ACOUSTIC-EMISSION STUDY OF FAILURE MECHANISMS IN TIC THERMAL BARRIER COATINGS

The failure mechanisms of thick plasma-sprayed TiC coatings subjected to severe thermal shocks were investigated using acoustic emission (AE), detected during and after the thermal shocks. The AE signal was triggered at the onset of the thermal shock and was completely captured for time durations up to 3 s. Signals were then studied and quantified directly by integrating the square of the waveform signals (i.e. calculating the electrical energy). This method was compared with the more classical ringdown counting and with the maximum-amplitude method. The electrical energy demonstrated the best results that could be correlated to physical events. Besides the coating segmentation, two modes of delamination were observed: a sudden delamination during heating due to the bending of the coating under compressive stresses and a progressive delamination during cooling after plastic deformation had taken place at a high temperature during the thermal shocks.