Friction and wear in micro- and nanomachines

The prediction and characterization of multilength-scale tribological phenomena is challenging, yet essential for the advancement of micro- and nanomachine technology. Here, we consider theoretical underpinnings of multiasperity friction, review various approaches to measure micro- and nanoscale friction and discuss the effect of monolayer coatings to reduce friction and wear. We then overview a theoretical framework known as rate-and-state friction (RSFrate-and-state friction (RSF)), in which friction is considered to be a continuous function of velocity and interface state. A microscale test platform that is used to measure friction over multiple decades of velocity and normal load is presented and results are reported. Using the RSF framework, we quantitatively predict and validate the transition from stick-slip to steady sliding, enabling the creation of a microscale kinetic phase diagram. Next, we take a brief look at continued progress in spinning micromachine motor technology. Finally, we discuss wear- and tribopolymer-related phenomena in micro- and nanoswitches, which are promising devices to complement transistors due to their low on resistance and steep subthreshold swing. We anticipate great progress towards reliable, contacting micro- and nanomachines by linking theory and experiment to nano- and microscale tribological phenomena and by improving the testing, materials and processing methods used to characterize these phenomena.