Investigation On Adhesion And Friction Behaviors Of γ-Fe/DLC Heterogeneous Interface

Due to its good mechanical properties and corrosion resistance,austenitic stainless steel is one of the important structural materials for nuclear power,marine and petrochemical engineering equipment,accounting for about 70%of the total amount of stainless steel.Diamond like carbon（DLC）is widely used in the field of machining as a coating because of its excellent wear resistance and self lubrication.However,when machining austenitic stainless-steel parts with DLC coated tools,severe friction and wear are easy to occur,resulting in early failure of the coating.Therefore,investigation on adhension and friction behaviors ofγ-Fe/DLC（simplifed as Fe/C）heterogeneous interface is of great significance to solve the wear problem between DLC coating and steel materials.In this paper,the adhesion and friction behaviors of Fe/C heterogeneous interface were studied by first-principles and molecular dynamics methods.At the same time,the effects of passivated atoms（H,F,P,S）,typical non-metallic doped atoms（B,N,Si）and extreme friction conditions（high frequency and high load）on adhesion and friction behaviors of Fe/C heterogeneous interface were analyzed.In addition,DLC films were prepared by magnetron sputtering technoleogy,and their basic mechanical and tribological properties were also studied.The results show that when the interaction energy between Fe/C heterogeneous interfaces is less than removal energy of the top layer ofγ-Fe,the Fe atoms can be transferred to the C layer.The friction barriers of Fe/C（100）,（110）and（111）heterogeneous sliding interfaces along minimum energy path are 1.45,0.48 and 0.42 J/m²,respectively,and the corresponding ideal shear strengths(τ_MEP)are 17.97,8.36 and 6.06GPa,respectively,which indicates that Fe/C（111）heterogeneous interfaces are most prone to sliding.The fluctuation of potential energy curve also increases with the increase of normal load（1-5 n N）.In addition,the surface morphology of the prepared DLC film is composed of bumps of different sizes and has good hydrophobicity.The friction coefficients of the DLC films under nomal load（1,3,5 N）are stable at about 0.1,0.10 and0.20,respectively.Passivated atoms（H,F,P,S）can inhibit the material transfer between Fe/C heterogeneous interfaces and weaken the friction barrier on the potential energy surface.The energy barriers of Fe/C-H,Fe/C-F,Fe/C-P and Fe/C-S along the minimum energy path（MEP）are 0.06,0.03,0.15 and 0.12 J/m²,respectively,and the corresponding energy barriers are 0.5,0.3,1.5 and 1.2 GPa,respectively.Compared with the corresponding energy barriers（0.42 J/m²）and（6.06 GPa）of clean Fe/C heterogeneous interface,passivated atoms can reduce the sliding resistance of Fe/C heterogeneous interface,and the weakening effect of H and F atoms on the energy barrier of Fe/C heterogeneous interface is stronger than that of P and S atoms.With the increase of normal load（1-5 n N）,the potential barrier of the potential energy curve of Fe/C heterogeneous interface with passivated atoms gradually increases.Among them,the Fe/C heterogeneous interface passivated by H and F atoms is easier to slide and the friction coefficient is smaller.Non-metallic atoms（B,N,Si）only weakly inhibit the material transfer of Fe/C heterogeneous interface.The lowest friction potential barriers of potential energy surfaces for Fe/C interface with B,N and Si are 1.15,2.84 and 1.75 J/m²,respectively,which are lower than potential barriers of undoped Fe/C interface（3.07 J/m²）,indicating that non-metallic atom can improve the friction potential barrier of Fe/C heterogeneous interface.In addition,when sliding along MEP,the friction barrier of Fe/C heterogeneous interface doped with B,N and Si atoms can be lower than 1 J/m² and lower than the corresponding value of undoped Fe/C interface（1.45 J/m²）.The increase of load（1-5 nn）makes the potential energy curve of Fe/C heterogeneous interface with B,N and Si more and more sharp,while the corresponding average friction coefficient fluctuates only slightly.Amorphous carbon has weak anisotropy due to the non-uniformity of structural density and sp³/sp² distribution.At the same time,the shear strain near the indentation expands unevenly in all directions during the indentation process.Under extreme friction conditions（high frequency and high load）,the increase of load can improve average friction coefficient of Fe/C heterogeneous interface,while the sliding velocity has little effect on the average friction coefficient.By analyzing the content of sp² hybrid bond near the friction interface,it is found that the increase of load can increase the proportion of sp²at the friction interface,but the sliding speed has no effect on it.The average friction coefficients of Fe/C（110）,（110）and（111）interfaces under different friction conditions are 0.18～0.44,0.18～0.39 and 0.20～0.11,respectively.Therefore,adjusting the load and the preferred orientation of Fe structure can improve the friction properties of Fe/C heterogeneous interface.