Thermal Barrier Coatings

Basic Technology and Aircraft Engine Experience. The concept of a thermal barrier coating is simply that a ceramic material of low thermal conductivity decreases the heat flux to the metal surface. In the case of cooled airfoils (Figure 1), coating thicknesses are restricted by the complex geometry, but the thermal barrier effect is enhanced by internal cooling such that temperature differences (between the surface of the coating and the underlying metal) up to 300°F are achievable. The lower metal temperature increases the lifetime of the airfoil, and/or the designer can take advantage of the thermal barrier effect to increase performance and efficiency of the engine through higher turbine inlet temperatures and higher rotational speeds.

Fig. 1. Schematic diagram of thermal barrier coating concept.

The microstructure of a state-of-the-art plasma sprayed thermal barrier coating is illustrated in Figure 2. The outer layer is stabilized zirconia, and the MCrAlY bond coat provides oxidation protection to the substrate (needed because ZrQ^ is highly pervious to oxygen) as well as a mechanical bonding effect due to its rough surface. Coatings of this type are routinely used in hot section components

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2. Typical thermal barrier coating microstructure.

of aircraft gas turbines and have operated successfully for thousands of hours in both commercial and military engines. Hence there is a broad experience base relating performance and durability to composition, processing history, and resultant microstructure of thermal barrier coatings. The most extensive contribution to this experience base is undoubtedly the work of the National Aeronautics and Space Administration (NASA), including both their own experimental studies (1-3) and their sponsorship of contract, research and annual Thermal Barrier Coatings workshops (4-7).

Illustrative of a critical composition effect is the dependence of spall resistance on yttria content as presented in Figure 3. The effect is not straightforward (the optimum'composition is only partially stabilized, and the criticality of the 8% level is not predictable from basic thermochemistry), but it demonstrates the type of background knowledge which contributes to useful engineering application of thermal barrier coatings.

Cycles to failure

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