Surface Friction and Adhesion

Control of fiber friction is essential to the processing of fibers, and it is sometimes desirable to modify fiber surfaces for particular end-uses. Most fiber friction modifications are accomplished by coating the fibers with lubricants or finishes. In most cases, these are temporary treatments tiiat are removed in final processing steps before sale of the finished good. In some cases, a more permanent treatment is desired, and chemical reactions are performed to attach different species to the fiber surface, e.g. siliconized slick finishes or rubber adliesion promoters. Polyester s lack of chemical bonding sites can be modified by surface treatments that generate free radicals, such as with corrosive chemicals (e.g. acrylic acid) or by ionic bombardment with plasma treatments. The broken molecular bonds produce more polar sites, thus providing increased surface wettability and reactivity. 

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Surface Friction and Adhesion. Control of fiber friction is essential to the processing of fibers, and it is sometimes desirable to modify fiber surfaces for particular end uses. Most fiber friction modifications are accomplished by coating the fibers with lubricants or finishes. In most cases, these are temporary treatments that are removed in final processing steps before sale of the finished good. 

This chapter and the two that follow are introduced at this time to illustrate some of the many extensive areas in which there are important applications of surface chemistry. Friction and lubrication as topics properly deserve mention in a textbook on surface chemistiy, partly because these subjects do involve surfaces directly and partly because many aspects of lubrication depend on the properties of surface films. The subject of adhesion is treated briefly in this chapter mainly because it, too, depends greatly on the behavior of surface films at a solid interface and also because friction and adhesion have some interrelations. Studies of the interaction between two solid surfaces, with or without an intervening liquid phase, have been stimulated in recent years by the development of equipment capable of the direct measurement of the forces between macroscopic bodies. 

Yamada S and Israelachvili J N 1998 Friction and adhesion hysteresis of fluorocarbon surfactant monolayer-coated surfaces measured with the surface forces apparatus J. Rhys. Chem. B 102 234-44... 

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) ... 

Wear. Ceramics generally exhibit excellent wear properties. Wear is deterrnined by a ceramic s friction and adhesion behavior, and occurs by two mechanisms adhesive wear and abrasive wear (43). Adhesive wear occurs when interfacial adhesion produces a localized Kj when the body on one side of the interface is moved relative to the other. If the strength of either of the materials is lower than the interfacial shear strength, fracture occurs. Lubricants (see Lubricants and lubrication) minimize adhesion between adj acent surfaces by providing an interlayer that shears easily. Abrasive wear occurs when one material is softer than the other. Particles originating in the harder material are introduced into the interface between the two materials and plow into and remove material from the softer material (52). Hard particles from extrinsic sources can also cause abrasive wear, and wear may occur in both of the materials depending on the hardness of the particle. 

The friction from the repulsive pinning center is of particular interest because it is contrary to the common belief that friction must result from attractive interactions between sliding surfaces. The results presented in Fig. 17(a) demonstrate that friction can be created by purely repulsive interactions. What really matters is the instability of the sliding body and energy dissipation, rather than the attractive or repulsive nature of interactions. This may also shed a light on the efforts to explore the correlation between friction and adhesion. 

Israelachivil et al. [25] proposed a phenomenal model for describing the interrelations between friction and adhesion. Consider the system shown in Fig. 28, where a spherical molecule slides over a corrugated solid surface. The scenario is somehow like pushing the wheel of a cart over a road paved with cobblestones, so it is also known as the cobblestone model. 

The frictional and adhesion forces between the abrasive particles and wafer surfaces were experimentally measured using alumina and silica slurries with and without citric acid. Although citric acid did not affect the zeta potential of the silica particles, it resulted in a more negative zeta potential of the alumina particles due to the adsorption of the negatively charged citrate ions onto the alumina surfaces. The highest particle adhesion force was measured in an alumina slurry without the addition of citric acid. However, the alumina slurry with the addition of citric acid had the lowest particle adhesion force due to the adsorption of citrate ions onto the alumina surfaces. Although citrate ions could easily adsorb onto alumina particles, the silica particles did not appear to benefit in terms of reduced frictional force when in citric acid solutions. 

For wall friction measurements, a wall coupon is inserted between the rings, and powder in the upper ring alone is sheared against a coupon of interest. Wall friction and adhesion, both static and dynamic, may be assessed against different materials of construction or surface finish. 

Contact mechanics is both an old and a modern field. Its classical domains of application are adhesion, friction, and fracture. Clearly, the relevance of the field for technical devices is enormous. Systematic strategies to control friction and adhesion between solid surfaces have been known since the stone age [1]. In modern times, the ground for systematic studies was laid in 1881 by Hertz in his seminal paper on the contact between soHd elastic bodies [2]. Hertz considers a sphere-plate contact. Solving the equations of continuum elasticity, he finds that the vertical force, F , is proportional to where S is the indentation. The sphere-plate contact forms a nonlinear spring with a differential spring constant k = dF/dS oc The nonhnearity occurs because there is a concentration of stress at the point of contact. Such stress concentrations - and the ensuing mechanical nonhnearities - are typical of contact mechanics. 

The presence of a closely packed layer of adsorbed molecules at a polymer surface will have a marked effect on many of its surface properties, such as friction, adhesiveness, and wettability. These properties of an adsorbed film have already foimd some application in industry. Allan [1] has demonstrated that small amounts of oleylamide incorporated in polyethylene foil will diffuse to the surface of the foil and greatly reduce the friction and adhesion between sheets of the plastic. A committee from the Piedmont Section of the American... 

Static friction is sometimes defined as the force needed to start the motion between two surfaces. It is clear, however, that if a shear force is applied to two contacting surfaces and no motion takes place it may be because the surfaces arc adhering and thus it is neccs.sarv to distinguish between friction and adhesion. A car licence holder on a windscreen illustratc.s the dilemma perfectly. There is a very strong resistance to shear, but is this entirely due to friction ... 

It might be expected that inclusion of small particles in a powder could decrease porosity if they fit into voids between larger particles. In practice, the opposite effect is usually observed. The explanation for this follows from consideration of the main factors that determine closeness of packing and flow properties of powders the size, shape, and surface properties of the particles. It is often found that the effects of surface properties outweigh the others because they govern the friction and adhesion between particles. As the size of particles decreases, the ratio of surface to volume increases, thus magnifying frichonal resistance. Other factors that may contribute to increased friction or stickiness are the presence of liquid films and electrical charge effects. 

Similarly, a book on the Adhesion of piles in stiff clays is more about frictional sliding of the pile as it is pulled out of the clay, rather like the fiber pullout test described in Chapter 16. However, once the pile is removed, clay may then be seen adhering to the steel surface. Clearly, both friction and adhesion can be observed simultaneously in this case, as shown in Fig. 2.2(d). 

The detachment of microparticles when a dust-covered surface is tilted will be opposed by both frictional and adhesive forces this is not taken into account in Amontons law [29]. 

If a continuous water film is present, such as that established in plows with a water channel between the plow surface and the soil, friction and adhesion are reduced. The construction of such a water channel, however, which essentially involves the passage of water from a container attached to the plow into the space between the plowshare and moldboard through several openings, is quite cumbersome, and the machine requires constant feed of water, which may not be justified in practice. 

If a dc current is fed to a plow sunk into the soil in such a manner that the plowshare and moldboard become the cathode, electroosmosis will tend to move the soil water toward these components and wet them. A water film is thus formed between the plow surface and the soil particles, reducing the friction and adhesion. 

Fibers rub against each other in assemblies when stretched, bent or sheared. Textile materials also rub against tissues and bones when used as implants. The friction and adhesion involved in such contacts cause surface damage, which adversely affects the long-term performance of an implant. Also, the particles generated by the friction/adhesion processes can cause a reaction and lead to inflammation of the surrounding tissues. Consequently, the ability of polymers and fibers to resist abrasion damage is an important... 

Friction and adhesive forces have been used widely as measurable properties [67], which can be obtained easily by SPM (Scanning Probe Microscopy) as described above. However, other properties such as surface potentials or contact potential difference (CPD) [29, 58-62], phases in tapping mode AFM [68, 69], and fluorescence intensities under SNOM [70-76] can be used for CFM [17,67,77]. 

Dependence of friction and adhesion on surface chemical species on samples has... 

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