In Fig. 1 grosscale microstructures of the reaction zones between CMAS and both, Gd2Zr2O7 (GdZ) and La2Zr2O7-3wt% Y2O3 (LaZ) EB-PVD TBCs are shown. The situation is quite different from the zirconia based compositions as the formation of crystal-line reaction products is promoted via rapid CMAS/TBC interaction at the column tips as well as within the inter-columnar gaps. Column tips of GdZ were dissolved and reaction products reprecipitated within a 3 to 5µm thick reaction layer which is retained along the column tips and the rims of wider inter-columnar gaps. Restriction of the reaction to the column rims becomes evident in Fig. 1 b which displays a composite image of an EDS silicon map with the corresponding SEM image. CMAS constituents were not detected inside the columns. The more feathery and open-columnar structure of GdZ (as compared to 7YSZ) is not necessarily detrimental for CMAS mitigation, but on more narrow inter-columnar gaps the infiltration depth is slightly lower than within larger gaps. Small probe microanalysis data of the reaction products agreed well with the data given in suggesting the presence of Ca-Gd-apatite of needle-like morphology and a globular cubic zirconia phase. For these annealing conditions, a mitigation of CMAS infiltration inside inter-columnar gaps was observed at a depth of 60 to 80µm from the coating’s surface.
Fig. 1: SEM cross sections of Gd-zirconate (a and b) and La-zirconate (c) TBCs upon annealing at 1260°C, 2 h under CMAS attack. (b) composite SE image/Si map indicative of the reaction zone in the outer column rim. Arrows mark the inter-columnar gaps.
A variety of reaction scenarios and microstructures was found on LaZ TBCs, ranging from complete disintegration of the TBC top region and fully reacted compact layers as shown in Fig. 1 c) up to reactions that took mainly place on the outer column rims inside and along larger inter-columnar gaps. The latter reaction is similar to GdZ where this setting is predominantly found. For LaZ, the reaction layer depth of around 10µm is somewhat thicker than on GdZ, even under the scenario of exclusive reaction on column rims. The morphology of LaZ processed via EB-PVD is characterized by a more compact microstructure of branching columns having small diameters and quite narrow inter-columnar gaps, and on the other hand frequently occurring larger columnar gaps of varying width that possibly developed by the large shrinkage of the material during manufacture. As a consequence, CMAS can penetrate locally quite deep into the coating in those gaps, occasionally reaching the substrate surface in case of very large inter-columnar gaps. Inside inter-columnar gaps of moderate width crystalline reaction products are formed up to a depth of 30 to 50µm inhibiting further CMAS infiltration.
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