Wear

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.

Atomistic Origins of Friction

This chapter covers the current state of knowledge about how the shear strength (the force needed to slide one surface over another) originates at the atomic level. For adhesive friction, friction originates from the forces needed to move the atoms on one surface over the atomic structure of the opposing surface; the simplest model for adhesive friction is the cobblestone model. The Frenkel–Kontorova model, the Prandtl–Tomlinson model, and molecular dynamic simulations are typically used to show how the atomic structure of the surfaces leads to static friction. One exciting aspect of these friction models is the prediction of superlubricity or negligible friction for incommensurate sliding surfaces, which is now being realized in experiments. Also discussed is why superlubricity is not observed in real-life situations. As atoms and molecules slide over surfaces, kinetic friction originates from phonon and electronic excitations, which are typically studied using the quartz crystal microbalance (QCM).

Surface Roughness

When two surfaces are brought into contact, they first touch where the summits of the surface asperities make contact. Consequently, surface roughness or topography strongly influences those physical phenomena associated with contact: friction, adhesion, and wear. This chapter discusses techniques for measuring the roughness of surfaces and the parameters frequently used to characterize this roughness. As atomic force microscopy (AFM) and optical interferometry are currently the predominant tools for characterizing roughness, these techniques are discussed at some length. Examples are given for determining not only the standard roughness parameters (the standard deviation of surface heights, the mean radius of curvature of asperity summits, waviness, and the average and rms of surface heights), but also for determining the surface roughness power spectrum, which has gained importance in recent tribology theories. The topography of self-affine fractal surfaces is also discussed along with the tribological importance of these surfaces.

Lubrication in Tight Spots

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.

Lubrication

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.

Surface Energy and Capillary Pressure

The energies associated with surfaces—surface energy, interfacial energy, surface tension, and work of adhesion—drive many surface and interfacial phenomena including tribological ones such as adhesion and friction. This chapter discusses the physical origins of surface energies for liquids and solids, and how the concepts of capillary pressure, capillary condensation, wetting, and work of adhesion are derived from surface energy. Further, this chapter covers the different methods for measuring surface energies, including the most common method for solid surfaces: contact angle measurements of liquid droplets on surfaces. This chapter also introduces how surface energies and surface tensions lead to adhesion and adhesion hysteresis between contacting surfaces, which is followed up in the subsequent chapters on surface forces.

Mechanical Properties of Solids and Real Area of Contact

This chapter covers how the material around contacting asperities deforms when two surfaces touch and the resulting stresses in the materials. The beginning portion of the chapter is devoted to how these stresses and deformations originate at the atomic level. Next, discussed are the elastic and plastic deformations that can occur when a single asperity contacts a flat surface, such as a Hertzian contact. The discussion of plasticity leads naturally to the discussion of hardness. A major portion of this chapter is devoted to estimating the solid–solid contact area between rough contacting surfaces, which can be due to both elastic and plastic deformations. This discussion of contact area is centered around the Greenwood and Williamson model and the Persson theory of the contact mechanics of rough surfaces.

Measuring Surface Forces

This chapter focuses on the two experimental techniques—the surface force apparatus (SFA) and the atomic force microscope (AFM)—that are commonly used for measuring molecular level forces that act between two surfaces at small separation distances. The first part of this chapter covers the fundamental principles of SFA and AFM design. The second half of this chapter illustrates the application of AFM to measuring surface forces with examples the measurement of van der Waals forces, atomic level repulsive forces, frictional forces, electrostatic double-layer forces, and meniscus forces from liquid films and from capillary condensation.

Surface Forces Derived from Surface Energies

As it more practical to measure the forces acting between two contacting surfaces then the energies of surfaces, this chapter covers those surface forces that are derived from surface energies. The starting point is Derjaguin’s approximation, which relates the energy between two flat surfaces to the force in other geometries: sphere/flat, sphere/sphere, and crossed cylinders. Next is a discussion of the surface forces in dry contacts with no liquid menisci around the contact points. This discussion covers the cases where adhesion causes significant deformation (JKR theory), where deformation is insignificant (DMT theory), and the cases in between. How surface roughness impacts adhesion is also discussed. The second half of this chapter deals with how liquid menisci around contacts contribute to adhesion forces, both for the sphere-on-flat geometry and for contacting rough surfaces.

Introduction

This chapter discusses why the scientific field encompassing friction, lubrication, adhesion, and wear is called tribology and how recent scientific advances are now enabling us to understand the rich interactions occurring between the atoms and molecules at contacting surfaces. This chapter also outlines the history of tribological science and tribology’s impact on technology, the economy, and everyday life. Success stories from MEMS, disk drives, automotive, and nanoimprinting industries are used to illustrate how nanoscale tribological science (often referred to as nanotribology) is helping to develop important new technologies. Examples considered include nanoscale contact switches and mechanical relays. The chapter concludes with a discussion around Feynman’s caveats on how friction and adhesion at the atomic and molecular level present major challenges for developing atomic scale machines.