Co-doping of air plasma-sprayed yttria- and ceria-stabilized zirconia for thermal barrier applications

Co-dopants of either Yb ³⁺ or Ca ²⁺ were incorporated into 7.6 mol% YO ^1.5-ZrO ₂ (7.6YSZ) and 12 mol% CeO ₂-ZrO ₂ (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 Yb ³⁺ segregation was detected at the grain boundary of either as-sprayed 2Yb/7.6YSZ or 2Yb/12CeSZ coatings. Dilatometer measurements showed that 2 mol% Yb ³⁺ 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 ZrO ₂ (identified presently as t′-ZrO ₂), while the 5Ca/7.6YSZ coating was a non-equilibrium composition of cubic ZrO ₂. After a heat treatment of 100 h at 1200°C, the 2Yb/7.6YSZ coating was completely t′-ZrO ₂, while the baseline and Ca ²⁺ co-doped 7.6YSZ coatings showed evidence of partitioning. Therefore, it appears that co-doping of 7.6YSZ with 2 mol% Yb ³⁺ increases the stability of t′-ZrO ₂, whereas co-doping with 2 mol% Ca ²⁺ decreases the stability of t′-ZrO ₂. The volume fraction of m-ZrO ₂ in the baseline 12CeSZ coatings was estimated to be 88% after a 100 h heat treatment at 1200°C. 2 mol% Yb ³⁺ or Ca ²⁺ 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.