(13f) Screening Structure-Property Relationships in Lubricating Monolayer Films through Molecular Dynamics Simulation

Nanoscale devices featuring surfaces in sliding contact exhibit high surface area-­to-volume ratios and surface energies that promote stiction and surface wear, leading to device failure and design limitations. Alkane-­based surface monolayers have been shown to reduce stiction while yielding low coefficients of friction and as such have been proposed as a potential lubrication solution to minimize such limitations. The tribological performance of monolayer lubricants is closely linked to their chemistry, with, for example, terminal group functionality and chain length shown to alter adhesion and frictional performance of monolayer films. However, only a small subset of the vast parameter space available to monolayer chemistry has been examined to-date. Wide screening of various chemistries is thus desirable to obtain a thorough understanding of the links between chemistry, structure, and optimal tribological properties.

To this end, we have developed the molecular simulation and design framework (MoSDeF) software suite [1] that provides scripted, flexible, and reusable routines to facilitate screening over chemical/structural parameter spaces. Using MoSDeF, screening of functionalized monolayer films featuring sixteen distinct terminal groups and five chain lengths has been performed. Both single- and mixed-component systems (where the top and bottom monolayers feature different chemistries) have been studied, with the goal of identifying trends between monolayer chemistry and tribology to aid in the design of better-lubricating films. Structure-property relationships are obtained by assigning a series of chemical descriptors to each terminal group and examining correlations with adhesion and coefficient of friction. We detail how characteristics such as asphericity and hydrophobicity influence the tribology of the monolayer films and provide insight into the characteristics of an “ideal” monolayer chemistry. These relationships are further related to underlying friction mechanisms via analysis of monolayer structural properties such as orientational chain order, average tilt angle, and gauche defects.

References

[1] “MoSDeF” [Online]. Available: https://github.com/mosdef-­hub.