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CHAPTER 2 LITERATURE REVIEW are the reaction zone and un-reacted zone [40]. In the reaction zone, columnar gaps and pores are filled by molten CMAS glass [41-43]. In contrast, there is little damage happening in the un-reacted zone. During thermal cycling, the top coat becomes denser and results in higher thermal mismatch with the substrate. This increases the internal stress of YSZ top coat and leads to the eventual spallation [35,44]. Figure 2.8 Cross-sectional image of TBC top coat penetrated by CMAS [42] Besides the penetration through the columnar gaps and pores, CMAS may also penetrate into the vertical cracks. Vertical cracks are generated when YSZ is sintered, which help the YSZ top coat to release strain energy [45,46]. However, vertical cracks also provide a pathway for liquid CMAS and facilitate the spallation of the top coat. Moreover, the liquid CMAS can dissolve the YSZ and extract the yttrium from stabilized zirconia, which means that when engine is in operation, liquid CMAS starts to extract yttrium, until its concentration of yttrium is low enough that the metastable tetragonal YSZ turns into tetragonal zirconia [40]. During the cooling process, the tetragonal phase zirconia turns to monoclinic phase, which led to a 4 Page 32PDF Image | Volcanic Ash Degradation on Thermal Barrier Coatings
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