Bond coat surface rumpling in thermal barrier coatings

Long term durability of thermal barrier coatings is determined by the mechanisms responsible for the nucleation and,growth of interfacial cracks between the coating and the substrate. One such mechanism is the progressive roughening or "rumpling" of the bond coat surface upon thermal cycling in superalloy-bond coat systems. The present work is inspired by a recent rumpling experiment [1] pointing to the important role played by the bond coat stresses in the rumpling process. A thermodynamic analysis is presented in this paper based on a bond coat stress-driven surface diffusion mechanism to explain the rumpling process. The surface chemical potential variation required for such a diffusion process is induced by the changes in the surface energy and the elastic strain energy in the system during rumpling. It is found that the amplitude of surface fluctuations with a wavelength higher than a threshold value monotonically increases with thermal cycling, while the amplitude for fluctuations with wavelength lower than the threshold value monotonically decreases. A range of wavelengths is identified to have a high monotonic amplitude increasing rate, resulting in characteristic wavelengths of rumpled surface upon thermal cycling. Effect of various input parameters on these wavelength values is analyzed. It is shown that under certain conditions, the range of wavelengths obtained from the model agree with that observed in several experimental studies. (C) 2002 Published by Elsevier Science Ltd on behalf of Acta Materialia Inc.