Dual coating layers to enhance oxidation resistance for refractory high-entropy alloys

Abstract

Refractory high-entropy alloys (RHEAs) possess excellent high-temperature mechanical properties, making them have the promising potential for applications at temperatures higher than Ni-base superalloys. However, their development has been limited by their poor oxidation resistance, which demands an urgent and effective improvement. In particular, the oxides of RHEAs generally suffer from severe pest phenomenon between 500 °C and 800 °C. In this study, the low-density, strong and ductile BCC Mo0.3Nb1.3TiZr RHEA was designed and prepared, and the isothermal oxidation mechanism was studied at mainly 800 °C.To effectively improve the oxidation resistance of the alloy, dual coating layers with inner-layer aluminide and outer-layer silica protection were designed and investigated. Firstly, pack cementation was used to prepare aluminide coating on RHEA. Secondly, the sol-gel method was used to deposit silica films over the aluminide coating. With single aluminide coating deposited on the base metal, the weight gain was reduced from 92.2 mg/cm2 of uncoated specimen to 4.5 mg/cm2 at 800 °C for 24 h. With the dual coating layers, no pest phenomenon was observed between 700 °C and 900 °C and the weight gain dropped sharply from 88.4 mg/cm2 of aluminized specimen to 2.8 mg/cm2 at 800 °C for 48 h. The aluminide coating and SiO2 film successfully adhered to the surface of the RHEA. In addition, the SiO2 film reduced the consumption of aluminide coating. Cracking, accelerated oxygen penetration, and pest phenomenon were not observed. This dual coating layers are very promising in solving the poor oxidation resistance of RHEAs.