Using an atomic force microscope, we have observed atomic-scale features on the frictional force acting on a tungsten wire tip sliding on the basal plane of a graphite surface at low loads, < 10-4 N. The atomic features have the periodicity of the graphite surface and are discussed in terms of a phenomenological model for the tip motion described by the sum of a periodic tip-surface force and the spring force exerted by the wire.

Atomic-scale friction of a tungsten tip on a graphite surface.

Surface Forces and Surface Interactions

This paper describes a study of the adhesion between elastic solids and the effect of roughness in reducing the adhesion. The experiments were carried out between optically smooth rubber spheres and a hard smooth flat surface of Perspex which could be roughened to various degrees. The radius of the rubber spheres was varied by a factor of 8, their elastic modulus by a factor of 10, while the centre line average (c.l.a.)of the roughened Perspex surface was varied from 0.12 to 1.5 μm. The results show that c.l.a. roughnesses which are small compared with the overall deformation occurring at the region of the rubber-Perspex contact can produce an extremely large reduction in adhesion. The effect is more marked for rubbers of higher modulus. On the other hand the curvature of the sphere (over the range examined) has little influence. For this reason and because the analytical problem of a sphere on a rough flat is extremely complicated a theoretical analysis has been developed for the simpler case of a smooth flat in contact with a rough flat surface. As in Greenwood & Williamson (1966) the rough surface is modelled by asperities all of the same radius of curvature and with heights following a Gaussian distribution of standard deviation σ. The overall contact force is obtained by applying the contact theory of Johnson, Kendall & Roberts (1971) to each individual asperity. The theory predicts that the adhesion expressed as a fraction of the maximum value depends upon a single parameter, 1/Δ e ,which is the ratio between a and the elastic displacement δ C that the tip of an asperity can sustain before it pulls off from the other surface. The analysis shows that the adhesion parameter may also be regarded as representing the statistical average of a competition between the compressive forces exerted by the higher asperities trying to prize the surfaces apart and the adhesive forces between the lower asperities trying to hold the surfaces together. Although the theory is derived for two nominally plane surfaces it is found to fit the experimental results for a sphere on a flat reasonably well.

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The effect of surface roughness on the adhesion of elastic solids

The fundamental mechanisms of solid lubrication are reviewed with examples from well-known solid lubricants like the transition metal dichalcogenides and diamond-like carbon families of coatings. Solid lubricants are applied either as surface coatings or as fillers in self-lubricating composites. Tribological (friction and wear) contacts with solid lubricant coatings typically result in transfer of a thin layer of material from the surface of the coating to the counterface, commonly known as a transfer film or tribofilm. The wear surfaces can exhibit different chemistry, microstructure, and crystallographic texture from those of the bulk coating due to surface chemical reactions with the surrounding environment. As a result, solid lubricant coatings that give extremely low friction and long wear life in one environment can fail to do so in a different environment. Most solid lubricants exhibit non-Amontonian friction behavior with friction coefficients decreasing with increasing contact stress. The main mechanism responsible for low friction is typically governed by interfacial sliding between the worn coating and the transfer film. Strategies are discussed for the design of novel coating architectures to adapt to varying environments.

Solid lubricants: a review

Structural superlubricity, a state of ultralow friction and wear between crystalline surfaces, is a fundamental phenomenon in modern tribology that defines a new approach to lubrication. Early measurements involved nanometre-scale contacts between layered materials, but recent experimental advances have extended its applicability to the micrometre scale. This is an important step towards practical utilization of structural superlubricity in future technological applications, such as durable nano- and micro-electromechanical devices, hard drives, mobile frictionless connectors, and mechanical bearings operating under extreme conditions. Here we provide an overview of the field, including its birth and main achievements, the current state of the art and the challenges to fulfilling its potential.

Structural superlubricity and ultralow friction across the length scales

This paper describes a study of the adhesion between clean surfaces, in a vacuum of 13nPa (10$^{-10}$ Torr), the influence of hardness, ductility, chemical bonding and adsorbed vapours. The geometry was that of crossed cylinders so that the contact region could be well localized and changed from experiment to experiment. In addition, electrical resistance measurements (in the case of conducting solids) could be used to follow the formation and breaking of contact. The joining load was very small (of order 1 mN) with the result that, even for the softest metals the deformation during loading was primarily elastic. The observed adhesion, however, is found to depend markedly on the ductility of the solid. With hard elastic solids the adhesion is very small indeed: much smaller than that predicted by theory and the results suggest that this may be due to fine-scale surface irregularities scarcely larger than atomic dimensions. With metals with a limited number of slip planes the adhesion is higher but appreciably less than would be expected from the area of the junctions formed. With metals such as gold and copper the adhesion is high and equal to the tensile strength of the interfacial junctions. Adhesion of soft metals to a hard solid such as titanium carbide can be high and when the surfaces are separated fragments of metal are found attached to the harder surface. With other hard solids such as sapphire or diamond the adhesion of copper is appreciably less, indicating that the bonding between copper and these covalent materials is intrinsically weak. Experiments at room temperature show that argon and hydrogen have no detectable effect on adhesion, whereas oxygen has. If one of the pair is copper, oxygen reduces the adhesion if the exposure is relatively heavy. With iron a minute exposure of oxygen reduces the adhesion to a very small value. It is suggested that with copper an adsorbed film is formed which is relatively ineffective. By contrast, on iron, minute iron oxide nuclei are formed which reduce the amount of metal-metal contact and also interpose an interfacial region of very limited ductility.

Adhesion between Clean Surfaces at Light Loads

SOLIDS with a layer-like structure have long been materials of interest in frictional studies In general they are found to possess a low coefficient of friction and have often been used as dry lubricant films We have recently studied the friction of graphite at very small loads (∼ 1 to 400 mg) The experiments were carried out in a scanning electron microscope so that surface deformation (to a resolution of about 500 A) could be observed as it occurred during sliding In this way it was possible to study directly the marked difference in sliding behaviour for sliding on the basal and on the edge planes The slider was generally a tungsten stylus, tip radius about 1 µm and was loaded on to the other, flat, surface using a delicate galvanometer movement (A similar device used in micro-hardness studies has been described by Gane1) The flat specimen could be traversed under the stylus thus keeping the stylus within the field of view of the microscope The specimen stage was carried on a stiff double spring and the frictional force was determined by measuring, with a differential capacitance device, the deflexion of the spring The sensitivity of the friction measurements was 005 mg and the sliding speed was 07 µm s−1

N. Gane

Papers

Adhesion between Clean Surfaces at Light Loads

Micro-friction of Graphite