Abradable seal coatings operate in highly demanding environments and are required to exhibit certain properties, including abradability, corrosion resistance, high-temperature stability, erosion resistance, and sufficient bond strength with the underlying material. To meet this complex combination of requirements, such coatings are largely composed of metal phases (e.g., Ni, Cu, Al, and various alloys) and self-lubricating non-metal phases (e.g., graphite, hBN diatomite, and high molecular polymers) with high porosity . Metal phases provide high strength, oxidation resistance, and hot-corrosion resistance. Self-lubricating non-metal phases mainly provide lubrication for coatings and enhance porosity to reduce the hardness of coatings.
The purpose of this investigation was to evaluate the microstructure, high-temperature oxidation behaviour, and hardness of a seal coating under controlled exposures at 750℃ with different exposure times.The microstructures and high-temperature corrosion behaviours of coatings under controlled exposure at 750℃with exposure time of 2, 6, 40, 80, 200, and 1000 h were investigated. The main results are as follows. The coating microstructure is composed of FeNi3, Ni, and hBN. The high-temperature oxidation process includes three stages:the fast stage, transition stage, and slow stage. a-Al2O3 peaks appear after 2-h exposure. As exposure time increases, the diffraction peak intensity of a-Al2O3 begins to decrease and Cr2O3 peaks appear after 200 h. After 1000 h, Cr2O3 becomes dominant with a small amount of remaining a-Al2O3. The main oxides of a-Al2O3 and Cr2O3 are formed around the pores in the coating initially. The inner oxide layer is enriched with higher concentrations of Cr, Ni, and Fe after 200 h. Finally, the hardness of the coating increases with exposure time increasing.
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