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HfO2–Sm3TaO7 ceramics are prepared through a solid-state reaction method. The X-ray diffraction and structural refinement show that the phase structures of HfO2–Sm3TaO7 ceramics are an ordered orthorhombic phase and the space groups are belonging to Ccmm. The degree of the structural disorder increases with increasing HfO2 content. The solid solution mechanism reveals that Hf4+ exists in the form of interstitial ions that cause crystal expansion when the doping content is less than 4 mol%. When the doping concentration of HfO2 ≥ 4 mol%, the Hf4+ ions can substitute an equal number of Sm3+ and Ta5+ ions. The phase transition of Sm3TaO7 ceramics is removed with increasing HfO2 content, and the 8 mol% HfO2–Sm3TaO7 ceramics have a high thermal expansion coefficient of 10.2 × 10−6 K−1 at 1200 °C. The 2 mol% HfO2–Sm3TaO7 ceramics have the lowest thermal conductivity (1.03 W/m K at 900 °C), which is lower than previous research of the 7–8 YSZ. The outstanding thermophysical properties of HfO2–Sm3TaO7 ceramics indicate that they are potential thermal-barrier coating materials.
SmTaO4 ceramics have excellent high-temperature phase stabilities and mechanical properties and show great potential for use as next-generation thermal barrier coating (TBC) materials. CeO2–SmTaO4 ceramics are prepared via high-temperature solid–state reaction. It retains a single monoclinic phase structure. Ce4+ was reduced to Ce3+ by high-temperature deoxidation, and the Ce3+ ions substitute for an equal number of Sm3+ ions. The CeO2–SmTaO4 ceramics had lower thermal conductivities [1.09–2.75 W/(m K)] than yttria-stabilized zirconia (YSZ) [2.1–2.7 W/(m K)] at 100–800 °C, which decreased dramatically with increasing temperature. SmTaO4 doped with 2% CeO2 had lower thermal conductivity [1.09 W/(m K), 800 °C] than SmTaO4 [1.42 W/(m K), 800 °C] and 2% ZrO2-doped SmTaO4 ceramics [1.22 W/(m K), 800 °C]. The low thermal conductivity is attributed to Ce3+ substitution for an equal number of Sm3+ ions, and because Ce3+ ions are the strongest phonon scattering centers, they can decrease the phonon mean free path effectively. The thermal expansion coefficient of 8% CeO2–SmTaO4 ceramics is approximately 10.3 × 10−6 K−1 at 1200 °C, which is slightly higher than that of both YSZ (10.0 × 10−6 K−1) and SmTaO4 (9.58 × 10−6 K−1). The outstanding thermophysical properties indicate that CeO2–SmTaO4 ceramics are potential TBC materials.
The cyclic oxidation experiment of yttria-stabilized zirconia coatings deposited on NiCoCrAlYHf alloys by air plasma spraying was investigated at 1050 °C in air and in air containing water vapor. The results revealed that water vapor has a great influence on the oxidation resistance of the thermal barrier coatings (TBCs). Compared with the samples oxidized in air atmosphere, TBCs oxidized in air containing water vapor had a longer lifetime. It was also found that different atmospheres could lead to different HfO2 formation positions, which could decrease the rumpling in the oxide layer. In particular, after the coatings on Hf-doped NiCoCrAlY were first pretreated in air containing water vapor for 24 h at 1050 °C, the lifetime of the pretreated coating was doubled compared to the coating in laboratory air only. The water vapor pretreatment of the coatings could be an important method for optimizing the lifetime of TBCs.
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