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

Influence of subsurface microstructure on the running-in of an AlSi alloy

The friction and wear behavior of a lubricated AlSi11Cu3 disk in contact with a 100Cr6 pin was studied by a radionuclide-assisted pin-on-disk tribometer. It is well known that shear forces change chemistry and microstructure of the near-surface material, thereby influencing friction and wear. To better understand the influences of the microstructure on the running-in behavior, disks with different silicon phase morphologies were tested under constant stressing conditions. Topography, chemistry and microstructure of pin and disk were characterized before and after tribological testing by white light interferometry, X-ray photoelectron. spectroscopy, Auger electron spectroscopy and focused ion beam microscopy. Wear of pins was measured with a radionuclide technique to resolve ultra-low wear rates. To characterize near-surface deformations, Ga ion markers were implanted and Auger electron spectroscopy was applied to follow their shear-induced displacements. To monitor subsurface shear, the deformation of columnar markers was analyzed. The results were discussed using Godet's third body model. Furthermore, the results allowed to us extend the model to systems operated under ultra-low wear rates

How do liquids confined at the nanoscale influence adhesion?

Liquids play an important role in adhesion and sliding friction. They behave as lubricants in human bodies especially in the joints. However, in many biological attachment systems they acts like adhesives, e.g. facilitating insects to move on ceilings or vertical walls. Here we use molecular dynamics to study how liquids confined at the nanoscale influence the adhesion between solid bodies with smooth and rough surfaces. We show that a monolayer of liquid may strongly affect the adhesion.Comment: 5 pages, 9 color figures. Some figures are in Postscript Level 3 format. Minimal changes with respect to the previous version. Added doi and reference to the published article also inside the pape

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 thorough knowledge of the water film thickness present on the surfaces of these devices with an accuracy in the nm range. In this work, the thickness of an ultra-thin water layer was measured by distance tunnelling spectroscopy and distance dynamic force spectroscopy during desorption in an ultra-high vacuum system, from about 2.5 nm up to complete desorption at 1E-8 mbar. The tunnelling current as well as the amplitude of vibration and the normal force were detected as a function of the probe-sample distance. In these experiments, a direct conversion of the results of both methods is possible. From the standpoint of surface science, taking the state-of-the-art concerning adsorbates on surfaces into consideration, dynamic force spectroscopy provides the most accurate values. The previously reported tunnelling spectroscopy, requiring the application of significantly high voltages, generally leads to values that are 25 times higher than values determined by dynamic force spectroscopy

Non-linear double-peeling: Experimental vs. theoretical predictions

The double peeling of detachment of non-linear adhesive tapes from a flat Poly(methylmethacrylate) (PMMA) surface has been investigated from both experimental and theoretical point of view. Double peeling tests show that, as the detachment process advances, the peeling angle stabilizes on a limiting value {\theta}lim corresponding to a critical pull-off force Fc above which the tape is completely detached from the substrate. This observed behavior is in good agreement with results obtained following the new theory of multiple peeling and taking into account the hardening-softening non-linear behavior of the experimentally tested adhesive tapes and clarifies some aspects of the experimental data. In particular, the theoretical model shows that the value of the limiting peeling angle depends on the geometry of the adhesive tape as well as on the stiffness properties and on the interfacial energy {\Delta}{\gamma}. Finally, theoretical predictions confirm that solutions with a peeling angle lower than {\theta}lim are unstable.Comment: 14 pages, 8 figure

On the nature of surface roughness with application to contact mechanics, sealing, rubber friction and adhesion

Surface roughness has a huge impact on many important phenomena. The most important property of rough surfaces is the surface roughness power spectrum C(q). We present surface roughness power spectra of many surfaces of practical importance, obtained from the surface height profile measured using optical methods and the Atomic Force Microscope. We show how the power spectrum determines the contact area between two solids. We also present applications to sealing, rubber friction and adhesion for rough surfaces, where the power spectrum enters as an important input.Comment: Topical review; 82 pages, 61 figures; Format: Latex (iopart). Some figures are in Postscript Level

Mechanics of Reversible Unzipping

We study the mechanics of a reversible decohesion (unzipping) of an elastic layer subjected to quasi-static end-point loading. At the micro level the system is simulated by an elastic chain of particles interacting with a rigid foundation through breakable springs. Such system can be viewed as prototypical for the description of a wide range of phenomena from peeling of polymeric tapes, to rolling of cells, working of gecko's fibrillar structures and denaturation of DNA. We construct a rigorous continuum limit of the discrete model which captures both stable and metastable configurations and present a detailed parametric study of the interplay between elastic and cohesive interactions. We show that the model reproduces the experimentally observed abrupt transition from an incremental evolution of the adhesion front to a sudden complete decohesion of a macroscopic segment of the adhesion layer. As the microscopic parameters vary the macroscopic response changes from quasi-ductile to quasi-brittle, with corresponding decrease in the size of the adhesion hysteresis. At the micro-scale this corresponds to a transition from a `localized' to a `diffuse' structure of the decohesion front (domain wall). We obtain an explicit expression for the critical debonding threshold in the limit when the internal length scales are much smaller than the size of the system. The achieved parametric control of the microscopic mechanism can be used in the design of new biological inspired adhesion devices and machines

Surface structure and frictional properties of the skin of the Amazon tree boa Corallus hortulanus (Squamata, Boidae)

The legless locomotion of snakes requires specific adaptations of their ventral scales to maintain friction force in different directions. The skin microornamentation of the snake Corallus hortulanus was studied by means of scanning electron microscopy and the friction properties of the skin were tested on substrates of different roughness. Skin samples from various parts of the body (dorsal, lateral, ventral) were compared. Dorsal and lateral scales showed similar, net-like microornamentation and similar friction coefficients. Average friction coefficients for dorsal and lateral scales on the epoxy resin surfaces were 0.331 and 0.323, respectively. In contrast, ventral scales possess ridges running parallel to the longitudinal body axis. They demonstrated a significantly lower friction coefficient compared to both dorsal and lateral scales (0.191 on average). In addition, ventral scales showed frictional anisotropy comparing longitudinal and perpendicular direction of the ridges. This study clearly demonstrates that different skin microstructure is responsible for different frictional properties in different body regions