THERMAL BARRIER COATINGS BASED ON ZrO₂ SOLID SOLUTIONS

Abstract

The standard material of the ceramic layer in a thermal barrier coating (TBC)—a solid solution of ZrO₂ stabilized with (6–8 wt.%) Y₂O₃ (YSZ)—approaches the temperature limit of its application (<1200 °C) because the ZrO₂ t' phase sinters and undergoes t' ZrO₂ → T-ZrO₂ + F-ZrO₂ phase transformations to form M-ZrO₂ at elevated temperatures. A new generation of ceramics for TBCs is needed to increase the operating temperature (up to 1600 °C), efficiency, and productivity of gas turbine engines. The overview paper analyzes research efforts focusing on the development of TBCs using solid solutions of ZrO₂ with rare earth and titanium oxides. When Y₂O₃ in YSZ is partially substituted by CeO₂, TiO₂, La₂O₃, Sc₂O₃, Gd₂O₃, Nd₂O₃, Yb₂O₃, Er₂O₃, and Ta₂O₅, ceramics with high phase stability (ZrO₂ t' phase being retained in the coating) up to 1500 °C, lower thermal conductivity, and required fracture toughness and resistance to sintering but shorter thermal fatigue life than that of standard YSZ are produced. The concepts of greater tetragonality of the ZrO₂ t' phase (ceramics in the ZrO₂–CeO₂–TiO₂ system) and a 'multicomponent defective cluster' (ceramics in the ZrO₂–Y₂O₃–Nd₂O₃ (Gd₂O₃, Sm₂O₃)–Yb₂O₃ (Sc₂O₃) system) explain how the operating temperature of the TBC ceramic layer increases to 1350 ºС and 1600 ºС, respectively. The thermal conductivity of TBC ceramics in the binary ZrO₂−CeO₂, ZrO₂−Er₂O₃, ZrO₂−Sm₂O₃, ZrO₂−Nd₂O₃, ZrO₂–Gd₂O₃, ZrO₂–Dy₂O₃, and ZrO₂–Yb₂O₃ systems is lower than that of YSZ. Ceramics with high phase stability and low thermal conductivity were produced in the ternary ZrO₂–Sc₂O₃–Gd₂O₃, ZrO₂–CeO₂–Gd₂O₃, ZrO₂–YbO_1.5– TaO_2.5, and ZrO₂–Yb₂O₃–TiO₂ systems. An integrated approach is needed to choose the composition of the ceramic layer based on the ZrO₂ solid solution, select the coating technique, and improve the coating architecture to design economical TBCs with balanced properties.