| As an amorphous coating with excellent mechanical and frictional properties,diamond-like carbon(DLC)films have been widely used as protective coatings for various workpieces to improve surface wear resistance and reduce friction.However,the friction mechanism of DLC films is more complicated and their friction properties are easily affected by hydrogen elements.Moreover,it is difficult to observe the microstructural conditions between the contact interfaces on a macroscopic scale.This leads to the fact that its tribological behavior and mechanism are not fully understood.Molecular dynamics(MD)simulation methods,which can study interatomic interactions from the microscopic scale,have become an effective method to study material friction problems from the atomic scale today.To this end,in this thesis,the following aspects of research are carried out for DLC films:Firstly,this thesis uses three potential functions commonly used for the empirical potential of the solid carbon phase,i.e.,Tersoff,Rebo-II,and Airebo potential,to model amorphous carbon with densities between 2.6 and 3.2 g/cm~3.After that,the effects of different potential functions on the bonding state of DLC films were investigated.The hybridization,radial distribution function G(r)and bond length bond angle distribution of the carbon coating were also analyzed.The results show that although the carbon films prepared by Tersoff potential have the highest sp~3 hybridized bonds and the lowest Airebo potential,the difference between the three is not significant.Moreover,the hybridization structures in the DLC films prepared by the Rebo-II and the Airebo potential are closer to the actual bond lengths compared to the Tersoff potential.However,like the Tersoff potential,the Rebo-II potential only considers covalent bonding interactions between atoms and does not consider non-bonding interactions.In contrast,the Airebo potential contains both covalent bonding interactions and non-bonded long-range intermolecular interactions,which is beneficial for studying frictional interface systems.Therefore,the Airebo potential is more suitable for use in MD simulations of DLC films.Secondly,after determining the empirical potential function used as the Airebo potential,a 3D friction simulation system consisting of two hydrogen-free DLC films was established using DLC films with a density of 3.0 g/cm~3.The effects of phase transition and the number of C-C bonds between interfaces on the friction force during sliding were analyzed,and the intrinsic mechanisms behind the different frictional behaviors were discussed.The results show that the periodic shear deformation and interfacial fracture occurring in the system during the low-speed sliding process lead to the sawtooth fluctuation variation of the friction force during the sliding process.This is the reason why the variation of the number of C-C bonds between the interfaces does not directly affect the friction force.In contrast,at high speed,the friction force is mainly controlled by the number of interfacial C-C bonds and is influenced by the graphitization phase change.In addition,the high temperature hardly affects the average friction force of DLC films during the friction process.However,at low speed,the increase of temperature will lead to the decrease of frictional force fluctuation.And at high speed,the friction force remains consistent at different temperatures due to the constant number of interfacial bonding and the degree of graphitization phase transition.Finally,in this thesis,H atoms were used to passivate the surface of DLC films,and then the friction model of H-passivated DLC films was established using the DLC films after surface H passivation as the study object.The effects of different degrees of surface H passivation on the frictional behavior of self-matched sub-DLC films were investigated.The results showed that the number of C-C bonds between the interfaces of DLC films was significantly reduced after surface H passivation,and the friction force value was significantly decreased.In addition,sliding speed,normal load and temperature have significant effects on the friction reduction effect of surface H passivation.During the sliding process under different conditions,when the passivation of H atoms between the contact surfaces was good,only a small number of C-C bonds existed between the contact surfaces,at which time there was a significant frictional interface during the sliding process and the sliding occurred between the H atoms,which resulted in the DLC films exhibiting low frictional behavior.However,when the passivation effect of the surface H-atoms was destroyed,strong interactions occurred between the contact interfaces and the number of C-C bonds formed increased significantly,which led to a significant increase in the frictional force and thus reduced the frictional properties of the DLC films. |
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