In-Process Exothermic Reaction in High-Velocity Oxyfuel and Plasma Spraying with SiO<SUB>2</SUB>/Ni/Al-Si-Mg Composite Powder

Abstract

Reactive thermal spraying, in which thermodynamically stable compounds are formed by expected inprocess reactions, has attracted considerable attention as a result of the wide availability of in situ composite coatings. Such in-process reactions occur differently in high-velocity oxyfuel (HVOF) and plasma spraying because of differences in the flame temperature and speed. In the current study, a composite powder of SiO2/Ni/Al-Si-Mg was deposited onto an aluminum substrate to fabricate in situ composite coatings by both spraying methods. The coating hardness sprayed with Al-Si-Mg core powder increases with silicon and magnesium content, whereas the coatings by HVOF spraying show higher hardness than those by plasma spraying. In the present reactive spraying, the exothermic reaction of SiO2 with molten Al-Si-Mg alloy leads to composite materials of MgAl2O4, Mg2Si, and Al-Si matrix. Moreover, a rapid formation of aluminide (NiAl3), which is introduced by an exothermic reaction of plated nickel with Al-Si-Mg core powder, enhances the reduction of SiO2, especially in HVOF spraying. A series of in-process reactions proceed mainly during splat layering on a substrate instead of during droplet flight even in plasma spraying. Plasma-sprayed composite coatings become much harder because of the great progress of in-process reactions.