(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).