Microstructure and heat transfer phenomena in ceramic thermal barrier coatings

Comparably thick Y2O3-partially-stabilized ZrO2 thermal barrier coatings were deposited by two different techniques, air plasma spray (APS) and electron beam physical vapor deposition (EB-PVD), on the same type of substrate. Microstructure and grain texture, as studied by SEM and XRD, were markedly different. The complex microstructure of the APS coatings, made of curled lamellar grains, was replaced in EB-PVD coatings by long columnar grains, aligned along the growth axis, with strong grain texture. Average porosity and other average or intrinsic properties, such as density and specific heat, were nearly the same for all studied coatings; phase composition ranged between 0 and 6 wt% of the m phase in a prevalent t'-phase matrix. The main difference was in the shape and orientation of porosity with respect to the thermal flux direction, which was responsible for the different thermal diffusivity that was three times higher in EB-PVD than in APS coatings. An appropriate modeling of the heat diffusion process, including open and closed porosity with orientation and shape factors, could explain the observed diffusivity values.