Solid surfaces plastic flow

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. 

Even if no perceptible motion occurs (see later, however), application of a force leads to microdisplacements of one surface relative to the other and, again, often a large increase in area of contact. The ratio F/W in such an experiment will be called since it does not correspond to either the usual ns or can be related semiempirically to the area change, as follows [38]. We assume that for two solids pressed against each other at rest the area of contact Aq is given by Eq. XII-1, A W/P. However, if shear as well as normal stress is present, then a more general relation for threshold plastic flow is... 

At the instant of contact between a sphere and a flat specimen there is no strain in the specimen, but the sphere then becomes flattened by the surface tractions which creates forces of reaction which produce strain in the specimen as well as the sphere. The strain consists of both hydrostatic compression and shear. The maximum shear strain is at a point along the axis of contact, lying a distance equal to about half of the radius of the area of contact (both solids having the same elastic properties with Poisson s ratio = 1/3). When this maximum shear strain reaches a critical value, plastic flow begins, or twinning occurs, or a phase transformation begins. Note that the critical value may be very small (e.g., in pure simple metals it is zero) or it may be quite large (e.g., in diamond). 

In powder metallurgy, the powdered material to be worked is pressed in a mold, then heated to increase the rate of diffusion. The temperature required to obtain flow of the material may be significantly below the melting point. As the powder becomes more dense and less porous, the vacancies move to the surface to produce a structure that is even less porous and more dense. In addition to diffusion, plastic flow and evaporation and condensation may contribute to the sintering process. As sintering of a solid occurs, it is... 

As follows from the hydrodynamic properties of systems involving phase boundaries (see e.g. [86a], chapter 2), the hydrodynamic, Prandtl or stagnant layer is formed during liquid movement along a boundary with a solid phase, i.e. also at the surface of an ISE with a solid or plastic membrane. The liquid velocity rapidly decreases in this layer as a result of viscosity forces. Very close to the interface, the liquid velocity decreases to such an extent that the material is virtually transported by diffusion alone in the Nernst layer (see fig. 4.13). It follows from the theory of diffusion transport toward a plane with characteristic length /, along which a liquid flows at velocity Vo, that the Nernst layer thickness, 5, is given approximately by the expression,... 

Surface Viscosity Molecular films on liquid surfaces may be either readily mobile or slow to flow under the action of a two-dimensional stress. Some surface films show surface plasticity and behave as solids until a critical stress is applied. These films exhibit surface viscosity. 

Suspension films separating two solid surfaces, and Wetting films separating a solid or liquid from a vapour. The minimum water content for which a small sample of soil or similar material will barely flow in a standardized test method. Also termed the upper plastic limit . See also Atterberg Limits, Plastic Limit, Plasticity Number. Light non-aqueous phase liquid. See Non-aqueous Phase Liquid. 

Plastic deformation (strain). When two surfaces of ductile materials are placed in contact and the load exceeds the elastic limit of one of the two materials, plastic deformation or strain occurs. The plastic deformation of one surface when two surfaces are in solid-state contact can occur in the presence or absence of lubricants. In fact, in some instances, the presence of lubricants can increase the deformability of the solid surfaces by a mechanism such as the Rehbinder effect. Plastic deformation of the solid surface is, therefore, observed in the presence of lubricants. Plastic deformation is accommodated by the generation of slip lines for dislocation flow in the solid surface. Dislocations are line defects in the solid and they are site of higher energy state on the surface. Thus, they interact or react more rapidly with certain chemical agents than do the bulk surfaces (Buckley, 1981 Lunarska and Samatowicz, 2000). 

Another key problem with diffusive sintering is that it can occur by several diffusion mechanisms as shown in Fig. 9.33. Viscous and plastic flow are two simple possibilities but another five are readily distinguished, including solid-state diffusion, grain boundary diffusion, surface diffusion, gas phase transport, and liquid layer transport. These inevitably form a neck at the particle contact, to reduce the sharp curvature of the contact region. 

Under some conditions, especially close to the melting point, many materials usually considered to be sohds will exhibit sufficient plastic flow in the surface that capillary forces will slowly, but within a reasonable time frame, come into play to move the surface toward equilibrium, or at least a lower energy situation. A prime practical example of such action is the sintering of solids. If a solid powder—metallic, crystalline, or amorphous—is heated to some temperature below its melting point, usually, but not always, with some applied pressure, sintering or fusion of adjacent particles will occur (Fig. 7.3). 

Phase transitions with decreasing density were achieved also in experiments involving an irradiation of substances [238], ball milling of crystals [239, 240], and static compression with shear [241-243]. Each of these processes involves a partial or total amorphization of the solid, by generating numerous defects, creating ultra-tine particles with highly active surfaces, or causing plastic flows, respectively. 

The repeated impact of liquid against a solid surface using an intermittent jet at sufficiently high speeds leads to a form of erosion. At impact, a liquid drop produces a very high compressive stress in the vicinity of the area of contact, and this is followed by outward, radial flow of liquid at very high speed which shears and erodes the surface. Erosion-corrosion in alpha-brass comprises two parts. The initial progressive plastic indentation is followed by the formation of an annulus where pits gradually form. 

The discussion up to this point has focused on the role of free surfaces and internal interfaces, such as grain boundaries, in mass diffusion. Surfaces produced internally in the material as a consequence of permanent deformation and damage induced by stress can also serve, in some cases, as paths along which enhanced atomic diffusion may occur. In amorphous solids undergoing active plastic flow, such increased atomic mobility along shear bands can result in the formation of nanocrystalline particles locally at the bands. An example of such crystallization process is illustrated in this section for the case of a bulk amorphous metallic alloy subjected to quasi-static nanoindentation at room temperature. 

The described scheme, illustrating the loss of crack stability due to the action of external extension stresses, is valid only in the case of an ideal brittle fracture of the solid. In more complex cases of fracturing involving plastic flow, the real work of the formation of new surfaces, the effective surface free energy, o, includes the work associated with plastic deformations, that is, the energy of distortion, w, in the vicinity of a crack ... 

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