section epub:type=”chapter”> This chapter discusses the frictional properties of materials in contact. This is of considerable importance in mechanical design. Frictional forces are undesirable in bearings because of the power they waste; and wear is bad because it leads to poor working tolerances and ultimately to failure. On the other hand, when selecting materials for clutch and brake linings—or even for the soles of shoes—the aim is to maximize friction but still to minimize wear. But wear is not always bad—for example, in operations such as grinding and polishing, the aim is to achieve maximum wear with the minimum of energy expended in friction. Without wear, one could not write with chalk on a blackboard or with a pencil on paper. The chapter examines the origins of friction and wear and explores case studies that illustrate the influence of friction and wear on component design. When two materials are placed in contact, any attempt to cause one of the materials to slide over the other is resisted by a friction force. The chapter presents the data for coefficients of friction and lubrication. Even when solid surfaces are protected by oxide films and boundary lubricants, some solid-to-solid contact occurs at regions where the oxide film breaks down under mechanical loading, and adsorption of active boundary lubricants is poor. This intimate contact will generally lead to wear. Wear is normally divided into two main types: adhesive and abrasive wear. The chapter concludes with a discussion of surface and bulk properties. We now look at the frictional properties of materials in contact, and the wear that results when such contacts slide. This is of considerable importance in mechanical design. Frictional forces are undesirable in bearings because of the power they waste; and wear is bad because it leads to poor working tolerances, and ultimately to failure. On the other hand, when selecting materials for clutch and brake linings—or even for the soles of shoes—we aim to maximize friction but still to minimize wear, for obvious reasons. But wear is not always bad: in operations such as grinding and polishing, we try to achieve maximum wear with the minimum of energy expended in friction; and without wear you could not write with chalk on a board, or with a pencil on paper. In this chapter and the next we examine the origins of friction and wear and then explore case studies that illustrate the influence of friction and wear on component design. As you know, when two materials are placed in contact, any attempt to cause one of the materials to slide over the other is resisted by a friction force (Figure 29.1). The force that will just cause sliding to start, Fs, is related to the force P acting normal to the contact surface by where μs is the coefficient of static friction. Once sliding starts, the limiting frictional force decreases and we can write where μk (<μs) is the coefficient of kinetic friction (Figure 29.1). The work done in sliding against kinetic friction appears as heat. These results at first sight run counter to our intuition—how is it that the friction between two surfaces can depend only on the force P pressing them together and not on their area? In order to understand this behavior, we must first look at the geometry of a typical surface. If the surface of a fine-turned bar of metal is examined by making an oblique slice through it (a “taper section” which magnifies the height of any asperities), or if its profile is measured with a profilometer, it is found that the surface looks like Figure 29.2. The figure shows a large number of projections or asperities—it looks rather like a cross-section through Switzerland. If the metal is abraded with the finest abrasive paper, the scale of the asperities decreases but they are still there—just smaller. Even if the surface is polished for a long time using the finest type of metal polish, micro-asperities still survive. So it follows that, if two surfaces are placed in contact, no matter how carefully they have been machined and polished, they will contact only at the occasional points where one set of asperities meets the other. It is rather like turning Austria upside down and putting it on top of Switzerland. The load pressing the surfaces together is supported solely by the contacting asperities. The real area of contact, a, is very small and because of this the stress P/a (load/area) on each asperity is very large. Initially, at very low loads, the asperities deform elastically where they touch. However, for realistic loads, the high stress causes extensive plastic deformation at the tips of asperities. If each asperity yields, forming a junction with its partner, the total load transmitted across the surface (Figure 29.3) is where σy is the compressive yield stress. In other words, the real area of contact is given by Obviously, if we double P we double the real area of contact, a. Let us now look at how this contact geometry influences friction. If you attempt to slide one of the surfaces over the other, a shear stress Fs/a appears at the asperities. The shear stress is greatest where the cross-sectional area of asperities is least, that is, at the contact plane. Now, the intense plastic deformation in the regions of contact presses the asperity tips together so well that there is atom-to-atom contact across the junction. The junction, therefore, can withstand a shear stress as large as k approximately, where k is the shear-yield strength of the material (Chapter 12). The asperities will give way, allowing sliding, when or, since k ≈ σy/2, when Combining this with Equation (29.3), we have This is just the empirical Equation (29.1) we started with, with μs ≈ 1/2, but this time it is not empirical—we derived it from a model of the sliding process. The value μs ≈ 1/2 is close to the value of coefficients of static friction between unlubricated metal, ceramic, and glass surfaces—a considerable success. How do we explain the lower value of μk? Well, once the surfaces are sliding, there is less time available for atom-to-atom bonding at the asperity junctions than when the surfaces are in static contact, and the contact area over which shearing needs to take place is correspondingly reduced. As soon as sliding stops, creep allows the contacts to grow a little, diffusion allows the bond there to become stronger, and μ rises again to μs. If metal surfaces are thoroughly cleaned in vacuum it is almost impossible to slide them over each other. Any shearing force causes further plasticity at the junctions, which quickly grow, leading to complete seizure (μ > 5). This is a problem in outer space, and in atmospheres (e.g., H2) that remove any surface films from the metal. A little oxygen or H2O greatly reduces μ by creating an oxide film that prevents these large metallic junctions forming. We said in Chapter 25 that all metals except gold have a layer, no matter how thin, of metal oxide on their surfaces. Experimentally, it is found that for some metals the junction between the oxide films formed at asperity tips is weaker in shear than the metal on which it grew (Figure 29.4). In this case, sliding of the surfaces will take place in the thin oxide layer, at a stress less than in the metal itself, and lead to a corresponding reduction in μ to a value between 0.5 and 1.5. When soft metals slide over each other (e.g., lead on lead, Figure 29.5) the junctions are weak but their area is large so μ is large. When hard metals slide (e.g., steel on steel) the junctions are small, but they are strong, and again friction is large (Figure 29.5). Many bearings are made of a thin film of a soft metal between two hard ones, giving weak junctions of small area. White metal bearings, for example, consist of soft alloys of lead or tin supported in a matrix of stronger phases; bearing bronzes consist of soft lead particles (which smear out to form the lubricating film) supported by a bronze matrix; and polymer-impregnated porous bearings are made by partly sintering copper with a polymer (usually PTFE) forced into its pores. Bearings such as these are not designed to run dry—but if lubrication does break down, the soft component gives a coefficient of friction of 0.1 to 0.2 which may be low enough to prevent catastrophic overheating and seizure.
Friction and Wear
Publisher Summary
29.1 Introduction
29.2 Friction between Materials
29.3 Coefficients of Friction
