Electrochemical Processes During Friction

In contrast to the processes commonly considered in electrochemistry, electrochemical processes occur during friction under conditions of moving and deforming discrete contacts of individual microasperities. The participation of electrolytes as a liquid layer in the friction pair leads to potential leaps of 9 3 and 933 the metal-solution interface and to the contact potential difference 9 2 ill metal contacts (see Fig. 1.7) [22]. As a result, a short-circuited galvanic microelement appears with a probability of redox reactions on its electrodes. 

Taking into account the discreteness of the friction contact, heterogeneity of the friction surfaces, micro-inhomogeneity of the metal structure, local plastic deformations and difference in energy between the states of fresh surface and areas covered with oxide films, the friction pair can be treated as an intricate multielectrode system. 

Oscillations may exert a strong effect on adsorption processes in the frictional contact. Adsorption of particles on the electrode with a certain potential is known [23] to occur at a finite speed. Under low oscillation frequencies the adsorption manages to follow the potential and participate in the variation of the interfacial layer structure. At high frequencies the adsorption mechanism does not work, giving place to electrostatic charging of the layer as a condenser, i.e. the generation of the double electric layer (DEL). A mechanical model of the interfacial DEL has been elaborated by Shepenkov [24]. It follows from the model that, if a periodic mechanical force acts on the double layer from the side of the liquid or electrode, the electrode potential will vary periodically with the same excitation frequency. 

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The Kinetics of Electrochemical Processes During Friction of Inhibited Plastics... 

Two interrelated trends can be distinguished in the present day research into electrochemical processes during metal friction (i) the use of electrochemical parameters to derive information on contact processes and (ii) friction and wear monitoring of metals via the regulation of the electrode potential of tribosystems. The second trend, which presupposes an intrusion into the course of the electrochemical processes on the frictional contact, has been actively developing in the recent two or three decades, especially after the discovery of selective transfer under friction, whose nature is largely electrochemical. The topic of the present chapter is connected exactly with this trend. 

L.S. Pinchuk, V.A. Goldade and A.S. Neverov. On the kinetics of electrochemical processes during metal-polymer pair friction in electrolytes. Soviet... 

Mechanochemical wear is a complex process during which electrochemical solution of metals catalyzes fatigue failure. Friction, in turn, activates electrochemical corrosion. As can be seen, the process of mechanochemical wear of metals in electrolytes has a combined fatigue-electrochemical nature. 

Electrochemical processes that occur on the actual contact spot during friction have been comprehensively studied [30] to reveal the mechanism of mechanochemical wearing. Friction was modeled as an intermittent interaction of a sample with a single surface asperity. The asperity slides over a... 

Model experiments were carried out to study the relation between friction and kinetics of electrochemical processes occurring during polarization of metal-polymer pairs [62]. The experiments employed a pendulum tribometer 1 (Fig. 4.14a) whose advantage is the presence of only one friction pair for examination at a time. The tribometer consists of a pendulum 2, a support 3, and a prism 4 on which the pendulum hangs. Support 3 is made as a vessel containing an electrolyte into which the friction surface in the form of one of the prism faces is immersed. The pendulum is set in motion at a constant initial amplitude. Attenuation of oscillations is recorded in terms of contracting amplitudes of the sinusoidal signal from the inductive pickup 6, into which the bow-shaped core 5 is in turn inserted as the pendulum oscillates. 

As has been indicated previously (see Sect. 4.2), polymer parts in friction joints add specific features to electrochemical processes in the friction zone. The polymer components acquire the properties of surfactants during rubbing against metals, which change the electrochemical activity of the metals. In this connection, the effect of the liberation of Cl from inhibited plastics on electrochemical processes in the friction zone has been studied [128]. 

Formation of the tribofilm layer on friction surfaces occurs under the effect of the field in the electrochemical metal,-lubricant-metal2 system, owing to formation of electro-potential (emf), forming free copper tribofilm (Shpenkov, 1995a). Since the process of tribofilm formation takes place during the friction process, disintegration of the reverse micelles takes place in a tribochemical reaction, where a redox reaction occurs, and copper oxide reduces to free copper. 

Sorooshian J, Hetherington D, Philipossian A. Effect of process temperature on coefficient of friction during CMP. Electrochem Solid State Lett 2004 7(10) G222-G224. 

The analysis of corrosion factors during polymer-metal pair wear has proved that the main path is electrochemical protection of the metal counterbody neutralization of corrosion agents formed in the friction zone and suppression of corrosion processes in the polymer-metal contact. These directions are realized by the means illustrated in Fig. 4.6 [37]. They are subdivided into two groups according to the use of special substances or physical fields and power effects. 

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