Thermal Barrier Coatings Based on ZrO 2 Solid Solutions
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PROTECTIVE AND FUNCTIONAL POWDER COATINGS THERMAL BARRIER COATINGS BASED ON ZrO2 SOLID SOLUTIONS E.V. Dudnik,1,3 S.N. Lakiza,1 I.N. Hrechanyuk,2 A.K. Ruban,1 V.P. Redko,1 I.O. Marek,1 V.B. Shmibelsky,1 A.A. Makudera,1 and N.I. Hrechanyuk1 UDC 541.1+621.762:546-31 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 (32% GdO1.5 reacted with the TGO layer to form a porous GdAlO3 intermediate phase, which affected their integrity and, finally, the TBC protective function. Calculational (finite elements) and experimental methods showed that the APS ZrO2–4 wt.% Dy2O3 coating with large globular pores had lower thermal conductivity and higher thermal fatigue life than YSZ did [125]. 190
The EB-PVD ZrO2–Gd2O3 and ZrO2–Dy2O3 coatings contain gadolinium zirconate (GdZ) and ZrO2 stabilized by Dy2O3 (DySZ). The single-layer DySZ and GdZ coatings have longer service life than the standard 7YSZ material does. In addition, the DySZ coating does not interact with the TGO layer [126]. The paper [127] studied ZrO2 materials doped with 3–10 mol.% Yb2O3 (YbSZ). The t-ZrO2 phase sintered at 1600C was identified in the samples containing 3–6 mol.% Yb2O3. In the range 300–1280C, the thermal conductivity of 7YSZ decreases from 3.0 to 2.38 W/(m · K) and the thermal conductivity of YbSZ (3–10 mol.% Yb2O3–ZrO2) remains much lower than that of 7YSZ. According to [126], two groups of samples can be identified. When zirconium dioxide is stabilized with 3–4 mol.% Yb2O3, the thermal conductivity in the range 300–1280C varies from 2.35–2.3 to 2.0 W/(m · K). When ZrO2 is stabilized with 5–10 mol.% Yb2O3, the thermal conductivity changes from 1.85 to 1.75 W/(m · K). The thermal conductivity of the sample containing 5 mol.% Yb2O3 is 24% lower than that of the starting 7YSZ coating. The same conclusion was made in [127] for the sample containing 10 mol.% Yb2O3, though its thermal conductivity remained much lower than that of 7YSZ but increased from 1.7 to 1.85 W/(m · K) in the temperature range in question. After the dense ZrO2–8 mol.% YbO1.5 material was aged for 192 h at 1150 and 1250°C, the t-ZrO2 F-ZrO2 + T-ZrO2 phase transformation occurred. The high starting fracture toughness (~87 J · m–2) decreased by ~56% at early ageing stages at both temperatures and gradually increased in the ageing process at 1150°C and more intensively at 1250°C. Transformation hardening activated in 192 h at 1250°C. Ferroelastic hardening was predominant in the absence of transformation hardening [27]. The ZrO2–Yb2O3 TBC demonstrated excellent thermal barrier properties at temperatures up to 1100°C. However, the coating had relatively low corrosion resistance to fuel combustion products and relatively low hightemperature oxidation resistance at elevated operating temperatures [122]. In the ZrO2–Y2O3 system, the greatest possible tetragonality of the nontransformable t-ZrO2 phase is equal to 1.0203. The difference betw
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