热障涂层太赫兹无损检测技术研究进展

Research Progress in Terahertz Non-destructive Testing of Thermal Barrier Coatings

太赫兹技术是近年来兴起的一种先进无损检测方法,因其优异的非接触、非电离、高精度和可实现自动化在线检测等技术优势,目前已经在热障涂层性能评价和寿命预测方面展现出巨大的潜力。简要介绍了热障涂层太赫兹无损检测技术的发展概况,分别从陶瓷厚度测量,TGO和冲蚀监测,孔隙、裂纹及应力状态表征方面进行重点分析和总结。指出目前利用太赫兹技术进行单层和多层陶瓷顶层厚度的监测技术已经较为成熟,可以逐步推广到工程应用中。在TGO监测方面,围绕利用太赫兹时域反射峰的时间差和振幅衰减进行TGO厚度监测取得了一定进展,但是检测精度有待进一步提高。在冲蚀监测方面,已经可以较为精确地对冲蚀造成的减薄进行定量检测,不过虽然已经可以对涂层内部渗入的CMAS进行定性判定,但是定量表征方面尚缺乏试验研究。在孔隙监测方面,探索了孔隙率大小对热障涂层的太赫兹频段光学参数变化的影响,提出相对展宽比是在孔隙率测量方面较折射率、消光系数和介电常数,既保留较高测量精度又兼具工程实用性的表征参量,但是目前对孔隙的微结构特征缺乏全方面表征。在裂纹监测方面,通过小波降噪及频域加Hanning窗函数滤波进行反卷积处理,已经可以分辨平行裂纹的位置和宽度,但是对于多条裂纹的监测尚有难度。在应力状态表征方面,发现折射率的改变与应力变化呈现线性关系,并且根据折射率的变化可以反推出光学应变系数,进而获取应力情况,但是目前反射式测量系统还需要进一步开发。最后针对目前热障涂层太赫兹无损检测技术应用现状、存在的问题及未来发展方向进行了总结和展望。

Terahertz(THz)technology is an advanced non-destructive testing method that has emerged in recent years.Owing to excellent characteristic of non-contact,non-ionization,high-precision,and automated online detection,it has now shown great potential in thermal barrier coating(TBCs)performance evaluation and life prediction.The development of terahertz non-destructive testing technology for thermal barrier coatings was introduced briefly and analysis and summary of ceramic thickness measurement,TGO and erosion monitoring,and pore,crack and stress state characterization were carried out emphatically.It is pointed out that the current technology for monitoring the thickness of single-layer and multi-layer ceramic top layers using terahertz technology is mature enough and can be gradually extended to engineering application.In terms of TGO monitoring,some progress has been made on TGO thickness monitoring by the time difference and amplitude attenuation of terahertz time-domain reflection peaks,but the detection accuracy needs to be further improved.In terms of erosion monitoring,the thickness loss caused by erosion can be quantitatively detected accurately.Although CMAS that has penetrated into the coatings can be detected qualitatively,there is still a lack of experimental research on the quantitative characterization.In terms of pore monitoring,the effect of the porosity on the change of the terahertz optical parameters of the thermal barrier coating has been explored.It is proposed that the relative broadening ratio is a more proper measure of the porosity than the refractive index,extinction coefficient and dielectric constant for engineering application.Nevertheless,the microstructure characteristics of the pore still lack a comprehensive characterization.In terms of crack monitoring,the positions and widths of parallel cracks can be distinguished by deconvolution filtering combined Hanning window function with wavelet denoising,but it is still difficult to monitor multiple cracks.In terms of stress state characterization,it is found that the change in refractive index has a linear relationship with the stress change,and the optical strain coefficient can be derived from the change in refractive index to obtain the stress state,but the current reflective measurement system needs further development.Finally,the current status,existing problems and future development direction of terahertz nondestructive testing for thermal barrier coatings are summarized and prospected.