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sensitive to an element such as oxygen to surface coverages of as little as one hundredth of a monolayer and analyzes to a depth of four or five atomic layers. AES analysis uses a beam of electrons just as with LEED, but the electron energy is higher, usually 1500 to 3000 eV. Incident electrons strike the sample surface, penetrate the electron shells of outermost surface atoms, and cause ejection of a second electron called an Auger electron. The ejected electron carries with it an energy characteristic of the atom from which it came. Measuring the energy of the ejected electron identifies its elemental source. The electron energies detected can be recorded on a strip chart or an oscilloscope (see References for details). An ordinary iron surface with normal surface contaminants will yield an Auger spectrum such as Figure 6a which contains peaks for sulfur, carbon, oxygen, and iron. The carbon and sulfur have two possible origins: impurities in the bulk iron which have segregated to the surface or adsorbates such as carbon monoxide, or carbon dioxide from the environment. The oxygen peak results from iron oxides present on the surface or adsorbates such as carbon compounds or water vapor. The three iron peaks originate from iron oxides and the iron metal. If the surface of Figure 6a is bombarded with argon ions, contaminants can be knocked off leaving only iron with the spectrum in Figure 6b. The added low energy iron peak at the left end of the spectrum is easily lost when the surface is contaminated. The shapes of Auger peaks can provide considerable information on the source of an element such as carbon, as demonstrated in Figure 7. The upper carbon Auger peak arises Volume II21 FIGURE 4.Atomic packing on var- ious crystallographic planes and in various directions. For face-centered cubic materials. (a) Appearance of several crystal planes; (b) the (111) plane. Copyright © 1983 CRC Press LLC from carbon which segregated on the surface of the molybdenum. The second peak is from adsorbed carbon monoxide, while the third peak is from graphite. X-Ray Photoemission Spectroscopy (XPS) XPS is a surface tool which can determine the molecular structure from which an element came. XPS was formerly called electron spectroscopy for chemical analysis (ESCA). 22 CRC Handbook of Lubrication FIGURE 5. Three LEED patterns from an iron (110) surface, (a) Carbon contaminant; (b) argon bombarded; and (c) clean surface (110V). FIGURE 6. Auger spectra for an iron surface (a) before and (b) after sputter cleaning. a b c b a Copyright © 1983 CRC Press LLC With XPS a monochromatic X-ray beam is used as the energy source. The beam causes ejection of electrons with kinetic energies characteristic of the surface atoms. Aspectrum of the elements present is obtained by plotting the total number of electrons ejected as a function of kinetic energy. XPS gives binding energies of the elements which enables identification of the compounds in which these elements exist. The binding energy of the electrons ejected from the surface is determined by their chemical environment and is roughly a function of the atomic charge. The binding energy measured with XPS will be altered by changing the particular elements bound to the element being examined. Elemental sulfur has a characteristic binding energy of 162.5 eV. Negatively charged S −2 has a lower binding energy. When oxygen is bound to the sulfur, the sulfur binding energy increases. Further, the SO 4 −2 structure has a greater binding energy than SO 3 −2 which can be used to distinguish between sulfur bound in these two states. Other Techniques Over 70 surface tools have been developed for analysis and chemical characterization. A few more commonly used techniques are indicated by their acronyms in Table 1. The nondestructive techniques are nuclear back scattering spectroscopy (NBS) and electron mi- croprobe (EM). Auger electron spectroscopy (AES), X-ray photoemission spectroscopy (XPS or ESCA), ion-scattering spectroscopy (ISS), and appearance potential spectroscopy (APS) are destructive only if sputter etching or depth profiling is used. Two techniques which are destructive are secondary ion mass spectroscopy (SIMS) and glow discharge mass spectroscopy (GDMS). These techniques detect the species sputtered from the surface (see book by Kane and Larrabee in the References for more details). Note from Table 1 that they both detect all elements except hydrogen and helium, provide excellent chemical identification, and have sensitivities of surface elements to as little as 0.01 mono- layer. Their disadvantage is that they must be operated in a vacuum system. Probably the most versatile tool is the scanning electron microscope (SEM). It is extremely useful in obtaining a view of features on a surface such as asperities, surface irregularities, and topography where adhesion and wear have occurred. When SEM has incorporated into it X-ray energy dispersive analysis, both topography and chemistry can be determined. The X-ray analysis is not a surface analytical tool, but it can provide considerable information where material transfer takes place in adhesion or sliding. An SEM photomicrograph of an Volume II 23 FIGURE 7. Auger electron spectra of car- bon. (a) Segregated at a Mo(110) surface during initial cleaning (labeled Mo-C); (b) CO on a clean Mo(110) surface (labeled Mo- CO); and (c) in graphite. Copyright © 1983 CRC Press LLC 24 CRC Handbook of Lubrication TABLE 1 COMPARATIVE TABLE FOR THE VARIOUS TECHNIQUES USED FOR THE CHEMICAL CHARACTERIZATION OF SURFACES NBS EM AES XPS ISS SIMS GDMS APS Destructive to sample No No No No No Yes Yes No (in general) Elements that can be Heavy Z ≥ 4 Z ≥ 3 Z ≥ 3 Z ≥ 3 All All except Z ≥ 3 detected He, Ne Elemental F G E E E G G E identification a Sensitivity (typical, in 50 5 −0.01 <0.01 −0.01 <1 ~1 ≤0.1 monolayers) Detectability (i.e., NA 100 <1 NA NA 1 100 NA ppm) b Results are (in Abs Abs Abs Abs Abs Abs Abs Abs principle) c Depth probed (in Å) 10 4 10 4 —10 5 15—20 15—75 3 ~5 × 10—10 4 ~10 10 4 Depth distribution of Yes Yes Y/d No Yes Yes Yes Y/d elements d Chemical (i.e., binding) No Yes Yes Yes No No No Yes information a E, Excellent; G, good; F, fair. b NA, Not applicable. c Rel, Relative; Abs, absolute. d Y/d, Yes, if destructive. From Kane, P. F. and Larrabee, G. B., Eds., Characterization of Solid Surfaces, Plenum Press, New York, 1974. With permission. Copyright © 1983 CRC Press LLC aluminum surface is shown in Figure 8a after sliding on an iron surface. The photomicrograph reveals surface topography while the X-ray map for iron reveals the white patch in Figure 8b where iron is detected on the aluminum wear surface. PROPERTIES OF SURFACES Metallurgy and Crystalline Structure The crystal structure of ideal surfaces has already been examined in Figure 4. All engi- neering surfaces vary from this ideal and have grain boundaries which develop during solidification as large defects which exist in the solid and extend to the surface. They do not possess a regular structure, are highly active regions, and on the surface are very energetic. Lesser defects include subboundaries, twins, dislocations, interstitials, and vacancies. Subboundaries are low-angle grain boundaries and usually occur where there is only a slight mismatch in orientation of adjacent grains on either side of the boundary. When the crystal lattices of adjacent grains are slightly tilted one toward the other, there is a tilt boundary. Where the lattices remain parallel but one is rotated about a simple crystallographic axis relative to the other with the boundary being normal to this axis, a twist boundary develops. The twin boundary occurs where there is only a degree or two of mismatch with the twins being mirror images. They are frequently seen on basal planes of hexagonal metals with deformation. Dislocations are atomic line defects in crystalline solids. They may be subsurface and terminate at the surface or they may be in the surface. Edge dislocations are entirely along a line where an extra half plane of atoms exists. Screw dislocations form along a spiral dislocation line. Small angle boundaries or subboundaries are generally composed of edge dislocations. These defects in crystalline solids cause them to deviate markedly from the theoretically achievable strengths of ideal crystals. Some of the crystalline surface defects are presented schematically in Figure 9. The vacant lattice site was seen on a real surface in the photomicrograph of Figure 2b. An interstital atom is crowded into the crystal lattice of Figure 9a. Edge and screw dislocations and a small angle boundary are also shown. Worn surfaces generally have undergone a high degree of strain and may contain large amounts of lattice distortion and defects such as dislocations. While initial dislocations cause a reduction in strength, their multiplication and interaction during deformation increase surfacial strength. Microhardness is generally higher in grain boundaries than in grains. With plastic deformation, the strain generally produces a reduction in recrystallization Volume II25 FIGURE 8. (a) Electron image of aluminum rider wear scar; (b) iron Kαmap of aluminum rider. a b Copyright © 1983 CRC Press LLC temperatures of material at the surface. The combination of strain and temperature can then bring about surface recrystallization which has an annealing effect. This process relieves lattice strain and stored energy, with a sharp reduction in the concentration of surface defects. In a dynamic, nonequilibrium system such as encountered in sliding, rolling, or rubbing, surface layers may be strained many times, recrystallized, and then strained again. Solid State Bonding What holds atoms or molecules in various arrangements and imparts to solids their basic cohesive strength? The answer lies in the bonding. Bonding in crystalline solids can be of four types, as shown in Figure 10: van der Waals, ionic, metallic, and covalent. Van der Waals forces, the weakest holding solids together, are attributed to nothing more than fluctuations in the charge distributions within atoms or molecules. These forces can be represented in bonding the atoms of an inert gas together when solidified. Very littleenergy is required to accomplish sublimation. An ionic bond is very strong, and some high-strength solids are held together by it. This bond is represented in Figure 10 by sodium chloride. Electrons are transferred from the metal to the nonmetal and the resulting ions are held together by the electrostatic forces; developed. Aluminum and magnesium oxides are two tribological solids with this bonding. With metals, the valence electrons are taken away from individual atoms to form a sea of electrons. This results in positively charged ions immersed in electrons. This bonding gives metals their good thermal and electrical conduction characteristics. The fourth type of bonding is covalent where electrons are simply shared. This is indicated in Figure 10 by the overlapping of carbon atoms in diamond (the hardest material and most resistant to deformation). At the same time, the covalent bond is found in organic molecules in polymers and lubricants where it is relatively weak. No other bonding type possesses such a wide range of strengths. 26CRC Handbook of Lubrication FIGURE 9. Crystalline defects in solids. Copyright © 1983 CRC Press LLC CHEMISTRYOF SURFACES Clean Surfaces Very clean surfaces are extremely active chemically. Acopper atom which lies in a (111) plane in the bulk of the solid will have a coordination number of 12: it is bonded to 12 nearest neighbors. That same copper atom at the surface will, however, have a coordination number of only 9 with only 9 nearest neighbors. The energy normally associated with bonding to three additional atoms is now available at the surface. This energy expressed over an area of many atoms is referred to as the surface energy. Surface energy is also the energy necessary to generate a new solid surface by the separation of adjacent planes. The energy required for separation is a function of the atomic packing. For example, for copper the atomic packing density is greatest in (111) planes (greatest number of nearest neighbors within the plane). As a result, bonding forces between adjacent (111) planes is least and the surface energy of new (111) surfaces generated, say by cleavage, is less than for the (110) and (100) planes. This lesser binding strength is also a function of the distance between adjacent planes, it being greater between adjacent (111) planes than between (110) and (100) planes. Because surface atoms have this unused energy, they can interact with each other, with other atoms from the bulk, and with species from the environment. Not bound as rigidly as atoms in the bulk, surface atoms can alter their lattice spacing by reconstruction, as depicted schematically in Figure 11. By use of LEED, this process has been found to occur in some crystalline solids but not in others. In solids containing more than a single element, atoms from the bulk can diffuse to the surface and segregate there. In a simple binary alloy, solute atom can diffuse from near surface regions to completely cover the surface of the solvent. This has been observed for many binary systems including aluminum in copper, tin in copper, indium in copper, alu- minum in iron, and silicon in iron. One hypothesis for the segregation mechanism is that the solute segregates on the surface because it reduces the surface energy. Asecond theory is that the solute produces a strain in the crystal lattice of the solvent, and this unnatural lattice state ejects solute atoms from the bulk. Chemisorption In addition to the solid interacting with itself at the surface, the surface can interact with Volume II27 FIGURE 10.The principle types of crystalline binding forces. Copyright © 1983 CRC Press LLC the environment. This interaction alters the surface chemistry, physics, metallurgy, and mechanical behavior. If a metal surface is very carefully cleaned in a vacuum system and then a gas such as oxygen admitted, the gas will adsorb on the metal surface. Except with inert gases, this adsorption results in chemical bonding in a chemisorption process indicated schematically in Figure 11. Once adsorbed, these films are generally difficult to remove. Where the species adsorbing on a clean surface is an element, adsorption is direct. Surface atoms of the solid retain their individual identity as do atoms of the adsorbate, yet each is chemically bonded to the other. When the adsorbing species is molecular, chemisorption may be a two-step process, first dissociation of the molecule upon contact with the energetic clean surface followed by adsorption of the dissociated constituents. Chemisorption is a monolayer process. Bond strengths are a function of chemical activity of the solid surface (surface energy), degree of surface coverage of that adsorbate or another adsorbate, reactivity of the adsorbing species, and its structure. The higher the surface energy of the solid surface, the stronger the tendency to chemisorb. In general, the high-energy, low-atomic density crystallographic planes will chemisorb much more rapidly than will the high-atomic density, low-surface energy planes. Hydrogen sulfide will adsorb more readily on (110) and (100) surfaces of copper than on (111) surfaces. The metal surface has an effect. Copper, silver, and gold are noble metals and many of their properties are similar. Yet, oxygen will chemisorb relatively strongly to copper, weakly to silver, and not at all to gold. Reactivity of the adsorbent is also important. Of the halogen family fluorine will adsorb more strongly than chlorine, chlorine than bromine, and bromine than iodine. The structure of the adsorbing species is also significant as can be demonstrated with simple hydrocarbons. If ethane, ethylene, and acetylene are adsorbed on an iron surface, tenacity of the chemisorbed films is in direct relation to the degree of bond unsaturation. Acetylene is much more strongly bound to the surface than ethylene, which in turn is more strongly bound than ethane. The carbon to carbon bonds break on adsorption and bond to the iron. The greater the number of carbon to carbon bonds, the greater the resulting number of carbon to iron bonds. Compound Formation Compound formation on tribological surfaces is extremely important. The naturally oc- curring oxides present on metals prevents their destruction when sliding on other solids. Extreme pressure additives and many antiwear materials placed in oils perform by compound formation with the surface to be lubricated. Once present on a surface, chemisorbed films often interact with that surface to form chemical compounds. The surface material and the adsorbate form an entirely new substance with its own characteristic properties. The process continues by diffusion of both the solid surface material and the environmental species into the film. The compound can grow in thickness on the surface if the film is porous and allows for two-way diffusion as shown in 28 CRC Handbook of Lubrication FIGURE 11. Possible surface events. Copyright © 1983 CRC Press LLC Figure 11. An example is the oxidation of ironin moist air which continues to consume iron. In contrast, oxidation of aluminum to form aluminum oxide results in a thin, dense oxide of 120 Å which retards diffusion and film growth. Environmental Effects Chemical, physical, and metallurgical properties of atomically clean metal surfaces are markedly altered by foreign substances. This is extremely important because most real surfaces are not atomically clean but have film(s) present on their surface (Figure 1). The wide variations found in the literature for surface properties of materials can be attributed to the effect of these films. Presence of oxides on metal surfaces has been observed to produce a surface hardening effect. One explanation for this hardening is that the oxygen pins dislocations which emerge at the surface, impeding their mobility. Other surface films increase ductility. For example, water on alkali halide crystals will allow an otherwise brittle solid to deform plastically. This effect is also observed with ceramics. Magnesium oxide (MgO) is normally very brittle with a surface hardness in the clean state of about 750 kg/mm 2 . Figure 12 presents the hardness of MgO as a function of indentation time in dry toluene and moist air. The increased surface ductility in the presence of water is striking, and the difference increases with increasing indentation time. This change with time makes the film effect a true surface property and not simply a lubricating effect produced by the water. In the 1920s Rehbinder found that certain organic molecules on the surface of solids produced a softening. Such substances as oleic acid in vaseline oil were examined. This surface softening by lubricating substances can be very beneficial in certain instances such as in arresting the formation of fatigue cracks in bearing surfaces. REFERENCES Introduction 1. ASTM,Symposium on the properties of surfaces, ASTM Mater. Sci. Ser. 4, 1963. 2. SCI, Surface Phenomena of Metals, Monograph No. 28, Society of Chemical Industry, London, 1968. 3. Anon., Conference on clean surfaces, Ann. N.Y. Acad. Sci., 101, 583, 1963. 4. Adamson, A. W., Physical Chemistry of Surfaces, 2nd ed., Interscience, New York, 1967. 5. Gatos, H. C., Ed.,The Surface Chemistry of Metals and Semiconductors, John Wiley &Sons, New York, 1960. 6. Blakely, J. M., Ed., Surface Physics of Materials. Vols. 1 and 2, Academic Press, New York, 1975. Volume II29 FIGURE 12. Illustration of the effect of time on microhardness of MgO in tol- uene and in moist air (after Westbrook). 21 Copyright © 1983 CRC Press LLC Method of Characterization of Surfaces 7. Kane, P. F. and Larrabee, G. B., Eds., Characterization of Solid Surfaces, Plenum Press, New York, 1974. 8. Bunshah, R. F., Ed., Technique of Metals Research, Vol. 2, Techniques for the Direct Observation of Structure and Imperfections, Part 2. Interscience, New York, 1969. 9. Blakely, J. M., Ed., Surface Physics of Materials, Materials Science Series, Vols. 1 and 2, Academic Press, 1975. 10. Somoraji, G. A., Principles of Surface Chemistry, Prentice-Hall, Englewood Cliffs, N.J., 1972. 11. Proc. 2nd Int. Conf. on Solid Surfaces. II, Jpn. J. Appl. Phys., Suppl. 2, 1974. 12. Muller, E. W. and Tsong, T. T., Field Ion Microscopy, American Elsevier, New York, 1969. Properties of Surfaces 13. Ehrlich, G., Atomistics of metal surfaces, Surface Phenomena of Metals, Monograph No. 28, Society of Chemical Industry, London, 1968, 13. 14. Hayward, D. O. and Trapnell, B. M. W., Chemisorption, 2nd ed., Butterworths, Washington, D.C., 1964. 15. Ferrante, J. and Buckley, D. H., A review of surface segregation, adhesion and friction studies performed on copper-aluminum, copper-tin, and iron-aluminum alloys, ASLE Trans., 15(l), 18, January 1972. 16. Burke, J. J., Reed, N. L., and Weiss, V., Eds., Surfaces and Interfaces 11, Physical and Mechanical Properties. Syracuse University Press, New York, 1968. 17. Westwood, A. R. C. and Stolaff, N. S., Eds., Environment-Sensitive Mechanical Behavior, Metallurgical Society Conference, Vol. 35, Gordan and Bovach Science Publishers, New York, 1966. 18. Jenkins, A. D., Ed., Polymer Science, A Materials Science Handbook, Vols. 1 and 2, North-Holland, Amsterdam. 1972. 19. Buckley, D. H., Definition and Effect of Chemical Properties of Surfaces in Friction, Wear, and Lubrication, NASA TM-73806, National Aeronautics and Space Administration, Washington, D.C., 1978. 20. Likhtman, V. I., Rehbinder, P. A., and Karpenko, G. V., Effect of Surface-Active Media on the Deformation of Metals, Chemical Publishing Company, New York, 1960. 21. Westbrook, J. H., Ed., Mechanical Properties of Intermetallic Compounds, John Wiley & Sons, New York, 1960. 30 CRC Handbook of Lubrication Copyright © 1983 CRC Press LLC [...]... plane measurement of coefficient of friction COEFFICIENT OF FRICTION Measurement of Friction Measurement of the coefficient of friction involves two quantities, namely F, the force required to initiate and/or sustain sliding, and N, the normal force holding two surfaces together Some of the earliest measurements of the coefficient of friction were done by an arrangement of pulleys and weights as shown... rotations of the flat plate These variations may exceed 10 or 20% of the average force trace (particularly when there is stick-slip) and have often been explained in terms of the stochastic or statistical nature of friction Variations are usually largest when small values of N are used and are reduced at high values of N, where contact pressures approach the state of fully developed plastic flow Analysis of. .. adhesion theory of friction are Bowden and Tabor.1 An early model from this school began with the idea that the force of friction is the product of Ar, the summation of the microscopic areas of contact, and the shear strength, Ss, of the bond in that region; i.e., F = ArSs To complete the model, the load, N, was thought to be borne by the tips of asperities, altogether comprising a total area of contact,... pressure of contact, N = ArPf, where Pf is the average pressure of contact on the asperities Altogether, the coefficient of friction is taken as Copyright © 1983 CRC Press LLC 36 CRC Handbook of Lubrication Ss is usually approximately Y/2 where Y is the yield strength of the material in tension Pf is usually no more than 3Y Thus, the ratio Ss/Pf is about 1/6, which is not far from 0.2, a value often found... roughness of 10 µin Neither of the Tabor models or the interlocking theory explain the influence of close lateral proximity of asperities which imposes a limit on the high value of µ This is the case in metal working where there is high-contact pressure Copyright © 1983 CRC Press LLC Volume II FIGURE 1 37 String-pulley-weight measurement of coefficient of friction FIGURE 2 Tilting plane measurement of coefficient... LLC 34 CRC Handbook of Lubrication Table 1 COEFFICIENT OF ADHESION FOR VARIOUS METALS search suggests that real contact area between nominally flat surfaces increases more neary as the 0.8 power of applied load.7 Adhesion and Peeling In the above model of the elastic sphere pressing against an elastic flat plate, the radius and area of contact increase as the normal load increases As a matter of practical... coefficient of friction As technology developed, it became possible to measure the coefficient of friction to a Copyright © 1983 CRC Press LLC 38 CRC Handbook of Lubrication FIGURE 3 One-piece device for measuring pin-on-flat coefficient of friction Strain gages on flexible sections 1 and 2 measure normal force; strain gage at 3 measures friction force by bending of the beam high accuracy under a wide range of. .. of adhesion untenable The influence of a cycle of loading and unloading of a sphere on a flat plate with and without adhesion may be seen in the illustration of a rubber ball pressed against a rigid flat surface As each increment of load is added, a ring of larger diameter of contact forms between the ball and flat plate The reverse occurs upon progressive removal of the load If the flat surface were... such a high coefficient of friction, as tire rubber on most road surfaces Typical data are shown in Figure 712 for both dry roads and roads wetted by a moderate rainfall Temperature — There is usually little effect on the coefficient of friction of metals until Copyright © 1983 CRC Press LLC 40 CRC Handbook of Lubrication FIGURE 5 General effect of sliding speed on coefficient of friction for metals... shear stress in the sphere would produce a new increment of strain in the direction of the resultant of the initial normal force and the applied shear force Thus, the shear force causes a further normal strain in asperities with the effect of increasing the area of contact If adhesion increases in proportion to the area of contact, the area of contact will grow in proportion to the average shear stress . high-contact pressure. 36 CRC Handbook of Lubrication Copyright © 1983 CRC Press LLC COEFFICIENT OF FRICTION Measurement of Friction Measurement of the coefficient of friction involves two quantities,. 1960. 30 CRC Handbook of Lubrication Copyright © 1983 CRC Press LLC FRICTION K. C. Ludema DEFINITION OF FRICTION The usual engineering definition of friction is resistance to relative motion of contacting bodies bonding over 40CRC Handbook of Lubrication FIGURE 5.General effect of sliding speed on coefficient of friction for metals and other crystalline solids (e.g., ice). FIGURE 6.Influence of sliding speed