Oxidation behaviour of nickel-coated graphene films reinforced laminated CoCrFeMnNi high-entropy alloy in steam environment at 900 °C

In this study, nickel-coated graphene films (GFs (Ni))-reinforced CoCrFeMnNi high-entropy alloy (HEA) matrix composites with pearly laminated structure were prepared using mechanical ball milling and vacuum hotpressing sintering. The high temperature oxidation behaviour of the prepared composites was investigated under a steam atmosphere at 900 degrees C. The experimental results demonstrate that the oxidation kinetic curves exhibit a two-step parabolic law. Both steps are steady-state oxidation processes. The value of kp for the initial steady-state oxidation stage is larger than that of the subsequent stage. The difference between them possibly being that additional oxides formed in the scale after 48 h further slow down the corrosion process. The oxidation products are mainly dominated by the outer layers Mn3O4, the intermediate layers Mn3O4 and MnFe2O4, and the inner layers MnCr2O4 and Cr2O3 are also included. Notably, significant areas of Mn- and Cr-poor elements are observed in the matrix adjacent to the oxidation layer. In this study, the reason for the capability of nickel plating on the surface of GFs to improve the oxidation resistance of composites was discussed. The presence of Ni plating was found to enhance the wettability and compatibility of the GFs-matrix interface. The anti-oxidation property of HEA composites can be effectively improved by adding an appropriate amount of GFs (Ni) and the composite with 5 vol% GFs (Ni) shows the highest oxidation resistance. However, excessive addition of GFs (Ni) led to more serious aggregation and an increase in interface defects, which negatively impacted the oxidation resistance of the composites. Meanwhile, the laminated structure changes the diffusion path of oxygen ions, the oxygen ions diffuse faster in the "mortar" part parallel to the laminated direction. Therefore, the anisotropy of the laminated structure gives the material a superior resistance to high-temperature oxidation in the perpendicular direction to the laminate layers.