热障涂层折射率与厚度的太赫兹无损检测

Terahertz Nondestructive Testing of Refractive Index and Thickness of Thermal Barrier Coating

精确提取陶瓷层(Top coat,TC)与热生长氧化层(Thermally grown oxide, TGO)层在太赫兹频段的折射率是进行热障涂层(Thermal barrier coatings,TBCs)太赫兹无损检测研究的重要条件。由于对涂层样品只能采取反射式测量,所以首先比较了反射式与传统透射式测量条件下提取样品太赫兹光学参数及厚度的结果,随后利用反射式太赫兹时域脉冲成像系统提取等离子体喷涂的8YSZ热障涂层(TBCs)中TC层与TGO层的折射率,并依据所提取折射率进一步对TC层的厚度分布进行测量及成像。试验结果表明在材料中衰减较小的有效频段下反射式测量同样可以精确提取样品的折射率以及厚度,反射式测量TC层的平均折射率为5.23,TGO层的折射率为2.91,TGO的主要成分α-Al2O3的折射率为2.85。TBCs样品中TC层的平均厚度为257μm,从TC层厚度的太赫兹图像中可观察到TC与粘结层(Bond coat,BC)界面的不均匀程度。反射式太赫兹无损检测可精确提取TBCs中TC与TGO的折射率和厚度,这对于TBCs中裂纹和气泡等缺陷的识别以及TGO生长太赫兹检测具有重要意义。

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 8 YSZ 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-Al2 O3, 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.