Formation Mechanism And Properties Of In-situ Formed DLC Film On M50NiL Steel Surface By Plasma Carburizing

M50NiL martensitic steel,as a new generation steel for aerospace bearings,has a composite microstructure with case-hardened surface and a relatively tough core after surface treatment.Traditional high-temperature gas carburizing for steel is giving way to low-temperature plasma carburizing due to its limitations related to performance,reliability and economy for future aircraft engines.However,the information on the properties of plasma carburized M50NiL steel is unavailable.Diamond-like carbon coatings have continuously attracted attention due to their superior tribological properties with low friction coefficient,high hardness and superior performance in resisting wear.Combination of DLC film and chemical heat treatment is an approach to improve the tribological properties of iron-based alloys.However,the multi-step technology is complex and time-consuming.Moreover,the quality of product is hard to control.Therefore,in this study,low-temperature plasma carburizing for M50NiL steel was conducted in order to improve the wear resistance and decrease the coefficient of friction for aerospace engine.DLC film can form simultaneously during plasma carburization,resulting in a combination of DLC film and carburized layer through a single-step process.The mechanical and tribological properties were characterized.First-principles calculations were conducted to clarify the formation mechanism of in-situ DLC film by plasma carburizing.The influences of gas composition and treatment temperature on properties of plasma carburized layer were investigated.The phases on the surface of carburized layer are carbon expanded martensite?cementite and a few of Fe₃O₄ phase.As the creasing of carbon-containing gas,the intensity of Fe₃C phase is increased while the intensity of??_c phase is decreased.Correspondingly,the layer thickness increases and then decreases.The influence of gas composition on matrix hardness is negligible.The main phase on the surface and subsurface is??_c phase for the specimens carburized at 500?and 550?.The carbon content of the surface layer is lower than that of the subsurface layer.The??_c phase on the surface layer is transformed to Fe₃C phase as the temperature decreases to 400?and 450?while the??_c phase on the subsurface layer remains unchanged.The??_c phase is transformed to Fe₃C phase due to carbon supersaturation as time prolonging when carburized at 450?.The carbon content on the surface decreases as time increasing due to sputtering effect of plasma when carburized at 500?.The matrix becomes softer due to tempering effect after carburizing,especially at high temperature.The microstructure evolution in the carburized layer demonstrates that plasma carburizing is a diffusion-controlled process.Activation energies of carbon diffusion into martensite and cementite were calculated,which are 64.5 kJ.mol^-1?120.7 kJ.mol^-1,respectively.Results demonstrate that DLC film tends to form on the surface of specimen after plasma carburizing in high fraction of carbon-containing gas at low temperature for long duration.The smooth DLC film forms on surface of 0.3C specimen with 53%sp³ content and hardness of 13.942 GPa.The wear and corrosion resistance is better than other specimens.The best DLC film was obtained on the specimen plasma carburized in 0.15C at 400?for 12 h.The film is 0.8?m thickness with 54.3%sp³ content and hardness of 13.227 GPa.The friction coefficient is stable and low?about 0.25?and correspondingly the wear rate is low(1.97×10^-6mm³N^-1m^-1).The wear mechanism for the specimen transforms from severe adhesive wear to micro cutting and oxidation wear after plasma carburizing.In-situ growth of DLC film only occurs when Fe₃C phase becomes main phase on the plasma carburized layer.The inductive effect of Fe₃C phase as substrate on DLC film was verified by the first-principles calculation.Moreover,the properties of the alloyed Fe₃C phase were predicted through first-principles analysis.The cohesive energy of Fe₃C is lowered by doping Cr,Mo,Ni or V,attributed to the stronger binding between carbon and Cr,Mo,Ni,V,compared to that of Fe.The formation enthalpies of alloyed cementite with Cr,Mn,Mo or V are all lower than that of pure cementite,suggesting that the doping of Cr,Mn,Mo or V into cementite can increase the phase stability of cementite.The alloyed Fe₃C?100?surface is the least stable,followed by?010?,and the alloyed Fe₃C?001?surface is the most stable.Ni stabilizes the cementite surface while doping Mn,Mo,Cr or V decreases the stability of the cementite surfaces.The cementite?100?plane is the most preferred plane for carbon atom adsorption,followed by?001?and?010?plane.Doping Cr,Mn,Mo and V endow cementite with higher absolute adsorption energies.The formation energies of carbon with sp³ bond decrease in the order from cementite?100?,through?001?to?010?,indicating that?010?surface is the best candidate for DLC film growth.Moreover,compared to Fe₃C,doping Mn,Mo,Cr or V does not favor DLC film growth,but doping Ni does.