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CHAPTER 4 VOLCANIC ASH AND ITS PENETRATION MECHNISMS IN TBCS The reason why volcanic ash shows a faster penetration might be related to its composition and phase structure. The viscosity of volcanic ash might be smaller than artificial CMAS due to the 7 more composition inside of volcanic ash (i.e. Na, P, K, Fe, Ti, Mn, and Ta) and its complicated phase structures. Because further research hasn’t been done, the exact reason is still needed to be confirmed. This is the reason why so many airline companies decided to cancel flights when the Eyjafjallajökull volcano erupted. The degradation mechanisms of volcanic ash attack on YSZ top coat was clearly shown in this chapter. Indeed, results showed that the degradation mechanisms of volcanic ash attack were quite similar to the CMAS attack on TBCs presented in previous publications [32,40,41,71]. Except for the detrimental physical penetration effects such as sintering and spallation made by molten ash, content of Y2O3 was depleted by molten volcanic ash. For the characterization of YSZ, zirconia is known as an unstable material, which undergoes the phase transformation between monoclinic phase and tetragonal phase under thermal cycles if there is no yttria to stabilize the tetragonal phase. Accompanied with the phase transformation, 4 - 6% of volume change in the unit cell occurs which is extremely harmful to the TBC lifetime [72]. Moreover, literature also claims that molten volcanic ash will react with ZrO2 to form ZrSiO4 as their reaction product [69], but this reaction product has not been found in our experiments. Page 68PDF Image | Volcanic Ash Degradation on Thermal Barrier Coatings
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