Ablation par laser pulse de revetements antierosion pour le domaine aeronautique.

摘要

Erosion resistant coatings (ERCs) are frequently used to protect aircraft engine components against erosion, and therefore, to extend their lifetime and reduce maintenance cost. However, after many hours in service, certain areas of the coating will begin to deteriorate. Given that such components are generally very costly, it is desirable to replace only the coating instead of the part itself.;This research is part of the MANU 4 project, supported by CRIAQ and NSERC, which aims to study the feasibility of stripping an erosion-resistant coating deposited on a titanium-based alloy with three different techniques: wet chemical etching, plasma etching and pulse laser ablation. This thesis focuses more specifically on the etching with a pulsed laser of a 20-mum thick TiAlN ERC deposited on a Ti-6Al-4V substrate. This work compares the suitability of two pulsed lasers: a femtosecond Ti:Sapphire laser emitting at 800 nm and a nanosecond KrF excimer laser centred at 248 nm. These two lasers were chosen since they are frequently used for micromachining applications and allow us to study the effect of the wavelength and pulse duration.;Preliminary findings have allowed us to identify four most critical variables that influence the etch rate and the surface roughness: (i) beam size, (ii) laser power, (iii) stage speed, and (iv) step distance between scanned lines. For each laser, optimal etching conditions were obtained by varying a single parameter at a time. Final results show that the higher energy per pulse offered by the excimer laser allows one to increase by one order of magnitude the etch rate, but almost doubles, from 1 mum to 1.8 mum, the surface roughness, in comparison with results obtained with the Ti:Sapphire laser.;Compared with other techniques, pulse laser ablation has the potential to offer very high selectivity. In this regard, plume emission spectroscopy was studied as an in situ technique to monitor the etching progress and determine the precise moment when the laser beam reaches the surface of the substrate. Despite chemical similarities between the coating and the substrate, we show that it is possible to monitor the decrease of the nitrogen line with respect to the titanium reference peak in order to diagnose, in real time, when the etching process should stop. Yet, more effort is still needed to define more precisely the etch stop criteria as a function of the applied laser fluence.;Finally, several surface characterization techniques were used to assess the effects of the stripping process on the surface of the substrate after ablation. Contact profilometry has confirmed that the surface roughness is increased to 1-2 mum, compared to the 0.6 mum average roughness of the coating before stripping. EDX and XRD measurements have also confirmed the presence of oxides at the surface of the substrate, a consequence of the oxidation of titanium during the ablation process at atmospheric pressure. Finally, nanoindentation measurements of the polished cross-section of the etched samples have shown that the hardness is not significantly affected during the etching process.