Ye Dongdong, Wang Weize, Zhou Haiting, Huang Jibo, Wu Wenchao, Gong Hanhong, Li Zhen
Opt Express. 2019 Sep 30;27(20):28150-28165. doi: 10.1364/OE.27.028150.
Porosity is one of the most important indicators for the characterization of the comprehensive performance of thermal barrier coatings (TBCs). Herein, we explored a fast, nondestructive porosity evaluation method based on the terahertz time-domain broadening effect. Different preparation process parameters were used to deposit the ceramic coatings, and the porosity ranged from 9.09% to 21.68%. Monte Carlo simulations were conducted to reveal the transitive relation between porosity and the terahertz time-domain broadening at different extinction coefficients and transmission distances. A transmission mode with an incidence angle of 0° was used to estimate the terahertz dielectric properties of ceramic coatings and the relative broadening ratio of terahertz pulses at different porosities. As a result, the Monte Carlo simulations showed that the time-domain broadening effect was enhanced when the extinction coefficient and transmission distances increased. As the porosity increased, the refractive index decreased and the extinction coefficient increased. The latter was more sensitive to minor porosity changes as demonstrated by linear fitting comparisons. Meanwhile, the relative broadening ratio increased when the porosity increased, and reserved the sensitivity of the extinction coefficient to porosity changes. The effect was more obvious on the relative broadening ratio which experienced multiple transmissions and reflections. Moreover, the relative broadening ratio could be obtained faster and in an easier manner compared to the dielectric parameters in both the transmission and reflection modes, based on single-step tests with the use of actual terahertz wave inspection. Finally, this study proposed a novel, convenient, online, nondestructive, and noncontact porosity evaluation method that could be potentially utilized to evaluate the integrity of TBCs in gas turbine blades.
孔隙率是表征热障涂层（TBCs）综合性能的最重要指标之一。在此，我们探索了一种基于太赫兹时域展宽效应的快速、无损孔隙率评估方法。采用不同的制备工艺参数沉积陶瓷涂层，孔隙率范围为9.09%至21.68%。进行了蒙特卡罗模拟，以揭示在不同消光系数和传输距离下孔隙率与太赫兹时域展宽之间的传递关系。使用入射角为0°的透射模式来估计陶瓷涂层的太赫兹介电性能以及不同孔隙率下太赫兹脉冲的相对展宽比。结果表明，蒙特卡罗模拟显示，当消光系数和传输距离增加时，时域展宽效应增强。随着孔隙率增加，折射率降低，消光系数增加。通过线性拟合比较表明，后者对微小孔隙率变化更敏感。同时，随着孔隙率增加，相对展宽比增大，并保留了消光系数对孔隙率变化的敏感性。这种效应在经历多次透射和反射的相对展宽比上更为明显。此外，与透射和反射模式下的介电参数相比，基于实际太赫兹波检测的单步测试可以更快、更简便地获得相对展宽比。最后，本研究提出了一种新颖、便捷、在线、无损且非接触的孔隙率评估方法，该方法有可能用于评估燃气轮机叶片中热障涂层的完整性。
