Mechanical Property Degradation Induced by Elevated Temperature Environmental Attack in MCrAlY Coatings of Gas Turbine Blades
This paper presents the influence of in-service environmental attack on the mechanical properties and microstructure/composition of plasma sprayed MCrAlY coatings over Ni based superalloy substrates in gas turbine (GT) blades using a small punch (SP) testing technique and scanning Auger microprobe analysis. SP tests on near surface CoNiCrAlY coatings demonstrated strong dependence of mechanical degradation on the elevated temperature environmental condition. In-service operation under the combined fuels of kerosene and liquefied natural gas (LNG) led to a two-fold increase in the ductile-brittle transition temperature over coatings observed mainly under LNG because of more extensive oxidation and grain boundary sulfidation. In CoCrAlY coatings of GT blades operated at higher temperatures than the CoNiCrAlY ones under LNG fuels, substantial oxidation/carbonization and nitridation occurred in near surface and interface regions of concave coatings, respectively, but not in convex coatings. Brittle cracks in the near surface and interface of concave coatings more easily initiated up to 950 °C than in the convex coatings. It was found that the oxidation/carbonization and nitridation in the concave CoCrAlY coatings produced a greater ductility loss than the oxidation/sulfidation in the CoNiCrAlY coatings.
KeywordsCombustion Fatigue Porosity Nickel Sulfide
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