Study On Microstructure Anisotropy And Mechanical Properties Of WC-12Ni Coatings By High-velocity Oxy-fuel Thermal Spraying

Thermal spraying technology has gradually been widely used in the industrial field.However,due to the technological characteristics of molten particle deposition in thermal spraying process,the coating often has anisotropic structure,which leads to the uneven mechanical properties of the coating in all directions,and it is difficult to meet the needs of higher performance applications.This paper USES the supersonic flame spraying technology,the martensitic precipitation hardening WC-Ni coating was prepared on stainless steel substrates,studied the kerosene fuel and oxygen flow rate,spraying distance and other process parameters,as well as the coating thickness on the coating deposition morphology,porosity,phase structure,the influence law of anisotropy,the organization characteristics of microstructure on the mechanical behavior of the coating are analyzed influence mechanism.it provides the basis for optimization of spraying process parameters and improves the mechanical properties of coating.Under the condition that the proportional flow coefficient of kerosene fuel and oxygen is fixed asλ=1.05,the adjustment range of kerosene flow rate is 19.3～24.5 L/h and oxygen flow rate is 690～875 L/min（NLPM）.The particle temperature ranges from 1852 to 1910℃,and from 1912 to 1974℃,and the particle velocity ranges from 821 to 823 m/s,853 to 863 m/s,and 900 to 910 m/s,were obtained to prepare WC-12Ni coatings with different thickness of 100to 500μm.The spraying process parameters on the coating surface,vertical cross section and parallel cross section the influence of three different surface coating microstructure characteristics,analysis its anisotropy,accordingly three surface coating mechanical properties evaluation,establish the WC-12Ni coating layer microhardness,fracture toughness and the interface fracture toughness and other mechanical properties and microstructure of anisotropic coating corresponding relation,the main research results are as follows:（1）The effect of spraying parameters on anisotropy of microstructure characteristics of the coating was studied.Considering the direction of spray gun scanning deposition process,the microstructure characterization of the coating surface,vertical cross section and horizontal cross section was carried out respectively.The porosity of the coating surface was in the range of 0.46～0.60%.The porosity is slightly less than 0.45～0.74%on the vertical and parallel surfaces of the coating.With the increase of spraying particle velocity,the residual stress of surface of coating increases first and then decreases.The residual stress of coating deposited at higher spraying particle temperature is higher than that deposited at lower temperature.In addition,the WC retention rate decreases first and then increases with the increase of particle velocity at the two temperature ranges,and the WC retention rate is 75.8～77.7%at 1912～1974℃and 78.0～82.5%at 1852～1910℃.（2）The effect of spraying parameters on the anisotropy of the microhardness of the coating was studied.When the coating thickness was 200μm,the microhardness of the coating surface increased with the increase of the particle velocity at the higher spraying particle temperature,and increased first and then decreased at the lower spraying particle temperature.The microhardness of vertical plane and parallel plane increases first and then decreases with the increase of particle velocity.At fixed spraying parameters of kerosene flow of 22.7 L/h and oxygen flow of 811 NLPM（particle velocity of 821 m/s and particle temperature of 1974℃）,100～500μm coatings with different thickness were prepared.The microhardness of the coating surface,vertical plane and parallel plane increased first and then decreased,reaching the maximum value at 200μm,12.07 GPa on the surface,11.75 GPa on the vertical plane and 10.64GPa on the parallel plane.Under the condition of the same thickness,the microhardness of the surface is the largest,the microhardness of the vertical plane is the second,and the microhardness of the parallel plane is the smallest.（3）To study the spraying process parameters on the coating and the interface fracture toughness anisotropy,the influence of coating on the surface of the fracture toughness increased with the increase of particle velocity and decreases,vertical and parallel surface after the fracture toughness of the coating are presented to reduce the increasing trend,under the condition of the same particle temperature,speed,surface coating fracture toughness is the largest,parallel surface times,vertical plane is minimal.With the increase of coating thickness,the coating fracture toughness increases first and then decreases,and reaches a maximum value of 11.61 MPa·m^1/2 at 200μm.The coating fracture toughness increases in both vertical and parallel planes.When the coating thickness reaches 300μm,the fracture toughness of the coating on vertical plane and parallel plane is higher than that on surface.The fracture toughness of vertical plane and parallel plane decreases first and then increases with the increase of particle velocity.With the increase of coating thickness,the fracture toughness of vertical plane and parallel plane decreases first and then increases.When the coating thickness is 300μm,the fracture toughness of vertical plane and parallel plane is the lowest,which is 4.45 MPa·m^1/2 and4.38 MPa·m^1/2,respectively.（4）The corresponding relationship between the anisotropy of microstructure characteristics and the anisotropy of mechanical properties of the coating is analyzed.With the decrease of porosity,the microhardness value of the coating increases.The porosity of the surface is larger than that of the vertical plane and horizontal plane,resulting in a smaller microhardness of the coating surface.With the increase of porosity,the fracture toughness of the coating increases.The fracture toughness of coating surface is larger than that of vertical plane and parallel plane.The fracture toughness of coating and interfacial fracture toughness decrease with the increase of microhardness.