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Thermal barrier coating systems

Thermal barrier coating (TBC) systems are an essential constituent of modern gas turbines since only these can achieve the high fuel gas temperatures necessary for efficient operation. During the volume expansion of a heated gas, these turbines transform the heat into kinetic energy. On the basis of very fundamental physical laws, a high temperature of the gas is decisive for a high efficiency of the energy transformation. Above approx. 600°C, the internal components of the turbine (usually metals) must be protected from the temperature because they do not withstand these conditions in the long run. This happens with thermal barrier coating systems.

The standard material that has been established worldwide for thermal barrier coatings is zirconia that has been partially stabilized with 7–8 wt% Y2O3 (YSZ). YSZ has a number of outstanding properties for this application, including low thermal conductivity, high expansion coefficients for reducing thermal stresses in the composite with the metallic substrate, and good fracture toughness.

Optimization of coating design and application of new processing technologies (see below) enables optimized materials microstructure which still expands the capabilities of this system for the application.

In addition to the development of extremely efficient YSZ layers, another area of work at IEK-1, Forschungszentrum Jülich, focuses on developing new materials for thermal barrier coatings. This is necessary because of the limited temperature stability of YSZ. Long-term applications are limited to temperatures of approx. 1200 °C because higher temperatures cause phase transitions and amplified compacting of the layers. Of particular interest are pyrochlores such as Gd2Zr2O7, complex perovskites, or (hexa-)aluminates. Due to the relatively low fracture toughness of these new TBC materials, the double-layer systems developed at Jülich are usually used. As figures show below, the bond coat is first covered with a layer of the tough YSZ, and then with a layer of the new ceramic.

WDS: Microstructure of double-layer TBC systems

The microstructure of double-layer TBC systems with an intermediate layer of YSZ and the top layer of advanced ceramics: pyrochlore Gd2Zr2O7 manufactured by APS process (left), complex perovskite La(Al0.25Mg0.5Ta0.25)O3 manufactured by SPS process (right).