Experiment and simulation of plasma evolution induced by variable power laser paint removal based on LIBS spectral analysis

In order to elucidate the evolution of laser-induced plasma and its mechanistic role in the removal efficiency during laser cleaning of composite coatings on civil-aircraft skin, a Q-switched nanosecond laser was employed to ablate an acrylic–polyurethane coating system from Al-alloy substrates at tunable average powers (30, 40, and 50 W). The macro and micro morphology of the paint removal surface was analyzed using multiple techniques. Laser‑induced breakdown spectroscopy (LIBS) was used to identify the excited species and to determine the maximum electron temperature and electron density. The transient COMSOL model of electron temperature field and density field was established. Combined with the high-speed image of laser plasma, the evolution of laser-induced plasma under three power conditions and its influence on the paint removal process were revealed. The results show that with the increase of laser power, the thickness of residual paint layer gradually decreases, the distance between ablation pits on the cleaned surface gradually increases, and the maximum temperature and electron density of plasma gradually increase; Paint neutralizer (NaOH) and functional particles (BaSO4 and CaCO3) are the main sources of the induced plasma, and Na is the dominant element of the induced plasma; The relative errors between the simulated and LIBS‑determined values are less than 3.2% for the maximum electron temperature and less than 4.0% for the maximum electron density. Key findings reveal that residual plasma surviving after pulse termination shields subsequent pulses, which constitutes the fundamental origin of the characteristic “pit–spacing” periodic morphology observed on the cleaned surface. By integrating multiple diagnostic techniques, this study clarifies the pivotal role of plasma shielding in laser cleaning, thereby providing a theoretical foundation for optimizing high-quality, high-efficiency laser-cleaning protocols of aeronautical composite coatings.