Preparation And Wear Resistance And Antifriction Properties Of Surface Micro-pit Composite MoS₂ Nickel-based Coating

Laser melting hard coating is one of the important means to solve the problem of insufficient friction performance of material surface,but it is difficult to balance the wear resistance and friction reduction performance of hard coating.The self-lubricating function of hard coating can be achieved to a certain extent by adding solid lubricant for preparing coating during laser cladding.However,the addition of solid lubricant usually reduces the strength of hard coating,which is manifested as the decreases of coating hardness and impact toughness,and even the generation of air gap,cracks and other defects,seriously affecting the formation of coating.In order to solve the problems that the wear resistance and friction reduction performance of hard coatings are difficult to take into account simultaneously and the mechanical properties of coatings decrease,a new coating structure with good wear resistance,friction reduction and mechanical properties is proposed in this paper,which provides an important basis for the coating design with uniform wear resistance and friction reduction performance.Firstly,in this paper,the nickel-based coating（LC）was prepared by laser melting coating on the surface of 42Cr Mo bearing steel substrate（S）.Then,the ordered micro-pit texture was prepared on the coating surface by EDM technology.Finally,the solid lubricant MoS₂was coated on the micro-pit texture of the coating surface for preparing the surface micro-pit composite MoS₂nickel-based coating（LCT）.The microstructure of LC and LCT samples were analyzed by advanced test technology.The results show that LC and LCT samples have no defects such as pores and cracks,and the coating is metallurgically bonded with the substrate.The coating takesγ-Ni（Fe）solid melt as the matrix phase,and generates a series of complex compounds such as Fe Ni3,（Cr,Fe）₇C₃,Cr B,BFe₂.The hardness of the coating is 3.5 times higher than that of the substrate,reaching 545HV_0.2;the surface pits of LCT sample are well formed,and the solid lubricant was completely filled to the bottom of the pits,and the filling effect is good.Secondly,LCT samples with different parameters were designed and prepared,and the effect of texture surface density on LCT samples was mainly studied.The results show that under certain working conditions,the friction coefficient and wear rate of LCT decrease with the increases of texture surface density in the range of surface density（8.72%-19.63%）.Under4 N and 0.1 m/s working conditions,the friction coefficient of the surface micro-pit composite MoS₂nickel-based coating with 19.63%surface density（LCT19.63%）is the lowest,reaching0.36.Compared with LCT8.72%and LCT12.56%samples decreased by 23.4%and 16.3%respectively.The higher the surface density of micro-pit texture is,the more solid lubricants can be stored per unit area.In the friction process,the friction surface is easy to form a lubricating film,which reduces the friction coefficient and wear rate of the sample and improves the friction reduction performance of the material.This paper studies the friction and wear behavior of S sample,LC sample and LCT sample under different working conditions,and explores the variation of friction coefficient and wear rate of each sample with working conditions.The results show that under different working conditions,the LCT sample shows excellent wear resistance and friction reduction performance compared with the S and LC samples,and has lower friction coefficient and wear rate.With the increases of load,the friction coefficient and wear of LCT sample decreases at first and then increase.When the speed increases from 0.1m/s to 0.3 m/s,the friction coefficient of the sample decreases,showing better friction reduction performance.When the speed further increases,the friction coefficient of the sample increases,and the self-lubricating performance of the sample decreases.Excessive speed will destroy the surface lubrication film of the sample.Finally,this paper analyzes the wear mechanism of S sample,LC sample and LCT sample under different working conditions,and focuses on the wear resistance,friction reduction mechanism and film formation mechanism of LCT sample.The results show that the wear modes of S and LC are mainly oxidation wear and abrasive wear,and the surface lubrication of LCT has three stages:initial stage,running-in stage and stable stage.With the continuous friction process,the lubricant is squeezed and dragged into the friction surface in the micro-pit,and the lubricant accumulation is formed on the surface.Under the further action of friction,the loss of lubricant on the friction surface is balanced with extrusion,and the friction pair can remain in a long-term stable stage,and the lubrication film formed in this stage is the most complete.There is an obvious MoS₂layer structure on the worn surface of the LCT sample,and a lubricating film is formed on the worn surface to achieve the purpose of reducing friction of the sample.The surface micro-pit composite MoS₂nickel-based coating combines the characteristics of micro-pit texture for storing abrasive particles,lubricants and high bearing capacity of the coating,and has good wear resistance and friction reduction performance.