Effect Of Microstructure And Surface Morphology Of Bond Coat On Growth Behavior Of Oxide Layer In Thermal Barrier Coating

The thermal efficiency of an aero-engine is directly related to the operating temperature of its hot-section components.However,the operating temperature of superalloys used in hot-section components is already near their maximum service temperature.To further enhance the thermal efficiency of aero-engines,it is necessary to spray a thermal barrier coating on the surface of the superalloy substrate to meet the requirements of heat insulation,oxidation,and corrosion protection at higher temperatures.Under high-temperature oxidation conditions,a thermally grown oxide(TGO)layer consisting of a dense α-Al2O3 layer and a loose porous mixed oxide layer is generated in the thermal barrier coating,which can effectively improve the oxidation resistance of the superalloy.The growth and failure behavior of the TGO layer are highly dependent on the microstructure and surface morphology of the bond coat.Therefore,this paper conducted an in-depth study on the growth behavior of TGO in MCr Al Y bond coat(Co-32Ni-21Cr-8Al-0.5Y,wt %)prepared by high-velocity oxygen fuel(HVOF),low-pressure plasma spraying(LPPS),and air plasma spraying(APS)from the microstructure and surface morphology.The main conclusions are as follows:1.The spraying method determines the microstructure and phase composition of the bond coat.The initial β phase content in the bond coat sprayed by LPPS was the highest,and a denseα-Al2O3 layer can be formed during oxidation.After isothermal oxidation for 1000 h,the thickness of α-Al2O3 in the sample sprayed by LPPS was greater than that in the sample by HVOF,and its oxidation resistance was also better.When the β phase was completely oxidized and exhausted,the growth of TGO was mainly dominated by the growth of loose and porous mixed oxides,causing large-scale delamination of the coating.The bond coat sprayed by HVOF contained a large amount of inner strip-shaped oxides.This structure can hinder the diffusion of aluminum atoms,inhibit the formation of dense α-Al2O3 during the oxidation process,and cause the preferential growth of loose and porous mixed oxides in the early stage of oxid ation,which eventually cause the premature delamination of the thermal barrier coating.2.The spraying method has a significant influence on the surface morphology of the bond coat and the growth process of the TGO.In the bond coat sprayed by HVOF,there are a large number of unmelted powders,which are the preferred location for oxide spalling.Through grinding the surface of the HVOF-sprayed bond coat and observing the oxide growth behavior,it was found that the thickness of the α-Al2O3 layer was not affected by the surface roughness in the early stages of oxidation.However,after 1000 h,the roughness had a decisive influence on crack propagation,leading to a large amount of oxide spalling.In contrast to HVOF,the surface roughness of the coating sprayed by APS was lower,but it contains a higher content of mixed oxides and residual stresses,which is the main reason for the failure of the APS-sprayed coating.In the coating sprayed by LPPS,a significant amount of α-Al2O3 was still observed after isothermal oxidation for 1000 h.Due to its suitable surface roughness and the lowest content of mixed oxides,no obvious delamination was found on the surface of the bond coat,only the formation of cracks was observed.