Frictional Aging and Sliding Bifurcation in Monolayer-Coated Micromachines

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 rate (F)Over dot(n) after a hold time t(h) at a hold load F-h. When coated by a monolayer of FOTAS (CF3C5F10C2H4Si(N(CH3)(2))(3)), we find that there is no unique coefficient of static friction mu(s) but instead that mu(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.