To study the high-temperature (isothermal) interaction between the APS 2ZrO2·Y2O3(ss) coatings and the CMASs, CMAS paste was applied over a small circular area (~1 cm diameter) at the center of the free-standing coating disk (~2.5 cm diameter) surface at 90 mg·cm-2 loading. The higher initial CMAS loading ensured a final CMAS loading of~30 mg?cm?2(same as the initial loading used in the earlier lower temperature studies because molten CMAS spreads easily at 1500°C. Also, the use of larger samples, relative to the ones used in the previous studies,18,19precluded the flow of the molten CMAS glass over the edges. These samples were then heated to 1500°C at a heating rate of 10°C?min?1in a box furnace(CM Furnaces, Bloomfield, NJ), and were held for 1 or24 hours, and then cooled to room temperature in the furnace, with a maximum cooling rate of 10°C·min-1Thesesamples, along with as-deposited APS 2ZrO2·Y2O3(ss)coatings, were sectioned and the cross-sections were pol-ished to a 1-lm finish using standard metallographic procedures. APS 2ZrO2?Y2O3(ss) coating control samples without CMAS were also heat treated at 1500°C for various durations (1-100 hours) to study the effect of heat treatment on the microstructure. The densities of the heat treated samples were measured using the Archimedes method. The cross-section of the control sample exposed to 1500°C for 24 hours was polished to a 1-lm finish formicrostructural observation.
All cross-sections were characterized using scanning electron microscopes (SEMs; LEO 1530VP, Carl Zeiss, Munich, Germany or Helios 600, FEI, Hillsboro, OR), bothequipped with a electron-dispersive spectrometer (EDS), operated in backscattered electron mode with 20 kV accelerating voltage. For some samples, electron probe microanalysis (EPMA; SX-100, Cameca Instruments, Inc., Gennevilliers, France) operated at 15 kV in the wave-length-dispersive spectroscopy (WDS) mode was used to obtain elemental maps and the corresponding electron images. Additional point and area measurements (both EDS and WDS) were also made to confirm the mapping results. Transmission electron microscopes (TEMs; 2100F,JEOL, Peabody, MA or CM20, FEI, Hillsboro, OR) operated at 200 kV acceleration voltage, and equipped with EDS, were used for more detailed characterization. Selected-area electron-diffraction patterns (SAEDPs) were collected and indexed according to standard practices. The TEM specimens were extracted using a focused ion beam(FIB; Helios 600, FEI, Hillsboro, OR) from specific locations, in conjunction with the in situ lift-out technique.
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