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Several years ago, the idea arose to write a general book on tribology. Students often requested a suitable book for the study of tribology and there were. FRICTION, WEAR, LUBRICATION A TEXTBOOK IN TRIBOLOGY K.C Ludema Professor of Mechanical Engineering The University of Michigan Ann Arbor. Industrial Significance of Tribology. 3. Origins and Significance of Micro/ Nanotribology. 4. Organization of the Book. 6. References.

An approximate value of Tg may also be marked on curves of damping loss energy loss during strain cycling versus temperature. The damping loss peaks are caused by morphologic transitions in the polymer. For example, PVC shows three peaks over a range of temperature. This transition is thought to be the point at which the free volume within the polymer becomes greater than 2. These take place at lower temperature and therefore at smaller free volume since the side chains require less free volume to move.

The glass—rubber transition is significant in separating rubbers from plastics: The glass transition temperature for polymers roughly correlates with the melting point of the crystalline phase of the polymer. The laboratory data for rubber have their counterpart in practice. For a rubber sphere the coefficient of restitution was found to vary with temperature, as shown in Figure 2.

The sphere is a golf ball. An example of visco-elastic transforms of friction data by the WLF equation can be shown with friction data from Grosch see Chapter 6 on polymer friction. See Problem Set question 2 f. In the polymers this behavior is attributed to dashpot-like behavior. In metals the reason is related to the motion of dislocations even at very low strains, i. Thus there is some energy lost with each cycle of straining.

Some typical numbers for materials are given in Table 2. Hardness indenters should be at least three times harder than the surfaces being indented in order to retain the shape of the indenter. Indenters for the harder materials are made of diamonds of various configurations, such as cones, pyramids, and other sharp shapes. Indenters for softer materials are often hardened steel spheres.

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Loads are applied to the indenters such that there is considerable plastic strain in ductile metals and significant amounts of plastic strain in ceramic materials. The size of indenter and load applied to an indenter are adjusted to achieve a compromise between measuring properties in small homogeneous regions e. For ceramic materials and metals, most hardness tests are static tests, though tests have also been developed to measure hardness at high strain rates referred to as dynamic hardness.

Hardness testing of these materials is done with a spring-loaded indenter the Shore systems, for example. Note that each system offers several combinations of indenter shapes and applied loads. This value changes with time so that it is necessary to report the time after first contact at which a hardness reading is taken.

Typical times are 10 seconds, 30 seconds, etc. Automobile tire rubbers have hardness of about 68 Shore D 10 s. Notice the stress states applied in a hardness test. With the sphere the substrate is mostly in compression, but the surface layer of the flat test specimen is stretched and has tension in it. Thus one sees ring cracks around circular indentations in brittle material. The substrate of that brittle material, however, usually plastically deforms, often more than would be expected in brittle materials.

In the case of the prismatic shape indenters, the faces of the indenters push materials apart as the indenter penetrates. Brittle material will crack at the apex of the polygonal indentation. This crack length is taken by some to indicate the brittleness, i.

See the section on Fracture Toughness later in this chapter. Hardness of minerals is measured in terms of relative scratch resistance rather than resistance to indentation. The Mohs Scale is the most prominent scratch hardness scale, and the hardnesses for several minerals are listed in Table 2.

See Problem Set question 2 g. Strictly, these stresses are not material properties, but they may influence apparent properties. Bars of heat-treated steel often contain tensile residual stresses just under the surface and compressive residual stress in the core. When such a bar is placed in a tensile tester, the applied tensile stresses add to the tensile residual stresses, causing fracture at a lower load than may be expected.

Compressive residual stresses are formed in a surface that has been shot peened, rolled, or burnished to shallow depths or milled off with a dull cutter. Tensile residual stresses are formed in a surface that has been heated above the recrystallization temperature and then cooled while the substrate remains unheated. Residual stresses imposed by any means will cause distortion of the entire part and have a significant effect on the fatigue life of solids.

See Problem Set question 2 h. Generally, stresses less than the yield point of the material are sufficient to cause fatigue fracture, but it may require between and cycles of strain to do so. Gear teeth, rolling element bearings, screws in artificial hip joints, and many other mechanical components fail by elastic fatigue.

If the applied cycling stress exceeds the yield point, as few as 10 cycles will cause fracture, as when a wire coat hanger is bent back and forth a few times. More cycles are required if the strains per cycle are small.

Failure due to cycling at stresses and strains above the yield point is often referred to as low-cycle fatigue or plastic fatigue. There is actually no sharp discontinuity between elastic behavior and plastic behavior of ductile materials dislocations move in both regimes though in high cycle or elastic fatigue, crack nucleation occurs late in the life of the part, whereas in lowcycle fatigue, cracks initiate quickly and propagation occupies a large fraction of part life.

There are several relationships between fatigue life and strain amplitude available in the literature. A convenient relationship is due to Manson in reference number 3 who suggested putting both high-cycle fatigue and low-cycle fatigue into one equation: The measuring of low-cycle fatigue properties is tedious and requires specialized equipment.

Part of the answer is seen in the observation that large structures are more likely to fail in a brittle manner than are small structures. Many materials do have the property, however, of being much less ductile or more brittle, to refer to the absence of a generally useful attribute at low temperatures than at higher temperatures. Furthermore, when high strain rates are imposed on materials as by impact loading, many materials fracture in a brittle manner.

It was to examine the latter property that impact tests were developed, such as the Charpy and Izod tests, for example. These tests measure a quantity somewhat related to area under the stress—strain curve i.

In ductile metals this was eventually found to be due to the influence of dislocation motion. However, dislocations do not move very far in glasses and other ceramic materials. The weakness in these materials was attributed to the existence of cracks, which propagate at low average stress in the body. Fracture mechanics began with these observations and focused on the influence of average stress fields, crack lengths, and crack shapes on crack propagation. Later it was found that the size of the body in which the crack s is are located also has an influence.

Studies in fracture mechanics and fracture toughness sometimes said to be the same, sometimes not are often done with a specimen of the shape shown in Figure 2. When rejoining of the crack walls restores the system to its original state, that energy per unit area is called the surface free energy. Another part of fracture mechanics consists of calculating the stresses at the tip of the crack. This is done in three separate modes of cracking, namely, Mode I where P is applied as shown in Figure 2.

K is not a stress concentration in the sense of a multiplying factor at a crack applied upon the average local stress. Rather, it is a multiplying factor that reflects the influence of the sizes of both the crack and the plate in which the crack is located.

Values of K have been calculated for many different geometries of cracks in plates, pipes, and other shapes, and these values may be found in handbooks. Cracking will occur where K approaches the critical value, Kc, which is a material property. The value of Kc is measured in a small specimen of very specific shape to represent the basic unmultiplied part size. In very brittle materials the value of K may be calculated from cracks at the apex of Vickers hardness indentations. The indenter is pyramidal in shape and produces a four-sided indentation as shown in Figure 2.

Cracks emanate from the four corners to a length of c. The value of Kc is calculated with the equation: The consequence of structure size may be seen in Figure 2. The stress required to initiate a crack is higher than the stress needed to propagate a crack: In ductile materials the crack tip is blunt and surrounded by a zone of plastic flow.

Typically, brittle ceramic materials have values of Kc of the order of 0. Adapted from Felbeck, D. However, as the crack in a large structure of steel begins to propagate faster, the plastic zone diminishes in size and amount of energy adsorbed diminishes. The crack accelerates, requiring still less energy to propagate, etc.

The calculations above refer to plane strain fields. For plane stress the calculated values will be one third those for plane strain. Correspondingly, Kc will be higher where there is plane stress than where there is plane strain. The wearing of material is also a response to applying stresses including chemical stresses. The mechanical stresses in sliding are very different from those imposed in standard mechanical tests, which is why few of the existing models for material wear adequately explain the physical observations of wear tests.

This may be seen by comparing the stress state in a flat plate, under a spherical slider with those in the tests for various material properties. Three locations under a spherical slider are identified by letters a, b, and c in the flat plate as shown in Figure 2. Possible Mohr circles for each point are shown in Figure 2. Note that location b in Figure 2. Circles d and e in Figure 2. Only the approximate axes with the shear and cleavage limits for two different material phases including locations of the Mohr circle for these tests are given.

Two observations may be made, namely, that the stresses imposed on material under a slider are very different from those in tensile and fracture toughness tests, and the stress state under a slider varies with time as well. The reader must imagine the mode of failure that will occur as each circle becomes larger due to increased stress.

It should be noted that the conclusions available from the Mohr circle alone are inadequate to explain the effects of plastic deformation versus brittle failure. The consequence of plastic flow in the strained material is to reconfigure the stress field, either by relieving the progression toward brittle failure, or perhaps by shifting the highest tensile stress field from one phase to another in a twophase system.

Further, plastic flow requires space for dislocations to move glide. Asperity junctions and grain sizes are of the order of 0. If local contact stress fields are not oriented for easy and lengthy dislocation glide, or for easy cross slip, that local material will fracture at a small strain, but may resist fracture as if it had a strength 10 to times that of the macroscopic yield strength.

An important property of material not included in Figure 2. Perhaps fatigue properties could be shown as a progressive reduction in one or both of the failure limits with cycles of strain. Ferry, J.

Bureau of Standards, J. Pushkar, A. Felbeck, D. Bonding between atoms may be described in terms of their electron structure.

In the current shell theory of electrons it would appear that the number of electrons with negative charge would balance the positive charge on the nucleus and there would be no net electrostatic attraction between atoms.

However, within clusters of atoms the valence electrons those in the outer shells take on two different duties. The average energy state of these delocalized electrons is lower than the energy state of valence electrons in single atoms, and this is the energy of bonding between atoms.

These energy states can be detected most readily by spectroscopic measurements. These are often referred to as cohesive bonding systems.

Those elements that readily conduct heat and electricity are referred to as metals. The valence electrons of metallic elements are not bound to specific nuclei as they are in ceramic and polymeric materials. The atoms are therefore not highly constrained to specific locations or bond angles relative to other atoms.

The Covalent Bond: When two or more atoms ions of the same charge share a pair of electrons such that they constitute a stable octet, they are referred to as covalently bonded atoms. For example, a hydrogen atom can bond to one other hydrogen or fluorine or chlorine etc. Some single atoms will have enough electrons to share with two or more other atoms and form a group of strongly attached atoms.

Oxygen and sulfur have two covalent bonds, nitrogen has three, carbon and silicon may have four. To dislodge covalently bonded atoms from their normal sites requires considerable energy, almost enough to separate evaporate the atoms completely. The bond angles are very specific in covalent solids. The carbon—carbon bond, as one covalent material, may produce a threedimensional array. In this array the bonds are very specific as to angle and length. This is why diamond is so hard and brittle.

When a single atom is brought down to a plane containing covalently bonded atoms, the single atom may receive either very little attention, or considerable attention depending on the exact site upon which it lands. The Ionic Bond: Some materials are held together by electrostatic attraction between positive and negative ions.

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Where the valence of the positive and negative ions is the same, there will be equal numbers of these bonded ions. Where, for example, the positive ion has a larger charge than do surrounding negative ions, several negative ions will surround the positive ion, consistent with available space between the ions.

Recall that the positive ion will usually be smaller than the negative ion. Actually, the ion pairs or clusters do not become isolated units. Ionic bonds are very strong.

They can accommodate only a little more linear and angular displacement than can the covalent bonds. Again, two surfaces of ionic materials may adhere with high strength, or a lower strength depending on the lattice alignment. Many atomic combinations cannot be accommodated to satisfy covalent or ionic bonding structures. Molecules are groups of atoms usually described by giving examples. Generally, crystalline and lamellar solids groups of atoms are not referred to as molecular.

Several different molecules may be made up of the same atoms, such as nitrogen, oxygen, or chlorine gases. Three types of hydrocarbon molecules are shown in the sketch below: These three molecules are based on the carbon atom. Carbon has four bonds which are represented by lines, the single line for the single strength bond and the double lines for the double strength bond.

Hydrogen has one bond and oxygen has two. Within the molecule, the atoms are firmly bonded together and are arranged with specific but compliant bond angles.

Actually, the molecules are not two dimensional, but rather each CH2 unit is rotated a certain amount relative to adjacent ones around the carbon bond. These molecules are not completely independent units, but rather are bonded together by the weak forces of all nearby resonating electrons.

Note that the center of positive charge in the acetone molecule coincides with the middle C atom, whereas the oxygen ion carries a negative charge. This separation of charge centers makes the acetone molecule a polar molecule. The other two molecules are nonpolar. The van der Waals Bonds: Attractive forces of atoms extend a distance of 3 or 4 times the radius of an atom, though the forces at this distance are weak. When atoms are assembled as molecules these forces are enhanced in proportion to the size of molecules, and enhanced further by any polarity that exists in some molecules.

In large molecular structures such as the polymers, these forces bind the molecules together and constitute a major part of the strength of the polymer material. The strength is much less than that of the ionic, covalent, and metallic bonds however.

The discussions on atomic bonding often focus on simple systems. In engineering practice, parts sliding against each other are often dissimilar. A brass sliding on steel, with no adsorbed layers present, might be expected to bond according to the rules of the metallic bond system, and similarly with the covalent, ionic, and van der Waals systems.

It is possible that solubility of one metal in the other may enhance adhesion and thereby influence friction and wear but not significantly at temperatures below two thirds of the MP in absolute units. Adhesion experiments with ceramic materials have not yielded high bond strength, probably because of the difficulty in matching lattices as perfectly as required. However, when two different ceramic materials are rubbed together, there is an increased probability that some fortuitous and adequate alignment of lattices occurs to form strong bonds.

Debris is also formed and these particles also bond to one or another of the sliding surfaces. Layers of debris sometimes form such compact films as to reduce the wear rate. Disparate Bonds: Again, when polyethylene is rubbed against clean glass or metal, a film of the polymer is left behind, indicating that the adhesive bond between the polymer film and glass or metal is about as strong as the cohesive bonds within the polymer.

In general some disparate systems might be expected to bond well because the surfaces of all materials have different structures and energy states than do the interiors. Where there is reasonable lattice matching there could then be high bond strength.

This is the subject of current research in materials science, and few guidelines are yet available. The net force, or energy, is usually described as the sum of two forces, an attraction force and a repulsion force. The force of attraction is related to the inverse of the square of the distance of the separation of the charges.

Atoms in a large three-dimensional array cannot be arranged with zero force between them. Rather, the nearest neighbors are too close and the next nearest are farther apart than the spacing which produces zero force.

Most metals are arranged in either the body-centered cubic, the face-centered cubic, or the hexagonal close-packed array. These three arrays are shown in Figures 3. Table 3. These and a few other arrays are also found in ionic and covalent materials.

Atomic arrangement in the body-centered and face-centered cubic lattice arrays. The cubic array is one of several ways to designate the position of atoms. For some purposes the unit cell uc is identified. The uc for the FCC array is composed of the atom in one corner plus the atoms in the center of adjacent faces.

For still other purposes a set of the cross-hatched planes is used to indicate the direction in which crystals will shear. Atoms on the octahedral planes are shown for two arrays.

The smallest distance between atom centers occurs across the body diagonal diagonally across the cross hatched plane in Figure 3. Thus the radius of the iron atom is 0. The size of atoms changes either when combined with atoms other than their own type, or when their neighbors are removed. The iron atom when combined with oxygen as FeO has a radius of 0. These are referred to as ion radii. The iron ion has a positive charge and is smaller than the atom.

A negative ion is larger than the same atom. The iron atom in the body-centered cubic form has eight neighbors. The atoms across the face diagonal are spaced most closely, producing an atom radius of 0. The FCC atoms have 12 near neighbors. See Problem Set question 3 a. Some of the defects are missing atoms, or perhaps excess atoms, singly or in local groups.

One type of defect is the dislocation in the crystalline order. The edge dislocation may be shown as an extra plane as shown in Figure 3. Orderly crystal structure exists above, below, and to the sides of the dislocation.

This process continues and the dislocation extra plane continues to move to the left. Much less shear stress is required for stepwise, single atom displacement than if all atoms were to be displaced at once, actually by about a factor of The presence of movable dislocations in metal makes them ductile. When the motion of dislocations is impeded by alloy atoms or by entanglement e. When there are no dislocations, as in a perfect crystal, or where dislocations are immobile as in a ceramic material, the material is brittle.

In Figure 3. A sufficiently high value of stress will simply separate planes of atoms. If this separation occurs along large areas of the simple crystallographic planes it is called cleavage.

Actually separation can occur along any average direction, still occurring along atomic planes. Cleavage strength and shear strength are seen as two independent properties of materials.

See Problem Set question 3 b. Out-of-plane adjustments are made to retain a structure that is somewhat compatible with the face-centered cubic substrate.

The higher state of energy of surface and near surface atoms is achieved by adding energy from outside to separate planes of atoms. That energy can be recovered by replacing the separated atoms, which is directly analogous to bringing magnets of opposite polarity into and out of contact. This process may not be totally irreversible if some irreversible deformation and defect generation has taken place. In the perfectly reversible process, the energy exchange is referred to as the surface free energy.

Where there is some irreversibility in the process, the new surface has increased its surface energy, some of which may be recovered by replacing the separated body, but not all.

The recovery of any amount of energy by replacing the separated body is the basis for adhesion. The surface of a solid has some unsatisfied bonds which can be satisfied by bringing any atom into the area of influence of the unsatisfied bond.

Adsorption is always accompanied by a decrease in surface energy. There are two classes of adsorption, namely, physical and chemical. Physical adsorption, involving van der Waals forces, is found to involve energies of the order of magnitude of that for the liquefaction of a gas, i.

Chemisorption involves an energy of activation of the order of chemical reactions, i. It is irreversible, or reversible with great difficulty. Actually, chemisorption involves two steps — physical adsorption followed by the combining of the adsorbate with substrate atoms to form a new compound. There are several theories and a number of isotherms indicating whether or not, and how vigorously, various adsorption processes may take place.

For this purpose one can also use handbook values of the heats of formation compounds formed from gases, as shown in Table 3. Copper nitride is not listed, so nitrogen very likely forms only a physically adsorbed layer.

The existence of attached gas and nonmetallic or intermetallic layers on solid surfaces is beyond dispute: Impinging atoms or molecules will readily attach or adsorb. The oxygen in the layer later forms oxide on metals. This complex layer shields or masks potentially high adhesion forces between contacting solids and significantly influences friction and wear.

The most mysterious characteristic of the literature on the mechanics of friction and wear is the near total absence in consideration of adsorbed films, in the face of overwhelming evidence of the ubiquitous nature of adsorbed films.

Perhaps the problem is that the films are invisible. The films do form very quickly. Following is a calculation to show how quickly a single layer forms. Begin with the assumption of Langmuir that only those molecules that strike a portion of the surface not already covered will remain attached; all others will reevaporate i.

The results are shown in the third column in Table 3. The second and successive layers adsorb more slowly depending on many factors.

Water adsorbs up to 2 to 3 monolayers on absolutely clean surfaces: Oxidation begins as quickly as adsorption occurs. Some experiments were done with annealed steel in a vacuum chamber, controlled to various pressures. The steel was fractured in tension, the two ends were held apart for various times, touched together, and then pulled apart again to measure readhesion strength.

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During the touching together, the relative amount of transmission of vibration at ultrasonic frequency through the partially reattached fractured ends was measured. The amount of exposure to gas bombardment is given in terms of Torr-sec. A load of 0. The adsorbed gas appeared to act as a liquid in these experiments. See Problem Set question 3 c. Several processes will be described. One of the more common surface making processes is done with a hard tool on metals which are usually softer than 40 Rc in lathes, milling machines, and drilling machines.

Steels as hard as 60 Rc can be cut with very hard tools such as cubic boron nitride. Material removal in a lathe is done by a tool moving usually from right to left while a cylinder rotates.

The finished surface is somewhat like a very shallow screw thread, depending on the rate of tool motion and the shape at the end of the tool. For some uses, this roughness of the cylinder along its length, i. For many uses, however, the roughness in the direction of cutting is more important, particularly when using tools designed to minimize the feed marks. The mechanics of cutting is usually represented as being done with a perfectly sharp tool edge. Such tools are difficult to make as is seen in the difficulty in getting very sharp points for use in scanning tunnel microscopes or field ion microscopes.

These dimensions are equivalent to to 60, atoms. The cutting action of conventional tools can best be visualized by observing the cutting of fairly brittle metal such as molybdenum. Figure 4. As the tool advances against the material to be removed it exerts a stress upon the material ahead of it. This crack also moves into a diminishing stress field and stops. The region below the cracks shows the shape of surface left by the crack sequences, which the heel of the tool alters further.

The sliding of the heel of the tool over a newly formed metal is a particularly severe form of sliding, producing very high friction. The tool burnishes the surface, pushing high regions downward, which causes valleys to rise by plastic flow. It shears the high regions so that tongues of metal become laps and folds lying over the lower regions.

The result is a very severely deformed surface region that is particularly vulnerable to corrosion. This severe deformation extends about 5R to 10R into the surface. The surface is rough, but the laps and folds are relatively easily removed by later sliding. This is one reason why new surfaces wear faster during first use and why surfaces need to be broken in.

The above illustration uses brittle properties of material initially to explain how cracks propagate ahead of the tool but suggests plastic behavior under the heel of the tool. The latter is reasonable in brittle material because the material under the tool has large compressive stress components imposed.

Initially ductile material does not fracture in the manner shown in Figure 4. Fracture is likely to follow the interfaces between two phases so that the resulting surface topography will be affected by the sizes of grain and phase regions.

Burnishing by the heel of the tool produces the same effect as described above. The burnishing action is severe, resulting in a hardening of the surface layer. Rolled sheet, plate, bar, et al. Hot rolling of metal is done at temperatures well above the recrystallization temperature and usually results in a surface covered by oxide and pock marks where oxides had been pressed into the metal and then fallen off.

Cold rolling is usually done after thick scales of oxides are pickled off in an acid. It produces a smoother surface. There is some slip between rollers and sheet, which roughens the sheet surface, but this effect can be reduced by good lubrication. Extrusion and Drawing: These processes can also be done hot or cold. The effect of oxides is the same as in rolling although the billets for extrusion and drawing are often heated in nonoxidizing atmospheres to reduce these effects.

In any case, sliding of the deforming metal, polymer, and unsintered ceramic materials against hard dies usually steel will produce very rough surfaces unless the process is well lubricated. Most cold-forming processes leave the surface of the processed part strained more in shear than the substrate has been strained. This produces surface hardening, but more important it produces compressive residual stresses in the surface with tensile residual stresses in the deeper substrate.

See the section titled Residual Stress in Chapter 2. Electrospark Erosion: This process applicable mostly to metals melts a small region of the surface and washes some molten metal away. Just below the melt region the metal goes through a cycle of heating and cooling, leaving that region in a state of tensile residual stress. See Residual Stress, Chapter 2. Grinding and Other Abrasive Operations: Removal of material by abrasive operations involves the same mechanics as in cutting with a hard tool.

The major difference is the scale size of damage and plastic working. The abrasive particles grit in grinding wheels, hones, and abrasive paper are small but rounded primarily, and they produce grooves on surfaces. The abrasive particles cut remove very little material but they plastically deform the surface severely, as may be seen by the fact that abrasive operations require between 5 and 10 times more energy to remove a unit of material than do operations using a hard tool.

Abrasive operations leave surfaces somewhat rough and severely cold-worked with residual stresses. Cold operations produce compressive residual stresses, but high severity grinding can produce tensile residual stresses. See Problem Set questions 4 a and b. In cutting and grinding, the deformation comes from the fact that the cutting edges of tools and abrasive particles are rounded rather than perfectly sharp. Localized heating and cooling, as in grinding, can produce tensile stresses.

An example of the intensity of these stresses can be seen in Figure 4. Adapted from Koster, W. The crystallographic structure of the metal is hidden by a layer of severely deformed metal. The structure of polished surfaces was studied by Sir George Beilby. He therefore suggested that this layer might be amorphous, and it became known as the Beilby Layer. Later work showed that this layer consists of very fine crystallites probably including embedded polishing compound and reaction products, and is not amorphous at all.

Its thickness is defined by the process used to form it. Beneath the very severely deformed region are gradations of less deformed material. These states of deformation are illustrated at the end of this section. Above the solid surface yet another phenomenon occurs, namely, oxidation and adsorption. See Problem Set question 4 c. The practical range of roughness of commercial surfaces is given in Table 4. Table 4. Atomic models lose their significance in the face of such great roughnesses.

However, since the majority of surfaces that come into contact with each other have relatively rough surfaces, we shall spend most of our time with such surfaces. From various sources we may estimate the thickness of various layers on a tool-cut surface that has been exposed to atmosphere for a day or two. This indicates the early growth rate of oxides. In contrast, Figure 4. Koster, W. Bowden F.

The pressure on those points is therefore very high. We can make some assumptions about this area of contact if we make some assumptions about the nature of asperities.

The first looks briefly at lubricant and bearing material properties. Then the classical regimes of lubrication from hydrodynamics to boundary lubrication are described. The last few chapters examine a few specific practieal aspects of engineering tribology, such as seals, metal working and rolling element beatings. The book ends with a series of "problems" which, no doubt have entertained students of the Engineering Department of the University of Cambridge as strongly as they will entrance the reader.

The field that this book encompasses is very broad and this is both a strength and a weakness. Its strength is that it means ElsevierScienceS,A, that Engineering Tribology is a useful compendium for the graduate engineer since it contains an introduction to many of the tribologieal problems which he or she may encounter in professional life.

Its weakness is that each topic can, necessarily, be tackled only cursorily and that the author cannot bring to all areas the same originality and flair.

This means that whilst some parts of the book, notably the last few chapters, are thorough and state of the'art, others are slightly old fashioned and dull.

This is probably an unjust criticism since I suspect that the field of Tribology has grown so large that the subject can no longer be comprehensively and lovingly covered by one person in a single book, as in the halcyon days of Cameron's Principles of Lubrication. Tools Get online access For authors. Email or Customer ID. Forgot password? Old Password. New Password. Your password has been changed.

Returning user. Request Username Can't sign in?The other stresses can be plotted as shown in Figure 2. Their effects could be considered those of lubrication, though to formalize concepts in this topic it would be necessary to characterize the thin films in terms of their thickness and viscosities. These bond systems are described in Chapter 3.

Friction and wear usually cost money, in the form of energy loss and material loss, as well as in the social system using the mechanical devices.

Impinging atoms or molecules will readily attach or adsorb.

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