Being the 7th most abundant element on the earth, magnesium and its alloys have become very promising in numerous industrial applications because of some of their distinguished properties, including high thermal conductivity, high strength/weight ratio, good machinability etc. However, some inferior properties especially the poor wear and corrosion resistance have obstructed a widespread use. As one of the most effective solutions, thermal spraying can deposit an adequate protective coating on the substrate, but the necessary grit-blasting operation before spraying leads to high grit-residues on the magnesium surface therefore degrading the deposit performance. The PROTAL® process, combining a laser surface preparation with the deposition stage, can avoid this kind of disadvantage. This paper presents the possibility of employing PROTAL® to prepare a resistant Ni-Cr coating on a magnesium substrate. The coating adhesion and interface microstructure were especially investigated.

Although it is widely used prior to the spraying stage to eliminate the surface contaminants and to improve the splat contact with substrate, a preheating operation should be precisely controlled to keep the metallic substrate away from an excessive oxidation. In most cases, the oxide scale can deteriorate the interface adhesion. A nanosecond pulsed laser treatment is a powerful tool to remove the surface oxide layers. It is also the fundamental principle of the PROTAL process that combines a laser surface preparation and the thermal spraying process. In this paper, a nanosecond pulsed laser was used to remove the thermally induced oxide while keeping the advantage of preheating. It is shown that the particle splashing was suppressed and the deposit adhesion was significantly improved resulting from the laser cleaning effects. The mechanism of oxide removal is also investigated.