The 2ZrO2⋅Y2O3(ss) TBC shows dramatic arrest of both CMAS compositions. Figs.1–3 compare the cross-sections of 2ZrO2⋅Y2O3(ss) TBCs after attack by sand-CMAS and Na-rich CMAS at 1340 ◦C for 1 min, 1 h and 24 h, respectively.
Fig. 1.BSE-SEM micrographs of 2ZrO2-⋅Y2O3(ss) 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. Ap, Z, and N indicate apatite, reprecipitated ZrO2(ss) and Na-rich crystalline phases, 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 2ZrO2-⋅Y2O3(ss) TBCs after attack by (A and C) sand-CMAS and (B and D) Na-rich CMAS after 1 h 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. Ap, Z, and N indicate apatite, reprecipitated ZrO2(ss) and Na-rich crystalline phases, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 3.BSE-SEM micrographs of 2ZrO2⋅Y2O3(ss) TBCs after attack by (A and C) sand-CMAS and (B and D) Na-rich CMAS after 24 h 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.)
After 1 min, a layer of sand-CMAS remains on the surface and shows negligible penetration into the grain boundaries (Fig. 1A and C). The composition of the sand-CMAS in the surface layer is similar to the original CMAS composition with the addition of a small amount of Y and Zr, as reported in Table 1. EDS measurements confirm that the reaction produces Y-depleted ZrO2 reprecipitated grains (Table 1) and long apatite rods (Table 1). The reaction regions found after 1 min and 1 h exposure times are similar, except for the volume of CMAS remaining on the surface (Figs. 1A and 2A). The reaction between 2ZrO2⋅Y2O3(ss) and the sand-CMAS at 1340 ◦C for 24 h has been reported previously by the authors, and the results are included here for comparison. After 24h, no CMAS remains on the surface and CMAS has penetrated ~20 μm into the coating (measured from the original surface; Fig. 3B). Apatite, Y-depleted ZrO2 grains, and a small number of unidentified, Y-rich grains were observed near the reaction front (Fig. 3C). Tables 1 and 2 lists the compositions of the apatite and Y-depleted ZrO2, respectively, at the three exposure times. The Y3+content increases in both reaction products as the reaction proceeds and the CMAS on the surface decreases.
In contrast, the Na-rich CMAS does not produce apatite after 1 min exposure. After 1 min, the Na-rich CMAS shows negligible penetration into the 2ZrO2⋅Y2O3(ss) coating (Fig. 1B and D). A Na-rich crystalline phase is identified at the TBC/CMAS interface and the EDS-measured composition is reported in Table 3. After 1 h, Na-rich CMAS has penetrated ~60 μm and apatite is observed in addition to the Na-rich crystalline phase on the surface (Tables 2 and 3, Fig. 2D). A thin layer of Y-depleted ZrO2 grains is found near the surface of the TBC (Table 1).
The BSE-SEM micrograph in Fig. 3 shows the 2ZrO2⋅Y2O3(ss) TBC after attack by Na-rich CMAS for 24 h at 1340 ◦C. The CMAS penetration depth, measured considering the original thickness of the coating (~350 μm), is 50–60 μm, similar to the penetration depth after 1 h. Three main phases are found within the reaction region and identified by EDS (Tables 1–3): Y-depleted c-ZrO2(ss), Ca–Y-apatite, and a CaO-rich silicate phase. The Na-rich crystalline phase is no longer observed and the reaction region appears similar to that found after the sand-CMAS attack at 24 h (Table 3). The ZrO2 grains do not show a significant change in the Y content between 1 h and 24 h exposure to Na-rich CMAS, but the grains contain greater Y-content than the sand-CMAS at all exposure times. (Note that the composition of the ZrO2 grains in the reaction zone after 1 min could not be identified with SEM/EDS measurements because the grains were too small and surrounded by the Na-rich crystalline phase, which interfered with the EDS signal.) The apatite compositions observed after 1 h and 24 h exposure to Na-rich CMAS are similar (Table 2).
Table 1
Compositions (cation basis, at%) of reprecipitated fluorite grains found in the reaction regions of the 2ZrO2⋅Y2O3(ss)/TBC cross sections after reactions with sand-CMAS and Na-rich CMAS (1340 ◦C).
Table 2
Compositions (cation basis, at%) of apatite grains found in the reaction regions of the 2ZrO2⋅Y2O3(ss)/TBC cross sections after reactions with sand-CMAS and Na-rich CMAS (1340 ◦C). The theoretical composition of Ca4Y6(SiO4)6O2 and Ca2Y8(SiO4)6O2 are listed for comparison.
Table 3
EDS compositions (cation basis, at%) of crystalline phases in the 2ZrO2-⋅Y2O3(ss)/Na-rich CMAS interaction region at various exposure times (1340 ◦C).
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