Effect of nanocrystalline grain size and point defects on the properties of yttria, tantala and niobia doped zirconias for TBC applications

The current study examines the effect of nanocrystalline grain size and point defects on the properties of yttria-, tantala-, and niobia-doped zirconias for potential improvements over the 6–8YSZ for TBC (Thermal Barrier Coating) applications.; In pure, dense monoclinic zirconia (grain size 63 nm) a strong temperature dependence of the thermal conductivity indicates that phonon-phonon scattering controls thermal conductivity even down to such small grain sizes. A small, albeit significant, increase in thermal conductivity is observed on increasing the grain size to ∼200 nm suggesting that there might be a small grain size effect. Thermal conductivity vs temperature (100–1000°C) data on yttria doped nanocrystalline zirconias (grain size = 30nm–400nm and 60–100% dense) is flat, which is representative of phonon scattering by defects. Subsequent comparison of data for different grain sizes and yttria contents suggests that the responsible defects are yttria-induced oxygen vacancies, not grain boundaries in spite of sizes as small as 30 nm.; Following the above result, the systems ZrO₂-Y₂O ₃-Ta₂O₅, ZrO₂-Y₂O₃ -Nb₂O₅ and ZrO₂-Ta₂O ₅ were chosen to examine whether specific substitutional defects—in addition to, or instead of, oxygen vacancies—can provide similar or greater reductions in conductivity and improvements in TBC relevant properties over the 6–8YSZ. The results show that Nb-Y and Ta-Y co-doped zirconia samples containing the two dopants in equal concentrations and therefore only substitutional defects, produce single phase tetragonal materials that are far more stable at temperatures up to 1500°C than the 6–8YSZ; have thermal conductivities very close to that of the conventional 6–8YSZ; can be deposited as coatings by plasma spraying; have comparable coefficient of thermal expansion and ∼50 GPa lower Young's modulus.; Nb+5/Ta+5-Y+3 doped samples, which contain an excess of yttria are found not to be stable in single phase form. On aging at 1500°C they split into a mixture of a tetragonal phase that is similar to the one in the intentional no vacancy compositions and a cubic phase containing the excess yttria.; Finally, hot corrosion studies by NaVO₃-V₂O ₅ mixtures show that the tantala doped orthorhombic zirconia is more resistant to acidic mixtures and less resistant to basic mixtures than the 6–8YSZ. (Abstract shortened by UMI.)...