Fig. 1. Structure diagram of preset defects and coating sample. (a) Preset defects on metal; (b) structure of coating sample with defects

Fig. 2. Photos of coating samples.(a) Preset defects; (b) sample 1#; (c) sample 2#

Fig. 3. Schematic diagram and photo of reflection-type THz-TDS system.(a) Schematic of reflection-type THz-TDS system;(b) photo of THz-TDS and scanning system

Fig. 4. Propagation model of terahertz wave in multilayer media

Fig. 5. Optical parameters of the coating at terahertz band.(a) Refractive index; (b) extinction coefficient

Fig. 6. Schematic diagram of terahertz wave propagation in the coating.（a）Without bonding defects；（b）with bonding defects

Fig. 7. THz time-domain simulation results of coatings.(a) 80-μm-thick coating without bonding defects; (b) 80-μm-thick coating with bonding defects; (c) 50-μm-thick coating without bonding defects; (d) 50-μm-thick coating with bonding defects

Fig. 8. Relationship between skewness of the second order echo and defect thickness

Fig. 9. THz-TDS signals of coating samples.(a) Sample 1#; (b) sample 2#

Fig. 10. Terahertz imaging results of coating samples. (a) Peak-to-peak imaging of sample 1#; (b) minimum imaging of sample 1#; (c) skewness imaging of sample 1#; (d) peak-to-peak imaging of sample 2#; (e) minimum imaging of sample 2#; (f) skewness imaging of sample 2#