Article Article
Millimeter Wave Thickness Evaluation of Thermal Barrier Coatings (TBCs) Using Open-Ended Waveguide Probes

Nondestructive testing (NDT) of thermal barrier coatings (TBCs) is a critical and ongoing topic of research and development. In particular, inspection techniques that determine the thickness of ceramic topcoat and thermally grown oxide (TGO) are of interest. This work investigates the utility of open-ended rectangular waveguide probes in the millimeter wave frequency range of 26.5–110 GHz for evaluation of topcoat and TGO thicknesses through a compressive set of electromagnetic (EM) simulations. In addition, these EM simulations are used to illustrate the influence of probe size and TBC substrate curvature on the complex reflection coefficient properties and the subsequent thickness estimation. The impact of volumetric porosity level on the same is also investigated. A standing-wave probe at V-band (50–75 GHz) is constructed and used to measure the topcoat thickness on three button-type TBC samples. This probe eliminates the need for using expensive and bulky vector network analyzers (VNA), which is quite desirous from a practical point-of-view. The experimental results indicate the capability of estimating the topcoat thickness to within ±15 μm (0.6 mils).



1. N. P. Padture, M. Gell, and E. H. Jordan, Science. 296(5566), 280–284 (2002 Apr). DOI: 10.1126/science.1068609.

2. A. J. K. M. Firdaus et al., Microwave nondestructive evaluation of thermal barrier coated turbine blades using correlation analysis, in 2016 46th European Microwave Conference (EuMC), Oct. 2016, pp. 520–523. DOI: 10.1109/EuMC.2016.7824394.

3. S. Parthasarathi, B. R. Tittmann, and E. J. Onesto, J. Therm. Spray Technol. 6(4), 486–488 (1997 Dec). DOI: 10.1007/s11666-997-0035-3.

4. B. Cao, E. Cai, and M. Fan, Mater Eval. 79(2), 125–135 (2021 Feb). DOI: 10.32548/

5. R. Zoughi, Microwave Non-Destructive Testing and Evaluation (2000). DOI: 10.1007/978-94-015-1303-6.

6. M. Sayar, D. Seo, and K. Ogawa, NDT E Int. 42(5), 398–403 (2009). DOI: 10.1016/j.ndteint.2009.01.003.

7. G. N. Jawad and M. F. Akbar, IEEE Access 9, 98561–98572 (2021). DOI: 10.1109/ACCESS.2021.3095105.

8. X. Q. Cao, R. Vassen, and D. Stoever, J. Eur. Ceram. Soc. 24(1), 1–10 (2004 Jan). DOI: 10.1016/S0955-2219(03)00129-8

9. M. T. Lanagan et al., Mater. Lett. 7(12), 437–440 (1989 Mar). DOI: 10.1016/0167-577X(89)90047-5.

10. J. Sebastian, A. Scaria, and D. G. Kurian, IOP Conf. Ser.: Mater. Sci. Eng. 330, 012043 (Mar 2018). DOI: 10.1088/1757-899X/330/1/012043.

11. J. Gao, X. Xiong, and Y. Gao, J. Mater. Sci. Mater. Electron. 28(16), 12015–12020 (2017 Aug). DOI: 10.1007/s10854-017-7011-6.

12. A. Case, M. T. Al Qaseer, and Zoughi, R., Microwave Inspection of Thermal Barrier Coating (TBC) Systems, Proceedings of the American Society for Nondestructive Testing (ASNT) Research Symposium, pp. 30–33, St. Louis, MO, June 20-23, 2022.

13. D. M. Pozar, Microwave Engineering, 4th ed. (Hoboken, NJ: John Wiley & Sons, 2012).

14. M. Kempin et al., IEEE Trans. Instrum. Meas. 63(6), 1524–1534 (2014 Jun). DOI: 10.1109/TIM.2013.2291952.

15. M. T. Ghasr et al., IEEE Trans. Instrum. Meas. 64(10), 2802–2812 (2015 October). DOI: 10.1109/TIM.2015.2426352.

16. A. H. Sihvola, Electromagnetic Mixing Formulas and Applications, (London Angleterre: Institution of Electrical Engineers, 1999).

17. M. T. Ghasr, D. Simms, and R. Zoughi, IEEE Trans. Instrum. Meas. 58(5), 1505–1513 (2009 May). DOI: 10.1109/TIM.2008.2009133.

18. M. Dvorsky, M. T. Al Qaseer, and R. Zoughi, Microwave Surface Conductivity Measurement Using an Open-Ended Circular Waveguide Probe, 2022 IEEE International Instrumentation and Measurement Technology Conference (I2MTC), 2022, pp. 1–6, DOI: 10.1109/I2MTC48687.2022.9806657.

19. M. A. Abou-Khousa et al., IEEE Trans. Instrum. Meas. 59(3), 534–542 (2010 March). DOI: 10.1109/TIM.2009.2024701.

20. M. T. Ghasr, R. Zoughi, and G. Steffes, Estimation of Corrosion Thickness under Paint using a Simple Millimeter Wave Probe, Proceedings of the 3nd International Conference on Electromagnetic Near-Field Characterization & Imaging (ICONIC 2007), St. Louis, USA, 27–29–88 (June 2007), pp. 83–88.


Usage Shares
Total Views
60 Page Views
Total Shares
0 Tweets
0 PDF Downloads
0 Facebook Shares
Total Usage