Molecular dynamics simulation of vibrational friction force due to molecular deformation in confined lubricant film

The lubrication by thin film has become a very important role in micro machine, magnetic storage device and so on. As the thickness of lubricant film becomes thinner to several nanometers, the conventional law of lubrication becomes unable to use. Nonequilibrium molecular dynamics simulation (NEMD) was carried out to investigate the dynamic behavior of thin lubricant film confined between walls. The model used in these simulations is composed of two solid walls and fluorocarbon polymer lubricant. One of the walls is supporting a load and at the same time moving at constant velocity. Results indicate that the frictional behavior of confined lubricant varied with load; velocity field in the film retain liquid like structure under low load conditions, on the other hand, under high load conditions lubricant film becomes solidified and periodical stick and slip motion is observed at the layer near the wall. At the same time periodically vibrating friction force is observed. In this case, radius of gyration of lubricant molecules also changes periodically. It is concluded that the periodical vibration of friction force is caused by stick-slip with molecular deformation.