Mechanochemical wear

Structural inhibited plastics have found application in the sealing of movable and stationary metal-polymer joints operating in hostile environment. They are most efficient in weakly and moderately loaded friction joints, plain bearings and seals. Their main role in metal-polymer friction joints is the reduction or suppression of mechanochemical wear. 

Mechanochemical wear is an intricate process in which corrosive damage and mechanical wear are interrelated [1,2]. The role of corrosive and mechanical factors in corrosion-induced wear of metals in electrodes is estimated in different ways. Some authors consider this process as purely corrosive, others... 

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. 

Let us examine the chief factors affecting the kinetics of mechanochemical wear of metals in electrolytes. 

Corrosion rates and those of mechanochemical wear of stainless steels and alloys widely applied in friction joints of chemical equipment are presented in Table 4.2 [30]. Owing to the formation of passivating protective films on contact with hostile media, these materials display high corrosion resistance. As can be seen from the table, corrosion rates grow during friction by a factor of thousand. Under such conditions, the material wears largely due to corrosion even in a weak solution of sulfuric acid for both sliding friction over a softer material (PE) and under abrasive action (ceramics). 

In addition, the mechanical action intensity affects the mechanochemical wear rate [31]. As the pressure in the steel-graphite friction pair rises from 0.05 to 0.4 MPa, the rate of mechanochemical wear increases from 0.15 to... 

Table 4.2. Corrosion rates (C) and mechanochemical wear rates (/) of steels and alloys at friction over PE and ceramics in 1 N solution of H2SO4 at p = 0.4 MPa [30]...

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... 

It should be emphasized that use of inhibited plastics in metal-polymer friction joints assists in a number of cases in solving the problem of fighting mechanochemical wear through the suppression of corrosion processes in the friction zone by Cl liberated by the polymer counterbody. Yet inhibited plastics are not the only means of corrosion inhibition in metal-polymer joints. 

Means of Inhibiting Mechanochemical Wear in Polymer-Metal Pairs... 

Wear inhibitors (WI) are substances able to alleviate mechanochemical wear in friction joints [18]. The notion of WI helps to generalize numerous means of fighting corrosion in machines and mechanisms independently of their friction joint design and composition. 

Electrical polarization of friction joints is an effective means of fighting mechanochemical wear. Polarization parameters are chosen so as to transfer the metal into a stable passive state. In other words, the metal potential maintained during operation of the friction joint is different from a standard electrode one. The sources of such polarization can be metal-polymer parts and joints of machines and equipment containing M1-P-M2 pairs. 

An example of successful use of the polarized state of polymers for abating mechanochemical wear of friction parts is a sleeve hearing, illustrated in Fig. 4.20 [70]. The friction joint consists of a shaft 1 and a bushing consisting of three coaxial bushes installed in the body 5. The middle bush 4 is made... 

To lower the friction and mechanochemical wear in friction joints it is possible to use solid lubricants [71] containing fuming-oxides (FO) as an active modifier of nonferrous metals. The solid lubricants represent fine-dispersed sublimation products of lead smelting slag. 

Many researchers believe that the products of tribochemical transformations of polymers and metals used as corrosion inhibitors may enhance the wear resistance of metal-pol3mier friction joints operating in hush media. A friction joint can be theoretically presented as a reactor continuously generating products able to lower mechanochemical wear. For practical purposes we should know the regularities of these tribochemical transformations in macromolecules and their relation with the nature and surface parameters of the frictional metal counterbody. 

Thus, it is evident that tribochemical reactions are critical for the formation of antifrictional films on the friction surface of the metal counterbody. A new phase (transfer film or third body) functioning as a wear inhibitor protects metal parts first of all from fatigue failure as well as against adhesive and mechanochemical wear. 

Fig. 4.21. Functional interrelation of the factors promoting mechanochemical wear of the metal-polymer friction pair...

As can be seen, the problem of improving resistance of the metal-polymer friction pair to mechanochemical wearing is determined by the governing cycle of tribochemical reactions that intensifies wear and elimination of conditions stimulating their generation. 

Suppression of Mechanochemical Wear of Metals by Inhibited Plastics 307... 

Proceeding from the above, it can be stated that the concept of mechanochemical wear of metal-pol3mier friction pairs put forward in [110,116] can serve as the basis for expanding the range of methods for fighting wear. 

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