Frictional coefficient variations

Figures 20.8 and 20.9 show the fiiction coefficient and specific wear rate variation of pure PTFE and PTFE/nanoserpentine composites as a function of shding velocity under a normal load of 2.85 MPa for 60 min. Despite the reversed U shape of the friction coefficient variation for pure PTFE, it is evident that the change of sliding velocity has no significant effect on the friction coefficient of both materials, especially for the PTFE/nanoserpentine composites. In aU wear tests, the friction coefficient of materials varies from 0.090 to 0.114. As for the wear resistance of...

Figure 20.8 Friction coefficient variation of pure PTFE and PTFE/nanoserpentine composites as a function of sliding velocity.

Fig. 10 shows the friction coefficient variation vs. the reciprocating cycles for the salt and for the amide. For the salt, the friction coefficient value remained low and steady at approximately 0.23 throughout the 100 reciprocating cycles. The friction coefficient with the amide increased with the number of cycles. 

Hardness is a measure of a material s resistance to deformation. In this article hardness is taken to be the measure of a material s resistance to indentation by a tool or indenter harder than itself This seems a relatively simple concept until mathematical analysis is attempted the elastic, plastic, and elastic recovery properties of a material are involved, making the relationship quite complex. Further complications are introduced by variations in elastic modulus and frictional coefficients. 

This formula is another variation on the Affinity Laws. Monsieur s Darcy and VVeisbach were hydraulic civil engineers in France in the mid 1850s (some 50 years before Mr. H VV). They based their formulas on friction losses of water moving in open canals. They applied other friction coefficients from some private experimentation, and developed their formulas for friction losses in closed aqueduct tubes. Through the years, their coefficients have evolved to incorporate the concepts of laminar and turbulent flow, variations in viscosity, temperature, and even piping with non uniform (rough) internal. surface finishes. With. so many variables and coefficients, the D/W formula only became practical and popular after the invention of the electronic calculator. The D/W forntula is extensive and eomplicated, compared to the empirieal estimations of Mr. H W. 

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. 

In order to examine the nature of the friction coefficient it is useful to consider the various time, space, and mass scales that are important for the dynamics of a B particle. Two important parameters that determine the nature of the Brownian motion are rm = (m/M) /2, that depends on the ratio of the bath and B particle masses, and rp = p/(3M/4ttct3), the ratio of the fluid mass density to the mass density of the B particle. The characteristic time scale for B particle momentum decay is xB = Af/ , from which the characteristic length lB = (kBT/M)i lxB can be defined. In derivations of Langevin descriptions, variations of length scales large compared to microscopic length but small compared to iB are considered. The simplest Markovian behavior is obtained when both rm << 1 and rp 1, while non-Markovian descriptions of the dynamics are needed when rm << 1 and rp > 1 [47]. The other important times in the problem are xv = ct2/v, the time it takes momentum to diffuse over the B particle radius ct, and Tp = cr/Df, the time it takes the B particle to diffuse over its radius. 

Figure 21 Friction coefficient for differently oriented Ni(100)/Ni(100) interfaces. Rough surfaces have a 0.8 A rms variation in roughness added to the atomically smooth surfaces. Reproduced with permission from Ref. 85.

Table I summarizes the friction coefficients obtained over the course of the study. Where a range of friction coefficients is indicated, this reflects the variability of friction coefficient as a function of sliding speed. No appreciable variation as a function of load was ever apparent.

For all materials, the adhesive mechanism and the plastic deformation should be the main processes. During testing, the variation in friction coefficient values could be influenced by the specific AM behavior. Since AM corresponds to the crosslink density of a composite, for a qualitative assessment it can be concluded that the crosslink density decreases with increasing absorbed dose. PTFE500kGy-EPDM showed much lower AM and f90 values. It can be inferred that the state of cure is strongly dependent on the irradiation dose absorbed by the PTFE powder. 

Chapter D is concerned with intrinsic viscosity and translational friction coefficient. Published data for the molecular-weight dependence of these quantities of polypeptides in helieogenic solvents and helix-breaking solvents are summarized, and the variations of these quantities during the helix-coil transition are described. 

The linear approximation (7.9) is insufficient to describe the variation of the friction coefficient at large velocity gradients. In this case, approximation (7.9) can be generalised (Pokrovskii and Pyshnograi 1990, 1991) to become... 

There are no variations in the friction coefficient according to age, gender, and ethnicity. The effect of age is still under debate. 

FIGURE 33.1 Variation of the friction coefficient versus time during the recording of an experiment on the forearm. The two curves correspond to two different loads applied on the sliding pad. The maximum of the curves corresponds to the static friction coefficient and the asymptotic value to the kinetic coefficient.3... 

FIGURE 33.2 Variations of the friction coefficient versus time after application of different cosmetic preparations. There is an initial decrease due to lipids followed by a progressive increase due to hydration of the SC.3... 

Bahun, C.J. and Jones, J.R., Influence of Load, Speed and Coating Thickness on the Wear Life of a Bonded Solid Lubricant, Lubric. Eng., 25, 351, (1969). Hopkins, V., Discussion, p. 6, on Rabinowicz, E., Variation of Friction and Wear of Solid Lubricant Films with Film Thickness, ASLE Trans., 10, 1, (1967). Whitehouse, G.D., Nandan, D. and Whitehurst, C.A., The Effect of Film Thickness on Friction Coefficients for Solid Lubricants CaF2, MoSj and Graphite, ASLE Trans., 13, 159, (1970). 

We present here the results of such a systematic investigation on the dependence of the self-diffusion coefficient of flexible polymer chains as a function of P and N, conducted on polydimethylsiloxane (PDMS). This model polymer is well above its glass temperature at room temperature (Ta = - 120°C), so that one can expect that spurious effects associated with the variation of the free volume and of the local monomer-monomer friction coefficient with the molecular weights of the chains are minimised. 

Fig. 34. The variation of the micro-hardness (left curve) and friction coefficient (right curve) with the dose (40 keV Ar onto steel) (after ))...

The implantation of a considerable dose of metals or non-metals in materials led to large changes of the friction coefficient. The effect of Ar" is certainly mainly due to the creation of a new surface topography or to variations of the natural oxide layer of the material. Non-inert ions as Pb, Mo and Se showed some chemical effect dependent on the type of element introduced. The greatest effect came from lead implantation and was interpreted as a change in the plastic properties of the junction events. Eventually, also the formation of PbO at the surface should be considered. 

Measurement of physical parameters (compaction, ejection and residual forces) in the tabletting process and the effect on the dissolution rate. Drug Dev Ind Pharm 1986 12 1329-1346. Baichwal AR, Augsburger LL. Variations in the friction coefficients of tablet lubricants and relationship to their physicochemical properties. J Pharm Pharmacol 1988 40 569-571. 

Phillips et al. [27] studied the surface chemistry of IL-lubricated steel/steel sliding contacts under temperature variation from room temperature to 300 C. This study was focused on understanding the high-temperature stability of the liquids in contact with metal under tribological stress [27]. Some Fe samples were oxidized to Fe Oj and FOjO via thermal evaporation prior to treatment with ILs. The metallic and oxidized Fe samples were then reacted with ILs at elevated temperatures. Results showed that the friction coefficient of different fluorinated ILs was below... 

An integration over all orientations of the vector R has been effected in these equations. The first term arises from momentum transport from mass motion and can be neglected in comparison with the second term. The indicated integration of Eq. 42 has been carried out numerically. That of Eq. 43 proved to be too sensitive to variation in the radial distribution function g0(2) to yield reliable results. In order to obtain the shear viscosity r), it is now necessary to evaluate the frictional coefficient f. A solution in series was obtained by Kirkwood25 but its numerical evaluation is too unwieldy for practical calculations. The analysis, however, stiggests the following estimate for the frictional coefficient ... 

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