Influence of defocus distance on surface characteristics in nanosecond pulsed laser cleaning of high-thickness paint layers

Laser cleaning is a promising alternative to conventional sandblasting for marine coating removal because it reduces secondary waste and contaminant emissions. However, removal of thick paint systems requires high energy input, which can cause substrate remelting and surface deterioration. In this study, a nanosecond pulsed laser was used to remove a thick marine paint system (∼1000 µm) coated on KE36 shipbuilding steel. The effect of defocus position on coating removal depth, surface morphology, and substrate integrity was systematically investigated under different pulse overlap numbers, pulse energies, and scanning pitches. Increasing the positive defocus distance enlarged the beam diameter and redistributed the laser energy over a wider interaction area, thereby reducing the peak energy density. As a result, the removal depth decreased, while the cleaned surface became smoother and more uniform. Cross-sectional observations showed that focused irradiation provided high removal capability but induced remelting and sharp interfacial features, whereas defocused irradiation effectively suppressed substrate remelting and edge damage. The reduction in removal depth caused by defocusing could be partially compensated for by increasing the pulse overlap number and pulse energy and by decreasing the scanning pitch. Surface roughness decreased from approximately 18 µm under the focused condition to about 11 µm under defocused conditions. These results demonstrate that defocus control is an effective strategy for balancing coating removal efficiency and substrate preservation during pulsed laser cleaning of thick marine paint systems.