Improved friction performance is an important objective of equipment manufacturers for meeting improved energy efficiency demands. The addition of friction-reducing additives, or friction modifiers (FMs), to lubricants is a key part of the strategy. The performance of these additives is related to their surface activity and their ability to form adsorbed layers on the metal surface. However, the extent of surface coverage (mass per unit area) required for effective friction reduction is currently unknown. In this article, we show that full coverage is not necessary for significant friction reduction. We first highlight various features of surface adsorption that can influence the surface coverage, packing, and free energy of adsorption of organic FMs on iron oxide surfaces. Using stearic acid in heptane and hexadecane as model lubricant formulations, we employ a combination of experiments and molecular dynamics (MD) simulations to show how the dimerization of acid molecules in the bulk solvent and the crystallographic orientation of the surface modifies surface adsorption. In addition, we show that the solvent can strongly influence the adsorption kinetics, and MD simulations reveal that hexadecane tends to align on the surface, increasing the energy barrier for the adsorption of stearic acid to the surface. Furthermore, we present a combined approach using MD and molecular thermodynamic theory to calculate adsorption isotherms for stearic acid on iron oxide surfaces, which agrees well with experimental data obtained with a quartz crystal microbalance (QCM). Our results suggest that while the friction of systems lubricated with organic FMs decreases with increasing coverage, complete coverage of the surface is neither practically achievable nor necessary for effective friction reduction for the systems and conditions studied here.

The absorbance and fluorescence spectra of Nile Red (NR) were examined in a series of nonpolar solvents comprising linear alkanes and a range of poly alpha olefins (PAO). These solvents span a 1000-fold range in viscosity and possess very similar dielectric constants and refractive index properties. A high-energy double peak with vibronic structure is observed in both fluorescence and absorbance spectra, possibly indicating that a locally excited (LE) state is accessed in these solvents. In addition, a red shift in peak position is observed with increasing refractive index; however, it is unaccompanied by any changes in Stokes shift. This shift is attributed to changes in the high-frequency polarizability of the solvent, which is a function of the refractive index. Finally, an increase in quantum yield with viscosity is also observed.

Mo- and S-based lubricant additives reduce friction in boundary lubrication through the formation of molybdenum disulfide (MoS2) during operation. However, the fundamental mechanisms of MoS2 formation are still not fully understood, in part because direct experimental measurement is challenging during the crystallization process. Previously, reactive molecular dynamics simulations were used to model the formation of crystalline MoS2 by compressing and heating amorphous material consisting of Mo and S. Here, the authors test the robustness of these models to capture the crystallization process under different simulation conditions and with different reactive force fields. Lastly, a reactive force field that contains parameters for Mo, S, and O was modified to enable it to capture MoS2 crystallization in the presence of oxygen.

Reactive molecular dynamics simulations of MoS2 crystallization from amorphous precursor materials showed that crystal domain size decreased because of excess S or O, relative to the stoichiometric case. Simulation results were corroborated by comparison of calculated limiting domain sizes to experimental measurements of MoS2 crystals grown from thermal decomposition of molybdenum dithiocarbamate. Then, the simulations were used to evaluate two previously proposed domain growth mechanismsthermodynamic and kinetic; both were shown to contribute to MoS2 domain growth and, importantly, to stopping growth at a limiting size. It was shown that S-rich or O-containing precursor materials can inhibit grain growth (i) thermodynamically, by increasing the amount of S at domain edges which decreases boundary energy, making them more stable and lowering the driving force for growth, and (ii) kinetically, by decreasing the probability of Mo–S interactions at domain edges that would otherwise contribute to domain growth. The simulations explain how each of these mechanisms determines the effect of precursor composition on MoS2 domain size and, further, suggest avenues for tunable MoS2 synthesis to achieve application-specific domain size.

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