Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

Abstract

Nonequilibrium molecular dynamics (NEMD) simulations have provided unique insights into the nanoscale behaviour of lubricants under shear. This review discusses the early history of NEMD and its progression from a tool to corroborate theories of the liquid state, to an instrument that can directly evaluate important fluid properties, towards a potential design tool in tribology. The key methodological advances which have allowed this evolution are also highlighted. This is followed by a summary of bulk and confined NEMD simulations of liquid lubricants and lubricant additives, as they have progressed from simple atomic fluids to ever more complex, realistic molecules. The future outlook of NEMD in tribology, including the inclusion of chemical reactivity for additives, and coupling to continuum methods for large systems, is also briefly discussed.

Shear heating, flow, and friction of confined molecular fluids at high pressure

“…Although lower values ofγ are desirable to overlap with those used in tribology experiments and real components, 58 they are not computationally feasible for extensive parameter studies. 23,32 The T was controlled using a Langevin thermostat 59 set to 350 K, which is representative of the EHL regime. 5 The thermostat acted only in the direction perpendicular to both sliding and compression (y).…”

“…This is difficult to obtain from experiments alone, and confined nonequilibrium molecular dynamics (NEMD) simulations 22 have provided insights into this behavior under EHL conditions. 23 For example, comprehensive confined NEMD simulations have studied the shear heating, flow, and friction behavior of atomic Lennard-Jones (LJ) fluids at high P andγ. [24][25][26] As observed experimentally, [18][19][20] transitions between Couette flow and different forms of shear localization were reported as P was increased.…”

“…28 Bulk NEMD simulations of lubricant molecules have shed light on their nonlinear rheological behavior under EHL conditions; [29][30][31] however, shear localization cannot be captured in such simulations since a linear velocity profile is imposed on the system. 23,28 Recently, the shear localization and friction of realistic lubricant and traction fluid molecules have also been investigated with confined NEMD simulations under EHL conditions. [32][33][34] These simulations have demonstrated links between the flow and friction behavior, which can both deviate significantly from that commonly assumed in macroscale models for EHL.…”

“…32 The use of a UA force-field was thus considered to be an acceptable trade-off between accuracy and computational expense for the wide parameter study conducted here. 23,31 In the α-Fe(100) slabs, interactions were described by the embedded-atom method (EAM) with parameters taken from ref. 49 For the interactions between the Fe atoms and the fluids, initial LJ parameters were modified from ref.…”

Scale-Dependent Friction-Coverage Relations and Non-Local Dissipation in Surfactant Monolayers

“…16 Molecular dynamics (MD) simulations can also provide atomic-level insights into the structure and friction behaviour of OFM films in tribological contacts. 35 For example, NEMD simulations have been used to study a range of OFMs (carboxylic acids, amides, esters) with C 18 tailgroups adsorbed on atomically-smooth α-Fe 2 O 3 surfaces 36,37 and α-Fe with nanoscale roughness features. 38,39 In these studies, OFM monolayers with higher surface coverage (Γ = 4.3 nm −2 ) generally showed lower friction (Γ = 1.4 nm −2 ).…”