Influence of counterbody surface hardness of a friction pair "steel-steel" on tribological behaviour of zinc nanopowder in oil/ Plieno ir plieno trinties poros kontrkuno kietumo itaka tribologiniam cinko nanodaleliu alyvoje poveikiui.

Jankauskas, V. ; Belyaev, S.

1. Introduction

One of the means to increase reliability and longevity of machine parts and devices is improvement of the quality of lubricating materials by introducing to them special additives made on the basis of nanoparticles of metals or alloys. In greases and oils nanoparticles are used as an antiwear and/or antifriction agent [1]. Many works have shown that addition of nanopowder of copper, bismuth and other metals as well as their combinations may noticeably improve tribological properties of lubricants [2-5]. These additives are not the universal means for reducing friction and wear. As an example may be zinc oxide nanopowder added to synthetic lubricant when it has caused an increase in wear in a steel friction pair in research tests [5].

These contradictory results are caused by the lack of systematization of equivalent studies. In particular, the factors which influence the efficiency of a lubricating effect in additives are seldom considered in research. These factors are numerous and they are determined by kinematics and by operation characteristics of a friction pair. In research [6] these factors are divided into physicochemical, operational, kinematic and technological questionable. Efficiency of additives is well analysed as to the influence of physicochemical and operational factors. The role of the rest of the factors remains vague [6, 7].

The objective of this research is to determine the influence of zinc nanopowder additive in mineral oil on the values of friction and wear of machine steel details when using specimens of different hardness for sliding friction pairs with a point contact. The given factor--hardness combination of specimens--is a design-technological factor. The mutual arrangement of specimen materials in due hardness may correspond to either so-called direct friction pair (movable specimen is harder than the fixed one) or reverse friction pair (movable specimen is softer than the fixed one). In higher kinematic friction pairs (line or point contact) the combination of materials "hard--hard" is also suitable [8]. Nano-resolution lubrication characteristics are influenced by the direction of processing traces of the surface [9].

Tribotechnical lubricant characteristics being linked to numerous physicochemical processes undergoing in friction and its environment, the structure of surface friction has been studied and the composition of its elements has been determined in this research.

2. Methodology of experiments

Experiments have been carried out at the room temperature, at relative humidity of air of 50% with a tri bometer CSM Instruments according to the scheme "Ballon-Disk" (Fig. 1). Disks are made of carbon steel, AISI 1080 (C--0.8%, Si - 0.25%, Mn - 0.25%, S - 0.01%, P 0.01%). They were used for a reverse friction pair in the state they were delivered (185 HB). Some of the disks were thermally treated to get friction pair with equal hardness (63 HRC - for disk and ball). Roughness of all operating surfaces of disks was [R.sub.a] = 0.1 Lim. Balls were made of steel AISI 52100 (C - 1.01%, Cr - 1.46%, Mn - 0.33%, Si - 0.27%), hardness 63...65 HRC. Diameter of the balls 3.0 mm, roughness [R.sub.a] = 0.1 L m.

[FIGURE 1 OMITTED]

All experiments were carried out at sliding speed 0.22 m/s and loaded by 10 N for 0.5 h. The sliding path of a ball over the disk surface was 400 m.

Mineral oil I-20 (GOST 20799-88) was used for standard lubricant. Zinc nanopowder was produced by electric wire explosion. The average size of nanopowder particles--100 nm. Oil-soluble concentrate of zinc nanopowder in mineral oil was obtained by magneticturbulent apparatus HPMS TK22. The concentrate of zinc nanopowder was added in proportion of 22:1 (by volume) to mineral oil and mechanically mixed to prepare metal-containing (suitable for further modification) oil. The final concentrate of zinc nanoparticles in modified oil was 5 g/l.

Calculation of contact stresses according to Hertz theory under the indicated loading conditions of contact geometry and materials showed that the maximal contact stresses were 2.32 GPa, mean - 1.55 GPa. Disks wear was evaluated from the width of friction path (track) by optical microscope Axiovert 200 MAT, and also from the depth of the track by contactless surface analyzer-profilometer Stil3D Micromeasure. Prior and after the experiments the specimens were cleaned in an ultrasound tub CT-420C. The surfaces of friction were analyzed by the methods of optical microscopy (Axiovert 200 MAT) and microradiography (adapter-analyzer Camebax Microbeam). Concen trating profiles of the elements on the surface areas of disks were determined by the layer-by-layer method of Auger-spectroscopy on spectrometer "Schhuna-2". The energy of electrons of probing beam was 3 keV. Ionic etching of the surfaces of specimens was done by argon ions. The inclination of the beam to the surface normal was 70[degrees]. During the analysis the probing beam was running up in the raster at a diameter of 0.3 mm. The analysis place was chosen by the image on the TV monitor.

3. Results and discussion

The effect of nanoparticles on friction coefficient of disks with different hardness is shown in Figs. 2 and 3.

In Fig. 2 it is seen that when lubricating with modified oil the dependence of the coefficient of friction in a friction pair with thermally non-treated disk has an obvious tendency to decrease compared to the case when nonadditivated mineral oil was used, Fig. 2, b. However, the presence of sporadic surges of friction coefficient appearing from time to time during the experiment corresponds to both cases.

[FIGURE 2 OMITTED]

When the surfaces of equal hardness are in frictional coupling, the tendency of decrease in the values of friction coefficient does not hold in both cases of using different lubricants, Fig. 3, a, b.

In a case when mineral oil was used (Fig. 3, b) the values of friction coefficient were unstable along the whole friction run resulting in its specific fluctuating curve. Addition of zinc nanopowder has differently affected the behaviour of friction coefficient - its values were also unstable along the friction run, moreover, this instability developed due to frequent emergence of peak values similar to those in the former dependences (Fig. 3), here the maximum has reached the value of 0.117.

Table 1 presents wear rates of the disks with different hardness tested with and without nanoparticles. When the friction pairs with a tempered (hardened) disk were lubricated with modified oil, it can be noted that the width of the wear path has increased by 20.5% compared to the case when the mineral oil was used. An opposite took place in the friction pair with a thermally non-treated disk. Then lubrication with modified oil caused reduction in the wear path width by 10%.

[FIGURE 3 OMITTED]

Table 1
Results of wear measurement (* values correspond to
thermally treated specimens)

Test lubricant            Mean wear scar    Cross-section of
                          width of disk    wear scar (trace),
                            samples, um    [micro][m.sup.2]2

Standard mineral oil           147 195*           99.0 18.3*
Mineral oil with Zn          132 (-10%)        80.1 (-19.1%)
nanoparticles             235* (+20.5%)       20.6* (+12.6%)

The further surface analysis (profilographing) of friction disks in the direction perpendicular to sliding has also shown that addition of zinc nanopowder has contributed to wear decrease when the friction pair was operating with a thermally non-treated disk. Depth of the wear path has significantly decreased, Fig. 4.

Surface analysis (profilographing) of the friction surfaces of hardened disks has revealed no difference between the values of the depth of analogous profiles, Fig. 5.

It may be connected with insignificant penetration of a ball to the coupling surface of the same hardness during the friction process. When the hardness was equal, immobile specimens (balls) wear out more intensively than the mobile ones (disk).

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Thus, introduction of zinc nanopowder into oil clearly affects the wear of steel surfaces depending on the difference in their hardness and it reduces (insignificantly) the coefficient of friction compared to the conventional oil only in operation of the friction pair of non-hardened specimens.

Fig. 6 presents optical images of friction surfaces of disks. In thermally non-treated specimens the friction surfaces do not show any obvious external difference among each other. Mat colour of friction surfaces indicates the presence of the surface films which have been formed during the friction process. A darker part of friction surfaces of hardened disks (Fig. 6, c, d) also indicates the formation of films and this process was more intensive. The friction surface lubricated with nanoparticles contain ing oil (Fig. 6, c) differs from the similar friction surface by a more evolved contour. Numerous defects in the form of scars of 2.4 Lim width distributed in the sliding direction are evident on the images; less defects are noticed on the friction surface of the specimen tested in mineral oil (Fig. 6, d).

In Auger spectral study it was estimated that oxygen containing thin layers (films) have been probably formed of non-stoichiometric composition. Layer-by-layer ionic etching of the modified surface made it possible to obtain concentrating elements profiles which show a considerable change (decrease) in the oxygen content in terms of penetrating it from the surface deeper into the specimens at a distance of some dozens of nanometers, Fig. 7. It concerns especially the thermally treated specimens because a considerable change in the oxygen composition took place in the surface volumes of about 20 nm thickness. If at the very surface of thermally non-treated specimen the concentration of oxygen was 40% (AT), then at the treated one it was 60%, Fig. 7. In both thermally treated and non-treated specimens tested in mineral oil correspondingly the same condition of elements composition was found. Consequently, oxidation intensity of steel surfaces of the layers was not attributed to the presence of zinc nanoparticles in mineral oil.

[FIGURE 6 OMITTED]

No zinc particles were detected on any friction surface either by Auger spectroscopy or by microradiogra phy.

[FIGURE 7 OMITTED]

The obtained data enable us to conclude the following. Under the selected conditions of a frictional contact, the influence of additives was based on the appearance of indirect effects of chemical interaction of nanoparticles with lubricant oxygen during the friction process, and also on the deformation character of these particles getting to the point of the direct contact. It is known that addition of metals or their salts enables to catalyze or to inhibit the oxidation process of mineral oils and plastic lubricants [11]. Results of the earlier performed analysis on determination of antioxidant stability of base oil and oil with zinc additive have shown that the particles have also a catalytic effect [12].

Consequently, zinc nanopowder in mineral oil has to promote the reduction in oxygen concentration (in molecular or bounded form) which is always present in commercial oils [10]. Oxygen access to the lubricated friction surfaces decreases. In the absence of the other active surface elements in lubricants, the number of local destruction points (secondary structures) in oxide film usually increases. Then numerous points of their abrasion, cracks and peeling-off appear. Nevertheless, the optical images of friction surfaces presented in Fig. 6 do not confirm the presence of secondary structures damages in the specimens tested with both modified and mineral oil. In spite of a catalytic effect of nanopowders, the amount of oxygen in the zone of friction was sufficient. It is indicated by the Auger-spectroscopy data, Fig. 7. This situation results from the method of lubricant supply to the friction zone during the experimental operation - friction pairs were lubricated by shallow immersion of the disk into the lubricating tub. According to the literature on this subject, oxidation of oil as a stimulator of the oxide films formation practically has no expressed value in such friction pairs (owing to considerable lubricants saturation with air in a thin lubricating layer) [10].

Apart from the catalytic effect, the nanopowder particles entering the friction zone at unlimited access of oxygen could determine the oxidation rate of surface layers (due to their high reaction capacity to gasses and liquids) and, consequently, they determine chemical composition and extent of damage of secondary structures. For example, zinc powder particles under friction can exert a corrosion influence on iron (steel). Inducing intensive oxidation of iron (steel) and without entering permanent responses with it, zinc nanoparticles may lead to the formation of oxides which are non-resistant to the wear [11]. In contrast to copper powder particles, the zinc particles can enter the low temperature reaction with oxygen and can abrasively affect the oxidized surfaces [11, 12]. Supposing the formation of oxide ZnO in the process of friction, it should be noted that it is not as hard as the conventional iron oxides. But in friction pairs "steel-steel" (even in nanocrystalline state) it frequently plays the role of abrasive particles under boundary lubrication and dry friction [12].

It should be mentioned that oxygen diffusion in steel increased at the contact of friction surfaces of equal hardness, see Fig. 7, a. It occurred due to more intensive frictional heat-up of thermally treated steels possessing high mechanical properties. In thermally treated material the deformation scale decreases (owing to the higher ultimate strength and yielding point), whereas saturation with oxygen increases in the volumes of local, close to the surface materials due to the repeated overstrain of a ferrite component of sorbite mixture and higher temperature at the contact. Consequently, there is a greater probability of oxidation of zinc particles between hard surfaces an as a sequence - their negative influence on the wear and friction coefficient of the specimens. Emergence of sporadic surges of friction coefficient when lubricating with modified oil is connected with the hitting of the direct contact point of oxidized agglomerates of zinc nanoparticles. At the same time, when lubricating with mineral oil the emergence of surges of the values on the corresponding curve (see Fig. 2 b) may be caused either by microhardening of the friction surfaces or by steel wear products hitting the contact. Nevertheless, during the experiments made with a thermally non-treated disk, the zinc particles had a positive influence on durability (wear resistance) of a coupling, probably because of a lower friction heat-up of the contact indicated by a moderately expressed oxidation of local surface volumes of steel AISI 1080 at the same depth level. In that case emergence of the particles might have been purely mechanical. During friction when lubricants contain metallic powder, oxidation phenomena might be dominated by a mechanical effect caused by the nanocrystalline state of added particles. For example, the copper surface layer with a nanocrystalline structure was noticed practically not to harden, in contrast to the surface layer of copper of polycrystalline structure [13]. In connection with this the wear rate of materials significantly differed. Dislocation activity of nanocrystals go down because of crushing of a grain and under the load of friction plastic deformation takes place in them during the intergrain sliding. Under friction, weak hardening of materials of nanocrystalline structures probably primarily corresponds to the rotating type of deformation, not to the shifting one.

4. Conclusions

1. In accordance with the obtained results, the introduction of zinc nanoparticles into lubricating oil during the operation of a friction pair "steel-steel" with thermally non-treated counterbody (disk) has resulted in reduction in wear of tested specimens. During the operation of a friction pair having thermally treated disk the wear of specimens has considerably increased. In the first case the values of friction coefficient have changed slightly, having in mind general tendency of their decrease during the experiment, however in case of the thermally treated disk these values as a whole remained similar to those which were obtained when lubricating the oil without zinc particles.

2. Under the contact stresses up to 2.32 GPa when lubricating with oil containing nanopowder particles, no protecting zinc containing layers are formed on the friction pairs "steel-steel".

3. Lubrication action of a metallic additive takes place due to the effect related to the interaction of zinc nanoparticles with lubricant oxygen and mechanical deformation of these particles striking the spots of the direct contact.

Acknowledgements

This research was funded by Research Council of Lithuania.

Received March 02, 2010

Accepted June 03, 2010

References

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V. Jankauskas *, S. Belyaev **

* Lithuanian University of Agriculture, Studentu 15, 53361 Kaunas-Akademija, Lithuania, E-mail: vytenis.jankauskas@lzuu.lt

** Tomsk State University of Architecture and Building, Solyanaya sq. 2, 634003 Tomsk, Russia, E-mail: sabvt@rambler.ru

Table 1
Results of wear measurement (* values correspond to
thermally treated specimens)

Test lubricant            Mean wear scar    Cross-section of
                          width of disk    wear scar (trace),
                            samples, um    [micro][m.sup.2]2

Standard mineral oil           147 195*           99.0 18.3*
Mineral oil with Zn          132 (-10%)        80.1 (-19.1%)
nanoparticles             235* (+20.5%)       20.6* (+12.6%)