For situations where high friction is not explicitly needed, lubricants are used to reduce friction and wear to acceptable levels. Lubricants function mainly by introducing a layer of solid or liquid material with low shear strength between two sliding surfaces. This chapter covers the basic regimes of lubrication: hydrostatic, hydrodynamic, elastohydrodynamic, mixed, and boundary. Viscosity is the most important physical parameter describing a lubricant, and it is thoroughly discussed in this chapter. Slippage of lubricants and other liquids against solid surfaces is also discussed. The chapter also discusses the basic mechanisms and types of bearings that provide hydrodynamic and elastohydrodynamic lubrication.

Friction, lubrication, adhesion, and wear are prevalent physical phenomena in everyday life and in many key technologies. The goal of this book is to incorporate a bottom up approach to friction, lubrication, and wear into a versatile textbook on tribology. This is done by focusing on how these tribological phenomena occur on the small scale—the atomic to the micrometer scale—a field often called nanotribology. The book covers the microscopic origins of the common tribological concepts: roughness, elasticity, plasticity, friction coefficients, and wear coefficients. Some macroscale concepts (like elasticity) scale down well to the micro- and atomic scale, while other macroscale concepts (like hydrodynamic lubrication) do not. In addition, this book also has chapters on topics not typically found in tribology texts: surface energy, surface forces, lubrication in confined spaces, and the atomistic origins of friction and wear. These chapters covered tribological concepts that become increasingly important at the small scale: capillary condensation, disjoining pressure, contact electrification, molecular slippage at interfaces, atomic scale stick-slip, and bond breaking. Numerous examples are provided throughout the book on how a nanoscale understanding of tribological phenomena is essential to the proper engineering of important new technologies such as MEMS, disk drives, and nanoimprinting. For the second edition, all the chapters have been revised and updated, with many new sections added to incorporate the most recent advancements in nanoscale tribology. Another important enhancement to the second edition is the addition of problem sets at the end of each chapter.

This chapter discusses the interesting phenomena that happen when the thickness of a lubricant film is reduced to nanoscale dimensions. For liquid lubricants sandwiched between two solid surfaces, the interesting phenomena associated with confined liquids include: molecules forming a layered structure, enhanced viscosity, and solidification. In boundary lubrication, an adsorbed monolayer resists penetration of contacting asperities and sliding takes place over the low shear strength surface of the boundary lubricant. The absence of boundary lubrication can lead to cold welding where adhesion at the interface leads to ultra-high friction and seizure. The last part of this chapter discusses how capillary and disjoining pressures lead to the formation of lubricant menisci around contacting asperities from a thin lubricant film on one of the surfaces and how these menisci influence adhesion and friction. The kinetics of meniscus formation from capillary condensation and its impact on friction are also discussed.

This chapter introduces friction as it manifests itself in everyday life. The chapter begins with Amontons’ law (1699) that friction is proportional to the loading force between contacting surfaces (the proportionality constant is called the coefficient of friction). The two primary mechanisms for unlubricated friction are adhesive friction and plowing friction, with the predominant mechanism generally being adhesive friction. Adhesive friction is proportional to the real area of contact; for rough surfaces, this contact area is proportional to the loading force, providing a physical underpinning for Amontons’ law. Processes like the nanoscale flow of atoms and molecules around contact points results in the force needed to induce sliding (static friction) being higher than the force needed to maintain sliding (kinetic friction). Friction decreasing with increasing velocity leads stick-slip motion of the sliding surfaces, where the slip distance can be as short as the distance between atoms.

This chapter outlines common mechanisms that contribute to wear, which is broadly defined to be any form of surface damage caused by rubbing one surface against another. Such wear mechanisms include delamination wear, adhesive wear (where adhesion followed by plastic shearing plucks the ends off the softer asperities, typically described by Archard’s law), abrasive wear (where hard particles or asperities gouge a surface and displace material), and oxidative wear (where surfaces react with atmospheric oxygen prior to being worn). Sliding conditions often determine which wear mechanism dominates, with the main factors being temperature, sliding velocity, oxidation, plasticity, loading force, and mechanical stresses. How wear rates respond to changes to these factors can be diagramed on a wear map. The last part of the chapter discusses how transition state theory can describe nanoscale wear by atomic attrition, and how plasticity and fracture occur at the nanoscale.