R₂O₃+Er₂O₃稳定ZrO₂的新型TBC材料

For a gas engine, high pressure turbine blades suffer severely environmental condition, high temperature, high pressure, thermal shock, etc. Thus, thermal barrier coatings (TBCs) have been investigated and applied on the surface of blades for oxidation, erosion and corrosion resistant. For the consideration of increasing operational temperature for new generation gas engine, turbine blades with traditional 7-8YSZ thermal barrier coating has been unable to meet the application temperature above 1200 ℃. Therefore, a new stabilizer for zirconia with high thermal phase stability at elevated temperature (above 1200 ℃) is highly desirable. The main purpose of this paper is to find new stabilizers for zirconia, which can replace conventional 7-8YSZ for application temperature above 1250 ℃. For synthesizing the rare earth oxides doped erbia-stabilised zirconias and 8YSZ (as the reference material), erbium(III) nitrate pentahydrate, ytterbium(III) nitrate pentahydrate, dysprosium(III) nitrate hydrate, gadolinium(III) nitrate hexahydrate and zirconyl chloride octahydrate were chosen as raw materials and citric acid was selected as organic complexing agent. Methanol was used as solvent. Firstly, certain amounts of raw materials determined by specific stoichiometric ratios were dissolved in methanol. The rare earth nitrates solutions were slowly dropped into zirconyl chloride octahydrate solution under magnetic stirring at 50 ℃. After stirring for 30 min, citric acid solution was added, at a molar ratio of citric acid/cation ions=0.6/1.0. Then the forerunner was stirred at 50 ℃ for 3 h, followed by heating at 80 ℃ until dried gel was obtained. The dried gel was placed into an alumina crucible and heated at 500 ℃ by 2 ℃·min^-1 for 2 h to remove carbon contents from organic components, then heated up to 950 ℃ by 2 ℃·min^-1 for 2 h to completely remove the remaining impurities, and cooled down with cooling rate of 10 ℃·min^-1 to room temperature. Finally, the obtained materials were ground into ceramic powders using a mortar for phase analysis. In order to understand the thermal phase stability and chemical compatibility with TGO layer, the as-synthesized powders are mixed with alumina powder and compressed into the pellets, which are heat-treated at 1400 ℃ for different durations to investigate their chemical compatibility with alumina and thermal phase stability at this high temperature. X-ray diffraction (XRD) patterns of as-synthesized powders of mono-rare earth oxides doped erbis-stabilized zirconia showed the non-transformable tetragonal phase, distinguishing from the peaks (004) nearly 73.4° and (400) nearly 73.9° and (006) and (600) peaks. Rietveld refinement was applied on the measured XRD patterns, where the 2θ collected from 25° to 150° to simulate the lattice parameters, a (a=b) and c. When Gd₂O₃, Dy₂O₃ and Yb₂O₃ oxides replaced some ZrO₂ in the crystals, the lattice constant a decreases from 0.36187 to 0.36151 nm; otherwise, c increases from 0.51671 to 0.51551 nm. If compared with pure erbia-stabilized zirconia (E5Z), √2c/a of E2.5Y2.5Z was very close to E5Z, due to the similar radius between Er³⁺ (0.1004 nm) and Yb³⁺ (0.0985 nm). But √2c/a became larger when the dopants are Dy³⁺ (0.1027 nm) and Gd³⁺ (0.1053 nm), which indicated that the doping enlarges the lattice constant in c-direction. For thermal phase stability and chemical compatibility with alumina, it was observed that m-phase was firstly detected in E2.5Y2.5Z, E2.5D2.5Z and E2.5G2.5Z after 30 h heat-treatment. The content of m-phase in E2.5D2.5Z rised dramatically from 6.07% to 26.28% when the heating duration increased from 30 to 50 h. The similar increase was observed in E2.5G2.5Z (19.24% to 23.06%). Furthermore, the conventional 8YSZ showed poor thermal phase stability, the content of m-phase of which increased from 4.35% to 28.67%. For E5Z, m-phase appeared after longer heating duration (50 h) with the content about 2.84%. In contrast, the m-phase was found in E2.5Y2.5Z after 30 h, but it increased very slowly from 1.40% to 3.35%. So E5Z and E2.5Y2.5Z showed lower phase transformation rates than E2.5D2.5Z and E2.5G2.5Z. The thermal conductivity of conventional 8YSZ was 1.99 W·m^-1·K^-1 at room temperature, and the values for E5Z, E2.5Y2.5Z, E2.5D2.5Z and E2.5G2.5Z were 1.55, 1.73, 1.61 and 1.57 W·m^-1·K^-1, respectively. The thermal conductivity of stabilized zirconia was smaller than that of conventional 8YSZ. E5Z, E2.5D2.5Z and E2.5G2.5Z had the similar CTEs from 100 up to 900 ℃, 9.0×10^-6~11.0×10^-6 K^-1. But E2.5Y2.5Z showed slightly higher CTEs than the others, especially above 300 ℃. The reason for this change could be related with lattice distortion due to the different radius of dopants. Additionally, the CTEs of stabilized zirconia were similar to that of conventional 8YSZ (10.5×10^-6~11.45×10^-6 K^-1). The CTEs of 8YSZ in this work was also very close to the reported value (10.5×10^-6~11.0×10^-6 K^-1). Forerbia based rare earth oxides-stabilized zirconia, XRD results confirmed that the powders had non-transformable tetragonal phase distinguished from (004) and (400) peaks. The most importantly, all specimens exhibited much lower thermal conductivities than conventional 8YSZ and also they had comparable coefficients of thermal expansion with 8YSZ. Furthermore, E5Z and E2.5Y2.5Z had better thermal phase stability than 8YSZ, which was confirmed and could be used above 1200 ℃. In addition, it was found that the thermal phase stabilities of these candidate materials were mainly related to √2c/a ratio, which indicated that the cation diffusion was also dependent on crystal structure of stabilized zirconia.