Molecular Simulation Study On The Adhesive Friction And Influence Mechanism Of Silicon Incorporation Of Amorphous Carbon Films

Amorphous carbon films can be used as solid lubrication and coating materials,which are popupar in many engineering applications.In addition,silicon incorporation thechnique can improve the stability of amorphous carbon films in the working environment with high temperature,and silicon incorporation has also effects on the friction behaviors of films.The friction mechanism of amorphous carbon films is complex which is influenced by the composites and working environments of films,and it is also confused about the friction origin of silicon-incorporated amorphous carbon films.Previous experiments found that the formation of the third-body materials(transfer layer,wear debris and tribolayer etc.)is the key to understand the friction origin of amorphous carbon films.Nevertheless,it is difficult to have the high time and spatial resolved insights to understand the friction behviors of interfacial materials at small scale from usual experimental methods.Using molecular dynamics simulation method,the formation of third-body materials has been found in previous studies on the friction of amorphous carbon films,which is related with the adhesive friction behaviors of friction systems.However,it is still need much works to do in the study of adhesive friction and third-body materials of amorphous carbon films,and it is still unclear about the friction mechanism of silicon incorporated amorphous carbon films.Under these backgrounds,this thesis aims at understanding the adhesive friction origin of amorphous carbon films and effects from silicon incorporation,using molecular dynamics simulation method.The major studies and results of this thesis are summarized as follows:(1)The deposition simulations for the growth of amorphous carbon films have been performed,and the structures and properties of several films deposited with different deposition energies and silicon incorporation were analyzed,in which a modified radial dsitribution function method were proposed for the analysis of the structure in local region.The study found that some properties of films are dependent with the deposition energy and silicon incorporation;In specifically,the density,fraction of sp~3 hybridization and internal stress of films can be improved with increasing deposition energies,while the increasing silicon incorporation seemed have opposite influences on these properties,in the range of the energies and silicon incorporation considered in this study.(2)The simulations for the nanoscale adhesive friction of amorphous carbon films and effects from slicon incorporation have been performed,and several analysis approaches were proposed to quantify the dynamical friction behaviors.Firstly,the friction responses analysed from the evolutional behaviors of interfacial strucures and variations of friction forces were studied;In the initial running-in period,the formation of interfacial junctions and transfer layers were observed because of adhesive friction,and performed relatively stable in following period;In the steady-state friction period,the different patterns of friction responses have been found,from the analysis results of transfer layers and friction forces for different frictions systems under different loading conditons.Secondly,the friction origin of friction systems in steady-state friction period was studied;The results indicated that the general adhesive friction behavior of each friction system is originated from the dynamics of interfacial material in junction region,and the material accommodating most shear strain in junction region was named as tribolayer;Besides,the spread of transfer layer is the deformation results of tribolayer,and the variation characteristics of friction force is attributed to the deformation and stability of tribolayer;Moreover,the study found that the dynamics behaviors of tribolayers were influenced by the loading conditions and silicon incoroporation,which is responsible for the friction responses for friction systems of amorphous carbon films.(3)The adhesive friction mechanism of amorphous carbon films in steady-stage period was studied.Firstly,a mechanism named annealing-freezing was proposed for the stick-slip motion of tribolayer,from the analysis of the mean-square displacements of atoms in tribolayer region,combined with analysis methods of dynamical temperature distribution and radial distribution functions.Then,the deformation origin of tribolayer was studied,from the structural analysis of short range orders and clusters;The results indicated that the stick-slip deformation of tribolayer is related with the transformation of the atomic structures with short range order,and the deformation stability of tribolayer is related with the evolutions of the atomic clusters.Finally,the dynamics behaviors of tribolayers were explained,and the friction mechanisms for friction systems of amorphous carbon films and effects from silicon incorporation were revealed,from the comprehensive analysis of friction forces,strucutres and deformation responses.