Engineering Tribology 2E Episode 10 pptx

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 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. 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. 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) BOUNDARY AND EXTREME PRESSURE LUBRICATION 385 where: U is the sliding velocity [m/s]; υ is the kinematic viscosity [m 2 /s]; W is the load [N]. The apparatus used to measure friction was a reciprocating steel ball on a steel plate, oscillating at short amplitude and high frequency as shown schematically in Figure 8.34. The value of the constant in equation (8.4) was found by measuring the loads and velocities where oil film collapse, manifested by a sharp increase in temperature (Figure 8.34), occurred during lubrication by plain mineral oil. Assuming that the constant is only a function of film geometry and independent of the lubricant it is possible to calculate the viscosity of the soap film. An example of the experimental results obtained with 0.3% stearic acid in hexadecane is shown in Figure 8.35 [62]. Force transducer Reciprocating motion variable frequency Amplitude fixed e.g. 2 mm Test steel plate Controlled electric heating Steel ball Load µ 0.5 0.1 Temperature Friction transition temperature a) b) Leaf springs FIGURE 8.34 Experimental principles involved in detecting viscous soap layers during reciprocating sliding, a) schematic diagram of the test apparatus, b) sharp increase in friction temperature indicating collapse of lubricating film (adapted from [62]). It can be seen from Figure 8.35 that the calculated viscosity is in the range between 200 - 2000 [cS] which is similar to the viscosity of a soap under the same temperature. The limitation associated with this mode of lubrication is that like chemisorption, reaction with an oxidized metallic substrate is a pre-requisite. Steels and other active metals such as copper or zinc would probably form soap layers whereas noble metals and non-oxide ceramics are unlikely to do so. · Amorphous Layers It is known from common experience that the process of sliding involves grinding which can reduce the thickness of any interposed object. Lumps of solid can be ground into fine powders and, at the extreme, a crystal lattice can be dismantled into an amorphous assembly of atoms and molecules. This process is particularly effective for brittle or friable substances. As discussed already, many lubricant additives function by reacting with a substrate to form a deposit or film of reacted material which is inevitably subjected to the process of comminution imposed by sliding. This material, finely divided (i.e. as very fine particles) or with an amorphous molecular structure, can have some useful load carrying properties and can also act as a lubricant. 386 ENGINEERING TRIBOLOGY 0.01 0.005 0.02 0.05 0.1 150 200 250 Temperature [°C] Velocity [m/s] Kinematic viscosity [cS] 200 500 1000 2000 5000 υ 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 can be illustrated by a bubble raft analogue of a crystal lattice. Each bubble is analogous to an atom and, when closely packed, the bubbles resemble a crystal lattice if regular and an amorphous distribution if irregularly arranged [64]. An example of a bubble raft model of a sliding interface is shown in Figure 8.36. Material close to the sliding surfaces tends to be crystalline because of the tendency to align with a plane surface. The bulk of the material, however, is amorphous because the shearing caused by sliding does not follow exact planes parallel to the sliding direction. Instead, transient ripples of shear waves completely disrupt any pre-existing crystal structure as shown in Figure 8.37. Amorphous layers of phosphates containing iron and zinc have been found in steel sliding contacts when zinc dialkyldithiophosphate (ZnDDP) was used as a lubricant additive [65,66]. The formation of these amorphous layers is associated with anti-wear action by the ZnDDP for reasons still unclear. Finely divided matter as small as the colloidal range of particles has been shown to be capable of exerting a large pressure of separation between metallic surfaces [67]. Very little pressure is required to compress a spherical powder particle to a lozenge shape, but when this lozenge shaped particle is further deformed to a lamina, the contact pressure rises almost exponentially. This can be visualized by considering the indentation of a layer of powder supported by a hard surface using a hemispherical punch. Initial indentation requires little force but it is very hard to penetrate the powder completely. The deformation process of a soft spherical powder particle is illustrated schematically in Figure 8.38. When the compression force is sufficiently large, the soft material is entrapped within the harder surface as illustrated in Figure 8.38. The resultant strain in the hard material may cause permanent deformation which could be manifested by scratching and gouging [67]. The compression tests reported were performed without simultaneous sliding. The films deposited by ZnDDP presumably have the ability to roll and shear within the sliding contact while individual ‘lumps’ of material are not further divided into smaller pieces. BOUNDARY AND EXTREME PRESSURE LUBRICATION 387 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]. 388 ENGINEERING TRIBOLOGY Light load Hard metal Hard metal Soft powder Debris or film product Higher load Extreme load Metal deformation Debris entrapment FIGURE 8.38 Deformation process of a soft material between two harder surfaces. These deposited films of solid powder or amorphous material on metallic surfaces would not suffer from the drawback of desorption at a limiting temperature or viscosity loss with increasing temperature as adsorbed films do. The mechanism which is involved in formation of these films evidently forms the basis of the high temperature lubricating properties of ZnDDP as compared to fatty acids. This research, however, is very recent and the current models will most probably be revised in the future. A further intriguing aspect of phosphorus-based additives is the spontaneous formation of deposits under a full elastohydrodynamic film [68,69]. Both pure phosphonate esters and solutions of phosphonate esters in paraffin tested by optical interferometry showed an increase in the EHL film thickness from about 200 [nm] to 400 [nm] over 2 [hours] of testing at 100°C. An example of this increase in EHL film thickness versus rolling time for a 3% solution of didodecyl phosphonate in purified mineral oil is shown in Figure 8.39. Surface analysis of the rolling track of the EHL contact revealed that the deposited layer was composed of a polymerized network of iron phosphate with some organic groups included. This indicated that the iron had reacted with the phosphorous additive to form a thick layer on the surface. The layer was extremely viscous and waxy in consistency and almost insoluble in organic solvents. Polymerization by cross-linkage between phosphate and iron atoms was also detected. It therefore seemed possible that an irregular network of repeating phosphate and organic groups and iron atoms formed to create an amorphous structure. This particular process may be the only confirmed observation of a so called ‘friction polymer’ [70]. Friction polymer in general terms refers to the polymerization of hydrocarbon lubricants in contact with metal whose oxide film has been removed by friction. A clean metallic surface, particularly of steel, is believed to have strong powers of catalysis which can induce the formation of hydrocarbon polymer films on the worn surface. These films are believed to reduce friction and wear. 8.5 HIGH TEMPERATURE - HIGH LOAD LUBRICATION MECHANISMS A lubrication mechanism acting at high temperature and high load is generally known as lubrication by sacrificial reaction films or ‘Extreme Pressure lubrication’ (often abbreviated to E.P. lubrication). This mechanism takes place in lubricated contacts in which loads and speeds are high enough to result in high transient friction temperatures sufficient to cause desorption of available adsorption lubricants. When desorption of adsorbed lubricants occurs, another lubrication mechanism based on sacrificial films is usually the most effective means available of preventing seizure or scuffing. The significance of temperature has led to the suggestion that this mode of lubrication be termed ‘Extreme Temperature Lubrication’ [6] but this term has never gained wide acceptance. It seems that the term ‘Extreme Temperature’ is too ambiguous for practical use since oils are never used at extreme temperatures and furthermore the contact stresses under which E.P. lubricants are effective BOUNDARY AND EXTREME PRESSURE LUBRICATION 389 and commonly used considerably exceed the limiting contact stresses of many high temperature lubricants. For example, sulphur-based E.P. additives used in 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]. 200 300 400 0 100 200 Time [minutes] Measured film thickness [nm] FIGURE 8.39 Increase in EHL film thickness due to chemical reaction between the iron substrate and the phosphonate additive [68]. Model of Lubrication by Sacrificial Films Current understanding of E.P. lubrication is based on the concept of a sacrificial film. The existence of this film is difficult to demonstrate as it is thought to be continuously destroyed and reformed during the wear process. However, a great deal of indirect evidence has been compiled to support the existence of such sacrificial films. The model of lubrication by a sacrificial film between two discs is illustrated in Figure 8.40. The main effect of severe contact loads is to remove the oxide film from asperity peaks during contact with opposing surfaces. As already mentioned, the oxide-free surface of most metals is extremely reactive. If a lubricant additive containing sulphur, chlorine or phosphorus is present then a sulphide, chloride, phosphide or phosphate film rapidly forms on the exposed or ‘nascent’ surface. The adhesion between opposing asperities covered with these films is much less than for nascent metallic surfaces and this forms the basis of the lubricating effect. The asperities are able to slide past each other with the minimum of damage and wear while the film material is destroyed by the shearing that inevitably occurs. If this mechanism fails, asperity adhesion and severe wear occurs as described in the section on ‘Mixed Lubrication and Scuffing'. In general terms the lubrication mechanism by sacrificial films depends on rapid film formation by a reactive E.P. additive and on sufficient time and temperature for the reaction films to form. The evidence for the formation of sacrificial films has gradually been gathered over time. It was originally observed 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 390 ENGINEERING TRIBOLOGY Region of EHL contact Virtually grown film Fully grown film Film produced by quiescent corrosion Reactive sulphur or other element from lubricant Bare unprotected surface Film destruction FIGURE 8.40 Model of lubrication by a sacrificial film. sufficiently advanced to detect traces of iron sulphide on the steel surface [76]. Films of iron sulphide were then produced on steel surfaces to test their lubricating effect and it was found that their survival time in a sliding contact was very short [77]. When the temperature of rubbing steel specimens was deliberately lowered below the ‘E.P. start temperature’ (i.e. the minimum temperature for which the E.P. reaction becomes effective, producing a lubricating effect), any lubricating effect rapidly disappeared even though it has been well developed at a higher temperature [78]. In a more elaborate test, the friction characteristics of a carbon steel pin sliding against a stainless steel ball were compared with those of a stainless steel pin sliding against a carbon steel ball while lubricated by an E.P. oil. The load capacity of the stainless steel pin sliding against a carbon steel ball was higher than when the friction pair materials were exchanged. Static corrosion tests of the sulphur additive outside of the sliding contact showed that stainless steel did not react or corrode as rapidly as plain carbon steel [78]. Since the pin is subjected to a very intense wearing contact a sacrificial film is unlikely to form on its surface whatever the material. It is therefore preferable for the ball to be made of reactive material, i.e. carbon steel, to allow a sacrificial film to form on the surface outside the sliding contact giving higher load capacity and better wear resistance. A model of this film formation is schematically illustrated in Figure 8.41. Additive Reactivity and Its 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 balls until seizure occurs. This test is commonly used in characterizing lubricating oils. Exact comparisons of additive chemical reactivity and E.P. performance are rather rare in the literature because of the limited practical need for them and experimental difficulties involved. One test measuring corrosion by E.P. additives and load carrying capacity was conducted on a ‘hot wire’ and ‘four-ball’ test rigs simultaneously [80]. The operating principle of a hot wire corrosion apparatus is shown in Figure 8.42. A wire submerged in a bath of the test oil is heated by electric current to induce corrosion of the wire. Since the corrosion product, e.g. iron sulphide, usually has a much higher resistivity than the metal wire, the increase in resistance provides a measure of the depth of the corrosion. The temperature of the wire, which also affects its resistance, is held constant during the test. There is a short period of time required for the wire to reach a steady temperature. The corrosion rates are usually sufficiently slow so that it can be assumed that no corrosion occurs [...]... 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Film with Bubbles Soap Raft Model, Proc 14th Leeds-Lyon Symp on Tribology, Interface Dynamics, Sept 1987, editors: D Dowson, C.M Taylor, M Godet and D Berthe, Elsevier, 1988, pp 3-9 65 M Belin, J.M Martin, J.L Mansot, Role of Iron in the Amorphization Process in Friction-Induced Phosphate Glasses, Tribology Transactions, Vol 32, 1989, pp 410- 413 66 J.M Martin, M Belin, J.L Mansot, H Dexpert and P Lagarde,... 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 402 ENGINEERING TRIBOLOGY µ 0.3 0.2 0.1 0 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... 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 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... 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 can also act as a lubricant. 386 ENGINEERING TRIBOLOGY 0.01 0.005 0.02 0.05 0.1 150 200 250 Temperature [°C] Velocity [m/s] Kinematic viscosity [cS] 200 500 100 0 2000 5000 υ FIGURE 8.35 Relation. acids on EHL film thickness was also detected [61]. Film thickness or separation distance 382 ENGINEERING TRIBOLOGY versus rolling speed under EHL lubrication by pure hexadecane and hexadecane with. other acids, the friction transition temperature was much lower, between 120°C and 160°C. 384 ENGINEERING TRIBOLOGY Thick Films of Soapy or Amorphous Material Almost all additives used to control