Real-time crack detection during extremely noisy APS process by advanced acoustic emission measurement system

Numerous cracks can be observed in the top coat of thermal barrier coatings (TBCs) deposited by the atmospheric plasma spraying (APS) method. These cracks can be classified into vertical and horizontal ones and they have opposite impact on the properties of TBCs. Vertical cracks reduce the residual stress in the top coat and provide strain tolerance. On the contrary, horizontal cracks trigger delamination of the top coat. In this study, an advanced acoustic emission (AE) measurement system was developed to realize in-process separate detection of generation of vertical and horizontal cracks during extremely noisy APS process. The output signal of piezoelectric AE sensors were continuously recorded and analyzed by strong noise reduction and event detection functions of this system. Vertical cracking and horizontal cracking could be identified by the occurrence timing of an AE event. During the deposition process by repeated spraying, vertical cracks were detected during heating by one pass of spraying to the sample, and horizontal cracks occurred during interval between two passes of sprayings.