Experimental and analytical evaluation of damage processes in thermal barrier coatings

Progressive damage evolution was evaluated using microscopy on samples subjected to a series of di€erent thermal cycle pro®les. Fickʹs law was used to describe the thermally grown oxide (TGO) buildup during early cycles. A correlation of damage thickness and oxide thickness for di€erent thermal cycle pro®les was established. The importance of this correlation is that, for a given thermal cycle pro®le and number of cycles, the oxide thickness can be calculated analytically using Fickʹs law, and from this thickness one can determine the interlayer separation (damage thickness). Both oxidation kinetics and interlayer separation (delamination) appear to have signi®cant 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 ®nally 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