Instrumentation, Techniques, and Measurement

Nondestructive characterization of thermal barrier coating by noncontact laser ultrasonic technique

[+] Author Affiliations
Yang Zhao, Jianwei Chen, Zhenzhen Zhang

Laser Institute of Shandong Academy of Science, NDT Lab, 28789 Jingshi East Road, Jinan 250103, China

Opt. Eng. 54(9), 094104 (Sep 17, 2015). doi:10.1117/1.OE.54.9.094104
History: Received April 21, 2015; Accepted August 10, 2015
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Abstract.  We present the application of a laser ultrasonic technique in nondestructive characterization of the bonding layer (BL) in a thermal barrier coating (TBC). A physical mode of a multilayered medium is established to describe the propagation of a longitudinal wave generated by a laser in a TBC system. Furthermore, the theoretical analysis on the ultrasonic transmission in TBC is carried out in order to derive the expression of the BL transmission coefficient spectrum (TCS) which is used to determine the velocity of the longitudinal wave in the BL. We employ the inversion method combined with TCS to ascertain the attenuation coefficient of the BL. The experimental validations are performed with TBC specimens produced by an electron-beam physical vapor deposition method. In those experiments, a pulsed laser with a width of 10 ns is used to generate an ultrasonic signal while a two-wave mixing interferometer is created to receive the ultrasonic signals. By introducing the wavelet soft-threshold method that improves the signal-to-noise ratio, the laser ultrasonic testing results of TBC with an oxidation of 1 cycle, 10 cycles, and 100 cycles show that the attenuation coefficients of the BL become larger with an increase in the oxidation time, which is evident for the scanning electron microscopy observations, in which the thickness of the thermally grown oxide increases with oxidation time.

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© 2015 Society of Photo-Optical Instrumentation Engineers

Citation

Yang Zhao ; Jianwei Chen and Zhenzhen Zhang
"Nondestructive characterization of thermal barrier coating by noncontact laser ultrasonic technique", Opt. Eng. 54(9), 094104 (Sep 17, 2015). ; http://dx.doi.org/10.1117/1.OE.54.9.094104


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