Non-destructive thermographic monitoring of crack evolution of thermal barrier coating coupons during cyclic oxidation aging

The diffused microscopic cracking of the interface between a thick (>300μm) air plasma spray (APS) thermal barrier coating (TBC) and a bond coat cannot be clearly identified by non-destructive evaluation and testing techniques. In this work, a semi-quantitative estimation of cracks at the TBC interface is obtained from thermal diffusivity values measured on coupons subjected to thermal cycling using a single-side thermographic technique. In fact, during thermal cycling, two phenomena occur: sintering, which promotes a significant increase in thermal diffusivity, and cracking, representing an additional thermal resistance which causes an apparent decrease in thermal diffusivity. The results refer to an experimental activity carried out on 28 APS TBC samples cyclic aged at six different fractions of their lifetime. For each sample, the thermal diffusivity was measured at fixed lifetimes, and the evolution of the cracked fraction of the interface was estimated by adopting a two-dimensional inversion model. Furthermore, at each of the six lifetime fractions, some samples were destructively characterized by image analysis, and the results were compared with the estimations given by the inversion model. Good agreement between the non-destructive estimations and image analysis results was obtained. Moreover, a figure of merit incorporating both the cracked fraction and the crack thickness is also proposed for ranking the damage.