Selective pulsed laser stripping of TiAlN erosion-resistant coatings: Effect of wavelength and pulse duration

The lifetime of titanium-alloy aerospace parts can be greatly enhanced by the application of erosion-resistant coatings (ERC). After many hours in service, the coating will inevitably begin to deteriorate. To permit re-coating and repair of the parts, the coating should be removed. We present here a first attempt to strip TiN-based, 20 μm-thick, coatings while comparing the suitability of two pulsed lasers: a femtosecond Ti:Sapphire laser emitting at 800 nm, and a nanosecond excimer laser centered at 248 nm. Following initial comparison of ablation thresholds for each laser, we optimized four critical laser parameters: power, beam diameter, stage speed and step distance. The results indicate that surface roughness can be improved by 35% with the femtosecond laser in comparison with the nanosecond laser, while the higher energy per pulse offered by the latter one accelerates the etch rate by one order of magnitude. Surface composition analysis confirmed full removal of the coating and higher oxidization of the substrate with the excimer laser. Nanoindentation measurements on the polished cross-section of stripped samples show that the ablation process does not influence the hardness of the remaining substrate. To increase selectivity, plume emission spectroscopy has been investigated, in order to control, in real time, the material removal in regions where the coating thickness may vary.