Piezo-spectroscopic mapping of the thermally grown oxide in thermal barrier coatings

Mapping of the effects of residual stress on the luminescence spectra from the thermally grown aluminium oxide (TGO) in thermal barrier coatings (TBCs) was conducted using a Raman microprobe in conjunction with a motorised stage. Maps of various spectral parameters such as peak shift, peak width ratio, peak height ratio and peak separation were generated for different spot sizes and mapping pitches. The optimum lateral resolution was found to be <5 μm and is determined not only by the spot size, but also by the crystallite size in the TGO. Analysis of the spectral parameters reveals a preferential crystallographic orientation of the α-Al2O3 with the c-axis perpendicular to the substrate surface. Spectral parameters were found to deviate significantly from their “usual” values if the analysed volume contains more than one significant stress level. In particular the width and height ratios of the R1 and R2 lines provide good indication of local damage.