Ordering of a Thin Lubricant Film due to Sliding

A thin lubricant film confined between two substrates in moving contact is studied using Langevin molecular dynamics with the coordinate- and velocity-dependent damping coefficient. It is shown that an optimal choice of the interaction within the lubricant can lead to minimal kinetic friction as well as to low critical velocity of the stick-slip to smooth-sliding transition. The strength of this interaction should be high enough (relative to the strength of the interaction of lubricant atoms with the substrates) so that the lubricant remains in a solid state during sliding. At the same time, the strength of the interaction should not be too high, in order to allow annealing of defects in the lubricant at slips.

Figure 1

The model: the light spheres show the lubricant atoms, and the dark spheres show the substrate atoms. The atoms in the outermost layers of the substrates correspond to the rigid parts of the substrates while the dynamics of the other substrate atoms is fully simulated. The load and shear are applied to the rigid part of the top substrate. The rigid part of the bottom substrate is fixed. (a),(b) The configuration before and after reordering, correspondingly (at the beginning and at the end of the dependence shown in Fig. 3) for the system with $V_{ll}=0.5$ driven with the velocity $v_s=0.1$. Each panel has side and bottom views; in the latter there is only one layer of substrate shown, and the atomic radii are adjusted in order to visualize clearly the (in)commensurability between the lubricant and the substrate. (The figures were produced with Visual Molecular Dynamics software [9].)

Figure 2

Typical time dependencies of the spring force for regimes of stick-slip motion [(a) and (c)] and smooth sliding [(b) and (d)] for the soft lubricant [$V_{ll}=0.2$ (a),(b)] and the hard lubricant [$V_{ll}=1$ (c),(d)]. The driving velocities are the following: (a) $v_s=0.3$ and (b) $v_s=1$ for the soft lubricant, and (c) $v_s=0.03$ and (d) $v_s=0.1$ for the hard lubricant.

Figure 3

Reordering of the lubricant: spring force, velocity of the top substrate, lubricant width, and effective lubricant temperature as functions of time at $v_s=0.1$ for the $V_{ll}=0.5$ system. Configurations before reordering (at stick in the stick-slip regime) and after it (at smooth sliding) are shown in Fig. 1 top and bottom, correspondingly.

Figure 4

Static $f_s$ and kinetic $f_k$ frictional forces for three values of the driving velocity ($v_s=0.1$, 0.3, and 1 as shown in legend) as functions of the interaction amplitude $V_{ll}$ in the semilogarithmic scale. The “error bars” show deviation of the simulation results in different runs.