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Microscale Experimental Techniques for Thermal Barrier Coating Development
Figure 1: A schematic and in-test image of a Pratt and Whitney F100 military jet engine. The orange areas on the schematic denote the hottest section of the engine; this is where thermal barrier coatings are used.
Figure 2: Comparison between as received and near end of life bond coat properties: the ultimate strength of the thermally cycled bond coat is reduced significantly during service.
Description
Globalization, environmental awareness, and fuel efficiency are some of the most important factors driving the development of today’s land and aero-based turbines. In recent years, thermal barrier coatings (TBC) have been developed to successfully protect the components in a turbine that are exposed to the highest temperatures (Figure 1). As a result of this, TBCs are now commonly used in gas turbines and jet engines to increase the overall system operating temperature, efficiency, and lifetime.
TBCs are dynamic, multi-layered, metallic-ceramic structures consisting of a superalloy substrate, an aluminum-rich bond coat, a thermally grown oxide (TGO), and a ceramic top coat. The structural integrity of the turbine blade is provided by the single-crystalline superalloy. Environmental protection of the superalloy is accomplished with a 50-100 mm thick intermetallic bond coat. The ceramic top coat is a porous Yttrium-Stabilized-Zirconia (YSZ) layer specifically designed to thermally insulate the bond coat and superalloy from the extremely hot gases passing through the engine. Spallation and failure of TBCs are governed by a sequence of crack nucleation, propagation, and coalescence steps that are related to the interactions between the various layers during thermal cycling. Improved knowledge of the mechanical behavior of each layer as a function of lifetime is therefore a key factor in developing the more efficient turbines and jet engines of the future. Figure 2 shows the mechanical response of a 100 mm thin bond coat layer in two different states. Prof. Kevin Hemker’s group is collaborating with researchers in other universities and aerospace industries to perform a wide variety of mechanical tests on each of the layers of a TBC. These tests form the basis of performance and lifeing models.