Yb-Doped ZrO2 for Thermal Barrier Coatings: A Common Compositional Boundary at 1300 °C

Yttria-stabilized zirconia (YSZ) thermal barrier coatings suffer from t′ phase destabilization and insufficient thermal insulation above 1200 °C. In this work, YbO1.5-stabilized ZrO2 powders (xYbSZ, x = 4–12 mol% YbO1.5 on a cation basis, equivalent to 2–6 mol% Yb2O3) were synthesized by chemical co-precipitation, consolidated by spark plasma sintering, and evaluated at 1300 °C in terms of phase stability, sintering behavior, thermal conductivity, and fracture toughness. A common compositional boundary near 8 mol% YbO1.5 was identified across all four responses. 8YbSZ retained the metastable t′ phase with a monoclinic content below 10 mol% after 300 h at 1300 °C, whereas grain coarsening accelerated markedly and the thermal conductivity reduction efficiency per unit doping at 1000 °C was approximately halved beyond this composition, with κ decreasing from 2.41 to 1.96 W·m−1·K−1 across the series. The toughness gain produced by thermal treatment fell from 34% (4YbSZ) to about 10% (10–12YbSZ) as the dominant toughening mechanism shifted from transformation and microcrack toughening (4–6 mol%) to ferroelastic domain switching (8 mol%), with both being lost in the compositions in which the cubic phase predominated. These results identify 8 mol% YbO1.5 as the optimal composition balancing phase stability, sintering resistance, thermal insulation, and mechanical integrity for TBC applications at 1300 °C.