Recent research on tribology in Japan

Wear, 100 (1984)

RECENT TOSHIO

543 - 560

RESEARCH

543

ON TRIBOLOGY

IN JAPAN

SAKURAI

Tribology

Laboratory,

Shimocho

I O-l 6, Isogo-ku,

Yokohama

(Japan)

Summary

Recent research on tribology in Japan is outlined. There have been many papers on the following problems. In fluid film lubrication, the effects of surface irregularities on the lubricating performance and increase in friction in high speed bearings due to turbulent flow are highlighted to date. Elastohydrodynamic lubrication and its related problems such as traction play an important role in the lubrication of cams and followers, rolling contact bearings and traction drives. The rheology of lubricants at high pressures is one of the most important properties governing lubricated concentrated contacts. Regarding basic studies on wear, the mechanism of the adhesive wear process is being studied. It is interesting to investigate the deformation behaviours of materials and structures by finite element methods. The friction and wear characteristics of plastics and their composites have been studied in basic terms. The chemical reactions of lubricating oil additives with frictional surfaces play important roles in lubricated concentrated contacts. Recently there have been many studies on the correlation between the lubricating properties and the chemical reactivity of lubricant additives. It is very interesting that there are many papers on the mutual effects between lubricant additives.

1. Introduction

Since the Japan Society of Lubrication Engineers (JSLE) was established in 1955, it has promoted meetings and encouraged the publication of papers of high quality on tribology and has played a much more active role in various tribology projects. On the occasion of the 20th anniversary of the JSLE, the JSLE-ASLE International Lubrication Conference was held in Tokyo on June 9 - 11, 1975. The Conference stimulated conspicuously young tribologists in Japan. A further JSLE international tribology conference is to be held in Tokyo on July 8 - 10, 1985, to celebrate the 30th anniversary of the foundation of the JSLE. The JSLE meetings are held twice a year, in the spring and the autumn, and at these meetings 80 - 90 papers are usually presented and discussed. Eleven research committees are 0043-1648/84/$3.00

0 Elsevier Sequoia/Printed

in The Netherlands

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provided which are sponsored by the JSLE to investigate the following projects: (1) effect of temperature on the life of grease; (‘2) investigation of future aspects of lub~cation technology; (3) con~mination study; (4) transient phenomena followed by friction; (5) lubrication by solid materials; (6) gas lubrication; (7) rolling fatigue; (8) seals; (9) wear and antiwear materials; (10) tribology in plastic working; (11) lubrication and wear in marine engines. In the following sections, recent research on tribology in Japan will be outlined. 2. Hydrodynamic

lubrication

In fluid film lubrication, the effects of surface irregularities on lubricating performance and the increase in friction in high speed bearings due to turbulent flow are h~~gh~d to date. Nakahara and coworkers have performed a study of the influences of surface roughness on frictional characteristics in hydrodynamic lubrication using an apparatus consisting of a glass disk and pads having various known surface roughnesses [ 1 - 31. The friction with transverse roughness was found to be higher than that with a smooth surface and longitudinal roughness [ 13. This inclination became marked when pads having a small aspect ratio were used [2]. The Pat+Cheng average flow model was applied to the finite width rectangular pivoted thrust bearing with regular onedimensional surface roughness. The calculated results were compared with the experimental findings mentioned above and a qualitative correlation was obtained [ 21. They also found that the contactinitiat~g Sommerfeld number for transverse and lon~tud~~ roughness increased and decreased respectively with increasing pad aspect ratio. The increase in friction with decreasing Sommerfeld number occurred without solid-to-solid contact which was confirmed by a contact resistance method [31. Running-in is one of the most important problems in practical machines, and the effects of a truncation of the asperities on the friction and m~imum film thickness have been studied [ 2 1. The results showed that surfaces having a trapezoidal roughness gave less friction than those having rectangular profiles when the peak-to-valley rot&messes were the same. This is an important factor in running-in, at least in the mixed and/or hydrodynamic regime, as well as the decrease in surface roughness [ 21. Mitsuya has conducted a theoretical analysis of the influences of surfaces with two-dimensional isotropic or anisotropic roughness on full film lubrication by means of a mixed average film thickness [ 41. In high speed bearings, the increase in friction due to the transition into the turbulent region is a serious problem. Kate and Hori have developed a turbulent lubrication theory for journal bearings with a turbulence energy dissipation model (k-e model) [ 53. This theory can be applied to bearings in which the pressure gradients or the eccentricity ratios are very high. The present theory agrees well with the experimental results, even when the eccentricity ratio is higher than 0.95 and the pressure gradient is very large.

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Friction reduction in the turbulent region is achieved by the addition of a small amount of polymer into the oil. This is well known as “Tom’s effect”. Fukayama et al. have carried out a study of friction reduction with respect to the effects of polymer substances, polymer concentration, bearing clearance and journal eccentricities using a journal bearing test rig over a wide range of Reynolds’ number in the superlaminar regime [6]. In the experiments, the friction of polymer solutions was less than 40% of that of the newtonian fluid in turbulent flow under the best conditions. The calculated friction factor for Couette flow was in good agreement with the corresponding experimental value. Degradation of high polymers has also been investigated experimentally, and it was found to be affected by polymer concentration. In analysing high speed bearings, consideration of the effect of inertia on bearing performance is another important problem. Mori and coworkers have developed the analysis of lubricant inertia effects on the static and dynamic performance of a plane inclined slider pad in laminar flow [ 7 - lo]. This analysis was carried out by using the modified Reynolds equation derived by averaging out all the inertia terms, including the unsteady terms in the Navier-Stokes equations, across the film thickness. These analyses have been evaluated by experiments with good accuracy. It is considered that lubrication of piston rings and cylinder liners is governed by fluid film lubrication throughout most of the operating cycle. Furuhama et al. have measured the oil film thicknesses between piston rings and cylinder liners in an actual diesel engine [ 111. They have revealed that the oil film thickness under actual operating conditions was thinner than the theoretical value and this was attributed mainly to an insufficient supply of oil. Wakuri et al. have performed a study of the relationship between the squeezing velocity of the oil film and the position of initiation of oil film separation on the basis of observations of the dynamic behaviour of a reciprocating slider which simulated the piston ring [ 121.

3. Elastohydrodynamic

lubrication

Elastohydrodynamic lubrication (EHL) and its related problems such as traction play an important role in the lubrication of gears, cams and followers, rolling contact bearings and traction drives. The lubricating behaviour of polymer-thickened oils, which have been developed to improve the viscosity-temperature relationship under concentrated contacts, are important in practical machines. There are a few papers on this subject [13 - 161. Sakurai and coworkers have carried out measurements of oil film thickness and friction of several polybutene solutions under pure sliding conditions using the optical interference method [13]. The EHL oil film thickness of the oils containing polymer was nearly equal to that of the base oil; however, it had a higher viscosity owing to the addition

546

of the polymer. The friction coefficients of polymer solutions were higher than those of the base oil at low sliding speeds, but they decreased with increasing speed to that of base oil. Yoshida et ~2. have measured the EHL film thicknesses of several polymethacrylate (PMA)-thickened oils under nominally pure rolling conditions [14]. Within the wide range of average molecular weights of the PMAs (40 000 - 900 000), the oil film thickness decreases with an increase in the average molecular weight of the PMA even though the ambient viscosities are the same. They have also conducted further experiments for previously degraded PMA-thickened oils by using a sonic shear tester at a frequency of 10 kHz. It is very interesting that the EHL film thickness increase with a decrease in ambient viscosity resulted from a molecular weight decrease. Aihara and Dowson have studied film formation under pure rolling concentrated contacts using a two-disc machine [15,16]. It was invariably found that the initial grease film thickness exceeded that of the base oil, but that this advantage was lost as the test progressed. In due course the grease film thickness became smaller than that of the base oil with the equilibrium ratio being dependent on the rolling speed [ 161. The influence of the amount of grease available in the inlet on the equilibrium film thickness was examined. With minute and fixed initial amounts of grease available, good correlation was obtained between the measured film thickness and the predictions of EHL theory, allowing for starvation in the inlet region. With larger quantities of grease, reverse flow occurred in the inlet but the equilibrium film thickness normally approached the value predicted by the zero reverse flow inlet boundary condition, particularly at high speeds. Observations of the inlet region with a travelling microscope suggested that the meniscus formed by grease generally resulted in a greater degree of inlet starvation than that for oil. Other factors, such as inlet shear heating or breakdown of the grease thickeners, were also investigated. A new formula for the estimation of grease film thickness was proposed, which was obtained by multiplying the film thickness of the base oil by a factor of 0.5 - 0.7 which varied with frictional conditions and the grease used [ 161.

4. Rheology

of lubricants

Lubricant rheology, especially at high pressures, is one of the most valuable properties governing the lubrication of concentrated contacts. Knowledge of the viscosity at a given temperature and a given pressure over wide ranges is regarded as being of major importance and many attempts have been made to determine it. It is well known that there are two basic models of liquid viscosity: (i) the Eyring model and (ii) the free volume model. Yasutomi et al. have carried out an analysis of the dependence of lubricant viscosity on temperature and pressure on the basis of the free volume model [17,18]. They introduced a pressure dependence of the viscosity in the Williams-Landel-Ferry (WLF) equation, which is one of the

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free volume equations concerned with the temperature dependence of viscosity. The modified WLF equation obtained, in which the pressure effects on viscosity are given in terms of the pressure dependence of the glass transition temperature Tg and of the thermal expansivity of of the free volume, is as follows:

logh.U, 01 = log pe-

Cl {T - T#‘)~W’) G + {T - TgP)V’(~)

where C, and Cz are well-known WLF constants, and cc, is the viscosity at Tg. T,(P) and F(P) are functions for describing the pressure dependence of Tg and crf respectively. This equation predicts the variation in viscosity of different types of lubricants over wide ranges of temperature and pressure with good accuracy [ 171. Furthermore, Yasutomi et al. applied this equation to the variation in viscosity p(T, P) for two series of binary blended lubricants containing a common synthetic diester (di-2-ethylhexyl sebacate) in a polyphenyl ether (5P4E) and in a naphthenic mineral oil. Dilatometric observations and the viscosity analysis indicate that the relations needed to predict the pressure functions in the blend can be obtained in the modified equation for those of the components. These relationships allow us to estimate p(T, P) of a binary blended lubricant without measurements of the high pressure viscosity of the blend [ 181. Tamai and coworkers have made an extensive effort to correlate rheological properties of lubricants with their chemical structure [ 19 - 231 in terms of the flow activation parameters such as the flow activation volume AV* obtained from Eyring’s transition theory as below:

(1) They found an empirical additivity rule (eqn. (1)) between AV* and the molecular structure of mineral oils as functions of the average number of carbon atoms in aromatic rings, naphthenic rings and paraffinic chains analysed by the Waterman method. They also investigated the flow activation quantities for synthetic esters [20, 211, and alicyclic compounds as traction fluids [22]. The effects of the flexibility of a molecule, the introduction of ethyl groups and the existence of cyclohexyl rings on the parameters AV* and AV*/V are argued to elucidate the modes of segmental flow. In addition, they tried to apply the same concept to polymer-blended lubricants as rolling oils to interpret the lubricity of the fluids by use of the flow activation parameters [ 231. Matsubara et al. have carried out a torsional braid analysis and employed the torsional oscillation viscometer to clarify the viscoelastic properties of oils and greases at low temperatures [ 24 - 261. Hoshino has studied the relationship between the flow properties of greases and torque in rolling bearings both at low temperatures and low speeds and at high temperatures and high speeds [27,28].

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Katafuehi and Takano have studied the electrical characteristics between lubricated frictional surfaces using a ball-on-ring friction apparatus. They found that electrical rectification phenomena existed when electrical currents were applied between the lubricated frictional surfaces. This was affected by oil viscosity, sliding speed, temperature, load and lubricant additives [ 29 I 331. Kimura and Muraki investigated experimentally and analytically the traction behaviour of EHL films [34 - 371. The behaviour of EHL films under twodimensional rolling contact was analysed following Johnson’s non-linear Maxwell model, and a convenient formula was presented to provide the maximum traction coefficient in the isothermal and non-isothermal regions. This formula was fitted to the Johnson-Cameron traction curve. Consequently, traction behaviour can be estimated with sufficient accuracy under arbitrary conditions; thermal effects can be included. The influence of the physical properties of lubricating oils on the maximum traction coefficient can be examined through this formula.

5. Basic studies of wear Sasada et al. have suggested that the mutual material transfer and the growth of transferred material in the rubbing surfaces play an important role in the adhesive wear process [38]. They observed using a pin-on-disk type of friction machine that the pin experienced intermittent motion in which it was lifted up by the growth of the transferred particle and dropped suddenly by the removal of it. They also observed the cross section of the frictional materials in which transferred material existed between them by using X-ray microanalysis. It is seen that the particle is press-slide flattened in shape and has a hairline mixed structure containing metals of both mating surfaces. The maximum lift of the pm is nearly equal to the size of the particles produced. Norose and Sasada have confirmed that mutual transfer and growth occur under lubricated frictional conditions [39]. Kayaba and Kato have considered the metal transfer (they called it “adhesive transfer”) and the adhesive wear process from another point of view [ 40 J, They observed friction junctions using scanning electron microscopy and introduced fractography to analyse the transferred fragments. They revealed that two modes of adhesive transfer existed. In the first mode, successive slip occurs on the slip planes within the prow and forms an extrusion called a “slip tongue”. A shear crack subsequently propagates from the root of the slip tongue. In the second mode, successive ploughing takes place at the head of the wedge, accompanying the shear crack growth at its trailing end. It was concluded that adhesive wear occurred as the result of the two modes of adhesive transfer. Enomoto studied the subsurface structure of MgO crystals frictionally damaged on their (001) faces by scratching with diamond stylus in the (100) direction and using a scanning electron microscope in both the cathode

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luminescence and the secondary electron modes [41]. Observations of the cross section revealed that the subsurface beneath the sliding contact consisted of four characteristic material zones: zone I was severely deformed with a high defect density which caused quenching of the luminescence; zone II was a plastically deformed intermediate zone showing enhanced luminescence; zone III was a slipped zone which on etching showed a dislocation pit pattern; zone IV consisted of non-deformed crystalline material. The normalized depth d/a, where d is the depth of each deformed zone and a is the track halfwidth, was independent of the load. Kimura et al. have performed wear tests under pure rolling conditions using a four-roller machine [42]. Even under lubricated pure rolling conditions, a finite amount of wear was observed as the oil film thickness was comparable with or less than the resultant surface roughness. A common feature was found in the wear characteristics when either the surface roughness or oil film thickness was varied: the effective surface roughness, defined as the difference between the resultant roughness (peak-to-valley roughness) and the oil film thickness, was found to correlate with both the amount of wear and strain hardness. The finite element method (FEM) is a general program for the investigation of the deformation behaviour of materials and structures. Ohmae and coworkers have applied the FEM to the surface deformation process due to friction and wear [43, 441. They regarded the wear process as plastic deformation and fracture phenomena in the subsurface layer on the basis of observations of the friction track of pure aluminium. In an FEM analysis each triangular element is set to represent a dislocation cell in order to computer simulate accurately the deformation processes as well as the fracture of the material. The effects of the oxide film and the work-hardened layer were also discussed from the results of computer simulations. As a result of these results, they proposed a new model of wear which was based on low cycle fatigue of the material at and around the surface [45,46]. A zooming analysis of the FEM was applied to the study of the wear process of pure copper. The inputs of the elastic-plastic analytical computer program of the FEM were obtained from friction experiments on pure copper. Stress analysis showed that the compression stress which exists at the front of sliding contacts had penetrated to a deep region, while the tensile stress lying to the rear was limited to the surface region. The results of the FEM and electron fractography have led to the conclusion that wear was a fracture of materials at the surface, primarily caused by friction-induced plastic deformation [ 471. Sasada and coworkers have conducted a series of investigations of twobody and three-body abrasive wear [48 - 521. When pure metals rubbed against an emery paper under four lubrication conditions (with no lubricant, MO&, liquid paraffin and the liquid paraffin containing stearic acid) the highest abrasive wear rate was obtained under lubrication with the liquid paraffin containing stearic acid and the lowest abrasive wear rate was obtained without lubrication. These findings are explained by means of the

550

cutting action of abrasive grains, i.e. the lubricant decreases the friction between the rake face and the cutting chips (wear debris), but the rate of wear is increased. 6. Applied studies related to wear In practical machines it is well known that vibration due to various sources has a great influence on friction and wear characteristics. Terauchi and coworkers have studied the influence of vibrational amplitude under a fixed frequency of 22 kHz on friction and wear using a pin-on-disk apparatus [ 53 - 561. They revealed that aspects of friction and wear varied with changes in vibrational amplitude and that a critical amplitude existed. At small amplitudes, the amount of wear and the value of the torque were large and increased rapidly with time and agreed nearly with those at no vibration. An increase in the amplitude reduced the amount of wear and the torque up to the critical amplitude of around 4 - 6 I.cm. Above this, a marked decrease in wear was observed and the frictional torque was also reduced and its time dependence disappeared. Surface roughness in the low wear region was small and ripples due to vibration were observed; meanwhile, in the high wear region the surface roughness was large and scratch flaws were observed. Terauchi and coworkers also conducted an analysis of vibrational parameters and the wear mechanism. Taga and Nakajima have studied the friction and wear characteristics of ternary Cu-Sn-P alloys and revealed that the amount of phosphorus in ternary alloys has marked effects on friction and wear [57]. The results showed that the decrease in both the coefficient of friction and the rate of wear became conspicuous with the increase in quantity of CuaP coexisting in the matrix, its amount increasing with the content of phosphorus. The structural changes in the surface of the specimen due to heating in a vacuum were observed by using Auger electron spectroscopy and X-ray photoelectron spectroscopy. It was seen that the surface concentration of phosphorus strongly increased after heating at 573 K, whereas the diffusion of tin atoms was markedly retarded. This is not the same as in binary Cu-Sn alloy in which tin segregates to the rubbing surface and reduces friction and wear. Nakajima and coworkers have also investigated the formation of selflubricating materials of nickel-base ternary alloys during friction and heating [58]. Several ternary Ni-M-S (M 2 MO, Nb or Fe) systems were arranged as a function of the content of M and sulphur and rubbed against 0.5 wt.% C quenched steel. X-ray diffraction revealed that sulphides or complex sulphides such as MoS, or FeNb,S, formed during friction or annealing, and they were recognized to be useful friction modifiers. These materials were found to have transformed from non-stoichiometric into stoichiometric compounds during friction. Tanaka and Miyazaki have studied the wear behaviour of several magnetic and non-magnetic materials and audio heads rubbed against magnetic tapes [ 591. It is concluded that the mechanism of wear by magnetic tape is

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not entirely due to the abrasive action of the magnetic powder in the tape but is also partially adhesive in nature. Mizuno and coworkers have studied the frictional behaviour of the metal-working process in dies and soft copper sheet systems [60,61]. They studied the effects of lubricant viscosity at various pressures and sliding velocities on lubricating conditions, and they revealed that surface textures and lubricating conditions in the interface were affected by the viscosity of the lubricant under pressure and the relative speed.

7. Scoring Yamamoto et al. have investigated the effects of surface roughness and surface oxide films on scoring under rolling and sliding motion with respect to a hardened steel and an annealed steel [62,63]. They suggested that the formation of oxide film on the surface of the higher speed disk played an important role in preventing the occurrence of severe scoring. They also found that the ferrite phase of annealed carbon steel squeezed out on the disk surfaces resulted in the formation of an oxide film.

8. Wear of plastics and their composites Hironaka and coworkers have performed wear studies of high density polyethylene (HDPE) and isotactic polypropylene (PP) rubbed against steel under dry and water-lubricated conditions [64,65]. The morphological study of the worn surfaces revealed that lamellar crystals linked by tie molecules were highly oriented along the sliding direction (the crystallographic c axis orientation) in the case of HDPE. In the PP-steel system, plastic deformation of the surface of PP took place during frictional contact both in the air and in water, giving rise to the formation of thick meandering fibrous aggregates with their longitudinal axes nearly perpendicular to the sliding direction and their fibrils aligned parallel to the sliding direction. Tanaka and Kawakami have studied the friction and wear characteristics of polytetrafluoroethylene (PTFE)-based composites incorporating various fillers [66]. The results showed that the friction of PTFE-based composites was generally independent on the type of fillers and was similar to that of unfilled PTFE. With respect to the wear-reducing action of the filler, fibre and particle fillers of suitable size were more effective than lamellar solid lubricants and very small hard particles. Yamaguchi and coworkers have carried out friction and wear tests on several composites based on melamine resin, polyimide, polyphenylene oxide and 30% glass-fibrecontaining silicone resin for brake shoe materials [67]. Kimura et al. have studied the friction and wear characteristics of carbon-carbon composites as brake materials for aeroplanes [ 681.

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Hironaka and coworkers have prepared a model for an artificial joint with a hydrogel structure similar to articular cartilage and, to examine its frictional behaviour, poly(viny1 alcohol) (PVA) dissolved in water was grafted by y irradiation onto preirradiated HDPE [69]. The friction coefficient in water decreased from 0.04 (for HDPE) to 0.02 by the PVA graft, indicating that the hydrogel structure was effective in the reduction of the friction coefficient in water. 9. Rolling contact

fatigue

Rolling contact fatigue is a serious problem and it is important to achieve a longer life of machine elements such as rolling contact bearings and gears. Studies of rolling contact fatigue have thus been performed by many research workers. Traction forces acting between two mating bodies influence rolling contact fatigue life. Soda and Yamamoto have studied the effects of tangential traction on rolling contact fatigue using a four-roller machine [70]. The results indicate that the fatigue life decreases when traction is applied in the same direction as that of rolling. When the traction is reversed (negative traction), the life increases compared with that obtained with zero traction. Akaoka and Nitanai also revealed the same inclination when a small negative traction was applied [71]. However, when a larger negative traction was applied, the fatigue life decreased slightly. Ishibashi and Hosoyama have studied the influence of certain factors (slide-roll ratio, lubrication and surface roughness) on rolling fatigue life in steel and bronze systems [ 72, 731. Kikuchi et al. have conducted fatigue tests on rolling contact bearings made of ceramics [ 741. Hirano et al. have revealed that molecular weight distributions of mineral oils influence bearing life [ 751. It is well known that fatigue life can be extended with higher viscosity lubricants; however, blended oils characterized by wide-ranging molecular weight distributions exhibited longer life compared with base oils with a narrow range of distributions corresponding to the same viscosity grade. Takata et al. have investigated the influence of lubrication with oil or grease on the fatigue life of roller bearings [76]. Results showed that the effect of lubrication with grease on the fatigue life of roller bearings is equal to that with oil if the grease film thickness is calculated by the aforementioned formula demonstrated by Aihara and Dowson [15]. Taki has performed a study of the effects of oil viscosity and its additives on the pitting durability of gears [ 771. It was concluded that the pitting durability was proportional to the lubricant viscosity raised to the power 3/10 - 4/10. However, polymer-thickened oil gave only 50% effectiveness compared with the predicted life. Further, S-P-type extreme pressure additives exert a negligible effect on the pitting durability of a gear tooth surface. Fujita and Yoshida also confirmed the same tendency in polymerthickened oil [78]. The effects of MO& on fatigue life are twofold; in the relatively mild region it increases the life, otherwise it

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reduces the life [ 781. In practical machines, lubricants contain many foreign particles such as wear debris, sludge and dust which greatly reduce the rolling fatigue life. Muro and coworkers have conducted rolling fatigue tests under lubrication with oils which contained several types of debris, and they also studied the improvement under such conditions [79]. They revealed that heat treatment can increase the rolling fatigue life not only under debriscontaminated lubrication but also under non-contaminated situations. 10. Adsorption and chemical reaction The chemical reactions of lubricating oil additives with frictional surfaces play important roles in boundary lubrication. However, the chemical reactions are highly complex because interactions exist between lubricants, including additives, and metal surfaces, the action of oxygen or water as one of the reactants and tribochemical effects. Sakurai and coworkers have emphasized the importance of the chemical reactions in the lubrication of concentrated contacts for many years [80 - SS]. Recently, in Japan, there have been many studies of the correlation between the lubricating properties and the chemical reactivity of lubricant additives. Kawamura and Fujita investigated the chemical reactivity of extreme pressure additives with SVJ-2 steel baaed on differential thermal analysis and correlated it with the wear properties using a cross-pin type of tester [89, 901. The chemical wear by elemental sulphur in n-hexadecane in a steelcopper sliding system correlated with the rate constant of static film formation in the hot-wire method [91]. Hirata and Watanabe have suggested that the elemental composition ratio S:P of sulphur and phosphorus of the surface film formed with S-P-type extreme pressure additives responds to wear and friction behaviour and that the S:P ratio should increase with increasing severity of the friction conditions [92,93]. Tamai and coworkers investigated the reaction of extreme pressure additives with mild steel during and after machining [94]. From an X-ray photoelectron spectroscopic analysis of the reaction products formed on the friction surfaces, they described the reaction of the thermal decomposition products of extreme pressure additives with steel surfaces and the mechanochemical activity of steel surface as followed by exoelectron emission. In boundary lubrication, the adsorption of lubricant additives on the friction surfaces is an important function in the reduction of friction and wear. It has been well known that oiliness agents adsorb on metal surfaces and form protective films to reduce metallic contact, contributing to friction and wear reduction. This effect is correlated with the heat of adsorption of additives on metal surfaces, i.e. the activity of adsorption on metal surfaces should be discussed. The adsorption activity of FeS and Fe,Oa surfaces for organic polar compounds is much higher than that of Fe203 [87,951, and the freshly formed aluminium surface is highly active with respect to adsorption [ 961. Fundamen~~y, the adsorption behaviour of additives on metal surfaces can

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be investigated by ellipsometry [97]. The combination of a fatty acid and an organic sulphur compound showed a synergistic effect in providing less wear and a smoother surface than did the sulphur compound alone [98]. The action of surface smoothing is concerned with the friction modification of surfaces caused by applying the binary additive system. Thus it promotes EHL conditions. Fatty acids adsorb strongly on the sulphide film surface formed with sulphur compounds to provide an effective EHL film.

11. Engine oil additives Modern engine oils usually contain many kinds of additives such as friction modifiers, antioxidants, detergents-dispersants and viscosity index improvers. The mechanism of engine oil performance is highly complex because of interactions among the additives, and it is not well understood. Inoue and Watanabe investigated the interactions between various detergents and zinc di~yldithiophospha~ (ZDP) by using osmometry and explained that no interaction was found between metallic detergents and ZDP, but succinimides were found to interact strongly with metallic detergents and ZDP [99 - 101]. Hamaguchi et ai. [ 1021 improved engine oils by incorporating a molybdenum dithiocarbamate [ 821 as a friction modifier for engine oils offering fuel savings which were compatible with emission control devices and provided for extended oil drain intervals. In addition, there have been studies of valve train wear [ 103 - 1051, acid neutralization of overbased detergents in which the mechanism of neutralization in HCI solution has become more clear [106,107] and antiwear properties of ZDP in used engine oils [108,109]*

12. Emulsion lubrication There have been only a few reports of emulsion lubrication in spite of the greater demands for advantages such as good fire resistance, large cooling capacity and low costs. Sakurai has recently reviewed the studies of emulsion lubrication [llO]. Hamaguchi et al. investigate the EHL film thickness formed by emulsions in a rolling point contact using an optical interference method [ill]. From the relationship between the load-carrying capacity and the oil concentration and pH of emulsions [ 1121, the maximum seizure load was obtained at a concentration that corresponded to the critical micelle concentration and was provided at the maximum pH. This suggests that the micelle formation and pH value of emulsion have an important influence on the lubricity of emulsions. In practice, there have been some studies of hot rolling emulsions of aluminium [1X3], cutting fluids [114] and the separation of emulsified water in waste lubricating oil 11151,

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13. Solid lubricants The recent studies of solid lubricants are as follows: the adsorption characteristics on the cleavage and edge surfaces of MO& by Auger electron spectroscopy [ 1161; the wear and friction behaviour of compressed compacts of MoSz [117]; solid lubrication by melamine cyanurate which is thermally stable up to 300 “C [ 1181; niobium sulphide synthesized by the reaction of niobium pentoxide with hydrogen sulphide [ 1191; the effects of solid lubricants on the formation of soap micelles in lithium soap grease and its phase behaviour [ 1201.

Acknowledgments The author would like to thank Dr. S. Hironaka and Mr. K. Yoshida for kind support to publish this paper.

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