Substrate alloy element diffusion in thermal barrier coatings

The application of 7 wt.%-yttria partially stabilized zirconia (7-YSZ) by electron beam physical vapor deposition (EB-PVD) produces a columnar coating which is adherent to a thermally grown oxide (TGO). The condition of the TGO is of considerable interest since this oxide layer binds the TBC to the underlying bondcoat, which is the typical site for TBC failure. Representative samples of a second-generation single crystal Ni-based superalloy were coated with MDC-150L (an outward growth type low-activity high temperature single-phase platinum aluminide) and MDC-620 (7-YSZ TBC by EB-PVD) were cyclic tested at 1408 K and then evaluated by metallography, SEM, and X-ray diffraction. Metallographic inspection reveals that samples had mixed modes of failure. These modes of failure include TGO–bondcoat interface, intra-TGO, and TGO–TBC interface failure for a given specimen. SEM analysis and X-ray mapping of ‘extended life’ samples (>1000 h) reveals solutioning of TGO and TBC up to 2.5 μm and enrichment of chromium, aluminum, and nickel within the TGO and also between columns throughout the thickness of the ceramic topcoat. Diffusion and segregation of these elements appears to have occurred along column boundaries throughout the entire thickness of the TBC (∼125 μm). SEM analysis also reveals that the outer portion of TBC spheroidized and sintered. X-Ray diffracta show Ni–alumina spinel and tetragonal or monoclinic phases of zirconia. Diffusion mechanisms and phase formations will be discussed with suggestions for further work.