(757g) Molecular Dynamics Study on the Effects of Nanoscale Roughness on the Wear of Alkylsilane Monolayers

Surface functionalization by silane-based monolayers represents a promising lubrication scheme for micro- and nanoelectromechanical systems (MEMS/NEMS) in which high surface area-to-volume ratios can readily induce wear of the high-energy silica surfaces. However, rapid degradation of monolayer coatings has hindered the effectiveness of these materials to date. Studies of the durability of monolayers with varying interfacial binding strengths suggest a prominent mechanism of wear may be through mechanically induced scission of surface bonds [1,2]. As a result, monolayer durability has been shown to be highly sensitive to surface features, such as roughness and crystallinity. To ascertain the feasibility of monolayer coatings for MEMS/NEMS lubrication, a comprehensive understanding of the wear process, including conditions that promote degradation and how these might be overcome, is imperative. Molecular dynamics simulations provide a means to analyze these systems at the atomic-scale. In previous work, molecular dynamics simulations of alkylsilane monolayers were used to show that even at the atomic level, chains located at â??peaksâ? on the surface (positive deviations from the surface mean) are preferentially removed under shear [3]. Nanoscale roughness, emerging from surface asperities, is likely to have an even greater effect on monolayer stability, as a result of the high local pressures arising at asperity contacts. This is exemplified through AFM measurements, which mimic a single-asperity contact, where irreversible monolayer damage has been shown to occur beyond a critical normal load as a result of the breaking of surface bonds [4].

In this work, we examine the wear of alkylsilane monolayers at both single- and dual-asperity contacts. A pseudo-reactive potential is developed to model interfacial bonding, and bond strengths are tuned to investigate the necessary strength required to sustain the high local pressures under these conditions. The influence of monolayer structure (e.g., composition and chain length) on monolayer durability is examined. Our results reveal greater durability as monolayer chain length is increased, agreeing with experiment, but also show that these conditions exhibit a higher friction coefficient, as a result of increased molecular plowing. The effects of surface deformation under single- and dual-asperity contact are also explored.

[1]  B. D. Booth, S. G. Vilt, C. McCabe, and G. K. Jennings, â??Tribology of Monolayer Films: Comparison between n-Alkanethiols on Gold and n-Alkyl Trichlorosilanes on Silicon.,â? Langmuir, vol. 25, no. 17, pp. 9995â??10001, Sep. 2009.

[2]  S. P. Pujari and H. Zuilhof, â??Highly wear-resistant ultra-thin perfluorinated organic monolayers on silicon(111) surfaces,â? Appl. Surf. Sci., vol. 287, no. December, pp. 159â??164, Dec. 2013.

[3]  A. Z. Summers, C. R. Iacovella, M. R. Billingsley, S. T. Arnold, P. T. Cummings, and C. McCabe, â??Influence of Surface Morphology on the Shear-Induced Wear of Alkylsilane Monolayers: Molecular Dynamics Study,â? Langmuir, 2016.

[4]  B. Bhushan, T. Kasai, G. Kulik, L. Barbieri, and P. Hoffmann, â??AFM study of perfluoroalkylsilane and alkylsilane self-assembled monolayers for anti-stiction in MEMS/NEMS,â? Ultramicroscopy, vol. 105, no. 1â??4, pp. 176â??188, Nov. 2005.