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Some aspects of boundary lubrication in the local contact of friction surfaces
Wear, 126 (1988) 69 - 78
SOME ASPECTS OF BOUNDARY LUBRICATION IN THE LOCAL CONTACT OF FRICTION SURFACES M. V. RAYIKO and N. F. DMYTRYCHENKO Kiev Institute
of Civil Aviation
(Received January 5,1988;
Engineers,
252058,
Kiev-58,
Komarova
1 (U.S.S.R.)
accepted February 9, 1988)
Summary Boundary lubrication and its related phenomena play an important role in the lubrication of highly loaded concentrated contacts. In this paper, the non-viscous boundary films, which are produced by various oils and greases in the local contact of friction surfaces, are carefully studied over a long time by measuring the voltage drop in the normal glow discharge regime between the two disks; this simulates gear tooth contact conditions. The boundary films (adsorption and chemical formation) are the result of physical and chemical adsorption, real chemical reactions between lubrications, in~lud~g those between additives and friction surfaces, the action of oxygen as one of the active reactants and the~omech~ic~ effects. Studies on the lubricating properties of boundary films have been performed by many research workers. However, it is a serious and important problem to achieve a longer life for machine elements using antiwear effects in the formation of these films.
1. Introduction The el~tohy~odyn~ic (EHD) lub~~ation theory has been extensively developed in recent years in order to describe lubrication processes in the concentrated contact of friction pairs that are widely used in the modern engineering. However, many data and the practice of applying additives indicate the essential role that the lubricating processes plays in the boundary processes. The measurements of lubrication film thickness, frictional losses, wear and temperature on roller models and industrial gear reducers enable us to clarify a number of questions concerning the role of boundary processes which are not explained by the existing theory. The thickness of the lubricating film was determined by measuring the voltage drop in the normal glow discharge regime. This method was proposed by Brix [I] and has been developed for many years at the Kiev Institute of Civil Aviation Engineers. 0043.1648/88/$3.50
0 EIsevier Sequoia/Printed
in The Netherlands
70
Disk machines MU-lM, MU-1MP and SMC-2 were used together with closed-cycle two-stage tooth-gear reducers which were installed on special stands and industrial types of gear reducer. The samples for the disk machines were in the form of cylindrical steel 45 disks, the diameter of which changed in relation with the sliding required. Involute and Novikov’s (different types of gearing) tooth gears were installed in the gear reducers used. All the experimental stands were equipped with a current collector and with systems used for measuring the frictional losses, temperature, load and frequency of rotation. The current collector was used to measure the film thickness of the lubricating film by determining the voltage drop in this layer in the normal glow discharge regime [l, 21. To determine the relationship between the voltage drop and the thickness of the lubricant layer in micrometres, we used non-rotatable cylindrical disks which had previously formed boundary layers, the distance between them being measured with an accuracy of 0.1 pm; the special device with rotating disks was calibrated with the help of X-rays, and the special unit with profiled disks was designed in the U.S.S.R. [ 31. The relationship between oils and the self-generating organic film (SOF) formed was established within the range of measurement error. Five oils were applied in the experiments (additives were not used): oil 1 had a viscosity of 20 mm2 s-i at 50 “C; oil 2 had a viscosity of 45 mm2 s-i at 50 “C; oil 3 had a viscosity of 50 mm 2 s-i at 50 “C; oil 4 had a viscosity of 20 mm2 s-l at 100 “C; oil 5 had a viscosity of 32 mm2 s-l at 100 “C. These oils contain various separate hydrocarbons: paraffin, naphthene and aromatic substances. 2. Experimental results and discussion The measurement on two-stage reducers, i.e. in real machines under a wide range of conditions (e.g. peripheral speed of 1 - 70 m s-l) and using different oils, has enabled us to compare theoretical and experimental lubrication film thicknesses (Fig. 1). Two areas exist which have different responses to external conditions (these areas are denoted A and B in what follows). The experimental values of h are less than or equal to the theoretical values in area A. This deflection can be explained easily by considering the loose tooth contact, dynamic load influence and essential friction surface roughness (the teeth were not subjected to a final processing). No experimental value of h exceeding the theoretical value was recorded in area A. The main hydrodynamic laws hold here, and h depends on speed to a power of 0.490 (the average value), on viscosity to a power of 0.386 - 0.454 (the value of 0.386 was calculated for different temperatures and a single type of oil, and the value of 0.454 for a single temperature and different oil types). These data show that the lubrication effect depends more on the temperature than on the individual oil properties. The situation is very different in area B. The values of h measured in most experiments (regimes) exceeded those calculated, although the states of
71
Fig. 1. Comparison of experimental h and theoretical h, values of the oil film thickness: line 1, coincidence of experimental and theoretical; line 2, upper limit of results which are compared; line 3, lower limit of results which are compared. Tests were done with oils 3 - 5 over a wide range of conditions.
the friction surfaces were identical. The observed improvement of the lubricating effect over that predicted with the EHD theory in the area of thin lubricat~g films (up to 1 - 1.5 pm) is of particular interest since it takes place under conditions often accompanied by intensive wear and faults. Area B is characterized by a considerable decrease in speed and in the influence of viscosity. The value of h depends on speed to a power of 0.2 or less and in some regimes, it does not depend on speed and viscosity at all. The influence of the load decreased also, indicating the growth of oil rigidity. It can be stated that the fern-thicken~g effect is caused by the nonhydrodynamic lubricating processes. The effect was observed for most conditions examined but was not inevitable. It can be simulated by modification of the oil composition and friction conditions. The experiments on preliminary run-in and refined cylinder rollers imitating toothing show that the film thickness increased slowly with respect to the calculated value. The process of film formation resulted not only in film thickening but also in a higher load resistance (Fig. 2(a)). The film formed demonstrated the after effect when the contact conditions were changed; this does not correspond to the properties of viscous liquid flow. This effect can be considered as the adaptation of the lubricating film to new conditions and results from o~~ization of the film structure. The relation between the shear rate S and the shear stress r was established to define the rheological state of the lubricating films in the rolling contact with various sliding speeds of rolling. It presents the complete Ostwald curve for the formed films only. The intervals of elastic lubricating film deformation at rather a small sliding speed were also observed as well as the intervals at ~~rrnedia~ speeds which correspond to elastic structure destruction and the viscous flow intervals. The last type was of a non-linear character because of the intensive heat emission at the corresponding sliding rate [4].
72
h.Mm,
0
I
I
I
IO
20
30
I
(a)
(b) Fig. 2. (a) Variation in the film thickness under loading (oil 2, 25 - 28 “C; uz = 0.96 m s-l, us = 0.08 m s-l): curve 1, after formation of a boundary-lubricating layer on friction surfaces; curve 2, before the beginning of the oil film formation. (b) The pattern of oil film thickness variation with time (after-effect): curve 1, load increase; curve 2, load decrease.
More examples can be considered, but that described above still shows that the improvement that the lubrication effect has on the thin films is produced by the non-viscous films of special structure and special rheological nature. These films determine the lubrication effect over a wide range including gears operating at a speed of up to 8 - 10 m s-l and a temperature over 60 - 80 “C. The non-viscous boundary films can be formed as the result of physical and chemical adsorption and real chemical reactions. Their structures range from liquid crystals to real crystals. Results on organic solid-like oil film formation in local contact have been published by Rayiko [ 51. These films have become known as friction polymers [6 -171, but not all the polymers produced during the lubrication contact fix themselves on the friction surfaces. The term self-generating organic films seems to be more correct. This type of formation is shown in Fig. 3. We have studied SOFs by IR techniques and consider that this formation consists of a mixture of molecules of low molecular masses. SOFs are formed on the contacting surface at the boundary-lubricating state and differ from physically adsorbed boundary layers and from the layers which are formed as a result of reactions between active additives and the metal used. They differ in composition, physicomechanical properties, structure, bonding energy, antifriction and antiwear characteristics. SOFs are
(a) Fig. 3. Scanning electron photomicrographs
(b) of SOFs on the specimen.
formed as a result of the oxidation of lubricating material components, the destruction of molecules and the polymerization due to catalysis of metalorganic substances formed during friction. The presence of an SOF was detected by means of a spectral analysis of the frictional films. In the experiments the formation and changes in SOF thickness were determined from the electrical ~h~ac~risti~s of the contact which reflected some pec~i~ties in the rheology and mechanical characteristics of these films. SOF formation was finally detected by the specific colours of the friction surfaces which were due to the SOFs. The electrical method of measurement enabled us to determine the advantages of such films with respect to the outer non-viscous films; the surfaces separate at most loaded points and under most heavy conditions. SOFs produce some increase in the coefficient of friction under certain conditions but protect the surface against wear better than do other viscous and nonviscous films [IS]. Investigations using rollers show that the antipitting resistance can be essentially increased if SOF generation is guaranteed. Figure 4 presents the results obtained on low hardness steel rollers without an SOF (at a contact temperature of 25 - 28 “C), and with the SOF at 75 - 90 “C. The second test conditions are much worse from an overall point of view. If there is no SOF on the surface, then conditions at which pitting starts correlate well with the theoretical data (curves A and B). However, pitting was not recorded at all under the conditions for SOF formation, although the number of cycles (reduced) was more than 15 times greater than the calculated value. The SOF had protected the surface against pitting even when the contact tension was twice as much. For hardened and carburized steel covered with an SOF, pitting started at a number of cycles which was a factor of 4 - 6 higher [19]_ The oil temperature was kept constant in these tests. The SOF emerging on the steel rollers of mean hardness gave, on average, an increase in the number of cycles.
74
Fig. 4. Antipitting effect of the SOF (oil 3): 0, crumbling of specimen without an SOF (25 - 28 “C); 0, x, specimen with an SOF (75 - 90 %I!), experiments were interrupted before crumbling appears; points 1 - 3, steel, Brine11hardness of 140 HB, uz = 0.96 m s-t, us = 0.00 m s--r; points 4 - 6, steel, Brine11 hardness of 180 HB, VI: = 0.48 m s-l, us = 0.04 m 6-l; points 7 - 9, steel, Brine11 hardness of 180 HB, IQ = 0.96 m s-r, us = 0.06 m 6 -‘. line A, fatigue curve for steel of Brine11 hardness 140 HB; line B, fatigue curve for steel of Brine11hardness 180 HB.
The influence of the SOF on wear was investigated using thermoprocessed alloyed steel gears under industrial operation conditions. Wear was reduced to a half when the formation of an SOF is guaranteed by lubricator selection. The same effect was observed using rollers operating in the startstop mode. Figure 5 illustrates the antiscoring effect of the SOF. Curve 1 shows the change in h without an SOF, and curve 2 the change in h after the formation of a stable SOF. The separate measurement of the thickness of the lubricating film components enables us to determine the change in hydrodynamic component thickness (curve 3).
Fig. 5. Antiscoring effect of the SOF (oil 4; ux = 0.96 m 6-l; us = 0.08 m s-l; UH = 600 MPa): curve 1, f, heating at a rate of 5 “C min-‘; curve 2, f, heating at a rate of 1 “C min-‘; curve 3, h, change in lubricating layer thickness under heating (the SOF thickness is not taken into account).
75
The state of the friction surface which is covered with the SOF is characterized by the constant electrode potential. This shows the constant heterogeneity of the metal surface, the thermodynamic stability, and the smaller misorientation of the crystal lattice. The thickness of the plastically deformed layer is the most marked feature of the influence of the SOF. It is 30 - 40 pm without an SOF and approximately 10 times less with an SOF (under the same conditions) [ 191. The main reasons for the improvement in the state of the friction surface are as follows. The SOF reduces the concentration of tension on the surface asperities since it covers them with a solid-like film. It reduces the t~gential tension in metal by encapsulat~g the shearing strain. The SOF protects the surface against corrosive substances dissolved in the oil (e.g. oxygen). Self-regulation is one of the most valuable properties of the SOF. As the operating conditions become more severe (i.e. as the temperature, load or sliding speed increases) the generation of an SOF is enhanced. For instance, the SOF thickness is constant in the temperature interval 120 220 “C for the majority of oils and gases tested. Tests on various metal friction pairs show that all types of lubricators, mineral and synthetic oils, plastic and semiliquid greases are able to form SOFs, but to a different extent. Formation is the result of contact tension and sliding which lead to the destruction of molecules and surface metal activation. Temperatu~ has an essential influence. For every lubricator, there exists an operating condition at which the most intensive SOF forms and which has the highest antiwear effect. This depends on the hydrocarbon composition of the lubricator. Tests of individual naphthene, paraffin and aromatic series hydrocarbons show that the temperature and sliding speed under which SOF is generated depend on both the molecular mass and the series. The measurement of wear and frictional moment confirm these data. For the hydrocarbons tested, the temperature of SOF formation ranges from 100 to 200 “c [ZO]. Figure 6 demonstrates the correlation between the SOF thickness and the coefficient of friction on the one hand and temperature on the other hand for hydrocarbons and mixtures of hydrocarbons. Curve 3 shows the model for the interaction of hydrocarbons in natural oil. The area of SOF destruction which corresponds to hydrocarbons of larger molecular mass decreases in the figure. The SOF formation is influenced greatly by gases which saturate oil [21]. Forced oil aeration increases the oxygen content; nitrogen bubbling decreases the oxygen concentration essentially and slows down SOF generation, reducing the film thickness. Stable SOFs are not formed when argon is bubbled through oil, thereby ensuring that the oil contains hardly any oxygen at all. The ability to hydrocarbons to form SOFs is influenced by gases in the same way [ 221. The change in lubricating film thickness inside the SOF formation area in a rolling and sliding contact is shown in Fig. 7. The decrease in thickness is forced by slowing down {and stopping) the SOF generation. SOF thickening
76
5)
400
4P
300
3P
2al
2P I,0 0
‘NJ -60
-20
20
60
100
440
b”C
Fig. 6. Oil film discharge voltage AU (curves 1 - 3) and frictional losses T (curves la, 2a and 3a) due to hydrocarbons and a mixture of hydrocarbons as functions of temperature (steel samples; Brine11 hardness, 200 HB; ux = 1.41 m s-r; u, = 0.23 m SC’; OH = 500 MPa): curves 1 and la, H-octane; curves 2 and 2a, H-octadecane; curves 3 and 3a, Hoctane + 10% H-octadecane.
Fig. 7. Oil film discharge voltage AU as a function of molecular mass M of paraffinic hydrocarbons (vx = 2.8 m s-r; oH = 500 MPa): curve 1, aeration by argon; curve 2, aeration by nitrogen; curve 3, aeration by air.
stops when the molecular mass of the lubricator reaches 300 and this coincides with the data of Bowden and Tabor [23] on the values for the coefficient of friction. These values stop decreasing at the same molecular mass and become stable at larger molecular masses. The experiments with rollers which simulated gear tooth contact conditions enabled us to establish that the wear intensity and frictional losses are effected by the interaction between lubrication films of a different nature. The measurements show that the thicknesses of the hydrodynamic films are summed, and this improves the lubrication effect. The interaction of the boundary films is more complex. Both adsorption film and SOFs are formed at the local contact. First the adsorption films emerge and then (depending
on the conditions) they are replaced by SOFs which are optimal for the local contact. The replacement includes a change in the mechanism of i~te~ction between the lubricator and the metal and a change in the structure and rheology of the lubricating films. This process is not accompanied by wear in stable and nearly stable conditions if the type of oil is properly selected. Under unstable conditions (e.g. heating, stop-start mode of operation, or spinning motion), this process results in a temporary worsening of the lubricating effect and may cause wear and scoring depending on the type of oil.
3. Conclusions Me~~ements of the thickness of lubricating layer of steel units and elements of tooth reduction gears and rolling bearings in local contact allow the following conclusions to be drawn. (1) There exist two regions: the region of hydrodynamic influence and the region in which the lubricating actions of the boundary process occur. In the first region the essential laws of EHD theory are observed and the effectiveness is lower than estimated, The second region is wider than thought by many specialists. It includes low roiling speeds and increased temperatures. The effectiveness of this region is not determined by the speed and viscosity. (2) Boundary-lubricating layers are characterized by a non-viscous rheological state, an increased loading ability, after-action effects, a long period of formation and sensitivity to ambient conditions. These characteristic features are connected to the structural organization of the materials and the character of their bonds to the metal used. (3) Local contacts of metals during rolling with sliding, when lubricated with petroleum and synthetic lubricants, are characterized by stable boundary chemosorbtion layers of definite type by SGFs. (4) The formation of SUFs on friction surfaces changes the state of the surface layer, leads to an increase in the resistance to crumbling and reduces wear owing to rubbing-off. Acknowledgments The authors wish to thank their colleagues who participated in this work. They are grateful to Dr. R. Gohar, Dr. G, Wan and Mr. P. Garden (Imperial College of Science and Technology) for their interest, many helpful comments and help, References 1 V. H. &ix,
Electrical study of boundary lubrication, Lubrication of Gear Teeth, Tekhnika,
2 M. V. Rayiko,
Aircr. Eng., 19 (1947) Kiev, 1910.
R94.
78 3 U.S.S.R. Patent N 1023226, 19 -1 M. V. Rayiko, V. V. Zaporoshets et al., Rheological characteristics of zone contact and structure of friction surfaces, Trenie Iznos Mash., 6 (4) (1983) 1008 - 1015. 5 M. V. Rayiko, Properties of lubricating films in the high temperature, Proc. KIGVF, 1954, p. 12. 6 H. W. Herman and T. F. Egan, Organic deposits on precious metal contacts, Bell Syst. Tech. J., 37 (1958) 739 - 776. 7 R. S. Fein and K. L. Kreus, Chemistry of boundary lubrication of steel by hydrocarbons, ASLE Trans., 8 (1965) 29 - 38. 8 E. E. Klaus and H. D. Bieber, Effect of some physical and chemical properties of lubricants on boundary lubrication, ASLE Trans., 7 (1964) 1 - 10. 9 10 11 12
13
14 15 16 17 18 19 20
21 22 23
D. Godfrey, Chemical changes in steel surfaces during extreme pressure lubrication, ASLE Trans., 5 (1962) 57. S. W. Chaikin, On friction polymer, Wear, 10 (1967) 49 - 60. G. F. Vinogradov, V. V. Arkharova and A. A. Petrov, Anti-wear and anti-friction properties of hydrocarbons under heavy loads, Wear, 4 (1961) 274 - 291. P. I. Sanin, A. B. Vipper, E. S. Shepeleva, V. L. Lashkhi, Yu. A. Lozovoy and A. A. Markov, Interaction of anti-wear additives with friction surfaces, Wear, 30 (1974) 249 265. Y. S. Zaslavsky, R. N. Zaslavsky, M. I. Cherkashin, E. S. Brodsky, I. M. Lukashenko, V. A. Lebedevskaya, S. B. Nikishenko and K. E. Belozevova, Some aspects of friction polymer formation chemistry, Wear, 30 (1974) 267 - 273. A. F. Aksenov, Trenie and Iznos in Hydrocarbons, Mashinostroenie, Moscow, 1971. H. G. Stinton et al., A study of friction polymer formation, ASLE Trans., 3 (1979) 355 - 360. S. M. Hsu and E. E. Klaus, Some chemical effects in boundary lubrication, ASLE Trans., 2 (1979) 135 145. H. Okabe et al., Tribochemical surface reaction and lubricating oil film, ASLE Trans., 1 (1979) 65 - 70. M. V. Rayiko, Some properties of lubricating films under heavy loads, Trenie and Iznos of Machine Elements, Rio Aeroflot, 1959, pp. 227 - 240. M. V. Rayiko and V. P. Kadomskii, Anti-wear properties of solid films forming by mineral oils, Mashinovedenie, 4 (1972) 111 - 117. M. V. Rayiko and V. A. Stadnic, Interaction of hydrocarbons at formation selfgenerating organic films in the local contact of friction surfaces, Frict. Lubr. Mach., 2 (1984) 291 - 292. N. F. Dmytrychenko et al., The application of inert gases during running-in of friction surfaces, Tekhnol. Organ. Proizvod., 1 (1982) 54 - 55. N. F. Dmytrychenko et al., The role of inactive petroleum products in boundary lubrication, Probl. Trenia Iznashivaniya, 1 (1982) 112 - 117. F. P. Bowden and D. Tabor, Friction and Lubrication of Solids, Vols. 1, 2, Clarendon, Oxford, 1964.
SOME ASPECTS OF BOUNDARY LUBRICATION IN THE LOCAL CONTACT OF FRICTION SURFACES M. V. RAYIKO and N. F. DMYTRYCHENKO Kiev Institute
of Civil Aviation
(Received January 5,1988;
Engineers,
252058,
Kiev-58,
Komarova
1 (U.S.S.R.)
accepted February 9, 1988)
Summary Boundary lubrication and its related phenomena play an important role in the lubrication of highly loaded concentrated contacts. In this paper, the non-viscous boundary films, which are produced by various oils and greases in the local contact of friction surfaces, are carefully studied over a long time by measuring the voltage drop in the normal glow discharge regime between the two disks; this simulates gear tooth contact conditions. The boundary films (adsorption and chemical formation) are the result of physical and chemical adsorption, real chemical reactions between lubrications, in~lud~g those between additives and friction surfaces, the action of oxygen as one of the active reactants and the~omech~ic~ effects. Studies on the lubricating properties of boundary films have been performed by many research workers. However, it is a serious and important problem to achieve a longer life for machine elements using antiwear effects in the formation of these films.
1. Introduction The el~tohy~odyn~ic (EHD) lub~~ation theory has been extensively developed in recent years in order to describe lubrication processes in the concentrated contact of friction pairs that are widely used in the modern engineering. However, many data and the practice of applying additives indicate the essential role that the lubricating processes plays in the boundary processes. The measurements of lubrication film thickness, frictional losses, wear and temperature on roller models and industrial gear reducers enable us to clarify a number of questions concerning the role of boundary processes which are not explained by the existing theory. The thickness of the lubricating film was determined by measuring the voltage drop in the normal glow discharge regime. This method was proposed by Brix [I] and has been developed for many years at the Kiev Institute of Civil Aviation Engineers. 0043.1648/88/$3.50
0 EIsevier Sequoia/Printed
in The Netherlands
70
Disk machines MU-lM, MU-1MP and SMC-2 were used together with closed-cycle two-stage tooth-gear reducers which were installed on special stands and industrial types of gear reducer. The samples for the disk machines were in the form of cylindrical steel 45 disks, the diameter of which changed in relation with the sliding required. Involute and Novikov’s (different types of gearing) tooth gears were installed in the gear reducers used. All the experimental stands were equipped with a current collector and with systems used for measuring the frictional losses, temperature, load and frequency of rotation. The current collector was used to measure the film thickness of the lubricating film by determining the voltage drop in this layer in the normal glow discharge regime [l, 21. To determine the relationship between the voltage drop and the thickness of the lubricant layer in micrometres, we used non-rotatable cylindrical disks which had previously formed boundary layers, the distance between them being measured with an accuracy of 0.1 pm; the special device with rotating disks was calibrated with the help of X-rays, and the special unit with profiled disks was designed in the U.S.S.R. [ 31. The relationship between oils and the self-generating organic film (SOF) formed was established within the range of measurement error. Five oils were applied in the experiments (additives were not used): oil 1 had a viscosity of 20 mm2 s-i at 50 “C; oil 2 had a viscosity of 45 mm2 s-i at 50 “C; oil 3 had a viscosity of 50 mm 2 s-i at 50 “C; oil 4 had a viscosity of 20 mm2 s-l at 100 “C; oil 5 had a viscosity of 32 mm2 s-l at 100 “C. These oils contain various separate hydrocarbons: paraffin, naphthene and aromatic substances. 2. Experimental results and discussion The measurement on two-stage reducers, i.e. in real machines under a wide range of conditions (e.g. peripheral speed of 1 - 70 m s-l) and using different oils, has enabled us to compare theoretical and experimental lubrication film thicknesses (Fig. 1). Two areas exist which have different responses to external conditions (these areas are denoted A and B in what follows). The experimental values of h are less than or equal to the theoretical values in area A. This deflection can be explained easily by considering the loose tooth contact, dynamic load influence and essential friction surface roughness (the teeth were not subjected to a final processing). No experimental value of h exceeding the theoretical value was recorded in area A. The main hydrodynamic laws hold here, and h depends on speed to a power of 0.490 (the average value), on viscosity to a power of 0.386 - 0.454 (the value of 0.386 was calculated for different temperatures and a single type of oil, and the value of 0.454 for a single temperature and different oil types). These data show that the lubrication effect depends more on the temperature than on the individual oil properties. The situation is very different in area B. The values of h measured in most experiments (regimes) exceeded those calculated, although the states of
71
Fig. 1. Comparison of experimental h and theoretical h, values of the oil film thickness: line 1, coincidence of experimental and theoretical; line 2, upper limit of results which are compared; line 3, lower limit of results which are compared. Tests were done with oils 3 - 5 over a wide range of conditions.
the friction surfaces were identical. The observed improvement of the lubricating effect over that predicted with the EHD theory in the area of thin lubricat~g films (up to 1 - 1.5 pm) is of particular interest since it takes place under conditions often accompanied by intensive wear and faults. Area B is characterized by a considerable decrease in speed and in the influence of viscosity. The value of h depends on speed to a power of 0.2 or less and in some regimes, it does not depend on speed and viscosity at all. The influence of the load decreased also, indicating the growth of oil rigidity. It can be stated that the fern-thicken~g effect is caused by the nonhydrodynamic lubricating processes. The effect was observed for most conditions examined but was not inevitable. It can be simulated by modification of the oil composition and friction conditions. The experiments on preliminary run-in and refined cylinder rollers imitating toothing show that the film thickness increased slowly with respect to the calculated value. The process of film formation resulted not only in film thickening but also in a higher load resistance (Fig. 2(a)). The film formed demonstrated the after effect when the contact conditions were changed; this does not correspond to the properties of viscous liquid flow. This effect can be considered as the adaptation of the lubricating film to new conditions and results from o~~ization of the film structure. The relation between the shear rate S and the shear stress r was established to define the rheological state of the lubricating films in the rolling contact with various sliding speeds of rolling. It presents the complete Ostwald curve for the formed films only. The intervals of elastic lubricating film deformation at rather a small sliding speed were also observed as well as the intervals at ~~rrnedia~ speeds which correspond to elastic structure destruction and the viscous flow intervals. The last type was of a non-linear character because of the intensive heat emission at the corresponding sliding rate [4].
72
h.Mm,
0
I
I
I
IO
20
30
I
(a)
(b) Fig. 2. (a) Variation in the film thickness under loading (oil 2, 25 - 28 “C; uz = 0.96 m s-l, us = 0.08 m s-l): curve 1, after formation of a boundary-lubricating layer on friction surfaces; curve 2, before the beginning of the oil film formation. (b) The pattern of oil film thickness variation with time (after-effect): curve 1, load increase; curve 2, load decrease.
More examples can be considered, but that described above still shows that the improvement that the lubrication effect has on the thin films is produced by the non-viscous films of special structure and special rheological nature. These films determine the lubrication effect over a wide range including gears operating at a speed of up to 8 - 10 m s-l and a temperature over 60 - 80 “C. The non-viscous boundary films can be formed as the result of physical and chemical adsorption and real chemical reactions. Their structures range from liquid crystals to real crystals. Results on organic solid-like oil film formation in local contact have been published by Rayiko [ 51. These films have become known as friction polymers [6 -171, but not all the polymers produced during the lubrication contact fix themselves on the friction surfaces. The term self-generating organic films seems to be more correct. This type of formation is shown in Fig. 3. We have studied SOFs by IR techniques and consider that this formation consists of a mixture of molecules of low molecular masses. SOFs are formed on the contacting surface at the boundary-lubricating state and differ from physically adsorbed boundary layers and from the layers which are formed as a result of reactions between active additives and the metal used. They differ in composition, physicomechanical properties, structure, bonding energy, antifriction and antiwear characteristics. SOFs are
(a) Fig. 3. Scanning electron photomicrographs
(b) of SOFs on the specimen.
formed as a result of the oxidation of lubricating material components, the destruction of molecules and the polymerization due to catalysis of metalorganic substances formed during friction. The presence of an SOF was detected by means of a spectral analysis of the frictional films. In the experiments the formation and changes in SOF thickness were determined from the electrical ~h~ac~risti~s of the contact which reflected some pec~i~ties in the rheology and mechanical characteristics of these films. SOF formation was finally detected by the specific colours of the friction surfaces which were due to the SOFs. The electrical method of measurement enabled us to determine the advantages of such films with respect to the outer non-viscous films; the surfaces separate at most loaded points and under most heavy conditions. SOFs produce some increase in the coefficient of friction under certain conditions but protect the surface against wear better than do other viscous and nonviscous films [IS]. Investigations using rollers show that the antipitting resistance can be essentially increased if SOF generation is guaranteed. Figure 4 presents the results obtained on low hardness steel rollers without an SOF (at a contact temperature of 25 - 28 “C), and with the SOF at 75 - 90 “C. The second test conditions are much worse from an overall point of view. If there is no SOF on the surface, then conditions at which pitting starts correlate well with the theoretical data (curves A and B). However, pitting was not recorded at all under the conditions for SOF formation, although the number of cycles (reduced) was more than 15 times greater than the calculated value. The SOF had protected the surface against pitting even when the contact tension was twice as much. For hardened and carburized steel covered with an SOF, pitting started at a number of cycles which was a factor of 4 - 6 higher [19]_ The oil temperature was kept constant in these tests. The SOF emerging on the steel rollers of mean hardness gave, on average, an increase in the number of cycles.
74
Fig. 4. Antipitting effect of the SOF (oil 3): 0, crumbling of specimen without an SOF (25 - 28 “C); 0, x, specimen with an SOF (75 - 90 %I!), experiments were interrupted before crumbling appears; points 1 - 3, steel, Brine11hardness of 140 HB, uz = 0.96 m s-t, us = 0.00 m s--r; points 4 - 6, steel, Brine11 hardness of 180 HB, VI: = 0.48 m s-l, us = 0.04 m 6-l; points 7 - 9, steel, Brine11 hardness of 180 HB, IQ = 0.96 m s-r, us = 0.06 m 6 -‘. line A, fatigue curve for steel of Brine11 hardness 140 HB; line B, fatigue curve for steel of Brine11hardness 180 HB.
The influence of the SOF on wear was investigated using thermoprocessed alloyed steel gears under industrial operation conditions. Wear was reduced to a half when the formation of an SOF is guaranteed by lubricator selection. The same effect was observed using rollers operating in the startstop mode. Figure 5 illustrates the antiscoring effect of the SOF. Curve 1 shows the change in h without an SOF, and curve 2 the change in h after the formation of a stable SOF. The separate measurement of the thickness of the lubricating film components enables us to determine the change in hydrodynamic component thickness (curve 3).
Fig. 5. Antiscoring effect of the SOF (oil 4; ux = 0.96 m 6-l; us = 0.08 m s-l; UH = 600 MPa): curve 1, f, heating at a rate of 5 “C min-‘; curve 2, f, heating at a rate of 1 “C min-‘; curve 3, h, change in lubricating layer thickness under heating (the SOF thickness is not taken into account).
75
The state of the friction surface which is covered with the SOF is characterized by the constant electrode potential. This shows the constant heterogeneity of the metal surface, the thermodynamic stability, and the smaller misorientation of the crystal lattice. The thickness of the plastically deformed layer is the most marked feature of the influence of the SOF. It is 30 - 40 pm without an SOF and approximately 10 times less with an SOF (under the same conditions) [ 191. The main reasons for the improvement in the state of the friction surface are as follows. The SOF reduces the concentration of tension on the surface asperities since it covers them with a solid-like film. It reduces the t~gential tension in metal by encapsulat~g the shearing strain. The SOF protects the surface against corrosive substances dissolved in the oil (e.g. oxygen). Self-regulation is one of the most valuable properties of the SOF. As the operating conditions become more severe (i.e. as the temperature, load or sliding speed increases) the generation of an SOF is enhanced. For instance, the SOF thickness is constant in the temperature interval 120 220 “C for the majority of oils and gases tested. Tests on various metal friction pairs show that all types of lubricators, mineral and synthetic oils, plastic and semiliquid greases are able to form SOFs, but to a different extent. Formation is the result of contact tension and sliding which lead to the destruction of molecules and surface metal activation. Temperatu~ has an essential influence. For every lubricator, there exists an operating condition at which the most intensive SOF forms and which has the highest antiwear effect. This depends on the hydrocarbon composition of the lubricator. Tests of individual naphthene, paraffin and aromatic series hydrocarbons show that the temperature and sliding speed under which SOF is generated depend on both the molecular mass and the series. The measurement of wear and frictional moment confirm these data. For the hydrocarbons tested, the temperature of SOF formation ranges from 100 to 200 “c [ZO]. Figure 6 demonstrates the correlation between the SOF thickness and the coefficient of friction on the one hand and temperature on the other hand for hydrocarbons and mixtures of hydrocarbons. Curve 3 shows the model for the interaction of hydrocarbons in natural oil. The area of SOF destruction which corresponds to hydrocarbons of larger molecular mass decreases in the figure. The SOF formation is influenced greatly by gases which saturate oil [21]. Forced oil aeration increases the oxygen content; nitrogen bubbling decreases the oxygen concentration essentially and slows down SOF generation, reducing the film thickness. Stable SOFs are not formed when argon is bubbled through oil, thereby ensuring that the oil contains hardly any oxygen at all. The ability to hydrocarbons to form SOFs is influenced by gases in the same way [ 221. The change in lubricating film thickness inside the SOF formation area in a rolling and sliding contact is shown in Fig. 7. The decrease in thickness is forced by slowing down {and stopping) the SOF generation. SOF thickening
76
5)
400
4P
300
3P
2al
2P I,0 0
‘NJ -60
-20
20
60
100
440
b”C
Fig. 6. Oil film discharge voltage AU (curves 1 - 3) and frictional losses T (curves la, 2a and 3a) due to hydrocarbons and a mixture of hydrocarbons as functions of temperature (steel samples; Brine11 hardness, 200 HB; ux = 1.41 m s-r; u, = 0.23 m SC’; OH = 500 MPa): curves 1 and la, H-octane; curves 2 and 2a, H-octadecane; curves 3 and 3a, Hoctane + 10% H-octadecane.
Fig. 7. Oil film discharge voltage AU as a function of molecular mass M of paraffinic hydrocarbons (vx = 2.8 m s-r; oH = 500 MPa): curve 1, aeration by argon; curve 2, aeration by nitrogen; curve 3, aeration by air.
stops when the molecular mass of the lubricator reaches 300 and this coincides with the data of Bowden and Tabor [23] on the values for the coefficient of friction. These values stop decreasing at the same molecular mass and become stable at larger molecular masses. The experiments with rollers which simulated gear tooth contact conditions enabled us to establish that the wear intensity and frictional losses are effected by the interaction between lubrication films of a different nature. The measurements show that the thicknesses of the hydrodynamic films are summed, and this improves the lubrication effect. The interaction of the boundary films is more complex. Both adsorption film and SOFs are formed at the local contact. First the adsorption films emerge and then (depending
on the conditions) they are replaced by SOFs which are optimal for the local contact. The replacement includes a change in the mechanism of i~te~ction between the lubricator and the metal and a change in the structure and rheology of the lubricating films. This process is not accompanied by wear in stable and nearly stable conditions if the type of oil is properly selected. Under unstable conditions (e.g. heating, stop-start mode of operation, or spinning motion), this process results in a temporary worsening of the lubricating effect and may cause wear and scoring depending on the type of oil.
3. Conclusions Me~~ements of the thickness of lubricating layer of steel units and elements of tooth reduction gears and rolling bearings in local contact allow the following conclusions to be drawn. (1) There exist two regions: the region of hydrodynamic influence and the region in which the lubricating actions of the boundary process occur. In the first region the essential laws of EHD theory are observed and the effectiveness is lower than estimated, The second region is wider than thought by many specialists. It includes low roiling speeds and increased temperatures. The effectiveness of this region is not determined by the speed and viscosity. (2) Boundary-lubricating layers are characterized by a non-viscous rheological state, an increased loading ability, after-action effects, a long period of formation and sensitivity to ambient conditions. These characteristic features are connected to the structural organization of the materials and the character of their bonds to the metal used. (3) Local contacts of metals during rolling with sliding, when lubricated with petroleum and synthetic lubricants, are characterized by stable boundary chemosorbtion layers of definite type by SGFs. (4) The formation of SUFs on friction surfaces changes the state of the surface layer, leads to an increase in the resistance to crumbling and reduces wear owing to rubbing-off. Acknowledgments The authors wish to thank their colleagues who participated in this work. They are grateful to Dr. R. Gohar, Dr. G, Wan and Mr. P. Garden (Imperial College of Science and Technology) for their interest, many helpful comments and help, References 1 V. H. &ix,
Electrical study of boundary lubrication, Lubrication of Gear Teeth, Tekhnika,
2 M. V. Rayiko,
Aircr. Eng., 19 (1947) Kiev, 1910.
R94.
78 3 U.S.S.R. Patent N 1023226, 19 -1 M. V. Rayiko, V. V. Zaporoshets et al., Rheological characteristics of zone contact and structure of friction surfaces, Trenie Iznos Mash., 6 (4) (1983) 1008 - 1015. 5 M. V. Rayiko, Properties of lubricating films in the high temperature, Proc. KIGVF, 1954, p. 12. 6 H. W. Herman and T. F. Egan, Organic deposits on precious metal contacts, Bell Syst. Tech. J., 37 (1958) 739 - 776. 7 R. S. Fein and K. L. Kreus, Chemistry of boundary lubrication of steel by hydrocarbons, ASLE Trans., 8 (1965) 29 - 38. 8 E. E. Klaus and H. D. Bieber, Effect of some physical and chemical properties of lubricants on boundary lubrication, ASLE Trans., 7 (1964) 1 - 10. 9 10 11 12
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14 15 16 17 18 19 20
21 22 23
D. Godfrey, Chemical changes in steel surfaces during extreme pressure lubrication, ASLE Trans., 5 (1962) 57. S. W. Chaikin, On friction polymer, Wear, 10 (1967) 49 - 60. G. F. Vinogradov, V. V. Arkharova and A. A. Petrov, Anti-wear and anti-friction properties of hydrocarbons under heavy loads, Wear, 4 (1961) 274 - 291. P. I. Sanin, A. B. Vipper, E. S. Shepeleva, V. L. Lashkhi, Yu. A. Lozovoy and A. A. Markov, Interaction of anti-wear additives with friction surfaces, Wear, 30 (1974) 249 265. Y. S. Zaslavsky, R. N. Zaslavsky, M. I. Cherkashin, E. S. Brodsky, I. M. Lukashenko, V. A. Lebedevskaya, S. B. Nikishenko and K. E. Belozevova, Some aspects of friction polymer formation chemistry, Wear, 30 (1974) 267 - 273. A. F. Aksenov, Trenie and Iznos in Hydrocarbons, Mashinostroenie, Moscow, 1971. H. G. Stinton et al., A study of friction polymer formation, ASLE Trans., 3 (1979) 355 - 360. S. M. Hsu and E. E. Klaus, Some chemical effects in boundary lubrication, ASLE Trans., 2 (1979) 135 145. H. Okabe et al., Tribochemical surface reaction and lubricating oil film, ASLE Trans., 1 (1979) 65 - 70. M. V. Rayiko, Some properties of lubricating films under heavy loads, Trenie and Iznos of Machine Elements, Rio Aeroflot, 1959, pp. 227 - 240. M. V. Rayiko and V. P. Kadomskii, Anti-wear properties of solid films forming by mineral oils, Mashinovedenie, 4 (1972) 111 - 117. M. V. Rayiko and V. A. Stadnic, Interaction of hydrocarbons at formation selfgenerating organic films in the local contact of friction surfaces, Frict. Lubr. Mach., 2 (1984) 291 - 292. N. F. Dmytrychenko et al., The application of inert gases during running-in of friction surfaces, Tekhnol. Organ. Proizvod., 1 (1982) 54 - 55. N. F. Dmytrychenko et al., The role of inactive petroleum products in boundary lubrication, Probl. Trenia Iznashivaniya, 1 (1982) 112 - 117. F. P. Bowden and D. Tabor, Friction and Lubrication of Solids, Vols. 1, 2, Clarendon, Oxford, 1964.