Precision paint stripping of multi-layer aerospace coatings on CFRP using femtosecond lasers: ablation mechanisms and substrate integrity

Damage-free removal of multi-layer paint coatings from carbon fibre reinforced polymer (CFRP) structures remains a significant challenge in the aerospace industry. Conventional chemical, mechanical and nanosecond (ns) laser paint removal methods pose risks of substrate damage, slow processing, and environmental hazards. Femtosecond (fs) laser processing offers high precision and minimal thermal impact, but its application in aerospace paint stripping has been limited due to an incomplete understanding of removal efficiency, ablation mechanism, and substrate response. This work establishes a comprehensive experimental framework to quantify layer-specific ablation thresholds, removal efficiency, and substrate integrity during fs laser stripping of multilayer aerospace coatings. Single-shot and raster-scanned ablation tests were performed to identify the ablation thresholds of the topcoat, primer, and CFRP substrate, while systematically evaluating the effects of fluence, repeat passes, and overlap ratios on removal depth. Multi-scale characterisation, including optical microscopy, SEM/EDS, high-precision coordinate measuring system and micro-CT, was employed to evaluate surface morphology, coating-substrate transition behaviour, and subsurface fibre integrity. The results show that the fs ablation thresholds for both coating layers are below 0.25 J/cm², significantly lower than those reported for ns laser paint stripping, enabling selective and energy-efficient paint removal. The paint removal depth increases linearly with the number of passes (N), but nonlinearly with laser fluence (F), and depends strongly on overlap ratios (η_x and η_y). An optimised parameter set (F = 1.91 J/cm², N = 4, η_x = 85 %, and η_y = 85 %) achieved complete paint layer removal of ∼60 μm with no detectable subsurface damage. The mechanistic analysis indicates that fs laser ablation is governed by strong-field ionisation, Coulomb explosion, and confined thermal effects, resulting in an extremely small heat-affected zone (HAZ). The process-mechanism insights and validated parameter conditions provided in this study offer practical guidance for implementing fs laser paint removal in aircraft maintenance and repair operations, addressing a critical knowledge gap in aerospace surface engineering.