Frictional Aging and Sliding Bifurcation in Monolayer-Coated Micromachines

Abstract

Using a high-performance polycrystalline-silicon micromachined actuator that also functions as a friction tester, we have found frictional forces that cannot be explained by Amontons' law with a constant coefficient of friction. In our friction test, a constant tangential force is applied while normal load is ramped down at a rateḞ n after a hold time t h at a hold load F h . When coated by a monolayer of FOTAS (CF 3 C 5 F 10 C 2 H 4 Si(N(CH 3 ) 2 ) 3 ), we find that there is no unique coefficient of static friction μ s , but instead that μ s depends on all three of these parameters. The dependence on t h implies static friction aging, but the rate of static friction aging can be suppressed by greater hold force. When sliding motion begins, we have identified a critical normal force to shear force ratio such that any motion initiating above the critical ratio proceeds with time-dependent frictional creep over several hundred nanometers, whereas any motion initiating below the critical ratio proceeds with a large inertial jump. These effects demonstrate that contact aging effects extend from the micrometer to the nanometer scale and are relevant to micromachined interfaces.[2008-0174]

Frictional aging, de-aging, and re-aging in a monolayer-coated micromachined interface

“…In our test protocol, we maintain a constant tangential force while ramping down the normal force, and look for the onset of motion, which can be measured to Ϯ5 nm resolution. Static friction measurements reported in a recent paper 16 indicated a comparatively large factor of three dependence of s on the normal force rampdown rate.…”

“…Because s varied over such a wide range, 16 it hardly seems the most appropriate way to characterize the frictional response of the system. We postulate in this paper that a more fundamental measure of friction is provided by instantaneously dropping the normal load and then measuring the time for motion to initiate.…”

“…16͒ in terms of the de-aging concept. Applying the measured empirical release time dependence in a simple model, we quantitatively relate the previous s measurements 16 to the present t r measurements by introducing a "re-aging" process. The measurements of t r also are in agreement with our previous observations that static friction decreases as hold force increases 16 and we therefore comment on the likely controlling mechanism.…”

Direct Measurement of Intermediate-Range Casimir-Polder Potentials

“…Furthermore, they have important technological implications for the development of micromachines with nanoscale moving parts [5,6]. Today, Casimir forces between solids can be measured with high precision [7][8][9]. These measurements are all done with objects that are large compared to the relevant distances where Casimir forces become dominant.…”

Evolution of Wear Characteristics and Frictional Behavior in MEMS Devices

“…The data presented here is for oxide-coated devices, which were found [32] to have shorter lifetimes than monolayer-coated devices. Other work we have reported [23,24,[34][35][36][37] was taken on nanotractors monolayer coatings applied according to the procedure detailed by Hankins et al [33]. For those tests, friction results are highly consistent in the first few hundreds of tests.…”