Gadolinia stabilized zirconia as a possible material for thermal barrier coatings

The objective of the work was to investigate the use of Gd2O3 as an alternative stabilizer for ZrO2-based thermal barrier coating (TBC) materials. The current state-of-the-art TBC material, Y2O3-stabilized ZrO2 (4 mole percent Y2O3), referred to as YSZ, suffers from limited durability at use temperatures above 1200 °C. Gd2O3 is one of the most effective additions for reducing the thermal conductivity of ZrO2 but its effect on the sintering and phase stability is unclear. The effect of Gd2O3 concentration (4-20 mole percent) on the sintering, grain growth, phase composition, and thermal conductivity of ZrO2 solid solutions was measured and the data were compared with those for YSZ. Powders prepared conventionally by calcination of coprecipitated powders and by plasma spraying were used in the sintering and phase transformation experiments. The powders were hot-pressed to produce dense samples for grain growth and thermal conductivity measurements. The techniques used to obtain the required data included dilatometry, quantitative X-ray diffraction, scanning electron microscopy, laser flash diffusivity and differential scanning calorimetry. The conventional ZrO2-4 mol% Gd2O3 sintered more slowly than the baseline YSZ. The sintering rate of Gd2O3 stabilized ZrO2 as a function of Gd2O3 content showed a maximum at 8 mol%. For the plasma-sprayed Gd2O3-stabilized ZrO2 powders, only the compositions with 4 mol% Gd2O3 contained the non-equilibrium tetragonal phase (t’ phase). Phase stability analysis of plasma-sprayed YSZ and ZrO2-4 mol% Gd2O3 indicated that the t’ phase in YSZ possesses significantly better resistance to partitioning to the equilibrium tetragonal and cubic phases. The thermal conductivity of dense Gd2O3-stabilized ZrO2 was lower than that for YSZ and decreased significantly with increasing Gd2O3 concentration. Plasma-sprayed ZrO2-4 mol% Gd2O3 sintered more slowly and had a lower thermal conductivity than plasma-sprayed YSZ. However, it had lower stability toward partitioning to the equilibrium tetragonal, monoclinic and the cubic phase from the t’ phase --Abstract, pages iii-iv