Laser ultrasonic evaluation of bonding layer in thermal barrier coating

Abstract

A noncontact laser ultrasonic method is provided for nondestructive characterization of bonding layer (BL) in thermal barrier coating (TBC). A physical mode of thin multi-layered structure is established for the case of received ultrasonic waveform consisting of reverberant overlapping echoes. Then, the transmission coefficient of the mode was derived in this work. Experiments have been performed on TBC specimens using the proposed method. The specimens are produced by electronic beam physical vapor deposition (EBPVD) method. A pulsed laser with width of 10 ns was used to generate ultrasonic while a two wave mixing (TWM) interferometer was employed to receive the ultrasonic signals. Wavelet soft-threshold method (WSTM) was introduce to improve the signal to noise ratio of laser ultrasonic testing TBC. The transmission coefficient spectrum of TBC was obtained to measure the longitudinal velocity of BL. Further, the numerical fitting method is used to determine the attenuation of BL. The proposed method is used to evaluate TBC after 1, 10 and 100 times, and the experimental results are in according with the SEM observations. This method is able to nondestructively characterize BL in TBC system, and is important to practical engineering application.

Nondestructive measurements of residual stress in air plasma‐sprayed thermal barrier coatings

“…The limitations of these techniques in the applications to TBCs are summarized as follows. Neither ultrasonic waves nor THz waves are able to transmit throughout the YSZ topcoat because of the large signal attenuation caused by the porous microstructure 20‐24 . In the active thermography method, the temperature difference between the delaminated and nondelaminated areas in TBCs is usually not evident for accurate detection 25‐27 .…”

“…Neither ultrasonic waves nor THz waves are able to transmit throughout the YSZ topcoat because of the large signal attenuation caused by the porous microstructure. [20][21][22][23][24] In the active thermography method, the temperature difference between the delaminated and nondelaminated areas in TBCs is usually not evident for accurate detection. [25][26][27] The signal of TGO thickness detected by the eddy current testing method depends on the microconstituents in TGO, such as Al 2 O 3 , NiO, and Cr 2 O 3 , which complicate the interpretation of the acquired signal for reliable detection.…”