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High entropy alloy attracts great attention for its high thermal stability and corrosion resistance. A CoCrFeNiMn high-entropy alloy coating was deposited on grey cast iron through plasma transfer arc cladding. It formed fine acicular martensite near the grey cast iron, with columnar grains perpendicular to the interface between the grey cast iron substrate and the cladding layer as well as dendrite in the middle part of the coatings. Simple FCC solid solutions present in the coatings which were similar to the powder’s structure. The coating had a microhardness of 300 ± 21.5 HV0.2 when the cladding current was 80 A for the solid solution strengthening. The HEA coating had the highest corrosion potential of −0.253 V when the plasma current was 60 A, which was much higher than the grey cast iron’s corrosion potential of −0.708 V. Meanwhile, the coating had a much lower corrosion current density of 9.075 × 10−7 mA/cm2 than the grey cast iron’s 2.4825 × 10−6 mA/cm2, which reflected that the CoCrFeNiMn HEA coating had much better corrosion resistance and lower corrosion rate than the grey cast iron for single FCC solid solution phase and a relatively higher concentration of Cr in the grain boundaries than in the grains and this could lead to corrosion protection effects.

Thermal barrier coatings with multi-sized porous structure at micro and nano scales were prepared with hollow spherical YSZ powders and polypropylene powders through atmospheric plasma spraying. The thermal conductivities of the multi-sized thermal barrier coatings after a long-term serving at high temperature were tested through laser flash heating method. Meanwhile, the physical models of thermal barrier coatings with multi-sized porous structure at micro and nano scales were constructed through Ansys Workbench. The evolutions of thermal conductivity of thermal barrier coatings with multi-sized pores after long-term service at 1100 °C were investigated through computation. It was found that the thermal conductivity of the coating increased with the extension of the serving time. When the serving time reached 60 days, the thermal conductivity of the coating tended to be stable and close to the compacted bulk. The computational results were consistent with the tested ones, which could be helpful to explain the thermal conducting evolution in thermal barrier coatings with multi-sized porous structure at nano and micro scales after long-term serving at high temperature.

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Gd and Yb elements have high chemical stability, which can stabilize the solid solution in ZrO2. Gd2O3 and Yb2O3 have high melting points, and good oxidation resistance in extreme environments, stable chemical properties. Therefore, Gd2O3 and Yb2O3 were added to ZrO2 to stabilize oxides, improve the high temperature stability, and effectively decrease the thermal conductivity at high temperature. In this work, 5 wt% Yb2O3 and 5 wt%, 10 wt%, 15 wt% Gd2O3 were doped into 8 wt% Y2O3 stabilized ZrO2 (8YSZ) powders as thermal barrier coating materials, and sintered at 1650 °C for 6 h, 12 h, 24 h. The effects of Gd2O3 addition on the microstructure, density, thermal conductivity, hardness, and fracture toughness of Gd2O3-Yb2O3-Y2O3-ZrO2 (GYYZO) bulk composite ceramics were investigated. It was found that the densification of the 8YSZ bulk and GYYZO bulk with 15 wt% Gd2O3 reached 96.89% and 96.22% sintered at 1650 °C for 24 h. With the increase of Gd2O3 addition, the hardness, elastic modulus and fracture toughness of the GYYZO bulk increased and the thermal conductivity and thermal expansion coefficient of the GYYZO bulk decreased. GYYZO bulk with 15 wt% Gd2O3 sintered at 1650 °C for 24h had the highest hardness, elastic modulus and fracture toughness of 15.61 GPa, 306.88 GPa, 7.822 MPa·m0.5, and the lowest thermal conductivity and thermal expansion coefficient of 1.04 W/(m·k) and 7.89 × 10−6/°C at 1100 °C, respectively. The addition of Gd2O3 into YSZ could not only effectively reduce the thermal conductivity but also improve the mechanical properties, which would improve the thermal barrier coatings’ performances further.