Na2O is volatile at high temperatures, and is the most likely the cause of the large voids in Region 1 of the 7YSZ TBC (Fig. 1B) after attack by Na-rich CMAS for 24 h (1340 ◦C). This is supported by the observed decrease in Na+in the system with an increase in the exposure duration (Table 1), and in a separate experiment a significant loss of Na2O is also measured after the glass frit was heat treated for 24 h at 1340 ◦C. The resulting voids destroy the coating’s structure and cause the free-standing coating to bend. Bending implies delamination from the substrate in a TBC system, which has been observed by Shan et al.
Fig. 1.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 original TBC/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.)
Table1
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.
If present during the reaction, Na2O poses a unique threat to coatings. However, the extent of which it is responsible for TBC failure in operational gas-turbine engines is hard to estimate at this time. Na2O may volatilize before interacting with the TBC and, thus, have no effect on the TBC failure rate. Its influence may also go undetected if it evaporates after the attack but before post-failure analysis. Regardless of the true threat Na2O-enriched CMAS plays in TBCs, the basic and glass-modifying nature of Na2O make it useful for exploring the mitigation capabilities of Y-based coatings.
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