Evaluation on the microstructure and thermal infrared emissivity of Ca²⁺-doped ytterbium chromate coating deposited by saucer-shaped porous feedstocks for high-temperature thermal protection

A high infrared emissivity coating with porosity of 16.2 % was deposited on Ni-based alloy by saucer-shaped porous feedstocks using the atmospheric plasma spraying technique. The solid-phase reaction method was employed to synthesize Ca²⁺-doped YbCrO₃ ceramic powders, and then the saucer-shaped porous feedstocks were obtained by electrostatic spraying accompanied by heat treatment, and the conventional spray drying feedstocks were used as comparison. Compared with the conventional structural coating, the coating achieved by saucer-shaped feedstocks shows higher porosity and surface roughness due to the insufficient melting of the feedstocks with larger diameters in the plasma flame. Therefore, the coating achieved by saucer-shaped feedstocks exhibits a bimodal organization similar to that of nanostructured thermal barrier coatings. Compared with the conventional coating, the coating with new structure shows a higher emissivity value of 0.9460 at 1–14 µm waveband at room temperature due to the higher surface roughness (7.12 μm). Moreover, the average emissivity of coatings with roughness ranging from 0.93 µm to 7.12 µm is all above 0.91. More importantly, although the infrared emissivity of all coatings with different roughness decreases with the increase of temperature, they still remain above 0.77 at 1–14 µm when the temperature reaches 1200 °C.