Lubricants friction coefficient

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. 

Steel Counterface. Wear and friction against steel, for all composites except LlWl, indicate excellent lubrication. Friction coefficients are near the minimum, and wear rates near the maximum, of the "low friction, low wear" regime delineated for a wide variety of polyimides by Fusaro (y. 

Lubricant number Structure of lubricant Friction coefficient Melting point /°C... 

If the surfaces are separated only by adsorbed molecules or oxide layers, boundary lubrication is acting. In systems where the thickness of the lubrication layer is of the order of the surface roughness and intermittent contact of the surfaces occurs, we talk about mixed lubrication. Friction coefficients are typically 0.01—O.Z... 

Titanium disulfide has been proposed as a soHd lubricant. The coefficient of friction between steel surfaces is 0.3, compared to only 0.2 for molybdenum disulfide. However, because it does not adhere strongly to metal surfaces, TiS2 is generally less effective than molybdenum sulfide. 

Boundary lubrication is perhaps best defined as the lubrication of surfaces by fluid films so thin that the friction coefficient is affected by both the type of lubricant and the nature of the surface, and is largely independent of viscosity. A fluid lubricant introduced between two surfaces may spread to a microscopically thin film that reduces the sliding friction between the surfaces. The peaks of the high spots may touch, but interlocking occurs only to a limited extent and frictional resistance will be relatively low. 

It has been a dream for a tribologiest to create a motion with a super low friction or even no friction between two contact surfaces. In order to reduce friction, great efforts have been made to seek materials that can exhibit lower friction coefficients. It is well known that friction coefficients of high quality lubricants, e.g., polytetrafluoroethylene (PTFE), graphite, molybdenum disulphide (M0S2), etc., are hardly reduced below a limit of 0,01,... 

Fig. 18—Friction coefficient with different substrates [48]. Lubrication paraffin liquid Load 2 N.

Fig. 19—Friction coefficient and load on different substrates [48]. Velocity 15.6 mm/s Lubricant paraffin liquid.

Figure 40 shows the relationship between the friction coefficient and the sliding speed for PEGs with different concentrations of UDP. It is clear that the friction coefficient of all lubricants is about 0.22 at the speed of about 0.1 mm/s. With increasing sliding speed, it drops sharply in the speed range from 0.1 mm/s to 15 mm/s, and then maintains about 0.03 when the speed is more than 15 mm/s. 

In the past decade, effects of an EEF on the properties of lubrication and wear have attracted significant attention. Many experimental results indicate that the friction coefficient changes with the intensity of the EEF on tribo-pairs. These phenomena are thought to be that the EEF can enhance the electrochemical reaction between lubricants and the surfaces of tribo-pairs, change the tropism of polar lubricant molecules, or help the formation of ordered lubricant molecular layers [51,73-77]. An instrument for measuring lubricant film thickness with a technique of the relative optical interference intensity (ROII) has been developed by Luo et al. [4,48,51,78] to capture such real-time interference fringes and to study the phenomenon when an EEF is applied, which is helpful to the understanding of the mechanism of thin film lubrication under the action of the EEF. 

Soft metals such as In, Ag, Sn, Pb, and Au can lead to reasonably low friction coefficients, when used as solid lubricants, due to their low shear strength. The metals were generally applied as thin films prepared by the vacuum deposition process. Especially, in applications to the high temperature conditions where liquid lubricants fail due to the evaporation, the thin films of soft metals can provide effective protection to the surfaces in sliding. 

The process of transition from hydrodynamic to boundary lubrication can be described qualitatively by plotting the measured friction coefficients against film thickness, which depends on the operational conditions, such as load, sliding velocity and lubricant viscosity. A typical diagram known as the "Stribeck Curve is schematically shown in Fig. 27, in which the friction coefficients are given as a function of, ... 

Because PTFE has low adhesion, high lubricity, and low friction coefficient [14], it is not difficult to understand PTFE film has a lower friction coefficient factor and higher wear rate than PTFE/Si3N4 multilayers under the current experi-... 

Practically, aU data of friction measurements on wet tracks in the speed range of hydrodynamic lubrication exist as tire skid measurements. Figure 26.10 shows the results of a braking test on wet, finely structured concrete using a smooth tire and measuring the friction coefficient as function of... 

FIGURE 26.24 Braking force as function of slip using the brush model with (dry) a constant friction coefficient and (wet) a friction coefficient which depends on the slip speed because of wet lubrication. Braking stiffness 90,000 N, dry friction coefficient 1.2, load 4500 N, speed at the onset of braking 40 km/h, critical speed 210 km/h. 

Metallic soaps such as zinc stearate or calcium stearate are used as lubricant, which reduces the friction coefficient of the surface of the thermosensitive layer resulting in smooth running on recording machines. 

Liquid-liquid solvent extraction, 21 399 Liquid lithium, 15 131 Liquid low density polyethylene, 20 205 Liquid lubricants, for extreme environments, 15 256 Liquid lubricated system, coefficient of friction in, 15 209 Liquid magnesium, 15 336 Liquid manometers, 20 646-647 Liquid MDI, 25 462. See also MDI [4,4 -methylenebis(phenyl isocyanate)] Liquid melamine resins, 15 773 Liquid membrane extraction, 10 766 Liquid membranes, 15 800, 814-815 supported, 16 28... 

Result A friction coefficient less than or equal to 0.175 indicates the fuel passes the lubricity evaluation. A friction coefficient greater than 0.175 indicates the fuel fails the lubricity evaluation. Wear scar diameter on the test ball is determined. A wear scar diameter of 440pm or less is considered acceptable for distillate fuel. 

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