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Electron microscope image of a commercial bond coat after thermal cycling shows how the thermally grown oxide "rumples", causing spallation of the ceramic top coat and failure of the entire TBC system. |
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The new ISU composition after the same number of thermal cycles shows essentially the same surface topography as the initial surface (i.e., no "rumpling"). |
The reason is a new bond coat for thermal-barrier coatings, or TBCs. The discovery could result in significantly better reliability and durability for turbine blades, thus extending engine life.
Commercial thermal-barrier coatings consist of three layers: The first is a bond coat based on nickel-aluminum compounds. The bond coat goes directly on the turbine blade. The second layer is a thin, thermally grown oxide, which forms as aluminum in the bond coat oxidizes. The third layer, a thin (around half millimeter) ceramic top coat, has low thermal conductivity and thus acts as a barrier against heat. Dan Sordelet, an Ames Laboratory senior scientist, explains that the ability of the bond coat to oxidize and form a continuous, slow-growing and adherent TGO layer is critical to creating a resilient and reliable thermalbarrier coating.
Cracking or breaking of the TGO layer is a main cause of failure in TBC systems. Also, at temperatures around 1,100°C (2,013°F) and above, the aluminum in the bond coat begins to diffuse into the substrate, changing the overall composition.
"If enough aluminum diffuses into the substrate, there's a phase change in the crystal structure that can lead to largescale distortion of the bond-coat surface and subsequent failure of the TBC system," says Sordelet. "Initially, there is a very thin TGO layer sitting on a flat bond-coat surface. If the bond coat continues to lose aluminum so phase transformations take place, conditions will change from thin and flat to thin and "rumpled"' Stresses develop and the likelihood of the top coat coming off increases rapidly."
Sordelet and Brian Gleeson, director of Ames' Materials and Engineering Physics Program, experimented on various nickel-aluminum-platinum alloys to improve reliability in TBC systems. They found that adding platinum makes the alloys highly resistant to oxidation, forming a tenacious, slow-growing TGO scale. "With the [further] addition of hafnium, oxidation rates dropped by as much as an order of magnitude," Sordelet says, resulting in superlow growth rates. In current aluminumrich bond coat alloys, only a very small amount (e.g., <0.1 wt.%) of zirconium or hafnium may be added to improve oxidation before causing catastrophic oxidation failure.
In commercial coating production, it is extremely difficult to uniformly distribute such a small amount of metals economically. But the new nickel-rich bond coat exhibited low oxidation rates over a wide concentration, from 0.5 to 4 wt.% hafnium. This gives the new coating a greater processing window, which researchers say is highly desirable in the coating industry.