Study of sulfur distribution in a NiPtAl bondcoat

Abstract

International audienceThe degradation of coatings used to protect turbine blades is closely linked to spalling resistance, which depends on the stability of the protective oxide scale produced by oxidation of the bond coat. In the present study, TEM microstructural observations associated with SIMS analyses were performed, according to different experimental conditions, to describe the microstructural and chemical changes occurring in a NiPtAl bond coat deposited on a nickel based superalloy, as well as elemental segregation at the Al2O3/NiAlPt interface. Interfacial sulfur segregation is well known to be responsible for alumina spallation during exposure at high temperature, this phenomenon often being linked to cavity formation and growth. Sulfur detection was achieved using the SIMS technique which enables S segregation to be detected at the oxide/BC interface or within the bond coat layer. The purpose of the present study was to compare the degree of S segregation, at the scale/BC interface and within the NiPtAl alloy, for different Ni based alloys (two S contents) and different oxidation conditions (isothermal and cyclic). The results obtained showed that, at the TGO/BC interface, the concentration of voids depends on the initial sulfur content in the superalloy for isothermal treatments. On the contrary, after cyclic tests, interfacial sulfur enrichment increases while the interfacial porosity fraction remains constant. These results agree with the proposal that sulfur segregation occurs at both cavity surfaces and intact interfaces