Compressive Creep Testing of Thermal Barrier Coated Nickel-Based Superalloys

Turbine airfoils have complex geometries and during service operation are subjected to complex loadings. In most publications, results are typically reported for either uniaxial, isothermal tensile creep or for thermal cyclic tests. The former generally provide data for creep of the superalloy and the overall performance, and the later provide data for thermal barrier coating (TBC) spallation as a function of thermally-grown oxide (TGO) thickness, surface roughness, temperature, and thermal mismatch between the layers. Both tests provide valuable data, but little is known about the effect of compressive creep strain on the performance of the superalloy/protective system at elevated temperatures. In conjunction with computational model development, laboratory-scale experimental validation was undertaken to verify the viability of the underlying damage mechanics concepts for the evolution of TBC damage. Nickel-based single-crystal Rene´ N5 coupons that were coated with a ∼150–200 μm MCrAlY bond coat and a ∼200–240 μm 7-YSZ APS topcoat were used in this effort. The coupons were exposed to 900, 1000, and 1100°C, for periods of 100, 300, 1000 and 3000 hours in slotted silicon carbide fixtures. The difference in the coefficients of thermal expansion of the Rene´ N5 substrate and the test fixture introduces thermally induced compressive stress in the coupon samples. Exposed samples were cross-sectioned and evaluated using scanning electron microscopy (SEM). Performance data was collected based on image analysis. Energy-dispersive x-ray (EDX) was employed to study the elemental distribution in the TBC system after exposure. To better understand the loading and failure mechanisms of the coating system under loading conditions, nanoindentation was used to study the mechanical properties (Young’s modulus and hardness) of the components in the TBC system and their evolution with temperature and time. The effect of uniaxial in-plane compressive creep strain on the rate of growth of the thermally grown oxide layer, the time to coating failure in TBC systems, and the evolution in the mechanical properties are presented.