Molecular Dynamics Simulation Of Nano-Scale Friction Behavior

Increasingly miniaturization of modern machinery and electronics systems, the sizehas reached nanometer scale. With the significant increasing of surface-to-volume ratioof smaller devices, the surface forces such as adhesion and friction become a key factorto determine its performance and life, so study of nano-scale friction behavior becomemore important. On nanometer scale, the space between two interfaces is very small andsurface wear need to reduce to a minimum, especially when the two surfaces may becontact, it is difficult for continuous contact theory to analyze the micro-scale frictionbehavior, so a lot of methods appear to solve this problem. Molecular dynamicssimulation method has been shown have great advantages in analysis of microscopicphenomena, and already has a wide range of applications.In this paper, large-scale molecular dynamics methods are performed to studyfriction behavior at nano-scale, the main contents show as follows:â‘ Dry friction between rigid spherical tips with different atomic-scale roughtopography and the elastic plane substrate are simulated. For nonadhesion and adhesion,the relationship between the load and friction, load and real contact area, as well asfriction and real contact area are studied, the decisive factor to friction on nanometerscale are studied and analyzed. The results show that in the absence of adhesion, frictiondetermined by the load (load-controlled friction), the friction-load relationship is linearfor all tips, while there is no definite relationship between friction and real contact area.In addition, the relation between real contact area and load for the smooth tips and theamorphous tip is consistent with the Hertz model and the G-W model respectively.However, for adhesive case, the friction behavior has changed, the real contact areadetermines the friction, there is a linear relationship between friction and real contactarea, the friction load relation is no longer linear. The friction-load relation and theload-real contact area relation of all tips agree with M-D model. The decisive factor onfriction has changed from non-adhesion to adhesion.â‘¡Friction between rigid spherical tips(as same as dry friction) and a flat elasticsubstrate covered with a fluid monolayer of adsorbed chain molecules are simulated.We compare the contact stress distribution with that in dry contact, the effect of theadsorbed film on contact stress distribution are studied. We also compare friction-loadrelationship for different sliding speeds, effect of sliding velocity on friction force are discussed. We discuss the friction-load relationship of different systems fornon-adhesion and adhesion respectively, the decisive factor on friction with adsorbedfilm are studied. The simulation results show that the adsorbed film can weaken thesensitivity of rough morphology on contact stress distribution, the contact stressdistribution of different tips are in good agreement with the Hertz model. The adsorbedfilm diminished the sensitivity of sliding speed to friction, the difference becomesmaller between different sliding velocity with adsorbed film. The adsorbed film alsoweak the impact of adhesion on friction, the friction-load relationship are both linear fornonadhesion and adhesion.