Friction zone

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. 

The wear ratio of the materials tested that corresponds to the wear rate of 0.25 yUm/h is characterized by a wide range of values from 0.2 up to 2.1, which depend to a large extent on the temperature in the friction zone. Thus by using the dependence of the wear ratio on the temperature it is possible to forecast the performance of a true friction unit as it had been similarly done elsewhere (18, 2l). 

Figure 2. Dependence of friction zone temperature (T) (curve 2) and the coefficient of friction (f) (curve l) upon PV.

When metal parts rub in an electrolyte, it is possible to form short-circuit galvanic microelements (Fig. 1.7). Potentials 1E3 and 2E3 appear at the metal-electrolyte interface and contact potential difference 1E2 in the contact sites of the parts. The electromotive force of these elements promotes electrode processes on the friction surfaces. The processes appear even though lEs = 2E3. because of the galvanic elements resulting from crevice corrosion in the friction zone. 

P, M and L are the pol3mier, metal and lubricating medium P, 1/, Cl are the frictional factors, i.e. pressure, velocity and the presence of Cl in the friction zone TCRP are the tribochemical reaction products. The latter can fulfill the function of wear inhibitors (WI) during physical-chemical interactions with the inhibitor on the metal friction surface or form neutral wear products (NWP) affecting neither corrosion nor friction. The task is how to transform these products into useful ones during friction. 

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. 

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. 

In its true sense, any component of a tribosystem promoting suppression of unfavorable tribochemical processes in the friction zone, irrespective of whether it was impregnated or formed internally, can be related to WI [34]. WI can be formed both as a result of physical-chemical interactions of polymers and metals, or physical fields and energy effects on a part or friction joint. This can be, e.g. thermochemical or radiation-thermal treatment, ion implantation, superposition of electrical and magnetic fields, shifts of electrode potentials, passage of electrical current, etc. 

Magnetic fields can be effectively applied to retain insulating lubricating layers possessing magnetic properties within the friction zone. These include magnetic fluids and some fusible metals and alloys referred to as corrosion inhibitors. 

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. 

Fig. 4.14. Scheme of a pendulum tribometer (a) and electrochemical cell for polarization of the friction zone (b) (1) tribometer (2) pendulum (3) support (4) prism (5) bow-shaped core (6) inductive pickup (7) rectifier unit with a generator (8) amplifier (9) self-recording potentiometer (10) voltage stabilizer (11) electrolyte (12) platinum electrode (13) metal tray (14) electrolytic switch (15) vessel with electrolyte (16) reference electrode (17) potentiostat... 

The problem of designing reliable and compact sources of electrical polarization of friction joints is still a weak point of the technique. Polarization of the friction zone by extneral voltage sources has not found wide enough application since they complicate the structure of the machines. So, it is important to find some alternative sources of polarization proceeding from design and technological peculiarities of the friction joint. 

The friction joint has been tested using a shaft on a bush friction machine with 2cm friction area, 0.35 MPa load and 2.4 m/s sliding velocity. A 40-mm-diameter shaft has been made of a carbon steel of 40-45 HRC hardness and 0.8-1.0 xm surface roughness. The outer bush material was aluminum, the inner was copper, the polymer layer was 200-p.m-thick PVB. A 0.1 N solution of NaCl was fed into the friction zone, the wear rate was determined by weighing. The test results are presented in Table 4.7. 

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]. 

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]. 

Proceeding from the above data we can state that electrochemical processes in the metal-pol3mier friction zone can be regulated with the help of Cl, and hence friction and wear of metal-pol3mier joints can be monitored as well. 

The development of inhibited polymer materials for antifrictional purposes has been a new approach to the problem of raising performance and durability of friction joints operating in hostile environments. As experience has shown, protective inhibited lubricants are not always efficient for movable joints where the lubricant is squeezed with time from the friction zone. During long-term operation of a friction joint it is necessary that Cl be continuously supplied to the friction zone and the inhibited antifrictional plastics have proved to be most suitable for this purpose [37]. 

These antifrictional materials of INHAM grade display higher wear resistance under friction in the liquid hostile media compared to non-inhibited plastics. For comparison, the properties of an antifrictional material based on PA6 and molybdenum disulfide without a Cl in its composition are presented in Table 4.13. Its wear resistance is seen to fall short of the INHAM materials since corrosion processes are not suppressed in the friction zone of this material with the metal. 

For a quantitative description of tear process, the Jurkov s equation was used, within which the relaxation time, x, was replaced with the tear rate, /, x with - the value of unitary tear, and the constants y and a with X and X, respectively, where X - constant P - specific friction pressure and p - friction coefficient in the friction zone. In this case, the durability equation can be written as ... 

The effectiveness of additives in the friction zone is connected with their surface activity. Therefore, additives are being sought that, under friction conditions, form a lubricant film that leads to a decrease in the coefficient of friction and wear and has a high load-carrying capacity. In addition, the additives should be efficient at low concentrations, such as 1%. 

Typically, not only a high surface hardness is desired, but also a lower coefficient of friction, which among other factors affects the temperature in the friction zone. Examples include surgical tools, where high temperatures in the bone tissue must be avoided to minimize bone necrosisThe lowering of the friction of biomaterials is also of interest in many other medical devices, such as catheters, needles, stents, joint prostheses. 

Figure 1. Schematic representation of the different steps in the sample preparation a) generation of the tribofilm (Cameron-Plint tester, friction zone dimensions 6x8 mm), b) positioning the altuninum mask (stripes of 1mm width), c) Ar ion etching, d) removal of the mask.

On the fnction image (Figure 5.a), one can notice a low friction zone, in the centre of the track, corresponding to the presence of the tribofilm. The low friction effect of the tribofilm lasts for 220 cycles. After that stick-slip occurs. 

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