Study On The High-temperature Oxidation Behavior And Mechanism Of NiAlX(X=Hf、Y) Bond Coatings With Different Phase Structures

As a typical high-temperature protective coating for hot-end components such as aero-engines and gas turbines,the thermal barrier coatings bond coating aims to improve the high-temperature oxidation resistance of the components.The conventional MCrAlY bond coating can no longer meet the current service environment of thermal barrier coatings,while NiAlX（X=Hf,Y and other reactive elements）bond coating is regarded as one of the most promising materials for bond coating because of its excellent high-temperature oxidation resistance.Usually Ni-based high-temperature alloy matrix consists of（γ+γ’）phase,while the conventionalβ-phase NiAlX coating is less tough due to its high Al content and is prone to interdiffusion with the matrix and precipitation of topologically close-packed phases（TCP）,thus weakening the high-temperature stability of the coating;γ’phase NiAlX coating has a similar phase structure with high-temperature alloy,in the matrix interface will not occur interdiffusion zone,and can improve the coating high-temperature strength and toughness.In order to make the bond coating have good resistance to high-temperature oxidation and mechanical properties at the same time,（β+γ’）biphasic andγ’phase NiAlX（X=Hf,Y）coatings are designed.In addition,the oxidation mechanism and properties ofβandγ’phases,as typical storage Al phases in the coatings,are important guidelines for the application of the coatings.The（β-phase,γ’phase）NiAlX（X=Hf,Y）alloys with different phase structures and coating targets were prepared by vacuum arc melting,followed by the preparation of NiAlX（X=Hf,Y）coatings with different phase structures（（β+γ’）biphasic,γ’phase）on DZ125 alloy by EB-PVD,and the NiAlX alloys and coatings were vacuum annealed.Then the TGO phases,morphology and composition,stress state,and fold degree of the materials after high-temperature oxidation were characterized by X-ray diffractometer,scanning electron microscope,Raman spectrometer and laser confocal microscope,and finally the oxidation growth mechanism of NiAlX alloys with different phase structures and the cyclic oxidation behavior of the coatings were analyzed and summarized.The details of the study and the main conclusions are as follows:（1）The isothermal oxidation properties of NiAlX（X=Hf,Y）alloys with different phase structures at 1200°C was investigated.On theβ-phase NiAlHf alloy,sub-stableθ-Al₂O₃ is formed first,followed by phase change toα-Al₂O₃ and stable growth;on theγ’phase Ni₃AlY alloy,Ni O and spinel are formed on the surface first,followed by Al₂O₃on the bottom layer,and Ni O growth is inhibited,and the final TGO structure is spinel on the surface layer+Al₂O₃ on the bottom layer.Different phase structure alloys have different resistance to high-temperature oxidation,according to the aerospace industry standard,β-phase alloy belongs to complete oxidation resistance grade,andγ’phase alloy belongs to oxidation resistance grade.（2）The cyclic oxidation performance of NiAlX（X=Hf,Y）coatings with different phase structures at 1100°C was investigated.In both cases,the TGO is always dominated by Al₂O₃ with a diffuse distribution of Hf-rich oxides;on the（β+γ’）biphasic NiAlHf coating,interdiffusion occurs at the coating/substrate interface,and the TGO surface has low thermal stress（～-2.943GPa）and large folds（～1.143μm）,without spalling.On theγ’phase Ni₃AlY coating,there is no interdiffusion zone between coating/substrate,the TGO surface is less folded（～0.531μm）,but the stress is higher（～-3.692 GPa）,so the TGO shows slight spalling.According to the aerospace industry standard,γ’is slightly weaker than the（β+γ’）biphasic coating,but both belong to the complete oxidation resistance grade.（3）The microscopic growth behavior of TGO after oxidation of alloy substrate surface coatings with different Hf concentrations was investigated.The coatings based on DZ125 all showed slight internal oxidation and Hf-rich oxide interfacial polarization phenomenon,while the coatings based on NiAlHf alloy did not show this phenomenon,and in addition,this phenomenon was more significant at 1200°C and even caused TGO flaking on the surface of the coatings.The NiAlX bond coating with different phase structures has a strong selectivity to the substrate,and the correct selection of a substrate with better interoperability is the key to ensure the high-temperature oxidation resistance of this coating.In summary,the NiAlX（X=Hf,Y）coating surface TGO is steady-stateα-Al₂O₃ for a long time,which has excellent resistance to high-temperature oxidation and can serve in more severe environments and has good prospects for application.