Preparation, Microstructure and Mechanical Properties of Mo Layer and CoCrFeNiMn High Entropy Alloy Hard Coating Layer

To address the issue of low hardness and limited service life of the Invar alloy in practical applications, this study employs double-glow plasma surface alloying (DGPSA) technique to fabricate Mo and CoCrFeNiMn hard coating layers on the surface of the Invar alloy. The phase structure, microstructure, and element distribution of the two coating layers were investigated using X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectroscopy (EDS). The effects of loading strain rate on surface hardness, elastic modulus, and creep behavior of the two hard coating layers were systematically studied via nanoindentation. The thickness of the Mo coating layer is approximately 8.3 μm, with a dense and uniform internal structure and a body-centered cubic (BCC) structure. The thickness of the CoCrFeNiMn coating layer is approximately 10 μm, with some internal porosity and presents a face-centered cubic (FCC) structure. The nanoindentation tests show that the hardness of the Mo and CoCrFeNiMn coating layers is 15.49 and 8.18 GPa, respectively, while their elastic modulus are 278.70 and 227.12 GPa. The two hard coating layers significantly enhance the surface hardness and elastic modulus of the Invar alloy, and exhibit sufficient toughness. Hardness of the two coating layers increases with increasing strain rate, showing a pronounced strain rate sensitivity, while the elastic modulus remains relatively stable. Additionally, the creep behavior of the two coatings layers is influenced by the applied strain rate, with nanoindentation creep primarily governed by dislocation motion. The modification effect of the Mo coating layer is superior to that of the CoCrFeNiMn coating layer.