Stress measurements in YTZP ceramic using GHz and THz radiation

We have measured stress distributions in ceramic thermal barrier coatings using reflection-based polariscopes working at GHz and THz frequencies. Such coatings are routinely used in aerospace, automotive and energy applications. Understanding stress distributions within these coatings during manufacture and after use is vital to improving their performance. Stress optic coefficient measurements of bulk YTZP ceramic in reflection are compared to previously measured values in transmission. We then report in-situ calibration of the stress optic coefficient of YTZP coatings on metal substrates in reflection. Finally, we imaged the stress concentration around sub-surface defects in thermal barrier coated samples. We do not use time domain spectroscopy, representing a departure from more traditional THz imaging and metrology approaches.