Preparation And Tribological Properties Of Laser Cladding High Entropy Alloy Coatings And Ion Sulfurized Solid Lubricating Films

High entropy alloy is generally composed of 5 to 13 main elements.And the atomic percentage of each element is between 5%and 35%.High entropy alloy is an ideal coating material with excellent properties such as high strength,high hardness,excellent wear resistance,excellent corrosion resistance and excellent thermal stability.In this paper,Al_0.5Co Cr Fe Ni Ti_x(x=0.2,0.5,0.7)and CoCrFeNiMo_x(x=0.5,1.0,1.5)high entropy alloy coatings were prepared on 45#steel substrate by laser cladding technology.Composite sulfide solid lubricating films were prepared by ionic sulfuration on CoCrFeNiMo high entropy alloy coatings.The microstructure and tribological properties of high entropy alloy coatings and sulfide solid lubrication films were studied by X-ray diffractometer,optical microscope,electron probe and friction and wear tester.The phase structure of Al_0.5Co Cr Fe Ni Ti_0.2 high entropy alloy coating is simple BCC and FCC solid solution structure.With the increase of Ti content,the phase structure of high entropy alloy coatings are simple BCC solid solution structure;the lattice constant increases and the grains are refined gradually.High entropy alloy coating has planar crystals at the junction with the substrate.The bottom of the high entropy alloy coating has columnar crystals with obvious growth direction,and the middle and upper part are equiaonal crystals.The contents of Al,Co,Cr and Ni within and between grains are not significantly different.There are intercrystalline segregation in Ti and intracrystalline segregation in Fe.The phase structure of CoCrFeNiMo_0.5 high entropy alloy coating is simple FCC solid solution structure.With the increase of Mo content,the phase structure of high entropy alloy coatings are simple FCC and BCC solid solution structure;the lattice constant increases and the grains are refined gradually.High entropy alloy coating has planar crystals at the junction with the substrate.The bottom of the high entropy alloy coating has columnar crystals with obvious growth direction,and the middle and upper part are equiaonal crystals.The contents of Co and Cr within and between grains arre not significantly different.There are intercrystalline segregation in Mo and intracrystalline segregation in Fe and Ni.The CoCrFeNiMo high entropy alloy coating formed a sulfide solid lubrication film on the surface after ionic sulfuration treatment.The CoCrFeNiMo high entropy alloy sulfurized layer is a kind of porous surface layer composed of spherical nanoparticles superposed with each other randomly.The results of microzonal composition analysis show that the sulfide particles are formed first in the grains of the coating.The XRD and XPS analysis of the sulfurized layer show that the main sulfides of the CoCrFeNiMo high entropy alloy sulfurized layer are Co S,Fe S and Mo S₂,and the grain size of the sulfurized layer is 20.9 nm.With the increase of Ti content,the microhardness of Al_0.5Co Cr Fe Ni Ti_x(x=0.2,0.5,0.7)high entropy alloy coatings gradually increase from 400HV_0.2 to 600HV_0.2;the friction coefficient decrease from 0.471 to 0.455;and the wear weight loss decrease from 0.0028g to0.0017g.The wear mechanism of Al_0.5Co Cr Fe Ni Ti_x(x=0.2,0.5,0.7)high entropy alloy coatings are adhesive wear,and the surface of high entropy alloy coatings are oxidized during friction and wear.With the increase of Mo content,the microhardness of CoCrFeNiMo_x(x=0.5,1.0,1.5)high entropy alloy coatings gradually increase from 500HV_0.2 to 1100HV_0.2;the friction coefficient decrease from 0.330 to 0.274;and the wear weight loss decrease from 0.0040g to 0.0018g.The wear mechanism of CoCrFeNiMo_0.5 high entropy alloy coating is abrasive wear,and the wear mechanism of CoCrFeNiMo_1.0and CoCrFeNiMo_1.5 high entropy alloy coatings are adhesive wear.The surface of high entropy alloy coatings are oxidized during friction and wear.The friction coefficient of CoCrFeNiMo_0.5 high entropy alloy coating is lower,because the plasticity of CoCrFeNiMo_0.5 high entropy alloy coating is better than that of Al_0.5Co Cr Fe Ni Ti_0.5high entropy alloy coating.And the thermal oxidation resistance of CoCrFeNiMo_0.5 high entropy alloy coating is better than that of Al_0.5Co Cr Fe Ni Ti_0.5 high entropy alloy coating under the condition of 750°C for 24 hours.Under the same condition of friction and wear,the friction coefficient of CoCrFeNiMo high entropy alloy coating is 0.295,and the wear weight loss is 0.0024g.A thin layer is adhered to the worn surface,and the wear mechanism is adhesive wear.And there is a serious oxidation in the local area.The friction coefficient of the sulfurized layer is 0.260,and the wear weight loss is only 0.0005g.There are slight abrasions on the worn surface,and the presence of sulfide particles on the surface reduces the degree of oxidation.The sulfide solid lubricating film is easy to deform and can avoid the direct contact of friction pair.Under the action of friction and heat,the sulfide decomposes and produces active sulfur atoms to diffuse inward,forming the influence zone of sulfur element.Combine with Fe and Mo in the cladding layer to generate sulfide solid lubrication phase again,and continue to reduce friction lubrication.Although the thickness of the layer is thin,the supporting role of CoCrFeNiMo high entropy alloy with high hardness and the effect of sulfur element influence zone can effectively extend the action time of the layer.