Engineering Tribology Episode 2 Part 7 potx

BOUNDARY AND EXTREME PRESSURE LUBRICATION 375 resistance of a sliding contact occasionally giving an overestimate of permissible loads and sliding speeds [41]. Stage 1 Stage 2 Large frictional heat Thermally induced film collapse (desorption, viscosity loss, catalytic degradation, etc.) Stage 3 Adhesion Surface roughening Wear particles Coefficient of friction Stage 1 Stage 2 Stage 3 Seizure Lubricated friction Beginning of instability First signs of scuffing Critical event Large heat release Propagation to next asperity FIGURE 8.23 Sequence of events leading to scuffing. For these reasons, one reliable means of avoiding scuffing is to apply relatively conservative gear design standards. Also, as will be discussed later, E.P. lubricants can prevent scuffing when the limit of adsorption lubrication is reached. There is, however, no quantitative measure available for estimating, for example, how much extra load can be applied if special anti-scuffing lubricants are used. Interpretation of scuffing in terms of friction transition temperatures has been the object of extensive studies [e.g. 42-44]. Initially a series of tests on a model friction apparatus with low TEAM LRN 376 ENGINEERING TRIBOLOGY Lubricant's critical temperature C Lubricant's critical temperature B Lubricant's critical temperature A EHL film pressure profile Momentary desorption Some scuffing risk (difficult to define) Sustained desorption High scuffing risk Temperature profile C B A Asperity collision transients Small scuffing risk No desorption Temperature levels FIGURE 8.24 Model of transient and steady state temperatures in a mixed lubrication sliding contact. sliding speed to suppress frictional temperature rises was conducted [42], followed by the testing of the same theory on tribometers with higher sliding speeds [43,44]. The following model for scuffing based on the thermodynamics of adsorption was developed [44]: lnC = −E a / (RT t ) + constant (8.3) where: C is the concentration of the additive in the solvent base stock [%wt]; E a is the adsorption heat of the additive on the metallic surface [kJ/kmol]; R is the universal gas constant [kJ/kmolK]; T t is the friction transition temperature [K]. According to this model, when the transition temperature is exceeded, damage to the adsorbate film is more rapid than film repair so that the adsorption film is progressively removed. High friction and wear are then inevitable. For a narrow range of experimental conditions agreement between the model (8.3) and experimental data was obtained. In Figure 8.25 the relationship between the concentration of fatty acids of varying chain length dissolved in purified inert mineral oil and transition temperature is shown [42]. The data provides a linear plot which is in agreement with theory. The gradient of the graph which is a measure of the heat of adsorption is also approximately the same as the heat of adsorption determined by more exact tests. On the other hand, it has been found that a 1% solution of oleic acid improved the lubrication capabilities of white oil even though the transition temperature for oleic acid was clearly exceeded [45]. In other words, this indicates that fatty acids do not function merely by adsorption lubrication and some new theories are necessary. An attempt to estimate the critical temperature in EHL contacts where scuffing or film failure is likely to occur based on heat of adsorption, sliding speed and melting point of lubricant has been made [35,113,114]. The practical applications of the expression found are extremely TEAM LRN BOUNDARY AND EXTREME PRESSURE LUBRICATION 377 limited because the data referring to one of the variables, i.e. melting temperature of the lubricant, is only available for pure compounds, not for mixtures, which commercial oils are. 0.1 1 10 100 290 300 310 320 330 340 350 10 5 /Transition temperature [K −1 ] Concentration [% wt] 50 20 5 2 0.5 0.2 Capric acid Myristic acid Oleic acid Stearic acid FIGURE 8.25 Relationship between friction transition temperature and concentration of some adsorption additives [42]. It should be realized that an EHL pressure field also affects both the concentration of additives within the EHL contact and the mechanism of adsorption lubrication by raising the critical temperature for desorption [115,125]. It has been found that under EHL pressure the concentration of additives in plain mineral oil (but not certain synthetic oils) tends to decline to less than half the bulk oil concentration [125]. Although the causes of this effect remain unclear, the implications for additive function within the EHL contact and its consequences on scuffing appear very significant. According to Langmuir's theory of adsorption, elevated pressure increases the fraction of the surface covered by adsorbate for any given temperature. The critical temperature for scuffing to occur is modelled as the temperature where the fractional surface coverage by adsorbate declines to less than half of the available atomic sites on the surface [116]. The critical temperature is approximately 150°C in slow speed sliding experiments but is between 300°C and 400°C in full scale scuffing tests where a substantial EHL pressure field is present [115]. Thus the equations which attempt to predict the critical temperature of scuffing according to Blok's theory but which do not allow for the pressure dependence might give incorrect results. The increase in critical desorption temperature with pressure suggests a mechanism of combined instability in a lubricated contact. Consider an experiment conducted on a ‘two disc’ apparatus where the discs are subjected to a progressive increase in load until scuffing occurs. Assume that initially the EHL pressure is low and the desorption temperature is close to that obtained in low speed tests. As the load is increased the contact temperatures, frequency of asperity contact and hydrodynamic pressure increase. The critical desorption temperature also increases so that effective adsorption lubrication is maintained. At some level of load, however, a limiting EHL pressure is reached. The limiting EHL pressure may either be due to lubricant characteristics or may be determined by the hardness of the disc materials. Further increases in load merely tend to increase the contact area or shift a greater proportion of the load onto asperity contacts. At this stage, two events may occur; either TEAM LRN 378 ENGINEERING TRIBOLOGY there is direct desorption of the adsorbate lubricating films caused by excessive contact temperature or there is a progressive collapse in EHL pressure caused by the asperity interference. When pressure declines the adsorbed films become unstable. The collapse in pressure can be limited to asperity contacts only while the bulk pressure field remains unaffected. If there is a localized reduction in hydrodynamic pressure then the critical desorption temperature will precipitately decline to allow local desorption of the adsorbate film. Scuffing will then be initiated from localized adhesive contacts between asperity peaks denuded of adsorbate film. This mechanism of combined instability is illustrated in Figure 8.26. Stable operation Load increasing Scuffing Uncontrolled scuffing Critical temperature Contact temperature EHL pressure Direct mechanism of adsorbate film instability and removal Stable operation Load increasing Scuffing Uncontrolled scuffing Critical temperature Contact temperature Indirect mechanism of adsorbate film instability and removal No macroscopic change in contact temperature during initiation of scuffing EHL pressure Localised decline in EHL pressure and critical temperature FIGURE 8.26 Combined instability in EHL pressure field and adsorbate lubricating films as cause of scuffing. As may be deduced from Figure 8.26, it is still unclear whether any collapse in EHL or micro- EHL induces desorption and scuffing or whether desorption occurs first and the resultant surface damage causes the cessation of EHL. A phenomenon of catalytic oil decomposition is believed to contribute to scuffing too. At high temperatures found in a heavily loaded EHL film, whenever asperity contact occurs, i.e. when the EHL film thickness becomes comparable to the combined surface roughness of the contacting surfaces, the exposure of nascent surfaces worn by asperity interaction may directly affect the base oil of the lubricant which may have severe consequences for the lubricant film. It is known that a major feature of nascent surface is its elevated catalytic activity compared to quiescent, oxidized metal [17,119-121]. Nascent surface typically catalyzes decomposition reactions of organic compounds found in oil to release low molecular weight products that are often gaseous [119]. Such catalysis can have a destructive effect on the TEAM LRN BOUNDARY AND EXTREME PRESSURE LUBRICATION 379 lubricating capacity of an oil. It has been suggested that scuffing occurs when there is sufficient nascent surface exposed by mechanical wear to cause the rate of chemical degradation of oil inside the contact to exceed the rate at which it can be replenished [121]. When a critical rate of degradation is reached, contact closure by partial failure of lubrication further reduces the supply rate of oil. This causes a sharp transition to unstable lubrication and scuffing from stable lubrication below a critical load. The catalysed decomposition products are unlikely to facilitate lubrication since they do not possess the physical capacity to sustain the extremely high shear rates prevailing in the EHL contacts. These products tend to accumulate between asperity contacts and when their concentration becomes high enough, lack of lubrication occurs leading to scuffing [115]. In addition, the decomposition products most probably surround the contact excluding fresh lubricating oil or else under the influence of extreme frictional heating they react and chemically bind both sliding surfaces. This last effect could cause a catastrophic rise in friction levels. The schematic illustration of this ‘catalytic model of scuffing’ is shown in Figure 8.27. An important method of controlling scuffing would appear to be the prevention of nascent metal surface by covering the sliding surfaces with coatings of non-metallic materials such as ceramics. Steel gears coated with titanium nitride and carbide have been found to offer good scuffing resistance in gearbox tests compared to uncoated steel gears [122]. Selection of a stable lubricant is also important since the decomposition of perfluoroalkyether lubricating oils has been found to initiate scuffing and wear of metal surfaces [123,124]. Load Load Rotation Rotation Oil Lubricated contact Surface 1 Surface 2 Hot, reactive nascent surface catalyses decomposition of oil Oil film collapses when sufficient degradation of oil occurs Collision of asperities and formation of nascent surface Figure 8.27 Schematic illustration of catalytic model of scuffing of metal surfaces lubricated by an oil. In other more mechanically orientated studies, it was also found that operational parameters such as loading history and run-in procedure have a strong influence on scuffing and measured critical temperatures [46-48]. The critical temperature appears therefore to be a function of many parameters not just pressure, heat of adsorption and sliding speed. Despite the poor understanding of scuffing, research in this area has become scant in recent years. This may be due to the fact that scuffing belongs to ‘industrial tribology’ which has a relatively low priority compared to other aspects of tribology [49]. · Metallurgical Effects The effect of alloying and heat treatment to produce a specific microstructure also exerts a major influence on whether a low coefficient of friction can be obtained by oil-based lubrication. Frictional characteristics of steel-on-steel contacts versus temperature for two steels, a martensitic plain carbon steel and an austenitic stainless steel, are shown in Figure 8.28 [50]. Both steels are lubricated by mineral oil. It can be seen from Figure 8.28 that the coefficient of friction of the austenitic stainless steel rises sharply at 160°C reaching values greater than unity by 200°C. In favourable contrast, the TEAM LRN 380 ENGINEERING TRIBOLOGY 0 0.5 1.0 0 100 200 Temperature [°C] µ 18/8 Stainless steel 0.6%C Tool steel FIGURE 8.28 Frictional characteristics of plain carbon and stainless steels versus temperature under mineral oil lubrication [50]. coefficient of friction of the plain carbon martensitic steel remains moderate in the range of 0.2 - 0.3. The difference between these two steels can be explained in terms of reactivity since the austenitic steel is considered to be less reactive than the martensitic steel because of the latter's greater lattice strain. The greater reactivity causes more rapid formation or repair of oxide films and re-adsorption of surfactant films under conditions of repeated sliding contact. In another study [52] it was found that austenitic steels have lower friction transition temperatures than martensitic steels. Similar tests conducted with additive enriched oils revealed that low alloy steels exhibit lower coefficients of friction than high alloy steels [51]. It appears that both the phase of the steel and the alloying content are the controlling factors in lubricant performance. For example, chromium was found to raise the scuffing load for austenitic steels [53] while the contrary effect was found in other cases [54-56] where martensitic and ferritic steels were tested. In a comprehensive study where the effect of different alloying elements on scuffing resistance was tested, it was found that irrespective of the alloying elements the microstructure has controlling effect on scuffing load. For example, ferrite gives the highest scuffing loads and since martensite and cementite are less ‘reactive’ they lower the scuffing load [57]. Austenite is the most unsuitable phase and gives very low scuffing loads. The failure load for austenites is less than one tenth of the failure load for ferritic steels [57]. Thus hardening of steels does not provide increased protection against scuffing since this induces martensite with a corresponding reduction in ferrite. · Interaction Between Surfactant and Carrier Fluid In the model of adsorption lubrication discussed so far, the fatty acid or surfactant was either applied neat to the test surface or as a solution in an inert fluid. In practice, the ‘carrier fluid’ or ‘base stock’ can also influence the lubrication mechanism. It was found that the heat of adsorption of stearic and palmitic acid on iron powder were up to 50% greater with hexadecane as the carrier fluid than with heptane [58]. The heat of adsorption dictates the friction transition temperature. For example, if hexadecane is used as a carrier fluid in preference to heptane, a higher friction transition temperature can be expected. This aspect of adsorption lubrication has also been relatively neglected, partly because of the difficulty in manipulating mineral oil as a carrier fluid. With the adoption of synthetic oils TEAM LRN BOUNDARY AND EXTREME PRESSURE LUBRICATION 381 which offer a much wider freedom of chemical specification, systematic optimization of the heat of adsorption may eventually become practicable. 8.4 HIGH TEMPERATURE - MEDIUM LOAD LUBRICATION MECHANISMS There has always been much interest in oil based lubrication mechanisms which were effective at high temperatures. The primary difficulty associated with lubrication is temperature, whether this is the result of process heat, e.g. a piston ring, or due to frictional energy dissipation, e.g. a high speed gear. Once the temperature limitations of adsorption lubrication were recognized the search began for ‘high temperature mechanisms'. Although these mechanisms have remained elusive some interesting phenomena have been discovered. Two basic mechanisms involved in high temperature lubrication at medium loads have been found: chain matching and formation of thick films of soapy or amorphous material. Chain matching is the modification of liquid properties close to a sliding surface in a manner similar to the ‘low temperature - low load’ mechanism but effective at far higher temperatures and contact pressures, and dependent on the type of additive used. The thick colloidal or greasy films are deposits of material formed in the sliding contact by chemical reaction. They separate the opposing surfaces by a combination of very high viscosity and entrapment in the contact. Chain Matching Chain matching refers to the improvement of lubricant properties which occurs when the chain lengths of the solute fatty acid and the solvent hydrocarbon are equal. This is a concept which is not modelled in detail but which has periodically been invoked to explain some unusual properties of oil-based lubricants. In a series of ‘four-ball’ tests the scuffing load was found to increase considerably when the dissolved fatty acid had the same chain length as the carrier fluid lubricant [43]. An example of scuffing load data versus chain length of various fatty acids is shown in Figure 8.29. Three carrier fluids (solvents) were used in the experiments, hexadecane, tetradecane and decane of chain lengths of 16, 14 and 10 respectively. The maximum in scuffing load occurred at a fatty acid chain length of 10 for decane, 14 for tetradecane and 16 for hexadecane. To explain this effect, it was hypothesized that a coherent viscous layer forms on the surface when chain matching occurred. This is similar to the ‘low temperature - low load’ mechanism discussed previously except that much higher contact stresses, > 1 [GPa], and higher temperatures, > 100°C, are involved and furthermore the mechanism is dependent on the type of additive used. It was suggested that when chain matching occurs, a thin layer with an ordered structure forms on the metallic surface. The additive, since it usually contains polar groups, may even act by bonding this layer to the surface. If the chain lengths do not match then a coherent surface structure cannot form and the properties of the surface-proximal liquid remain similar to those of the disordered state of bulk fluid as shown in Figure 8.30. To support this argument, the near surface viscosity under hydrodynamic squeeze conditions was measured and a large viscosity was found when chain matching was present [43]. The relationship between the viscosity calculated from squeeze rates versus distance from the surface for pure hexadecane and hexadecane plus fatty acids of varying chain length is shown in Figure 8.31. Although chain matching has been confirmed in other studies [59,60] many researchers have failed to detect this effect and still remain sceptical [33]. Recently, however, an influence of fatty acids on EHL film thickness was also detected [61]. Film thickness or separation distance TEAM LRN 382 ENGINEERING TRIBOLOGY versus rolling speed under EHL lubrication by pure hexadecane and hexadecane with stearic acid present as a saturated solution is shown in Figure 8.32. 0 1 2 Seizure load [kN] 01020 Number of carbon atoms in saturated additive 51525 A B C Hexadecane Tetradecane Decane FIGURE 8.29 Scuffing loads as a function of fatty acid chain length for various aliphatic hydrocarbon carrier oils [43]. Substrate Substrate Substrate Substrate Chaotic liquid state Ordered layer Ordered layer Bonding to surface to anchor viscous layer Chain-matching effective No chain-matching = Carrier or solvent oil = Additive (fatty acid) FIGURE 8.30 Model of chain matching. It can be seen from Figure 8.32 that EHL film thicknesses for pure hexadecane and a hexadecane solution of stearic acid diverge significantly. At very low speeds hexadecane gives no residual film on the surface while the stearic acid/hexadecane solution gives separation of about 2 [nm]. This effect can be attributed to an adsorbed layer of stearic acid. As speed increases and an EHL film is generated the film thickness for both lubricating liquids becomes the same and the effect of stearic acid is diminished. TEAM LRN BOUNDARY AND EXTREME PRESSURE LUBRICATION 383 0 0.1 0.2 η [P] 0 102030 Gap [nm] C 16 C 18 C 14 Pure hexadecane FIGURE 8.31 Viscosity versus distance between squeezing surfaces for pure hexadecane and hexadecane with dissolved fatty acids of chain lengths 14, 16 and 18 [43]. 0.2 0.5 1 2 5 10 20 0.001 0.002 0.005 0.01 0.02 0.05 0.1 0.2 0.5 1 Speed [m/s] Separation [nm] Stearic acid/ hexadecane (saturated) Pure hexadecane FIGURE 8.32 Effect of dissolved fatty acid on EHL film [61]. The effects of various fatty acids on friction, i.e. lauric, palmitic and stearic acid added to hexadecane, were tested under heavily loaded conditions between sliding steel surfaces [62]. At low friction a layer of adsorbate, thicker than a monolayer, was detected by contact resistance measurements. After the friction transition temperature was exceeded and the friction coefficient rose, this layer seemed to decline to negligible values. However, the highest friction transition temperature of about 240°C was recorded when the chain length of the fatty acid matched that of the hexadecane, i.e. at 16 which corresponds to palmitic acid. For the other acids, the friction transition temperature was much lower, between 120°C and 160°C. TEAM LRN 384 ENGINEERING TRIBOLOGY Thick Films of Soapy or Amorphous Material Almost all additives used to control friction and wear can react chemically with the worn metallic surface. This means that in addition to adsorbate films and viscous surface layers, a layer of reaction product can also form on the sliding contact surface. It is virtually impossible to control this process once the additive is present in the oil. Until recently this aspect of additive interaction was hardly considered since the reaction products were usually assumed to be extraneous debris having little effect on film thicknesses, friction and wear. Recently, however, the idea of films thicker than a mono-molecular adsorbate layer but thinner than the typical EHL film thickness has been developed [62,63,67-69]. The thickness of this film is estimated to be in the range of 100 - 1000 [nm] and the limitations of desorption at high frictional temperatures have been avoided. The consistency or rheology of these films varies from soapy, which implies a quasi-liquid, to a powder or amorphous solid. · Soap Layers Soap layers are formed by the reaction between a metal hydroxide and a fatty acid which results in soap plus water. If reaction conditions are favourable, there is also a possibility of soap formation between the iron oxide of a steel surface and the stearic acid which is routinely added to lubricating oils. The iron oxide is less reactive than alkali hydroxides but, on the other hand, the quantity of ‘soap’ required to form a lubricating film is very small. Soap formation promoted by the heat and mechanical agitation of sliding contact was proposed to model the frictional characteristics of stearic acid [62,63]. In the theory of adsorption lubrication, it was assumed that only a monolayer of soap would form by chemisorption between the fatty acid and underlying metal oxide, e.g. copper oxide and lauric acid to form copper laurate. No fundamental reason was given as to why the reaction would be limited to a monolayer. The soap formed by the reaction between a fatty acid and metal is believed to lubricate by providing a surface layer much more viscous than the carrier oil as shown schematically in Figure 8.33 [62]. Steel Fe + Fatty acid ⇒ Fe based soap e.g. Ferrous stearate Oil layer Fatty acid Viscous soap layer ≈0.1µm Heat FIGURE 8.33 Formation of a viscous soap layer on steel by a reaction between iron and a fatty acid in lubricating oil. The presence of a viscous layer functioning by the mechanism of hydrodynamic lubrication was deduced from electrical contact resistance measurements [62]. When there was a measurable and significant contact resistance, the thick viscous layer was assumed to be present. Dependence on hydrodynamic lubrication was tested by applying the Stribeck law. According to the Stribeck law, the following relationship applies at the limit of hydrodynamic lubrication: logU + logυ − logW = constant (8.4) TEAM LRN [...]... properties and can also act as a lubricant TEAM LRN 386 ENGINEERING TRIBOLOGY 20 00 Velocity [m/s] 0.1 1000 0.05 500 0. 02 υ Kinematic viscosity [cS] 5000 20 0 0.01 0.005 150 20 0 25 0 Temperature [°C] FIGURE 8.35 Relation between temperature, sliding speed and viscosity of the soap layer formed in sliding contact during lubrication by stearic acid in hexadecane [ 62] The process of amorphization of interposed material... that when wear tracks and contacts were lubricated by oils containing sulphur, the sulphur accumulated in the heavily loaded regions [ 72 - 74 ] The concept of an iron sulphide film was then proposed [75 ] and later confirmed when surface analysis was TEAM LRN 390 ENGINEERING TRIBOLOGY Bare unprotected surface Region of EHL contact Film produced by quiescent corrosion Virtually grown film Reactive sulphur... additive molecules) adsorb onto positive points on the surface [ 87] The electron emission is associated with initial oxidation of the surface by TEAM LRN 394 ENGINEERING TRIBOLOGY Contaminants impede adsorption of oxygen Adsorbed water and contaminant layer Primary barrier to oxygen: solid state diffusion required for the reaction 4Fe + 3O2 ⇒ 2Fe2O3 to occur Oxide Metal Normal oxidized surface Very weakly... discussed above, chemical reactions and film TEAM LRN 396 ENGINEERING TRIBOLOGY O2 Slow diffusion of oxygen in oil S SSS SSS S S S S S S SS SSS SS S S Rapid sulphidization Slow formation of mostly Fe2 O3 (equilibrium product) Mostly oxide Sulfide/oxide mixture Non-sacrificial corrosion product film Short lifetime sacrificial film FIGURE 8. 47 Formation and structure of the E.P film in the presence of... The mean of several measurements is usually taken [N]; κtest oil is the corrosion constant of the test oil [m2/s]; κwhite oil is the corrosion constant of the white oil [m2/s] The corrosion constant ‘κ ’ can be deduced from the Wagner parabolic law of high temperature corrosion,i.e.: d 2 = κt (8 .7) where: d is the average depth of corrosion [m]; t is the corrosion time [s] An example of the plot of ‘K’... pin-on-disc sliding tests ensure moderate wear rates up to 2 [GPa] while methyl laurate (an adsorption lubricant) fails at about 1.3 [GPa] and allows scuffing Plain mineral oil shows an even lower ability to operate under high contact stress, resulting in excessive wear rates at contact pressures below 1 [GPa] [71 ] Measured film thickness [nm] 400 300 20 0 0 100 20 0 Time [minutes] FIGURE 8.39 Increase in EHL film... BOUNDARY AND EXTREME PRESSURE LUBRICATION 3 87 Crystalline Amorphous layer Crystalline FIGURE 8.36 Bubble raft analogy of crystal/amorphous structure of the material separating sliding surfaces [64] FIGURE 8. 37 Rippling shear fronts under sliding using the bubble raft analogy as a mechanism of destruction of the crystal lattice [64] TEAM LRN 388 ENGINEERING TRIBOLOGY Light load Higher load Hard metal... concentrations from the wear scars of a four-ball test is shown in Figure 8.49 The critical load is defined as the maximum load which permits smooth sliding up to 20 0°C 4 Critical load [kN] 3 2 1 0 0 1 2 Ratio of sulphur to oxygen surface concentration at 20 0°C FIGURE 8.49 Influence of sulphur to oxygen ratio in the wear scar film on the critical load [96] When only a film of oxides is present then the critical... Effect on Lubrication In order for an E.P additive to effectively form sacrificial films it must be chemically active and react with worn metallic surfaces [75 ,79 ] An ‘active’ E.P additive gives a higher seizure load than a ‘mild’ E.P additive [76 ] The seizure load is the load sufficient to cause seizure of the balls in a ‘four-ball’ test In this test one ball is rotated under load against three stationary... reactivity becomes a function of hydraulic pressure [81] The ‘hot wire’ method is an adequate and effective method for demonstrating a general relationship between chemical activity and TEAM LRN 3 92 ENGINEERING TRIBOLOGY lubricating effect for a wide range of additives However, an exact comparison between similar additives also requires an independent confirmation by other methods To specifically evaluate . 16 and 18 [43]. 0 .2 0.5 1 2 5 10 20 0.001 0.0 02 0.005 0.01 0. 02 0.05 0.1 0 .2 0.5 1 Speed [m/s] Separation [nm] Stearic acid/ hexadecane (saturated) Pure hexadecane FIGURE 8. 32 Effect of dissolved. 0.1 1 10 100 29 0 300 310 320 330 340 350 10 5 /Transition temperature [K −1 ] Concentration [% wt] 50 20 5 2 0.5 0 .2 Capric acid Myristic acid Oleic acid Stearic acid FIGURE 8 .25 Relationship. the heavily loaded regions [ 72 - 74 ]. The concept of an iron sulphide film was then proposed [75 ] and later confirmed when surface analysis was TEAM LRN 390 ENGINEERING TRIBOLOGY Region of EHL contact Virtually