Effects of rare-earth oxide doping on the thermal radiation performance of HfO2 coating

Abstract In this study, the REO-HfO 2 (REO = Tb 4 O 7 , Gd 2 O 3 and Sm 2 O 3 ) coatings and pure HfO 2 coatings were prepared by atmospheric plasma spraying. The chemical compositions, morphologies, infrared radiation performance and thermal resistances of the coatings were systematically investigated. The experimental results showed that the Tb 4 O 7 -HfO 2 , Gd 2 O 3 -HfO 2 , Sm 2 O 3 -HfO 2 and pure HfO 2 coatings had infrared emissivity values of 0.863, 0.852, 0.854 and 0.621, respectively, at room temperature. Based on the phase analysis, the higher infrared emissivity of the REO-HfO 2 coatings could be attributed to the fact that the newly formed RE 2 Hf 2 O 7 (RE = Tb, Gd and Sm) phase, which had a defective fluorite-type structure, and the RE 3+ ions enhanced the lattice absorption and electron absorption. Additionally, the Tb 4 O 7 -HfO 2 coating exhibited a relatively higher infrared emissivity than those of the Gd 2 O 3 -HfO 2 and Sm 2 O 3 -HfO 2 coating over the wavelength range of 1–15 μm, which was due to the relatively higher vibrational frequency of the Tb O bond in RE 2 Hf 2 O 7 (RE = Tb, Gd and Sm) and the transformation of Tb 3+ into Tb 4+ in the Tb 4 O 7 -HfO 2 system. In addition, the REO-HfO 2 ceramic coatings exhibited excellent thermal resistance, which could withstand high-temperature treatment at 1600 °C for at least 50 h without undergoing a phase change and exfoliation, and the infrared emissivity at different temperatures hardly changed after thermal treatment.