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Volcanic Ash Degradation on Thermal Barrier Coatings and Preliminary Fabrication of Protective Coatings
[Thesis]. Manchester, UK: The University of Manchester; 2014.
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Abstract
Thermal barrier coatings (TBCs) made of yttria stabilized zirconia (YSZ) have been applied to aero engines industry since 1970s. However, because of the increasing operational temperature, TBCs are suffering from molten foreign deposits known as calcium-magnesium-alumino-silicate (CMAS). Molten CMAS sinters YSZ top coat and shortens the lifetime of TBCs. Alumina has been widely proved to prevent CMAS from degradation and is the most common material chosen to avert a CMAS attack in state-of-the-art TBC technology.This study uses real volcanic ash to study the degradation process of TBCs and potential mitigation mechanisms. The results show that, similar to CMAS, volcanic ash severely penetrates the thickness and fills the columnar gaps of the TBC top coat. It is also found that the yttria content of the YSZ top coat decreases substantially with high temperature exposure to volcanic ash, which has a detrimental effect on the phase stability of YSZ. In terms of mitigation, volcanic ash reacts with alumina around 1310 °C, forming anorthite (CaAl2Si2O8), magnetite (Fe3O4), and spinel (Al1.75Mg0.889Mn0.351O4) as reactive products, which, according to the literature, have melting temperatures above that of the volcanic ash studied and the typical values reported for CMAS. Since the new melting temperatures are now above the typical surface operating temperature of turbine components, the melting-induced penetration of volcanic ash and CMAS can be considerably suppressed.For the protective coatings, both YSZ and Al2O3 could be made by using sol-gel based spray coating. However, different grade of thermal expansion between TBC layers and protective coating during heat treatment will break the structure of protective coating which is needed to be conquered in the future.