Segregation behavior at TGO/bondcoat interfaces

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.