Abstract: The accurate extraction of the refractive index of top coat (TC) and thermally grown oxide (TGO) layer in terahertz band is an important condition for terahertz nondestructive testing of thermal barrier coatings (TBCs). In view of the fact that only reflection measurement can be used for coating samples, we compare the results of THz optical parameters and thickness extraction under the condition of reflection measurement and traditional transmission measurement. Then, the refractive index of TC and TGO in plasma sprayed 8YSZ thermal barrier coating (TBCs) is extracted by using the reflection terahertz time domain pulse imaging system, and the thickness distribution of TC layer is further measured and imaged according to the extracted refractive index. The results show that the refractive index and thickness of the sample can also be accurately extracted by reflection measurement in the effective frequency band with small attenuation. The average refractive index of TC and TGO is 5.23 and 2.91 respectively. The refractive index of α-Al₂O₃, the main component of TGO, is 2.85. The average thickness of TC in TBCs is 257 μm. The inhomogeneity of interface between TC and bonding layer (BC) can be observed from THz image of TC thickness. Reflection THz nondestructive testing can accurately extract the refractive index and thickness of TC and TGO in TBCs, which is of great significance for the identification of cracks and bubbles in TBCs and the THz detection of TGO growth.

Key words: terahertz, thermal barrier coatings, thermally grown oxide, refractive index, thickness, non-destructive testing