Acoustic emission monitoring and damage mode discrimination of APS thermal barrier coatings under high temperature CMAS corrosion

Molten calcium-magnesium-alumino-silicate (CMAS) corrosion is the most dangerous failure mode for thermal barrier coatings (TBCs). The real-time monitoring and mode discrimination of the corrosion failure process are desirable to understand the corrosion failure mechanism of TBCs. In this paper, the failure mechanism of the TBCs subjected to molten CMAS attack is investigated with acoustic emission (AE). Based on the number and amplitude, and compared to the substrate and the TBCs without CMAS, more serious damage is found to occur in the TBCs attacked by the molten CMAS. Possible failure modes are discriminated by wavelet analysis, and the results indicate that there are four distinct frequency bands corresponding to surface vertical cracks, sliding interface cracks, opening interface cracks, and substrate deformation in the TBCs suffered high temperature molten CMAS corrosion. In fact, the frequency band of sliding interface crack is associated with two crack types, including the crack at the YSZ/BC interface and the parallel crack in the ceramic coating induced by the compressive stress. These two cracks dominate the whole failure process, and ultimately result in the spallation of coatings.