(529h) Atomistic Molecular Dynamics Study of Thin Film Based Nanoscale Lubrication Schemes

Nanoscale lubrication is commonly achieved using monolayers or thin films of polymers. Silica-based micro- and nanoelectromechanical systems (MEMS and NEMS) typically rely on dense alkylsilane/perfluoralkylsilane monolayers¹, whereas hard disk drives (HDDs) rely on sparse thin films of flexible perfluoropolyether (PFPE) polymers, such as Fomblin Zdol². The focus of this study is to understand the role of non-ideality and polymer properties on lubrication behavior in thin film systems. Dense alkylsilane monolayers are studied to understand the role of surface structure and non-ideality, and sparse Zdol films are studied to examine chain flexibility and length. We have developed a synthesis mimetic simulation scheme (SMS) using the ReaxFF reactive force field³ to generate alkylsilane monolayers assembled on realistic hydroxylated amorphous silica surfaces. When compared to idealized systems (including monolayers assembled on crystalline silica substrates and non-hydroxylated amorphous silica substrates), the SMS-based systems yield higher coefficients of friction, which are more consistent with experimental data⁴. This trend toward higher friction results primarily from processing-induced atomic-scale surface roughness of the silica and secondarily from a non-regular in-plane arrangement of chains in the monolayer. Simulations of Zdol thin films demonstrate that the normal load a film can sustain increases as film thickness (i.e. number of polymers per film and polymer length) increases, whereas the coefficient of friction depends on grafting density rather than film thickness; this increase in friction as grafting density decreases is likely a result of sparse films’ inability to fully prevent substrate-substrate interactions. 

¹Chandross, M., Grest, G., and Stevens, M. “Friction between Alkylsilane Monolayers: Molecular Simulation of Ordered Monolayers,” Langmuir, 18, 8392-8399 (2002).

²Choi, Junho, Kawaguchi, Masahiro, and Kato, Takahisa. “Nanoscale lubricant with strongly bonded phase and mobile phase.” Tribology Letters, 15, 353-358 (2003).

³van Duin, A., Dasgupta, S., Lorant, F., and Goddard, W. “ReaxFF: A Reactive Force Field for Hydrocarbons,” Journal of Physical Chemistry, 105, 9396-9409 (2001).

⁴Bush, B. G., Del Rio, F. W., Jaye, C., Fischer, D. A., and Cook, R. F. “Interfacial Mechanical Properties of n-Alkylsilane Monolayers on Silicon Substrates”, Journal of Microelectromechanical Systems, 22, 34-43 (2013).