Research And Application Of Nanoindentation By Molecular Dynamics

With the rapid development of nanotechnology,the size of the research object is getting smaller and smaller,and the micro/nano mechanical properties of the material have received extensive attention.Due to the high precision and small damage to materials,nanoindentation has been widely used in the characterization of micro/nano-mechanical properties in micro-domains,MEMS,thin-film materials and so on.However,due to the limitation of instrument conditions,such as indenter tip passivation,material surface fluctuations,and environmental interference,the nanoindentation experimental study has resulted in a significant drop in test accuracy as the indentation depth decreases.On the other hand,this experimental method is difficult to study the evolution of internal defects in materials.Molecular dynamics(MD)simulation can show the dynamic behavior of atoms in real time.It is a powerful tool for studying the intrinsic mechanism of indentation response and has been rapidly developed in recent years.However,due to the variety and complexity of material systems,it is still in its infancy.Therefore,in this paper,single crystal aluminum,single crystal silicon,and amorphous carbon films are used as representative materials to study the dislocation reactions,phase transformation and deformation of amorphous phases during nanoindentation by MD.The nanoindentation of single crystal aluminum(100)was simulated by MD,and the process of dislocation initiation,movement and reaction was analyzed.The new detached mechanism of the prismatic dislocation loop has been studied,that is,reactions happen between Shockley partial dislocations,and then a stair-rod forms;the resultant stair-rod reacts with a Shockley partial dislocation,and then a new Shockley partial dislocation forms;annihilation occurs between Shockley partial dislocations and finally a prismatic dislocation loop is released.In addition,the depth of indentation and the holding time have a significant effect on the mechanical properties of monocrystalline aluminum and the movement of the prismatic dislocation loops.The MD simulation of nanoindentation for single crystal silicon shows that there is pop-out in the force-displacement curve which is caused by the phase transition of the Si-II phase identified by atomic coordination analysis.With the increase of penetration depth and the decrease of the indenter radius,it is found that extrusion of materials,resulting in the disappearance of pop-out.With the increase of loading/unloading velocity,the load corresponding to the occurrence of pop-out shows a decreasing trend,which is consistent with the experimentally reported results.Through the virtual heat treatment,pressure increase and quenching of diamond film/silicon substrate,a molecular dynamics model of amorphous carbon film(often called Diamond-like Carbon,DLC)was established.The nanoindentation of amorphous carbon film was simulated and analyzed.The results show that DLC has a significant protective effect on silicon substrate.The mechanical response of the film changes significantly with the size of the indenter,the depth of penetration and the indentation velocity.