Cold-spraying coupled to nano-pulsed Nd-YaG laser surface pre-treatment

The effect of Al2017 substrate pre-treatment using pulsed laser ablation on adhesion strength of cold-sprayed Al coating is examined. A high energy pulsed laser beam was coupled with a cold-spray gun to result in laser ablation of the substrate surface a few milliseconds prior to the deposition. The influence of the laser fluence and repetition rate on substrate surface morphology and physico-chemical properties are investigated. Coating-substrate interfaces were observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of thin foils which were prepared using focused ion beam (FIB). Adhesion strength was evaluated by means of finite element method (FEM) of LAser Shock Adhesion Tests (LASAT). The results are compared to samples prepared on as received substrate, and pre-treated by two conventional methods, namely polishing and grit-blasting. It is shown that the coating-substrate interface is significantly improved when pulsed laser ablation is performed at optimized parameters. No oxide layer was found at the coating-substrate interface on laser ablated sample while two oxide layers were found on the as-received sample indicating that particle impingement transformed the native alumina layer in an amorphous Al oxide phase. The observations allow concluding that bonding of cold spray Al particles on Al2017 substrate requires either the removal of the native oxide layer or its transformation in an amorphous Al oxide phase.