Detection of AE Events due to Cracks in TBC during Spraying Process

Abstract

Cracks may occur inside a top coating or at an interface between the top and bond coating of the thermal barrier coating (TBC) during atmospheric plasma spraying (APS) process. The acoustic emission (AE) method is suitable for detection of the generation and propagation of these cracks because it is an in-situ non-destructive evaluation technique. However, AE monitoring of APS process is prevented by large acoustic noise at elevated temperature on a specimen due to heating by the plasma jet. Therefore, a non-contact laser AE method with a newly developed measurement and analysis system was used for monitoring of the top coating process. Several types of noise reduction processes such as soft-thresholding were applied to continuously recorded AE waveform. Experiments were conducted with various preheating temperatures and scanning speeds of the plasma torch. AE events due to cracking were detected only during spraying, on the other hand no events were detected after spraying. The effect of preheating temperature and scanning speed of the plasma torch on the development of cracks were estimated. A correlation was observed between the density of delamination cracks and the number of AE events.