Effect Of Mo-Gray Cast Iron Shell-Core-Structured Powder On The Microstructure And Wear Resistance Of Plasma-Sprayed Coating

Lightweight of vehicles is the most effective way to achieve high-speed transportation,energy saving,and environmental protection.Aluminum alloy is the best material for vehicles to achieve lightweight.However,aluminum alloy is extremely easy to be worn and thus be out of work in service.To prolong the lifetime of the aluminum alloy parts,a wear-resistant gray cast iron coating with high performance and low cost can be prepared by plasma spraying on its surface.However,the limited interlamellar bonding of plasma sprayed coatings makes it easy to crack along the nobonded interfaces between splats under friction conditions,resulting in delamination of splats and deteriorated wear.Therefore,improving the interlamellar bonding has become the major research focus in the preparation of gray cast iron coatings.There are two methods can be used to improve the interlamellar bonding of the coating.One is increasing the collision particle temperature and the other is increasing substrate temperature.However,for the aluminum alloy substrate,increasing the substrate temperature can cause the substrate oxidation and deformation,so increasing the particle temperature is a more appropriate way to improve the bonding.Recent studies had shown that spraying using the cladded particles with low-melting point coated with high-melting point material can significantly increase the surface temperature of the sprayed particles and improve the bonding between the particles.Therefore,it is expected that when molybdenum-gray cast iron shell-core structured powders are heated by plasma jet,molybdenum not only reduces the oxidation of the gray cast iron,but also increases the temperature of the iron particles by heat transfer from molybdenum shell to gray cast iron core.Based on these,in this study molybdenum-gray cast iron shell and core structured powders were firstly prepared by mechanical ball milling method,and then this powder and gray cast iron powder were employed to be the starting materials to prepare the coatings by atmospheric plasma spraying.Besides,the effects of ball milling speed,ball milling time and additives on the surface morphology and cladding state of molybdenum-gray cast iron shell core structured powders were studied.Based on the numerical simulation of the heating behavior of molybdenum-gray cast iron shell core structured powder particles in plasma jet,the effects of powder surface cladding state on the microstructure,phase composition and wear resistance of the coatings were experimentally studied.The optimum process parameters for the preparation of graphite-grey cast iron shell core structured powders by mechanical ball milling method are as follows,the addition amount of molybdenum is 20 wt.%,the addition amount of 2%concentration PVA solution is 0.4g,the ball milling speed is 400 r/min,the ball milling time is 4h,and the ball-to-material ratio is 10:1.The molybdenum-gray cast iron shell core structured powders achieved a maximum coverage rate of 77.2%under the optimal ball milling parameters.The fluidity of gray cast iron powders and the molybdenum-gray cast iron shell-core structured powders are 23s/50g and 45s/50g,respectively,indicating their suitable for thermal spraying.The numerical simulation results revealed that the coverage rate has a positive correlation with the core temperature of particles,which means that when the coverage rate of particle increases,the core temperature raises accordingly.When the powder coverage rate is 25%,50%and 75%,the core temperature of the particles is 924.8℃,1024.8℃,and 1475.4℃respectively.In addition,the core temperature of particles is positively correlated with the thickness of cladding layer.So when the layer thickness increases,the core temperature of the particle raises accordingly.When the layer thickness is 1μm,2μm,3μm,4μm and 5μm,the core temperature of the particle is 1365.5℃,1525.1℃,1936.6℃,2309.1℃and 2694.2℃respectively.The research results of the microstructure and wear resistance of coatings show that the oxides deteriorates the interlamellar bonding of gray cast iron coating.In comparison,the porosity and the oxidation degree of the molybdenum-gray cast iron coating are reduced,the cohesion within the coating is improved,and the interlamellar bonding is greatly strengthened.The friction coefficients of gray cast iron coating and molybdenum-gray cast iron coating are stabilized at 0.42±0.05 and 0.26±0.03,and the wear rates are 1.696×10^-3mm³/N·mand1.162×10^-3mm³/N·m and,respectively.The wear mechanism of gray cast iron coating is mainly abrasive grain wear and oxidative wear.The wear mechanism of the molybdenum-gray cast iron coating is a small amount of oxidative wear.