Experimental investigation of the bond-coat rumpling instability under isothermal and cyclic thermal histories in thermal barrier systems

Reliable life-prediction models for the durability of thermal barrier coatings require the identification of the relative importance of various mechanisms responsible for the failure of the coatings at high temperatures. Studies of these mechanisms in subsystems of thermal barrier coatings can provide valuable information. In the present work, we undertake an experimental study of 'rumpling', or progressive roughening of the bond-coat (BC) surface in the bond coat-superalloy systems upon high-temperature exposure. Thermal cycling and isothermal experiments are carried out on a platinum aluminide BC, a nickel aluminide BC, and an NiCoCrAlY BC deposited on an Ni-based superalloy in air and in vacuum. Cyclic experiments are conducted in air for different levels of initial roughness of the BC surface. Upon thermal cycling in air, the BC surfaces with a wide range of initial roughness rumple to comparable characteristic wavelengths and amplitudes, indicating the insensitivity of the rumpling phenomenon to the initial surface fluctuations. Observations of the rumpled BC surfaces show that the stresses in the BC provide driving force for the rumpling process. On comparing the experimental observations with existing rumpling models in the literature, it is concluded that the thermally grown aluminium oxide and the microstructural changes in the BC have a limited role in inducing rumpling. Plausibility of several possible deformation mechanisms is discussed. It is shown that several of the rumpling observations in the current work can be explained by a BC stress-driven surface-diffusion model from the literature.