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CHAPTER 4 VOLCANIC ASH AND ITS PENETRATION MECHNISMS IN TBCS 4.1.2 Melting Point of Volcanic Ash The melting point of volcanic ash plays an important role in determining the type of degradation induced on TBCs. If the melting temperature of the volcanic ash is higher than the surface temperature of the engine turbine, there should be no or very little penetration happening between volcanic ash and YSZ top coat of TBCs. Figure 4.3 shows the result of DTA analysis on volcanic ash. It is noticed that a sharp valley has been formed at the temperature around 1235 °C, which can be explained by the transformation from solid phase to liquid phase (i.e. melting). Besides, another experiment has been carried out for better understanding of the melting temperature of volcanic ash. Isothermal heat treatments at four different temperatures 1150 °C, 1200 °C, 1250 °C, and 1300 °C have been done on volcanic ash for 30 minutes and their surface morphology and surface roughness were shown in Figure 4.4 - Figure 4.7. By comparing the heat treated samples, obvious change of their surface morphology and surface roughness were obtained. If volcanic ash starts melting, its structure would transform from rough particles to a much flatter plane. Figure 4.4 shows the volcanic ash image took after 1150 °C of heating. It shows that the surface was inhomogeneous and the volcanic ash was sintered already. By observing the surface roughness, a 291.6 μm height difference from the highest point to the lowest point was noticed. By referring to Figure 4.5, it can be seen that volcanic ash had partially started melting at 1200 °C because its surface structure became smoother and its surface morphology looked much more homogenized after Page 51PDF Image | Volcanic Ash Degradation on Thermal Barrier Coatings
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