Tribological Examination of Alkylsilane Monolayer Films via Molecular Dynamics Simulation
Summers, Andrew Zachary
The lifetimes of micro- and nanoelectromechanical systems (MEMS/NEMS), devices featuring both electronic and mechanical components with feature lengths on the micro- to nanoscale, are threatened by friction and wear, often imposing signficant design constraints that increase device complexity and limit device application. Surface functionalization by monolayer films has been proposed as a potential solution to this problem, providing a dense, "brushlike" coating of chains that helps prevent direct contact between surfaces and helps lower frictional and adhesive forces. However, durability concerns have prevented monolayer films from becoming a widespread lubricant for MEMS/NEMS, as these films have been shown to exhibit short lifetimes under shear. Fortunately, the chemistry of monolayer films is highly tunable, providing optimism that the structure of these lubricants can be adjusted to meet the tribological demands of MEMS/NEMS devices. To this end, molecular dynamics (MD) simulation provides a useful platform for studying friction and wear mechanisms of monolayers at the atomic level and with precise control of system variables. In this dissertation, MD is used to examine several tribological aspects of monolayer films. First, the effect of substrate morphology on the wear of alkylsilane monolayers is analyzed, where it is observed that surface roughness and non-ideality of the in-plane ordering of chain binding sites promotes reduced durability; however, increases in chain length are shown to help mask these effects. Next, the effect of monolayer density on friction mechanisms is examined for films under contact by a model nanoscale asperity, where the liquid-like response of lower density films is found to promote reduced frictional forces. Screening of functionalized monolayer films, facilitated by the Molecular Simulation Design Framework (MoSDeF), is then performed to analyze the effect of terminal group chemistry on monolayer tribology. Terminal group characteristics with the greatest influence on friction and adhesion are extracted from structure-property models. Finally, a coarse-grained model for amorphous silica is presented, which should aid in reducing the computational cost of future monolayer screening.