Lubricant layer thickness decreasing with

Step 2 Loading The separation of slider and substrate (lubricant layer thickness) is a function of lubricant viscosity and pressure (normal load) on the contact. In addition, aging eiSects may become important since a liquid can migrate or be squeezed-out from high-pressure contacts with time (116, 117). As a result, areas of direct contact between slider and substrate and, simultaneously, adhesion forces acting between both surfaces can be reintroduced in case the lubricant layer thickness decreases with either contact pressure or time (Figure 6). 

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. 

In general, this hydrodynamic effect (lubrication effect) cannot be neglected. It is particularly important for thin double layers ( a 1) because the range of viscous coupling scales with the particle size a, whereas the double layer thickness scales with K. Similarly, the contribution of the hydrodynamic effect to the stability ratio is increased when the Hamaker constant A131 is decreased. In the absence of non-viscous interactions and external forces, the viscous coupling will even inhibit a particle collision. 

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. 

The review also deals with boundary lubrication in terms of decreasing lubricant film thickness from the electrically charged double layer (ca lOoA thick), the monomolecular layer of amphipathic molecules (20-40A thick) to the very thin adsorbed layers of small molecules that may be only 5 10A thick. Reference is also made to chemical modifications of the surface and to the behavior of polymers containing a small amount of dissolved additives. It is evident that, in the boundary lubrication of rubbers and polymers, certain aspects are fairly well understood. On the other hand our knowledge Is rather fragmentary. We need a more comprehensive picture of the physical and chemical reactions of lubricants with polymer surfaces, the properties of the surface films formed, and their mechanical and thermal stability as a function of speed, load and temperature. 

The starved regime is characterised by a lack of lubricant in the inlet of the contact. An incomplete oil film profile is imposed in the inlet. Experimentally, this oil profile corresponds to the oil profile found in the outlet of the previous pass. From a numerical point of view, it constitutes a boundary condition and the meniscus position is treated as a free boundary problem. 9 represents the ratio of the oil layer thickness Hou) and the total gap H) between the surfaces 9 = Hoii/H) see figure 2. In the inlet, the gap H between the surfaces decreases as X increases while the oil thickness Hou remains constant. Hence the fractional film thickness 9 increases steadily until a step occitts ( = 1). The lack of lubricant delays the pressme generation which starts at the point where the full film is formed (at the meniscus position). Figure 2 (obtained numerically with the model presented in appendix A), shows the variar tions of the different parameters the pressure P, the fractional film 9, and the gap H between the surfaces along the X axis, in the plane Y = 0. The shape of the oil layer in the same plane is shown in figure 3. 

Recently, we reported that sub nanometer-thick Ag films deposited on Si (111) surface showed good lubricity under ultrahigh vacuum (UHV) enviromnent [4, 5]. In these cases. Si wafer with a very smooth surface was selected as the substrate, and the counter surface was finely polished diamond, which is not reactive with Ag. As a result, adhesive force and displacement taking place in the Ag layer could be minimized, and the coefficient of fiiction decreased to the 0.01 range. The film thickness effect might be apparent in such an ultra-thin region ... 

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