Progressive damage assessment in thermal barrier coatings using thermal wave imaging technique

Thermal wave imaging (TWI)—a noncontact nondestructive method was used to evaluate Electron Beam Physical vapor-deposited (EB–PVD) thermal barrier coating (TBC), ZrO2–8 wt.% Y2O3 deposited on a Rene N5 single-crystal substrate with platinum aluminide bond coat to determine the efficacy of this technique to assess the integrity of TBC over its lifetime. In addition, the technique was used to monitor the development of specific failure modes such as coating delamination after various thermal cycles utilizing the thermal wave amplitude signals. Contrary to the commonly held view that coating delamination continues to grow (amenable to fracture mechanics analysis) over the lifetime, the present study confirms that coating delamination and buckling occurs only toward the end of life and has a very short span. It is consistent with observations that oxide growth and its breakdown due to diffused microcracking under the TBC layer consumes most of the life under thermal cycling as evidenced by increasingly higher thermal wave amplitude with increasing number of thermal cycles with absence of coating delamination and buckling until near the end of TBC life. Results from this investigation clearly point out the strength of the TWI technique for TBC damage assessment and health monitoring. Saturation of thermal wave amplitude can be considered as an indicator of the end of useful coating life.