Removal mechanisms and substrate morphology evolution in UV picosecond laser cleaning of radar absorbing coatings on aluminum alloys

Laser cleaning with a UV picosecond laser offers an effective solution for removing degraded radar absorbing coatings. This study systematically investigates the cleaning process, focusing on efficacy, substrate modifications, and removal mechanisms across a range of scanning speeds. Complete coating removal is achieved at 1500 mm/s, resulting in a clean surface with negligible oxidation. Lower scanning speeds result in spherical microstructures and coating residue, whereas extensive ablation prevails at 500 mm/s. The spherical microstructures increase surface roughness, which exhibits a non-monotonic dependence on scanning speed. At 1000 mm/s, surface roughness peaks at 2.52 μm, accompanied by a contact angle of 46.86° and a surface energy of 55.20 mJ/m². Corrosion resistance, determined by both wettability and microstructure, is worst at this speed and optimal at 1500 mm/s. The removal mechanism shifts from predominantly photochemical decomposition and plasma shock at 1500 mm/s to progressively dominant thermal ablation at lower speeds. At 500 mm/s, thermal ablation becomes the dominant mechanism, leading to coating carbonization and substantial residue formation. Notably, the near-surface crystal structure of the substrate remains unaltered after processing at the optimal speed of 1500 mm/s, confirming the negligible internal impact of the UV picosecond laser cleaning process.