Non-equilibrium phase behavior and friction of confined molecular films under shear: A non-equilibrium molecular dynamics study

Abstract

The phase behavior of a confined liquid at high pressure and shear rate, such as is found in elastohydrodynamic lubrication, can influence the traction characteristics in machine operation.Generic aspects of this behavior are investigated here using Non-equilibrium Molecular Dynamics (NEMD) simulations of confined Lennard-Jones (LJ) films under load with a recently proposed wall-driven shearing method without wall atom tethering [Gattinoni, Maćkowiak, Heyes, Brańka and Dini, Phys. Rev. E 90, 043302 (2014)]. The focus is on thick films in which the nonequilibrium phases formed in the confined region impact on the traction properties. The nonequilibrium phase and tribological diagrams are mapped out in detail as a function of load, wall sliding speed and atomic scale surface roughness, which is shown can have a significant effect. The transition between these phases is typically not sharp as the external conditions are varied. The magnitude of the friction coefficient depends strongly on the nonequilibrium phase adopted by the confined region of molecules, and in general does not follow the classical friction relations between macroscopic bodies, e.g., the frictional force can decrease with increasing load in the Plug-Slip (PS) region of the phase diagram owing to structural changes induced in the confined film. The friction coefficient can be extremely low (∼ 0.01) in the PS region as a result of incommensurate alignment between a (100) face-centered cubic wall plane and reconstructed (111) layers of the confined region near the wall. It is possible to exploit hysteresis to retain low friction PS states well into the Central Localization high wall speed region of the phase diagram.Stick-Slip behavior due to periodic in-plane melting of layers in the confined region and subsequent annealing is observed at low wall speeds and moderate external loads. At intermediate wall speeds and pressure values (at least) the friction coefficient decreases with increasing well depth of the LJ potential between the wall atoms, but increases when the attractive part of the potential between wall atoms and confined molecules is made larger.2

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…In these simulations, only the first one or two layers of fluid were "locked" to the surface (due to strong wall interaction) and the rest of the fluid was sheared. This is distinct from the shear localisation behaviour observed in the high pressure NEMD simulations described above [186][187][188] where the "unsheared" region extends well beyond the direct influence of the wall atoms. Subsequent confined NEMD simulations of L−J fluids between one crystalline and one amorphous wall by Butler and Harrowell [190][191][192], L−J glasses (mixture of different sized L−J atoms to frustrate crystallization) by Varnik et al [193], as well as L−J glasses and amorphous polymers by Rottler and Robbins [194] have shown various forms of shear localisation.…”

“…4). As the shear rate is increased in the CL region, the central liquid region widens until eventually a Couette profile is recovered [186][187][188]. Under the lowest shear rates studied, stick-slip friction behaviour was observed due to the commensurability of the walls and the solid-like fluid [188].…”

“…In the last few years, NEMD simulations of confined L−J liquids sheared at GPa-level pressures have revealed a range of nonequilibrium phase behaviour [186][187][188]. These simulations were designed to mimic the conditions found in the EHL regime, which is particularly important for machine components that roll and slide together, for example; rolling bearings, gears, constant velocity joints and cam/follower systems [41].…”

“…Above a certain pressure, the NEMD simulation [186][187][188] flow profiles transitioned from Couette-like to those indicative of shear localisation (or banding). Nonequilibrium phase diagrams were constructed for L−J fluids to map the changes in behaviour with pressure and shear rate (or sliding velocity).…”

“…As the shear rate is increased in the CL region, the central liquid region widens until eventually a Couette profile is recovered [186][187][188]. Under the lowest shear rates studied, stick-slip friction behaviour was observed due to the commensurability of the walls and the solid-like fluid [188]. This mirrors the stick-slip behaviour [102] observed for more strongly confined films at ambient pressure (Section 2.2.2), supporting the postulate due to Robbins and Smith [162] that confinement-induced phase transitions observed in SFA experiments and highly confined NEMD simulations may be closely related to the pressure-induced transitions observed in macroscopic tribology experiments.…”