High Temperature Tribological Properties of CoCrFeNiNb_x High-Entropy Alloy Coatings by Laser Cladding

High entropy alloys (HEAs) have been received more and more attention in recent years. Multi-components with equal or near-equal molar ratio are adopted to form solid solutions with severe lattice distortion in HEAs. This novel compositional design strategy provides the possibility to achieve the goal of balancing the high strength and high ductility, which is recognized as a challenge for traditional alloys. Also, HEAs have been considered as a novel kind of material for high-temperature applications due to their excellent mechanical properties, superb high-temperature stability and outstanding oxidation resistance. As a ductile HEA, CoCrFeNi alloy with a single FCC phase is investigated by many researches in terms of the microstructure and performances. However, the strength of CoCrFeNi HEA is low at high-temperature, and its high-temperature performances are not ideal, which limits its further application. According to the well-known cocktail effect, CoCrFeNi HEA was doped by the refractory element Nb for the purpose of improving its high-temperature properties in this work. CoCrFeNiNb_x (x=0, 0.25, 0.5, 0.75, 1.0) HEA coatings were successfully prepared on the surface of 45 steel by laser cladding technology. The phase composition and the microstructure of the HEA coatings were observed by X-ray diffractometer and scanning electron microscope equipped with energy disperse spectroscopy. The effect of Nb element on the phase composition and microstructural evolution of the HEA coatings were analyzed. The microhardness of the HEA coatings was measured by a Vickers hardness tester, and the effects of temperature on wear resistance and wear mechanism of the CoCrFeNiNb_0.75 HEA coating were studied by a friction and wear tester. The results showed that the microstructure of CoCrFeNi HEA coating consisted of a single FCC phase, and Nb doping resulted in the formation of Laves phase with HCP lattice structure. As the Nb content increased, the volume fraction of Laves phase increased and the lattice constant of FCC phase increased first and then decreased. In addition, the microstructure of CoCrFeNiNb_x coatings evolved from a single cellular FCC solid solution phase (x=0), to hypoeutectic structure (x=0.25), to eutectic structure (x=0.5), to hypereutectic structure (x=0.75, 1.0), and the primary phase in the HEA coatings changed from FCC solid solution phase to Laves phase. With the increase of Nb content, the microhardness of CoCrFeNiNb_x increased remarkably first (x=0 to 0.75) and then maintained (x=0.75 to 1.0) at a high level, and the CoCrFeNiNb_0.75 HEA coating had the highest average microhardness (574 HV), indicating that a proper amount of Nb doping effectively improved the microhardness of the coating. The improvement of microhardness was attributed to the interaction of solid solution strengthening, second phase strengthening, and a large number of new interfaces in the lamellar eutectic structure, which hindered the dislocation movement. The volume wear rate of the CoCrFeNiNb_x HEA coating showed a trend of first increasing and then decreasing, reaching the maximum volume wear rate of 2.2×10⁻⁴ mm³/(N·m) at 400 ℃. The main wear mechanisms of CoCrFeNiNb_0.75 HEA coating were oxidation wear and mild abrasive wear at room temperature , and oxidation wear at 400 ℃ and 800 ℃. The XPS results showed that the oxides at 800 ℃ on the worn surface of CoCrFeNiNb_0.75 HEA coating were CoCrO₄, NiCr₂O₄, CoO, Co₃O₄, Cr₂O₃, Fe₂O₃, NiO and Nb₂O₅. Because the oxides with high PBR (Pilling-Bedworth Ratio) were preferentially produced according to the calculation of Gibbs free energy, the oxide layer predisposed to breakup, limiting the wear resistance of the HEA coating at 400 ℃. Moreover, with the production and interaction of a large number of oxides, a dense oxide enamel layer formed on the wear surface at 800 ℃, which played a good role in reducing friction and anti-wear.