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CHAPTER 2 LITERATURE REVIEW Compared with β-NiAl bond coat, γ+γ’ bond coat provides better compatibility with typical superalloy substrate and is more durable against mechanical damage because of its higher yield strength and creep resistance [29,30]. The fabrication process of γ+γ’ bond coat has a lower cost because it doesn't need aluminizing. 2.1.4 Thermal Grown Oxide Thermal grown oxide is the layer which is formed at high temperature due to oxidation of bond coat and is used to protect against oxidation when the engine is in operation and it is the main factor to control the lifetime of TBC. Since the structure of YSZ top coat, i.e. oxygen conductive and porous, the oxygen can easily penetrate into the bond coat layer when engine is in operation. Besides, the high ionic diffusivity of oxygen in ZrO2 will also lead the oxidation of bond coat layer. Thus, a protective thermal grown oxide plays an important role to protect the substrate superalloy of engine turbine from environmental damage. For the requirement of TGO layer, TGO layer needs to be stable during thermal cycling and it also needs to have a good adhesion with both the bond coat and YSZ layers. For the state-of-the-art TGO, a thin layer alumina which is grown from the bond coat during the heat treatment can provide a good function to fit that purpose. Generally, the protective alumina layer is based on α phase alumina because it provides good stability at high temperature, good adhesion and low oxygen diffusivity [23]. Generally, TGO layer develops large compressive stress which is coming from the thermal shrinkage mismatch with the metallic substrate Page 29PDF Image | Volcanic Ash Degradation on Thermal Barrier Coatings
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