Application of Cr 3+ photoluminescence piezo-spectroscopy to plasma-sprayed thermal barrier coatings for residual stress measurement

Abstract

Cr 3+ 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-Al 2O 3 with Cr 3+ solute) layer buried under Y 2O 3-stabilized ZrO 2 (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 Cr 3+ photoluminescence signal through the YSZ. In the other three types of TBCs, that thickness was limited to about 50 μm. However, non-TGO Cr 3+ 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 Cr 3+ photoluminescence. To identify the sources of attenuation of the Cr 3+ 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-Al 2O 3 slab was placed underneath a variety of thin YSZ plates containing varying amounts of porosities, pore sizes, Y 2O 3 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-Al 2O 3 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.