Friction Modifier Behaviour in Lubricated MEMS Devices

Low viscosity fluids could provide reliable lubrication for certain microelectromechanical system's (MEMS) applications where high-sliding speeds and/or high sliding distances occur. However, while the use of low viscosity fluids leads to reduced hydrodynamic friction, high boundary friction can be a significant issue at low entrainment speeds. This article describes a series of tests of low viscosity fluids, blended with a friction modifier additive so as to provide a combination of both low hydrodynamic and low boundary friction at MEMS scales. The low viscosity fluids tested were hexadecane, low viscosity silicone oil, toluene and water. With the exception of water, the addition of the organic friction modifier octadecylamine to all these lubricating fluids produced a significant reduction in boundary friction. For a MEMS contact lubricated with silicone oil for instance, boundary friction was reduced from 0.5 to close to 0.05. The presence of the amine dissolved in the toluene had the effect of reducing boundary friction from 0.75 to 0.55; this was further reduced to 0.25 after the specimens had been immersed in the toluene-additive blend for 48 h. A water-soluble additive, diethylamine, was added to de-ionized water, at 0.1% by weight concentration. Although an initial reduction in boundary friction was observed (0.45-0.25), under these conditions the rapid onset of severe wear negated these effects. It is suggested that corrosion of silicon by water, followed by abrasion, is the cause of this accelerated wear.