Lubricating layer

Salmeron M B 1993 Use of the atomic force microscope to study mechanical properties of lubricant layers MRS Bulletin XVIII-5 20... 

Clays have plate-like molecules with charges on their surfaces (Chapter 16). The charges draw water into the clay as a thin lubricating layer between the plates. With the right moisture content, clays are plastie they can be moulded, extruded, turned or carved. But when they are dried, they have sufficient strength to be handled and stacked in kilns for firing. 

One of the major themes of boundary lubrication is to transfer the shear stress at the interface of direct solid contact to somewhere inside the lubricating layer, to achieve low friction and high wear resistance. In this sense, materials with low shear strength, such as liquid films, soft metals, and lamella solids, can be employed as candidate lubricants. 

Hydropolymer gel has been considered as a possible candidate for an artificial articular cartilage in artificial joints because it exhibits very low friction when it is in contact with a solid. The origin of such low friction is considered to be associated with the water absorbed in the gel [83-86], some of which is squeezed out from the gel under the load and serves as a lubricant layer between the gel and solid surface, resulting in hydrodynamic lubrication [87, 88]. Although the structural information about the interfacial water is important to understand the role of water for the low frictional properties of hydrogel in contact with a solid and the molecular structure of lubricants other than water at solid/solid interfaces have been investigated theoretically [89-91] and experimentally [92-98], no experimental investigations on water structure at gel/solid interfaces have been carried out due to the lack of an effective experimental technique. 

Humidity. There are two conceivable effects of humidity. Particularly with respect to agricultural products the moisture content can influence the hardness and elasticity. For example, Segler (1951) observed that dry peas are more sensitive to breakage than wet ones. Moreover, Wyszynski and Bridgwater (1993) have reported that a lubricating layer of moisture on the particle surface can reduce the particle degradation. 

With some concentrated suspensions of solid particles, particularly those in which the liquid has a relatively low viscosity, the suspension appears to slip at the pipe wall or at the solid surfaces of a viscometer. Slip occurs because the suspension is depleted of particles in the vicinity of the solid surface. In the case of concentrated suspensions, the main reason is probably that of physical exclusion if the suspension at the solid surface were to have the same spatial distribution of particles as that in the bulk, some particles would have to overlap the wall. As a result of the lower concentration of particles in the immediate vicinity of the wall, the effective viscosity of the suspension near the wall may be significantly lower than that of the bulk and consequently this wall layer may have an extremely high shear rate. If this happens, the bulk material appears to slip on this lubricating layer of low viscosity material. 

Example 11.4. McGuiggan et al. [492] measured the friction on mica surfaces coated with thin films of either perfluoropolyether (PFPE) or polydimethylsiloxane (PDMS) using three different methods The surface forces apparatus (radius of curvature of the contacting bodies R 1 cm) friction force microscopy with a sharp AFM tip (R 20 nm) and friction force microscopy with a colloidal probe (R 15 nm). In the surface force apparatus, friction coefficients of the two materials differed by a factor of 100 whereas for the AFM silicon nitride tip, the friction coefficient for both materials was the same. When the colloidal probe technique was used, the friction coefficients differed by a factor of 4. This can be explained by the fact that, in friction force experiments, the contact pressures are much higher. This leads to a complete penetration of the AFM tip through the lubrication layer, rendering the lubricants ineffective. In the case of the colloidal probe the contact pressure is reduced and the lubrication layer cannot be displaced completely. 

Depending on the thickness of the lubricating layer, we distinguish between two different lubrication regimes. In hydrodynamic lubrication the lubrication layer is thicker than the maximum height of the surface asperities resulting in a complete separation of the friction partners. In boundary lubrication the lubrication layer is typically only a few molecular layers thick and therefore thinner than the surface roughness. In many practical applications we are between the two extremes, which is referred to as mixed lubrication. 

For a fixed geometry, the friction force depends solely on the viscosity of the lubricant. We could try to decrease the viscosity of the lubricant to reduce friction. There is, however, a limit to this The lubrication film thickness must always be kept higher than the surfaces asperities. Otherwise the surfaces will come into direct contact, resulting in much higher friction. Therefore, it is common to use an oil with a viscosity that is just high enough to maintain a continuous lubrication layer. 

In gears and ball bearings, extremely high local pressures occur, at least for short periods of time for which the lubricant has no time to flow away from such a high-pressure region. Under these conditions, the above theory of hydrodynamic lubrication would predict a lubrication layer thickness that is smaller than the surface corrugation. However, experiments show that fluid film lubrication still holds under such conditions. To understand this phenomenon, we have to take two effects into account, which lead to elasto-hydrodynamic lubrication ... 

At low sliding velocities and high loads, the lubricating film is squeezed out of the gap. This leads to so-called boundary lubrication. Friction coefficients under these conditions are typically 100 times higher than under hydrodynamic lubrication conditions, but still substantially smaller than for dry friction under UHV conditions. This is due to the fact that the surfaces are still wetted by molecular layers of the lubricant, even under conditions where the local stress is high enough to deform the surface asperities. Under these conditions friction depends more on the chemical constitution of the lubrication layer than on its viscosity. 

Excessive load, reduced fluid viscosity, and low speed can cause a breakdown of the lubrication layer to the point of a metal-metal contact. Direct adhesive contact can lead to molecular interaction strong enough to tear material out of the surface. This process is also known as cold welding . Metal surfaces under ambient conditions are usually covered with oxide layers that prevent cold welding at moderate loads. 

Three theories were proposed to explain wall-slip (a) adhesive failure at the wall, (b) cohesive failure within the material as a result of disentanglement of chains in the bulk and chains absorbed on the wall, and (c) the creation of a lubricating surface layer at the wall either by a stress-induced transition, or by a lubricating additive. If the polymer contains low molecular weight components or slip-additives, their diffusion to the wall will create a thin lubricating layer at the wall, generating apparent slip. 

The last term takes account of slippage at the boundaries. Buckingham assumes that there is a very thin lubricating layer (of thickness 8 and fluidity m") between the plastic material an4 the wall. Examination of Buckingham s equation indicates that flow at pressures less than h increases directly as the pressure. At pressure ho shearing begins and the relation is shown by a curved line AB in Figure 91. 

At the interface between solids and organic solvents, however, specific attractions between the solid and the adsorbed substances may come into play and produce considerable adsorption. Some such cases are of importance in lubrication long-chain fatty acids, and some of their salts, are adsorbed from solution in hydrocarbon oils at the surface of many metals, and the result is a boundary lubricating layer (see Chap. VI). 

On the other hand, adhesion, especially for particles of mixed sizes, may lead to the formation of a coating of fine powders on the surface of coarse particles, facilitating the fluidization of these otherwise hard-to-fluidize particles by virtue of the acquisition of this lubricating layer of rollerbearing-like surface deposit. 

The yield stress of a foam depends to a considerable extent on the character of foam interaction with the tube walls or the cylindrical surface of the viscometer, used in the study of its rheological properties. At low flow rates and smooth tube walls the maximum shear stress of the foam layers contacting the wall can be less than the shear stress of the foam matrix (shear of bubble layers). Hence, the foam flow will occur as a movement of a continuous medium in a cylinder covered with a thin lubricating layer of thickness 2-10 pm [9,16], In this case t0 is ca. 1 Pa, that is, much less than its theoretical value. 

Increasing the foam expansion ratio from n = 18 to n - 200 leads to a decrease in the flow rate which can be explained with the reduction of the lubricating layer thickness, i.e. from 5=15 p.m at n = 18 to 8 = 4.8 at n = 200. Further increase in the expansion ratio (n = 300 to 1000) does not change the flow rate but the reproducibility of the results in this range is worsened. Such a course of the v(n) and S(n) dependences is related to the intensive foam collapse during drainage. This is confirmed both by visual observations and expansion ratio measurements, performed at the tube outlet. 

Slip with additives containing mixture. This shows the role of the additives, the wall material and the surface state of the wall. It has been studied by various authors [6-8] who have shown that the additive migrates towards the wall, forming a thin lubricating film and/or a thin film of low viscosity between the wall and the polymer. The polymer may then slip on this fine lubricating layer. 

In some cases the presence of slip is fairly obvious, as are its causes. For example, when an aqueous foam is sheared between smooth surfaces, the water in the foam can easily form a lubricating layer at the wall, leaving the bulk of the foam less sheared than intended (Yoshimura and Prud homme 1988 Khan et al. 1988). Gelled colloidal suspensions are elastic materials that contain solvents capable of lubricating rheometer tool surfaces, and slip is a problem (Buscall et al. 1993 Persello et al. 1994). In these and other cases, slip can be counteracted in a number of ways, for example by using roughened rheometer surfaces (Khan et al. 1988 Buscall et al. 1993),... 

---