For thin (<200 micron) air plasma spray (APS) and electron beam physical vapor deposition (EB-PVD) ceramic thermal barrier coatings (TBCs), some non-destructive techniques indicate damage at the bond coat-TBC interface during either ageing or cyclic oxidation tests. However, no technique is available for thick (>250 micron) APS TBCs. Several applications of transient thermography for detecting macroscopic coating detachments are reported in the literature. Most of these techniques work properly when a sound TBC area is available for being compared to the defected one. On the other hand, if diffused microscopic cracking of the interface all along the inspected area is expected, these techniques do not allow a clear integrity assessment of the interface. In fact, owing to the heat diffusion, each single crack is too small to produce a thermal indication on the TBC surface and for the same reason no sound reference area can be clearly identified.In this work, a semi-quantitative estimation of cracks at the interface of TBCs is obtained from thermal diffusivity values measured on coupons subjected to thermal cycling by using a single side thermographic technique. In fact, during thermal cycling, two phenomena occur: sintering that promotes a significant increase of thermal diffusivity, and cracking that, representing an additional thermal resistance, causes an apparent decrease of thermal diffusivity.The reported results refer to an experimental activity carried out on twenty-eight 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 2-D inversion model. Furthermore, at each of the six lifetime fractions some samples were destructively characterised by image analysis and the results were compared to the estimations given by the inversion model. A good agreement between the non destructive estimations and image analysis results has been obtained. Moreover a figure of merit incorporating both the cracked fraction and the crack thickness is also proposed for ranking the damage.