Article Article
Nondestructive Evaluation of Coating Defects and Uniformity Based on Terahertz Time-Domain Spectroscopy

Structural coatings are widely used because of their excellent mechanical and thermal properties. To evaluate defects and uniformity in coatings, both qualitatively and quantitatively, a terahertz time-domain spectroscopy (THz-TDS) detection technique is proposed. The thermal barrier coating is selected as a typical single-layer coating structure for quantitative defect detection. A wavelet noise reduction method is used on the acquired raw signals to eliminate noise while retaining detailed information. The peak value of the preprocessed signal is used as a feature parameter for imaging, and the automatic binarization threshold segmentation technique is used to describe the defects quantitatively. The automotive coating is selected as a typical multilayer coating structure for uniformity detection. The time-frequency characteristics of a strongly superimposed signal are analyzed; the peak-to-peak value is used as a feature parameter for imaging, and the peak-to-peak 3D imaging is then used to characterize the coating uniformity, enabling fast and intuitive acquisition of the coating state. The statistical characteristics of the standard deviation and range are used to evaluate the uniformity of each layer of the automotive coating. The results show that the uniformity of the clean coating is optimal. The results of a subsequent thickness inspection using an eddy current gauge are consistent with those of the terahertz technique. The results demonstrate that THz-TDS can effectively detect defects and uniformity in coatings.



Chen, H.-L.R., B. Zhang, M.A. Alvin, and Y. Lin, 2012, “Ultrasonic Detection of Delamination and Material Characterization of Thermal Barrier Coatings,” J Therm Spray Technol, Vol. 21, pp. 1184–1194,

Chen, Z., H. Huang, K. Zhao, W. Jia, and L. Fang, 2018, “Influence of Inhomogeneous Thermally Grown Oxide Thickness on Residual Stress Distribution in Thermal Barrier Coating System,” Ceram Int, Vol. 44, pp.16937–16946,

Choi, C.J., J.K. Lee, L.K. Kwac, and J.Y. Kim, 2010, “Development of Advance Thermal Barrier Coating in Gas Turbine,” Adv Mater Res, Vol.123–125, pp. 459–462,

Doleker, K.M., Y. Ozgurluk, Y. Kahraman, and A.C. Karaoglanli, 2021, “Oxidation and Hot Corrosion Resistance of HVOF/EB-PVD Thermal Barrier Coating System,” Surf Coat Technol, Vol. 409,

Gomez, C.J.J., R. Naraparaju, P. Mechnich, K. Kelm, U. Schulz, and C.V. Ramana, 2020, “Effects of Yttria Content on the CMAS Infiltration Resistance of Yttria Stabilized Thermal Barrier Coatings System,” J Mater Sci, Vol. 43, pp. 74–83,

Hu, B.B., and M.C. Nuss, 1995, “Imaging with Terahertz Waves,” Opt Lett, Vol. 20, No. 16, pp. 1716–1718,

Jenck, J.F., F. Agterberg, and M.J. Droescher, 2004, “Products and Processes for a Sustainable Chemical Industry: A Review of Achievements and Prospects,” Green Chem, Vol. 6, No. 11, pp. 544–556,

JJG, 2005, JJG 818: Verification Regulation of Magnetic and Eddy Current Measuring Instrument for Coating Thickness, Metrology & Measurement Industry Standard, Beijing Institute of Measuring and Testing

Krimi, S., G. Torosyan, and R. Beigang, 2017, “Advanced GPU-Based Terahertz Approach for In-Line Multilayer Thickness Measurements,” IEEE J

Sel Top Quantum Electron, Vol. 23, No. 4, pp. 1–12,

Li, Y., B. Yan, W. Li, and D. Li, 2016, “Thickness Assessment of Thermal Barrier Coatings of Aeroengine Blades via Dual-Frequency Eddy Current Evaluation,” IEEE Magn Lett, Vol. 7, pp. 1–5,

Lou, H.H., and Y.L. Huang, 2003, “Hierarchical Decision Making for Proactive Quality Control: System Development for Defect Reduction in Automotive Coating Operations,” Eng Appl Artif Intel, Vol. 16, No. 3, pp.237–250,

Ma, Z., Y. Zhao, Z. Luo, and L. Lin, 2014, “Ultrasonic Characterization of Thermally Grown Oxide in Thermal Barrier Coating by Reflection Coefficient Amplitude Spectrum,” Ultrasonics, Vol. 54, No. 4, pp. 1005–1009,

Miller, R.A., 1997, “Thermal Barrier Coatings for Aircraft Engines: History and Directions,” J Therm Spray Technol, Vol. 6,

Naumenko, D., R. Pillai, A. Chyrkin, and W.J. Quadakkers, 2017, “Overview on Recent Developments of Bondcoats for Plasma-Sprayed Thermal Barrier Coatings,” J Therm Spray Technol, Vol. 26,

Prendi, L., P. Henshaw, and E.K.L. Tam, 2006, “Automotive Coatings with Improved Environmental Performance,” Int J Environ Stud, Vol. 63, No. 4, pp. 463–471,

Su, K., Y.-C. Shen, and J.A. Zeitler, 2014, “Terahertz Sensor for Non-contact Thickness and Quality Measurement of Automobile Paints of Varying Complexity,” IEEE Trans Terahertz Sci Technol, Vol. 4, No. 4, pp. 432–439,

Tang, Q., J. Liu, J. Dai, and Z. Yu, 2017, “Theoretical and Experimental Study on Thermal Barrier Coating (TBC) Uneven Thickness Detection Using Pulsed Infrared Thermography Technology,” Appl Therm Eng, Vol. 114, pp. 770–775,

Tu, W., S. Zhong, Y. Shen, Q. Zhou, and L. Yao, 2014, “FDTD-Based Quantitative Analysis of Terahertz Wave Detection for Multilayered Structures,” J Opt Soc Am A, Vol. 31, No. 9, pp. 2285–2293,

Unnikrishnakurup, S., J. Dash, S. Ray, B. Pesala, and K. Balasubramaniam, 2020, “Nondestructive Evaluation of Thermal Barrier Coating Thickness Degradation Using Pulsed IR Thermography and THz-TDS Measurements: A Comparative Study,” NDT & E Int, Vol. 116,

van Mechelen, J.L.M., A.B. Kuzmenko, and H. Merbold, 2014, “Stratified Dispersive Model for Material Characterization Using Terahertz Time-Domain Spectroscopy,” Opt Lett, Vol. 39, No. 13, pp. 3853–3856,

Wasekar, N.P., L. Bathini, L. Ramakrishna, D.S. Rao, and G. Padmanabham, 2020, “Pulsed Electrodeposition, Mechanical Properties and Wear Mechanism in Ni-W/SiC Nanocomposite Coatings Used for Automotive Applications,” Appl Surf Sci, Vol. 527,

Wei, R.,E. Langa, C. Rincon, and J.H. Arps, 2006, “Deposition of Thick Nitrides and Carbonitrides for Sand Erosion Protection,” Surf Coat Technol, Vol. 201, No. 7, pp. 4453–4459, Wei, S., F. Wang, Q.B. Fan, and M. Zhuang, 2012, “Effects of Defects on the Effective Thermal Conductivity of Thermal Barrier Coatings,” Appl Math Model, Vol. 36, No. 5, pp. 1995-2002,

White, J., G. Fichter, A. Chernovsky, J.F. Whitaker, D. Das, T.M. Pollock, and D. Zimdars, 2009, “Time Domain Terahertz Non-destructive Evaluation of Aeroturbine Blade Thermal Barrier Coatings,” AIP Conf Proc, Vol. 1096, No. 1, pp. 434–439,

Yong, L., Z. Chen, Y. Mao, and Q. Yong, 2012, “Quantitative Evaluation of Thermal Barrier Coating based on Eddy Current Technique,” NDT & E Int, Vol. 50, pp. 29–35,

Zhai, M., A. Locquet, C. Roquelet, L.A. Ronqueti, and D.S. Citrin, 2020, “Thickness Characterization of Multi-layer Coated Steel by Terahertz Time-Of-Flight Tomography,” NDT & E Int, Vol. 116,

Zhang, X.-C., and J. Xu, 2010, Introduction to THz Wave Photonics, Springer New York, NY,

Zhao, Y., A. Shinmi, X. Zhao, P.J. Withers, S. Van Boxel, N. Markocsan, P. Nylen, and P. Xiao, 2012, “Investigation of Interfacial Properties of Atmospheric Plasma Sprayed Thermal Barrier Coatings with Four-Point Bending and Computed Tomography Technique,” Surf Coat Technol, Vol. 206, No.23, pp. 4922–4929,

Zhong, S., 2019, “Progress in Terahertz Nondestructive Testing: A Review,” Front Mech Eng, Vol. 14, pp. 273-281,


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