Oxidation/carbonization/nitridation and in-service mechanical property degradation of CoCrAlY coatings in land-based gas turbine blades

This article describes variations in the microstructure/composition and mec hanical properties in plasma sprayed CoCrAlY coatings and a modified Rene 8 0 substrate of gas turbine blades operated for 21,000 h under liquefied nat ural gas fuels. Substantial oxidation/carbonization occurred in the near su rface region of concave coatings, but not in the convex coatings. Aluminum and nickel/titanium-rich nitrides formed in near interface coatings and sub strates of concave side of blades, respectively. Small punch (SP) specimens were prepared from the different blade location to examine the variation o f the mechanical properties in the coatings. In SP tests, brittle cracks in the near surface and interface coatings of the concave side easily initiat ed up to 950 degrees C. The convex coatings exhibited higher ductility than the concave coatings and substrate and showed a rapid increase in the duct ility above 800 degrees C, Thus it is apparent that the oxidation/carboniza tion and nitridation in the concave coatings produced a significant loss of the ductility. The in-service degradation mechanism of the CoCrAlY coating s is discussed in light of the operating temperature distribution and compa red to that of CoNiCrAlY coatings induced by grain boundary sulfidation/oxi dation.