Application of Cr3+ photoluminescence piezo-spectroscopy to plasma-sprayed thermal barrier coatings for residual stress measurement

Cr3+ photoluminescence piezo-spectroscopy (CPLPS) is being developed as a non-destructive inspection technique for the measurement of residual stresses within the thermally grown oxide (TGO; consisting of α-Al2O3 with Cr3+ solute) layer buried under Y2O3-stabilized ZrO2 (YSZ) thermal barrier coatings (TBCs). In this study, CPLPS experiments were performed to measure residual stresses in TGOs buried under four different types of plasma-sprayed TBCs, as a function of TBC thickness (from 0 to full-thickness of 250 μm) using ‘taper-polishing’. In one type of TBC, the CPLPS technique could be used to measure TGO residual stresses, but was limited to a TBC thickness of less than 170 μm due to severe attenuation of the Cr3+ photoluminescence signal through the YSZ. In the other three types of TBCs, that thickness was limited to about 50 μm. However, non-TGO Cr3+ photoluminescence signals were obtained through thicker TBCs of these latter types. To identify the source of this non-TGO signal, chemical analyses and CPLPS of as-received and heat-treated plasma-spray feedstock powders were performed. It was found that these powders contained both Al and Cr, which upon heat-treatment created conditions for Cr3+ photoluminescence. To identify the sources of attenuation of the Cr3+ photoluminescence signal intensity in all types of TBCs, CPLPS was performed on a set of ‘model coatings’. These ‘model coatings’ consisted of monolithic ceramics, where a polycrystalline α-Al2O3 slab was placed underneath a variety of thin YSZ plates containing varying amounts of porosities, pore sizes, Y2O3 contents, and grain boundaries. It was found that porosity, grain boundaries, and most importantly splat boundaries, were the key factors that obstructed the observation of CPLPS from α-Al2O3 through YSZ. In order to alleviate this signal attenuation by pores and microcracks, plasma-sprayed TBCs were vacuum-impregnated with high-refractive-index materials (mineral oil or Stycast™ epoxy), allowing us to measure, for the first time, TGO residual stresses through full-thickness plasma-sprayed TBCs using CPLPS.