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CHAPTER 7 CONCLUSIONS AND SUGGESTED FUTURE WORK (3.71 wt.%) was much lower than as received TBC samples (8 wt.% Y2O3), which agreed with the understanding that the concentration of yttria can be depleted by molten volcanic ash. More details of volcanic ash/YSZ interaction have been found using DTA and XRD analysis. Reactions between volcanic ash and YSZ started at 410 °C which is lower than the surface temperature of engine turbine. In addition, phase transformation was found in XRD analysis. Accompanied with the depletion of yttria, zirconia was no longer stable and phase transformations occurred during thermal cycles. For zirconia, it stayed in tetragonal phase when temperature is higher than 1050 °C and returned to monoclinic phase in low temperature. Along with phase transformation, 4 - 6% volume change would occur which is extremely harmful for the TBC lifetime. For the protective mechanisms, alumina has been proved to have a good ability against CMAS and volcanic ash penetration. In penetration experiments, YSZ showed a very poor ability against volcanic ash penetration, even though the density of YSZ plate is much higher than YSZ top coat. Unlike YSZ, alumina showed a good ability to prevent volcanic ash penetration, which formed two layers after reacting with volcanic ash. The layer upon alumina plate is mainly made from magnetite and spinel, and the layer below volcanic ash is mainly made from anorthite. These three reaction products are able to protect TBCs against volcanic ash because their melting temperature is much higher than the Page 90PDF Image | Volcanic Ash Degradation on Thermal Barrier Coatings
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