Boundary lubrication regime

Thin him lubrication is essentially a transition lubrication regime between elastohydrodynamic lubrication and boundary lubrication regimes. Papers devoted to the investigations of this lubrication regime are not enough for engineering needs. In this section, a function to describe the viscosity distribution is proposed in order to attain predictive results and to describe the characteristics of TFL in the viewpoint of engineering. 

A qualitative model for lubrication of the condensed alcohol layer is not fully developed yet. We present here the best qualitative model based on the literature. As the applied pressure is very large and the scanning speed is very low in AFM, one can rule out the hydrodynamic lubrication even in the presence of condensed alcohol layer on the substrate. The AFM tip and substrate interface must be in the boundary lubrication regime. Figs. 7-10 show that the adhesion force reduction upon alcohol adsorption is accompanied by reduction of the friction force. Part of the adhesion force reduction is because of the surface tension decrease of the water layer on the substrate. When alcohol is dissolved in water, the surface tension decreases significantly because of segregation of alcohol molecules to the liquid-air interface. ... 

A complementary study of steel/steel contacts (100Cr6) [47] used an oscillation wear and friction tester to screen ionic liquids in the boundary lubrication regime at 30, 100 and 170°C at 200 N load. Results are summarized as follows (i) some promising ionic liquids gave lower friction and wear at all temperatures, compared to reference lubricants, (ii) chemical structures of ionic liquids have a high impact on their physico-chemical as well as on tribological properties and (iii) careful selection of both cation and anion, as a tailor-made ionic liquid, enables adaptation of ionic liquid structures to specific applications. 

Conventionally, adhesive wear occurs when two interacting surfaces are not sufficiently lubricated, which results in the adhesive transfer or removal of near-surface material. Surface adhesion is dependent on the nature of the contacting surface. Wear material transfer in this region has been called "solid-phase welding," which occurs at asperity contacts.1 A schematic of adhesive wear is shown in Figure 5.1.2 Adhesive wear is related to the materials and slurries used. The amount of slurry can shift contact from the boundary lubrication regime to the EHD/micro-EHD regime where surfaces are more separated. 

The quantized friction would be a direct result of the multiple discrete values of the adhesion-controlled contribution to the total friction, and could be a common feature for fluids able to form thin film-layered structures. The results described by Smith et al. could account for discontinuities in friction across thin films as a function of the number of layers, particularly under boundary lubrication regimes with varying liquid film thickness across the contact zone between rough surfaces. 

Wear is defined as the loss of material from a surface as the result of relative motion. In this chapter, the wear processes in polymer implants are discussed. Polymers are used in a wide variety of implants in the human body such as joint replacement implants, pacemakers, catheters and heart valves. Wear of polymer implants is almost exclusive to joint replacement implants, such as those used to replace the hip or knee. These implants involve the articulation of a metal or ceramic against a polymer. Typically these implants operate with a mixed or boundary lubrication regime and, therefore, there is contact between the bearing surfaces that can lead to the generation of wear debris. The chapter is divided into sections that cover implants, wear processes, polymers used in implants, the effect of wear debris on the body and, finally, likely future trends. 

Finally, in the boundary lubrication regime the average thickness of the liquid film is significantly smaller than the average surface roughness. 

A large number of the asperity contacts are formed and the lubricant loses most of its efficiency. The boundary lubrication regime is favored by a high normal foree, a low velocity and a low viscosity of the lubiieant. Under these conditions, high friction often leads to failure of the eontaet by seizure. 

The effect of polymer brushes in different lubrication regimes is seen in Fig. 4a and b, in which the Stribeck curves obtained from pin-on-disk and MTM measurements are plotted, respectively. The coefficient of friction is plotted against speed multiplied by viscosity for all HEPES-glycerol mixtures both in the presence and the absence of the polymer. As expected, the effect of polymer in reducing the friction is predominantly seen in the boundary-lubrication regime. The effect of polymer on the friction is also extended to the mixed-lubrication regime... 

Coefficient of friction versus load. The COF values measured in this work are high at all applied loads. This fact substantiates the assumption that the experiments have been carried out in the boundary lubrication regime [42]. Despite the high experimental error in the COF at low loads, a slight decrease in COF can be seen. 

Morina, A., Green, J.H., Neville, A., Priest, M. Surface and tribological characteristics of Uibofilms formed in the boundary lubrication regime with application to internal combustion engines. Tribol. Lett. 15, 443-452 (2003)... 

Friction and wear were measured using the Penn State sequential four-ball test. The sequential test was developed to evaluate hydraulic fluids [4-6]. This test operates in the boundary lubrication regime. Test variables included time, load, speed, and temperature. Both torque (friction) and wear were measured. The sequential test con-... 

Lubrication. Friction in the presence of a lubricant, which is commonly used to mitigate high fiiction contacts, is a velocity dependent phenomenon that can be illustrated with the so-called. Stribeck curve (7, 60, 61). Tbee regimes of lubricated fiiction are identified (1) boundary, (2) nuxed, and (3) hydrodynamic lubrication. Frictional problems, such as wear or firiction-force fluctuations, so-called stick-slip motion, are usually associated with the mixed and boundary lubrication regime. Either velocity of the moving counterfaces is too low, pressure on the fiiction contact is too... 

A molecularly thin lubricant layer which effectively prevents direct contact between slider and substrate is called a houndary luhricanf (61, 122, 123). Friction in the presence of a boundary lubricant (direct contact in slider/substrate interface prevented by boundary lubricant) should therefore not be confused with the above defined boundary lubrication regime (partial direct contact in slider/substrate interface). In fact, the boundary lubrication regime is either prevented or moved into a regime of higher normal load or lower velocity in the presence of a boundary lubricant. 

Yet another theory has emerged recently from the field of nanotribology. Surface force apparatus studies, combined with molecular dynamics simulations, of simplified model systems, such as molecularly flat mica separated by a few molecules thick lubricant layers, have identified a solid-melt transition as the cause for stick-slip motion to occur for such confined liquids (18, 106, 111-113, 144-149). A similarly confined liquid can be found in macroscopic friction systems in the boundary lubrication regime wherein thin lubricant layers are trapped between surface asperities in very close proximity. 

Stick-slip motion observed with syringes is associated with the nuxed and boundary lubrication regime. This is in agreement with current models of the occurrence of stick-slip motion for lubricated sliding systems (see Physics of Friction section). Possible causes for this phenomenon are discussed in more detail in the following. 

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