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Thermal Barrier Coatings


There are a myriad of different coatings used to protect a whole range of structural engineering materials from wear, corrosion, and thermal degradation; they also provide both lubrication and thermal insulation. Thermal barrier coatings operate in some of the most demanding environments, requiring complex structures to effectively withstand the harsh, high temperature operating conditions found in aircraft, diesel and industrial gas turbine engines, providing not only thermal protection to the metallic components beneath them, but also to protect them from oxidation, hot corrosion and wear damage. 

The development and application of thermal barrier coatings is driven by technological, economic and societal pressures to increase turbine efficiency by operating them at increasingly higher temperatures, reducing service intervention through the longer  life of the turbine alloy components due to lower thermal fatigue, and an overall more environmentally-friendly life cycle of the turbine engine.

  thermal barrier coatings (TBC)  

High performance thermal barrier coatings require the combination of multiple important material properties:

- High melting point

- No phase transformation between ambient and operational temperature

- Low thermal conductivity

- Chemical inertness

- Co-efficient of thermal expansion similar to the underlying alloy

- Excellent adhesion to the metallic substrate

- And low sintering rate of the porous microstructure

- A thermal barrier coating typically comprises four layers:

- Alloy substrate providing structural strength

- Bond coat for oxidation resistance

- A thermally grown oxide between top and bond coats

- An external ceramic topcoat providing the thermal insulation

4 mol% yttria stabilised zirconia, a stable variant of the tetragonal phase, is the preferred material for thermal barrier coating topcoat applications. For a ceramic, a fully dense yttria stabilised zirconia has one of the lowest thermal conductivities at elevated temperature, due to the high concentration of point defects in its lattice structure, which scatter heat-conducting phonons (or lattice waves). A high co-efficient of thermal expansion helps to relieve the stresses generated due to the similar thermal expansion of the underlying metal layer. Its relatively low density is important, keeping the rotating engine’s component weight to a minimum. A high melting point (~2700ºC) material with a hardness of 14GPa makes it ideally suited to aggressive, high temperature environments and resistant to both erosion and impact.


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