Friction plowing

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. 

Design and Operation The pan crusher (Fig. 20-30) consists of one or more grinding wheels or mullers revolving in a pan the pan may remain stationaty and the mullers be driven, or the pan may be driven while the iTuillers revolve by friction. The mullers are made of tough alloys such as Ni-Hard. Iron scrapers or plows at a proper angle feed the material under the mullers. 

A major limitation of the dissipative mechanisms involving multiplicative noise —and by extension the iGLE and WiGLE models— is that they involve equilibrium changes only in the strength of the response with respect to the instantaneous friction kernel. They do not involve a change in the response time of the solvent at equilibrium limits. Presumably the response time also changes in some systems, and the inclusion of this variation is a necessary component of the minimal class of models for nonstationary stochastic dynamics. Plow this should be included, however, is an open problem which awaits an answer. 

Another theory of the reason for increased friction in the presence of moisture was proposed by Gao et al . They found that in a humid environment molybdenum disulphide films were more readily thinned by sliding contact, due to increased ease of interlamellar slip. They suggested that adsorption of water softened the films, and that resulting increased deformation by plowing in sliding contact led to a poorly oriented film and thus to increased friction. However, they considered that this was a short-term reversible effect which was not in conflict with theories of chemical breakdown. Gao et al also poiinted out the possibility that an increase In friction is caused by capillary pressure effects of moisture at asperity contacts. 

Figure 1. Schematic representation of various possible friction mechanisms (a) Geometric interlocking of asperities with typical angle 0, (b) elastic deformation (stretched dashed bonds) to interlock atoms and/or macroscopic peaks, resulting in multiple metastable states, (c) defect pinning (circles), (d) pinning by an intervening layer of weakly bound material, (e) plastic deformation or plowing, and (f) material mixing or cold welding.

One of the commonly described mechanisms for producing friction is plowing (Fig. le) [31]. In this case a hard tip is indented into a softer material and plows a permanent groove into the material as it slides. The work needed to produce this plastic deformation of the substrate has to be provided by the frictional force. This mechanism clearly occurs whenever a substrate is scratched during sliding. This naturally leads to rapid wear, which may be desirable in the context of machining. 

Since the stresses in real asperities are not restricted to pure shear, 25 is probably too high a value for a. Tabor [5] argued that 3.3, the value obtained from experiments with indium, is so low because of the experimental difficulty of measuring the adhesion and because of the plowing contribution to the force of friction for such a soft metal. The value of ca. 12 from the results of Courtney-Pratt and Eisner [10] for platinum may be imprecise because of the influence of surface film error on the conductivity measurements. The value of 9 was selected for a as a useful magnitude representative of the metals usually encountered. Setting = 3i then gives... 

There are mechanisms other than the adhesion of asperity contacts that can result in phenomenologically observable friction. The two most important, from an engineering viewpoint, are plowing and hysteresis loss. 

In a broad sense, it may be considered that the main function of a lubricant is to keep the surfaces apart so that interaction (e.g., adhesion, plowing, and shear) between the solids cannot occur thus friction and wear can be reduced or controlled. 

In a scratch test, a diamond or WC, conical or pyramidal indentor is sheared across a specimen under a controlled normal load while monitoring the tangential force [117]. The adhesive bond and the tangential plowing force determine the total work performed by frictional forces. The coefficient of friction is determined from the ratio of the tangential to the normal load. 

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