Preparation And Characterization Of Thermal Barrier Coatings On Magnesium Alloy

Magnesium(Mg)alloys becomes the first choice material for light structure in automobile industry due to a series of advantages such as low density,high specific strength and stiffness,free machining and so on.Mg alloys are called as green engineering materials in the 21 st century and play an important role in the fields of aerospace,national defense military and transportation.However,the standard electrode potential of Mg alloys is-2.37 V,which is a highly electronegative metal with poor corrosion resistance.In addition,the melting point of Mg alloy is 650°C and the long-term service temperature is lower than 200°C,which restricts its application under high temperature conditions.Thermal barrier coating(TBCs)is an effective surface modification method to improve the corrosion resistance,thermal shock resistance and wear resistance of Mg alloys.However,the thermal effects of high temperature particles on the substrate during thermal spraying,as well as the large thermal expansion coefficient difference between the substrate and the bonding layer have an adverse impact on the performance of the coating.In this study,an interlayer with low melting point,moderate thermal expansion coefficient and good chemical compatibility was prepared between Mg alloy substrate and thermal barrier coatings.The influence of phase composition structure,surface topography and cross-section topography on the corrosion resistance and thermal shock resistance of the coating was studied.In order to optimize the structure of TBCs on the surface of Mg alloy,an interlayer with moderate thermal expansion coefficient and good bonding with the substrate was prepared on the surface of Mg alloy.Sprayed pure Al coating,pure Zn coating and electroless plated Ni-P layer acted as interlayer.The effects of different interlayers on the properties of the coatings were investigated.Compared with Zn coating,the surface of Al coating by HVOF is rough,and the existence of transverse crack defects reduces the bonding strength between Al layer and substrate.The Al-TBCs coating system falls off before the Zn-TBCs under the action of stress,but the corrosion resistance of the Al layer is superior to that of the Zn coating due to the protective effect of the dense oxide film Al2O3.The Ni-P coating has compact structure and good chemical compatibility with NiCrAlY bond coat.The formation of passivation film in the Ni-P coating improves the corrosion resistance of the substrate.Ni-p coating eliminates the thermal influence of thermal spraying flame flow on the substrate and effectively absorbs and weakens the thermal stress,which improves the thermal cycle life of the coating system.Mismatching of thermal expansion coefficient and galvanic corrosion of matrix at bonding interface are the main reasons of coating failure.The properties of the interlayer with different mass ratios were investigated to optimize the composition of the interlayer on the surface of magnesium alloy.The A15Z85,A30Z70 and A45Z55 interlayer is well combined with the substrate.Besides,there is no obvious thermal influence layer on the surface of the substrate.The surface roughness,corrosion resistance and thermal cycle life of the coating increase with the increase of Al content.