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HIPERCOAT Résumé de rapport

Project ID: G5RD-CT-2001-00573
Financé au titre de: FP5-GROWTH
Pays: United States

Multilayer TBC concepts

Thermal barrier coatings (TBCs) comprising bilayer or multilayer configurations have been proposed in the literature for a variety of reasons. Of particular interest here is the use of an interlayer of the standard yttria-stabilized zirconia (7YSZ) between a novel TBC material and the thermally grown oxide (TGO) that provides oxidation protection for the metallic component. Justification of this design has generally been based on the higher toughness and/or "adherence" of the 7YSZ material since spallation failures often occurs by crack propagation at or immediately above the TGO/TBC interface. The concept was extended in the HIPERCOAT program to alleviate the problem of diffusional interactions between novel TBC materials based on rare-earth zirconates and the TGO. In essence, the 7YSZ interlayer is supposed to act as a diffusion barrier, but it must also be strain tolerant so it should have a microstructure that provides high in-plane compliance. At issue is whether the segmented columnar pattern arising from EB-PVD deposition can be designed with the proper thickness to prevent inter-diffusion of the reactive oxides (Gd and Al) along its internal surfaces, and also sufficiently thin that one can take full advantage of the lower conductivity and enhanced sintering resistance of the zirconate TBC.

It was demonstrated in this program that a segmented columnar YSZ layer of thickness [O] 50µm could be an effective diffusion barrier at temperatures as high as 1200°C, with no significant bulk or boundary inter-diffusion detected by analytical transmission electron microscopy. It was also found that the zirconate grows epitaxially on the 7YSZ columns with no perturbations on the microstructure other than the sharp change in composition. (A graded composition is arguably less preferable because of concerns about phase stability of Gd-rich t' compositions.) The integrity of this interface bodes well for its durability in thermal cycling.

Informations connexes

Reported by

University of California, Santa Barbara
1361D Engineering II, Materials Department
93106-5050 Santa Barbara, California
United States