This review focuses on recent developments in the use of 2D nanomaterials for controlling the frictional properties of surfaces and interfaces. While materials such as MoS 2 and graphite have been investigated for some time, a host of other layered nanomaterials have emerged as alternatives for friction modification. These advanced lubrication schemes provide an opportunity to address growing needs in energy and materials efficiency and device compatibility, offering improved boundary and solid lubrication of contacting and sliding interfaces. Here, we describe both computational and experimental investigations of the mechanisms and implementations of 2D nanomaterials in the lubrication of interfaces.

Self-assembled monolayers (SAMs) of alkylsilanes have been considered as wear reducing layers in tribological applications, particularly to reduce stiction and wear in microelectromechanical systems (MEMS) devices. Though these films successfully reduce interfacial forces, they are easily damaged during impact and shear. Surface roughness at the nanoscale is believed to play an important role in the failure of these films because it effects both the formation and quality of SAMs, and it focuses interfacial contact forces to very small areas, magnifying the locally applied pressure and shear on the lubricant film. To complement our prior studies employing Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM) experiments in which silica nanoparticles are used to simulate nanoasperities and to refine our analysis of these films to a molecular level, classical molecular dynamics simulations have been employed to understand the impact of nanoscopic surface curvature on the properties of alkylsilane SAMs. Amorphous silica nanoparticles of various radii were prepared to simulate single asperities on a rough MEMS device surface, or AFM tips, which were then functionalized with alkylsilane SAMs of varying chain lengths. Factors related to the tribological performance of the film, including gauche defect density and exposed silica surface area, were examined to understand the impact of surface curvature on the film. Additionally, because the packing density of the films has been found to be relatively low for alkylsilane SAMs on surfaces with nanoscopic curvature, packing density studies were performed on simulated silica surfaces lacking curvature to understand the relative impact of these two important factors. It was found that both curvature and packing density affect the film quality; however, packing density was found to have the strongest correlation to film quality, demonstrating that greater priority should be given to the reduction of free volume within the films to improve their structural rigidity, to better passivate the underlying surfaces of the devices, and to improve the extent and accessibility of nondestructive dissipation pathways, all of which will lead to improved friction and wear resistance. While focused on silica nanoasperities, these MD simulations afford general approaches for studies of ligand effects on a range of surfaces with nanoscopic curvature such as metal oxide nanoparticles and quantum dots.

This document includes details on the synthesis of the molecule used in this study and its associate characterization along with additional details on the monolayer characterization by FTIR and XPS and additional details on the DFT calculations employed.

To achieve a better understanding of the mechanical effects of adsorbed films at surface contacts, methods were developed to map and examine the pressure distribution of nanoasperity contacts, modeled by molecular dynamics simulation. The methods employ smoothing functions to project the atomic forces obtained in contact simulation onto the contact plane for fitting to standard continuum contact models and subsequent analysis. Importantly, these methods allow for contact evolution between nanoscopic asperity-asperity contacts to be examined because these are the central load-bearing junctions at interfaces. To demonstrate the application and features of this approach, it was employed to examine the evolution of contact between silica nanoasperities, with an increasing density of octadecyltrichlorosilane (OTS) films employed as a model adsorbate film. Linearly increasing contact radius and linearly decreasing maximal pressure were observed as a function of the film packing density. Because contact between the underlying, high-energy silica surfaces is undesirable, the evolution of silica contact was also examined using these same methods. As more molecules were introduced into the contact, a sharp transition was observed from the narrow, high-pressure interaction between the underlying substrates, to a broad, substantially lower pressure interaction, indicating a sharp transition from the dry to lubricated condition. To study the dependence of these behaviors on contact morphology, silica nanoasperities in contact with a flat silica surface were also examined. Similar behavior, including the broadening of the contact area and the minimization of direct surface contact, were observed. The method developed herein is applicable to a variety of systems and can be employed to optimize surface protection and pressure redistribution by boundary lubricants. This method can also be extended to AFM adhesion measurements where a detailed understanding of the true contact area is critical for the quantitative determinations of molecular forces and local surface mechanics.

Detailed pressure and strain mapping of atomistic contact simulations elucidate the mechanical and tribochemical mechanisms of surface force modification with SAMs.