Chapter 7 - Thermal Barrier Coatings (TBCs)

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Metallic coatings for protection of structural alloys against oxidation and corrosion were covered in Chapter 6. There is another group of coatings called thermal barrier coatings (TBCs), whose function is to reduce component temperatures and thereby increase life. TBCs are generally a combination of multiple layers of coatings (Fig. 7.1), with each layer having a specific function and requirement. The topmost layer provides thermal insulation and consists of a ceramic, with low thermal conductivity, typically ZrO2, known as zirconia. As will be discussed later, zirconia has to be stabilized against polymorphic phase transformation. The ceramic insulating layer is deposited on the substrate alloy with an intervening oxidation-resistant metallic layer called the “bond coat.” The metallic coating is either a diffusion aluminide, such as platinum aluminide, or an overlay coating of general composition NiCoCrAlY, conforming somewhat to the substrate alloy composition. During the ceramic coating deposition, a thermally grown oxide (TGO), predominantly Al2O3, forms on the bond coat surface at the ceramic–bond coat interface. In effect, the TGO binds the ceramic layer to the bond coat. Thus TBCs are systems consisting of a ceramic coating, the TGO, and a metallic bond coat on the substrate alloy. The approximate thickness range of each layer is as follows: ceramic 125–1000μm, bond coat 50–125μm, and TGO 0.5 to about 10μm, the last depending on the duration of exposure of the TBC in the high-temperature environment of the coating process and in operation.