Study On Reaction Modification Mechanism,microstructure And Properties Of Thermal Spraying Coatings

Titanium and titanium alloys usually have low density,high specific strength and the superior corrosion resistance,however,the mechanical properties of the alloys may be reduced when temperature rises above 600?,as the strength of the alloys is seriously affected by high-temperature oxidation caused by the infiltration of oxygen.In order to improve the oxidation resistance of Ti alloys while maintaining their outstanding performance,a coating with outstanding oxidation resistance can be added to the Ti alloys.First,a pure Al coating and double layer coatings containing Al element were deposited on the surface of pure Ti substrates by arc spray and plasma spray technology.Furthermore,by modifying spray coatings with furnace heating treatment,a protective intermetallic compound coating was formed through in-situ reaction between the Al coating and the Ti substrate,and the Al containing double layer coatings.The results showed that,for Al/Ti specimen,after heating treatment,the Al element in the coating could react directly with the Ti element in the substrate and the TiAl₃,TiAl₂,TiAl and Ti₃Al were then formed and became protective coatings.This Ti-Al intermetallic compounds layer was found the thickest when the heat treatment condition in the furnace was 900?/5h;for Al/Ni/Ti specimen,after heating treatment,the NiAl₃,Ni₂Al₃ and NiAl intermetallic compounds were formed in situ and became protective coatings.This Ni-Al intermetallic compounds layer was found the thickest when the heat treatment condition in the furnace was 800?/15h;in addition,Al/NiAl,Al/NiCr and Al/NiCu coatings were also prepared on the surface of Ti substrate,and the phase transitions between these elements were further studied by heating test in furnace.Furthermore,in order to carry out theoretical analysis on the the phase change process of pure Al and Al/Ni composite coating on the surface of Ti substrate,the first principle calculation was also carried out in this paper.The order of effective heat of formation?EFH?of Ti-Al intermetallics was as follows:TiAl₃<TiAl₂<TiAl<Ti₃Al<0;and the order of effective heat of formation?EFH?of Ni-Al intermetallics was as follows:NiAl₃<Ni₂Al₃<NiAl<0.The preferential order of various compounds formation of these two systems was analyzed from the thermodynamics point of view.The calculated results were consistent with the experimental phenomena in this paper.Then,in order to test the high-temperature oxidation resistance of the intermetallic compounds generated during the in-situ reaction,the high temperature oxidation tests were carried out on these intermetallic compounds coatings.The results showed that the intermetallic compounds coatings of Al/Ti,Al/Ni/Ti,Al/NiAl/Ti,Al/NiCr/Ti and Al/NiCu/Ti specimens obtained after heat treatment,which all had certain resistance to the diffusion of oxygen at high temperature.However,the oxidation resistance of NiAl intermetallic was better than that of TiAl₃ intermetallic.Among all kinds of modified coatings,Al/NiCr modified coating had the best oxidation resistance at high temperature.Finally,the laser remelting method was also used to modify the pure Al and Al/Ni double coating on the surface of Ti substrate,and the possibility of in-situ formation of intermetallic compound protective coating by this method was studied.The results showed that,for Al/Ti specimen,only a small amount of TiAl₃ intermetallic was generated in the remelting reaction zone after laser remelting.But for Al/Ni/Ti specimen,there were more equiaxed Ni₂Al₃ phases and a few dendritic NiAl phases were generated in the remelting reaction zone after laser remelting.In addition,the oxidation test results showed that the modified Al coating and Al/Ni coating after the laser remelting treatment both had a certain anti high temperature oxidation ability,but was not as good as that of the Al and Al/Ni coating modified by heating in the furnace.