Research On The Microstructure And Formation Mechanism Of Plasma Nitrided-Carburized Layer On AerMet100 Steel

Ultra-high strength secondary hardening steel AerMet100 exhibits a combination of strength and toughness,and is mainly used for aircraft landing gears and gearbox gears.Nevertheless,the ever-increasing demands for aircraft operating life in harsh service environment also poses a new challenge to the performance of landing gears and gearbox gears.Especially when used as a gearbox gear,its surface hardness and wear resistance cannot adapt to long-term wear conditions.Therefore,in the present work,plasma nitriding and nitriding-carburizing dual process surface modification methods are used to prepare a gradient modified layer with a certain thickness on the surface of AerMet100 steel to improve tribological properties and further increase its service life.In addition,based on the experimental results,first-principles calculations are used to calculate the alloy elements-doped grain/phase boundaries of thermochemical treatment modified layer,and reveal the effects of alloy elements on the interface properties from the perspective of atomic and electronic structure.Therefore,the screening of favorable alloying elements can be realized,which provides theoretical support for future design and optimization of interfacial doping technology.In this thesis,the effects of plasma nitriding and nitriding-carburizing treatment on the microstructure of surface layer of AerMet100 steel with different process parameters were investigated,and the mechanical and tribological properties of the modified layer were characterized.In the plasma nitriding treatment of AerMet100steel,as the temperature increases,the thickness of nitrided layers increased significantly,and no white compound layer was formed on the surface,only diffusion layer can be observed.The surface phases are mainly composed of nitrogen-containing martensite andγ’-Fe₄N phases,wherein the 430℃and 460℃nitrided specimens are accompanied by a small amount ofε-Fe_2-3N phase.With the increase of the ratio of nitrogen and hydrogen,the thickness of nitrided layer increases slightly,and the relative content of theε-Fe_2-3N andγ’-Fe₄N phase increases.The prolongation of nitriding duration does not significantly change the surface phase constitutes.Based on the experimental data,the diffusion activation energy of nitrogen in martensite is calculated to be 68.11 kJ/mol.A huge accumulation of carbon beyond the nitrided layer,implying a remarkable carbon“push-ahead”effect by nitrogen.Whether theε-Fe_2-3N phase exists on the surface of nitrided layer has a certain influence on the redistribution of carbon in nitrided layer.The hardness of nitrided layer shows a gradient distribution and the surface hardness up to 1100～1200 HV.The effects of nitriding temperature on the hardness profile of nitrided layer is more significant than that of nitriding duration and atmosphere.The substrate hardness decreases with increasing nitriding temperature,430°C and 460°C nitrided specimens show obvious secondary hardening.The nitriding duration has little effect on the substrate hardness,and the nitriding atmosphere basically does not substantially affect the substrate hardness.For nitriding at different temperatures,the surface hardness of 430°C nitrided specimen is the highest,up to 12.68 GPa,and the 460°C and 490°C nitrided specimens have relatively better plastic deformation resistance and wear resistance,which realizes the simultaneous strengthening of the surface layer and the substrate.With the increase of nitrogen-hydrogen ratio,the relative content of brittleε-Fe_2-3N phase increases and the toughness and plasticity of the surface layer decreases.When the ratio of nitrogen to hydrogen is 2:2,it showed the best wear resistance.With the prolongation of nitriding duration,the surface hardness of nitrided specimen decreased,and the wear resistance was the best when nitriding at 490°C for 8 h.After nitriding,the main wear mechanism of the specimens changed from severe adhesive wear to more wear-resistant oxidative wear and slight abrasive wear.In the nitriding-carburizing treatment of AerMet100 steel:DLC coatings can be formed on the surface of carburized specimens with or without pre-nitriding(C₄₅₀and N₄₅₀+C₄₅₀),and the DLC coatings formed on N₄₅₀+C₄₅₀ specimen exhibit a smoother and finer morphology,higher hardness than that of C₄₅₀ specimen.Its higher sp³ content mean the coatings are more diamond-like.The N₄₅₀+C₄₅₀ specimen exhibits a higher surface hardness（～13.06 GPa）,a stable friction coefficient of 0.1,indicating more excellent wear resistance.The DLC coatings on the surface of C₄₅₀specimen is mainly formed by the synergistic induction and catalysis of Fe₃C phase andα’_C-Fe phase,while the formation of the DLC coatings on the surface of N₄₅₀+C₄₅₀ specimen mainly depends on the induced catalysis ofα’_（C+N）-Fe phase.The effect of alloying elements doping on grain/phase interfacial properties were predicted using first-principles calculations,and the corresponding influence mechanisms were revealed.Results show that in all Fe₃C（001）/diamond（111）interface structures,the Fe-HCP structure exhibits the highest interfacial stability and the largest interfacial adhesion W_ad（1.07 J/m²）,that is,the highest interfacial bonding strength,and is the most preferred stacking sequence.The doping of Al,Mn,Nb,Mo,Cr,Ti and V strengthens the interfacial bonding,and the strengthening ability increases sequentially,while the doping of Zr,Cu,Ni,Si and Co weakens the interfacial bonding,and the weakening ability decreases in turn.These show that doping of cementite with appropriate alloy elements can effectively improve the interfacial adhesion between it and the DLC coatings.In all γ’-Fe₄N（111）/α’_N（011）interface structures,the Fe-Bridge structure has the largest W_ad（2.325 J/m²）and the highest interfacial bonding strength,which is the most preferred stacking sequence.The doping of Cr has a strengthening effect on the interface,while the doping of Si,Zr,Cu,Al,Ni,Mn,Co,Nb,Ti,V and Mo all weaken the interfacial bonding,and the weakening effect decreases in turn.Mo and Zr are the most effective alloy doping elements for strengthening and weakening FeΣ3（111）[110]grain boundaries,respectively.The first-principles tensile test results show that the ultimate tensile strength of Mo-doped grain boundaries is about 420MPa higher than that of pure grain boundaries,that is,the bonding strength of grain boundaries is significantly improved.