Lifetime evaluation of a thick thermal barrier coated superalloy used in turbine blade

Abstract

This paper deals with evaluation of lifetime of a thick thermal barrier coated (TBC), AE-437A Ni base superalloy mostly employed for manufacturing compressor and stationary stator blades in aero turbines from accelerated creep tests. Severe high-temperature oxidation is likely the cause of the reduced lifetime of the bare substrate while accelerated creep experiments are carried out in an oxidizing environment. The bond coat was capable of supporting some load and the apparent equivalence of 800 °C stress rupture properties of thickly and thinly coated AE-437A was not anticipated since the thick TBC coating with its 160 μm bond coat represents an ∼20% increase in cross-sectional area for a 3 mm × 4 mm sample; whereas thin TBC with a 100 μm bond coat has a ∼12% increase in cross-section for the same initial sample size. Delamination of top coat layer and bond coat from the substrate occurred at very high creep stress, whereas delamination of TBC from the bond coat was evident at intermediate creep stress level. It was found that furnace thermal cycling (10–50, 1-h thermal cycles) on the TBC samples prior to creep loading at 70 MPa/800 °C, drastically reduces the lifetime or rupture life of the TBCs. The damage generated by this micro-cracking is expected to be a primary life-limiting factor.