Experimental and analytical evaluation of damage processes in thermal barrier coatings

Abstract

Progressive damage evolution was evaluated using microscopy on samples subjected to a series of different thermal cycle profiles. Fick’s law was used to describe the thermally grown oxide (TGO) buildup during early cycles. A correlation of damage thickness and oxide thickness for different thermal cycle profiles was established. The importance of this correlation is that, for a given thermal cycle profile and number of cycles, the oxide thickness can be calculated analytically using Fick’s law, and from this thickness one can determine the interlayer separation (damage thickness). Both oxidation kinetics and interlayer separation (delamination) appear to have significant roles with respect to spallation. As early microcracks coalesce to form major delamination cracks or interlayer separation, the susceptibility for coating buckling is increased. The delamination cracks finally consume the TGO layer. Progressive microcrack linking is a possible mechanism to develop such critical delamination crack lengths. Physical evidence of buckling was found in specimens prior to complete spallation.