Large Scale Molecular Dynamics Simulations of Vapor Phase Lubrication for MEMS

Abstract

While alkylsilane monolayers reduce both adhesion and friction in MEMS, experiments and simulations have shown that they are easily damaged by momentary contact even at low loads. Vapor phase alcohols appear to provide a potential solution to this problem, reducing friction in MEMS with no noticeable wear, and allowing devices to run for billions of cycles without failure. The underlying mechanisms behind both the reduction in friction as well as the healing of damage are however unclear. We report on the results of large scale molecular dynamics simulations aimed at understanding the tribology of vapor phase alcohols in contact with amorphous silica substrates. The healing mechanism is investigated by simulating asperity contact with a model AFM tip in contact with a monolayer of propanol on an amorphous silica substrate. We find that because of the low vapor pressure, alcohol molecules removed by shear contact remain close to the substrate, moving around the contact region to replenish molecules removed from the damage site. For comparison, the tribology of propanol and water confined between two opposing flat silica surfaces is also studied.

Generalized Langevin equation: An efficient approach to nonequilibrium molecular dynamics of open systems

“…When these equilibrium thermostats are applied to nonequilibrium MD simulations, they introduce artifacts into the resulting trajectories in these simulations. For example, in nanotribology MD simulations, the most commonly used method to thermostat the system is by applying a Langevin thermostat only in the direction perpendicular to the shear plane of the system [43], but this method has limitations at high shear rates [43] which are required to study friction of low viscosity fluids [44].…”

“…For example, in nanotribology MD simulations, the most commonly used method to thermostat the system is by applying a Langevin thermostat only in the direction perpendicular to the shear plane of the system, 43 but this method has limitations at high shear rates 43 which are required to study friction of low viscosity fluids. 44 An exact and elegant solution to this problem is provided by the Generalized Langevin Equation (GLE). 45 Under rather general assumptions concerning the classical Hamiltonian of the open system and its interaction with its surroundings, assumed to be harmonic, one arrives at non-Markovian dynamics of the open system with multivariate Gaussian distributed random force and the memory kernel that is shown to be exactly proportional to the random force autocorrelation function.…”

Fundamentals of Friction and Wear on the Nanoscale

“…Various realistic models for lubrication layers in very specific contexts have been investigated with extensive MD simulations [162,[184][185][186][187]. During sliding, a thin lubricant film may be solid or liquid, depending on the interplay between the strength of interaction of lubricant molecules with the surfaces and between themselves.…”

Electrical Contact Resistance and Device Lifetime Measurements of Au-RuO2-Based RF MEMS Exposed to Hydrocarbons in Vacuum and Nitrogen Environments

“…This raises the question of the possible physical origins of the factor of the approximate doubling in the resistance: A dielectric material (for dodecane, K = 2) confined within the contact is expected to raise the resistance by many orders of magnitude [6,31] and depend closely on the number of molecules confined within. It is possible that the molecules, being physisorbed and mobile, are not actually within the contacts but squeezed out into the regions immediately outside of them [46][47][48][49][50][51]. Indeed, Patton et al [3] reported that contact force of 200 lN is high enough to remove SAM molecules outside of MEMS contacts.…”