Evaluation of the Lifetime and Effect of Furnace Thermal Cycling on Stress Rupture Behaviour of a Thermal Barrier Coated Superalloy Used in a Turbine Blade

Abstract

This paper highlights the trends recently observed while evaluating the lifetime of a 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 in air at 800 degrees C. Based on accelerated stress rupture properties, the TBC specimens had superior life in terms of rupture hours compared to that of the bare substrate as 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 effect of furnace thermal cycling on accelerated creep properties of the TBC samples at 70 MPa and 800 degrees C was also studied. It was observed that furnace thermal cycling (10 to 50, one hour thermal cycles) prior to creep loading, drastically reduces the rupture life of the TBCs. The life of thermal barrier coating prior to delamination and spallation is dominated by micro-cracking in both the thermally grown oxide and the yttria stabilized zirconia top coat. The damage generated by this micro-cracking is expected to be a primary life limiting factor. 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.