CMSX-4 with Thermal Barrier Coating under Thermal Gradient Mechanical Fatigue

Kurzfassung

Gas turbine blades with a thermal barrier coating are internally cooled. Between the hot surface and the cooled interior a thermal gradient develops, resulting, under constraints, in high multiaxial stresses. Conventional thermal fatigue or thermal mechanical fatigue testing can not fully simulate these complex multiaxial loading conditions, necessitating thermal gradient mechanical fatigue (TGMF) testing. A new TGMF set-up with a radiation furnace was developed, allowing controlled thermal and mechanical cyclic loading with, additionally, a thermal gradient over the wall of tubular specimens. The advantages of the furnace are high power (16 lamps a 1000 W), high heating and cooling rates, and lamp lives exceeding 8000 thermal cycles. The studied material system was the single crystalline superalloy CMSX-4 with a NiCoCrAlY bond coat and an yttria stabilized zirconia thermal barrier coating. Both coatings were applied by electron beam physical vapor deposition. TGMF tests with a maximal surface temperature of 1050°C, mechanical loads up to 400 MPa, and cycle numbers up to 9000 were performed. The tests resulted in microstructural changes and defects, reflecting in each case the particular temperatures, thermal gradients, and local stresses. For example, phase evolution of the metal coatings, cracks in the ceramic coating, and rafting of the substrate morphology were investigated. The observed morphology changes and defects were related to the stresses, which vary over the wall thickness of the hollow specimens, due to the thermal gradient and the mismatch of thermal expansion coefficients and elastic properties of substrate and coatings. The stress distribution was determined, adopting finite element calculations.