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CHAPTER 4 VOLCANIC ASH AND ITS PENETRATION MECHNISMS IN TBCS phase diagram shown in Figure 2.3 [1,12], phase transformation of YSZ occurred at around 1050 °C. Once the monoclinic phase YSZ transformed to tetragonal phase, it would be really difficult for YSZ to transform back to monoclinic phase because yttria stabilizes the phase of zirconia, allowing tetragonal phase able to stay in lower temperature for a long time. Thus, no monoclinic phase YSZ could be observed when heat treatment temperature was higher than 1050 °C. For the case of volcanic ash-YSZ mixed sample, the percentage of monoclinic phase kept increasing except for the sample heat treated at 950 °C. Compared with the as prepared sample, the percentage of tetragonal phase was 13% lower after heat treated at 490 °C. The increase in monoclinic phase can be explained by the depletion of yttria. When the heat treatment was carried out above 490 °C, the reaction between volcanic ash and YSZ caused the depletion of the yttria content, and subsequently led to transformation of monoclinic phase in YSZ at lower temperature. For volcanic ash-YSZ mixed sample heat treated at 950 °C, the monoclinic phase YSZ was 18% lower than volcanic ash-YSZ mixed sample thermal exposure at 490 °C. This is because monoclinic phase YSZ was already existed in as-prepared sample. For 490 °C heat treated sample, monoclinic phase YSZ could be separated as YSZ with yttria and YSZ with no yttria, which schematic diagram was shown in Figure 4.21. According to the previous DTA curve, it was demonstrated that the phase transformation point of YSZ was shifted from 1050 °C to 950 °C, all monoclinic phase YSZ powder transformed to tetragonal. However, when the temperature was cooling down, only Page 66PDF Image | Volcanic Ash Degradation on Thermal Barrier Coatings
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