Impurity segregation at the interfaces between thermally grown aluminum oxide and a few coatings deposited on single crystal Ni-based superalloys are reviewed.Results are compared with studies made at oxide/metal interfaces formed on model Al_2O_3-forming alloys.The coatings were NiPtAl on CMSX4 or AM1 with two different bulk sulfur contents,and NiCoCrAlY on PWA1484.Auger electron microscopy was used to study the chemistry at the TGO/coating interface after portions of the oxide scale were removed in ultra high vacuum by scratching the oxidized sample.The extent of oxide spallation in relation to the scratch width was utilized to evaluate the interfacial strength,which was then related to the interface chemistry,particular its sulfur level.Results show strong relationship between sulfur segregation and the alloy substrate composition.This relationship is discussed in terms of the effects of Pt,Hf,Y and Cr and the alloy sulfur content. Impurity segregation at the interfaces between thermally grown aluminum oxide and a few coatings deposited on single crystal Ni-based superalloys were reviewed. Results are compared with studies made at oxide / metal interfaces formed on aluminum-oxide-forming alloys. The coatings were NiPtAl on CMSX4 or AM1 with two different bulk sulfur contents, and NiCoCrAlY on PWA1484. Auger electron microscopy was used to study the chemistry at the TGO / coating interface after portions of the oxide scale were removed in ultra high vacuum by scratching the oxidized sample. The extent of oxide spallation in relation to the scratch width was utilized to evaluate the interfacial strength, which was then related to the interface chemistry, particular its sulfur level. Results show strong relationship between sulfur segregation and the alloy substrate composition. This relationship is discussed in terms of the effects of Pt, Hf, Y and Cr and the alloy sulfur content.