Segregation behavior at TGO/bondcoat interfaces
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The segregation of sulfur and other elements at the interface between thermally grown alumina and a few coatings have been reviewed and compared with studies made at oxide/metal interfaces formed on model alloys. The coatings studied were NiPtAl on CMSX-4 or AM1 with two different bulk sulfur contents, and NiCoCrAlY on PWA 1484. The segregation behavior at the oxide/PWA1484 interface was also reported. Auger electron microscopy was used to study the chemistry at the oxide/coating interface after portions of the oxide were removed in ultra high vacuum (UHV) by scratches made on the oxidized sample surface. The extent of oxide spallation in relation to the scratch width was utilized to evaluate the interfacial strength, which was then correlated with the interface impurity level. Results showed strong relationship between sulfur segregation and the composition of the alloy substrates. In addition to substrate sulfur content, the degree of sulfur segregation was most significantly increased by Cr co-segregation or decreased by Y doping of the coating. Pt and Hf could stop segregation only when present together. P was found as a significant segregand in one case where sulfur segregation was prevented by Y. These behaviors are discussed in terms of various thermochemical interactions in the bulk and at the interface.
KeywordsInterfacial Strength Al2O3 Scale Segregation Behavior Segregation Energy Sulfur Segregation
The author is grateful for the following people and company for providing samples: Snecma (Safran group) for the NiPtAl on AM1 through Dr. Regine Molins, Mr. Kenneth S. Murphy at Howmet Castings for the NiPtAl on CMSX4 through Dr. Vladimir Tolpygo and Mr. M. Maloney and Mr. D. Litton of Pratt and Whitney for the NiCoCrAlY on PWA1484 through Prof. Kevin Hemker. The permissions from Dr. Regine Molins of Ecole des Mines de Paris and Dr. Vladimir Tolpygo of Honeywell Aerospace to use some of their results from our prior collaborations are greatly appreciated. The Auger studies were performed at the Molecular Foundry, Lawrence Berkeley National Laboratory. Financial supports for the Molecular Foundry and for parts of this work are provided by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. This work is also supported in part by AFOSR under the MEANS-2 Program (Grant No. FA9550-05-1-0173).
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