Microstructural evolution of plasma spray physical vapor deposited thermal barrier coatings at 1150 °C studied by impedance spectroscopy

Plasma spray physical vapor deposition (PS-PVD) is a new promising technology for processing thermal barrier coatings (TBCs). In this study, the microstructural evolution of PS-PVD yttria-stabilized zirconia (YSZ) TBCs at 1150 °C was investigated by scanning electron microscopy, electron backscattered diffraction, and impedance spectroscopy. Bridging is observed between the secondary columns of the coatings during the first 8 h of heat treatment, while sintering necks are observed between the quasi-columns over the heat-treatment interval of 12-20 h. As the heat-treatment time increases to 100 h, the nanogaps between the secondary columns disappear and a large number of shrinkage pores are observed in the columns, leading to a typical cellular structure. Besides, the average grain size of the YSZ coatings increases from 0.20 (as-deposited) to 0.55 µm (after 50 h of heat treatment). The resistance of the YSZ grains increases while that of the YSZ grain boundaries decreases because of grain coarsening. During the first 4 h of heat treatment, the resistance of the thermally grown oxide (TGO) scale increases because of its thickening. However, its resistance decreases during the next 4 h of heat treatment mainly because of the change in its composition (from pure alumina to mixed oxides).