Acoustic emission source analysis of plasma sprayed thermal barrier coatings during four-point bend tests

Abstract

Plasma-sprayed thermal barrier coatings (TBCs), as-sprayed and pre-oxidized, were tested under four-point tensile bending conditions, and their acoustic emission (AE) responses were monitored by an advanced AE system. On the basis of an inversion processing of AE signals, the damage sources in the deposits were localized, identified and classified into three main cracking modes. Furthermore, cracking source parameters (i.e. rise time and crack volume) were estimated and used to determine the critical cracking or delamination events among the AE signals. Consequently, the damage progressions in the TBCs were elucidated by correlating the fracture source parameters to the strain curves in time domain. In the bending tests, vertical cracks were induced in the ceramic top layer at a low strain rate and delamination at the bond/top coat interface accounted for the spallation of top coat before failure. Pre-oxidized samples tended to crack early at a low tensile strain, and the AE sources were characterized by predominant shear fracture (Mode II) at the bond/top coat interface, corresponding to a degradation mechanism of microcracking before failure.