Figs. 1–3 compare the 7YSZ TBC cross-sections after attack by sand-CMAS and Na-rich CMAS after 1 min, 1 h, and 24 h, respectively, at 1340 ◦C. After 1 min, EDS and WDS measurements found that the sand-CMAS and Na-rich CMAS have penetrated ~210 μm and ~230 μm, respectively, into the 7YSZ TBCs, as shown in with red dashed lines in Fig. 1A and B. Within an hour, the sand-CMAS and Na-rich CMAS have fully penetrated the 7YSZ TBCs. The reaction products from the sand-CMAS and Na-rich CMAS are discussed in detail below.
Fig. 1.BSE-SEM micrographs of 7YSZ TBCs after attack by (A and C) sand-CMAS and (B and D) Na-rich CMAS after 1 min at 1340 ◦C. The red dashed lines in A and B
indicate the penetration depth of CMAS determined by EDS and WDS measurements. C and D are higher-magnification micrographs near the TBC/CMAS interface seen in A and B, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2.BSE-SEM micrographs of 7YSZ TBCs after attack by (A and C) sand-CMAS and (B and D) Na-rich CMAS after 1 h at 1340 ◦C. C and D are higher magnification micrographs near the original TBC/CMAS interface seen in A and B, respectively.
Fig. 3.BSE-SEM micrographs of 7YSZ TBCs after attack by (A,C, and E) sand-CMAS and (B, D and F) Na-rich CMAS after 24 h at 1340 ◦C. The red dashed lines in A and B indicate the positions where Raman spectra were collected. C and D are higher-magnification micrographs near the originalTBC/CMAS interface seen in A and B, respectively. E and F are higher magnification micrographs collected from the bottom half of the TBCs seen in A and B, respectively. Note that C and E are different magnifications than D and F. Regions 1 and 2 as described in the text are labeled in B. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 1A and C shows the backscattered electron (BSE) cross-sectional SEM images of the 7YSZ TBC after interaction with the sand-CMAS for 1min at 1340 ◦C. A layer of sand-CMAS remains on the surface and contains no crystalline phases. The EDS-measured composition of this layer is similar to the initial sand-CMAS composition and shows a low solubility of Y3+and Zr4+, as listed in Table 1. The 7YSZ TBC comprises small ZrO2 grains (white phase) and pockets of CMAS (dark gray phase) near the surface. After 1 h and 24 h exposure, the CMAS layer on the surface has disappeared, and larger volumes of CMAS are found between the ZrO2 grains. ZrO2 grains of similar size (<1 μm in diameter) and morphology are observed after the 1 h and 24 h exposure times with the sand-CMAS. These small grains primarily reside at the surface. Large, equiaxed grains dominate the microstructure deeper within the coating as shown in Fig. 3E. Previously, similar small-sized grains were observed on the surface after attack by the sand-CMAS in a 7YSZ TBC that was prepared with using different plasma-spray conditions. Transmission electron microscopy studies, in conjunction with EDS, identified the small grains to be Y-depleted m-ZrO2. Here, the crystal structure and, therefore, the relative stabilizer content (Y and Ca) were determined by Raman spectroscopy because the grain size was too small for accurate SEM/EDS measurements.
Table 1
The CMAS compositions (cation basis, at%) after interaction with 7YSZ TBCs at 1340 ◦C for 1 min, 1 h and 24 h measured using EDS.
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