Co-Doping of Air Plasma-Sprayed Yttria- and Ceria-Stabilized Zirconia for Thermal Barrier Applications
Co-dopants of either Yb3+ or Ca2+ were incorporated into 7.6 mol% YO1.5–ZrO2 (7.6YSZ) and 12 mol% CeO2–ZrO2 (12CeSZ) coatings by infiltrating porous spray-dried powders with salt solutions containing the appropriate co-dopant species prior to plasma spraying. Co-dopant concentration was varied from 2 to 5 mol%. Using a combination of transmission electron microscopy and energy-dispersive analysis, no secondary phase or Yb3+ segregation was detected at the grain boundary of either as-sprayed 2Yb/7.6YSZ or 2Yb/12CeSZ coatings. Dilatometer measurements showed that 2 mol% Yb3+ co-doped 7.6YSZ and 12CeSZ coatings shrank ∼0.6% during a 5 h soak at 1400°C, approximately the same contraction as the baseline coatings (i.e. not co-doped). X-ray diffraction results show that the as-sprayed 7.6YSZ, 2Ca/7.6YSZ, and 2Yb/7.6YSZ coatings comprised of non-transformable, non-equilibrium composition tetragonal ZrO2 (identified presently as t′-ZrO2), while the 5Ca/7.6YSZ coating was a non-equilibrium composition of cubic ZrO2. After a heat treatment of 100 h at 1200°C, the 2Yb/7.6YSZ coating was completely t′-ZrO2, while the baseline and Ca2+ co-doped 7.6YSZ coatings showed evidence of partitioning. Therefore, it appears that co-doping of 7.6YSZ with 2 mol% Yb3+ increases the stability of t′-ZrO2, whereas co-doping with 2 mol% Ca2+ decreases the stability of t′-ZrO2. The volume fraction of m-ZrO2 in the baseline 12CeSZ coatings was estimated to be 88% after a 100 h heat treatment at 1200°C. 2 mol% Yb3+ or Ca2+ co-doping limited the tetragonal to monoclinic phase transformation in 12CeSZ, with only 37% and 43% monoclinic phase observed, respectively, after a 100 h heat treatment at 1200°C; this was an improvement over the baseline 12CeSZ coating. As-sprayed 2Yb/7.6YSZ and 2Yb/12CeSZ coatings had slightly lower thermal conductivity than their baseline counterparts in the as-sprayed condition; after 100 h at 1200°C, their conductivity increased to that of the baseline coatings.
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