Phase evolution and thermophysical properties of plasma sprayed thick zirconia coatings after annealing

Yttria partially stabilized zirconia (YSZ) thick thermal barrier coatings were fabricated by Atmospheric Plasma Spraying (APS) and isothermally annealed at 1315 °C for different durations. The phase composition of as-sprayed and heat-treated free-standing coatings was investigated by X-ray Diffraction (XRD) and the Rietveld method was employed for quantitative phase analysis. High-temperature exposure of YSZ coatings produced the partial decomposition of metastable t′ zirconia phase and the corresponding increase in the amount of stable tetragonal t, cubic c and monoclinic m phases with increasing the aging time. The thermophysical properties of as-sprayed and annealed YSZ coatings, such as thermal expansion and heat capacity, were measured. The thermal expansion coefficient kept almost constant in-plane direction after heat treatment. Otherwise, it changed in through-thickness direction due to any structural changes and high-temperature sintering of the porous microstructure. The sintering also influenced the specific heat capacity Cp which increased with increasing the annealing time.