Load-dependence of friction

Figure 9, (a) Schematic illustration of the locus of energy dissipation in the case of interfacial dissipation, which scales as the 2/3 power of load, and "deformation dissipation, which scales linearly with load, (b) Load dependence of friction on a thin PMMA film. Plots of two-term (solid line) and one-term (dashed line) models containing 2/3-power and linear load dependences as indicated. 

Figure 8.17 Load dependence of frictional coefficient of Toray Resin Company Amiian PA 6 with additives [1],...

Fig. 7—Dependence of friction force signal of Au film and Si wafer on load.

Fig. 10—Dependence of friction force of PTFE, Si3N4, and PTFE/Si3N4 film on load.

Fig. 14—Dependence of friction force signal of a triacetic acid L-B film on load.

The discussion begins below with an overview of proposed energy dissipation mechanisms that lead to friction. This is followed by brief discussions of phenomenological friction laws that describe the dependence of friction upon normal load and sliding velocity. The dependence of friction on the symmetry of the surfaces that are in contact is discussed later. 

Figure 11.5 Dependence of friction on load for a single microcontact. The friction force between a silica sphere of 5 //in diameter and an oxidized silicon wafer is shown (filled symbols). Different symbols correspond to different silica particles. The solid line is a fitted friction force using a constant shear strength and the JKR model to calculate the true contact area (based on Eq. (6.68)). Results obtained with five different silanized particles (using hexamethylsililazane) on silanized silica are shown as open symbols. Redrawn after Ref. [467].

Figure 3. Dependence of friction factor on applied loading 1. a clay solution 2. a clay solution - SDBUR (1.0mass.%) 3. aclay solution - SDISUR (1.0 mass.%) - piperylenc bis- tetrasullide (0.1 mass.%) 4. aclay solution - SDBUR (1.0 mass.%) - piperylene bis- tetrasulfide in solvent (0.1 mass.%).

The linear dependence of friction on load established in solid friction, F = fiW, is explained in terms of the yielding mechanism i.e., the solid surface is not molecu-larly flat and the real contact area between two surfaces increases with an increase of load due to yielding. Thus, the friction has no dependence on the apparent contact area of the two solid surfaces, and Amonton s law holds [38]. 

Gong JP, Iwasaki Y, Osada Y (2000) Friction of gels. 5. negative load dependence of polysaccharide gels. J Phys Chem B 104 3423-3428... 

The schanatic of the dependence of friction torque on the applied load is presented in fig. 18.25. The following denotations were adopted ... 

Dependences of friction torque on time are presented in fig. 18.26, and the mean values of scuffing load, wear-scar diameters of the balls, and wear-scar profiles obtained after the tests in the presence of % aqueous solutions of SML/ESMIS mixtures are shown in figs. 18.27-18.29. 

FIGURE 18.25 Simplified dependence of friction torque on load. 

FIGURE 18.26 Examples of dependences of friction torque on time (loading) for water and 1% aqueous solutions of SML/ESMIS mixtures (four-ball tester, rotational speed of the spindle 500 rpm, load increase rate 409 N/s). 

Aqueous solutions (1%) of SML/ESMIS mixtures exhibited entirely different antiseizure properties than water. For SML/ESMIS mixtures, the obtained dependences of friction torque on time (a change in time is equivalent to increasing load) had a similar course. No sudden increase in friction torque was observed within the first seconds of the test. This shows that a relatively stable adsorbed film that effectively reduced the motion resistance was produced on the mating elements. Scuffing load was estimated from the dependences obtained (fig. 18.26). The results obtained are presented in fig. 18.27. The highest values were found for 1% aqueous solutions of mixtures with the 5 5, 3 7, and 1 9 ratios. These equaled, respectively, 2500 N, 2350 N, and 2300 N. Lower P values were obtained for the other compositions. For a solution containing the 7 3 SML/ESMIS mixture, the P, value was 2000 N, while for the ESMIS solution (0 10) the value was 1900 N. 

In the case of aqueous solutions of SML/ESMIS mixtures, the dependence of friction torque on load is different. After exceeding the load at which the lubricant film breaks (P,), relatively intense wear of the balls occurs. When the elements are not protected by a lubricant, adhesion bonding and seizure may easily occur. As the results obtained indicate (fig. 18.26), this is prevented by the action of aqueous solutions of the mixtures. After a short time, there is another increase in friction torque and, as one might expect, the lubricant film is reconstructed. 

Figure l Load dependence of the friction force at a constant sliding velocity of lOpm/s (0 loading, m unloading)... 

Figure 3 shows the total load dependence of the friction force measured for modulation amplitudes of 50nm and lOOnm. The contact location was arbitrarily chosen on the same surface. Both curves are described by the same Amontons law. The friction coefficient defined by the slope of the linear fit is )li=0.087 0.001. When plotted as the friction force versus the total load, the intercept is zero. It is important at this point to specify that the error associated to the friction coefficient arises from the fitting analysis of our data, which therefore, determines the precision of the experiment and not the overall acuracy of the experiment. Indeed, the main source of uncertainty in our measurements originates in the precision in the cantilever metrics measured by optical microscopy and SEM which is of the order of 3% to 5%. Some other sources (19,28), like the position of the laser spot on the backside of the cantilever affects the absolute accuracy of the friction measurements to an extend that is difficult to evaluate. We expect the overall accuracy on the friction measurement to be less than 60%(25). Nevertheless, since the crucial experimental conditions were optimized and kept constant from an experiment to the other, the comparison remains valid. 

Figure 3. Load dependence of the friction force measuredfor an oscillatory motion at constant speeds (lateral displacement amplitude 50nm and lOOnm at frequency 213Hz)...

Figure 6 illustrates the load dependance of the friction force in the steady state sliding regime when the tip describes an oscillatory motion at constant velocity. The friction coefficient defined by the slope is in agreement the friction coefficient obtained from figure 5. 

Figure 6 Load dependence of the friction force force at constant sliding velocity ( 33Apm/s, M 133.8pm/s)...

Fig. A. Load dependence of the coefficient of static friction Pg observed between the steel hemisphere, 0.24 cm in radius and the plate of polymers.

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