Preparation and Study of Ca/Tb Co-Doped HfO2 Infrared Coatings with Different Atomic Ratios

Abstract

In this study, HfO2 coatings co-doped with different Ca/Tb atomic ratios were prepared via the atmospheric plasma spraying (APS) method. The microstructure, infrared radiation properties, and high-temperature stability of the coatings were investigated. All of the doped coatings possessed a porous surface and were composed of two phases, namely the monoclinic HfO2 phase and the cubic HfO2 phase. In addition, the content of the cubic phases increased when raising the doping atomic ratio of Ca/Tb, suggesting that Ca could stabilize the cubic HfO2 phase more effectively. The results also show that the coating with a Ca/Tb atomic ratio of 1/0 (CT1 coating) had more excellent infrared radiative properties, whose total emissivity was 0.844 in the 0.75~6.5 μm band and 0.900 in the 6.5~15 μm band, respectively. The improvement in emissivity in the 0.75~6.5 μm band was mainly due to the impurity energy levels introduced via oxygen vacancy, which promoted the absorption of free carriers. And, in 6.5~15 μm, because the approximate masses of the Ca-O and Tb-O bonds were smaller than that of the Hf-O bonds, the infrared absorption of the lattice vibration shifted, favoring absorption below 10 μm. Moreover, Ca had a more significant strengthening effect than Tb in the whole band. In terms of high-temperature infrared radiation performance, the total emissivity of the CT1 coating at 2.5~25 μm increased as the temperature increased from 500 °C to 1100 °C, which might be attributed to the thermal-enhanced lattice vibration absorption. However, the emissivity of the CT1 coating at 3~5 μm was kept around 0.9 from 1100 °C to 2000 °C, owing to the fact that infrared absorption was more determined by the intrinsic width of the energy levels because of the weakening of the doping effect at high temperatures. In terms of thermal stability, the surface morphology and chemical composition of the CT1 coating were barely changed within 4 h of heat treatment at 2000 °C. The total infrared emissivity of the CT1 coating after 4 h of heat treatment was 0.826 in the 0.75~6.5 μm band and 0.895 in the 6.5~15 μm band, slightly lower than that before heat treatment, suggesting good thermal stability and good application prospects as a high-temperature infrared material.