns and concomitantly enhancing surface polarity. Roughness and feature depth increase with fluence; the water contact angle decreases to 8.0°; and x-ray photoelectron spectroscopy indicates higher fractions of oxidized carbon species, while the Raman I_D/I_G ratio remains essentially unchanged. In AA7075/CFRP single-lap-shear tests, the lap-shear strength increases from 5.9 to 19.6 MPa at 5.8 J cm−2, accompanied by a transition from interfacial adhesive failure to fiber-dominated mixed failure. Mechanistically, at an appropriate fluence, hierarchical micro-/nanostructures, together with enhanced surface polarity, enable synergistic mechanical interlocking and chemical coupling; by contrast, excessive fluence can introduce voids and microcracks and promote partial Cassie wetting. The non-contact, programmable process is compatible with complex geometries and automation, providing a high-throughput surface-engineering route for durable joining in CFRP–Al lap joints and stiffeners in aircraft, bonds between CFRP skins and aluminum honeycomb/cores; mixed-material joints in automotive body structures, and wind-turbine blade root–insert interfaces.