Reactively thermal sprayed coatings based on in-process reactions during flight and after impingement offer superior properties compared with conventional coatings because it has in-situ formed fine and uniformly dispersed stable hard phases. In the present work, composite powder composed of plated nickel, fine SiO 2 particulates and Al-Si-Mg core particles with water glass binder (SiO 2 /Ni/Al-Si-Mg) was deposited onto an aluminum substrate to fabricate composite coatings by using HVOF (high velocity oxyfuel), RF (radio frequency) and DC (direct current) plasma spraying methods. The amount and constituents of phases formed during reactive thermal spraying were found to be different depending upon the methods used where in-process reactions differently proceed. Consequently, reactively sprayed composite coatings mainly consist of Mg 2 Si, MgAl 2 O 4 , NiAl 3 and Al-Si matrix through the exothermic reaction of SiO 2 and nickel with molten Al-Si-Mg alloy. The depletion of magnesium in the composite powder is responsible for the obtained lower hardness of composite coatings sprayed by RF plasma spraying which offers the highest molten droplet temperature.

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 SiO 2 /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 SiO 2 with molten Al-Si-Mg alloy leads to composite materials of MgAl 2 O 4 , Mg 2 Si and Al-Si matrix. Moreover, a rapid formation of aluminide (NiAl 3 ), which is introduced by an exothermic reaction of plated nickel with Al-Si-Mg core powder, enhances the reduction of SiO 2 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.