Rehbinder Effects

Rehbinder and co-workers were pioneers in the study of environmental effects on the strength of solids [144], As discussed by Frumkin and others [143-145], the measured hardness of a metal immersed in an electrolyte solution varies with applied potential in the manner of an electrocapillary curve (see Section V-7). A dramatic demonstration of this so-called Rehbinder effect is the easy deformation of single crystals of tin and of zinc if the surface is coated with an oleic acid monolayer [144]. 

By increasing the surface area, screw dislocations moving at or through increase the surface area, hence the surface energy. As a result, surface active agents that affect the surface energy have an effect on near-surface screw dislocation motion (Likhtman, Rehbinder, and Karpenko, 1958). These effects are known collectively as Rehbinder Effects. See also see other papers, for example, Westwood (1963). 

In the 1920s and 1930s, Rehbinder (Rehbinder and Likhtman, 1957) recognized that the presence of certain organic acids on the surfaces of solids resulted in a surface softening or a reduction in the mechanical properties of solids. The Rehbinder effect produced increases in plasticity in the presence of surface active materials. 

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). 

Finally, the investigation of the specifics of the Rehbinder effect (see Chapter IX, 4) in polymer materials, e.g. the formation of nanofibrous structures in the course of polymer... 

IX.4. Physico-Chemical Phenomena in Processes of Deformation and Fracture of Solids. The Rehbinder Effect... 

When one talks about reversibility of the Rehbinder effect, the presence of a thermodynamically stable interface between mutually saturated solid phase and the liquid, as well as complete disappearance of these effects upon the removal of the medium (e.g. by evaporation) are implied. These features emphasize principal difference between the Rehbinder effect and corrosive action of the medium. At the same time, one has to realize that it is not possible to draw here a distinct border line. The term disintegration covers a broad range of processes from idealized cases of purely mechanical breaking to destruction by corrosion or dissolution. The Rehbinder effect, i.e. the lowering of strength due to adsorption and chemisorption, stress-caused corrosion, and corrosion fatigue, occupies some intermediate place between these extremes. All these phenomena represent a certain degree of combination between the mechanical work performed by external forces and chemical (physico-chemical) interaction with the medium. 

Let us turn to a detailed examination of factors belonging to Group I., and just briefly mention the role that the structure of a solid and conditions of deformation play. The factors listed in Group I are those through which the Rehbinder effect, i.e. the sharp decrease in strength due to adsorption, reveals in a most drastic way. We will also discuss possible applications of these effects, as well as means to prevent the harm that they may cause [9]. 

This scheme of the loss of crack stability due to the action of external tensile stresses is valid only in the case of an ideally brittle fracture. Further, while discussing the role that deformation conditions and the structure of solid play in the Rehbinder effect, we will extend these considerations to objects in which fracture is accompanied by significant plastic deformation. We will also discuss the nature of primary cracks, as well as conditions under which they appear. 

Higher temperature may result in a weaker Rehbinder effect as well. This occurs due to the facilitation of a plastic flow at elevated temperatures. Thermal fluctuations result in the relaxation of deformational microheterogeneities. As a result, at elevated temperatures local concentrations of stresses are too low to initiate the formation of primary microcracks. An increase in temperature thus often leads to a transition from brittle fracture in the presence of adsorption-active medium to plastic deformation. The decrease in the rate of deformation of a solid has an analogous effect slow deformation also results in an increased probability of the thermally activated relaxation of locally concentrated deformations and stresses. 

Deformation of a solid object in an adsorption-active medium under the conditions when no cracks develop and no fracture takes place, allows one to highlight another type of Rehbinder effect, namely the adsorption plasticizing of a solid [10]. In this effect the adsorption-active medium, while lowering the surface energy, also facilitates the development of new surfaces, which always occurs during deformation of solids. If some constant load is applied to the solid object, the presence of active medium increases the rate of plastic deformation, de/dt (Fig. IX-36, a). At constant deformation rate the deformation resistance decreases, i.e. the yield stress, P becomes lower (Fig. IX-36, b). 

Pertsov, N.V., Summ, B.D., The Rehbinder Effect, Nauka, Moscow, 1966... 

Before closing the book on lubrication, there are a few additional points of interest that should be mentioned briefly. Two potentially important ones from a practical standpoint are the so-called Rehbinder effect and weeping lubrication. The Rehbinder effect relates to the effects of the adsorption on the mechanical strength of materials. While there exists some uncertainty on the matter, there is significant evidence that the adsorption of surfactants or other materials onto surfaces, especially in cracks and surface flaws, can reduce the mechanical strength of the material. 

Unlike many practical problems, wear seems to be a complex phenomenon that has not lent itself to the formulation of useful generalizations or the generation of laws of wear. The safest things one can say about the subject is that wear increases with time of operation, with severity of operating conditions (e.g., load, speed, temperature), and appears to be more severe with soft and brittle materials than with hard surfaces. Unfortunately, there are many exceptions to those three generalizations. In addition, the existence of chemical wear, the Rehbinder effect, and other factors complicate matters... 

This model leads also to interesting comparison with embrittlement by surface active liquids (Rehbinder effect) or by intergranular segregation. As viscoelastic losses... 

These results show that there may be differences in the ability to prevent wear under extrane loading conditions, depending on the SML/ESMIS ratio employed and, hence, on the stability of the adsorbed film. This behavior of the SML/ESMIS mixtures can be explained on the basis of the so-called Rehbinder effect [130-133]. 

In the discussion of the reversibility of the Rehbinder effect, it was implied that there is a thermodynamically stable interface present between the mutually saturated solid phase and liquid medium and that the effect vanishes when the liquid medium is ranoved, for example, by evaporation. These two peculiarities make the Rehbinder effect principally different from the corrosion caused by the action of aggressive media. At the same time, one must realize that complete segregation is not possible various processes can cover a fairly broad spectrum from idealized cases involving purely mechanical failure to purely corrosive processes (or dissolution). The Rehbinder effect, which involves the adsorption-induced lowering of strength, stress-facilitated corrosion, and corrosive fatigue, often occupies intermediate positions in these series. In this type of phenomenon, the action of external forces and the action of chemically active media both contribute to the net result in certain proportions. 

INFLUENCE OF AN ACTIVE MEDIUM ON THE MECHANICAL PROPERTIES OF SOLIDS THE REHBINDER EFFECT... 

This section includes two large essays on the Rehbinder effect, which involves a reversible physical-chemical (adsorption) action on the part of the medium on the mechanical behavior of solids, with an emphasis on the two main aspects associated with the effect. The first is the nniversal nature and selectiveness of the Rehbinder effect as a function of the chemical nature of the solid phase and the dispersion medium. The second aspect is related to the dependence of the extent and specific type of the effect on the experimental conditions and the real (defect) structure of the subject material. These two essays contain a summary of the studies conducted at the Russian Institnte of Physical Chemistry and Moscow State University over the years. 

Role of the Actual Structure of the Solid and Role of External Conditions IN THE Manifestation of the Rehbinder Effect and the Deformation of Solids... 

FIGURE 7.17 The shift in a crystal due to a shift caused by an edge dislocation. (From Shchukin, E.D., Rehbinder effect, in Science and Mankind, 1970, pp. 336—367.)... 

Along with metals, the threshold of forced cold brittleness is also observed in solids of all other kinds, that is, covalent crystals (e.g., in the system germanium-gold), ionic substances (e.g., sodium chloride in the melted aluminum chloride), and molecular crystals (e.g., naphthalene in liquid hydrocarbon). In the other words, there is only a limited interval of optimum temperatures in which the Rehbinder effect is observed. At temperatures that are too low, the effect is retarded by the excessive starting brittleness and the solidification of the medium, while at temperatures that are too high, it is retarded by the excessive plasticity of the solid. This temperature dependence is one of the principal features of the Rehbinder effect, which makes it very different from the chanical or corrosive action of the medium, both of which intensify as temperature increases. 

The next section of this chapter describes the results of significant studies by Yushchenko et al. [71-74]. These studies provided insights into the molecular nature of the Rehbinder effect and constituted the first steps toward a numerical simulation of the elementary acts of deformation and bond rupture in the lattice of a solid. The studies also dealt with the influence of adsorption on this process (Sections 1.1 and 4.2). For detailed discussions of these molecular dynamic experiments, the reader is referred to the original works and reviews [71-83]. The quantum mechanical studies of Rehbinder effect were also done by Ab-initio calculations (c-c bond cleavage) [84]. 

A dynamic numerical experiment was used to study the molecular mechanism of the Rehbinder effect using molecular dynamic simulation [71-74]. A numerical experiment allows one to observe tine details of the process being modeled. However, the experiment is limited in terms of the size of the system (up to 10 particles) and the observation time (for argon this time is 10" ° s). Consequently, processes involving a large number of particles, or processes that take place over periods significantly exceeding the maximum time of dynamic-type numerical experiments, can t be modeled. 

While in a single-component system at selected temperatures and deformation rates, the observed sheer is plastic (Figure 7.34), in the presence of foreign atoms (adsorption-active medium), the formation of a brittle crack takes place, that is, one directly observes the Rehbinder effect (Figure 7.36). Each given experiment is characterized by its individual deformation, but in general falls into one of the three main categories ... 

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