Effect Of Nocolok Flux On The Interfacial Structure And Bonding Strength Between Thermally Sprayed Coating And Substrate

The bonding quality of thermal sprayed coating is the key factor that determinesthe coating property. It was reported that the strong bonding was obtained by using theNocolok flux surface preparation on aluminum substrate. In order to simulate the splatformation, an experiment was designed and conducted, in which a millimeter-sizedroplet fell freely and impacted on substrate based on analogy of Reynolds number.The microstructural characteristics and the bonding strength between splat andsubstrate were studied. The nickel coating was plasma sprayed on ZL101substratewith flux pretreatment. In order to investigate the effect of flux on the adhesion andmicrostructure of thermally sprayed coatings, the bonding strength and the interfacialmicrostructure of coatings, combined with the individual splat morphology wereanalyzed with different surface preparation. Meanwhile, the thermal conductionprocesses of the millimeter-sized droplet and thermal sprayed droplet have beenstudied numerically to explore the effect of flux preparation on the coating formation.The results showed that the substrate melting phenomenon was observed and themetallurgical bonding layer was formed between splat and substrate resulting in ahigher interfacial bonding strength. When the initial substrate temperature wasincreased and the impacted velocity of droplet was decreased, the degree of substratemelting was increased. In the case of substrate melted intensively, the degree ofmetallurgical bonding was increased in local area between splat and substrate whenthe flux laid on the substrate. But the interfacial microstructure had not been changedobviously. A higher melting degree of substrate would be observed, when the copperdroplet impacted on ZL101substrate with flux pretreatment.In the case of thermal sprayed splats on flat substrate (Ra=0.275μm) with fluxsurface preparation, the bonding area between splat and substrate would decrease andthe coating would fall off from the substrate with the effect of thermal stress. However,on rough substrate (Ra=11.656μm), the bonding strength of coating would beincreased obviously. Further research showed that in the case of no vaporization ofdroplet impacted on substrate with flux pretreatment, a higher bonding strength ofcoatings would observe with the initial substrate temperature and the droplettemperature were increased.The numerical results showed that increasing interfacial heat transfer coefficient, substrate temperature, and droplet temperature resulted in an increase in thepossibility of substrate melted. In the case of millimeter-sized droplet impacted onsubstrate with flux pretreatment, interfacial heat transfer coefficient was increased,resulted in a great melting degree of substrate. During the formation of thermalsprayed coatings, the interfacial heat transfer coefficient was increased with the effectof flux, which would provide a great opportunity of the interfacial metallurgicalbonding.