Microstructure and phase correlation of DySZ-Polyester abradable seal coating after thermal exposure

Abstract

Advanced gas turbine blade/shroud clearance control system requires abradable seal coatings with a high-temperature capacity above 1000 ℃ to accommodate the increasing turbine inlet temperature requirements. Dysprosia-stabilized-zirconia (DySZ) based abradable seal Coatings were fabricated on the Ni-based single-crystal superalloy via the atmospheric plasma spraying technique. The microstructure of the DySZ-based coating before and after thermal ageing was characterized using high-contrast SEM. The phase composition and distribution of the coatings were measured by XRD and Raman spectroscopy, respectively. Focused ion beam (FIB) and Scanning transmission electron microscope (STEM) technologies were employed for the phase confirmation. The results indicated that phase transformation from tetragonal to monoclinic ZrO2 was observed in the DySZ-Polyester after 300 h thermal exposure. Based on the different concentrations of Dy element in the monoclinic and tetragonal phases, the morphology and distribution of the monoclinic phase in the coating were observed intuitively using high-contrast BSE images and further corroborated by the Raman spectra mapping results. During heat treatment, the polyester added to the feedstock was oxidized to form amorphous carbon, which is mainly distributed in the gaps of interlamellar cracks and big pores. In addition, numerous Dy2O3 precipitates added to the feedstock were detected in the as-sprayed coating, and some reacted with ZrO2 to form the Dy2Zr2O7 phase during thermal spraying and thermal ageing. The correlation between Dy concentration and phase structure was then discussed.