Research progress in terahertz non-destructive testing of thermal barrier coatings

Terahertz (THz) technology is an advanced non-destructive testing method that has emerged in recent years. Owing to excellent characteristic of non-contact, non-ionization, high-precision, and automated online detection, it has now shown great potential in thermal barrier coating (TBCs) performance evaluation and life prediction. The development of terahertz non-destructive testing technology for thermal barrier coatings was introduced briefly and analysis and summary of ceramic thickness measurement, TGO and erosion monitoring, and pore, crack and stress state characterization were carried out emphatically. It is pointed out that the current technology for monitoring the thickness of single-layer and multi-layer ceramic top layers using terahertz technology is mature enough and can be gradually extended to engineering application. In terms of TGO monitoring, some progress has been made on TGO thickness monitoring by the time difference and amplitude attenuation of terahertz time-domain reflection peaks, but the detection accuracy needs to be further improved. In terms of erosion monitoring, the thickness loss caused by erosion can be quantitatively detected accurately. Although CMAS that has penetrated into the coatings can be detected qualitatively, there is still a lack of experimental research on the quantitative characterization. In terms of pore monitoring, the effect of the porosity on the change of the terahertz optical parameters of the thermal barrier coating has been explored. It is proposed that the relative broadening ratio is a more proper measure of the porosity than the refractive index, extinction coefficient and dielectric constant for engineering application. Nevertheless, the microstructure characteristics of the pore still lack a comprehensive characterization. In terms of crack monitoring, the positions and widths of parallel cracks can be distinguished by deconvolution filtering combined Hanning window function with wavelet denoising, but it is still difficult to monitor multiple cracks. In terms of stress state characterization, it is found that the change in refractive index has a linear relationship with the stress change, and the optical strain coefficient can be derived from the change in refractive index to obtain the stress state, but the current reflective measurement system needs further development. Finally, the current status, existing problems and future development direction of terahertz nondestructive testing for thermal barrier coatings are summarized and prospected. © 2020, Chongqing Wujiu Periodicals Press. All rights reserved.