Friction coefficient differences

Example 11.4. McGuiggan et al. [492] measured the friction on mica surfaces coated with thin films of either perfluoropolyether (PFPE) or polydimethylsiloxane (PDMS) using three different methods The surface forces apparatus (radius of curvature of the contacting bodies R 1 cm) friction force microscopy with a sharp AFM tip (R 20 nm) and friction force microscopy with a colloidal probe (R 15 nm). In the surface force apparatus, friction coefficients of the two materials differed by a factor of 100 whereas for the AFM silicon nitride tip, the friction coefficient for both materials was the same. When the colloidal probe technique was used, the friction coefficients differed by a factor of 4. This can be explained by the fact that, in friction force experiments, the contact pressures are much higher. This leads to a complete penetration of the AFM tip through the lubrication layer, rendering the lubricants ineffective. In the case of the colloidal probe the contact pressure is reduced and the lubrication layer cannot be displaced completely. 

In a free solution, the electrophoretic mobility (i.e., peiec, the particle velocity per unit applied electric field) is a function of the net charge, the hydrodynamic drag on a molecule, and the properties of the solutions (viscosity present ions—their concentration and mobility). It can be expressed as the ratio of its electric charge Z (Z = q-e, with e the charge if an electron and q the valance) to its electrophoretic friction coefficient. Different predictive models have been demonstrated involving the size, flexibility, and permeability of the molecules or particles. Henry s theoretical model of pdcc for colloids (Henry, 1931) can be combined with the Debye-Hiickel theory predicting a linear relation between mobility and the charge Z ... 

The [[ ]] quantities in these last two equations have the same arguments as the distribution functions with which they are associated. Note that Eqs. (10.5), (10.6), and (10.7) are valid for any species in a multicomponent mixture involving flexible macromolecules. Equation (10.7) is a generalization of the usual equation of continuity for P (r , Q , t) for Rouse chains (DPL, Eqs. (15.1-5)), in that It IS applicable to models of any connectivity and with bead masses and friction coefficients different from one another. One often sees the equation written without the momentum-space averages for the velocities in such instances the equation contains an inappropriate mixture of statistical and deterministic quantities. 

The friction coefficients of asbestos fibers are also different for chrysotile and amphiboles (when measured against the same material). Compared to glass fibers, the friction coefficients decrease in the order chrysotile, amphiboles, glass fibers. 

In a simulation it is not convenient to work with fluctuating time intervals. The real-variable formulation is therefore recommended. Hoover [26] showed that the equations derived by Nose can be further simplified. He derived a slightly different set of equations that dispense with the time-scaling parameter s. To simplify the equations, we can introduce the thermodynamic friction coefficient, = pJQ. The equations of motion then become... 

Friction coefficients will vary for a particular material from the value just as motion starts to the value it attains in motion. The coefficient depends on the surface of the material, whether rough or smooth, as well as the composition of the material. Frequently the surface of a particular plastics will exhibit significantly different friction characteristics from that of a cut surface of the same smoothness. These variations and others that are reviewed make it necessary to do careful testing for an application which relies on the friction characteristics of plastics. Once the friction characteristics are defined, however, they are stable for a particular material fabricated in a stated manner. 

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.

Fig. 38—Friction coefficient at different sliding speed [60]. Load 2 N, Concentration of UDP 0.3 %, Base oil PEG.

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. 

The friction coefficient, n, is assigned with different values in different friction conditions. Fj is the relative sliding velocity of two contacting bodies. 

Table 2 lists the results of the friction coefficient derived by different researchers under different test conditions [71,73,75-80]. From the table, it can be seen that the CNx films prepared by different techniques did not demonstrate surprising frictional behavior. 

As it is known, during the micro friction force measurement, it is difficult to obtain the real and precise friction force due to the small size of cantilever as well as the slight difference between cantilevers. According to the principle of FFM, the friction force signal can be used as the representative of the real friction force. Therefore, a friction coefficient factor is introduced in order to compare the relative micro friction characteristics among the samples. Corresponding to the relationship between the friction force and the friction... 

The Y-axis represents the magnitude of the friction signal force and the X-axis is the load. The slope of the trend line is dehned as the friction factor (friction force signal/load) which is used to express the relative friction coefficient (friction force/load). Experiments that have been done in the same monolayer L-B him but different scan ranges give similar results as shown in Fig. 24 and Fig. 25. The friction factors of this monolayer L-B him, 0.0265 and 0.0203, are similar. The topographies of these two areas are shown in Fig. 26. 

The friction coefficient of Sample 2 is quite different from the other samples this can be attributed to the surface difference. The previous research shows that the friction coefficient of DLC is related to the deposition parameter [29]. In this study, in order to evaluate the surface properties in the same condition, we designed Layer A as the outermost layer for all the multilayer samples. Among all the samples, only the surface of Sample 2 is from Layer B. 

Laser treatment is also an energy-efficient process. After treated by the C02-pulsed laser beam without photo initiator, the samples showed significant differences in hydrophilicity depending on the number of the laser pulses. The friction coefficient of the PDMS decreased drastically when the surface was treated with the laser, and low platelet spreading and aggregation was observed on the laser-treated PDMS surface. 

Master curves of the friction coefficient have been obtained for a wide range of rubber compounds on different types of tracks for dry and wet surfaces. 

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