THERMAL BARRIER COATINGS BASED ON ZrO2 SOLID SOLUTIONS

Abstract

The standard material of the ceramic layer in thermal barrier coatings (TBCs)—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. Ceramic materials for a new generation of TBCs need to be developed 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 metal 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 sintering resistance 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 1350oC and 1600oC, 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 have been produced in the ternary ZrO2–Sc2O3–Gd2O3, ZrO2–CeO2–Gd2O3, ZrO2–YbO1.5–TaO2.5, 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 effective TBCs with balanced properties.