Reaction mechanism of CaO-MgO-Al2O3-SiO2 (CMAS) on lanthanide zirconia thermal barrier coatings
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Higher operating temperature could improve energy efficiency in the gas turbine engines, and thus, thermal barrier system is applied to protect the superalloy blades from the high temperature. Thermal barrier system is a multi-layer system and the most significant part is the top layer, thermal barrier coating (TBC). TBC should not only protect the blades from the high temperature, but also resist the corrosion from the molten dust injected into the engines. Lanthanide zirconates are promising materials to replace currently used yttria stabilized zirconia as TBC materials due to its better thermal properties and higher corrosion resistance against molten dust. However, the corrosion resistance mechanism is still not clearly understood. This paper targets to provide an essential understanding in lanthanide zirconates materials’ corrosion resistance against molten dust. The molten dust was simulated by CaO-MgO-Al2O3-SiO2 (CMAS) and the materials simulated as TBC were [GdxSm(1-x)]2Zr2O7 (x = 0, 0.2, 0.4, 0.6,0.8 and 1.0). The reactions between CMAS and lanthanide zirconates were studied to understand the corrosion mechanism. Lanthanide zirconates ceramics were prepared by solid oxide sintering and co-precipitation method in pellet shape and molten dust were prepared by sintering raw oxides (CaO, MgO, Al2O3, SiO2) in stoichiometric ratio to get homogeneous glass powders. Then the CMAS powders were placed on top of lanthanide zirconates and sintered from 1200°C to 1500°C. All the samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS). CHAPTER 2 shows the experimental procedures, including how to prepare lanthanide zirconates and CMAS glass powders. CHAPTER 3 provides morphologies and properties of materials before corrosion. CHAPTER 4 discusses the CMAS corrosion properties on the lanthanide zirconates materials at higher temperature (>1250°C) and lower temperature (<1250°C). The trend in reaction products at various temperatures are also discussed in this chapter. Furthermore, the effect of porosities in lanthanide zirconates and CMAS composition on corrosion were studied in this chapter too. Effect of water flow on CMAS corrosion is discussed in Section 4.5. Then, the quantification of corrosion and function of reaction layer are shown in CHAPTER 5. In this chapter, corrosion on yttria stabilized zirconia was used to compare with corrosion on lanthanide zirconates.
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