Analysis of localized damage in EB-PVD/(Ni, Pt)Al thermal barrier coatings

The structural integrity of the thermally grown oxide (TGO) in the EB-PVD/(Ni, Pt)Al thermal barrier coatings was examined using photo-stimulated luminescence piezo-spectroscopy (PLPS). PLPS spectra exhibiting both a high stress component and a low stress (or stress-free) component were observed during thermal cycling (i.e. bimodal luminescence). The fraction of bimodal spectra increases initially, then decreases, and increases again when close to failure. It is shown that the bimodal luminescence originates from stress relaxation caused by localized damage. It is proposed that the initial increase of bimodal luminescence is related to cracking caused by the volume change associated with the phase transformation of TGO from θ to α-Al2O3; whereas, the increase at the final stage is due to TGO cracking and spallation. Area stress maps show the gradual accumulation of damage and indicate that PLPS is a useful tool for detection of the initiation and progression of TBC spallation.