Analysis of the Cr 3+ luminescence spectra from thermally grown oxide in thermal barrier coatings

Abstract

Photo-luminescence spectra from the alumina thermally grown oxide (TGO) in thermal barrier coating (TBC) systems (CMSX-4 substrates, Pt–Al or Pt diffusion bond coats (BCs), and electron-beam evaporation (EBPVD) YSZ) have been obtained after thermal exposure using a micro Raman instrument. The spectra were often composed of at least two contributing spectra corresponding to two different residual stress levels in the analysed volume, one from intact regions of the coating and the other from the regions where the stress has been relaxed by local damage. The composite spectrum was deconvoluted to give the two contributing spectra using a fitting procedure that preserves certain essential constraints in the parameters of the individual spectra. These constraints have been established as a function of stress level in the TGO. The deconvoluted spectra acquired from rectangular grids of analysis points have been used to map the distribution of the damaged regions. This approach can form the basis of a non-destructive technique for assessing the amount of accumulated damage and estimating the residual life of a TBC on an engine component.