Engineering Tribology Episode 2 Part 8 docx

400 ENGINEERING TRIBOLOGY disulphide and dilauryl hydrogen phosphate. By using these additives separately and together the effect of phosphorus, sulphur and phosphorus-sulphur on seizure load was found [109]. 0 100 200 300 400 012345 Sliding speed [m/s] Seizure load [N] Combined sulphur and phosphorus Sulphur only Phosphorus only FIGURE 8.52 Comparison of seizure loads for sulphur, phosphorus and sulphur-phosphorus enriched lubricants [109]. It can be seen that although the phosphorous additive by itself is ineffective as compared to the sulphur additive, the combination of phosphorus and sulphur is significantly better than either additive acting in isolation. Unfortunately the Timken test imposes severe sliding conditions on the lubricant which may not be representative of typical operating conditions of practical machinery. The IAE (Institute of Automotive Engineers) and IP (Institute of Petroleum) 166 gear tests conducted with the same additives revealed that the phosphorus- based additive allowed the same seizure or failure loads for a much smaller concentration than the sulphur-based additive [109]. The critical difference between the Timken test and the gear tests is the slide/roll ratio. The Timken test involves pure sliding while the gear tests only impose sliding combined with rolling. It appears that the sulphur originated surface films are more resistant to the shearing of pure sliding than films formed from phosphorous additives. The chemistry of steel surfaces after lubrication by sulphur-phosphorus oils was also studied [109,110]. Films found on wear scars formed under severe conditions, e.g. the Timken test, consisted mostly of sulphur. However, under milder load and lower slide/roll ratios, which are characteristic for general machinery, it was found that phosphorus predominates in the wear scar films. This pattern of film chemistry versus sliding severity is illustrated schematically in Figure 8.53. It can be seen from Figure 8.53 that a sulphur-phosphorus based lubricant provides considerable versatility in lubricating performance. The sulphur is essential to prevent seizure under abnormally high loads and speeds while phosphorus maintains low friction and wear rates under normal operating conditions. TEAM LRN BOUNDARY AND EXTREME PRESSURE LUBRICATION 401 Low friction property critical Anti-seziure property critical Phosphorus Sulphur Phosphorus based films Sulphur based films Low Phosphorous-sulphur composite films Moderate High (pure sliding)Slide/roll ratio: Moderate HighLoad severity: Moderate Steady Steady ShockLoad type: FIGURE 8.53 Dependence of sulphur-phosphorus wear scar film chemistry on severity of sliding conditions. The relative benefits of sulphur versus phosphorus can also be discussed in terms of their ability to provide effective lubrication under shock loading. It was found that sulphur based additives tend to provide better lubrication, i.e. maintain a moderate coefficient of friction, during a precipitate increase in load than phosphorus based additives [109]. Phosphorus based additives are characterized by a progressive decline in friction and accumulation of phosphorus on the worn surface. It appears that in these cases mechanisms other than sacrificial film lubrication may be involved. The most probable mechanism seems to be lubrication by an amorphous layer which was discussed previously. Temperature Distress Temperature distress is a term used to describe high friction occurring over a relatively narrow band of intermediate temperature in lubrication by an oil. An example of this effect is shown in Figure 8.54 which illustrates the friction coefficient versus temperature results from a four-ball test where the lubricant tested is white oil with tributylphosphate [111]. The tests were conducted at a relatively high contact stress of approximately 2 [GPa] and, to ensure negligible frictional transient temperatures, at a very low sliding speed of 0.2 [mm/s]. Friction, initially moderate at room temperature, rises to a peak between 100 - 150°C followed by a sharp decline at higher temperatures. This phenomenon is the result of a significant difference between the desorption temperature of surfactants from the steel surface and the lowest temperature where rapid sacrificial film formation can occur. In this test, the surfactants were relatively scarce consisting only of impurities or oxidation products in the white oil. In practical oil formulations, however, surfactants are carefully chosen so that the desorption temperature is higher than the ‘start temperature’ of sacrificial film lubrication. The concept of wide temperature range lubrication which is achieved by employing in tandem adsorption and sacrificial film lubrication is illustrated in Figure 8.55. It can be seen that when only the fatty acid is applied, the coefficient of friction is quite low below a critical temperature and then sharply rises. Conversely when the E.P. additive (in an E.P. lubricant) is acting alone, the coefficient of friction remains high below a critical TEAM LRN 402 ENGINEERING TRIBOLOGY 0 0.1 0.2 0.3 µ 0 100 200 300 Temperature [°C] FIGURE 8.54 Experimental friction characteristic of a phosphate E.P. lubricant versus temperature [111]. temperature and then there is a sharp drop. Effective lubrication, i.e. a low coefficient of friction over a wide range of temperatures, is obtained when these two additive types are combined. This model of temperature distress assumes that the mechanisms of adsorption and sacrificial film lubrication are entirely independent. The formation of partially oxidized sulphide films can influence the desorption temperature so that the range of temperature distress is not necessarily the exact temperature difference between desorption and sacrificial film formation acting in isolation. 0 0.1 0.2 0.3 0.4 0.5 µ Temperature T r Paraffin oil Fatty acid EP lubricant Mixture of EP lubricant and fatty acid EP lubricant reacts with the surfaces at temperature T r FIGURE 8.55 Co-application of adsorption and sacrificial film lubrication to ensure a wide temperature range of lubrication function [6]. TEAM LRN BOUNDARY AND EXTREME PRESSURE LUBRICATION 403 Speed Limitations of Sacrificial Film Mechanism As discussed in this chapter, sacrificial films formed on severely loaded surfaces require some finite period of time to reform between successive sliding contacts. In most research it is assumed that the formation time is so short that it does not exert a significant limitation on lubricant performance. It was found, for example, that E.P. additives were effective in raising the maximum load before scuffing only at low sliding speeds [112]. In low speed tests performed under pure sliding using a pin-on-ring machine, when an E.P. additive was present, the scuffing load was increased by a factor of 2 compared to that of plain oil. At higher speeds the E.P. additives had almost no effect on the scuffing load. It is speculated that at high speeds the sacrificial films did not form and as a result the E.P. additives were ineffective. Tribo-emission From Worn Surfaces Tribo-emission is a term describing the emission of electrons, ions and photons as a response to friction and wear processes. The mechanisms involved in tribo-emission are complex and not known in detail [130]. However, it is speculated that triboemission precedes and is even necessary for tribochemical reactions to occur in the tribocontact. The best researched is the emission of already mentioned low energy electrons (Figures 8.44 and 8.45), also called exoelectrons. One of the mechanisms proposed, involving tribo-emission of electrons, is described below. During wear surface cracks are generated as a result of severe deformation of the worn surface. In general, when a crack forms there is an imbalance of electrons on opposite faces of the crack [e.g. 126-128]. This imbalance is particularly evident in ionic solids which are composed of alternating layers of anions and cations. For example, when a crack develops in aluminium oxide, one side of the crack will contain oxide anions while the opposite side will contain aluminium cations. The narrow gap between opposing faces of a crack causes formation of a large electric field gradient (electric field gradient is controlled by the distance between opposite electric charges). This electric field is sufficient to cause electron escape from the anions [128]. It is believed that not all the electrons which escape from the anions are collected by the cations on the opposing crack face. This results in tribo-emission or the release of electrons into the wider environment under the action of sliding. The phenomenon is schematically illustrated in Figure 8.56. In dry sliding tests under vacuum, ceramics exhibit a strong tribo-emission of electrons because of their ionic crystalline structure while metals reveal a lesser tendency since the high electron mobility in a metal tends to equalize electron distribution on either side of the crack. Tribo-emission also occurs during sliding in air or under a lubricant but the electrons are not easily detected as their path length in air is much shorter than that in vacuum. Water and possibly other gases or liquids may influence tribo-emission of electrons by chemisorption on the exposed surfaces of the crack about to release electrons. Irradiation by high energy radiation such as gamma-rays appears to activate worn surfaces to significantly raise the level of tribo-emission, the detailed physical causes of this phenomenon are still poorly understood [129]. Tribo-emission of positive and negative ions, as well photons, has been detected during wear of ceramics in n-butane of various pressure [127]. In this case the wear mechanism was explained in terms of gas discharge due to high electric field generated on the wear surface when charges are separated. The ionized gas molecules may then recombine generating molecules different from the original gas. A completely different mechanism of tribo- emission was also suggested for a similar ceramic-diamond abrasive contact [130]. Tribo- emission from MgO scratched by diamond was attributed to excited defects created by abrasion in the solid phase. TEAM LRN 404 ENGINEERING TRIBOLOGY Crack formation Tribo-emission Generation of electric field – + + + + + – – – – Strong electric field Electron capture Electron escape (diversion of electron caused by thermal vibration of anion and cation) Cations Anions Figure 8.56 Schematic illustration of the mechanism of crack-induced tribo-emission. The tribo-emission accelerate chemical reactions such as oxidation or polymerization of the lubricant under boundary lubrication conditions [127] and is an example of mechanical activation. The tribo-emission is beneficial if it promotes formation of wear and friction reducing surface films but is harmful if these films or a lubricant are degraded to produce a sludge or other forms of debris. Therefore it is important to know whether the tribo- emission triggers the tribochemical reactions and whether these reaction products influence wear and friction characteristics. 8.6 BOUNDARY AND E.P. LUBRICATION OF NON-METALLIC SURFACES Most of the discussion on boundary and E.P. lubrication in this chapter refers to lubrication of metallic surfaces. Increased interest in the tribological applications of ceramics has resulted in more research into boundary lubrication of ceramics, especially at elevated temperatures. Both E.P. [131] and detergent-type additives [132] were found to form boundary lubricating layers on silicon nitride in the ‘four-ball’ tester. EDX analysis revealed, however, than the tribochemical reactions on silicon nitride were different from those found on steel surfaces when the same detergent-type additives were used. Since ceramics are less reactive than metals the effectiveness of typical adsorption and antiwear additives in many cases appears to be lower for ceramic-ceramic contacts than for ceramic-metal contacts [133]. Although a sacrificial iron phosphate film was detected on the silicon nitride surface when it was slid against steel with vapour phase lubrication of oleic acid and TCP, the triboreaction took place on the steel surface [133]. When self-mated silicon nitride was lubricated by the same vapour phase much higher wear was recorded. On the other hand, boundary lubrication by sacrificial films of oxides and hydroxides is much more effective for ceramics than for metals [117]. For example, silicon nitride can be lubricated by thin layers of silicon oxide and alumina by alumina hydroxide formed in the tribocontact. In contrast with E.P. sacrificial films on metal surfaces, ceramic oxides and hydroxides do not require high temperatures to be generated. More information on lubrication of ceramics can be found in Chapter 16. 8.7 SUMMARY Lubrication by chemical and physical interaction between an oil-based lubricant and a surface (usually metal) is essential to the operation of most practical machinery. Four basic forms of this lubrication are identified: (i) the formation of an ultra-viscous layer close to the worn surface, (ii) the shielding of an oxidized metal surface by a mono-molecular layer of adsorbed linear surfactants, (iii) the separation of contacting surfaces by entrapped layers of finely divided and perhaps amorphous debris and (iv) the suppression of metal to metal contact at extreme pressures by the temperature dependent formation of sacrificial films of corrosion product on worn metallic surfaces. Each lubrication mechanism has certain merits and disadvantages but they all contribute to the reduction of wear and friction under conditions where other lubrication mechanisms such as hydrodynamic and elastohydrodynamic TEAM LRN BOUNDARY AND EXTREME PRESSURE LUBRICATION 405 lubrication are ineffective. This is achieved by the addition of some relatively cheap and simple chemicals to the oil. It is possible to describe fairly precisely how a particular additive functions in terms of friction and wear control. However, the prediction of lubricant performance from chemical specification is still not possible and this constrains research to testing for specific applications. This task remains a future challenge for research. REFERENCES 1 C.M. Allen and E. Drauglis, Boundary Layer Lubrication: Monolayer or Multilayer, Wear, Vol. 14, 1969, pp. 363-384. 2 G.J. 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Reid, Liquid Phase Adsorption/Reaction Studies of Organo-Sulfur Compounds and their Load Carrying Mechanism, ASLE Transactions, Vol. 16, 1973, pp. 50-60. 103 D. Godfrey, The Lubrication Mechanism of Tricresylphosphate on Steel, ASLE Transactions, Vol. 8, 1965, pp. 1-11. TEAM LRN BOUNDARY AND EXTREME PRESSURE LUBRICATION 409 104 E.H. Loeser, R.C. Wiquist and S.B. Twist, Cam and Tappet Lubrication, Part III, Radio-Active Study of Phosphorus in the E.P. Film, ASLE Transactions, Vol. 1, 1958, pp. 329-335. 105 P.A. Willermet, S.K. Kandah, W.O. Siegl and R.E. Chase, The Influence of Molecular Oxygen on Wear Protection by Surface-Active Compounds, ASLE Transactions, Vol. 26, 1983, pp. 523-531. 106 M. Kawamura, K. Fujita and K. Ninomiya, Lubrication Properties of Surface Films Under Dry Conditions, Journal of JSLE., International Edition, No. 2, 1981, pp. 157-162. 107 P.V. Kotvis, L. Huezo, W.S. Millman and W.T. Tysoe, The Surface Decomposition and Extreme-Pressure Tribological Properties of Highly Chlorinated Methanes and Ethanes on Ferrous Surfaces, Wear, Vol. 147, 1991, pp. 401-419. 108 D. Ozimina and C. Kajdas, Tribological Properties and Action Mechanism of Complex Compounds of Sn(II) and Sn(IV) in Lubrication of Steel, ASLE Transactions, Vol. 30, 1987, pp. 508-519. 109 K. Kubo, Y. Shimakawa and M. Kibukawa, Study on the Load Carrying Mechanism of Sulphur-Phosphorus Type Lubricants, Proc. JSLE. Int. Tribology Conf., 8-10 July, 1985, Tokyo, Japan, Elsevier, pp. 661-666. 110 A. Masuko, M. Hirata and H. Watanabe, Electron Probe Microanalysis of Wear Scars of Timken Test Blocks on Sulfur-Phosphorus Type Industrial Gear Oils, ASLE Transactions, Vol. 20, 1977, pp. 304-308. 111 R.M. Matveevsky, Temperature of the Tribochemical Reaction Between Extreme-Pressure (E.P.) Additives and Metals, Tribology International, Vol. 4, 1971, pp. 97-98. 112 G. 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Transactions, Vol 39, 1996, pp 795 -8 02 1 32 R.S Gates and S.M Hsu, Silicon Nitride Boundary Lubrication: Effect of Sulfonate, Phenate and Salicylate Compounds, Tribology Transactions, Vol 43, 20 00, pp 26 9 -27 4 133 W Liu, E.E Klaus and J.L Duda, Wear Behaviour of Steel-on-Si3 N 4 and Systems with Vapor Phase Lubrication of Oleic Acid and TCP, Wear , Vol 21 4, 19 98, pp 20 7 -21 1 TEAM LRN 9 9.1 S O L I D L U... lubrication to molybdenum TEAM LRN 422 ENGINEERING TRIBOLOGY disulphide and for this reason interest has been limited, although there are some exceptions [e.g 39] µ 0.4 Unlubricated 0.3 0 .2 Lubricated with mineral oil 0.1 0 Indium film 4 µm thick 0 10 20 30 40 50 60 70 80 90 100 Load [N] FIGURE 9.14 Effect of indium surface film on the frictional characteristics of steel [2] Soft plastic metals can also...410 ENGINEERING TRIBOLOGY 1 28 C Kajdas, Physics and Chemistry of Tribological Wear, Proceedings of the 10th International Tribology Colloquium, Technische Akademie Esslingen, Ostfildern, Germany, 9-11 January, 1996, Volume I, (editor: Wilfried J Bartz), publ Technische Akademie Esslingen, 1996, pp 37- 62 129 Y Enomoto, H Ohuchi and S Mori, Electron Emission... steady state value between 2 - 4 [µm], and is maintained until failure of the film by the blistering process occurs TEAM LRN SOLID LUBRICATION AND SURFACE TREATMENTS µ 419 0.6 0.5 0.4 0.3 0 .2 0.1 0 0 500 1000 1500 20 00 Specimen temperature [°C] FIGURE 9.11 Friction coefficient of two types of graphite in a vacuum versus temperature [22 ] · Carbon-Based Materials Other than Graphite Apart from graphite, other... Ceramics During Sliding, Proceedings of the First Asia International Conference on Tribology, ASIATRIB' 98, Beijing, publ Tsinghua University Press, 19 98, pp 669-6 72 130 J.T Dickinson, L Scudiero, K Yasuda, M-W Kim and S.C Langford, Dynamic Tribological Probes: Particle Emission and Transient Electrical Measurements, Tribology Letters, Vol 3, 1997, pp 53-67 131 R.S Gates and S.M Hsu, Silicon Nitride... Figure 9.1 TEAM LRN 4 12 ENGINEERING TRIBOLOGY Initial position Position after sliding Planes of low shear resistance allow relative movement between lamellae Lamellar crystal structure of solid lubricant FIGURE 9.1 Mechanism of lubrication by lamellar solids This intuitive model of solid lubrication, which still has not been unequivocally demonstrated, was formally stated by Bragg in 1 9 28 [1] to explain... studied at high temperatures [35] Although these lubricants give satisfactory performance at temperatures above 26 0°C with a coefficient of friction about 0 .2, at temperatures below 26 0°C they exhibit a high coefficient of friction at low sliding velocities and poor adhesion to the substrate 9 .2. 2 REDUCTION OF FRICTION BY SOFT METALLIC FILMS Soft plastic metals such as gold, silver, indium and lead have... range from room temperature to about 80 0°C, where decomposition of the molybdenum disulphide to molybdenum metal and gaseous sulphur occurs [22 ] The effect of water on the frictional performance of molybdenum disulphide is only slight For example, it was found that when dry nitrogen was replaced by moist nitrogen the coefficient of friction increased from 0.1 to 0 .2 [23 ] In air, trace amounts of water... undistorted lamellae is less prone to blistering than distorted lamellae acting alone The TEAM LRN 4 18 ENGINEERING TRIBOLOGY model of the beneficial effect of graphite on the durability of solid lubricating films is illustrated schematically in Figure 9.10 Undamaged graphite lamellae Crinkled and oxidized MoS2 lamella Uncrinkled graphite lamellae maintain smooth movement FIGURE 9.10 Schematic illustration... [25 ] It was found that the lubricating performance of phthalocyanine in sliding steel contacts is quite similar to graphite but inferior to molybdenum disulphide [26 ] The load carrying mechanism of phthalocyanine depends on a visible film of material deposited on the surface, in a manner similar to graphite and molybdenum disulphide [26 ] On the other hand, it has also been found that for TEAM LRN 420 . Thin Solid Films, Vol. 180 , 1 989 , pp. 28 7 -29 1. 22 H. Okabe, T. Ohmori and M. Masuko, A Study on Friction-Polymer Type Additives, Proc. JSLE. Int. Tribology Conf., 8- 10 July 1 985 , Tokyo, Publ. Elsevier,. Mech. Engrs. Publ., London, 1963, pp. 70 -80 . 37 A. Dyson, Scuffing, A Review, Tribology International, Vol. 8, 1975, Part 1: pp. 77 -87 , Part 2: pp. 117- 122 . 38 H. Blok, Les Temperatures de Surface. Transactions, Vol. 24 , 1 981 , pp. 467-473. 34 E.P. Kingsbury, Some Aspects of the Thermal Desorption of a Boundary Lubricant, Journal of Applied Physics, Vol. 29 , 19 58, pp. 88 8 -89 1. 35 C.N. Rowe,