Acoustic emission monitoring and failure behavior discrimination of 8YSZ thermal barrier coatings under Vickers indentation testing

摘要

Acoustic emission (AE) is a dynamic non-destructive detecting technique, and could be a promising application for failure detection of plasma sprayed coatings. This work presents a thorough study for detecting and classifying different failure modes under Vickers indentation testing of 8 wt% Y2O3-stabilized ZrO2 (8YSZ) thermal barrier coatings (TBCs) by using acoustic emission technique. Cluster analysis and wavelet packet transform (WPT) were used to characterize the acoustic emission signals released from as-sprayed TBCs during indentation testing. Back-propagation (BP) neural network was used to study the effect of thermal exposure treatment on damage evolution of 8YSZ coatings. The results of cluster analysis showed that there were three types of AE signals associated with distinct failure modes for 8YSZ thermal barrier coatings under Vickers indentation testing. Based on wavelet packet transform, the dominant frequency bands of these three types of signals were concentrated at levels 0-156.25 kHz, 156.25-234.375 kHz and 312.5-390.625 kHz, respectively. Combined with scanning electron microscope (SEM) micrographs of residual Vickers impression, it can be found that 8YSZ coatings have undergone three processes: elastic-plastic deformation, micro-cracks propagation and debonding at the splat boundaries during the indentation testing. The BP neural network identification results showed that after high temperature thermal exposure treatment, the strain tolerance and service durability of 8YSZ thermal barrier coatings decreased while the more fracture signals are generated during the indentation process. The high temperature oxidation environment accelerated the degradation of the plasma sprayed 8YSZ thermal barrier coatings.