Thermal diffusivity measurement by thermographic technique for the non-destructive integrity assessment of TBCs coupons

For thin (< 200 μm) air plasma spray (APS) and electron beam physical vapor deposition (EBPVD) 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 (> 200 μm) APS TBCs.In this work, a semi-quantitative estimation of cracks at the interface of APS TBCs thicker than 300 μm is obtained from thermal diffusivity values measured by using a single side thermographic technique on coupons subjected to thermal cycling.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 idea presented hereinafter consists in removing the effects of sintering from apparent thermal diffusivity to estimate cracking at the interface.