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.

Understanding the nanoscale behavior of complex tribological systems is a significant challenge in lubrication development and is difficult to investigate experimentally. In this work we use non-equilibrium molecular dynamics (NEMD) simulations to examine the atomistic structure and friction properties of commercially relevant organic friction modifier (OFM) monolayers adsorbed on iron oxide surfaces and lubricated by a thin, separating layer of hexadecane. Specifically, acid, amide and glyceride OFMs with saturated and Z-unsaturated hydrocarbon tail groups, are simulated at various surface coverages and sliding velocities. At low and medium coverage, the OFMs form liquid-like and amorphous monolayers respectively which are significantly interdigitated with the hexadecane lubricant, resulting in relatively high friction coefficients. At high coverage, solid-like monolayers are formed for all of the OFMs which, during sliding, result in slip planes between well-defined OFM and hexadecane layers, yielding a marked reduction in friction coefficient. OFMs with saturated and Z-unsaturated tail groups are found to yield similar structure and friction behavior. OFMs with glyceride head groups yield significantly lower friction coefficients than amide and particularly acid head groups. For all the OFMs and coverages simulated, the friction coefficient is found to increase linearly with the logarithm of sliding velocity; however the gradient of this increase depends on the coverage. The structure and friction details obtained from these simulations agree well with experimental results and also shed light on relative tribological performance of these OFMs through nanoscale structural variations. This has important implications in terms of the applicability of NEMD to aid the development of new formulations to control friction.

For the successful development and application of lubricants, a full understanding of their complex nanoscale behavior under a wide range of external conditions is required, but this is difficult to obtain experimentally. Nonequilibrium molecular dynamics (NEMD) simulations can be used to yield unique insights into the atomic-scale structure and friction of lubricants and additives; however, the accuracy of the results depend on the chosen force-field. In this study, we demonstrate that the use of an accurate, all-atom force-field is critical in order to; (i) accurately predict important properties of long-chain, linear molecules; and (ii) reproduce experimental friction behavior of multi-component tribological systems. In particular, we focus on n-hexadecane, an important model lubricant with a wide range of industrial applications. Moreover, simulating conditions common in tribological systems, i.e., high temperatures and pressures (HTHP), allows the limits of the selected force-fields to be tested. In the first section, a large number of united-atom and all-atom force-fields are benchmarked in terms of their density and viscosity prediction accuracy of n-hexadecane using equilibrium molecular dynamics (EMD) simulations at ambient and HTHP conditions. Whilst united-atom force-fields accurately reproduce experimental density, the viscosity is significantly under-predicted compared to all-atom force-fields and experiments. Moreover, some all-atom force-fields yield elevated melting points, leading to significant overestimation of both the density and viscosity. In the second section, the most accurate united-atom and all-atom force-field are compared in confined NEMD simulations which probe the structure and friction of stearic acid adsorbed on iron oxide and separated by a thin layer of n-hexadecane. The united-atom force-field provides an accurate representation of the structure of the confined stearic acid film; however, friction coefficients are consistently under-predicted and the friction-coverage and friction-velocity behavior deviates from that observed using all-atom force-fields and experimentally. This has important implications regarding force-field selection for NEMD simulations of systems containing long-chain, linear molecules; specifically, it is recommended that accurate all-atom potentials, such as L-OPLS-AA, are employed.

Nonequilibrium molecular dynamics (NEMD) simulations have been used to examine the structure and friction of stearic acid films adsorbed on iron surfaces with nanoscale roughness. The effect of pressure, stearic acid coverage, and level of surface roughness were investigated. The direct contact of asperities was prevented under all of the conditions simulated due to strong adsorption, which prevented squeeze-out. An increased coverage generally resulted in lower lateral (friction) forces due to reductions in both the friction coefficient and Derjaguin offset. Rougher surfaces led to more liquidlike, disordered films; however, the friction coefficient and Derjaguin offset were only slightly increased. This suggests that stearic acid films are almost as effective on contact surfaces with nanoscale roughness as those which are atomically-smooth

From corrosion inhibition to lubrication, the interaction of surfactants with iron oxide surfaces is critical for a range of industrial applications. There is, however, still limited understanding of this process at the atomic-level. In this study, the adsorption of three surfactants which are commonly employed as lubricant additives (carboxylic acid, amide, mono-glyceride) on a α-Fe₂O₃(0001) surface is studied with density functional theory. The nature and strength of the adsorption for the different surfactants, as well as their propensity to deprotonate on the surface, is studied at a range of surface coverages. In agreement with the available experiments, strong chemisorption on α-Fe₂O₃(0001) is observed for all cases. Carboxylic acid and glyceride surfactants spontaneously deprotonate through the formation of a surface hydroxyl group. Glyceride surfactants form the most strongly adsorbed films at low and high surface coverage due to the presence of multiple functional groups which can all act as binding sites. However, the large size of the glyceride headgroup also means that glyceride surfactants are far more stable at low coverage, where the formation of multiple bonds with the surface is possible, than at high coverage. Conversely, carboxylic acid surfactant films have similar stability at low coverage and at high coverage, where van der Waals forces between proximal tailgroups stabilize the adsorption structures. The results of this study provide atomic-level insights which help to explain friction results from previous macroscopic tribology experiments and molecular dynamics simulations. They also facilitate the molecular design of new surfactants to maximize adsorption energy, surface coverage, and ultimately friction reduction on iron oxide surfaces.