Thermal conductivity and micromechanical properties of plasma-sprayed yttria-stabilized zirconia thermal barrier coatings

Plasma-sprayed ZrO2 – 8 wt% Y2O3 (8YSZ) is the most widely used kind of thermal barrier coating (TBC) to insulate components in the hot gas path of industrial gas turbines. The thermal conductivity of an 8YSZ TBC depends on its microstructure, which is influenced by the characteristics of the feedstock powder. Thus, in this paper, we studied the thermal conductivity of 8YSZ coatings deposited using agglomerated and sintered (A&S), fused and crushed (F&C) and hollow spherical (HOSP) feedstock powders, both in the as-deposited condition and after thermal cycling. The coating from the A&S powder, which exhibited greater porosity (≈22 %), also had lower thermal conductivity in as-deposited condition (0.54 W/(m·K)) compared to the coatings from F&C (0.89 W/(m·K)) and HOSP (0.70 W/(m·K)) powders. Its thermal conductivity also increased less than that of the other coatings after thermal cycling. Because ZrO2 with a higher Y2O3 content is attracting interest as the top layer of molten silicate-resistant TBCs, we also studied the thermal conductivity of bilayer systems with an 8YSZ bottom layer and a top layer of ZrO2 – 20 wt% Y2O3 (20YSZ) or ZrO2 – 55 wt% Y2O3 (55YSZ), all obtained from an A&S feedstock. We found comparable thermal conductivities of ≈0.5–0.6 W/(m∙K) and, again, a tendency to increase slightly after thermal cycling. Furthermore, because 20YSZ and 55YSZ are fully stabilized in the cubic form, therefore potentially more brittle than 8YSZ, we also measured the intrinsic indentation fracture toughness of these materials by pillar splitting. Both 20YSZ and 55YSZ had a similar toughness of 1.39 MPa√m, compared to 2.32 MPa√m for 8YSZ.