In-Process Exothermic Reaction in HVOF and Plasma Spraying with SiO2/Ni/Al-Si-Mg Composite Powder

Abstract

Abstract Reactive thermal spraying, in which thermodynamically stable compounds are formed by expected in-process reactions, has attracted considerable attention as a result of wide availability to in-situ composite coatings. Such inprocess reactions differently proceed in HVOF and plasma spraying because of differences in the flame temperature and speed. In the present study, composite powder of SiO2/Ni/Al-Si-Mg was deposited onto an aluminium 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 mainly proceed during splat layering on a substrate, instead of during droplet flight even in DC plasma spraying. Plasma sprayed composite coatings become much harder due to the great progress of in-process reactions.