Non-contact force

About: Non-contact force is a research topic. Over the lifetime, 77 publications have been published within this topic receiving 5176 citations. The topic is also known as: field force.

The Effect of a Tangential Force on the Contact of Metallic Bodies

Abstract: When two solid bodies are brought together, they are usually very heavily deformed in their region of contact Bowden & Tabor (1954) have discussed very fully how a consideration of this deformation may be used to explain friction They have shown that the interaction between the two bodies in the area of contact, where they come very close together on an atomic scale, is so strong that the application of a tangential force tending to slide two normally loaded bodies over each other nearly always produces deformation and rupture in the bodies themselves, rather than slip in the original interface We have studied experimentally the tangential movement of two bodies relative to each other and the size of the area of contact between them, when they are first loaded normally and then subjected to tangential forces too small to cause sliding We have examined the contact of like metallic specimens, using gold, platinum, tin, indium and mild steel The experiments showed that both the relative displacement of the two bodies and the area of contact between them are smooth, increasing functions of the tangential force, as long as this is increased monotonically from zero Any value of the tangential force, less than the force of static friction, gives rise to a certain equilibrium displacement Such displacements cannot therefore be called ‘sliding’ in the ordinary sense As the displacement increases the tangential force increases more and more slowly, tending asymptotically to the force of static friction We found that the tangential force usually closely approaches the force of static friction while the displacement is still small compared with the diameter of the area of contact The area of contact was found by measuring the electrical contact resistance Only if there are no insulating layers between the bodies does this resistance give a direct measure of the contact area The increase in contact area could therefore be clearly observed only with the noble metals With these the behaviour can be described in terms of the simple analysis given by McFarlane & Tabor (1950) of the yielding in the area of contact under combined normal and tangential loading With metals that bear an insulating oxide layer, the contact resistance gives information on the disruption of the layer We found that purely normal loading causes very little mechanical breakdown; marked breakdown occurs only when the tangential force is increased to a large fraction of that needed to cause sliding It was found that the changes both in displacement and area of contact produced by tangential loading are essentially irreversible A reversible (elastic) component of the displacements exists, but it is a small part of the total except when the tangential force is small Release and even reversal of the tangential force produces no further irreversible changes until the force is increased again, in either direction, to a numerical value as high as the highest it has previously reached It was further found that a lubricant does not essentially affect the deformation process for tangential forces less than those required to cause slip The lubricant acts by weakening the surface-interaction, so that slip occurs for a smaller value of tangential force Displacements corresponding to forces less than those required to cause sliding are approximately equal for lubricated and unlubricated specimens The experiments provide quantitative data on the micro-displacements before sliding The results can be interpreted in terms of the adhesion theory of friction They confirm the analysis of combined stresses given by McFarlane & Tabor and extend its applicability

Separation of interactions by noncontact force microscopy

Abstract: Quantitative measurements of frequency shift vs distance curves of ultrahigh-vacuum force microscopy in a noncontact mode are presented. Different contributions from electrostatic, van der Waals, and chemical interactions are determined by a systematic procedure. First, long-range electrostatic interactions are eliminated by compensating for the contact potential difference between the probing tip and the sample. Second, the long-range van der Waals contribution is determined by fitting the data for distances between 1 and 6 nm. Third, the van der Waals part is subtracted from the interaction curves. The remaining part corresponds to the short-range chemical interaction, and is found to decrease exponentially. A Morse potential is used to fit these data. The determined parameters indicate that the interaction potential between single atoms can be measured by force microscopy in a noncontact mode.