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Über dieses Buch

At the conclusion of the Conference on Tribology in the area of Wear Life Pre­ diction of Mechanical Components, which was held at the General Motors Research Laboratories and sponsored by the Industrial Research Institute, a very high pri­ ority recommendation was modeling of tribological systems. Since the appearance of the Conference Proceedings in 1985, the writers discussed the matter of modeling with Dr. Edward A. Saibel, Chief of the Solid Mechanics Branch, Engineering Sciences Division, U.S. Army Research Office. This discussion led to a proposal for the Workshop which resulted in this volume. The choice of proposal and Workshop name turned out to be more restricted than it needed to be. As such, the Workshop adopted the name for this volume, Approaches to Modeling of Friction and Wear. By design, the attendance was restricted to not more than 40 individuals so as to allow small group discussions. There were four panels which deliberated on the same questions after two invited area lectures. Section 1 contains the substance of the two lectures. Section 2 is the Workshop Summary which is a distillation of the four panel reports by the entire Workshop attendance. This was formally written up and edited by the eight panel session chairmen, i.e., each of the four panels met twice on two different questions under the leadership of a chairman for each session. Section 3 contains four brief position papers on the subject of the Workshop.



Invited Area Lectures


1.1. Friction and Wear from the Materials Science Vantage Point

Tribology, by its very nature, is a multidisciplinary subject which incorporates specialists in many different areas. However, the study of tribology has often polarized along set disciplinary lines, with each discipline rigidly adhering to its own terminology and research interests. Older explanations of friction and wear behavior have emphasized adhesive interactions of surface asperities. This approach has led to an emphasis on surface chemistry and on continuum mechanics, with much less attention given to the structures of the interacting materials themselves.
Gregory E. Vignoul

1.2. Modeling of Friction and Wear Phenomena

Modeling friction and wear is a challenge for the future, yet design engineers require that information now. Such information must be made available because of the considerable impact it will have on today’s technology. The problem is to decide whether it will be obtained with the help of Tribologists or in spite of them. If Tribologists, concerned with dry friction problems and wear, acknowledge this need, they can still play a part in introducing their understanding in the modeling [1]. If not computer codes will continue, as they do now, to offer formulae chosen without discrimination, but which, because of their very existence, will be used in industry at large setting a trend which will take years to reverse.
Maurice Godet

Workshop Summary


2. Workshop Summary

This summary represents the consolidated views of four panels during eight panel sessions, see APPENDIX A for the panels’ membership and chairmanship for Sessions I and II. Session I of all panels addressed the question of modeling from the materials science vantage point. Session II of all panels addressed the question of modeling of friction and wear in general.
F. F. Ling, C. H. T. Pan

Contributed Brief Position Papers


3.1. Tribology Modeling

The first step in approaching the analytical modeling of tribology requires that at least two questions be asked:
What is the end objective of the modeling?
Where are advances in tribology most needed?
Donald G. Flom

3.2. Modelling of Dry and Boundary Lubricated Contacts As Seen From The Hydromicist’s Perspective

A commonly held view among those reasonably familiar with tribology is that the modeling of a hydrodynamic thick-film contact, and even that of an elastohydrodynamic contact, is a “closed book”; whereas the modeling of a much thinner film, either boundary lubricated or nonlubricated, is effectively impossible.
John A. Tichy

3.3. Assessing Research Needs in Tribology

The whole picture of lubrication technology looks somewhat like a jigsaw puzzle which has pieces tabulated in Table I. For a given machine we wish to know its expected service life for a given lubricant as a simple example. This picture can get very complicated; for economical and/or critical operations, many factors must be considered and understood. Some of them will be mentioned here.
Julian J. Wu

3.4. Amontons and Coulomb, Friction’s Founding Fathers

As is now generally known, the idea that frictional force is proportional to normal force stems from a paper by Guillaume Amontons [1], published in the Mémoires de Mathématique et de Physique of the Académie Royale des Sciences in Paris for 1699, in the very first volume following renewal of the Academy’s royal charter. Presented on December 19 of that year, it was entitled “On the resistance caused in machines, by the friction of their component parts as well as by stiffness of the cords used there, and the way to calculate the one and the other.” I have inspected the paper in a second edition of the Mémoires published in Amsterdam in 1734 [2]. But rather than giving you my translation, I prefer to preserve some of the flavor of that time by quoting the translation in the abridgement of the Mémoires published in 1742 by two English academics [3].
Herbert Deresiewicz

Contributed Brief Scientific Papers


4.1. On Use of Surface Deformation Models to Predict Tribological Behavior

Friction and, to a greater extent, wear and surface damage from strong intersurface adhesions or plastic flow depend critically on deformation (of bearing materials). These manifestations of tribological behavior also depend on the other physical processes and on chemical processes and how these interact with system variables (including operating variables) and among themselves. Thus, deformation models can be useful for explaining, predicting or correlating tribological behavior only as a part of a model including the other significant processes for a particular wear regime(s).
Richard S. Fein

4.2. Modelling Tribochemistry

The influence of chemistry on the sliding and rolling wear of materials has been known for a long time [1]. This influence is exploited in the formulation of lubricants where anti-wear additives react to deposit protective coatings on the rubbing surfaces and extreme pressure (E.P.) additives prevent the seizing of gears by controlled corrosion that prevents the welding of metallic contacts. Chemistry also manifests itself in fretting wear of bolted junctions where low amplitude rubbing causes accelerated oxidation, in the strong influence of environment on the wear of ceramics and in oxidative wear. A common feature of all these phenomena is the much higher rate of chemical reaction on the rubbing surfaces than on the adjacent areas of the same pieces of material. This interaction of friction with chemical reaction is called tribochemistry and is the subject of this analysis.
Traugott E. Fischer

4.3. Role of Nanostructure of Adsorbed Layers in Lubrication

Surfactants and polymers added to boundary lubrication systems adsorb on solid surfaces and form a protective film. Effectiveness of boundary lubricants has often been attributed to the adsorption affinity and the integrity of the adsorbed film. Such adsorption is influenced by additives introduced into the system to reduce thermal degradation, corrosion, sludge formation, foaming etc. There are many interactions that can take place between the additives and the surfactants and the base oil leading in addition to adsorption effects, a number of interfacial and colloidal phenomena such as micellization, precipitation and solubilization as well as flocculation of particulate matter in the fluid [1]. Clearly the adsorption and integrity of the adsorbed film will depend on the molecular geometry of the interacting species. For example, among various saturated mono-, bi-, and tricyclic aromatics, alkanes, and polyethers tested as lubricating fluids by Hentschel [2], the “irregularly shaped molecules” yielded high traction coefficients whereas molecules with a “thread-like shape and minimum structural subunits or functional groups” yielded low coefficients. Evidently ability of molecules to undergo intramolecular as well as intermolecular motions is important in maintaining the integrity of the lubricating films.
Ponisseril Somasundaran

4.4. A Proposed Model for The Development of High Temperature Fluids for Lubrication of Ceramics

The following discussion will attempt to outline a plan for overcoming at least one current impasse in the utilization of engineering ceramics in high temperature sliding contacts and illustrate the possible role of the synthetic chemist coupled with other disciplines in designing new and effective tribological systems.
David A. Dalman

4.5. Simple Model of Metalworking Friction Under Extreme Pressure

This model was first introduced in the late 1960’s [1, 2]. We are revisiting it because of its simplicity on the one hand and for what questions it was able to answer on the other hand. During the mid 1960’s, the Materials Advisory Board of the National Research Council asked two questions regarding metalworking friction under high pressure. The prevailing understanding at the time was that friction under high pressure should have an upper limit in the shear strength of the metal being worked, as calculated from simple tensile test data [3]. The first question was why the aforementioned was often not borne out by experience.
Frederick F. Ling, Marshall B. Peterson

4.6. Use of Cutting Force in Disciplining Relations Between Abrasive Wear and Mechanical Properties

One of the simplest wear processes is two-body abrasion of ductile metals by hard abrasives, as in the case when a metal sample slides over an abrasive paper. In this situation, metal removal is by surface deformation and micromachining, and it would appear relatively simple to model the process from machining theory and mechanical behavior at large strains and strain rates. This has not yet happened, however, in any quantitative sense, although reasonably good quantitative explanations can be developed for most types of observed behavior.
Jorn Larsen-Basse

4.7. Large Plastic Deformation in Sliding Friction and Wear

It is well established that sliding friction involves large amounts of near- surface plastic deformation [1]. The deformation is caused by frictional tractions at the contact interface and is an integral part of the friction process, at least for ductile materials [2]. Microscopic examination of the highly deformed near- surface regions has shown that the plastic deformation is also responsible for most sliding wear through such mechanisms as void formation, crack nucleation and propagation, extrusion and other chip or particle formation processes [3,4]. It is therefore imperative that any models of friction and wear processes account for large near-surface plasticity effects.
Francis E. Kennedy

4.8. Friction with Solid Lubricant Films

A number of models have been proposed to describe solid film lubrication [1]. One of these, and the most traditional, suggests simple shear or continuous plastic deformation of an oriented film on the surface. This hypothesis is evaluated in this brief.
Marshall B. Peterson, Mike Kanakia

4.9. On The Role of Adhesion in the Wear Process

Tribology research, like that in many other scientific fields, depends upon progress in several different disciplines. Hence, progress in the understanding of friction and wear behavior is somewhat cyclical in nature. Based on limited information, theories are advanced which often cannot be proven or disproved with the experimental capabilities available at the time. With the arrival of each new generation of analytical tools, it is often possible to test existing theories and provide a new framework for understanding fundamental phenomena. Similarly, advances in one discipline can profoundly affect the progress in another.
James J. Wert

4.10. Possible Relationship Between Partial-Elastohydrodynamic Lubrication and Wear Modelling

In most lubricated contacts, the lubrication mode is partially elastohydrodynamic, that is, a significant portion of the load is carried by the asperity contacts. In this regime, wear phenomenon is influenced by how the asperities are protected by the lubricant or surface film during sliding.
Herbert S. Cheng

4.11. Sliding Systems With No Wear

There are certain systems in which the primary function of sliding contacts is not the transmission of motion or work. For example, the purpose of a sliding electrical contact is to provide a low resistance electrical path between two bodies in relative motion. The contact between the magnetic head and a floppy disk or a video cassette recorder tape is necessary to maximize the magnetic field stored in the magnetic domains in the coating on the disk or tape. In these systems, the normal load at the contacts is minimized within the other constraints of each system. One purpose for minimizing the normal load is to minimize wear.
Norman S. Eiss

4.12. Comparison of Wear Chip Morphology with Different Models of “Adhesive” Wear

At least six basic models of wear chip formation in unlubricated sliding between ductile materials exist, as indicated in Fig. 1. These are (a) breaking or shearing-off pieces of the softer material where asperities of the two sides collide (Fig. 1A); (b) shearing-off flakes of the softer material where local adhesion has occurred at contact spots (Fig. IB); (c) plucking out fragments of the softer material where strong adhesion, such as through cold-welding, has created a firm bond between the two sides at contact spots (Fig. 1C); (d) shearing-off of flakes wherein the relative rest between the two sides at the contact spots is not due to adhesive forces but to mechanical micro-roughness interlocking (Fig. ID); (e) flaking of a debris layer at contact spots (Fig. IE); (f) “Delamination” of flakes through sub-surface cracks parallel to the interface which spread through repeated passes of asperities (Fig. IF). “Wedge” or “Prow” formation is believed to occur through repeated action according to model (b).
Yu Jun Chang, Doris Kuhlmann-Wilsdorf

4.13. A Proposed Thermomechanical Wear Theory

The transmission of load and motion in a mechanical system rely on the relative contact movement between the tribological elements. However, tribological contacts induce surface tractions and wear takes place. Although wear processes in tribocontacts are still not fully understood, a major cause of wear is the interaction between the asperities of the contact surfaces. Various types of wear mechanisms have been considered [1, 2]. Adhesion, abrasion, fatigue and corrosion of the material are some of the important wear mechanisms.
Bond-Yen Ting, Ward O. Winer

4.14. Predictive Models for Sliding Wear

Wear is defined as “damage to a solid surface, generally involving progressive loss of material due to relative motion between that surface and a contacting substance or substances” [1]. Examination of worn machine elements indicates that the wear process is rather complex and can occur by various mechanisms [2]. Wear can be regarded as the result of the surface being stressed mechanically, thermally, chemically, or electrically. These processes may occur independently or simultaneously to cause material removal. When two surfaces are rubbed together, in the absence of any foreign abrasive particles, the wear process is classified as “sliding wear.” In sliding wear, various processes can operate to generate wear particles.
Said Jahanmir

4.15. Surface Deformation Considerations for Rolling with Incipient Sliding

Attention is focused on the material’s response to stress (normal and tangential) in connection with the modelling of material removal by wear as well as the resulting frictional force to produce sliding. Tribological models are confined generally to relative motion in pure sliding where the surface material of one body is in continual contact with the passing of the surface material of another body. The case of combined rolling and sliding (particularly incipient sliding), while important from a practical perspective, may also be very useful for tribological model development.
Lavern D. Wedeven

4.16. Some Thermal Implications on the Life of High Speed Rolling Element Bearings

Within Rolls-Royce, the subject of rolling contact fatigue and lubricated wear have been topics of interest for many years. Gear scuffing and roller bearing slip/wear were driving problems during the 1960’s, with Smith [1] investigating the parameters contributing to wear in high speed gas turbine roller bearings. With the lessening of our interest in geared transmission systems during the 1970’s, the gear work lost its priority, and solutions were found to the roller bearing slip/wear problems. At the same time, the possibility of fracture failure modes in rolling element bearings resulting from higher race hoop stresses became apparent.
Richard Nicholson

4.17. Profilometric Roughness and Contact Fatigue

Contact fatigue life prediction models have existed for half a century [1]. Recent refinements take into account the microstresses superimposed on the Hertz contact stress field as a result of roughness in the two contacting surfaces [2, 3, 4].These latter efforts are characterized by the use of some simple asperity geometry (rounded-top prisms in [2] and Greenwood-Williamson model using Hertzian second-order) asperity shapes in [4]), to calculate microstress fields under asperities. Given these stress fields, the models predict fatigue life (in cycles) by integration, over the stressed volume, of a relationship of the form:
$$ - \ell n(dS) = C{t^a}{z^b}{N^c}dV$$
where C is a constant, t a stress, z a linear dimension, dV a volume elements and dS a local survival probability, a, b and c are (positive or negative) numeric exponents.
Tibor E. Tallian

4.18. Two-Dimensional Dynamics of Coulomb Friction

The foundation of our ideas about dry friction were put forth by Amontons (1699) and Coulomb (1785), who postulated that the primary phenomenon in dry friction is the interaction of rough surfaces. Recently Villagio [1] argued that if the contacting bodies deform elastically, then Coulomb’s or Amontons’ law should not be accepted as a postulate but as a consequence of both the topology and the elastic deformation of the contact surfaces. These basis considerations are the subjects of several current investigations with potential near-future applications.
Iradj G. Tadjbakhsh

4.19. Microscopic Mechanisms of Tribological and Wear Processes: Molecular Dynamics Simulations

Although tribological phenomena are an everyday experience and have been observed and studied for a very long time [1], a detailed microscopic theory of tribological phenomena is lacking. Nevertheless a large body of empirical data has been collected and some phenomenological models have been developed [2]. The major reasons, from a theorist’s perspective, for this state of affairs may be attributed to: (i) the complexity of the phenomena, which involve processes occurring in materials under stress, the generation and propagation of defects, large structural deformations and properties of systems beyond the elastic regime, (ii) the nature of available experimental data, which until fairly recently was not obtained under controlled conditions on compositionally and structurally well characterized samples. Recent advances in computer simulation methods [3] and the advent and application of surface science experimental probes provide the impetus for the development of fundamental models, on the atomic scale, of tribological and wear phenomena. In this note we outline our recent investigations in this area using molecular-dynamics (MD) simulations, and demonstrate the potential of such investigations towards the development of a microscopic understanding of the physical processes which underly these phenomena.
Martin W. Ribarsky, Uzi Landman

4.20. On Fractal Dimension of Engineering Surfaces

One of the most important properties of machine elements is their surface roughness. As such, surface roughness profiles, and topography in general, have been measured. Various instruments have been developed for such measurements.
Frederick F. Ling, Salvadore J. Calabrese
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