Investigation On Mechanical Properties And Tribological Behaviors Of γ-Fe/CrN/DLC Films

DLC films have been widely applied in the surface modification of metal workpieces due to its high hardness,low friction and good abrasion resistance.However,a large internal stress will be generated during the deposition process,which may cause the peeling of the DLC from the substrate,and adversely reduce the service life of the workpieces.Therefore,it is significant to introduce a transition layer between the film and the substrate or to dope an effective alloy element in the DLC film to enhance its peeling resistance and improve the tribological properties of the workpieces.In this paper,austenitic stainless steel（γ-Fe）was used as the substrate,and CrN was used as the transition layer,on which a Cr-DLC film was deposited.Firstly,the interfacial properties betweenγ-Fe/CrN/DLC were calculated by first-principles method,and the effectiveness of CrN in improving the bonding strength ofγ-Fe and DLC films was analyzed.Then,molecular dynamics（MD）method was used to simulate the deformation behavior of DLC bulk material andγ-Fe/CrN/DLC film during nanoindentation,and the effects of DLC density,key parameters（size,quenching rate and potential function）and doping Cr atomic fraction on the residual stress of DLC film were analyzed.In addition,the friction behavior of DLC films during sliding friction with different Cr atomic fractions and surface deformation were simulated.Finally,the structure of the DLC films is characterized,and the mechanical and tribological properties are tested by experiment to verify the validity of the simulated results.Theγ-Fe/CrN interface and CrN/DLC interfaces were established by first principles method.In which,a semi-coherent interface can be formed between theγ-Fe（100）plane and the CrN（111）one,and a coherent interface formed between the C（111）plane and the CrN（111）one.In theγ-Fe（100）/CrN（111）interface model,the W_ad of N-2 interface is the largest,which is 1.703 J/m².Both Cr and N atoms on the CrN side of the interface participate in charge transfer.The chemical bonds at the interface are dominated by polar covalent bonds,supplemented by metal bonds.In the C（111）/CrN（111）interface model,the W_ad of Cr-C interface is largest,which is 5.217 J/m².The W_ad is increased after Cr element is doped at the interface.The doping of Cr atoms can promote the electron transfer between the interface atoms,which enhance the interaction between the atoms at the interface.Nanoindentation simulation of DLC bulk material andγ-Fe/CrN/DLC film were carried out by MD method.Both of Young’s modulus and hardness are increased with increasing density.In the DLC bulk material,it is found that the presence of sp²-C makes the DLC film have a long-range collective mechanical response.And the more sp²-C content is,the more obvious the long-range mechanical response is.In theγ-Fe/CrN/DLC film,the two different interfaces ofγ-Fe/CrN and CrN/DLC can effectively enhance the ability of the film to resist elastic deformation.Three DLC models with densities of 2.0 g/cm³,2.8 g/cm³ and 3.5 g/cm³ were established by considering the size effect,quenching rate and potential function type.For different densities,the residual stress is changed from tensile stress to compressive stress with increase of the density.The residual compressive stress of 2.8 g/cm³DLC is smallest.In addition,the potential function has a much larger effect on the residual stress of DLC structure than the size effect and quenching rate.The DLC films with different Cr contents（0 at.%～7.81 at.%）were calculated by ab initio MD.The residual stress is increased firstly,then is decreased and increased with increase of the Cr content.The residual stress is the smallest when the Cr content is 4.69 at.%,which is 2.168 GPa.The formation of Cr-C bonds improves the bonding strength of the structure,which enhance the interaction between the atoms.The higher residual compressive stress is mainly due to the distortion of bond angle（<109.467°）and bond length（<1.418（?））in the carbon grid.The sliding friction behavior of pure DLC and Cr_4.69at.%-DLC was simulated by MD method.Compared with pure DLC,the friction coefficient of Cr_4.69at.%-DLC is only increased by 0.065.Deeper（0.36 nm）and wider（2.1 nm）wear scars can be observed in Cr_4.69at.%-DLC.Friction causes an increase in the compressive stress on the contact surface,but the smaller the increase in compressive stress is,the smaller the friction coefficient is.During the friction process,the sp-C and sp³-C of the two films are decreased,and the sp²-C are increased,which indicate the sp-C and sp³-C hybrid bonds are converted to sp²-C hybrid bonds during the friction process.The sliding friction behavior of Cr-DLC films with different texture sizes and texture densities was simulated by MD.Compared with the texture density,the texture size has a more significant effect on the tribological properties,and both of them can decrease the friction coefficient to a certain extent.The residual compressive stress on the film surface is decreased with different texture densities.During the friction process,in addition to the conversion of sp³ hybrid bonds to sp² hybrid bonds,part of the sp hybrid bonds will also be converted to sp² hybrid bonds.The structural characterization and performance test of six DLC films were carried out by experiment.It is observed by SEM that the surface of all DLC films is small and dense spherical particles.The size of spherical particles is grew with the doping of Cr element.C exists as an amorphous form in the film,and the sp³（C-C）/sp²（C=C）ratio is decreased by doping Cr element,which causes the smaller residual stress in the Cr/CrN/Cr-DLC films.The doping of Cr element changed the deformation mechanism of the film during nanoindentation.In addition,the friction coefficient between Cr/CrN/DLC film and Cr/CrN/Cr-DLC film is close.Therefore,Cr doping can improve the cracking resistance on the basis of guaranteeing excellent tribological properties.