Friction pair

M. A. Frolov, I. V. Molyavko, A. I. Spivak, D. L. Rakhmankulov, V. R. Rakhmatullin, and N. A. Romanov. Lubricant for friction pairs working under heavy loads—contains mineral oil and additive in form of 2,4,8,10-tetra-oxaspiro-(5,5)-undecane, to improve anti-wear and anti-scratch properties. Patent SU 1817788-A, 1993. 

A 1% ethanol solution (10 cm3) of salicylalaniline was treated in the reactor for 40 min at a velocity of relative sliding of 0.8 m/sec and a loading of 50 kg/cm2 using a nickel (sample)-steel (counterbody) friction pair. The green solution formed was filtered and evaporated. Yield 15%. 

Si3N4 + SiC + BN + C Plates, rods, balls Friction pairs, ball bearings... 

In order to make ILs a more effective lubricant for steel/aluminum friction pair especially at higher loads to reduce corrosion and wear Al matrix, a series of phosphate ester functional group-containing ILs was designed and synthesized. Mu et al. [84-86] synthesized a new series of imidazohum cation-based ILs with 0,0-diethylphosphonyl groups on the alkyl side chain (Fig. 9.12), evaluated the tribological properties of the ILs, and discussed possible mechanisms of action. 

With polymeric materials in friction pairs the working capacity of the assembly is lost for three reasons i) temperature rise in the friction zone above the limiting value, ii) wear exceeding the tolerable limit, and iii) loads over the yield point of the polymeric material. 

Polymer-metal friction pairs are also characterized by frictional transfer of material. This implies the material displacement from the polymer part friction surface onto that of the metal counterbody. Macrotransfer is realized either as a fatigue detachment of polymer particles sticking to the counterbody or as galling, i.e. sticking of the viscous-flow fragments from the polymer surface layer, their extension and movement in the friction direction. 

The specificity of corrosion protection of the inhibited plastic-metal friction pair is liable to tribochemical transformation of the polymer matrix. The products of tribochemical reactions enter into interactions with the elements of the corrosion system and Cl liberated from the plastics ... 

In contrast to film materials, express estimates of the protective ability of inhibited structural plastics have been insufficiently elaborated so far. For example, the effect of inhibited plastics on the wear rate of metal samples has been studied by the authors of [100] using a pendulum tribometer containing a plastic-metal friction pair. Corrosion tests of systems such as inhibited structural plastic-metal counterparts are usually conducted in real operation regimes [44]. 

Chemical reactions on the friction surfaces may in fact appear in any kind of media, although their effect on the friction behavior and wear of the friction pair is often far from apparent. Chemical wear in its broadest sense could mean abrasive wear of the tribocomponents in a corrosive liquid [1,4]. Nevertheless, it would be unfair to relate chemical wear to an individual wear mode since it is just one of the contributing processes exerting a significant effect on other wear modes. 

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. 

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... 

Hence, the dominant factors affecting the wear rate of metals during friction in electrolytes are the corrosion activity of the medium, the pressure in the friction pair and the properties of the counterbody. 

Streams of high-energy particles effectively inhibit wear of a number of pol3mier-metal friction pairs in vacuum [39]. The main role of exposure is in the efficient cleaning of frictional surfaces from contaminants, above all of water molecules. Under the action of a stream of particles structural defects in the pol3mier surface layer are healed and concentration of corrosion-active radicals in the friction zone decreases. 

Frictional characteristics of MPE were studied within two directions. The first is connected with the effective use of polarization currents in Ml-P-M2 systems aimed at improved durability of friction pairs. The second direction is intended to improve wear resistance of pol3uner materials through their structural modification. 

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. 

The friction pair was polarized following a scheme shown in Fig. 4.14b. The working electrode was a metal tray 13 installed on a dielectric support 3. Prism 4 of the pendulum pressed against the bottom of the tray with electrolyte. As an auxiliary electrode, a platinum wire 12 was used. The reference... 

Fig. 4.15. Variation kinetics of polarizing currents of PA-steel friction pair. Numbers on the curves correspond to polarization voltage in volts. Solution of NaCl (10 mole/1) load - 0.3MPa Xi - start %2 - stop of the pendulum...

Proceeding from the above, current generated in M1-P-M2 systems is able to ensure stable polarization of the friction pair. Polarization takes place when such materials as PA, PPl, PVC, polyolefines and some other thermoplastics are used as the lining material. The polarization sources may appear in seals containing polymer linings, in friction joints whose parts are covered by polymer coats, in reinforced polymer articles and so on. These sources may perceptibly affect friction and wear of metal parts operating in hostile media or found between facings in the electrical contact. 

The Mechanochemical Concept of Wear of Metal-Polymer Friction Pairs... 

The nature of the metal counterhody significantly influences the wear of contacting polymers (Table 4.11). Under low loads and friction velocities tribological characteristics of metal-polymer friction pairs depend only slightly on the metal nature. As soon as the load-velocity regime becomes more severe, the role of this factor increses drastically [108]. 

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

This closes the cycle of unfavorable processes in the system and intensifies wear of the friction pair as a result of thermal and mechanical destruction of polyamide. 

Analogous cycles are observed in the polyacetal-steel and polyolefin-steel friction pairs. Physical-chemical processes can exert a favorable effect on the metal-polymer friction pair as well. For instance, metal-containing compounds of copper stearate and oleate types are formed in the contact zone as a result of tribochemical reactions when CFD copolymer rubs against copper. These compounds retard thermally destruction processes in CFD, serving as wear inhibitors (WI) for this system. 

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. 

In practice, the choice of WI is based on the analysis of service, design, economic and other factors. An efficient means of improving wear resistance of the polyamide-steel 45 friction pair turns out to be inhibition of the thermally oxidative and destructive processes in the polymer surface layers to avoid the formation of corrosion-active oxide compounds. It is possible to break this unfavorable cycle by the introduction of antioxidants into the polymer composition, thus disabling macroradicals through the reactions of mechanochemical synthesis, polyamide alloying by functional additives forming separating layers or more thermally stable products, and so on [108]. Application of WI with this aim abates undesirable thermally oxidative processes in polyamide... 

Wear resistance of the polyacetal-metal friction pair can be improved considerably by the introduction of higher fat acids or realizing their S3mthesis conditions in the friction zone. Passivation of metal surface layers by phos-phating formulations and epilamens may elevate wear resistance of friction bodies in which polyacetal, polyamide, fluoroplastics, and other substances rub against copper alloys, aluminum, chrome or titanium [108,117,118]. 

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. 

An analogous effect is observed for the PTFE-chromium pair under lubricated friction with Cl [121]. Wear of both components diminishes noticeably in this case. This is due to passivation of the metal friction surface induced by desorption of Cl components containing active functional groups. As a result, the catalytic effect of chromium oxide compounds on tribocracking of PTFE macromolecules diminishes, leading to a decrease in the friction pair wear. 

G.E. Lazarev, T.L. Kharlamova and V.I. Vereikin. Electrochemical methods for improving the wear resistance of graphite-steel friction pairs Inbricated by liquid aggressive media. Soviet J. Friction and Wear, 1985, Vol. 6, No. 1, pp. 89-92. 

V.A. Struk. Effect of composition and topography of surface compounds of metal counterbody on wear of metal-polymer friction pair. In Mechanics and Physics of Contact Interactions. Kalinin, Kalinin State University, 1979, pp. 69-76. 

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