Changes in friction coefficient

The coefficient of friction determinations have been made at specific times into the sliding wear tests. These have an associated uncertainty of approximately 10%, predominantly from the (unsteady) change in friction coefficient with time as the sample surface is modified by wear and vibration/noise associated with the friction monitoring transducer. 

Figure 2. Change in friction coefficient with the number of laps at 10 mm/s and 1 N for the lubricating solutions (a)...

Figure 18.10 presents examples of dependences of friction coefficient changes as a function of time for water and 0.01% aqueous solutions of SML/ESMIS mixtures. The highest friction coefficient values were observed for the tests in which pure water was the lubricant. Over the test duration, the coefficient of friction increased from a value of 0.2 to about 0.3. Over the course of measurement, in the case of all the solutions containing the SML/ESMIS mixtures, only a slight reduction in resistance to motion occurred, and no changes in friction coefficient values were observed. However, the compositions analyzed differed in their ability to reduce p. A set of mean friction coefficient values from the whole 900-s test can be seen in fig. 18.11. 

Figure 8.16 Change in frictional coefficients as a function of load and lubrication conditions (sliding speed 14 m/min) of Toray Resin Company Amilan CM1017 PA 6 plastics (MoS is molybdenum disulfide) [1],...

FIGURE 31.8 Change in the coefficient of friction of modified dicumyl peroxide/ethylene-propylene-diene monomer (DCPD/EPDM) with the concentration of trimethylolpropane triacrylate (TMPTA) at a fixed irradiation dose of 100 kGy. (G) Surface modified with 100 kGy, (A) Bulk modified with 100 kGy dose, ( ) Control EPDM rubber. (Erom Sen Majumder, P. and Bhowmick, A.K., Wear, 221, 15, 1998. With permission.)... 

During the course of a cyclic test, some changes in the coefficient of friction may arise as a consequence, for example, of the development of physicochemical interactions between the contacting surfaces. For glass in contact with glassy polymers such as poly(methylmethacrylate) or epoxies, an in-... 

The changes in friction and wear behaviour with change in temperature in air are presumably determined by oxidation. Figure 14.5 shows that at low temperatures both molybdenum disulphide and tungsten disulphide have similar coefficients of friction of about 0.07 to 0.08, but a marked increase in friction occurs at 400°C with molybdenum disulphide and 600"C with tungsten disulphide. The corresponding temperatures for increasing friction reported by Tsuya were 300 C and 450°C. 

FoiTn the physical point of view, roughening of fibre surface as seen by atomic force microscopy is responsible for changes in the coefficient of friction, top cohesion, spinnability, yam strength, etc., as well as for increase in felting resistance of wool. From the chemical point of view, the oxidation of the fibre surface and interaction with polymeric materials are the main factors responsible for improvements in various properties of plasma treated materials. 

The effect of phase structure on the coefficient of friction was studied for TRS 10-410 + isobutanol + salt + water system. The coefficient of friction was measured on aluminum-aluminum metal surfaces using the surfactant formulation at several salt concentrations. A significant change in the coefficient of friction was observed as the salt concentration was increased in the system because of the change from isotropic to anisotropic structure of the surfactant system. 

The presence of dispersed fillers in the polymer material in low amounts may intensify electrization, increase the residual charge and change the friction coefficient. Introduction of the filler in the electret state exerts a still stronger effect on polymer electrization on frictional interaction with metals. Depending on the direction of the field intensity vector formed by the filler particles, the field generated by triboelectrization can be attenuated or intensified. This means that the principle of the electret-triboelectrization superposition is realized [49], which can be used to regulate the parameters of frictional interactions. Thus, by the introduction of the electret filler, e.g. mechanically activated F-3 powder, it is possible to decrease the friction force (Fig. 4.9). 

If the actual solids conveying performance is as sensitive to the coefficient of friction as the theory indicates, small changes in the actual coefficient of friction can have a substantial effect on the entire extrusion process. This is particularly true for low values of the coefficient of friction against the barrel. The high sensitivity to the coefficient of friction will tend to result in unstable extruder performance. At high values of the barrel coefficient of friction the extruder performance will be less affected by changes in friction and the extruder will be more stable under these conditions. This explains why grooved feed extruders tend to be more stable than smooth bore extruders. 

Figure 10.6 illustrates how the presence of adsorbed oxygen changes the friction coefficient between two tungsten surfaces [1], In the absence of oxygen, in ultrahigh vacuum, the friction coefficient is close to/= 3. When oxygen is admitted, it decreases to about 1.3. 

The tests were carried out at a constant load of 2 kN, which is relatively small and corresponds to a moderate increase in the friction force moment (fig. 17.9). Examples of changes in the coefficient of friction as a function of time are given in fig. 17.16. 

FIGURE 17.17 Changes in the coefficient of friction of 1 wt% oxyethylated alcohol soln-tions as a function of (a) oxyethylation degree and (b) length and nature of the alkyl chain at a constant load of 2 kN. (Data obtained using tester T02.)... 

Changes in the coefficient of friction as a function of time observed for 1% aqueous solutions of SML/ESMIS mixtures are shown in fig. 18.20. Out of the lubricant compositions tested, the highest resistance to motion was observed for water. Its lubricating properties were so poor that, just a few seconds after starting the tester under the load of 3.0 kN, welding of the balls took place. Therefore, tests with pure water were carried out at a load of 2.0 kN. Under these conditions, the coefficient of friction was relatively high, fluctuating around 0.3. 

The hysteresis phenomenon was also observed by Luzhnov [133] when studying the static friction of soot powder (see Fig. III.4). In these experiments the upper branch of the hysteresis loop characterizes the change in the coefficient of static friction with increasing relative humidity and the lower with falling humidity, In the present case the failure of the curves to coincide is apparently associated with the replacement of dry by semi-liquid or liquid friction, which is always accompanied by a reduction in the friction coefficient. 

In this section, the friction and wear of PTFE-based composites with different nano-scaled fillers are explicitly discussed. The friction coefficients of PTFE-based composites with different nanoscaled fillers differ with each other because of the dissimilar physical and chemical properties of different types of nanofiUers. However, despite the different nanofiller type and content, the variation of friction coefficient between PTFE-based composites and pure PTFE is evident under different experimental conditions. On the one hand, this is caused by the very low friction coefficient of pure PTFE so that a further decrease in friction coefficient becomes a formidable issue. On the other hand, due to the material nature of the nanofillers—for instance the lubrication property of nano-EG significantly lowers the friction coefficient of PTFE/nano-EG composites while friction coefficient of PTFE/nanoserpentine composites barely changes, which is greatly related to the material nature of the nanofillers. Conversely, a dramatic reduction in wear rate is observed in all PTFE-based composites. It is believed that the strong interfacial interaction, high shear strength, enhanced load capacity, and extra lubrication effect of PTFE-based composites with nanoscaled fillers are responsible for the improvement of wear resistance. However, the specific enhancement mechanism remains unsolved. 

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