Molecular Dynamics Simulations of Alkylsilane Monolayers on Silica Nanoasperities: Impact of Surface Curvature on Monolayer Structure and Pathways for Energy Dissipation in Tribological Contacts
BW Ewers and JD Batteas, JOURNAL OF PHYSICAL CHEMISTRY C, 116, 25165-25177 (2012).
Self-assembled rnonolayers (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.
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