Controlling Nanoscale Friction through the Competition between Capillary Adsorption and Thermally Activated Sliding
C. Greiner, J.R. Felts, Z. Dai,
W.P. King, R.W. Carpick
ACS Nano, Band 6, Seiten 4305-4313
We demonstrate measurement and control of nanoscale single-asperity friction by using cantilever probes featuring an in-situ solid-state heater in contact with silicon oxide substrates. The heater temperature was varied between 25 - 790 °C. By using a low thermal conductivity sample, silicon oxide, we are able to vary tip temperatures over a broad range from 25±2 - 255±25 °C. In ambient atmosphere with ~30 % relative humidity, the control of friction forces was achieved through the formation of a capillary bridge whose characteristics exhibit a strong dependence on temperature and sliding speed. The capillary condensation is observed to be a thermally-activated process, such that in ambient air heating caused friction to increase due to the capillary bridge nucleating and getting larger. However, above tip temperatures of ~100±10 °C, friction decreased drastically, which we attribute to controllably evaporating water from the contact at the nanoscale. In contrast, in a dry nitrogen atmosphere, friction was not affected appreciably by temperature changes. In the presence of a capillary, friction decreases at higher sliding speeds due to disruption of the capillary, otherwise friction increases in accordance with the predictions of a thermally-assisted sliding model. In ambient atmospheres, the rate of increase of friction with sliding speed at room temperature is sufficiently strong that the friction force changes from being smaller than the response at 76±8 °C to being larger. Thus, an appropriate change in temperature can cause friction to increase at one sliding speed, while it decreases at another speed. This behavior was reproduced in real-time friction measurements where the temperature of the AFM cantilever was rapidly modulated while scanning. Using in-situ heating to control the presence of the nanoscale capillary and to modulate friction offers new opportunities for tip-based nanomanufacturing and is of importance in the field of micro- and nanoelectromechnical systems.