Ta₂O₅/Nb₂O₅ and Y ₂O₃ Co-doped Zirconias for Thermal Barrier Coatings

Zirconia doped with 3.2-4.2 mol% (6-8 wt%) yttria (3-4YSZ) is currently the material of choice for thermal barrier coating topcoats. The present study examines the ZrO₂-Y₂O₃Ta₂O ₅/Nb₂O₅ systems for potential alternative chemistries that would overcome the limitations of the 3-4YSZ. A rationale for choosing specific compositions based on the effect of defect chemistry on the thermal conductivity and phase stability in zirconia-based systems is presented. The results show that it is possible to produce stable (for up to 200 h at 1000°-1500°C), single (tetragonal) or dual (tetragonal + cubic) phase chemistries that have thermal conductivity that is as low (1.8-2.8W/m K) as the 3-4YSZ, a wide range of elastic moduli (150-232 GPa), and a similar mean coefficient of thermal expansion at 1000°C. The chemistries can be plasma sprayed without change in composition or deleterious effects to phase stability. Preliminary burner rig testing results on one of the compositions are also presented.