Water vapor retards grain growth during sintering of yttria-stabilized zirconia pellets and coatings

Yttria-stabilized zirconia (YSZ) is the benchmark thermal barrier topcoat, but its sintering response to water vapor is unclear. YSZ pellets of three initial porosities and free-standing plasma-sprayed coatings were heat-treated at 1300 °C in dry air, steam, or dry nitrogen. Contrary to reported promotion in simple oxides, steam retarded microstructural evolution. After 100 h the most porous pellet showed a 31% smaller grain size in steam, with apparent rate constants lower by factors of 2.8–4.2. A dry-nitrogen control in the same furnace changed the grain size by only 5.5%, isolating water vapor. Coatings likewise retained higher porosity and, at intermediate exposures, lower thermal conductivity. DFT calculations showed preferential OH binding at a grain-boundary Y 3+ site (−2.73 eV); a defect-equilibrium sensitivity analysis predicted 8–22% oxygen-vacancy depletion. These results support a mechanism in which OH-mediated grain-boundary stabilization through Y 3+ anchoring and vacancy-related defect modification leads to the retardation of sintering.