Nanofriction mechanisms derived from the dependence of friction on load and sliding velocity from air to UHV on hydrophilic silicon

TL;DR

This study investigates nanoscale friction mechanisms on hydrophilic silicon by analyzing how friction varies with load and velocity from air to ultra-high vacuum, revealing three distinct regimes related to water films and solid contact.

Contribution

It identifies and characterizes three different nanoscale friction regimes on silicon surfaces based on environmental conditions and load, advancing understanding of nanofriction mechanisms.

Findings

Friction regimes dominated by capillarity, water layer ordering, and solid-solid contact.

Friction behavior changes significantly during vacuum pump-down.

Different friction mechanisms can coexist depending on conditions.

Abstract

This paper examines friction as a function of the sliding velocity and applied normal load from air to UHV in a scanning force microscope (SFM) experiment in which a sharp silicon tip slides against a flat Si(100) sample. Under ambient conditions, both surfaces are covered by a native oxide, which is hydrophilic. During pump-down in the vacuum chamber housing the SFM, the behavior of friction as a function of the applied normal load and the sliding velocity undergoes a change. By analyzing these changes it is possible to identify three distinct friction regimes with corresponding contact properties: (a) friction dominated by the additional normal forces induced by capillarity due to the presence of thick water films, (b) higher drag force from ordering effects present in thin water layers and (c) low friction due to direct solid-solid contact for the sample with the counterbody. Depending on environmental conditions and the applied normal load, all three mechanisms may be present at one time. Their individual contributions can be identified by investigating the dependence of friction on the applied normal load as well as on the sliding velocity in different pressure regimes, thus providing information about nanoscale friction mechanisms.