Large Scale Molecular Dynamics Simulations of Vapor Phase Lubrication for MEMS

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.

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