High speed microtribology with quartz crystal resonators.

Quartz resonators in contact with metal-covered spheres have been used to study the tribology of micron-sized metal-metal contacts at high speed (1 m/sec and high frequency (12 MHz), while maintaining a shear amplitude in the nanometer range. The data aquisition is based on ring-down experiments, where the electrical excitation is periodically interrupted and the free decay of the oscillation is analyzed. At contact, an amplitude-dependent frequency and decay rate are found, indicative of an underlying nonlinear equation of motion. Using perturbation theory and the two-timing approximation, a nonlinear spring constant kappa(1)(x) and a nonlinear drag coefficient xi(1)(x) are explicitly derived. We find a local slip-to-stick-transition at a shear amplitude of 0.5 nm. Coating the gold sufaces with a self-assembled thiol monolayer removes the stick.