Friction macroscopic, tests

Surprising results, compared with macroscopic friction, were obtained for PDMS 6, with a higher nanoscale friction before extraction of free chains. The influence of extraction was also more pronounced for lower normal loads. Macroscopic tests previously showed a sHght increase in friction after extraction. 

Wear is a result of frictional work pWV, To remove polymer the strain must be large and hence is proportional to, say, tan 9 where is the mean asperity slope angle. The eflSciency of the work done per removal operation is inversely proportional to the toughness of the polymer which may be approximated as OyCy, where Gy and Cy are the stress and strains at rupture in a macroscopic test. The volume of material deformed at the interface will be inversely proportional to the hardness H, If the rate of wear is expressed as W/V then... 

Friction and Adhesion. The coefficient of friction p. is the constant of proportionality between the normal force P between two materials in contact and the perpendicular force F required to move one of the materials relative to the other. Macroscopic friction occurs from the contact of asperities on opposing surfaces as they sHde past each other. On the atomic level friction occurs from the formation of bonds between adjacent atoms as they sHde past one another. Friction coefficients are usually measured using a sliding pin on a disk arrangement. Friction coefficients for ceramic fibers in a matrix have been measured using fiber pushout tests (53). For various material combinations (43) ... 

After the preparation of the PMAA brushes, their lubrication ability under low contact pressures was tested in a neutral aqueous solution. It was shown that the macroscopic friction between polyelectrolyle bmshes of different molecular weights and soft, hydrophilic ox-PDMS pins was below the detection limit of the employed pin-on-disk tribometer over the entire speed range tested. While the PMAA bmshes could not be distinguished with n vs sliding speed plots, the long-term stability of short 15 nm PMAA bmshes was shown to be inferior to long bmshes (240 nm dry thickness). A further comparison between... 

Diffusion. The translational diffusion coefficient D is the most commonly measured transport property of polymer solutions, but as there are several distinct types of diffusion, care must be taken to interpret D properly. For c < c, Brownian motion of isolated chains in a homogeneous solvent defines the dilute solution diffusion coefficient Dq. As c increases toward c and above, chain-chain interactions modify the friction felt during chain motion. Under these conditions, the tracer- or self-diffusion coefficient Dtr is measured by tracking the path of a single chain in a macroscopically imiform mixture of chains and solvent. To distinguish the test chain from neighbors so that its path can be identified, the chain... 

We have stndied the macroscopic frictional properties of high-density polymer brushes prepared by surface-initialed ATRP of methyl methacrylate (MM A) [31] and hydrophilic methacrylates [32, 33] from silicon substrates. Friction tests were carried out using a stainless steel or glass ball as the sliding probe under a normal load of 100 MPa from the viewpoint of practical engineering applications. This chapter reviews the macroscopic frictional properties of polymer brushes under a high normal load, the dependence of solvent qnaUty, the effect of humidity on hydrophilic brnsh, and wear resistance, and we compare these with alkylsilane monolayers. 

Macroscopic friction tests on the NTS monolayer and bilayer in air were performed by sliding a stainless steel ball along a distance of 20 mm at a sliding velocity of 90 mm/min under a load of 0.2 N. Both thin films showed similar dynamic friction coefficients of 0.080-0.083, while a large difference was observed in wear resistance. The friction coefficient of the NTS monolayer increased after the 150 friction cycles, as shown in fig. 5.8a. On the other hand, the bilayer demonstrated a stable friction coefficient until 2200 friction cycles (fig. 5.8b). A wear track was clearly observed on the surface of the monolayer after 300 friction cycles (fig. 5.8c), while only a slight wear track was present on the surface of the bilayer, even after 1000 slidings, as shown in fig. 5.8d. Generally, a wear track should be formed by direct contact of the stainless steel probe with the silicon substrate at the point where... 

Macroscopic friction tests on PMMA brushes were carried out by sliding a stainless steel ball on the substrates at a rate of 90 mm/min in air under the normal load of 0.49 N at room temperature. In the case of a nonmodifled silicon wafer under a normal... 

Frederich established one of the first progressions [8]. The friction function which it defines results from on site tests. The fnction is a function of the rolling speed and the normal force applied to a wheel. Unfortunately, the relations of macroscopic causes (mechanical parameters) in microscopic effects (variation of the fnction) are not physically known. Therefore, Ohyama carried out laboratory experiments with layers injected into the contact where their shear stress were expressed as a function of the ratio of two coefficients static friction ps and kinematic pd [9]. The first coefficient was defined in the stick zone while the second was in the slip zone (Figure 4). Thus the total tangential force was the integral of the tangential stresses qi (x) and q2(x) of each zone. 

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