And lubricated friction

Frictional Forces and Lubrication Frictional forces are a measure of how much contact the pad makes with the surface, which is important in determining the abrasion mode. Friction and lubrication also affect the amount of heat that is produced during polishing and thus affect the temperature. 

Table 9.1 Examples of coefficients of friction between different materials for dry and lubricated friction [915].

The major conclusion of these studies is the close connection between the rolling and lubricated friction and the viscoelastic properties of the polymer g ye s been numerous theoretical treatments of the problem. and while the dissipation... 

F. P. Bowden and D. Tabor, The Friction and Lubrication of Solids, The Clarendon Press, Oxford, 1950. 

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. 

The force between two adjacent surfaces can be measured directly with the surface force apparatus (SEA), as described in section BT20 [96]. The SEA can be employed in solution to provide an in situ detennination of the forces. Although this instmment does not directly involve an atomically resolved measurement, it has provided considerable msight mto the microscopic origins of surface friction and the effects of electrolytes and lubricants [97]. 

Bowden F P and Tabor D 1964 Friction and Lubrication of Soiids Part II (Oxford Oxford University Press)... 

Yoshizawa H, Chen Y L and Israelachvili J N 1993 Recent advances in molecular level understanding of adhesion, friction and lubrication Wear 6B 161-6... 

The goal of lubrication is elimination of this wear and minimizing friction otherwise encountered in dry sliding. This is accompHshed ideally with complete separation of the mbbing surfaces with a full film of lubricant. When complete hill-film separation is impossible, surface chemical effects of a lubricating oil and its additives, or solid-film lubricants such as graphite and molybdenum sulfide, can assist. 

Elastohydrodynamic Lubrication (EHL). Lubrication needs in many machines ate minimized by carrying the load on concentrated contacts in ball and toUet beatings, gear teeth, cams, and some friction drives. With the load concentrated on a small elastically deformed area, these EHL contacts ate commonly characterized by a very thin separating hydrodynamic oil film which supports local stresses that tax the fatigue strength of the strongest steels. 

Dry lubricants are usually added to the powder in order to decrease the friction effects. The more common lubricants include zinc stearate [557-05-17, lithium stearate [4485-12-5] calcium stearate [1592-23-0] stearic acid [57-11-4] paraffin, graphite, and molybdenum disulfide [1317-33-5]. Lubricants are generally added to the powder in a dry state in amounts of 0.25—1.0 wt % of the metal powder. Some lubricants are added by drying and screening a slurry of powder and lubricant. In some instances, lubricants are appHed in Hquid form to the die wall. 

Lubricants protect die and punch surfaces from wear and bum-out of the compact during sintering without objectionable effects or residues. They must have small particle size, and overcome the main share of friction generated between tool surfaces and powder particles during compaction and ejection. They must mix easily with the powder, and must not excessively impede powder flow (see Lubrication and lubricants). 

Applications. The capabiHties of a gc/k/ms in separating and identifying components in complex mixtures is very high for a broad spectmm of analytical problems. One area where k information particularly complements ms data is in the differentiation of isomeric compounds. An example is in the analysis of tricresyl phosphates (TCPs) used as additives in a variety of products because of thek lubricating and antiwear characteristics (see Lubrication and lubricants). One important use of TCPs is in hydrauHc fluid where they tenaciously coat metal surfaces thereby reducing friction and wear. Tricresyl phosphate [1330-78-5] (7.2 21 exists in a variety of isomeric forms and the commercial product is a complex mixture of these isomers. 

Sulfuiized and sulfurchlorinated unsaturated compounds and meicaptans are used as lubricant additives (antiwear, friction modification, load-carrying, extreme pressure and temperature, corrosion inhibition, and antioxidants), refinery catalyst regeneration compounds, steel processing (annealing) aids, and vulcanization catalysts (see Lubrication and lubricants). 

SoHd lubricants ate added to help control high friction characteristics in high speed or heavy-duty appHcations where high temperatures are generated. Molybdenum disulfide [1317-33-5] M0S2, may be used alone or in a complex compound formed by grinding with fine natural graphite, and zinc sulfide [1314-98-3] ZnS. Other compounds include calcium fluoride, cryoHte [15096-52-3] Na AlF, rare-earth oxides, and metal sulfides, eg, iron, antimony, or zinc (see LUBRICATION AND LUBRICANTS). 

High temperature lubrication, as in some metal-forming processes, requites dry graphite. Although the coefficient of friction of graphite is higher than that of petroleum lubricants, it is often added as a safety measure should the carrier lubricant fail (17) (see Lubrication and lubricants). 

Tribological Behavior. Tribological performance of ceramics, which includes friction, adhesion, wear, and lubricated behavior of two soHd materials in contact, has been reviewed (52). 

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. 

Packiug may not provide a completely leak-free seal. With shaft surface speeds less than approximately 2.5 m/s (500 ft/min), the packiug may be adjusted to seal completely. However, for higher speeds some leakage is required for lubrication, friction reduction, and cooling. 

Other fields of surface study were of course developing the study of catalysts for the chemical industry and the study of friction and lubrication of solid surfaces were two such fields. But in sheer terms of economic weight, solid-state electronics seems to have led the field. 

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