Preparation and infrared emissivity of metal borides(metal=V,Mo,Fe) and MnO2 co-doped NiCr2O4 coatings

Abstract In this study, NiCr2O4 coatings co-doped with metal borides MBx (M = V, Mo and Fe) and MnO2 were synthesized by atmospheric plasma spraying. The phase composition, microstructure, infrared radiation performances as well as the thermal resistance of the coatings were investigated. All coatings possessed typically layer stacked microstructure, and the thickness and roughness (Ra) of the ceramic coatings ranging about 50-60 μm and 3.51-3.65 μm, correspondingly. Besides, the doped coatings still had a spinel structure. Since the metal ions and boron ions entered into the spinel lattice, which induced lattice distortion and enhanced the infrared activity of lattice vibrational absorption, the infrared emissivity of the coatings at 5-25 μm was increased effectively. The highest emissivity at 5-25 μm was obtained by V-doped coating, which was 0.8836 about 0.0175 higher than that of pure NiCr2O4 coating. However, the emissivity at 3∼5 μm was declined owing to compensating effect of high-valence ion doping, which reduced the hole's concentration and the intersub-band transition absorption. The lowest emissivity of 0.8664 was obtained with Mo-doped coating at 3-5 μm, which was 0.017 lower than that of pure NiCr2O4 coating. Attributed to the donor levels introduced through high-valance doping, the free electron's concentration increased, which promoted the transition absorption of free electrons from the impurity levels to the conduction band, resulting in higher emissivity at 0.75-2.5 μm. The highest emissivity of 0.935 at 0.75-2.5 μm was obtained with the Mo-doped coating, which was 0.02 higher than that of pure NiCr2O4 coating. As for high temperature performance, the VB2–MnO2 co-doped coating possessed the highest emissivity of 0.92 in 2.5∼25 μm at 1000 °C, and also showed good thermal resistance after 30 thermal cycles with water-cooling from 750 °C to 25 °C.