Smooth surfaces friction

In addition to chain molecules confined between two smooth surfaces, frictional forces were studied for grafted chains surrounded by solvent molecules [21] and for chain molecules sheared by topographically nonuniform solid surfaces [33], Tip-based simulations were also carried out to study frictional properties of confined thin films [88-92]. These topics are not covered in this review. 

Static friction, unlike kinetic friction, can be related to the equilibrium adhesion force or energy. Generally, for two molecularly smooth surfaces, friction can be described by Eq. (2) ... 

Considering the foregoing remarks, it is convenient to distinguish between the friction on the two types of surfaces by a suitable choice of names. Without going into the semantics, and in view of the fact that failure occurs at the surface of a nominally smooth solid, it seems appropriate to use "adhesive friction" to denote the friction of nominally smooth surfaces. Friction of sharp textured surfaces may be described as abrasive friction. The scope of the detailed discussion hereafter will be limited to the topic of adhesive friction. 

Flow Along Smooth Surfaces. When the flow is entirely parallel to a smooth surface, eg, in a pipe far from the entrance, only the shear stresses contribute to the drag the normal stresses are directed perpendicular to the flow (see Piping systems). The shear stress is usually expressed in terms of a dimensionless friction factor ... 

An air stream at approximately atmospheric temperature and pressure and containing a low concentration of carbon disulphide vapour is flowing at 38 m/s through a series of 50 mm diameter tubes. The inside of the tubes is covered with a thin film of liquid and both heat and mass transfer are taking place between the gas stream and the liquid film. The film heat transfer coefficient is found to be 100 W/mzK. Using a pipe friction chan and assuming the tubes to behave as smooth surfaces, calculate ... 

DLC coatings are already in production in several areas (optical and IR windows) and appear particularly well-suited for abrasion and wear applications due to their high hardness and low coefficient of friction. They have an extremely smooth surface and can be deposited with little restriction of geometry and size (as opposed to CVD diamond). These are important advantages and DLC coatings will compete actively with existing hard coatings, such as titanium carbide, titanium nitride, and other thin film... 

Homola, A. M., Israelachvili, J. N., McGuiggan, P. M., and Gee, M., "Fundamental Experimental Studies in Tribology The Transition from Tnterfacial Friction of Undamaged Molecularly Smooth Surfaces to Normal Friction with Wear, Wear, Vol. 136, No. 1,1990, pp. 65-83. 

Israelachvili, J. N., Adhesion, Friction and Lubrication of Molecularly Smooth Surfaces, Fundamentals of Friction, I. L. Singer and H. M. PoUock, Eds., Kluwer Academic Publishers, 1991,PP-351-385. 

Therefore, for a rough surface, although it is smooth macro-scopically, the friction coefficient is greatly affected by its surface morphology. For a smooth surface on atomic scale, 0 can be neglected and the friction coefficient is only related to the twist angle of the cantilever. 

The friction coefficient is defined as the tangential force acting on a sliding body to the ground reaction force. For rubbers this is a function of the ground pressure. Its dependence has been discussed sufficiently in the literature where it was shown that this is important for soft rubbers on smooth surfaces [2,3], but is of little influence for tire compounds on roads which are always sufficiently rough for the load dependence to be small if not completely absent [4,5]. 

This is not surprising since at a given speed the coarseness of a track (the average spacing of the asperities) influences the friction only on a logarithmic scale. Also the observed dependence of the friction coefficient on load of soft mbber compounds on smooth surfaces disappears for harder black or silica-filled treads compounds on rough surfaces. 

Figure 21 Friction coefficient for differently oriented Ni(100)/Ni(100) interfaces. Rough surfaces have a 0.8 A rms variation in roughness added to the atomically smooth surfaces. Reproduced with permission from Ref. 85.

In any particular wear situation several mechanisms are usually involved but one may predominate. Abrasive wear requires hard, sharp cutting edges and high friction. Fatigue abrasion occurs with rough but blunt surfaces and does not need high friction. Adhesion wear is less common but can occur on smooth surfaces. 

B12), and by Gazley (Gl), using air-water. It was shown that when a gas-liquid interface is smooth, energy transfer from gas to liquid is entirely dissipated in surface friction, but when a liquid surface is hydrodynamically rough, energy transfer from gas to liquid may amount to twice that to be expected on the basis of interfacial friction (Gl). Presumably this excess energy is dissipated in the formation and maintenance of surface waves. 

A pultrusion die is usually machined from tool steel and typically has a length of 600-1,500 mm. With the exception of the tapered entrance, the die normally has a constant-crosssection cavity with extremely smooth surfaces that are chrome-plated to lower friction and decrease wear. The die usually is sectioned to facilitate machining, inspection, cleaning, and so on. Most of the die is heated, and typically employs two or more independently controlled temperature zones, whereas the final section of the die may need active cooling. Because excess resin is squeezed out of the reinforcement at the tapered entrance to the die, also this section may require active cooling to prevent premature gelation of the resin (i.e., before it reaches the constant-cross-section cavity). 

The apparent area of contact between two surfaces is much larger than the actual area over which they touch, even if the surfaces appear smooth. The frictional force is proportional to the real contact area, so anything that changes the real contact area will change the force measured. 

Friction welding is the most common solid-state process. One type, direct-drive friction welding, is accomplished by spinning one piece rapidly against a second, stationary piece to produce frictional heating. When the metal is at a forging temperature below the solidus, rotation is stopped and axial force is applied. Heated metal, together with oxide and surface contamination, is squeezed out of the interface between the two pieces in the form of a metal flash, which can be trimmed to form a smooth surface. 

Conceptually, predecessors of the scanning force microscope are the surface force apparatus (SFA) [73,74] and the stylus profilometer [75,76]. The SFA enables measurement of normal and friction forces between molecularly smooth surfaces as small as 1 nN as a function of distance with a resolution of 0.1 nm. In addition to the local force measurement, the profilometer provides a topographic map of the surface by scanning the surface with a sharp probe. However, the profilometer is not suitable for structure characterisation because of the large radius of the tip (about 1 pm) and the low sensitivity of the force sensor (in the range of 1(T2 to 1(T5 N). 

Based on the measurements the following was established. The velocity of cylinder is independent of the friction coefficient (Figs. 4 and 5). This means that the smooth surfaced wooden or aluminum cylinders move with the same speed as the rough sand-paper covered one. Higher average velocities with 0.2 cm per movement was found in sequence 2 (where the middle examined cylinder moves upward), than in sequence 1 (where a cylinder in the middle moves down). 

In the previous sections, only the normal contact of two elastic spheres with perfect smooth surfaces (i.e., no tangential force) is considered. However, for oblique contact between two frictional spheres, tangential forces are encountered, and, consequently,... 

The friction coefficient is defined as the ratio of the tangential friction force, resisting relative motion of the surfaces, to the normal load pressing the surfaces together. In the case of high adhesion of wet smooth surfaces, the normal load shall be considered as a sum of the externally applied normal force and the internal adhesion (stiction) between surfaces. For simplification, the normal load is usually considered equal to the externally applied normal force. 

We now describe an approach for estimating the reaction temperature. Viewed at a microscopic level, this is a very complex stochastic thermal problem involving the frictional interaction of a relatively smooth surface with a rough one in the presence of a cooling lubricant that contains abrasive particles. Rather than treat this problem fully, we try to capture the most important features while neglecting other potentially interesting ones. The end result will be a simple estimate of the reaction temperature that can be used in a compact model. 

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