Nondestructive testing using an acousto-ultrasonic technique has been utilized to detect the change of material properties and provide early warning of failure of thermal barrier coating (TBC) systems. Testing was performed on Rene´ N5 and Haynes 230 coupons with an applied NETL-bond coat, as well as on coupons containing both an applied MCrAlY bond coat and 7-YSZ top coat. The coupons were subjected to either cyclic or isothermal testing at 1100°C. Ultrasonic testing was performed before and after thermal testing using piezoelectric sensors with dry contact on the surface of the coatings. Proof-of-concept test results indicated that changes in the properties of the ∼ 40 μ.m bond coat can be detected using the proposed technique. Waveforms generated via Pitch/Catch indicated minor changes within the bond coat applied to Rene´ N5 substrate after 400∼500 hours of cyclic oxidation at 1100°C. In contrast, marked differences in waveforms and travel time reflected significant crack formation and spallation of the bond coat from the Haynes 230 substrate. Finite element analysis (FEA) simulation of the wave propagation on a simplified TBC system with nonlinear effects was conducted. FEA results clearly show detection of a small embedded void incorporated to simulate delamination. Comparisons between experimental measurements and finite element simulations were used to estimate the material properties of the coatings and the substrate.

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