Lubrication and Lubricants

LUBRICATION AND LUBRICANTS 1. Introduction Lubrication is a process in which a film of lubricant is inserted between rubbing surfaces for the purpose of controlling friction and/or to reduce wear of the surfaces. These films are designed to minimize contact between the rubbing surfaces and to shear easily so that the frictional force opposing the rubbing motion is low. Lubricants may be liquids, solids, gases, or greases. Lubricating oils and greases contain refined or synthesized base oils from animal, vegetable, or mineral (petroleum) origin, and a variety of additives to improve their lubricating and other characteristics. Lubrication is a major component of tribology, defined as the science and technology concerned with interacting surfaces in relative motion, including friction, lubrication, wear, and erosion (1). Tribology and lubrication are ancient arts. In his splendid History of Tribology, Professor Dowson traces the development of these arts and sciences, and describes the outstanding artists and scientists responsible, from the paleolithic age to the end of the twentieth century. He reports archeological evidence that bitumen was used to lubricate potters wheels 5000 years ago. Water-lubricated sliding bearings were used in Egypt 2400 BC to transport large objects. A chariot wheel from 1400 BC was found with traces of tallow as lubricant, and the Chinese had lubricated metal wheel bearings with leather seals to hold the lubricant in place in the fourth century BC (2). The word ‘‘tribology’’ first appeared in Reference 3. This is often called The Jost Report, after H. Peter Jost, the chairman of the British Lubrication Engineering Working Group, which prepared the report. The word is derived from the Greek tribein, meaning ‘‘to rub,’’ and logos, meaning ‘‘reading’’ or ‘‘study.’’ Tribology is literally the study of rubbing. The Working Group defined it more precisely as ‘‘the science and technology of interacting surfaces in relative motion and the practices related thereto’’ (3). The Jost Report was part of an effort to focus attention on the ‘‘scientific, technological, economic and environmental issues’’ (2) involved in the study and practice of tribology. Another purpose was to bring together the many, and often splintered, engineering and scientific disciplines that deal with this technology. Dowson speculates that the dramatic progress in this field in the final third of the twentieth century may have been significantly influenced by these efforts in the 1960s (2). The 1966 report by the British Lubrication Engineering Working Group demonstrated to industry and government the impact of friction, wear, and lubrication on the nation’s economy, and the value of further research in tribology. That report showed, for example, that the most significant value of better lubrication (91%) comes from increased productivity, lower maintenance and replacement costs, and lower investment cost. Direct energy savings (5%), and savings in the cost of lubrication, in manpower and material (4%), account for the remainder. Tribology is a multidisciplinary science that embraces lubrication, friction, wear, properties of lubricants, surface characterization, bearing materials, and the selection and design of lubricating systems. The lubrication engineer would 1 Kirk-Othmer Encyclopedia of Chemical Technology. Copyright # 2015 John Wiley & Sons, Inc. All rights reserved. DOI: 10.1002/0471238961.1221021802151519.a01.pub3 2 LUBRICATION AND LUBRICANTS add to this list lubricant and coolant selection, plant lubrication and maintenance programs, and machine condition monitoring. As an example of the potential impact of the science of tribology, a study was recently completed that documents the opportunities for tribologists to reduce the energy needed to overcome friction in passenger cars. The study, ‘‘Global energy consumption due to friction in passenger cars’’ by Kenneth Holmberg; Peter Andersson, VTT Technical Research Centre of Finland; and Ali Erdemir, Argonne National Laboratory, Energy Systems Division, identified the following: In passenger cars, one-third of the fuel energy is used to overcome friction in the engine, transmission, tires, and brakes. The direct frictional loss, with braking friction excluded, is 28% of the fuel energy. In total, 21.5% of the fuel energy is used to move the car. Worldwide, 208,000 million liters of fuel (gasoline and diesel) were used in 2009 to overcome friction in passenger cars. This equals 360 million tons oil equivalent per year (Mtoe/a) or 7.3 million TJ/a. Reductions in frictional losses will lead to a threefold improvement in fuel economy as it will also reduce both the exhaust and cooling losses at the same ratio. Globally, one passenger car uses an average of 340 L of fuel per year to overcome friction, which would cost D 510, according to the average European gas price in 2011, and corresponds to an average driving distance of 13,000 km/a. By taking advantage of new technology for friction reduction in passenger cars, friction losses could be reduced by 18% in the short term (5–10 years) and by 61% in the long term (15–25 years). This would equal worldwide economic savings of D 174,000 million and D 576,000 million, respectively; fuel savings of 117,000 and 385,000 million liters, respectively; and CO2 emission reduction of 290 and 960 million tons, respectively. The friction-related energy losses in an electric car are estimated to be only about half those of an internal combustion passenger car. Potential actions to reduce friction in passenger cars include the use of advanced coatings and surface texturing technology on engine and transmission components, new low viscosity and low shear lubricants and additives, and tire designs that reduce rolling friction. The sciences of tribology and lubrication engineering are somewhat hidden, enabling technologies that are the underpinning of virtually all systems that have surfaces that rub against each other. Example applications that show the diversity of these systems include all mechanical machinery that utilize systems such as bearings, gears, hydraulics, or seals; biomechanical systems such as prosthetic joints, an eyelid sliding over a contact lens, various creams and salves for skin care, or hair conditioner to ease a comb through the hair; and hard drives in the magnetic recording industry for large-scale data storage. Today tribology is successful in saving energy, reducing friction and wear, improving efficiency and reliability of equipment, enhancing new designs, and so on. 2. Fundamentals of Lubrication Tribology, by definition, is concerned with interacting surfaces in relative motion. It is appropriate, therefore, to begin the discussion of lubrication fundamentals by describing the characteristics of tribological surfaces. LUBRICATION AND LUBRICANTS 3 2.1. TheNatureofInteractingSurfacesinRelativeMotion. Tribological surfaces are the load-bearing surfaces on the moving parts of machines. They include surfaces on crankshaft rod and main bearings, radial and thrust bearings on steam and gas turbines, cams and valve lifters, pistons and cylinders, natural and artificial hip joints, ball and roller bearings, machine tool slideways, cutting tools, magnetic information storage devices, and microelectromechanical systems (MEMS). Despite their appearance and finishing efforts, these surfaces are not perfectly smooth. There are microscopic irregularities, gently sloping hills and valleys called asperities on them. If an imaginary line is drawn through a real surface, such that the volume of all of the material above the imaginary surface is equal to the volume of voids below that surface, the roughness of the real surface, Ra, can be defined as Ra ¼ ðjy1 j þ jy2 j þ þjyn jÞ=n (1) where Ra is the center line or arithmetic average of the absolute distances, yi, from the imaginary surface (mean line) for a given sampling length (usually 0.80 mm). Roughness (Ra) values of machined surfaces range from 0.025 mm for ball bearing surfaces to 25 mm clearance surfaces on rough machine parts (4–6). The  roughness of computer hard disk surfaces is measured in angstroms (A) or nanometers (nm) (7). The total profile of a surface consists of a ‘‘waviness’’ and a roughness component. The parameter Ra, although it is the most common measure of surface roughness, is insensitive to the shape or waviness of the profile. A more useful parameter is the root-mean-square (rms) roughness, Rq: Rq ¼ 
 1=2 y21 þ y22 þ þy2n =n (2) where Rq is the rms deviation of yi from the mean line for a given sampling length (4). The rms roughness of computer hard disk surfaces is <2 nm (7). The most repeatable of the roughness parameters is the 10 points height, Rz. Rz ¼ ½ðP1 þ P2 þ þP5 Þ ðV 1 þ V 2 þ þV 5 ފ=5 (3) where Rz is the average distance between the five highest peaks (Pi) and the five deepest valleys (Vi) within the sampling length. It is also linked with the machining parameter S2/8r, where S is the feed rate and r is the tool radius (4). Another index of roughness is Rt, the maximum peak-to-valley height. Rt ¼ Rp þ Rv (4) where Rp is the maximum peak height and Rv is the maximum valley depth within the sampling length (4). The description of the test disk used in the ASTM D6425-11 test method for measuring friction and wear illustrates the impact of surface texture on these quantities (8) and is shown below and used with ASTM’s permission. 4 LUBRICATION AND LUBRICANTS Test Disk, AISI 52100 Steel, 62  1 HRC Hardness. The surfaces of the disk are lapped and free of lapping raw materials. The topography of the disk will be determined by four values: 0.005 mm < Rz > 0.65 mm; 0.035 mm < Ra > 0.050 mm; 0.020 mm < Rp > 0.035 mm; 0.050 mm < Rv > 0.075 mm. Four different measures of surface topography are specified for the disk in order to get acceptable reproducibility of the test method (9). Approximate roughness indexes obtained with various metalworking processes are as follows: Production process turning grinding milling boring honing lapping Rt, mm Ra, mm 4.00–25.0 2.00–6.0 1.50–20.0 0.50-20.0 0.03–1.0 0.03–0.6 0.500–3.0 0.400–0.8 0.200–2.0 0.050–1.6 0.015–0.2 0.015–0.1 Engineering surfaces also differ in composition from the underlying bulk material. A metal bearing, for example, will have a work hardened layer at the surface, over which an oxide layer forms, on top of which is an adsorbed layer of moisture and gases. When two such surfaces come in moving contact, their surface structures, compositions, and the interaction between opposing asperities account for a major portion of the friction between them and much of the wear that inevitably occurs (10–12). For example, without the oxide and adsorbed layers, coefficients of friction >4 have been measured in a vacuum of 0.133 mPa on surfaces cleaned by abrasive cloth and heated. The coefficient decreased considerably when oxygen was admitted to a pressure of 0.133 Pa (13). The oxide and adsorbed layers on metal surfaces can, therefore, be considered as lubricating films. 2.2. Friction. When two of these surfaces are brought together, they initially touch at the highest asperities. The load N normal to the surfaces at the contact points causes the asperities to deform until the pressure in the resulting contact areas just equals the yield pressure p of the asperities. The sum of these contact areas is the real contact area Ar. The yield pressure is equivalent to the Brinell hardness number (BHN), in consistent units, measured at the surface of the material (4): Ar ¼ N=p ¼ N=BHN (5) The real area of contact is a minute fraction of the total surface area. For example, with a typical bearing contact stress of 3 MPa and a bronze-bearing asperity yield pressure of 500 MPa, <1.0% of the nominal area would involve asperity contact. As the load on the surfaces increases, the asperities continue to deform, the softer surface more than the other. More of the asperities come in contact and the real area of contact grows. The opposing surfaces also tend to adhere or bond to LUBRICATION AND LUBRICANTS 5 each other in the contact area. The shear strength of these bonds depends on the time of contact and the difference in composition of the two surfaces. Sliding of one of these nonlubricated surfaces across the other requires a friction force F to displace the contacting asperities. This force includes several components, including a shear or adhesion component arising from bonding of the contacting asperities; a plowing or deformation component, arising from the interlocking of asperities; a lifting component to raise asperities over the roughness of the mating surfaces. The shearing component Fs may account for 90% or more of the total friction force. This component is proportional to the shear strength s of the asperity junctions: F s ¼ Ar s (6) More detailed descriptions of surface texture, surface structure, and composition and the real area of contact will be found in References 4–6, 12, 13, 15, and 16. 2.3. Coefficient of Friction. The coefficient of friction f of a pair of contacting surfaces is defined as the ratio of the total frictional force to the normal force or load. It can also be expressed as the ratio of shear strength s to the yield pressure p at the asperity junctions: f ¼ F=N ¼ s=p (7) If there is a lubricating film on the surfaces, the coefficient of friction is the ratio of the shear strength of the surface film sf to the yield pressure pm of the substrate or backing material: f ¼ sf =pm (8) If a shear force is gradually applied to one of two dry, nonlubricated surfaces in contact, the surface will not move until the force is great enough to overcome the shear strength of the asperity contacts. The ratio of the shear force required to start motion to the normal force on the surfaces is the static coefficient of friction. Once motion starts, less force is needed to keep the surface moving at a constant velocity. The coefficient of friction during sliding is the kinetic or dynamic coefficient. The static coefficient measured for a hard steel surface on another hard steel surface is 0.78. The dynamic coefficient measured for hard steel on hard steel is 0.42. When a thin film of light mineral oil is applied to these surfaces, the static coefficient drops to 0.23. The dynamic coefficient with a light oil film drops to 0.1. Adding a friction modifier to the oil can reduce or reverse the difference between the two coefficients. Adding stearic acid to the lubricant, for example, for hard steel on hard steel, reduces the static coefficient to 0.0052, which is lower than the dynamic coefficient, 0.029 (17). An extensive Friction and Wear Databank is found in References 17 and 18. Tables of the coefficient of friction values for a wide variety of material combinations are also available in References 19 and 20 and many other sources. These data, however, should be used with caution. The coefficient of friction varies with changes in humidity, gas pressure, time, temperature, sliding speed, surface quality, the shape of the contact region, the method of testing, and other variables. 6 LUBRICATION AND LUBRICANTS Table 1. Wear Coefficients for Various Sliding Combinationsa Material combination Wear coefficient, k zinc on zinc low carbon steel on low carbon steel copper on copper stainless steel on stainless steel copper on mild steel mild steel on copper phenolic resin on phenolic resin 53  10 7  10 11  10 7  10 5  10 17  10 7  10 a 3 3 3 3 4 5 6 For equation 9, from Ref. 14. Where high reliability is needed, the friction should be measured using a prototype device under design conditions (20). 2.4. Wear. The principal types of wear in sliding contacts are adhesive, abrasive, and corrosive wear. Fatigue wear occurs in concentrated contacts (ball and roller bearings, gears, cams, and automotive valve lifters) under the combination of sliding and rolling (21). The Archard equation reported by Rabinowicz (21) gives a simple, quantitative relationship for predicting the adhesive wear rate: V ¼ kNx=p (9) where V ¼ wear volume, k ¼ wear coefficient, N ¼ normal load, x ¼ sliding distance, and p ¼ yield stress or indentation hardness. Values of k for several unlubricated material combinations are shown in Table 1 (14). Others will be found in References 18 and 21–24. Adhesive wear is material separated or transferred during the shearing of asperity contacts. These wear particles, and other particulate surface contaminants that are hard enough to damage the surface, cause abrasive wear. Abrasive wear is the removal of material by ploughing, cutting, or scratching. Its rate generally obeys equation 9 and the wear coefficients tend to be higher than the adhesive coefficients (21). Corrosive wear is the wearing away of the products of galvanic or chemical corrosion of the surface. There is no simple equation that characterizes this type of wear. Current broader discussions of the friction and wear phenomena are found in References 13, 25, and 26. 2.5. Viscosity. Viscosity, or resistance to flow, is the most important property of a lubricating oil. It was defined by Newton (27) as the ratio of the shear stress, T, divided by the shear rate, dU/dh, in a fluid during flow. Newton’s law of viscosity is given in equation 10 (28): T ¼ F s =As ¼ m
dU=dh (10) The fluid in contact with the surface of the moving plate has the same velocity U as the plate. The fluid in contact with the stationary plate has zero velocity. There is, therefore, a shear stress T on the fluid equal to the force Fs required to keep the plate moving divided by the area As of the moving plate, and is proportional to the velocity gradient dU/dh of the fluid. The parameter Fs is a frictional force and LUBRICATION AND LUBRICANTS 7 the coefficient m is the dynamic viscosity of the fluid (29). The unit of dynamic viscosity in the System International (SI) is pascal second (Pa s). The customary unit is centipoise (cP), which is Pa s  10 3. Schoff and Kamarchik (30) describe methods of measuring the viscosity of a wide variety of materials. Standard methods for measuring the dynamic viscosity of lubricating oil can be found in References 31–34. Generally, however, it is easier to measure the kinematic viscosity n of a lubricating oil using a capillary viscometer. Kinematic viscosity is the dynamic viscosity divided by the density d of the fluid at the same temperature. n ¼ m=d (11) The customary unit for kinematic viscosity is centistokes (cSt), which is equivalent to millimeters square per second (mm2/s). The most common method for accurate measurement of kinematic viscosity over a wide temperature range is ASTM D445 (35). Older literature may report kinematic viscosities of lubricating oils in Saybolt Universal Seconds (SUS). ASTM D2161-10 has been retained for the purpose of calculating kinematic viscosity in centistokes from SUS or SFS data (36). Viscosity–Temperature Relationship. The MacCoull equation, also called the Walther equation, relates the kinematic viscosity v of a liquid to its temperature. log log Z ¼ A Bðlog TÞ (12) where Z ¼ (n þ 0.7) for 2.00 cSt  n  2  107 cSt; A and B ¼ constants and T ¼ absolute temperature,  K ¼ temperature,  C þ 273.2. It is applicable to homogeneous liquid lubricants with conventionally refined hydrocarbon base oils and is valid between the cloud point at low temperatures and the initial boiling point (340 C) at higher temperatures. (The cloud point is the temperature at which a cloud of wax crystals first appears when it is cooled under conditions prescribed in ASTM D2500.) This equation is the basis of the viscosity–temperature charts described in ASTM D341 (37). Oils with ester, phosphate, silicone, and synthesized hydrocarbon base oils follow the MacCoull (Walther) relationship over the range of 18 to 175 C to within 5%. Many esters, synthesized hydrocarbons, and low pour point mineral oils exhibit low temperature ( 40 to 54 C) viscosities substantially below MacCoull equation predictions (38). Viscosity Index. Another widely used and accepted measure of the variation of viscosity with temperature is the viscosity index (VI). The higher the VI, the lower is the variation of viscosity with temperature. The VIs for oils having values <100 are calculated by VI ¼ 100½ðL U Þ=ðL H ފ (13) where U ¼ kinematic viscosity at 40 C of the oil whose viscosity index is to be calculated; L ¼ kinematic viscosity at 40 C of a ‘‘0’’ VI oil with the same viscosity at 100 C as the unknown; H ¼ kinematic viscosity at 40 C of a ‘‘100’’ VI oil with the same viscosity at 100 C as the unknown. Values of H and L for viscosities of 2 cSt and above are given in ASTM D2270 (39). 8 LUBRICATION AND LUBRICANTS Equation 13 gives confusing results for VI values >100. Viscosity indexes of 100 or greater are calculated by the empirical formula (39): VI ¼ ½ððantilog N Þ 1Þ=0:00715 þ 100Š (14) where YN ¼ H/U and Y ¼ kinematic viscosity at 100 C of the oil whose viscosity index is to be calculated. Viscosity index is sometimes used as a measure of the quality of lubricating oil, especially for selecting base stocks for automotive engine oils and automatic transmission fluids (ATFs). This is, however, not always applicable. The role of viscosity index in base stock selection is described later. Several aromatic type fluids, polyphenyl ethers, aryl phosphate esters, and halogenated aromatic hydrocarbons, have negative VIs. Effect of Pressure on Viscosity. The lubricant film pressure in the concentrated contact areas of rolling element bearings, gears, cams, and so on can be as high as 2000–3000 MPa. The viscosity of the film at these pressures could be a million times higher than that of the lubricating oil at atmospheric pressure, as illustrated in Figure 1, or the lubricant may have solidified (38,40). The rate of viscosity increase with pressure of a liquid lubricant varies with its composition and chemical structure and with temperature and pressure. Traction drives found in some industrial machinery and the toroidal drives in some continuously variable transmissions (CVTs) depend on this property of traction fluids to transfer power. Generalized pressure–temperature–viscosity relationships from extensive data on petroleum and synthetic oils are described in References 14, 41, and 42. Fig. 1. Viscosity pressure curve for typical petroleum oils. —: paraffinic, ---: alicyclic, ": solid. LUBRICATION AND LUBRICANTS 9 Newtonian Versus Non-Newtonian Behavior. If the viscosity of a fluid subjected to shear is independent of the rate of shear or magnitude of the shear stress, it is a Newtonian fluid. Most industrial lubricating and hydraulic oils are Newtonian fluids. If the viscosity changes with shear stress or shear rate, the fluid is nonNewtonian. This behavior is typical of multigrade engine oils and other oils containing polymeric viscosity improvers. The rate of decline in viscosity in a non-Newtonian lubricating oil is initially slow, then reaches a maximum, and finally slows again. At very high shear rates, the viscosity tends to level out and approach that of the base oil. Grease also behaves in a non-Newtonian manner. At low shear rate, it acts like a high viscosity semisolid and ‘‘stays in place’’. In a bearing, under high shear rate, it acts more like its base oil and supports full-fluid lubricating films. 2.6. Lubrication Regimes. In the full-fluid film regimes, the moving, load-bearing surfaces are completely separated. There is no contact between them. Resistance to motion arises solely from the internal friction of the fluid, a function of its viscosity. Adhesive wear is absent. Wear may occur from surface fatigue or from contamination of the fluid with corrosive or abrasive substances. The coefficient of friction in a liquid lubricated system is dependent on, among other things, the lubricant viscosity, the relative speed of the surfaces, and the load on the surfaces. In a journal bearing, for example, the lubricant film thickness and the coefficient of friction are functions of the dimensionless Sommerfeld bearing characteristic number S:   S ¼ R2 =C2 ZN=P (15) where P is the average pressure on the bearing surface W/2RL, Z is the dynamic viscosity m of the fluid, and N ¼ is the rotational speed of the journal U/2pR. The relationship between the kinetic coefficient of friction and the dimensionless quantity ZN/P is illustrated in Figure 2 (10). This is known as a Stribeck curve. At values of ZN/P greater than 30, lubrication is in the full-fluid film regime. Friction increases with increasing ZN/P because of the increasing resistance to flow. As ZN/P is reduced, by reductions in the speed or viscosity, or by increases in load, the coefficient will reach a minimum value. Further reduction in ZN/P leads to partial breakdown in the fluid film and lubrication is in the mixed film regime. Friction increases as ZN/P decreases in this regime, as more and more of the load is carried by asperity contact and less by fluid-film pressure. Finally, a point is reached, as ZN/P gets smaller, where there is no fluid pressure, the rate of increase in the coefficient of friction starts to level out, and lubrication is in the thin-film boundary regime (43). Ideally, a bearing is designed to operate where the coefficient of friction is at its minimum. Hydrodynamic Lubrication Regime. Rohde (44) and Dowson (45) remind us that the basic mechanism of fluid-film lubrication was explained by Reynolds (46) in 1886, based on the earlier work of Petrov (47), and Tower (48). The fundamental requirements of lubrication in the hydrodynamic regime are the formation of a wedge-shaped film and generation of pressure in the film, sufficient to keep the surfaces apart, by the motion of the surfaces themselves. 10 LUBRICATION AND LUBRICANTS Fig. 2. Variation of coefficient of friction with ZN/P. The pressure distribution in the lubricating film of the sliding bearing illustrated in Figure 3 (29), moving with velocity U relative to a slanted stationary pad in a fluid with viscosity m, and assuming no flow out of the sides of the bearing, is (49) p ¼ 6mUxðh h2 Þ=h2 ðh1 þ h2 Þ (16) The total force P that the bearing will support per unit width is h P ¼ 6mUB2 =ðh1 i h2 Þ2 ½lnðh1 =h2 ފ 2½ðh1 h2 Þ=ðh1 þ h2 ފ (17) Fig. 3. Pressure distribution in a hydrodynamic film. Reprinted with permission from Reference 29. LUBRICATION AND LUBRICANTS 11 and the frictional force F required to move the slider at speed U is F ¼ ½2mUB=ðh1 h2 ފ½2 lnðh1 =h2 Þ 3ðh1 h2 Þ=ðh1 þ h2 ފ (18) The analysis of hydrodynamic fluid films assumes laminar flow in the film, that is, a Reynolds number Re, <1000. Re ¼ Urh=m ¼ Uh=n (19) where U ¼ velocity, r ¼ density, h ¼ average film thickness, m ¼ dynamic viscosity, and n ¼ kinematic viscosity. Transition from laminar flow to turbulent flow starts when the Re is 1000 and the flow is completely turbulent at a Re of 1600 (50). In a journal bearing, the wedge is formed because the diameter of the journal is smaller than that of the bearing. As the journal starts to rotate, its centerline moves away from that of the bearing. The rotating journal drags, or pumps the lubricant through the wedge, against the resistance to flow, and increases the pressure in the fluid until the journal is lifted off the bearing surface. The loadcarrying capacity W of a journal bearing is (51)  

W ¼ mUR2 L=C2 12pe= 2 þ e2 1 e2 1=2 (20) where m ¼ lubricant viscosity, U ¼ journal peripheral speed, R ¼ journal radius, C ¼ clearance ¼ bearing radius – journal radius, L ¼ axial length of the bearing, e ¼ eccentricity ¼ center offset distance e/clearance C. Bearings designed for hydrodynamic lubrication include journal and thrust bearings in steam and gas turbines, and main and rod bearings for automotive engine crankshafts. Elastohydrodynamic (EHD) Lubrication Regime. The shapes of sliding surface bearings designed for hydrodynamic lubrication have a high degree of conformity, a relatively large contact area, and low unit loading, such that the effect of pressure on viscosity can be neglected. In contrast, the surfaces in a rolling element bearing and on gear teeth are nonconforming. Surface contact is concentrated at a point in ball bearings or along a line in roller bearings and gears. Contact pressure, therefore, is high enough to cause elastic deformation of the contacting surfaces, forming a small area of contact. The viscosity of liquid lubricants entering the contact area increases exponentially and may solidify. Since the viscosity of the lubricant is affected by the elastic deformation of the surfaces, as well as other fluid properties, this lubrication regime is called elastohydrodynamic or EHD. The hydrodynamic pressure developed in the lubricant is sufficient to separate the surfaces at the leading edge of the contact area. As the lubricant is drawn into the contact, its pressure and viscosity increase further, keeping the surfaces apart. During contact, the fluid acts like an elastic solid, so that it cannot escape the contact except in the direction of rolling (52). As the lubricant comes out of the contact area, there is a sharp pressure spike, followed by a sudden pressure drop so extreme that it causes a bulge in the 12 LUBRICATION AND LUBRICANTS rolling surfaces. The minimum film thickness is at the location of this bulge. The pressure spike and sudden film restriction directly affect the rolling element fatigue life (53). The Dowson-Higginson equation for minimum film thickness (54,55) is 0:7

0:6
 hmin =R ¼ 1:6 aE0 mU=E0 R P=E0 0:13 (21) where R is the effective radius of the contacting surfaces, a is the viscosity– pressure coefficient, E0 is the effective Young’s modulus of the contacting surfaces, m is the dynamic viscosity at the inlet temperature and atmospheric pressure, U is the effective velocity, and P is the Hertz pressure at the line contact. Similar equations for film thickness in point contact are given by Dowson (2) and by Khonsari and Hua (53). The life of rolling element bearings is related to a film thickness parameter l. l ¼ h=s (22) where h is the calculated EHD film thickness and s is the composite surface roughness. In the absence of chemically active additives in the lubricating oil, damage to the bearing surface occurs, and reduces the life of the bearing, if the film thickness becomes less than the height of the surface asperities (l  1). The life of the bearing increases significantly at values of l > 1.5 (56). Hydrostatic Lubrication. In hydrostatic lubrication, fluid is pumped under pressure to the load-carrying bearing. Almost any fluid may be used, including gases (nitrogen, helium, and air), water, and liquid metals. The principal applications of hydrostatic lubrication are gas bearings, moving large masses on relatively small bearing areas and for startup of heavily loaded hydrodynamic bearings. Squeeze Films. Viscous lubricant films do not immediately collapse when sliding stops. During the time it takes for these films to be squeezed out of the contact area, they can support peak loads higher than those supported in steadystate operation. This time delay also provides damping for shock loads and shaft vibration. These squeeze films are important in rod bearings of reciprocating automotive engines, damping in turbomachinery, and in the lubrication of skeletal joints (hips, etc). Ludema (55) estimates the time delay for a viscous fluid squeezed out of an elliptical contact area as 1=h2 ¼ 1=h20 þ nh o  i 2W a2 þ b2 t =3pa3 b3 m (23) where a and b are dimensions of the ellipse, h0 is the original film thickness (for small values of h0 relative to a or b), m is the dynamic viscosity of the fluid, and W is the load that produces a film of thickness h after time t. Boundary Film Lubrication Regimes. At lower values of the Sommerfeld number (eq. 15), that is, when unit loads P are very high or when sliding speed N or viscosity Z are low, a full-fluid film cannot be generated. The film thickness is smaller than the height of asperities on the surfaces and the surfaces come in LUBRICATION AND LUBRICANTS 13 contact with each other. When there is no fluid pressure, and only a thin film of lubricant on the surfaces, lubrication is said to be in the boundary regime. Except in the case of starved lubrication, that is, inadequate or blocked lubricant supply, lubricating oils and greases usually provide lubrication in the mixed or partial film regime, where the load is supported by a combination of surface asperity contact and fluid film pressure, at lower Summerfeld numbers. Lubricants designed to operate in the mixed or partial film regime may contain additives that form lubricating films on the bearing surface by adsorption, chemisorption, or tribochemical reaction. The additives may be friction modifiers, antiwear additives, EP additives, or multifunctional additives. These films are discussed later under Section 2.8. 2.7. Micro- or Nanotribology. Micro- or nanotribology is the study of adhesion, friction, lubrication, and wear at scales ranging from atomic and molecular scales to microscale. These studies are important to understand interfacial phenomena in micro/nanostructures used in magnetic storage devices, micro/nanoelectro-mechanical systems (MEMS/NEMS), and other nanotechnology applications. Since macroscale surface contact occurs at numerous microscale asperities, studies in micro- or nanotribology are also valuable to provide the fundamental understanding of interfacial phenomena in macrostructures to bridge the gap between science and engineering. Topics in micro- or nanotribology are discussed in References 57–60. Micro- or nanotribology is an interdisciplinary field that is enabled by the development of the surface force apparatus (SFA), the tip-based microscopies such as atomic force microscopy (AFM) and friction force microscopy (FFM), and advanced computational techniques. SFA is commonly employed to study the properties of molecularly thin liquid films sandwiched between two molecularly smooth surfaces. These studies are important to fluid flow through narrow pores and thin film lubrication between two shearing surfaces. AFM and FFM can measure ultrasmall forces between a probe tip and an engineering surface. With this tool, a probe, the tip of which has a radius less than 10 nm (10 8 m), can be moved over a surface to measure surface topography, adhesion force, frictional resistance, boundary lubrication, and other phenomena. Molecular dynamics simulations are used to simulate sliding contact to provide insights into atomic scale energetics, structure, dynamics, and rheological aspects of tribological processes. These experimental and computational tools allow interfacial phenomena and lubricant properties to be studied with high resolution, down to molecular or atomic level. At these levels, the apparent surface interactions do not obey the same ‘‘laws’’ as those observed on a macroscopic scale (61,62). For example, SFA experiments and molecular dynamics simulations have shown that the properties of nanometer-thick lubricant fluids confined between two solid surfaces are different from those at bulk scales. Two crystalline surfaces can cause a liquid to order into a layered structure and solidify or freeze when the gap between the two surfaces is reduced to several times of the molecular diameter. Because of this, branched-chain molecules are better lubricants than straight-chain molecules that are prone to ordering and freezing. Experiments have also shown that the normal force between two surfaces can be modified by a thin liquid or polymer film. A principal application for micro- or nanotribology is computer hard disk  drives. The rms roughness Rq of the disk surface is less than 2 A. The surface is 14 LUBRICATION AND LUBRICANTS covered with a 3–4 nm carbon overcoat and a 1 nm film of partially chemically bonded perfluoropolyether-based lubricant with very low vapor pressure (nonvolatile). The flying height, that is, the air gap between the disk and the recording head, is about 3–5 nm (7). With the quest to further increase the recording density, the magnetic spacing and thus the flying height need to be further reduced, which poses significant tribological challenges for the interface design. Thermal fly height control is used to actively control flying height without reducing the lubricant and overcoat thickness. Another major application of micro- or nanotribology is MEMS/NEMS, which have applications in biomedical, aerospace, electronics, optics, and many others. A major technological challenge is to produce microscale devices that can survive sliding contacts. To effectively lubricate microscale devices, organized, dense molecular layers of long-chain molecule monolayers and thin films are deposited on the surfaces in contact by Langmuir-Blodgett (LB) deposition and by chemical grafting of molecules into self-assembled monolayers (SAMs) (63). LB films are physically bonded to the substrate by weak van der Waals attraction, while SAMs are chemically bonded via covalent bonds to the substrate. AFM and FFM have been used to study the lubricating effects of LB and SAM films. SAMs hold great promise for boundary lubrication of MEMS/NEMS since it offers a variety of chain length and terminal linking group. However, experiments on SAM-coated devices show that they are easily worn off under high load. Therefore, a wear-resistant conformal lubricating coating remains highly desirable. Several new methods of lubrication at a nanoscale are now being investigated. Vapor-phase lubrication has emerged as an effective lubrication method for MEMS/NEMS (64). Self-assembled dual layers (SADL) (65) are found to be more durable and effective in reducing friction coefficients than SAM. Nanomaterials are being widely explored for use as lubricants, such as carbon-based ‘‘bucky balls,’’ nanotubes, and nano-onions. The use of rounded or spherical nanoparticles can potentially allow an efficient rolling action instead of sliding action. Micro- or nanotribology solutions have enabled computer hard drives to have ever-increasing recording density and digital micromirror devices to operate reliably. However, solutions for microdevices to have sliding contact are still missing. The success of several breakthrough nanotechnologies, such as nanoimprinting and IBM millipede for high density storage, rely on the fundamental understandings gained from micro- or nanotribology. The study of micro- or nanotribology itself is also facing several major challenges. One challenge is the uncertainty associated with AFM force calibration and tip characterization and the other is associated with the time and length scale that the molecular dynamics simulations can handle. Perhaps the grand challenge in this field is linking the results obtained at the nanoscale with tribological phenomena occurring in the macroscopic world. 2.8. Lubricant Films. Layers of lubricating material, deposited on the rubbing surfaces, control friction and wear in the boundary portion of the mixed film regime. This material is initially dissolved or dispersed in the oil, and is deposited on the surface by adsorption, chemisorption, or tribochemical reaction. As portions of these films are rubbed off, they are replaced by additional material in the oil. LUBRICATION AND LUBRICANTS 15 A film of paraffin oil adsorbed on rubbing steel surfaces can reduce the coefficient of friction from 0.78 to 0.23 (17). Such a film will not, however, support much load or high temperature. A better strategy for friction modification is to add a surface-active polar molecule, with a long, nonpolar tail to the paraffin oil. These molecules, known as ‘‘friction modifiers’’ or ‘‘boundary lubricity additives,’’ can be naturally occurring substances derived from animal fats, such as lard oil, neatsfoot oil, tallow, and, in time past, sperm whale oil; vegetable oils, such as olive oil, palm oil, rapeseed oil, and soybean oil; or they may be synthesized molecules designed to have a particular structure and chain length. Typical structures are fatty alcohols, esters, amines, and acids with chain lengths of 16–22 carbon atoms. These additives are adsorbed or chemisorbed out of the nonpolar base oil and condense on the surface. The polar end sticks to the surface, and the nonpolar tails pack in as closely as possible and, because of the lateral cohesive forces (London forces) between them, form a strong solid film. This film resists the penetration of asperities and inhibits metal-to-metal contact (66). Fatty acids are chemisorbed on a metal surface, forming a metal soap without removing the metal atoms from the lattice structure of the surface.  With stearic acid, for example, the length of each soap molecule is 19 A, and  2 there can be little over four such molecules on each 100 A of surface (66). Adsorbed films and soaps are temperature sensitive. As surface temperature increases, it reaches the point where the film desorbs, gets disoriented, or melts. For this reason, some lubricants contain EP or antiwear additives, which are activated by the heat of friction at points of metal-to-metal contact during sliding. They react with the surface to form protective layers of solid compounds that prevent welding and catastrophic destruction of the metal surface. Both antiwear and EP additives usually contain one or more of the elements chlorine, phosphorus, or sulfur. These tribochemical reactions involve the controlled corrosion of the metal surface to prevent more serious damage under extreme loads and temperatures. This regime is often called the extreme pressure (EP) lubrication regime. The following are the examples of the type of chemicals used: (1) Zinc dialkyl or diaryl thiophosphate (ZDDP), an effective antiwear additive used in engine oils and hydraulic fluids. It has a relatively high activation temperature (150 C) and forms a tough, wear-resistant surface layer. It is not an effective friction modifier. (2) Mild EP additives with intermediate activation temperatures, such as the sulfur–phosphorous compounds used in automotive hypoid and industrial gear systems. (3) Highly sulfurized or halogenated compounds used in some metalcutting fluids that are activated at relatively low temperatures, in some cases at room temperature. 3. Lubricating Oil Base Stocks The most important component of a lubricating oil or grease is its base oil. Although often supplemented by additives, the base oil determines the flow characteristics of the lubricant, its oxidation stability (sludge and deposit-forming tendency), its volatility, and its corrosion potential. 16 LUBRICATION AND LUBRICANTS Most lubricant base oils are mixtures of paraffins (straight- or branchedchain hydrocarbons), naphthenes (cycloparaffins), and aromatics (alkyl benzenes and multiring aromatics), typically containing 20–40 carbon atoms per molecule. If paraffins predominate, the base stock is paraffinic. If naphthenes predominate, it is a naphthenic base stock. Paraffins and naphthenes are saturates, meaning that all of the carbon atoms in the hydrocarbon molecule are singly bonded to another carbon atom or to a hydrogen atom. These lubricant base stocks are manufactured by the distillation of selected crude oils, followed by further refining of the lube oil distillates by conventional separation or modern conversion processes. When refined by conventional separation processes, the type of base stock is crude specific, meaning that paraffinic base stocks come from paraffinic crude oils, and naphthenic oils come from naphthenic crudes. When refined by modern conversion processes, base stocks are less crude specific, since these processes are capable of converting naphthenic and aromatic compounds to paraffins (see also HYDROCARBONS). 3.1. Base Stock Classification. The American Petroleum Institute (API) defines a base stock as ‘‘ . . . a lubricant component that is produced by a single manufacturer to the same specifications (independent of feed source or manufacturer location), that meets the same manufacturer’s specification, and that is identified by a unique formula, product identification number, or both. Base stocks may be manufactured using a variety of different processes, including but not limited to distillation, solvent refining, hydrogen processing, oligomerization, esterification, and rerefining. Rerefined stock shall be substantially free from materials introduced through manufacturing, contamination, or previous use’’ (67). ‘‘A base stock slate is a product line of base stocks that have different viscosities but are in the same base stock grouping and from the same manufacturer’’ (67). ‘‘A base oil is the base stock or blend of base stocks used in an API-licensed oil’’ (67). API has also established five base stock categories, classified according to saturates content, sulfur content, and VI. The classification is shown in Table 2 (67). Base oils with higher saturates content are generally more resistant to, and easier to protect against, oxidation. They also have higher VIs. Paraffinic oils, at the same saturates level, have higher VIs than naphthenic oils. Sulfur compounds produce corrosive material when oxidized. Base stocks with a wide molecular weight range tend to be more volatile than those with a narrow range. Table 2. API Base Stock Categories API group I II III IV V Saturates content, mass% by ASTM D2007 <90 90 90 Sulfur content, mass%a and/or >0.03 and 0.03 and 0.03 polyalphaolefins (PAO) All other base stocks not included in Group I, II, III, or IVb VI by ASTM D2270 80  VI < 120 80  VI < 120 VI  120 a Measure sulfur content by the most recent version of one of the following test methods: ASTM D1522, ASTM D2622, ASTM D3120, ASTM D4294, and ASTM D4927. b Group V includes naphthenic hydrocarbon and rerefined base stocks. LUBRICATION AND LUBRICANTS 17 Base stocks in API Groups I–IV are paraffinic hydrocarbons and are manufactured in several viscosity grades. Naphthenic base stocks are in Group V (References 68–75). Typically, the saturates content of Group II and III base stocks is >99%, and the sulfur content is <15 ppm (<0.0015%). Group IV stocks are 100% saturates and contain no sulfur. The viscosity indexes of Group II stocks are typically 100–115, and those of Groups III and IV are 120–140. Group IV base stocks have the best low temperature flow characteristics because they contain no wax. Naphthenic stocks, with minimal wax contents, also have good low temperature flow properties. They also have the lowest viscosity indexes. Oxidation stability of the paraffinic stocks improves with each group number, as do volatility and deposit, sludge, and soot control. 3.2. Synthetic Base Stocks. The word ‘‘synthetic’’ is not part of the API classification. It is a marketing term, not a technical term. The application of the word synthetic is the subject of controversy because of the difficulty of defining it. It was originally used to differentiate between base stocks made by conventional crude oil refining processes and those synthesized from other chemicals. Polyalphaolefins in Group IV are still called synthetic, as are the diesters, polyol esters, polyglycols, and so on, in Group V. This definition has been blurred, however, by the availability of base stocks derived from petroleum feed stocks by modern conversion processes (76). ‘‘Synthetic’’ is an accepted description, for example, for Group III oils. 3.3. Base Stock Manufacturing Processes. Figure 4 lists some of the refining and conversion processes used to manufacture Groups I–III base stocks (77,78). Crude oil is first fractionated in an atmospheric distillation tower to produce light gases and fuel products. The residue, or bottoms, from the Fig. 4. Base stock refining options. Used with permission. 18 LUBRICATION AND LUBRICANTS atmospheric tower are then fractionated in a vacuum distillation tower to produce gas oil and lube oil fractions. The vacuum residue is separated with propane to produce asphalt and deasphalted cylinder oil. Solvent Extraction. In the solvent extraction process, lube oil feedstock from the vacuum tower flows upward through a treating tower, countercurrent to a stream of solvent. The solvent preferentially removes undesirable tars, resins, asphaltic compounds, and polycyclic naphthenes and aromatics. Solvents used include phenol, furfural, and N-methyl-pyrrolidone (NMP). Solvent is then stripped from both the aromatic extract and raffinate streams. Solvent extraction increases the VI and the stability of the raffinate (76,78). Hydrofinishing. Hydrofinishing is catalytic hydrogenation process that converts unstable compounds remaining after solvent extraction to stable ones. It converts some aromatics to naphthenes and removes some sulfur compounds and other trace materials. It is a relatively mild process compared to the modern hydrocracking process that causes major molecular changes (79). It is severe enough, however, to produce naphthenic base oils from unextracted distillates that do not require labeling as carcinogenic under OSHA’s Hazardous Substances Communication Standard. This makes them suitable for use as base oils for metalworking fluids, greases, and other industrial lubricating products. Dewaxing. Paraffinic distillates contain some high molecular weight, high melting point paraffin waxes. At temperatures below their melting point, these waxes crystallize and cause the oil to gel. Several methods are used to reduce the wax content of lubricating oil base stocks. In one method, the hydrofined raffinate is mixed at low temperatures with a chilled solvent, in which the heaviest waxes are insoluble. The wax crystallizes and is then separated from the mixture by cold filtration. Solvents used include propane, methyl ethyl ketone (MEK), methyl isobutyl ketone, and a mixture of MEK and toluene. Another method for removing wax from the raffinate is catalytic dewaxing. This method uses a shape-selective dewaxing catalyst to crack straight-chain or slightly branched waxes into naphtha and gas (78) (see also MOLECULAR SIEVES). Isomerization technology is used in the manufacture of Group II and III base stocks. In this process, high melting point straight-chain paraffins are isomerized in a shape-selective catalyst to branched-chain, lower pour-point lube molecules. The process also produces gasoline and diesel fuels (78,80). Hydrogen Processing. Hydrogen reforming, hydrocracking, and wax isomerization are modern refining processes that convert undesirable components of lube oil fractions into desirable components, rather than remove them. They are used instead of solvent extraction on vacuum tower distillates, or on raffinates from the extraction tower, to produce Group II and III lubricating oil base stocks with higher saturates content, lower sulfur content, and higher viscosity index. The principal reactions taking place in these processes are (76) saturation, ring opening, reforming (isomerization), cracking, desulfurization, and denitrogenation. Naphthenic and Group I paraffinic base stocks are refined by conventional separation processes and are crude specific. Groups II and III paraffinic stocks produced by modern conversion processes, hydrogen reforming, hydrocracking, catalytic dewaxing, and wax isomerization, are less crude specific (see also PETROLEUMREFININGPROCESSES). LUBRICATION AND LUBRICANTS 19 Table 3. Typical Properties of GTL Base Stocka,b Base stock properties  viscosity at 100 C, cSt viscosity index pour point,  C cold-cranking simulator at NOACK wt% composition, mass% iso-paraffins monocycloparaffins polycycloparaffins aromatics 25 C, cP ASTM GTL-5, typical properties Industry range, min–max D445 D2270 D97 D5293 D5800 4.5 144 21 816 7.8 4.0–5.0 120–141 24 to 12 729–2239 10.4–14.8 — high low low low 100 0 0 0 47.3–80.9 18.7–28.8 5.3–22.2 00–1.2 high low low low Value a See Ref. 84. Reprinted with permission. b 3.4. Polyalphaolefins. Group IV base stocks are poly(a-olefins) (PAO). They are produced by steam cracking hydrocarbons to produce ethylene; ethylene oligomerization to produce linear a-olefins (1-decene, 1-dodecene, or 1-tetradecene); oligomerization of linear a-olefin to form a mixture of dimers, trimers, tetramers, and higher oligomers; and hydrogenation of the unsaturated oligomer. The characteristics of the finished PAO are affected by the chain length of the olefin raw material, temperature, time and pressure, catalyst types, and concentration and distillation of the final product to remove the dimers (81,82) (see also OLEFIN POLYMERS, INTRODUCTION). 3.5. Polyinternal Olefins and Gas–Liquid Base Stocks. The Technical Association of the European Lubricants Industry, ATIEL, has established a Group VI category for polyinternalolefins (PIO). These are mixtures of linear and cyclic olefin isomers (83,84). The conversion of natural gas to liquids (GTL) by the Fischer-Tropsch process is a promising method for producing high quality lubricating oil base stocks. Table 3 shows the typical properties of a GTL base stock compared to those of current base stocks (85) (see also FUELS, SYNTHETIC, LIQUID FUELS). 3.6. Organic Esters. Organic esters synthesized by reacting dibasic acids with monoalcohols (diesters), or by reacting monoacids with polyhydric alcohols (polyol esters), have been used as lubricating oil base stocks for >50 years. They are branched hydrocarbon molecules with multiple ester linkages, which give them polarity. Their unique structure gives them excellent thermal and oxidation stability, good low temperature flow characteristics, low volatility, lubricity, detergency and dispersancy, and biodegradability. Esters have been used exclusively in aircraft turbine engine oils (jet engine oils) for >40 years. They are also the preferred base stock in refrigerator compressor lube oils used with R134 refrigerants. Other applications for ester base stocks include rotary screw air and process gas compressors, oven chain lubricants and gas engines. Typical structures of ester base stocks can be found in References 84, 86, and 87 (see also ESTERS, ORGANIC). 3.7. Polyglycols. Polyalkylene glycols (PAG) and polyethers are usually copolymers of ethylene oxide and propylene oxide. The oxide monomer sequence 20 LUBRICATION AND LUBRICANTS can be random or blocked and their solubility can be varied from oil soluble to completely water soluble. Their applications include gear and compressor lubricants, metalworking fluids, fiber lubricants, and fire-resistant hydraulic fluids (84,88) (see also HYDRAULIC FLUIDS). 3.8. Vegetable Oil Esters. Triglyceride esters are obtained from renewable sources: olive, soybean, rapeseed, canola, safflower, sunflower, meadowfoam, castor, and other vegetable oils. They are biodegradable and offer specific environmental benefits over hydrocarbon-based lubricants. Lubricating oils made with these base stocks and their derivatives are recommended in applications where lubricant leaks and spent lubricant can contaminate the environment. Examples include chain bar lubricants, two-cycle oils for outboard marine engines, hydraulic fluids for farm machinery, and rail curve greases. Most vegetable oil esters have a combination of saturated and unsaturated fatty acids attached to the three alcohol groups in glycerine. Highly saturated oils have good oxidation stability and poor low temperature flow properties. As the amount of saturation decreases, oxidation stability decreases and the low temperature flow properties improve. Advances in breeding technology can change fatty acid profiles and alter the physical properties of vegetable oils (89). 3.9. Biodegradable Base Stocks. Lubricant or base stock biodegradability is the extent to which the material can be broken down by living things (microorganisms) into innocuous products such as water, carbon dioxide, inorganic compounds, and methane. The least biodegradable of lubricant base stocks are silicone oils, polyphenyl ethers, perfluoro alkyl ethers, and alkylated aromatic oils. Naphthenic stocks and base stocks in API Groups I–IV also have relatively poor biodegradability. The most biodegradable base stocks are vegetable oil esters, followed by polyalkylene glycols, organic esters, and phosphate esters (90). 3.10. Other Base Stocks. Other chemical compounds used as lubricant base stocks include polybutenes (89), hydrocarbons obtained by the alkylation of naphthalene with a-olefins (75,92), alkylated aromatic hydrocarbons (93), silicones (93,94), phosphate esters (95), chlorotrifluoroethylene (96,97), polyphenylethers (97), and perfluoroalkyl polyether (97,98). 3.11. Rerefined Base Stock. In its definition of a base stock, the API states ‘‘Rerefined stock shall be substantially free from materials introduced through manufacturing, contamination, or previous use.’’ (67). Products made from rerefined stock are subject to the same stringent refining, compounding, and performance standards applied to virgin oil products. Rerefined oil may, in fact, have superior oxidation stability than virgin stocks because the easily oxidized compounds will have been reacted during previous use and then removed during reprocessing (76,99). In the rerefining process, used oil is preferably segregated by type, collected, and delivered to the reprocessing facility. The oils are then screened and inappropriate feedstocks rejected. Solid and other gross contaminants are then separated by, for example, propane precipitation, alcohol–ketone precipitation, and acid–clay filtration. The filtrate is dehydrated and then refractionated by vacuum distillation. The distillates are then finished by hydrotreating or clay filtration or both, and then vacuum distilled to obtain the desired viscosity grades (76,99). LUBRICATION AND LUBRICANTS 21 4. Additives for Lubricating Oil and Grease Advanced refining and manufacturing technologies have significantly improved the quality of base oils used in the manufacturing of lubricating oils and greases. They are more stable, have better low temperature flow properties, are less volatile, and less corrosive. Alone, however, they do not meet the requirements of modern lubricating oils and greases. Additives are chemical substances added to the base oil to impart or improve certain properties. They include oxidation inhibitors, rust and corrosion inhibitors, pour point depressants, viscosity (VI) improvers, detergents, dispersants, friction modifiers, EP agents, thickeners, emulsifiers, demulsifiers, bactericides, fungicides, and tackiness additives. Most oils and greases contain several different additives, many of them surface active. They can assist each other, resulting in a synergistic effect, or they can have antagonistic effects. Many additives have several functions (multipurpose additives). These additives and base stocks are the elements used by the lubricant designer to meet the increasingly critical requirements of equipment manufacturers and of the users of lubricating oils and greases (100,101). 4.1. Oxidation Inhibitors: Antioxidants. Hydrocarbons and many other components of lubricating oils and greases are subject to oxidation under the operating conditions in which they are used. The products of oxidation may be corrosive, they may be varnish, soot, or sludge that cause wear, interfere with proper operation of equipment, or plug oil passages, preventing the lubricant from getting to its intended application. Preventing or delaying the oxidation of lubricating oils and greases extends their operating lives. Oil change intervals for automotive engines have increased 10-fold over the past 50 years, even though engine operating temperatures have increased significantly over the same period. Many machine elements are ‘‘lubricated for life.’’ Steam turbine oils remain in use for decades without any oxidation effects. These are major economic benefits for machine operators. The oxidation process is a chain reaction with several different chemical actions. It is generally initiated by heat or light leading to the formation of free radicals, followed by, in the presence of air, the formation of peroxides, and then a variety of reactions, including polymerization. It is catalyzed, or speeded up, by the activity of metal surfaces. Metals employed in soap thickeners have been found to catalyze the oxidation of greases (102). Detailed mechanisms are described in References 103 and 104 (see also HYDROCARBONOXIDATION). Natural inhibitors (sulfur or nitrogen containing) present in some base stocks, or their oxygen-containing intermediates can impair the formation of free radicals. Molecules with carbon–carbon double bonds and aromatic residues can promote formation of radicals. Refining processes remove easily oxidized components from the base stock and make it more responsive to oxidation inhibitors. They also remove some natural inhibitors. Synthesized base stocks with few impurities (and no natural inhibitors) are less prone to oxidation and more responsive to antioxidants. A variety of additives are available that terminate the chain reaction in the early stages of oxidation. There are (1) free-radical scavengers, primary antioxidants that function by breaking the propagation step; (2) hydroperoxide decomposers, secondary antioxidants that function by inhibiting the branching step; (3) 22 LUBRICATION AND LUBRICANTS OH O ROO ROOH R R O O ROO R ROO R Fig. 5. Radical scavenging with hindered phenols. Reprinted with permission. metal deactivators and corrosion inhibitors that prevent catalysis of the initiation reaction (102). High quality lubricants may contain more than one of each type of these oxidation inhibitors. The combination of different antioxidants is synergistic. Each functions via a different mechanism, in a different temperature range, and at different rates. Each stabilizes different species, and one may act to regenerate another (103–105). There are two types of primary antioxidants or free radical scavengers: hindered phenols and aromatic amines. Both function by donating a hydrogen atom to the peroxide radical, ROO , to break the propagation step in the oxidation mechanism and form relatively inert products (102,103). In the presence of active antioxidants, oxidation proceeds very slowly until the inhibitors have no more available hydrogen atoms to contribute. At this point, the rate of oxidation increases dramatically and other measures are necessary to deal with the oxidation products. These include dispersants and detergent additives (discussed later), filtration, reconstitution of the additive package, and replacing the oil. Oil quality monitoring is often used to follow the progress of deterioration and determine when an oil change is needed (105,106). Figure 5 shows the way hindered phenols react with peroxide radicals to terminate the oxidation chain reaction (103,104). Chemicals used in this process include (102,103,107,108) 2,6-di-tert-butyl phenol (DTBP), 2,6-di-tert-butyl-p-cresol (DBPC or BHT), 3,5-di-tert-butyl-4-hydroxyanisole (BHA), 4,40 -methylenebis (2,6-ditert-butyl phenol), 3,5-di-tert-butyl-4-hydroxy-hydrocinnamic acid, C7–C9 branched alkyl ester, 1,6-hexamethylene bis(3,5-di-tert-butyl-4-hydroxy-hydrocinnamate). Examples of aromatic amine antioxidants include alkylated diphenylamines (ADPA), phenyl alpha diphenyl amine (PANA), alkyl-substituted PANA, and trimethyl quinoline derivatives (99,103,104). A variety of mechanisms is reported for the action of amine-type antioxidants. They act at both the initiation and propagation steps and are effective at lower concentrations and over a wider temperature range than are the hindered phenols (102–104). There is also evidence that some of the DBPC and PANA in the oil react directly with oxygen and those portions are not contributing to the termination of the chain reaction (105,109). LUBRICATION AND LUBRICANTS 23 Secondary antioxidants include sulfur-containing and organophosphorus compounds. Typical examples include dialkyl thioesters, metal dithiocarbamates, metal dithio phosphates, and aryl phosphites. They inhibit the branching step by decomposing hydroperoxides. Reyes-Gavilan and Odorisio (102), referring to several earlier works, report that, in general, (1) thioethers react with hydroperoxides to form sulfoxides, sulfones, and alcohols; (2) zinc dialkyl dithiocarbamates react with hydroperoxides to form sulfur acids that ionically catalyze hydroperoxide decomposition; (3) the reaction of zinc dialkyldithiophosphate (ZDDP) with hydroperoxides occurs through an O,O0 -dialkyl hydrogen phosphoro dithioate-catalyzed ionic mechanism; (4) hydroperoxide decomposition by phosphites is accomplished through a displacement reaction mechanism. The products of this reaction consist of an organic phosphate and an alcohol. Secondary antioxidants are best used in combination with primary antioxidants. They are not as effective as the latter when used alone. There are some multifunctional additives, in which sulfur is incorporated in the phenolic molecule, that are both primary and secondary antioxidants (102,103). 4.2. Metal Deactivators. Metal deactivators passivate the surface of copper, brass, and bronze alloys, or they chelate ions of copper, iron, manganese, cobalt, and other metals. Passivators, which lay a protective layer over the metal surface, include benzotriazole, 2-mercaptobenzothiazole, and tolutriazole derivatives. Chelators, which trap metal ions in the bulk of the substrate, include N,N0 disalicylidene-1,2-diaminopropane and ethylenediaminetetraacetic acid (EDTA) (100,103). Both types prevent metal catalysis at the oxidation initiation step. 4.3. Corrosion Inhibitors. Corrosion inhibitors in lubricating oils and greases form a protective layer on the surface of metals to prevent moisture and oxygen from easily reaching the surface. Natishan and Moran discuss the mechanism of corrosion of metals, mainly caused by electrolytic reactions (see CORROSION AND CORROSION CONTROL). Polyisobutenylsuccinic acid derivatives, amine phosphates, fatty acid amides of sarcosine, imidazoline derivatives, and sulfonates are a few of the chemicals used in lubricating oils as corrosion inhibitors (107,108,110). 4.4. Detergents and Dispersants. Detergent and dispersant additives are used principally in gasoline and heavy-duty diesel engine oils, automatic transmission fluids, and tractor hydraulic fluids. Their primary function is to keep the engine surfaces clean: They prevent deposits on hot surfaces such as pistons and rings; they neutralize corrosive oxidation products and other contaminants; they suspend insoluble oxidation products and debris in the oil, thus preventing them from clogging vital oil passages. They do not, however, remove deposits already on the surfaces. Detergents are made by reacting metal oxides or hydroxides with long-chain organic acids. The metals include calcium, magnesium, barium, and sodium. Acids include alkylbenzene sulfonic acid, alkylnaphthalene sulfonic acid, alkyl phenol, sulfur, or methylene-bridged alkyl phenol, alkyl salicylic acid, and polyisobutenyl phosphonic acid (111). Detergents may be neutral, basic, or overbased. Basic and overbased detergents have reserve alkalinity to neutralize acidic combustion products (usually formed from sulfur in the fuel) that leak or ‘‘blow by’’ the rings in an internal combustion engine. These are made by using excess metal base in the reaction and 24 LUBRICATION AND LUBRICANTS then blowing the product with CO2 to form metal carbonate. The carbonate is held in suspension in a micelle structure (111). Some detergents, particularly the phenates and salicylates, are detergent inhibitors, meaning that parts of the molecule are able to inhibit oil oxidation and bearing corrosion (112). Dispersants are metal-free and, therefore, ashless. They are higher in molecular weight than detergents and more effective in suspending oxidation products and debris. Some examples of dispersant chemistry are polyisobutenylsuccinimides, polyisobutenyl succinate esters, polyaminomethylpolyisobutylphenols, and bis(hydroxypropyl polyisobutenylthiophosphonate) (111). The polar ends of the dispersant molecules surround and attach themselves to the insoluble contaminant particles. Their oil-soluble, nonpolar tails keep the particles in suspension (111,112). Dispersancy is also built into multifunctional viscosity improvers based on olefin copolymers, polyacrylates, and styrene–maleic anhydride polymers (111). 4.5. ZDDP: Antiwear Additive, Antioxidant, and Corrosion Inhibitor. The ZDDP (Zinc Dialkyl Dithiophosphate or Zinc Diaryl Dithiophosphate) compounds have, for >50 years, been the most effective wear reducing additives in engine lubricating oils, hydraulic oils, and other lubricants. When two steel surfaces rub together, and the metal on one surface is in contact with the metal on the other surface, the frictional heat caused by the rubbing causes the ZDDP to react chemically with the surfaces to form very tough wear-resisting boundary lubricating films. These films do not shear easily, and have little effect on the coefficient of friction. The ZDDP films do rub off, however, and they are constantly being rebuilt by the chemical reaction of the metal with ZDDP. These films are particularly effective in concentrated sliding contacts such as those found in engine valve trains and in vane-type hydraulic pumps. Concentrated contacts have a very small contact area that results in extremely high pressure on the contact area. Some friction modifying additives work with ZDDP to reduce both friction and wear of the rubbing surfaces. The ZDDP compounds are multifunctional. They are secondary oxidation inhibitors, decomposing hydroperoxides in the branching step of hydrocarbon oxidation. They are also effective bearing corrosion inhibitors. This compound is manufactured by first reacting an alkyl or aryl alcohol with phosphorous pentasulfide (P2S5) and then neutralizing the resultant acid with zinc oxide. The alkyl alcohol may be primary alkyl, branched chain primary alkyl, secondary alkyl or, rarely, tertiary alkyl. As a rule, the secondary and tertiary alkyl derivatives are more effective as antiwear additives, but are the least stable. The aryl derivatives generally have the best thermal stability and the least potent antiwear activity (113,114). The ZDDP compounds, particularly those with lower thermal stability, can decompose at temperatures below those of rubbing steel surfaces and become corrosive to copper and copper alloys. This is generally not a problem in internal combustion engines, but it can be a problem in machinery using, for example, bronze bushings, pistons, or clutch plates. However, by the judicious selection of alcohols and careful control of the manufacturing process, ZDDP compounds are made that perform well in machines such as automatic transmissions and pistontype hydraulic pumps that contain parts made with copper alloys. LUBRICATION AND LUBRICANTS 25 Despite its effectiveness as a wear, oxidation, and corrosion inhibitor, and its relatively low cost, there are serious efforts to reduce or eliminate the use of ZDDP additives in engine oils. When the oil gets in the combustion chamber, the additive decomposes at high temperatures and, because a metal is present, leaves ash, a solid residue, and other deposits in the chamber. These deposits interfere with engine efficiency and emissions control. There is also evidence that ZDDP decomposition products poison the catalysts used to convert CO, NOx, unburned hydrocarbons, and volatile organic compounds to less harmful emissions. Zinc oxide, phosphorus pentasulfide, and a zinc pyrophosphate glaze have been found on catalyst surfaces that prevent passage of exhaust gases into the porous structure of the catalyst (115). (see also EMISSION CONTROL, AUTOMOTIVE). Lacking an acceptable emission systems protection test, the International Lubricant Standardization and Approval Committee (ILSAC) places maximum phosphorus and sulfur limits on the GF-4 engine oil specification. These limits are expected to be reduced further in the next-generation specification. 4.6. EP and Antiwear Additives. There is very little distinction between the so-called EP and antiwear additives. Both react with rubbing metal surfaces to form lubricating films, and they both inhibit wear. ZDDP additives have relatively high activation energy and are not effective until the surface temperature becomes >150 C. The EP additives are used in relatively high load, slow speed applications to prevent catastrophic failure of the surface. Mild EP additives used in automotive and industrial gear oils have lower activation temperatures than ZDDP. ‘‘Active’’ EP additives used in metal-cutting oils, such as elemental sulfur, chlorinated hydrocarbons, and some sulfurized esters, can react at room temperature. Table 4 lists some of the many compounds used as EP and antiwear additives in lubricating oils (108,116,117). Table 4. EP and Antiwear Additives zinc dialkyl and diaryl dithiophosphates zinc dialkyldithiocarbamate molybdenum dialkyldithiophosphate molybdenum di-n-butyldithiocarbamate antimony dialkyldithiophosphate antimony tris(dialkyldithiocarbamate) ethyl acrylate dialkyldithiophosphate methylene bis(dibutyldithiocarbamate) 2,5-dimercapto-1,3,4-thiadiazol, alkyl polycarboxylate amine phosphate and thiophosphates tricresyl phosphate trialkyl phosphates alkyl monoacid and diacid phosphates tertiarybutylphenyl phosphate dialkyl, trialkyl, and triaryl phosphites sulfurized olefins sulfurized oils and esters esters of thio- and dithio-phosphoric acid chlorinated paraffins antiwear, antioxidant, corrosion inhibitor antiwear, antioxidant, corrosion inhibitor, metal deactivator, color stabilizer antiwear, antioxidant, friction modifier, EP antiwear, antioxidant, friction modifier, EP antiwear, antioxidant, friction modifier, EP EP, antiscuff, antioxidant ashless antiwear, EP ashless EP, antioxidant ashless antiwear, antioxidant ashless antiwear, rust inhibition ashless antiwear, EP ashless antiwear, EP ashless antiwear, EP ashless antiwear, EP ashless antiwear, EP EP EP EP EP 26 LUBRICATION AND LUBRICANTS 4.7. Friction Modifiers: Boundary Lubricity Additives. Friction is a force that resists sliding, or slipping, of one surface across another. Static friction prevents sliding or causes the surfaces to move together, with no relative motion between the surfaces. When a shearing force high enough to overcome the static force is applied, the surfaces move relative to each other and the friction is kinetic. On unlubricated surfaces, and on surfaces lubricated with fluids that do not contain friction-modifying additives, static friction is higher than kinetic friction. Friction modifiers added to the oil tend to reduce both the static and kinetic friction forces and to make the static friction lower than the dynamic friction. Friction modifiers have two functions: to reduce energy consumption by machinery by lowering the kinetic friction and to eliminate shudder in automatic transmission torque converter clutches, chatter in limited slip automotive differential gear systems, and stick-slip in machine tool slideways. Shudder, chatter, and stick-slip are all the same phenomena, frictional vibration caused by oscillation between static and kinetic friction forces. The oscillation occurs at low relative speeds between surfaces in contact and when the static friction is higher than the kinetic friction. Inherent resilience in the applied shearing force also contributes to stick-slip. With a friction-modified lubricant, kinetic friction is higher than static friction and the frictional vibration is eliminated (118–120). Like many lubricating oil additive molecules, organic friction modifiers have a polar end that attaches to the rubbing surface and to a long-chain hydrocarbon tail. Sulfurized fats and phosphoric and thiophosphoric acid derivatives form boundary-lubricating film layers by chemical reaction with the surface material. These three also function as EP additives. Other friction modifiers, for example, fatty acids, carboxylic acid derivatives, esters, ethers, amines, amides, and imides form layers of boundary lubricating films by chemisorption. The polar heads on the surface are attracted to each other by hydrogen bonding and dipole–dipole interactions and the hydrocarbon tails are lined up perpendicular to the surface by van der Waals (London) forces. Another class is friction polymers formed at the asperities under the influence of contact temperature and load. These include partial complex esters, methacrylates, unsaturated fatty acids, and sulfurized olefins. Finally, there are metalloorganic types such as molybdenum dialkyldithiophosphate and molybdenum dialkyldithiocarbamate and solid lubricants such as graphite, molybdenum disulfide, and PTFE (121). 4.8. Pour Depressants. The pour point is the lowest temperature at which an oil is observed to flow, when cooled as prescribed in ASTM D97. At this point, wax crystals that have previously precipitated as solid needles and platelets agglomerate and cause the oil to gel, preventing its flow. Dewaxing and hydroisomerization processes remove most, but not all, of the waxes (high molecular weight n-paraffins) from Groups I–III base stocks. The resultant pour points of the base stocks vary from about 15 to about 25 C. Adding a pour depressant can further reduce the pour point another 30–40 C. Pour point depressants are branched-chain polymeric additives that coprecipitate with the wax crystals and prevent them from agglomerating. The side chains on these polymers, often called wax crystal modifiers, are about the same length as the wax molecules (122). The first pour depressants were alkylated naphthalenes. Phthalic acid dialkyaryl esters followed, and then long-chain alkyl acrylates and methacrylates. LUBRICATION AND LUBRICANTS 27 Polymethacrylates are probably the most widely used pour depressants today. Many other polymeric additives can be tailored to provide pour depressant properties in addition to their other functions (122–124). 4.9. Viscosity Improvers. Viscosity improvers are polymeric additives that are more soluble in oil at high temperatures than at low temperatures. The dissolved molecules expand or stretch out and considerably increase the viscosity of the oil at high temperatures. These molecules shrivel up and shrink, contributing very little to the viscosity of the oil at low temperatures. Viscosity improvers are added to low viscosity base oils, which have the necessary low temperature flow characteristics, to increase their viscosity and resultant lubricating ability at operating temperatures. Multigrade oils, oils containing viscosity improvers, are non-Newtonian oils. The high temperature viscosity of the oil undergoing high shear rates is lower than its viscosity at very low shear rates. This viscosity loss may be temporary and the oil will recover its original viscosity when the shear stress is relieved. Permanent viscosity loss occurs when the polymer itself is sheared. Automotive engine oil specifications control both temporary and permanent viscosity losses caused by shear stress. Viscosity-improving polymers include olefin copolymers from ethylene, propylene, and butylene monomers; polymethacrylates; styrene–butadiene copolymers; and hydrogenated polyisoprene. Many of these polymers can be modified to make them multifunctional, giving them pour depressing and/or dispersant properties in addition to viscosity improvement (122,125) (see also METHACRYLIC POLYMERS). 4.10. Foam Inhibitors. Foam in lubricating oils consists of bubbles of air on the liquid surface whose walls are thin liquid films. Strong foaming affects the lubricating properties of oils and enhances their oxidation. Foam is generated on mixing air with the oil during agitation and then separation of the air at the oil surface. Foam stability is a function of oil viscosity and surface tension at the oil– air interface. It is strongly affected by surface-active substances such as detergents, friction modifiers, EP additives, and corrosion inhibitors. Finely dispersed polydimethylsiloxane, which is insoluble or has borderline solubility in hydrocarbon oils, forms a monolayer on the liquid surface, lowering the surface tension and breaking the foam. Antifoam additives based on this silicone oil are widely used in automotive engine, transmission, and gear oils. Silicone oils, however, tend to stabilize air emulsions, where the air bubbles do not separate from the oil or separate very slowly. Entrained air in an oil circulating system is compressible and leads to spongy response in hydraulic systems and severe cavitation damage in bearings. Alkylacrylate and alkylmethacrylate copolymers, while not as effective as silicones for foam suppression, are preferred where air entrainment is a problem (126–128). 4.11. Demulsifiers. Steam turbine oil, paper machine oil, industrial gear oils, hydraulic oils, and oils for many other applications are subject to contamination by water. It is important that this water separates easily from the oil and drops to the bottom of the oil reservoir, where it can be drained off. If it does not, it can both mix and circulate with the oil, seriously degrading the oils lubricating capability. Demulsifiers are sometimes added to the oil formulation to facilitate water separation. They concentrate at the oil–water interface and 28 LUBRICATION AND LUBRICANTS promote coalescence of the water droplets. Chemicals used include anion-active compounds such as dinonylnaphthalene sulfonates and polyalkoxylated phenols, polyols, and polyamines (129,130). 4.12. Emulsifiers. Water-in-oil emulsions are used as coolants–lubricants in metalworking applications and as hydraulic fluids. Medium molecular weight sodium sulfonates (derived from natural and synthetic raw materials) are the most widely used emulsifiers in these fluids. Nonionic emulsifiers based on polyethylene oxide are also used, as are long-chain alkylammonium salts (131). 5. Lubricating Oils Lubricating oils are specifically formulated for virtually every type of machine and manufacturing process. The base stocks and the type and concentration of additives used for these oils are selected based on the requirements of the machinery or process being lubricated, the quality required by the builders and the users of the machinery, and the government regulation. Table 5 is a partial list of the principal applications for which lubricating oils are specifically formulated: Each of these oils has a unique set of performance requirements. In addition to proper lubrication of the machine or process, these requirements include maintenance of the quality of the lubricant itself, as well as the effect of the lubricant’s use and disposal on energy use, the quality of the environment, and on Table 5. Principal Applications for Lubrication Oils Automotive lubricating oils gasoline engine oils for passenger cars and light trucks for heavy duty automotive and industrial service for piston engines in general aviation service small 2-stroke and 4-stroke gasoline engines for outboard motors for scooters, mopeds, and motorcycles for lawn mowers and small tractors for chain saws and similar portable equipment diesel engine oils for heavy duty trucks, agricultural and construction vehicles for industrial cross-head and trunk piston diesel engines for railroad diesel engines for marine cross-head and trunk piston diesel engines gas engine oils gas turbine oils for aircraft jet engines in commercial aviation service for industrial gas turbine engines automatic transmission fluids gear oils for automotive manual transmissions for automotive differentials Industrial lubricating oils industrial gear oils pneumatic tool lubricating oil high temperature oils air and gas compressor oils for reciprocating compressors for rotary vane compressors for rotary screw compressors for refrigeration compressors machine tool way oils textile oils steam turbine oils hydraulic fluids paper machine oils food machinery oils steam cylinder oils metalworking fluids for metal cutting for metal rolling for metal drawing, forging, stamping, etc. LUBRICATION AND LUBRICANTS 29 the health of the user. The performance requirements of a few of these lubricating oils are discussed below. 5.1. Gasoline Engine Oils for Passenger Cars and Light Trucks. Internal combustion engines provide the best example of performance requirements dictating the formulation and quality of the engine oil. These engines have lubrication requirements in nearly every regime: hydrodynamic, elastohydrodynamic, squeeze film, EP, boundary, and mixed film. They operate at high oil and surface temperatures, must start up at very low temperatures, are subject to contamination (by fuels, combustion products, oxidation products, and dirt), and they must meet stringent fuel economy and exhaust emission standards. Their lubricating oils contain a wide variety of additives and their base oil requirements determine the type and quality of base stocks produced in lube refineries and chemical manufacturing plants. The performance requirements of gasoline engine oils for use in automobiles are a good example of the relationship between the performance requirements and the formulation of lubricating oils for a specific application. Gasoline engine oils for use in automobiles in the United States are licensed and certified to ensure that they meet the minimum performance standards established by industry technical specifications. The Engine Oil Licensing and Certification System (EOLCS), administered by the API, is a voluntary program that authorizes oil marketers to use the API Engine Oil Quality Marks–the API Service Symbol ‘‘Donut’’ and Certification Mark ‘‘Starburst’’ (132). This program is a cooperative effort between the oil industry and vehicle and engine manufacturers– Ford, General Motors, and Daimler Chrysler; the Japan Automobile Manufacturers Association (JAMA); and the Engine Manufacturers Association (EMA). Performance requirements, test methods, and limits are cooperatively established by vehicle and engine manufacturers, technical societies, [eg, the Society of Automotive Engineers (SAE) and the American Society of Testing and Materials (ASTM)], and industry associations (eg, the American Chemistry Council and API). An ongoing monitoring and enforcement program to ensure licensees adhere to industry technical specifications backs APIs Engine Oil Program (133). The API Service Symbol ‘‘Donut’’ is divided into three parts: (1) The top of the Donut shows the oil’s performance level for gasoline and/or diesel engines. The letter ‘‘S’’ followed by another letter (eg, ‘‘SM’’) refers to oil suitable for gasoline engines. The letter ‘‘C’’ followed by another letter and/or number (eg, CI-4) refers to oil suitable for diesel engines. (2) The center identifies the oil’s viscosity characteristics. The numbers and the letter ‘‘W’’ indicate the SAE viscosity grade. These grades are defined in SAE J300, May 2004 (134). The low-temperature ‘‘W’’ grades indicate how quickly an engine will crank and how well the oil will flow to lubricate critical engine parts at low temperatures. The grades without the ‘‘W’’ indicate the oil’s operating temperature flow properties. A multigrade oil (eg, 5W-30) provides good flow capability for low temperatures and adequate viscosity characteristics for high temperature lubrication. It meets the viscosity requirements of both the ‘‘W’’ grade and the other grade indicated (134). (3) The bottom of the Donut tells whether the oil has demonstrated energy-conserving properties in a standard test in comparison with reference oil. It may also include a supplemental category description (eg, CI-4 PLUS). The API Certification Mark ‘‘Starburst’’ (Fig. 6) is designed to identify engine oils recommended for a specific application (eg, gasoline engine service). An oil 30 LUBRICATION AND LUBRICANTS Fig. 6. API certification mark. may be licensed to display the Starburst only if the oil satisfies the most current requirements of the International Lubricant Standardization and Approval Committee (ILSAC) minimum performance standard for this application. Members of ILSAC include representatives of the Japanese Automobile Manufacturers Association, DaimlerChrysler Corporation, Ford Motor Company, and General Motors Corporation. An engine oil meeting the latest API service category may or may not meet all of the requirements of the latest ILSAC category. When it does meet both requirements, its container may display both symbols. The current ILSAC minimum performance standard, suitable for use in the current and all earlier model year automobiles, is ILSAC GF-5 Standard for Passenger Car Motor Oils (135). The corresponding API Service Category is API Service SN. Both were issued in 2009 and superseded the 2004 standards, ILSAC GF-4 (136) and API Service SM. Those, in turn, superseded ILSAC GF-3 and API Service SL, issued in 2001 and API Service SJ, issued in 1996. After November 2009, the Starburst can no longer be used for GF-4 oils. The performance required by ILSAC for GF-5 Engine Lubricating Oils is measured in a battery of full-scale engine tests and laboratory bench tests conducted by qualified independent laboratories. Tables 6–11 describe these engine and bench test requirements for ILSAC GF-5, GF-4, GF-3, and GF-2 and for API Service SN, SM, SL, and SJ (135,136). Engine test requirements in the ILSAC GF-4 Standard include the following: (1) The ASTM Sequence IIIG test measures viscosity increase, piston deposits at high operating temperatures, and the valve train wear. At the end of the test, the low temperature pumping viscosity of the oil is measured. It is run in a 1996/1997 3800 cc series II General Motors V-6 fuel-injected gasoline engine. After a 25-min break-in, it operates at 125 bhp, 3600 rpm, 150 C oil temperature for 100 hours (135,137). (2) The ASTM Sequence VG test (ASTM D6593-0414a) evaluates the oil’s ability to prevent sludge and varnish formation. It is run in a 1994 Ford V-8, Table 6. Engine Test Requirements, ASTM Sequence IIIG, IIIF, IIIE for Wear, Piston Deposits, and High Temperature Oxidation 31 ILSAC classification API service category Issue date: ASTM engine test sequence ASTM test procedure kinematic viscosity increase at 40 C, % average-weighted piston deposits, merits hot stuck rings cam plus lifter wear average, mm maximum per position, mm piston skirt varnish oil consumption, L average oil ring land deposits average engine sludge rating lifter sticking cam þ lifter scuffing low temperature pumping viscosity at end of test by ASTM D4684 (MRV TP-1) ILSAC GF-5 API SN 2009 SEQUENCE IIIG ASTM D7320 150 max ILSAC GF-4 API SM 2004 SEQUENCE IIIG 150 max ILSAC GF-3 API SL 2001 SEQUENCE IIIF 275 max 4.0 min 4 4 none none none none 60 max 60 max 20 max 30 max 64 8.9 min 5.1 max 3.5 min 9.2 min none none 9.0 min 5.2 max stay in grade or next higher grade stay in grade or next higher grade rate and report ILSAC GF-2 API SJ 1996 SEQUENCE IIIE ASTM D5533 375 max Table 7. Engine Test Requirements, ASTM Sequence VG and VE for Wear, Sludge, and Varnish 32 ILSAC classification API service category Issue date ASTM engine test sequence ASTM test procedure average engine sludge, merits average rocker cover sludge, merits average engine varnish, merits average piston skirt varnish, merits oil screen sludge (clogging), % area oil screen debris, % area hot stuck compression rings cold stuck rings oil ring clogging, % area cam lobe wear average, mm maximum per position, mm cam follower pin wear, cyl #8, avg, mm ring gap increase, cyl #1 and #8, avg, mm cylinder bore wear ring wear ILSAC GF-5 API SN 2009 SEQUENCE VG ASTM D6593 8.0 min 8.3 min 8.9 min 7.5 min 15 max rate and report none rate and report rate and report ILSAC GF-4 API SM 2004 SEQUENCE VG ASTM D6593 7.8 min 8.0 min 8.9 min 7.5 min 20 max rate and report none rate and report rate and report rate and report rate and report ILSAC GF-3 API SL 2001 SEQUENCE VG ASTM D6593 7.8 min 8.0 min 8.9 min 7.5 min 20 max rate and report none rate and report rate and report rate and report rate and report rate and report ILSAC GF-2 API SJ 1996 SEQUENCE VE ASTM D5302 9.0 min 7.0 min 5.0 min 6.5 min 20 max none — 15 127 max 380 max — — — — Table 8. Engine Test Requirements, ASTM Sequence IVA, VIII, IID, and CRC-L-38 for Valve Train Wear, Bearing Corrosion, Shear Stability, and Engine Rusting ILSAC classification API service category Issue date ASTM engine test sequence ASTM test procedure 33 average cam wear (7 position average) mm engine test sequence ASTM test procedure bearing weight loss, mg shear stability (10-h stripped viscosity) piston skirt varnish ASTM engine test sequence ASTM procedure avg. engine rust rating lifter sticking ILSAC GF-5 API SN 2009 SEQUENCE IVA ASTM D6891 ILSAC GF-4 API SM 2004 SEQUENCE IVA ASTM D6891 ILSAC GF-3 API SL 2001 SEQUENCE IVA ASTM D6891 ILSAC GF-2 1996 API SJ 90 max 90 max 120 max — SEQUENCE VIII ASTM D6709 26 max stay in grade SEQUENCE VIII ASTM D6709 26 max stay in grade SEQUENCE VIII ASTM D6709 26.4 max stay in grade CRC L-38 ASTM D5119 40 max stay in grade (see Ball rust bench test) (see Ball rust bench test) (see Ball rust bench test) SEQUENCE IID ASTM D5844 8.5 min none Table 9. Engine Test Requirements, ASTM Sequence VIB and VIA for Fuel Economy 34 ILSAC classification API service category Issue date ASTM engine test sequence ASTM test procedure SAE 0W-20 and 5W-20 viscosity grades % fuel economy improvement, phase 1 % fuel economy improvement, phase 2 SAE 0W-30 and 5W-30 viscosity grades % fuel economy improvement, phase 1 % fuel economy improvement, phase 2 % total fuel economy improvement SAE 10W-30 and all other grades not listed above % fuel economy improvement, phase 1 % fuel economy improvement, phase 2 % total fuel economy improvement a ILSAC GF-5 API SN 2009 SEQUENCE VID ASTM D6837a ILSAC GF-4 API SM 2004 SEQUENCE VID ASTM D6837a ILSAC GF-3 API SL 2001 SEQUENCE VIB ASTM D6837a 2.6 min 2.3 min 2.0 min 1.2 min 2.0 min 1.7 min 1.9 min 1.8 min 1.6 min 0.9 min 1.5 min 1.3 min ILSAC GF-2 API SJ 1996 SEQUENCE VIA ASTM D6202a 1.4 min 1.1 min 3.0 min 1.5 min 1.1 min 0.9 min 0.6 min 0.8 min 0.6 min All fuel economy values, phases 1 and 2, are determined relative to ASTM Reference Oil BC. 1.6 min 0.5 min Table 10. Bench Test Requirements for Catalyst Compatibility, Wear, and Volatility 35 ILSAC classification API service category Issue date Catalyst compatibility phosphorus content, mass%, ASTM D4951 SAE 0W-20, 5W-20, 5W-30, and 10W-30 only SAE 5W-30 and 10W-30 only sulfur content, mass%, ASTM D4951 or D2622 SAE 0W and 5W multigrades SAE 10W multigrades phosphorus volatility/retention, % 0.5 max 0.6 max 79 min 0.5 max 0.7 max none Wear phosphorus content, mass%, ASTM D4951 0.06 min 0.06 min 15 max 15 max NOACK Volatility, ASTM D5800 evaporation loss after 1 h at 250 C, % SAE 0W-XX SAE 5W-XX SAE 10W-30 SAE 15W-40 Volatility, Simulated Distillation, ASTM D6417 % volatilized to 371 C SAE 0W-XX SAE 5W-XX SAE 10W-30 SAE 15W-40 ILSAC GF-5 API SN 2009 ILSAC GF-4 API SM 2004 ILSAC GF-3 API SL 2001 0.08 max 0.08 max 0.1 max ILSAC GF-2 API SJ 1996 0.1 max NR 22 max 22 max 20 max 10 max 10 max 10 max 17 max 17 max 17 max 15 max 36 LUBRICATION AND LUBRICANTS Table 11. Bench Test Requirements for High Temperature Deposits, Filterability, Foam Tendency and Stability, Engine Rusting, and Flash Point ILSAC classification ILSAC GF-5 ILSAC GF-4 API service category API SN API SM Issue date 2009 2004 High temperature deposits, TEOST, ASTM D6335 Total deposits, mg 30 max 35 max Filterability Engine oil water tolerance test, ASTM D6794 % flow reduction with 0.6% 50 max water % flow reduction with 1.0% 50 max water % flow reduction with 2.0% 50 max water % flow reduction with 3.0% 50 max water ILSAC GF-3 API SL 2001 ILSAC GF-2 API SJ 1996 45 max 50 max 50 max 50 max 50 max 50 max 50 max 50 max 50 max 50 max 50 max 50 max Foam tendency/stability, ASTM D892 (Option A) SEQUENCE I 10/0 10/0 SEQUENCE II 50/0 50/0 SEQUENCE III 10/0 10/0 high temperature 100/0 100/0 10/0 50/0 10/0 100/0 10/0 50/0 10/0 200/50 Engine rusting, Ball rust test, ASTM D6557 average gray value 100 min 100 min Engine oil filterability test, ASTM D6795 % flow reduction 50 max 100 min  Flash point, ASTM D92, C 0W-XX 5W-XX 10W-30 15W-40 200 min 200 min 200 min 215 min 4.6-L fuel-injected gasoline engine. The procedure involves 54 cycles in 216 h, each cycle consisting of three different sets of operating conditions (135,137,138). (3) The ASTM Sequence IVA test (ASTM D6891-14) measures the average camshaft lobe wear in a 100-h test in a KA24E Nissan 2.4-L, four-cylinder engine (135,137,139). (4) The ASTM Sequence VIII test (ASTM D6709-14a) evaluates a lubricant’s performance in combating copper-, lead-, and tin-bearing corrosion and measures viscous shear stability at high operating temperatures. A 42 C.I.D. single-cylinder engine runs for 40 h at 3150 rpm. The oil temperature is raised to 143 C using an external oil heater (135,137,140). (5) The ASTM Sequence VIBVID test (ASTM D6837-13) measures the effects of multigrade engine oils on the fuel economy of passenger cars and light-duty trucks. It is run in a 1993 Ford 4.6-L V-8 engine on a dynamometer test stand (135,137,141). Maximum phosphorus and sulfur contents are specified in GF-5 for catalyst compatibility (135,142,143). Minimum phosphorus is specified for wear protection (137,142). Bench tests are specified for minimum volatility, maximum high temperature deposits, filterability, engine rusting, and foaming characteristics (137,144–150). LUBRICATION AND LUBRICANTS 37 Many of the engine test results are visual merit ratings. These are made by experts trained in the rating process using standards published in Coordinating Research Council (CRC) Manuals (151,152). 5.2. Heavy-Duty Diesel Engine Oils for Trucks, Agricultural, and Construction Vehicles. The driving forces for diesel engine oil development are engine durability, reduction of NOx, and soot emissions and compatibility with exhaust aftertreatment systems. As with gasoline engine oils, the performance of diesel engine oils is continually being upgraded to meet engine manufacturers’ needs, consumer requirements, and government regulations. Diesel engine oils are licensed and certified by the API in its Engine Oil Licensing and Certification System (EOLCS) (135). Performance requirements, test methods, and limits are cooperatively established by the ASTM Heavy Duty Engine Oil Classification Panel (HDEOCP) and the Diesel Engine Oil Advisory Panel (DEOAP). Both of these groups are made up of members of the Engine Manufacturers Association (EMA) and oil and additive company members of API (153). Table 12 is an example of the performance criteria and the engine and bench tests used to measure them, for the API ‘‘C’’ category diesel engine oils. This table illustrates the increasingly rapid growth of new performance requirements (categories), the number of tests, and the number of new engines required for certification of heavy-duty diesel engine oils (154–158). Detailed performance requirements and engine test details for European diesel oil categories are given in Reference 155. 5.3. Gas Turbine Oils. The performance requirements for aircraft jet engines are described in the military specifications MIL-PRF-7808L and MILPRF-23699F. These specifications require bench tests for physical, chemical, and performance characteristics, an accelerated endurance test in a turboshaft engine, and, if these requirements are met, a 150-h test on two different models of aviation gas turbines conducted in a test cell and a 500-h flight evaluation test (159,160). Critical bench test performance requirements in MIL-PRF-23699F include oxidation and corrosion stability (OCS) tests for 72 h each at temperatures of 175, 204, and, 218 C, a thermal stability and corrosivity test at 274 C, a test for gear load-carrying ability, and tests for bearing deposits and bearing corrosion. The maximum pour point specified is 54 C (160). The wide operating temperature range needed in this application is the reason that neopentyl polyol ester base stocks are used for aircraft jet engine oils. MIL-PRF-23699F fluids have a nominal viscosity of 5 cSt at 100 C. The nominal viscosity of MIL-PRF-7808L fluids is 3 cSt at 100 C. 5.4. Automatic Transmission Fluids. ATFs are highly complex fluids that perform many essential functions in an automatic transmission (161). They transfer power in the torque converter; provide hydraulic pressure to operate clutches and shift gears; lubricate bearings, gears, and bushings; remove heat from the transmission; and provide the right friction profile for proper operation of plate, band, and torque converter clutches. Automatic transmissions normally operate at temperatures of 75–95 C and start up at temperatures as low as 40 C. The ATFs must flow easily at low temperatures, be highly resistant to thermal and oxidative degradation at high temperatures, be noncorrosive toward all transmission components, be resistant Table 12. Performance Requirements of Four-Stroke Diesel Engines 38 API service category foaming characteristics (ASTM D892) foaming/settling, mL, max, SEQUENCE I SEQUENCE II SEQUENCE III ASTM corrosion test copper increase, ppm, max lead increase, ppm, max tin increase, ppm, copper corrosion by ASTM D130, max rating NOACK volatility (ASTM D5800) or distillation (ASTM D2887) evaporative loss at 250 C, 10W-30, % max evaporative loss at 250 C, other, % max SAE 10W-30, % volatility loss at 250 C, max SAE 15W-40, % volatility loss at 250 C, max SAE 10W-30, % volatility loss at 371 C, max SAE 15W-40, % volatility loss at 371 C, max shear stability(ASTM D6278) kinematic viscosity after shearing SAE XW-30, cSt min SAE XW-40, cSt min sooted oil MRV (ASTM D6896) 180-h sample from Mack T-11 or T-11a, viscosity at 20 C, mPa s, max high temperature/high shear viscosity (ASTM D4683), mPa s, min elastomer compatibility limits (ASTM D7216) nitrile/NBR volume change hardness tensile strength elongation silicone/VMQ CG-4 CH-4 CI-4 CJ-4 10/0 20/0 10/0 D5968 20 60 report 3 10/0 20/0 10/0 D5968 20 120 50 3 10/0 20/0 10/0 D6594 20 120 50 3 10/0 20/0 10/0 D6594 20 120 15 15 13 9.3 12.5 25,000 9.3 12.5 25,000 3.5 3.5 þ5/ 3 þ7/ 5 þ10/ TMC 1006 þ10/ TMC 1006 þ5/ 3 þ7/ 5 þ10/ TMC 1006 þ10/ TMC 1006 3 20 18 9.3 12.5 9.3 12.5 (continued) Table 12. (Continued) 39 API service category volume change hardness tensile strength elongation polyacrylate/ACM volume change hardness tensile strength elongation fluoroelastomer/FKM volume change hardness tensile strength elongation vamac G volume change hardness tensile strength elongation CG-4 CH-4 CI-4 þTMC 1006/ 3 þ5/ TMC 1006 þ10/ 45 þ20/ 30 þ5/ þ8/ þ18/ þ20/ 3 5 15 30 þ5/ 2 þ7/ 5 þ10/ TMC 1006 þ10/ TMC 1006 CJ-4 þTMC 1006/ 3 þ5/ TMC 1006 þ10/ 45 þ20/ 30 þ5/ þ8/ þ18/ þ10/ 3 5 15 35 þ5/ 2 þ7/ 5 þ10/ TMC 1006 þ10/ TMC 1006 þTMC 1006/ 3 þ5/ TMC 1006 þ10/ TMC 1006 þ10/ TMC 1006 40 LUBRICATION AND LUBRICANTS to foaming, and have specialized friction and wear properties. They may contain as many as 15 different additives and a mixture of base stocks in API Groups I–IV (119). The major automobile and truck manufacturers each develop their own specifications for automatic transmission fluids, often using proprietary test methods and test materials consistent with their own transmission designs. Current examples include General Motors Dexron III fluid, version H; Ford’s Mercon V and Mercon SP; Daimler Chrysler’s MS-9602 (ATFþ4); Toyota’s Fuel Saving WS ATF, and Detroit Diesel Allison’s TES-295 (162). These fluids are not necessarily interchangeable. Performance requirements specified by all manufacturers include friction characteristics, oxidation stability, low temperature flow, shear stability, EP and wear protection, antishudder, and friction durability. Fluids designed for use in continuously variable transmissions (CVTs) require special frictional characteristics. There are two types of CVT used in modern production vehicles: (1) belt-drive CVTs using a Van Doorne type steel push-belt or a link-plate chain running in hydraulically adjustable, variable width sheaves or pulleys. Torque is transferred between the sheaves and the belt by the frictional force provided by metal–metal contact. (2) Traction-drive CVTs using a movable roller between two disks with toroid-shaped cavities. Torque is transferred between the roller and disks by a fluid film in the elastic–plastic (EHD) flow regime. Fluids with high traction coefficients, two to three times higher than those of conventional ATFs, are required for efficient operation of the toroidal traction drive. Fluids that have been evaluated as base stocks for traction fluids include derivatives of the a-methyl styrene dimer of isomerized tricyclopentadiene and other multiring naphthenic structures (163,164). Transmission fluids for belt-drive CVTs require high metal–metal friction coefficients to provide adequate torque transfer between the belt and sheaves. They must also provide good antiwear and antipitting protection. Since CVTs use controlled slip torque converters and wet clutches for starting and reverse, these properties must be provided without sacrificing the antishudder and wet clutch performance of modern ATFs. They must also have the proper viscosity characteristics over the entire operating temperature range, excellent thermal and oxidation stability, and compatibility with other transmission components. 5.5. Automotive Gear Oils. Automotive and industrial gear systems are the principal applications for mild EP lubricants. These lubricants are formulated to prevent wear, fatigue pitting, and catastrophic failure (spalling, scoring, and scuffing) of gear tooth surfaces. They are also formulated to protect against thermal and oxidative degradation, rust, cuprous alloy corrosion, foaming, and oil seal deterioration (165). Automotive gear lubricants in API category GL-5 are designed for use in final drive axles (differential gears) in moderate to severe service. Gear oils in API category MT-1 are intended for use in nonsynchronized manual transmissions in heavy-duty trucks and buses (166). Test requirements for GL-5 gear lubricants include the following: CRC L-37 24-h dynamometer test, operated at high torque, low speed conditions, to measure gear scoring and wear (167). LUBRICATION AND LUBRICANTS 41 CRC L-42 2-h dynamometer test, operated at high speed with occasional shock loading, to measure gear scoring (167). CRC L-60-1 48-hour oxidation test with motored gears to measure viscosity increase and insolubles (167). CRC L-33 7 days in humidity cabinet after initial motoring phase to rate rusting (167). ASTM D130 to measure copper strip corrosion (168). ASTM D892 to measure foam tendency (169). Gear Oil viscosity grades are defined by the SAE J306 Automotive Gear Oil Viscosity Classification (165,166,170). The MT-1 requirements include additional CRC L-60-1 ratings, ASTM D5662-99 seal compatibility tests (161), ASTM D5579-04 high temperature cyclic durability test (172), a more severe copper corrosion requirement, and ASTM D4998-95(2003)e FZG tractor hydraulic fluid wear test (166,167,173). SAE J2360 and MIL-PRF-2105E, which have identical performance requirements, combine the requirements of GL-5 and MT-1 gear oils (165). Limited slip differentials contain friction clutches that operate to apply equal torque to each driving wheel under slippery conditions. Lubricants for these units require a supplemental friction modifier added to the GL-5 lubricant to prevent chatter and noise when the clutches engage (166). 5.6. Industrial Oil Viscosity Classification and Guide Recommendations. The ISO viscosity classification system for industrial oils, based on kinematic viscosity at 40 C, is shown in Table 13 (174,175). Other viscosity classification systems include ASTM D6080 (176) and ANSI/AGMA 9005 (177). Recommended ISO viscosity grades for hydrodynamic bearings operating between 15 and 60 C are given in Table 14 (178). Table 15 shows viscosity grade recommendations for industrial enclosed gears (179). The recommended viscosity grade for rolling bearing lubricants as a function of temperature and dN number are given in Reference 180. dN number is the product of the bore diameter and the rotational speed. The lubrication of gears, hydrodynamic thrust and radial bearings, rollingcontact bearings, and other machine elements, such as sintered metal sliding bearings, screws, wire ropes, seals, valves, springs, and pneumatic components, is discussed at length in Reference 181 (for an excellent discussion of hydraulic fluids, see HYDRAULIC FLUIDS). 5.7. Turbine Oils for Industrial Steam and Gas Turbines. Steam and gas turbines are prime movers for electric power generation, for large ocean-going vessels, for pumps and compressors, for processing sugar cane, for rolling steel, and in many other industrial processes. Lubricating oils for the thrust and radial shaft bearings on these turbines may also be used for the generators, reduction gears, compressors, and other machines that the turbines are driving. Compared to internal combustion engine oils, turbine oils have a relatively small number and concentration of additives. Their lives, however, are measured not in miles or hours, but in decades. Steam and gas turbine bearings are lubricated by an oil circulation system, which includes the oil, a pump, driven 42 LUBRICATION AND LUBRICANTS Table 13. Viscosity System for Industrial Fluid Lubricantsa Viscosity grade ISO VG 2 ISO VG 3 ISO VG 5 ISO VG 7 Midpoint viscosity, cSt at 40 C 2.2 3.2 4.6 6.8 Kinematic viscosity limits, cSt at 40 C min max 1.98 2.4 2.88 3.52 4.14 5.06 6.12 7.48 ISO VG 10 ISO VG 15 ISO VG 22 ISO VG 32 ISO VG 46 ISO VG 68 10 15 22 32 46 68 9.00 13.5 19.8 28.8 41.4 61.2 11.0 16.5 24.2 35.2 50.6 74.8 ISO VG 100 ISO VG 150 ISO VG 220 ISO VG 320 ISO VG 460 ISO VG 680 100 150 220 320 460 680 90.0 135 198 288 414 612 110 165 242 352 506 748 1,000 1,500 2,200 3,200 900 1,350 1,980 2,880 1,100 1,650 2,420 3,520 ISO VG 1000 ISO VG 1500 ISO VG 2200 ISO VG 3200 a Reprinted with permission. by the turbine shaft, an oil reservoir, a filter, an oil cooler, the bearings, and auxiliary equipment. Turbine oils must have excellent thermal and oxidation stability, protect bearing surfaces from corrosion, separate easily from water, and they may not entrain air or foam excessively. Some may also require load-carrying capacity beyond that afforded by a hydrodynamic lubricating film. ASTM defines three different types of turbine oil: Type I oils are ‘‘oils for steam, gas, or combined cycle turbine lubricating systems where the machinery Table 14. Suggested Viscosity Grade of Oil for Plain Bearings Operating between 15 and 60 C Loading on projected bearing area Speed range, rpm 5,000–10,000 2,000–5,000 1,000–2,000 500–1,000 300–500 100–300 50–100 <50 Light, up to 700 kPa 10 15 22 32, 46 68, 100 100, 150 150, 220 220, 320 Medium, 700–1700 kPa Heavy, >1700 kPa 32, 46 68, 100 100,150 220, 320 220, 320 320, 460 320, 460 460, 680, 1,000 460, 680, 1,000 LUBRICATION AND LUBRICANTS 43 Table 15. ISO Viscosity Grade Guide Recommendations for Enclosed Gears Ambient temperature 10 to 10 C 10–50 C 68, 100 68, 100 100, 150 100, 150 150, 220 150, 220 double reduction: shaft center distance up to 20 cm >20 cm 68, 100 100, 150 100, 150 150, 220 triple reduction: shaft center distance up to 20 cm 20–50 cm >50 cm input speeds >3600 rpm: all types and sizes 68, 100 100, 150 150, 220 46 100, 150 150, 220 220, 320 68 planetary units: diameter of housing up to 40 cm >40 cm 68, 100 100, 150 100, 150 150, 220 bevel gears, spiral or straight: cone distanceb up to 30 cm >30 cm gear motors and shaft mounted units 68, 100 100, 150 68, 100 150, 220 220, 320 150, 220 Type of gear parallel shaft units: input speeds to 3600 rpm single reduction: shaft center distancea up to 20 cm 20–50 cm >50 cm a Shaft center distance ¼ distance between centers of largest gear and its pinion. Cone distance ¼ one-half the outside diameter of the largest gear. b does not require lubricants with enhanced load carrying capacity. Type I oils are generally satisfactory for turbine sets where bearing temperatures do not exceed 110 C.’’ Type II oils are ‘‘oils for steam, gas, or combined cycle turbine lubricating systems where the machinery requires enhanced load carrying capacity.’’ Type III oils are ‘‘oils for heavy duty gas or combined cycle turbine lubricating systems where the lubricant shall withstand higher temperatures and exhibit higher thermal stability than Type I or Type II oils. Type III oils are formulated for use in turbine sets where bearing temperatures may exceed 110 C. The turbine lubrication systems using Type III oils may be equipped with a gearbox that may require the selection of oils that contain additional antiwear additives to impart the specified load carrying capacity (182).’’ Typical specifications for an ISO VG 32 and an ISO 46 Type III turbine oil are shown in Table 16. Premium turbine oils can be formulated for >10,000-h D 943 oxidation lives, RPVOT lives of >1000 min, D 1401 demulsibility of 41/39/0 in 3 min, and an FZG failure load stage of 12 (183). 6. Metalworking Fluids Production of durable goods such as automobiles and airplanes requires manufacturers to cut, shape, and bend a variety of different metals (examples include aluminum and steel alloys). Metalworking fluids provide lubrication and other 44 LUBRICATION AND LUBRICANTS Table 16. Requirements for Type III Turbine Oils Property ISO viscosity grade flash point,  C, pour point,  C water content, mass% air release, 50 C, minutes foaming characteristics SEQUENCE I, tendency/ stability, mL emulsion characteristics at 54 C, minutes to 3 mL emulsion copper corrosion, 3 h at 100 C rust-preventing characteristics oxidation stability, hours to neut. No. 2.0 RPVOT, minutes to 175 kPa pressure drop RPVOT, retention after nitrogen treatment, % 1000-h TOST sludge, mg elastomer compatibility SRE NBR 1, or SRE NBR 28P or SRE-NBR-28PX (168  2 h at 100 C  1 C), hardness change cleanliness as filled into turbine, rating Test method D2422 D-92 D-97 D6304 D3427 32 200 min 6 max 0.02 max 5 max 46 200 min 9 max 0.02 max 5 max D892 50/0 50/0 D1401 30 max 30 max D130 1 max D665, procedure B Pass D943 5000 1 max Pass 5000 D2272 750 min 750 min D2272, modified 85 85 D4310 ISO 6072 200 max 8 to 8 200 max 8 to 8 ISO 4406-99 18/16/13 max 18/16/13 max important functions to ensure that these machining operations are carried out in a cost-effective manner. 6.1. Definition of Metalworking. Metalworking is a general term that describes the processing of metal into various shapes and sizes. These steps are accomplished in a number of different ways. Metalworking operations typically begin with metal that has just been manufactured into rolls, bars, and strips. Machine shops conduct a series of operations to remove pieces at specific locations and to bend the metal into particular shapes in order to produce a metal part, which meets the specifications of the customer. 6.2. Metalworking Operations. Metalworking operations can be divided into four main categories. Metal Removal. Metal removal is the process involving the use of a cutting tool to remove metal from a workpiece. Examples include boring, broaching, drilling, grinding, milling (Fig. 7), sawing (Fig. 8), shaping, and tapping. Lubrication in metal cutting and grinding operations differs from lubrication in bearings and gears in several important ways: (1) The relative motion of the chip and workpiece surfaces on the rake face and clearance face of the cutting tool (Fig. 9) tends to move fluid away from the contact zone rather than into it. Lubrication, therefore, is always in the boundary regime. Cutting fluid enters the contact zone through a labyrinth of capillaries formed by asperities on the surfaces. It is drawn in by the pressure difference between the contact zone and the atmosphere. (2) As the chip shears off the workpiece, nascent metal surfaces are formed, which rub on the rake and clearance faces of the cutting tool. LUBRICATION AND LUBRICANTS 45 Fig. 7. Milling. Without an oxide coating, these surfaces are very reactive. Pieces of this active metal weld together to form a ‘‘built-up edge,’’ which causes poor quality of the machined surface and wear of the clearance face. Wear of the rake face occurs behind the built-up edge and shows up as a crater some distance from the tool tip. Lubricating additives in the cutting fluid must react with the nascent metal surfaces, at the temperature on the surfaces, to minimize formation of the built-up edge and prevent wear of the cutting tool. (3) The additives must also reduce Fig. 8. Sawing. 46 LUBRICATION AND LUBRICANTS Rake face Cutting tool Clearance face Direction of motion Built-up edge Fig. 9. Chip formation in metal cutting. friction force on the chip. The higher the friction force on the chip, the smaller the shear angle, the longer the shear plane, and the more force it takes to shear the metal and form the chip. About one-quarter of the work in metal cutting is done to overcome friction and about three-quarter is done to form the chip. (4) Finally, cooling is extremely important to prevent distortion of the machined surface and to extend the life of the tool. Tool life decreases exponentially as the temperature at the tool–chip interface increases (184): Tun ¼ K (24) where T is the tool life (min), u is the temperature at the tool–chip interface ( C), and n is an exponent dependent on the type of tool, with value usually between 20 and 30, and K is a constant dependent on the tool and workpiece material. The other functions of the cutting fluid are to provide corrosion protection and to flush chips and metal fines from the cutting area. This prevents the chips from damaging cutting tools, machine tools, and the finished workpiece. Metal Forming. Process changes the shape and size of a workpiece without the removal of any metal. Examples include blanking, drawing (Fig. 10), extrusion, forming, forging, rolling, spinning, and stamping. Metal Protecting. Fluids protect metal parts from corrosion. Typically, metal protecting lubricants are applied to the surface of a metal part for either a certain period of time until the next machining step or an indefinite duration. Metal parts treated with metal-protecting fluids are typically stored in stacks as shown in Fig. 11. Metal Treating. A metal part is heated to temperatures in excess of 500 C and then immersed in an inert, metal-treating fluid, which is maintained at a temperature between 90 and 200 C. The purpose of this process is to subject the part to a series of temperature changes that harden the metal (Fig. 12). Metaltreating fluids control the heat transfer process and minimize the formation of distortions in the metal. Physical characteristics affected include strength, hardness, ductility, and malleability. 6.3. Types of Metalworking Fluids. Four major types of metalworking fluids are available in the marketplace. They are known as straight (or neat) oils, emulsifiable oils (also known as soluble oils), semisynthetic fluids, and synthetic fluids. LUBRICATION AND LUBRICANTS Fig. 10. Drawing. Fig. 11. Metal parts treated with metal protecting. 47 48 LUBRICATION AND LUBRICANTS Fig. 12. Industrial quenching process. Straight (or Neat) Oils. Straight oils are primarily formulated with two different types of mineral base oils (naphthenic and paraffinic) supplemented by additives that increase the performance of the fluid. The preferred mineral base oil is naphthenics because this base stock is a superior solvent and is much more compatible with additives. The leading paraffinic oil used in straight oils is Group I. Additives used to supplement the fluid include those that provide boundary lubricity and act as extreme pressure agents. The extreme pressure additives are very important in straight oils and examples include chlorinated paraffins, sulfurized additives, and phosphate esters. Boundary lubricity additives include animal and vegetable oils, blown oils, and a variety of esters, including methyl esters, monobasic esters, and polyol esters. Other additives used in straight oils include antioxidants and metal deactivators. Emulsifiable (Soluble) Oils. The original name given to this metalworking fluid type, soluble oils, is a misnomer. Mineral oils are not soluble in water. The term emulsifiable oils is a much better way to characterize this fluid because they are milky macroemulsions formed when a concentrate containing naphthenic or paraffinic mineral oil with additives is further diluted in water by the end user. In most metal removal applications, water is the predominant component in the diluted emulsifiable oil making up at least 90% of this metalworking fluid type. The main component in an emulsifiable oil concentrate is mineral oil. Naphthenic oils are the preferred mineral oil because they exhibit superior LUBRICATION AND LUBRICANTS 49 solvency and additive compatibility. A large number of additives are used to supplement the mineral oil and to ensure the emulsifiable oil can function in an aqueous environment. Among the additives formulated in an emulsifiable oil are emulsifiers, extreme pressure additives, boundary lubricity additives, corrosion inhibitors, coupling agents, antifoams, antimicrobial pesticides, and metal deactivators. Two of the key additives used are emulsifiers and extreme pressure agents. Emulsifiers used include anionic surfactants (eg, sodium petroleum sulfonates) and nonionic surfactants (eg, alcohol ethoxylates). This additive type is needed to stabilize mineral oil and the other additives in the water medium. Emulsifiable oils are divided into two types: those that are formulated with extreme pressure agents and known as heavy-duty emulsifiable oils and those without extreme pressure agents that are known as general-purpose emulsifiable oils. Chlorinated paraffins are the dominant type of extreme pressure agents used in emulsifiable oils. The emulsion particles have sizes ranging from 1 to 10 mm. Semisynthetic Fluids. Semisynthetic fluids are prepared from a concentrate that typically contains both mineral oil and water. The mineral oil used is primarily naphthenic oil, although paraffinic oil can also be used. Mineral oil and water are the two components most widely used in the semisynthetic fluid concentrate. Other additive types are needed to boost performance, including boundary lubricity additives, emulsifiers, corrosion inhibitors, reserve alkalinity boosters, coupling agents, metal deactivators, antimicrobial pesticides, and antifoams. The two basic types of semisynthetic fluids are prepared with both high and low levels of mineral oil. Emulsifiers and extreme pressure agents represent two of the most important additives used in semisynthetic fluids. Emulsifiers used are from the anionic (eg, sodium petroleum sulfonate) and nonionic (eg, alcohol ethoxylate) classes of surfactants. The main extreme pressure agent used in semisynthetic fluids is chlorinated paraffins. When prepared and further diluted with water, semisynthetic fluids have a translucent appearance typical of a microemulsion. Emulsion particle sizes for semisynthetic fluids range from 0.2 to 1 mm. They are smaller than emulsifiable oils enabling light to pass through the fluid. Synthetic Fluids. Unlike other types of lubricants, the main component used in a synthetic metalworking fluid is not mineral oil. This means the main component can be prepared with water, a synthetic base stock (eg, polyalphaolefins), or a vegetable oil. The synthetic metalworking fluid concentrate is typically further diluted with water. Most commercial synthetic metalworking fluids are prepared with water supplemented by other additives that provide boundary lubricity, extreme pressure characteristics, corrosion inhibition, reserve alkalinity boosting, metal deactivation, coupling, and antifoam characteristics. Antimicrobial pesticides are also included in the formulation. The key boundary lubricity additive commonly used is an ethylene oxide, propylene oxide block copolymer. Particle sizes in synthetic metalworking fluids are less than 10 3 mm. 50 LUBRICATION AND LUBRICANTS Dyes are used in all metalworking fluid types so that end users can readily monitor the fluid during use. This is particularly important in synthetic metalworking fluids that have the appearance of a water solution when no dye is used. 6.4. Application of Metalworking Fluid Types. Straight oils are typically used in metalworking operations, which require higher levels of lubricity. These operations are typically run under slower speeds and do not require as much cooling. Synthetic metalworking fluids provide the best degree of cooling and are used mainly in light-duty, high speed metalworking operations where not as much lubricity is required. Emulsifiable oils and semisynthetic fluids are more versatile and can be used in most metalworking operations because they provide both lubricity and cooling. Semisynthetic fluids are becoming more popular because the microemulsion when formed is proving to be more durable than the macroemulsions generated when emulsifiable oils are diluted with water. Fluid durability is a major trend in metalworking fluids because end users seek to operate their fluids for longer time frames under very demanding conditions. 7. Lubricating Grease Lubricating grease is a mixture of a fluid lubricant and a thickener that is dispersed in oil. It is a non-Newtonian fluid that acts like a sticky solid when there is no shear stress on it and that flows when a shear force is applied. It is widely used in ball and roller bearings and other machine elements where liquid lubricants cannot be retained. Automotive applications include wheel bearings, alternator bearings, constant velocity driveshaft joints (CVJ), mechanical clutch mechanisms, and gearbox bearings. Steel mills have central lubricating systems filled with multipurpose EP greases that are pumped long distances and applied through complex dispensing systems. Electric motor bearings, couplings, and roll neck bearings are some of the applications for grease in this industry (185,186). Greases are classified by their consistency and by the type of thickener. The consistency of a grease is measured by cone penetration, that is, the distance, in mm/10 (penetration number), which a standard cone, acting under the influence of gravity, will penetrate a grease sample at 25 C under test conditions described in ASTM D217-02 (187). The National Lubricating Grease Institute (NLGI) classification system for greases, based on worked penetration, is as follows (185): NLGI consistency number 000 00 0 1 2 3 4 5 6 Worked penetration, mm/10 at 25 C 445–475 400–430 355–385 310–340 265–295 220–250 175–205 130–160 85–115 LUBRICATION AND LUBRICANTS 51 Worked penetration is the penetration of a sample immediately after it has been subjected to 60 double strokes in a standard grease worker. The thickener in a grease may play as important a role as the oil in lubrication. The first greases were lime soap greases, made by saponifying a fatty oil with a slurry of hydrated lime, Ca(OH)2, and then dispersing the soap in oil and heating to drive off excess water. These greases are highly resistant to water, but unstable at high temperatures. Soda soap greases, made with NaOH, are stable at high temperatures, but wash out in moist conditions. Lithium soap greases resist both heat and moisture. A mixed base soap is a combination of soaps, offering some of the advantages of each type (188). A complex soap is formed by the simultaneous reaction of an alkali with a high molecular weight fat or fatty acid and with a low molecular weight organic or inorganic acid. Non-soap thickeners include clays, silica gels, carbon black, and synthesized organic materials such as substituted ureas and polyureas (185–187). Lithium soap and lithium complex greases are the most widely used multipurpose greases. The fatty acid portion is most often 12-hydroxystearic acid, made from hydrogenated castor oil. Dimethyl azelate and sebacate have been used as complexing agents. The dropping point of lithium soap greases is in the range of 180–190 C, whereas the dropping point of lithium complex greases is in the range of 260–300 C. Dropping point is the lowest temperature at which a grease is sufficiently fluid to drip, as determined by the test method ASTM D566-02 or D2265-06. It is an indication of whether a grease will stay in a bearing at operating temperatures (189,190). Polyurea greases are made by reacting amines with isocyanates or diisocyanates in base oil. Because they have superior high temperature durability, water resistance, structural stability, and low torque performance, polyurea greases are used in sealed-for-life bearings in electric motors and alternators and water pumps and in constant velocity joints. Bentonite clay (sodium montmorillonite and hectorite) is used as a grease thickener after being coated with quaternary alkyl ammonium cations (see CLAYS, USES). Clay-thickened greases have good oxidation and excellent thermal and mechanical stability. They have been successfully used in applications where ambient temperatures >260 C. An example is roller bearings on conveyor belts in glass factories. The effect of shear rate on the apparent viscosity of three greases is shown in Figure 13 (191). Grease has a yield point or yield stress, and does not flow until that stress is exceeded. Given the high shear rates in most rolling bearings, the apparent viscosity of grease is nearly the same as the viscosity of its base oil (185,191). Table 17 is a grease application guide, comparing the properties of the several grease thickener systems (185). ASTM D4950-14 (195) covers lubricating greases suitable for the periodic relubrication of chassis systems and wheel bearings of passenger cars, trucks, and other vehicles. It includes specifications for two chassis lubricants, LA and LB, and three wheel-bearing lubricants, GA, GB, and GC. To facilitate easy and accurate identification of greases in these categories, NLGI has made available the ‘‘NLGI Certification Mark’’ to be displayed on grease packaging. These specifications and the certification marks are also described in the January 2000 revision of SAE recommended practice J310 (186). 52 LUBRICATION AND LUBRICANTS Fig. 13. Effect of shear rate on the apparent viscosity of three NLGI grade 1 greases at 25 C. 8. Solid-Film Lubrication There are several applications where liquids do not provide adequate lubrication. In aerospace applications, for example, wide temperature ranges ( 240 to 900 C) are encountered accompanied by high vacuum. The manufacture of glass products requires lubricants that work at the temperature of molten glass. Lubricants are also needed that can withstand chemical attack in a corrosive atmosphere and are resistant to acids, aggressive gases, liquid oxygen, fuels, and solvents. Electrical contacts and precision machinery that require low start-up friction and cannot tolerate contamination by lubricating oils and greases are further examples (193). For these applications, solid lubricating films are applied to the rubbing surfaces. The wide range of solid lubricants can generally be classified as either inorganic compounds or organic polymers (both commonly used in a bonded coating on a matching substrate), chemical conversion coatings, and metal films. Since solid-film lubricants often suffer from poor wear resistance and inability to self-heal any breaks in the film, the search continues for improved compositions. 8.1. Inorganic Compounds. The most important inorganic materials are layer–lattice solids in which the bonding between atoms in an individual layer is by strong covalent or ionic forces and those between layers are relatively weak van der Waal’s forces. Because of their high melting points, high thermal Table 17. Grease Application Guidea,b Properties Aluminum Sodium Calcium, conventional Calcium, anhydrous Lithium Aluminum complex Calcium complex Lithium complex Polyurea Organo clay 53 dropping point,  C 110 163–177 96–104 135–143 177–204 260þ 260 þ 260þ usable temperature,  c water resistance work stability oxidation stability rust protection pumpability (in centralized systems) oil separation appearance 79 max 121 max 93 max 110 max 135 max 177 max 177 max 177 max good–excellent poor excellent poor–fair fair poor–good good–excellent fair–good poor–excellent excellent good–excellent fair–excellent good good–excellent fair–excellent good–excellent good–excellent fair–excellent fair–excellent fair–good poor–good good–excellent good–excellent fair–excellent good–excellent poor–good good–excellent fair–excellent fair–good good good–excellent poor good–excellent poor–fair poor–excellent good–excellent poor–excellent fair–excellent poor–excellent fair–excellent good–excellent fair–good fair–excellent poor–fair fair–excellent good–excellent fair–excellent good–excellent poor–excellent good good smooth and clear fair–good smooth–fibrous good smooth and buttery EP grades available good–excellent smooth and buttery EP grades available good–excellent smooth and buttery EP grades available general uses for economy military multiservice good–excellent smooth and buttery EP grades available reversible multiservice industrial good–excellent smooth and buttery inherent EP and antiwear rolling contact bearings good–excellent smooth and buttery EP grades available reversible multiservice automotive and industrial good–excellent smooth and buttery adhesive and cohesive poor–good smooth and buttery EP grades available multiservice automotive and industrial multiservice automotive and industrial multiservice automotive and industrial high temperature (frequent relube) other properties principal uses thread lubricants 243 177 max 260þ 177 max a Multiservice includes rolling contact bearings, plain bearings, and others. Reversibility is the ability of a grease to return to its normal grease-like consistency after temporary exposure to temperatures near or above its dropping point. b Reprinted with permission. 54 LUBRICATION AND LUBRICANTS stabilities, low evaporation rates, good radiation resistance, and effective friction lowering ability, molybdenum disulfide (MoS2) [1317-33-5], and graphite [778242-5] are the preferred choices in this group. Hexagonal boron nitride and boric acid also provide excellent lubrication (194). Among other layer–lattice solids that find occasional use are tungsten disulfide (WS2) [12138-09-9], tungsten diselenide (WeS2) [12067-46-8], niobium diselenide (NbSe2) [12034-77-4], calcium chloride [10108-64-2], cadmium iodide (CdI) [7790-80-9], and graphite fluoride [11113-63-6] (Table 18) (198,201). Graphite is widely used as a dry powder or as a colloidal dispersion in water, petroleum oil, castor oil, mineral spirits, or other solvents. The water dispersions are used for lubricating dies, tools, metalworking molds, oxygen equipment, and wire drawing. Graphite dispersed in solvents is used for drawing, extruding, and forming aluminum and magnesium, as a high temperature lubricant for conveyors, and for a variety of industrial applications. Graphite alone is ineffective in vacuum since adsorbed water normally plays a decisive role in its lubricating ability. Its film-forming ability can be restored, however, by mixing with cadmium oxide or MoS2 and most organic materials, so that graphite may offer effective lubricating action when bonded to the surface with organics. Air oxidation commonly sets a use limit of 550 C, and high friction may occur in air with water desorption at >100 C (197). Molybdenum disulfide has increasingly supplanted graphite for three reasons: consistent properties in rigid specifications, independence from need for adsorbed vapors in providing lubrication, and superior load capacity (197). Like graphite, MoS2 has a layer–lattice structure in which weak sulfur–sulfur bonds allow easy sliding between each sulfur–molybdenum–sulfur layer. Molybdenum disulfide covered by MIL-M-7866 is the most common lubricant grade: It is purified from molybdenite ore and is essentially free of abrasive constituents (195). Petroleum oil and grease dispersions of MoS2 are used extensively in automotive and truck chassis lubrication and in general industrial use. Dispersions are also made with 2-propanol, polyalkylene glycols, other synthetic oils, and water for airframe lubrication, in wire drawing, and for splines, fastenings, gears, and fittings. Above 400 C, the MoS2 is oxidized to molybdenum trioxide, which has a significantly higher coefficient of friction. There are indications that, as rubbed films, both MoS2 and graphite may accelerate corrosion: MoS2 by hydrolysis to form corrosive acids and graphite by galvanic action. Various other soft materials without the layer–lattice structure are used as solid lubricants (201), for example, basic white lead or lead carbonate [598-63-0] used in thread compounds, lime [1305-78-8] as a carrier in wire drawing, talc [14807-96-6] and bentonite [1302-78-9] as fillers for grease for cable pulling, and zinc oxide [1314-13-2] in high load capacity greases. Graphite fluoride is effective as a thin-film lubricant up to 400 C and is especially useful with a suitable binder such as polyimide varnish (197). Boric acid has been shown to have promise as a self-replenishing solid composite (194,198). 8.2. Organic Polymers. These self-lubricating polymers are used primarily in three ways: as thin films, as self-lubricating materials (see BEARING MATERIALS), or as binders for lamellar solids (194,196,199). Coatings are typically applied in powder or dispersion form with thickness ranging upward from 25 mm. Table 18. Common Solid Lubricantsa Acceptable usage temperature,  C Minimum Material molybdenum disulfide (MoS2) In air 240 Maximum In N2 or vacuum 240 Average friction coefficient, f In air 370 In N2 or vacuum 820 In air 0.10–0.25 In N2 or vacuum 0.05–0.10 polytetrafluoroethylene (PTFE) 70 70 290 290 0.02–0.15 0.02–0.15 fluoroethylene-propylene copolymer (FEP) graphite 70 70 200 200 0.02–0.15 0.02 540 unstable in vacuum 0.10–0.30 0.02–0.45 370 1320 0.12–0.40 0.07 430 820 0.10–0.20 370 1320 240 55 niobium diselenide (NbSe2) tungsten disulfide (WS2) 240 240 tungsten diselenide (WSe2) lead sulfide (PbS) lead oxide (PbO) calcium fluoride–barium fluoride eutectic (CaF2–BaF2) antimony trioxide (Sb2O2) a See Ref. 195. 480 430 430 650 820 0.10–0.30 820 0.10–0.30 0.10–0.25 above 540 C 0.25–0.40 above 540 C Remarks low f, carries high load, good overall lubricant, can promote metal corrosion lowest f of solid lubricants, load capacity moderate, and decreases at elevated temperatures low f, lower load capacity than PTFE low f and high load capacity in air, high f and wear in vacuum, conducts electricity low f, high load capacity, conducts electricity (in air or vacuum) f not as low as MoS2, temperature capability in air a little higher same as for Ws2 very high load capacity, used primarily as additive with other solid lubricants same as for PbS can be used at higher temperatures than other solid lubricants, high f < 540 C high load capacity, used as corrosion inhibitor in MoS2 lubricants 56 LUBRICATION AND LUBRICANTS The polymer is then fused to the surface as a coating that provides lubricity, abrasion and chemical resistance, or release properties. PTFE is outstanding in this group. In thin films it provides the lowest coefficient of friction (0.03–0.1) of any polymer, is effective from 200 to 250 C, and is generally unreactive chemically. The low friction is attributed to the smooth molecular profile of PTFE chains that allows easy sliding (194,196). Typical applications include chemical and food processing equipment, electrical components, and as a component to provide improved friction and wear in other resin systems. Other polymers finding self-lubricating use are fluorinated ethylene–propylene copolymer (FEP), perfluoroalkoxy resin (PFA), ethylene–chlorotrifluoroethylene alternating copolymer (ECTFE), and poly(vinylidene fluoride) (PVDF) (199). With a useful temperature range up to 200 C, outstanding weatherability, and low friction, FEP finds use in chemical process equipment, roll covers, wire and cable, and as powder in resin-bonded products. Perfluoroalkoxy resin provides somewhat better mechanical properties than PTFE and FEP at temperatures up to 250 C. The ECTFE provides superior strength, wear resistance, and creep resistance from cryogenic temperatures to 165 C. Although fairly expensive, it is effective in its common use as a corrosion-resistant coating. Also having superior mechanical properties, PVDF is more commonly used for lining chemical piping and reactor vessels than as a lubricant. 8.3. Bonded Solid-Film Lubricants. Although a thin film of solid lubricant that is burnished onto a wearing surface is often useful for break-in operations, >95% are resin bonded for improved life and performance (202). Use of adhesive binders permits applications of coatings 5–20 mm thick by spraying, dipping, or brushing as dispersions in a volatile solvent. The performance properties of some commonly used bonded lubricant films are listed in Table 19 (199,202). For many moderate-duty films for operating temperatures <80–120 C, MoS2 is used in combination with acrylics, alkyds, vinyls, and acetate room temperature curing resins. For improved wear life and temperatures up to 150–300 C, baked coatings are commonly used with thermosetting resins, for example, phenolics, epoxies, alkyds, silicones, polyimides, and urethanes. Of these, the MIL-L-8937 phenolic type is widely used (196). Inorganic binders are used, usually with graphite or MoS2, for extreme conditions such as high vacuum, liquid oxygen, radiation resistance, and high temperatures (200). The most common binder systems are silicates, phosphates, and aluminates. Some silicon and titanate metalloorganics used for high temperature binders become inorganic on curing. An emerging class of ceramic bonded materials for aerospace applications use either graphite, a CaF2 BaF2 eutectic, or proprietary systems, often with a glass frit binder that is fused into a continuous film (195,200). Plasma spray coating avoids overheating damage to the substrate metal while achieving the melting point of at least one component in high temperature film compositions (195,197,202). The solid lubricant/binder ratio is a principal performance factor. High lubricant content usually gives minimum friction, while high binder content tends to give better corrosion resistance, hardness, durability, and a glossy finish (202). With commonly used MoS2–graphite and organic resin binders, the Table 19. Performance Properties of Typical Solid-Film Lubricantsa Organicb Inorganicb Thermo set Air dry 57 Specification MIL-L-8937 MIL-L-46010 composition lubricant binder application cure MoS2 phenolic spray 149 C MoS2/metallic epoxy spray 204 C MoS2/graphite silicone spray 260 C PTFE phenolic spray 204 C compatibility LOX OZ rocket fuel jet fuel hydrocarbons solvents radiation N/A N/A X X X X Fair N/A N/A N/A X X X N/A N/A N/A L X X X N/A 260 C 220 C 10 7 Pa 2500 lb gage Falex 1000 lb gage Falex 60 min <0.1 G. operating temperature air (high) 260 C air (low) 220 C vacuum 10 4 Pa load capacityc force test wear lifec load test time coefficient of friction corrosion resistance a MIL-L-23398 MIL-L-46009 MIL-L-81329 AMS2525A MoS2 MoS2/graphite graphite MoS2 spray ambient aerosol ambient MoS2/graphite silicate spray 204 C 149 C impingement 149 F N/A N/A N/A X X X N/A N/A N/A N/A X X X N/A N/A N/A N/A L L L X X L X X X V.G. X X X X X X V.G. X X X X X X V.G. 371 C 157 F 10 7 Pa 260 C 220 C N/A 176 C 220 C N/A 204 C 185 C N/A 649 C 240 C 10 3 Pa þ1093 C 240 C 10 7 Pa 400 C 220 C 10 7 Pa 2500 lb gage Falex 2500 lb gage Falex 150 lb LFW-1 2500 lb gage Falex 2500 lb gage Falex 1000 lb gage Falex 450 min <0.1 V.G. 1000 lb gage Falex 60 min <0.1 F 150 lb LFW-1 120,000 min <0.1 E 1000 lb gage Falex 120 min <0.1 G 1000 lb gage Falex 70 min <0.1 F 50 50 Falex 2 min <0.1 Falex 5 min <0.1 See Ref. 202. X ¼ compatible; L ¼ low; N/A ¼ not applicable. c Falex tests are all designed for English units and are, therefore, reported as such. b <0.1 AMS2526A 58 LUBRICATION AND LUBRICANTS Table 20. Typical Pretreatments for Various Substratesa Substrate aluminum copper and its alloys iron and steel stainless steel titanium Pretreatment vapor degrease plus anodize light abrasive blast plus chromate conversion vapor degrease light abrasive blast plus chromate conversion vapor degrease abrasive blast plus phosphate vapor degrease sandblast passivation alkaline cleaning abrasive blast fluoride phosphate or alkaline anodize a See Ref. 200. optimum lubricant/binder ratio is usually 1:1–4:1. With inorganic binding agents, the ratio is from 4:1 to as high as 20:1 and increases with high temperatures. Substrate Properties. It is clear from equation 8 that higher hardness of the substrate lowers friction. Wear rate of the film is also generally lower. Phosphate undercoats on steel considerably improve wear life of bonded coatings by providing a porous surface that holds reserve lubricant. The same is true for surfaces that are vapor- or sandblasted prior to application of the solid-film lubricant. A number of typical surface pretreatments are given in Table 20 to prepare a surface for solid-film bonding (200). Optimum surface roughness is usually 0.05–0.5 mm; a very smooth surface contains very little lubricant within its depressions, whereas rough peaks penetrate the lubricant to promote wear. Improved corrosion resistance may be obtained with a suitable subcoating surface conversion treatment or by inclusion of inhibitors in the coating. Plasma-Applied Coatings. Composite coatings consisting of, for example, NiCr as a binder, Cr2O3 as hardener, BaF2/CaF2 as high temperature lubricant, and silver as low temperature lubricant have been developed by NASA (203,204). Applied as a plasma spray, these coatings provide lubrication at temperatures from 85 up to 1000 C (195,204). Plasma-enhanced chemical vapor deposition (PECVD) is used to produce diamond-like carbon and near-frictionless carbon coatings on steel substrates. Thin-film carbon coatings applied by this technique are described as ‘‘nanosmooth’’ and have excellent friction modifying, wear reduction and antiscuffing characteristics (205–207). Vacuum deposition techniques, such as sputtering, ion plating, physical vapor deposition, pulse laser deposition, ion beam-assisted deposition, and plasma-enhanced chemical vapor deposition, produce solid lubricant films that strongly adhere to the substrate. When these advanced methods are combined with surface texturing and micropatterning, the films achieve much improved tribological properties (208). 8.4. Chemical Conversion Coatings. These involve inorganic surface compounds developed by chemical or electrochemical action (see METAL SURFACE LUBRICATION AND LUBRICANTS 59 CONVERSION TREATMENTS). One of the best-known treatments for steel is phosphating to coat the surface with a layer of mixed zinc, iron, and manganese phosphates. Other films are anodized oxide coatings on aluminum, oxalate on copper alloys, and various sulfides, chlorides, and fluorides. Although many of these films are not strictly solid lubricants, they are often effective for short-term wear resistance. For long-term effectiveness, they often provide a porous reservoir for liquid lubricants and increased life of organically bonded coatings. Diffusion provides an alternative procedure for generating a chemically modified surface, for example, sulfide surface films can be formed by immersing steel in molten mixtures of sulfur-containing salts such as sodium thiosulfate or sodium sulfide. Similar processes are employed for carburizing, nitriding, boriding, or siliconizing. Metalliding can introduce a new element into many metal surfaces from a molten fluoride bath. A number of hardening treatments, as well as flame-sprayed tungsten and titanium carbides, provide excellent wear resistance. Some of these also provide good bases for low shear strength films. 8.5. Metal Films. In many respects, soft metals such as gallium, indium, thallium, lead, tin, gold, and silver are ideal solid lubricants (195,201). They have low shear strength, can be bonded strongly to substrate metal as continuous films, have good lubricity, and have high thermal conductivity. Metal films can be applied by electroplating or by vacuum processes, for example, evaporation, sputtering, and ion plating (see METAL COATINGS, SURVEY). Melting points of gallium, indium, and tin are too low, and those of thallium and lead are borderline when high surface temperatures are generated by high speeds and loads. Gallium is a special case, that is, it is above its melting point under most conditions and is too reactive with many metals. It is effective, however, when applied in a vacuum with AISI 440C stainless steel and with ceramics (qv) such as boron carbide or aluminum oxide, which can be applied as undercoats (209). A number of metal films are used industrially. Copper and silver are electroplated on the threads for bolt lubrication. Slurries of powders of nickel, copper, lead, and silver are also used in commercial bolt lubricants. Tin, zinc, copper, and silver coatings are used as lubricants in metalworking, where toxicity has virtually eliminated lead as a lubricating coating (210). Gold and silver find limited use on more expensive workpiece materials such as titanium. Silver films are useful in a variety of other sliding and rolling contacts, in vacuum and at high temperatures, since silver forms no alloys with steels and is soft at high temperatures. Under severe conditions and at high temperatures, noble metal films may fail by oxidation of the substrate base metal through pores in the film. Improved life may be achieved by first imposing a harder noble metal film, for example, rhodium or platinum–iridium, on the substrate metal. For maximum adhesion, the metal of the intermediate film should alloy both with the substrate metal and the soft noble metal lubricating film. This sometimes requires more than one intermediate layers. For example, silver does not alloy to steel and tends to lack adhesion. A flash of hard nickel bonds well to the steel, but the nickel tends to oxidize and should be coated with rhodium before applying silver of 1–5-mm thickness. This triplex film then provides better adhesion and greatly increased corrosion protection. 60 LUBRICATION AND LUBRICANTS 9. Extreme Ambient Conditions 9.1. Gas Lubrication. Despite severe limitations, gas lubrication of bearings has received intensive consideration for its resistance to radiation, for high speeds, temperature extremes, and use of the working fluid (gas) in a machine as its lubricant. A primary limitation is, however, the very low viscosity of gases (211) that leads to a limiting load of only 15–30 kPa (2.2–4.4 psi) for most self-acting (hydrodynamic) gas bearings and up to 70 kPa (10 psi) for operation with external gas-lifting pressure in hydrostatic operation. Gases that have been used for bearing lubrication include air, hydrogen, helium, nitrogen, oxygen, uranium hexafluoride [7783-81-5], carbon dioxide, and argon [7440-37-1]. A useful property of gases is that their viscosity, and hence their capacity to generate hydrodynamic pressure P, increases with temperature, whereas the opposite is true for liquids. Gas viscosity is usually independent of pressure up to 1 MPa (10 atm). Hydrodynamic principles for gas bearings are similar to those involved with liquid lubricants except that gas compressibility usually is a significant factor (212,213). With gas employed as a lubricant at high speeds, start–stop wear is minimized by selection of wear-resistant materials for the journal and bearing. This may involve hard coatings, such as tungsten carbide or chromium oxide flame plate, or solid lubricants, for example, PTFE and MoS2. Because of the very small bearing clearances in gas bearings, dust particles, moisture, and wear debris (from starting and stopping) should be kept to a minimum. Gas bearings have been used in precision spindles, gyroscopes, motorand turbine-driven circulators, compressors, fans, Brayton cycle turbomachinery, environmental simulation tables, and memory drums. 9.2. Liquid Metals. If operating temperatures rise >250–300 C, where many organic fluids decompose and water exerts high vapor pressure, liquid metals have found some use, for example, mercury for limited application in turbines; sodium, especially its low melting eutectic with 23 wt% potassium, as a hydraulic fluid and coolant in nuclear reactors; and potassium, rubidium, cesium, and gallium in some special uses. Liquid metal selection is usually limited to the lower melting point metals such as those mentioned above. Liquid metal viscosity generally is similar to water at room temperature and approaches the viscosities of gases at high temperatures (Fig. 14). Hydrodynamic load capacity with both liquid metals and water in a bearing is about one-tenth of that with oil, as indicated in Table 21. The sodium–potassium eutectic is commercially available for use as a liquid over a wide temperature range. Because of its excessive oxidizing tendency in air, however, its handling and disposal is hazardous; it can be used only in closed vacuum or in an inert gas atmosphere of helium, argon, or nitrogen. In addition to the oxidation problem, bearing material selection is critical for liquid metal bearings. Tungsten carbide cermet with 10–20 wt% cobalt binder gave superior performance when running against molybdenum under heavy loads at low speeds at temperatures up to 815 C (214). A low melting (5 C) gallium–indium–tin alloy has been the choice for small spiral-groove bearings in vacuum for x-ray tubes at speeds up to 7000 rpm (215). LUBRICATION AND LUBRICANTS 61 Fig. 14. Viscosity versus temperature. Surface tension 30 times that of oil avoids leakage of the gallium alloy from the ends of the bearings. 9.3. Cryogenic Bearing Lubrication. Cryogenic fluids, such as liquid oxygen, hydrogen, or nitrogen, are used as lubricants in liquid rocket propulsion systems, turbine expanders in liquefaction and refrigeration, and pumps to transfer large quantities of liquefied gases (see CRYOGENIC TECHNOLOGY). Bearings operating in cryogenic fluids are amply cooled from the standpoint of dissipating the heat generated from friction. Unfortunately, the low viscosity of the fluids leads to marginal lubrication. For wear resistance and low friction, coatings of PTFE or MoS2 generally have been satisfactory. Use of low thermal expansion filler in PTFE helps 62 LUBRICATION AND LUBRICANTS Table 21. Typical Design Limits for Fluid-Film Hydrodynamic Bearings Item oil lubrication steady load electric motors steam turbines railroad car axles dynamic load automobile engine main bearings automobile connecting rod bearings steel-mill roll necks water lubrication gas bearings a Load on projected area, MPaa 1.4 2.1 2.4 24 34 34 0.2 0.02 To convert MPa to psi, multiply by 145. minimize cracking and loss of adhesion from metal substrates with their lower coefficients of expansion. Because of the low viscosities of cryogenic liquids, rolling element bearings seem better suited than hydrodynamic bearings for turbo pumps. The AISI 440C stainless balls and rings generally are preferred for their corrosion resistance over the more commonly used AISI 52100 steel. 9.4. Nuclear Radiation Effects. Components of a nuclear reactor system that are exposed to radiation and that require lubrication include control-rod drives, coolant circulating pumps or compressors, motor-operated valves, and fuel handling devices, and, of course, are exposed to varying amounts of ionizing (216). Degree of damage suffered by a lubricant depends primarily on the total radioactive energy absorbed, whether it is from neutron bombardment or from gamma radiation. The common energy unit for absorbed dosage, the gray (Gy), is equal to 10 5 J (100 ergs) absorbed per gram of material, or 0.01 Gy ¼ 1 rad. The first changes observed with petroleum oils (at 104 Gy dosage) are evolution of hydrogen and light hydrocarbon gas as fragments from the original molecule. Unsaturation results in decreased oxidation stability, cross-linking, polymerization, or scission (217). The trend is for increasing viscosity with increased dose for several petroleum oils (217). For many lubricant applications, a dose that gives a 25% increase in 40 C viscosity can be taken as a tolerance limit. Lower radiation absorption seldom changes the lubricant sufficiently to interfere with its performance. Greater dosage results in more rapid thickening, sludging, and operating trouble (218). The general range of tolerance limit of 1–4  105 Gy (1–4  108 rad) for petroleum oils in Table 22 tends to be somewhat higher than for synthetic oils (219). This is surprising in view of the excellent thermal and oxidative stability of methyl silicones, diesters, silicates, and some other synthetics. An exception is the high order of stability with synthetic oils consisting of aromatic hydrocarbons in which much of the absorbed energy appears to be transferred into harmless resonance in the aromatic ring structure. This reduces the degree of damaging ionization and free radical formation that occurs on a more general basis with the LUBRICATION AND LUBRICANTS 63 Table 22. Radiation Tolerance Limits of Several Oil Types Oil Tolerance limit, 106 Gya for 25% increase in 40 C viscosity petroleum synthetic diester MIL-L-6085 synthetic hydrocarbon phosphate ester poly(propylene oxide) alkylbenzene dimethyl silicone methyl phenyl silicone tetraaryl silicate a 1–4 1.1 2.5–4.5 0.4–0.6 1.0 5 <1 1 0.6 To convert Gy to rad, multiply by 100. chain-like structures in paraffinic oils or in the saturated ring structure of alicyclic oils. Oil life is reduced by both the radiation dose and by oxidation if oxygen (air) is present at high temperatures. Conventional greases consisting of petroleum oils thickened with lithium, sodium, calcium, or other soaps suffer significant breakdown of the soap gel structure at doses above 105–106 Gy (107–108 rad). Initial breakdown commonly involves increased softening of the grease to the point, where it may become fluid. At even higher doses, polymerization of the oil phase eventually leads to overall grease hardening. Some greases with radiation-resistant components, for example, polyphenyl ether oil and nonsoap thickeners, maintain satisfactory consistency for lubrication purposes up to 107 Gy (109 rad). 9.5. Lubrication with Glass. Softening glass is used as a lubricant for extrusion, forming, and other hot working processes with steel- and nickel-base alloys up to 1000 C; for extrusion and forming titanium and zirconium alloys; and less frequently for extruding copper alloys (210). Principal types of glasses used are pure fused silica [7631-86-9], 96% silica–soda–lime, borosilicates, and aluminosilicates [1327-36-2]. The glass composition is selected for proper viscosity, typically 10–100 Pa s (1000–10,000 P) at the mean temperature of the die and workpiece, to serve as a true hydrodynamic lubricant. Glass may be applied as fibers or powder to the die or hot workpiece, or as a slurry with a polymeric bonding agent to the workpiece before heating. Another method involves rolling heated steel billets across glass sheets, where the glass then wraps itself around the billet before passing to a die extrusion chamber. The U.S. Bureau of Mines has employed glass for forming ceramic materials at high temperatures (220). 9.6. Ionic Liquids. Ionic liquids are organic salts that remain liquid at room temperature. They have virtually no vapor pressure and, in applications such as aluminum cold rolling, can significantly reduce air emissions if used in place of hydrocarbon-based lubricants. Forces holding the molecules together are Coulombic or electrostatic, as opposed to the weaker van der Waals forces holding most liquids together. A variety of anions such as Cl , BF4 , and PF6 , to name a few, 64 LUBRICATION AND LUBRICANTS may be used. The properties of two of these liquids, 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide and 1-butyl-3-methylimidazolium hexafluorophosphate, are described in Reference 221 (see also MICROWAVE TECHNOLOGY). 9.7. Solid and Liquid Lubricants for Extreme Environments. A compilation of papers presented at a Symposium on Lubricants for Extreme Environments is given in Reference 222. 10. The Lubricants Market The American Fuel & Petrochemical Manufacturers (AFPM) reports annual lubricating oil sales of 2.4 billion gallons in the United States in 2002–2013. Worldwide Lubricant demand in 2003, without marine oils, was 36 million metric tons and remained relatively unchanged at 35.3 million metric tons in 2013. Lubricating oil base stock production in North America was 71 million barrels in 2003 and 59.8 million barrels in 2013. These numbers are broken down by region and product type in Table 23 (223,224). The sales of automotive oils and greases have been fairly level in the period 1998–2002. Sales of industrial oils have declined in this period. The number of base stock refineries in North America has fallen from 36 in 1998 to 30 in 2014. North American base stock capacity has increased from 264,500 barrels/day in 1998 to 283,850 barrels/day in 2014. Production of Group II and III base stocks has grown over the past 10 years, however, from 21 to 42% (223). Table 23. The Lubricants Marketa Worldwide lubricant demand (without marine oils) Asia–Pacific North America Western Europe Central and Eastern Europe Latin America Near and Middle East Africa total worldwide lubricant demand, millions of metric tons 2013 (%) 42 19 11 9 9 5 5 35.3 2003 (%) 31.2 23.2 13.4 13.4 8.7 4.9 5.2 36 Estimated North American base stock production 2014 (%) 2005 (%) paraffinic Group I paraffinic Group II paraffinic Group III polyalphaolefin Group IV naphthenic and other Group V total estimated North American base stock production, millions of barrels/day 45 31 11 4 9 1,057,145 65 16 5 3 11 941,055 Global lubricant demand by product 2013 (%) 2005 (%) Automotive oils Process oils Industrial oils Metalworking fluids and corrosion preventives Greases 56 10 26 5 3 63 17 14 3 2 a See Refs. 223 and 224. LUBRICATION AND LUBRICANTS 65 11. Environmental and Health Factors: Toxicology Conservation, health, safety, and environmental pollution concerns have led to the creation of wide-reaching legislation such as the U.S. Congress Energy Policy and Conservation Act, Toxic Substances Control Act (225), Solid Waste Disposal Act, the Used Oil Recycling Act of 1980, and subsequent implementation of many rules and regulations such as the OSHA Hazard Communication Standard. Continuing publications of new and proposed rules and regulations are available from the EPA (Washington, D.C.) and from the National Technical Information Service of the U.S. Department of Commerce (Springfield, Va.) (226). Regulations generally prohibit disposal of lubricants in streams, chemical dumps, or other environmental channels. Over one-half of disposed lubricants are burned as fuel, usually mixed with virgin residual, and distillate fuels (227). Waste aqueous metalworking fluids may be successfully treated by conventional means for removal of tramp oil, surfactants, and other chemical agents to provide suitable effluent water quality (228). 11.1. Lubricant Recycling. Considerable effort is underway to improve and expand recycling of lubricating oils. Although typical processes result in 80– 90% yield, questions remain regarding initial collection and the separation of used oil from water and other contaminants. Recycling treatment varies from simple cleaning to essentially the complete refining process used with virgin oil. Typical steps involved in purifying used petroleum lubricating oil are indicated schematically in Figure 15 (229). Fig. 15. Oil recycling flow diagram. 66 LUBRICATION AND LUBRICANTS 11.2. Reclamation. Reclamation involves simple separation of contaminants by gravity settling of water and dirt, centrifuging, filtering, and membrane techniques. With water-soluble cutting oils containing only a few percent of oil, chemical emulsion breakers are first added that consist of sulfuric acid and then aluminum sulfate as a coagulant. Polymers are sometimes added to speed the process. The separated oil then is decanted, skimmed, or centrifuged and is commonly burned. Generally, 1–5% reprocessed waste oil may be added to fuel and still meet EPA industrial furnace limits, that is, <5 ppm arsenic by mass, <2 ppm cadmium, <10 ppm chromium, <100 ppm lead, and <4000 ppm halogens and a flash point >18 C (226). 11.3. Reprocessing. The simplest operation involves flash distillation in an evaporator at 100–200 C in partial vacuum to remove water and low boiling contaminants, for example, gasoline and solvents. This is followed by treatment with fuller’s earth or other activated clay for removing oxidation products and most additives to produce a purified, light-colored oil, which, with suitable additives, is satisfactory for use as fuel, metalworking base stocks, noncritical lubricants, and concrete form oil. Some used oils, for example, hydraulic and transformer oils, can be reprocessed with a portable unit of capacity to their original oil quality directly at the equipment in which they are being used. 11.4. Rerefining. The technology currently attracting most attention for producing original quality lubricating oil depends on distillation in thin-film evaporators (TFE) (223). Thin-film evaporator processes usually involve a scheme similar to that shown at the bottom of Figure 15. The preliminary removal of water, solvents, and fuel is done in the same fashion as in most other recycling. Coking and fouling during distillation is avoided because the maximum temperature is maintained for only 2–5 s as the oil flows down the TFE wall under the influence of moving wiper blades; this is a small fraction of residence time in the packing or plates of a more traditional distillation tower. These variations involve batch operation with a single unit, or sequential distillation in multiple units to produce several lube fractions. Older rerefining units used 2–5 kg/L of activated clay at 40–70 C and higher temperatures in place of TFE to clean the oil (230). More elaborate chemical and hydro treating of used engine oils without a distillation step has been developed by Phillips Petroleum for processing 40,000 m3/year (10  106 gal/year). Establishment of a reliable feedstock supply is a critical consideration for larger rerefining plants. 11.5. Toxic and Hazardous Constituents. Questionable constituents of lubricating oils are polycyclic aromatics in the base oil plus various additives (231). Of refining steps used in preparing lubricating oil base stocks from toxic distillates, only effective solvent extraction, severe hydrogenation, or exhaustive fuming sulfuric acid treatment appear adequate to eliminate carcinogenicity. Mild hydrofinishing or light solvent extraction reduces carcinogenic potential, but does not necessarily eliminate it. Increasing the severity of hydrofinishing can eliminate carcinogenic compounds. Group IV base stocks, polyalphaolefins, are not expected to be carcinogenic since no polycyclic aromatics are present. Most additives for lubricants present little risk, but the following involve significant hazards: lead compounds, phenyl 2-naphthylamine, sodium nitrite LUBRICATION AND LUBRICANTS 67 plus amines, tricresylphosphate high in the o-cresol isomer, and chlorinated naphthalenes. A number of sulfur compounds used as additives cause skin irritation; however, properly refined base oils containing usual concentrations of these additives have a low degree of toxicity. Used motor oil has displayed increased carcinogenic activity over its new counterpart (226,231). Users should also avoid contact with lubricants, metalworking oils, and quench oils that are highly degraded, were in service at extremely high temperatures, or are contaminated with toxic metals or bacteria. The latest government regulations set forth under the Toxic Substances Control Act and in Public Health Service publications should be checked before formulating new lubricants. Users of lubricants should request Material Safety Data Sheets for each substance involved plus certification of compliance from vendors. Lubricant compounders should insist on similar information from their suppliers for any additive packages. Manufacturers of both additives and lubricants commonly make toxicity checks on commercial products. 11.6. Food Processing. Prior to September 1998, the U.S. Department of Agriculture (USDA), Food Safety Inspection Service (FSIS), working with the Food and Drug Administration (FDA), maintained a system to ensure minimal risk to the consumer from incidental or unintended contact with lubricants used in the food and beverage industries (see FOOD PROCESSING) (232). Upon satisfactory determination of nontoxicity, the USDA issued one of three ratings (233): H1 lubricants—food-grade lubricants used in the food processing environment, where there is a possibility of incidental food contact, for example, as by splashing or dripping from machinery above an edible product; H2 lubricants—nonfood-grade lubricants used on equipment and machine parts in locations where there is no possibility of food contact, for example, as in sealed gear boxes or machinery below a product line; H3 lubricants—food-grade lubricants, typically edible oils, used to prevent rust on hooks, trolleys, and similar equipment. These classes include a number of petroleum and nonpetroleum oils and greases. In September 1998, the USDA issued the Hazard Analysis and Critical Control Point (HACCP) protocol and eliminated the approval program. This action shifted the burden of assessing the risk and approval of lubricants to the food processor or manufacturer. Under the HACCP system, food processing facilities must show the FSIS inspectors ‘‘documentation substantiating the safety of a chemical’s (lubricant’s) use in a food processing environment’’ (232,233). In order to minimize disclosure of proprietary formulation details, lubricant manufacturers, with the approval of food processors, equipment builders, and the USDA, have developed a system whereby third party certifiers assume the role previously handled by USDA. This includes preauthorization of nonfood compounds, screening of product formulations, product registration, and the issuance of approval documents (234,235). The rating system used by USDA for lubricants will continue to be used. Food-grade lubricants rated H1 or H3 for incidental contact must be generally regarded as safe (GRAS), used in accordance with the provisions of a prior sanction or approval, or listed under 21 CFR 178.3570. 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