A molecular simulation study to the deformation Behaviors and the size effect of polyethylene during nanoindentation

Abstract

We present the results of the large-scale molecular mechanics (MM) simulation during the nanoindentation of polyethylene (PE). The PCFF force field was taken to describe the interaction in the indenter-substrate system. A monocrystalline silicon indenter in truncated conical shape was driven to penetrate into PE substrate on which a molecular dynamics equilibrium has been performed. To accurately compute the projected contact area used in calculation of hardness, we measured the contact depth via judging the initial contact point. A reasonable load-displacement curve based on the contact depth was depicted. As for the serrated shape of load-displacement curve, we supposed that the alternant occurrence of elastic and plastic deformation in the substrate during the indentation process is the main reason. We verified this supposition by discussing the loading and several unloading curves. Based on the obtained load-displacement curve, we established a model to compute the hardness. The model presents obvious size effect that the hardness initially increases with the increase of contact depth till it reaches a hardness peak and then gradually decreases with further increase in contact depth. We explained the increase of hardness at the initial loading stage. The results reveal that size effect is the intrinsic characteristic of PE during nanoindentation since the model was s deviation which could also cause size effect.