Composite Ceramics for Thermal-Barrier Coatings Produced from Zirconia Doped with Rare Earth Oxides

The thermal fatigue life of zirconia-based complex composite ceramics doped with a mixture of rare earth oxides was studied. Two concentrates of rare earth oxides were chosen (wt.%): 1) cerium- subgroup concentrate of composition 62.4 CeO₂, 13.5 La₂O₃, 10.9 Nd₂O₃, 3.9 Pr₆O₁₁, 0.92 Sm₂O₃, 1.2 Gd₂O₃, 0.24 Eu₂O₃, 2.66 ZrO₂, 1.2 Al₂O₃, 1.7 SiO₂, and 1.38 other oxides (light concentrate (LC)) and 2) yttrium-subgroup concentrate of composition 13.3 Y₂O₃, 1.22 Tb₄O₇, 33.2 Dy₂O₃, 8.9 Ho₂O₃, 21.8 Er₂O₃, 1.86 Tm₂O₃, 12.5 Yb₂O₃, 0.57 Lu₂O₃, and 6.65 other oxides (heavy concentrate (HC)). Two-layer metal/ceramic thermal-barrier coatings (TBCs) were deposited on gas turbine engine blades by electron-beam physical vapor deposition (EB-PVD) in one process cycle. The properties of ZrO₂–LC and ZrO₂–HC TBC ceramic top coats were compared to those of a standard yttria-stabilized zirconia layer (ZrO₂–Y₂O₃). The thermal fatigue experiment was performed by heating the samples to 1100°C in a muffle furnace for 5 min, holding them at this temperature for 50 min, and cooling in water for 5 min. The standard ZrO₂–Y₂O₃ layer withstood 138 thermal cycles, while the ZrO₂–LC and ZrO₂–HC layers withstood 161 thermal cycles. The porous microstructure of the ceramic layers developed during thermal cycling was found to depend on laminar microstructures acquired by the layers in the EB-PVD process. The number of spherical pores in the ZrO₂–LC and ZrO₂–HC layers was much higher than in the ZrO₂–Y₂O₃ layer. This increased their thermal fatigue life by 16% compared to the standard coating. An integrated approach to the choice of the ceramic top coat composition based on ZrO₂ solid solutions doped with natural rare earth oxide concentrates and of the technique for their deposition, as well as improvement in the coating architecture, will promote cost-effective TBCs with the properties required.