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 mathdotF _n after a hold time t _h at a hold load F _h. When coated by a monolayer of FOTAS (CF₃ C₅F₁₀C₂H₄Si(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.