Chapter 1

Introduction

1 Introduction

Friction is a naturally occurring resistance that opposes and hinders the accomplishment ofany activity. The phenomenon of friction is an intimate component of natural and tech-nological systems as it derives from the second law of thermodynamics that requires thepartial degradation of energy into heat every time there is a transformation of energy fromone form to another. In mechanical systems, friction is always encountered when two solidsare brought into close contact and then made to move relative to each other. Althoughfriction is not only useful but essential in some instances, in many cases it is undesirable asit results in waste. The waste associated with friction consists not only of the fraction of theenergy that gets transformed into heat but also of the material wastage that is created bythe process of wear. Wear is the gradual removal of material from a surface due to frictionalinteractions. Lubrication is the use of a third material to a friction couple with the purposeof mitigating the friction and wear interactions that would occur between two solids in theabsence of the lubricant. The discipline that studies the phenomena of friction, wear andlubrication is called Tribology.

2 Tribology

Tribology is the study of the thermal, mechanical and chemical interactions that occur whensolid surfaces in or near contact move in relation with each other. A typical tribological sys-tem consists of three participating materials, namely, two solids in partial mechanical contactand a third material that can be a fluid or a solid, located in the intervening space betweenthe contacting solids. Because of the multiplicity of processes encountered in tribologicalsystems, and the many size scales involved, Tribology is necessarily an interdisciplinary areaof study. Insights from the following disciplines are often essential for an understanding oftribological problems:

Physics and Chemistry

Materials Science and Engineering

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Surface Science and Engineering

Continuum (Solid and Fluid) Thermo-Mechanics

Concepts from Physics and Chemistry that shed light on observed tribological behaviorinclude considerations of atomic structure and atomic bonding, chemical reactivity and thephysical chemistry of surfaces and interfaces.

From the science and engineering of materials, concepts such as crystallographic struc-ture, crystalline defects, material microstructure (grains, grain boundaries, phases) providea useful framework for analysis at the micro and nanoscales. Moreover, the ability to ma-nipulate material microstructures (and properties) through processing provides a useful toolto understand and control tribological system.

The scientific and engineering study of surfaces is key since the solution of many tribo-logical problems requires an understanding of fundamental surface properties such as surfacestructure and surface roughness. One must be familiar with standard measurement and char-acterization techniques and also recognize that manufactured components carry with themon their surfaces characteristic imprints created by the specific manufacturing process usedin their production. In addition, adhesion and capillary phenomena can be critical in someapplications. Significant achievements in tribology have been made using the technology ofsurface coating for surface modification purposes.

Continuum Thermo-Mechanics is the study of deformation and flow of materials due toapplied forces and also of the transport of thermal energy due to heat fluxes and temperaturegradients. By disregarding the discrete, atomic nature of matter and using instead the notionof a continuous medium, the tools and methods of continuum thermo-mechanics have beenused to obtain solutions to many problems in nature, science and technology. Fundamentalequations derived from conservation principles provide a general foundation. Problems areeffectively tackled once the governing equations are coupled to suitable constitutive equationsof materials behavior and the resulting set is solved subject to selected initial and boundaryconditions. In the case of solid materials, constitutive equations are used to describe linearelastic, plastic, viscoelastic and viscoplastic deformation behavior, while Newtonian and non-Newtonian constitutive equations are used to describe the flow of fluids. Since a recurringfeature of many tribological systems is the existence of partial contact between the solids,localized stress intensification in the vicinity of contacting asperities is to be universallyexpected. The widespread use of lubricants in technology can be rationalized by investigatingthe flow behavior of lubricating fluids in the narrow gap between partially contacting solids.In fact, lubricant layers can be designed so as to completely hinder the two surfaces fromcontacting each other. The study of fluid dynamics is key for an understanding of lubricantaction. Since friction, wear and lubrication involve energy degradation, thermal effects are tobe expected in tribological systems. Thermal energy is generated in the vicinity of asperitycontacts. Heat is also produced by shearing in the interior of both solid and fluid materials.To understand the transport of thermal energy in tribological systems a heat transfer analysisis required. In general, solid deformation, fluid flow and heat transfer are often all involved

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in tribological systems and full understanding from the continuum point of view requiresconsideration of all three processes.

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