Syed Imad-Uddin Ahmed

Friction of thin water films: a nanotribological study

Abstract Lubricant thickness is known to influence the frictional properties between two contacting surfaces in relative motion. In this paper, the dependence of friction on water films with various thicknesses is examined using a scanning force microscopy (SFM) between a hydrophilic silicon tip and silicon flat that was rendered either hydrophilic or hydrophobic. Results indicate that the frictional properties are influenced by the ordering effects of water. The friction of a hydrophilic surface initially covered with 2.6 nm of water was examined as a function of the film thickness, which was separately determined using scanning tunneling microscopy and dynamic SFM. Capillary effects dominate the tribological properties for films that are between 3 and about 1 nm thick. For thinner films the friction properties can be explained by the ordering effect of water on the sample and the tip, which together cause an increased resistance to shear (higher viscosity). Further reduction of film thickness due to water desorption leads to a decrease of the friction force; this regime is dominated by cohesive forces arising from the solid–solid contact. For all applied forces in this study, liquid confinement between tip and surface does not occur. Rather, the sharp SFM tip apparently penetrates the water film with the tip apex making direct contact with the surface. The ordering effect of water occurs on the sample surface and at the sides of the SFM tip. These results, together with previous microtribological studies, highlight the significant differences existing between two contacting surfaces moving in relative motion in the micro- and nanoregime.

Nanofriction of silicon oxide surfaces covered with thin water films

Abstract A thin water film present on surfaces plays a central role in defining the micro- and nanotribological properties of a system. This paper presents a quantitative examination of the nanotribological effects of thin water films in ultra high vacuum (UHV) on OH-terminated (hydrophilic) and bare (no OH terminations, hydrophobic in vacuum) silicon oxide surfaces. Water film thickness was controlled by varying the water partial pressure in UHV. Friction was measured by scanning force microscopy (SFM) as a function of an external applied load. The surface energy and the shear stress of the nanotribological contact was then approximated by fitting the friction-load curves using the Derjaguin–Muller–Toporov (DMT) model. The surface energy as well as the adhesion force of the OH-terminated hydrophilic sample first decrease and later increase significantly at higher water partial pressures. No such dependence could be deduced from the friction-load curves at varying water pressures for the bare hydrophobic silicon oxide surface. However, at relatively high normal loads (pressures) and water partial pressures the bare hydrophobic silicon oxide is transformed to an OH-terminated surface. This transformation appears to occur only in the area of contact leading to the conclusion that it is friction-induced. This work shows that the chemical composition of the topmost surface layer defines the frictional behavior of the tribosystem.

HREELS Investigation of the Surfaces of Nanocrystalline Diamond Films Oxidized by Different Processes

This article reports on the use of high-resolution electron energy loss spectroscopy (HREELS) for the investigation of as-grown (hydrogen-terminated) and oxidized nanocrystalline diamond films (NCD) using chemical, physical, and electrochemical approaches. The results indicate that the nature and number of oxygen-related chemical groups generated on the NCD surface depend strongly on the oxidation process. A high concentration of C-O functions has been obtained on the NCD surface oxidized by rf (radio frequency) oxygen plasma, whereas the highest C═O/C-O ratio has been achieved by electrochemical oxidation. The NCD surface oxidized by rf plasma was totally free of C═O groups. Traces of surface hydroxyl groups (C-OH) have been detected upon annealing in air or through UV/ozone oxidation.

A comparative investigation of thickness measurements of ultra-thin water films by scanning probe techniques

The reliable operation of micro- and nanomechanical devices necessitates a precise knowledge of the water film thickness present on the surfaces of these devices with accuracy in the nanometer range. In this work, the thickness of an ultra-thin water film was measured by distance tunneling spectroscopy and distance dynamic force spectroscopy during desorption in an ultra-high vacuum system, from about 2.5 nm up to complete desorption at 10−8 mbar. The tunneling current and the amplitude of vibration and the normal force were detected as a function of the probe-sample distance. In these experiments, a direct comparison of both methods was possible. It was determined that dynamic force spectroscopy provides the most accurate values. The previously reported tunneling spectroscopy, which requires the application of significantly high voltages generally leads to values that are 25 times higher than values determined by dynamic force spectroscopy.

Nanofriction Mechanisms Derived from the Dependence of Friction on Load and Sliding Velocity from Air to UHV on Hydrophilic Silicon

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.