Abstract
This paper aims at investigating the microstructure and phases evolution of single crystal superalloy/high temperature protective coating during high temperature static oxidation, and exploring the influence of element interdiffusion behaviour on microstructure and phase evolution of the single crystal superalloy substrate. A NiCoCrAlY high-temperature protective coating was deposited on the Ni-based single-crystal superalloy by low-pressure plasma spraying technology. The coated samples were subjected to static oxidation for 200 h at a constant temperature of 1100 °C. Scanning electron microscope, energy dispersive spectrometer and X-ray diffraction were used to characterise the microstructure and phase after interdiffusion between the coating and the substrate at high temperature. The results showed that a dense thermally grown oxide layer was formed on the surface of the NiCoCrAlY coating after oxidation for over 100 h. The only interdiffusion zone was formed after oxidation for 50 h, while both interdiffusion zone and secondary reaction zone could be observed after oxidation for over 100 h. The thickness of interdiffusion zone and secondary reaction zone is increased with the extension of oxidation time, and the grain growth of topological close-packed phase in the secondary reaction zone is found. Al, Cr and Co in the coating diffuse from the coating to the substrate, while Ni and refractory materials like Ta, Mo, Re and W diffuse from the coating to the substrate. The interdiffusion of coating and substrate leads to the instability of γ/γ′ phase in the substrate, which finally results in the formation of W, Re and Cr-rich needle-like topological close-packed phase in the substrate.
Highlights
IntroductionAs the main materials used for advanced aero-engine blades, Ni-based single crystal superalloys possess excellent creep strength and fatigue resistance performance at high service temperature [1,2,3,4,5]
As the main materials used for advanced aero-engine blades, Ni-based single crystal superalloys possess excellent creep strength and fatigue resistance performance at high service temperature [1,2,3,4,5].In contrast, the oxidation resistance of Ni-based single crystal superalloys is poor for the increased service temperature in high thrust-ratio turbine engines, and one of the most effective ways is to apply with protective coatings like MCrAlY, PtAl coating [6,7,8,9,10]
According to the X-ray diffraction (XRD) pattern, the main phase of the as-deposited coating consists of low-pressure plasma spraying technology (LPPS) coating
Summary
As the main materials used for advanced aero-engine blades, Ni-based single crystal superalloys possess excellent creep strength and fatigue resistance performance at high service temperature [1,2,3,4,5]. The oxidation resistance of Ni-based single crystal superalloys is poor for the increased service temperature in high thrust-ratio turbine engines, and one of the most effective ways is to apply with protective coatings like MCrAlY, PtAl coating [6,7,8,9,10]. The huge differences in composition between coating and underlying substrate can lead to element interdiffusion during long-term high-temperature service, resulting in the segregation of insoluble elements such as Ta in the substrate and the precipitation of harmful topological close-packed. Few reports are available on the precipitation of topological close-packed phases during high temperature oxidation process
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