Friction measurement conditions

Much effort has been spent in understanding how skin friction changes with differing biological conditions and upon the application of various products to the skin surface. These studies have been of interest to various industries that manufacture products meant as skin topical agents because friction measurements can provide clues regarding the effectiveness of their products. 

Fig. 23 Contact fatigue behaviour of methylmethacrylate random copolymers under fretting conditions. White circle Glutarimide co-monomer (76%), black circle cyclohexyl malemimide co-monomer (25%). The maximum value of the tensile stress has been calculated at the edge of the contact, where cracks initiate, and for values of the coefficient of friction measured at crack initiation...

Because of the complex nature of the temperature and velocity dependence of the kinetic ice friction coefficient, it is important to measure ice friction under conditions that are as close as possible to those where the data will be applied. It is inadvisable to apply or extrapolate measurements and inferences made at low velocities to skating at much higher velocities. 

Staxing with this frictional analogy, the inclined plane method of measuring friction would gi c an apparent coefficient of friction, since conditions are not tighll) controlled (xelocilx. for example, cannot be. specified) and there is little chance of relating the result to other conditions. 

Several examples of experimental approaches to thin-film lubrication have been reported [3]. It is important in examining these techniques to make the distinction between methods that are used to determine lubricant film thickness under hydrodynamic or elastohydrodynamic conditions (e.g., optical interference, electrical capacitance, or x-ray transmission), and methods that are used to determine the occurrence or frequency of contact. As we will see later, most experimental studies of synovial joint lubrication have focused on friction measurements, using the information to determine the lubrication regime involved this approach can be misleading. 

Due to frictional boundary conditions, seizures of up to half the contact length and subsequently Coulomb friction with a constant friction value were assumed. However, this method is considerably more complex than a simple comparison of measured and calculated forces. 

Measurement of the external coefficient of friction of particulate polymers is very difficult because of the very large number of variables that influence the coefficient of friction. Many investigators have made elaborate measurements on the external coefficient of friction [24-32]. The result of this work is that many variables have been identified that affect the frictional behavior however, most measurement techniques do not yield accurate and reproducible results that can be used in the analysis of the extrusion process. The most elaborate measurements and the most meaningful results have probably been obtained at the DKl in Darmstadt, Germany [95]. It is possible to obtain reproducible results by very careful experimental techniques and special surface preparation of the metal wall. However, the frictional coefficients determined in this fashion are hardly representative of the frictional process conditions occurring in an extruder. 

Frictional forces and coefficients of Diction depend to a considerable extent on relative humidity, or in extreme cases on the presence of water. We are distinguishing between three conditions for frictional measurements ambient (65% RH), wet-with-water (www, the fiber is taken directly from the bath to the friction measurement), and wet-in-water (wiw, friction measurement in a liquid bath). There is also a significant effect of the nature of the capstan material. Hard rubber produces a higher coeffieient of frietion than aluminium, an effect that is more pronounced in the wet state (41). Similarly, the surface of the hair, especially its hydrophilicily, affects its frictional behavior. Bleaehing and permanent-waving treatments increase frictional forces (40). In both eases, seission of disulfide bonds in the outer layer of the cuticle leads to a softening of the surface, resulting in an increase in the area of real contact. 

One way of quantifying the extent of fluid film lubrication is in terms of the lambda ratio . A, which is defined as the ratio between the film thickness h and the out-of-contact, composite surface roughness of the sliding bodies [16]. In the current study, it was possible to calculate the lambda ratio directly, since the film thickness was measured. Therefore, since the contact conditions and tribo-pair were the same in both film thickness and MTM friction measurements, it was possible to determine the dependence of COF on... 

SRR varies from 0 to 200% with SRR = 0% ( bau = disk) representing pine rolling and SRR = 200% for complete sUding conditions. A SRR of 10% was used for all experiments to maintain the conditions of near-pure rofling. Using the manufacturer s software (PCS Instmments, MTM version 1.0, London, UK) the speed can be varied from 0 to2500 mm/ s. A load of 10 N was appUed (Hertzian contact pressur-e = 0.42 GPa) and the coefficient of friction measured as a function of the mean speed of the disk and the ball. A temperature of 25 °C was maintained by means of a water bath. New disks and balls were used for every measurement. 

Figure 2.14b shows the friction force measured on platinum patterns and the averaged scan pull-off force measured before and after each friction measurement for the same scanned area. In fig. 2.14b, both the friction and pull-off forces decreased as the asperity height increased, but not as rapidly as the forces with silicon groove depth (fig. 2.10b). The difference in contact conditions, as mentioned previously, possibly caused the differences in rates of decrease in the friction and pull-off forces. 

Friction properties were measured using a translation tribometer (fig. 12.1). The PDMS hemisphere was brought into contact with the substrate under a given normal force. The substrate was then moved at a given speed, and the tangential force, which corresponds to the friction force, was measured. At least five friction measurements were performed for each experimental condition to obtain a mean friction coefficient value. The friction coefficient p is defined as the quotient of the tangential (or frictional) force Ft divided by the applied normal force F, i.e., p = Fj/F. All data were collected at ambient conditions (25°C). 

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. 

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