Design And Preparation Of Anti-friction And Wear-resistance Functional Layer During Plasma Nitriding And Carburizing Duplex Treatment Of M50 Steel

As an important transmission part,bearings play a key role in the reliability and mechanical efficiency of the system.Compared with ordinary bearings,aerospace bearings service in more severe conditions and they are in urgent need of ultra-long fatigue life,ultra-high wear resistance,strength,and toughness.M50 steel and its variant M50Ni L steel are cal ed second-generation bearing steels which are currently the main steel for aerospace bearings.In addition,carbon nitride films are good candidates for applications in gearboxes and bearings due to their advantages such as high hardness,low friction coefficient,high wear resistance,and low cost.However,due to the large performance difference,it is easy to generate large residual stress between the carbon nitride film with higher hardness and the soft steel matrix,resulting in weak adhesion,which limits its application in bearing steel matrix.In this paper,a composite modification layer is designed which is composed of a nanostructure nitride layer and diamond-like carbon film.And the composite modified layer is in-situ prepared on the surface of M50 steel through nitriding and carburizing duplex treatment.The modified layer realizes the synergy of load bearing,wear resistance,and friction reduction,which significantly improves the wear resistance of bearing steel.The structure of the wear-resistant and friction-reducing functional layer is designed through thermodynamic calculation and first-principles simulation,which guides the selection of nitriding and carburizing processes.According to the double sublattice model,the mixed Gibbs free energy curve of the Fe-Cr-N ternary system with BCT structure was calculated.It was found that the quenched M50 steel had a stability limit during the plasma nitriding process,and the spinodal decomposition could occur to realize surface nano-crystallization.According to the thermodynamic calculation,the phase structure of the surface with nano-crystallization is designed as martensite（high nitrogen and low nitrogen martensite）,and nitride.The first-principles calculations show that both carbon atoms and nitride-forming elements in the solid solution increase the diffusion barrier of N atoms.Therefore,the selection of nitriding nanotechnology process parameters is as follows:the nitriding temperature range 410 ^oC-570 ^oC and N₂/H₂ as 1/7,1/3,and 3/1.The formation energy of sp² and sp³ bonds on（100）and（111）facets is negative based on first-principles calculations,implying that it is favorable for the growth of diamond/graphite-like carbon.Only strong carbide forming elements such as Cr and V can simultaneously improve the structural stability of Fe₅C₂ and the stability of（100）plane.Combined with the Fe-C-N ternary phase diagram,it is confirmed that the higher N content on the pre-nitridation surface is conducive to the formation of carbide Fe₅C₂,and the higher the carburizing temperature,the more conducive to the formation of HCP structure.The temperature range 410 ^oC-570 ^oC is chosen as the carburizing range to prepare a carburized layer including different nitrides or carbides.According to the parameters designed by thermodynamics,the plasma nitriding process was explored,and the microstructure and properties response of the nitrided layer to the temperature,atmosphere,time and pre-treatment were comprehensively investigated.The nitrided surface of quenched M50 steel is composed of nitrogen-containing martensiteα’,nitrides Fe₄N,Fe₃N,and a small amount of MN.The nitriding parameters may change the content and orientation of nitrides.Too low temperature,too short time and too high nitrogen-hydrogen ratio are not conducive to the thickening of the nitrided layer.When the temperature is above 530 ^oC,the time extends 12 h,and the nitrogen-hydrogen ratio is higher than 3/1,a compound layer will appear above the diffusion layer,and a coarse intergranular structure will be found inside the diffusion layer,leading to tissue deterioration of the layer.The surface and the subsurface synergistic strengthened after nitriding at the second hardening temperature（530 ^oC）.A thicker high-hardness zone is produced,the hardness gradient becomes gentle,and the friction coefficient and wear rate reach the lowest values of 0.39 and 1.32×10^-5 mm³N^-1m^-1.The effect of carburizing process parameters on the microstructure and properties of the composite layer is explored with the pre-nitriding microstructure as the initial state.The structure of the carbon nitride layer depends on the experimental conditions.When the carburizing temperature is 410-490 ^oC,some Fe₅C₂ nanoparticles are doped in the diamond-like carbon nitride layer with more sp³ bonds.The content of Fe₃C increases with the increase of carburizing temperature,time,and carbon content in the atmosphere,leading to the decrease of sp³ bonds in the carbon-nitrogen layer.It should be noted that the structure of the carbon nitride layer formed at high temperature（T≥530 ^oC）is graphitic-like while the structure of the carbon nitride layer formed for a long time（t≥12h）exhibits ful erene-like characteristics.The performance of the composite layer mainly depends on the structure of the carbonitride film.After the plasma is carburized at 450and 490 ^oC with C80 for 4 h,the samples own the excel ent comprehensive performance due to the carbonitride film with N-DLC structure such as the surface hardness of 12.65-19.88 GPa,the friction coefficient of 0.2～0.25,and the wear rate of 0.52-0.6×10^-5 mm³N^-1m^-1.The formation process of gradient structure and the mechanism of the wear-resistant and anti-friction of the functional layer are revealed.