MICROSTRUCTURAL CHARACTERIZATION OF PLASMA-SPRAYED ZIRCONIA THERMAL BARRIER COATINGS BY X-RAY-DIFFRACTION FULL PATTERN-ANALYSIS

Thick two-layered thermal barrier coatings made of Ni-Co-Cr-Al-Y and yttria-partially stabilized zirconia were plasma sprayed at controlled deposition temperature onto stainless steel substrates. During the spraying of the top coat, the deposition temperature was kept as constant as possible using a front air or liquid-argon-cooling system. The correlation between process parameters and microstructure was investigated by scanning electron microscopy and X-ray diffraction (XRD) analysis. The application of a new procedure for XRD data processing, based on the Rietveld method, made it possible to obtain good accuracy in the phase determination and quantitative evaluation of zirconia polymorphs. Moreover, the crystallite size and microstrain were refined simultaneously to phase percentages. Also the preferred orientation in the coating could be taken into account, adding useful information and increasing the reliability of all the other results. The influence on the coating microstructure of changing the deposition temperature and environment is also discussed.