Friction behavior

ISO 12957-1 Geosynthetics Determination of Friction Characteristics Part 1 Direct Shear Test 

ASTM D5321 Standard Test Method for Determining the Shear Strength of Soil— Geosynthetic and Geosynthetic—Geosynthetic Interfaces by Direct Shear 

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As might be expected, this simple picture does not hold perfectly. The coefficient of friction tends to increase with increasing velocity and also is smaller if the pavement is wet [14]. On a wet road, /x may be as small as 0.2, and, in fact, one of the principal reasons for patterning the tread and sides of the tire is to prevent the confinement of a water layer between the tire and the road surface. Similarly, the texture of the road surface is important to the wet friction behavior. Properly applied, however, measurements of skid length provide a conservative estimate of the speed of the vehicle when the brakes are first applied, and it has become a routine matter for data of this kind to be obtained at the scene of a serious accident. 

Substances in this category include Krypton, sodium chloride, and diamond, as examples, and it is not surprising that differences in detail as to frictional behavior do occur. The softer solids tend to obey Amontons law with /i values in the normal range of 0.5-1.0, provided they are not too near their melting points. Ionic crystals, such as sodium chloride, tend to show irreversible surface damage, in the form of cracks, owing to their brittleness, but still tend to obey Amontons law. This suggests that the area of contact is mainly determined by plastic flow rather than by elastic deformation. 

The fabric may also be given one or more of a number of other finishing treatments, either ia tandem with web formation and bonding or off-line as a separate operation, as a means of enhancing fabric performance or aesthetic properties. Performance properties iaclude functional characteristics such as moisture transport, absorbency, or repeUency flame retardancy electrical conductivity or static propensity abrasion resistance and frictional behavior. Aesthetic properties iaclude appearance, surface texture, and smell. 

Dutrowski [5] in 1969, and Johnson and coworkers [6] in 1971, independently, observed that relatively small particles, when in contact with each other or with a flat surface, deform, and these deformations are larger than those predicted by the Hertz theory. Johnson and coworkers [6] recognized that the excess deformation was due to the interfacial attractive forces, and modified the original Hertz theory to account for these interfacial forces. This led to the development of a new theory of contact mechanics, widely referred to as the JKR theory. Over the past two decades or so, the contact mechanics principles and the JKR theory have been employed extensively to study the adhesion and friction behavior of a variety of materials. 

P. Bordarier, B. Rousseau, A. H. Fuchs. A model for the static friction behavior of nanolubricated contacts. Thin Solid Films 330 21-26, 1998. 

DellaCorte, C., Static Dynamic Friction Behavior of Candidate High Temperature Materials, NASA, 1994. 

The adhesion hysteresis and its contribution to the friction have been studied extensively by means of SFA [39], which leads to an important conclusion that it is the adhesion hysteresis or the energy loss during the process of approach/separation, rather than the surface energy itself, that dominates the frictional behavior of boundary films. 

Even though the load carrying capacity and friction behavior are apparently of interest to the authors, they are not presented here because no general law can be constructed if there are no practically reliable parameters presented. This remains to be done in our further research. 

Phenomenal studies were made to observe the frictional behavior of L-B films and SAMs and its dependence on applied load and sliding velocity, which has been summarized in a review article by Zhang [33]. It has been confirmed that in comparison to the bare surface of the substrates, the friction on molecular films is significantly reduced, with friction coefficients in a range of 0.05-0.1. Friction forces are found... 

The Stribeck curve gives a general description for the transition of lubrication regime, but the quantitative information, such as the variations of real contact areas, the percentage of the load carried by contact, and changes in friction behavior, are not available due to lack of numerical tools for prediction. The deterministic ML model provides an opportunity to explore the entire process of transition from full-film EHL to boundary lubrication, as demonstrated by the examples presented in this section. 

It should be stressed that environment conditions strongly influence the friction behavior of NFC coatings. In Erdemir s work, super-low friction coefficient was attained only in vacuum or an inert gas environment [22,51]. When oxygen and moisture are presented, friction coefficient dramatically increased [22,52,53]. 

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. 

Heimberg, J. A., Wahl, K. J., Singer, I. L., and Erdemir, A., "Friction Behavior of Diamond-like Carbon Coatings Time and Speed Effects, Appl. Phys. Lett., Vol. 78, 2001, pp. 2449-2451. 

The shape of the so called master curve, however, depends strongly on the compound formulation and the type of contact surface in the friction process. Its detailed study gives an insight into the processes responsible for the frictional behavior. 

When the water film is squeezed out, the thick water layer is removed and the surfaces are separated by lubricant film of only molecular dimensions. Under these conditions, which are referred to as BL conditions, the very thin film of water is bonded to the substrate by very strong molecular adhesion forces and it has obviously lost its bulk fluid properties. The bulk viscosity of the water plays little or no part in the frictional behavior, which is influenced by the nature of the underlying surface. By comparing with the friction force of an elastomer sliding on a rigid surface in a dry state, Moore was able to conclude that for an elastomer sliding on a rigid surface under BL conditions, one can expect ... 

Attractive or repulsive interaction between two solid surfaces should play an important role in the interfacial frictional behavior [87,92-95]. From previous theoretical [89] and experimental investigations [87, 95], it was known that the attractive interaction result in a high friction and repulsive interaction results in low friction force. To characterize the interfacial molecular structure between two solids under electrostatic interaction is also important to elucidate the frictional properties of two solids. 

The friction behavior of PVA gel and solid is strongly dependent on the nature of the solid surface. While the friction is low when the PVA gel is in contact with a quartz surface, which is hydrophilic, it is very high when the PVA gel is in contact with a... 

The frictional behavior of 316 stainless steel is shown in Fig. 8 for sliding in air or in different atmospheres. No clear trend in the dependence of friction coefficient on environment was observed. 

J. Andersson, R.A. Erck, and A. Erdemir, Frictional Behavior of Diamondlike Carbon Films in Vacuum and under Varying Water Vapor Pressure, Surf. Coat. Technology 163-164,535-540 (2003). 

The concentration of a small molecule reactant inside the polymer coils can be lower than outside when one uses a poor solvent for the polymer. This results in lower local and overall reaction rates. In the extreme, a poor solvent results in reaction occurring only on the surfaces of a polymer. Surface reactions are advantageous for applications requiring modification of surface properties without affecting the bulk physical properties of a polymer, such as modification of surface dyeability, biocompatibility, adhesive and frictional behavior, and coatability [Ward and McCarthy, 1989]. 

This variation in friction behavior could be due to the specific apparent crosslink densities or AM values, as shown in Fig. 17. Both PTFE0kGy-EPDM and PTFE20kGy-EPDM have higher AM values and therefore have lower / initial values. Even a slight variation in AM results in different /Wiai values. As soon as AM decreases, as in the case of PTFE500kGy-EPDM, a significant increase in / , i l is... 

Fig. 40 Comparison of the friction behavior of PTFE-EPDM composites as a function of sliding distance...

Not all classes of fluids conform to the frictional behavior described in Section 6.3. This section will describe the commonly recognized types of liquids, from the point of view of flow behavior, and will summarize the data and techniques that arc used for analyzing friction in such lines. 

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