Chapter 2

Friction and Friction Materials

Abstract

This chapter introduces the role of dry sliding friction in conventional road vehicle disc and drum brakes. It describes how modern friction materials are designed, manufactured and used to rub against a mating body to form a’friction pair’, which generates consistent and reliable retarding torque to decelerate the vehicle. After introducing some historical context and the basic science of sliding friction, it focuses on the conventional automotive brake friction pair comprising resin-bonded composite friction material operating against a cast-iron rotor (brake drum or disc); other types of friction pair are only briefly described. The constituents of friction materials and their function are explained, an outline of processing practice is given, and examples of thermophysical properties are discussed. The coefficient of friction (?) between the friction material and the rotor is the most important brake design parameter and the influence of operating parameters, especially temperature, on ? and other properties is explained.

Keywords

Friction materialbrakepadliningcompositemanufacturepropertiescoefficient of friction

Introduction

The brakes of road vehicles have relied upon friction for hundreds of years. The use of frictional forces generated between two bodies in sliding contact to provide a retardation mechanism for moving bodies can be traced back in history almost to the origins of human endeavour (Dowson, 1979). Wheel bearings were first noted 5000 years ago, but the use of wheel brakes cannot be traced back this far; it is almost certain that frictional retardation was invoked by dragging, e.g. a log behind a horse-drawn cart when descending a steep hill, and parking while ascending a steep hill (for example, to allow the horse to rest) would be achieved by a ‘sprag’ to prevent its rolling back. A mechanism that pressed a friction pad against a rotating wheel was a subsequent technological advance, probably dating in England from the 1700s on horse-drawn carriages. Braking devices of this basic form were then utilised on railway carriages and trucks, and then on the first ‘horseless carriage’ road vehicles in Europe in the later 1800s (Newcomb and Spurr, 1989). Fundamental to the operation of the friction brake is the dynamic or sliding coefficient of friction (?) between the rotor and the stator components. (The symbol ? is universally used for the coefficient of friction; in this book it is used specifically to represent the dynamic or sliding coefficient of friction when the bodies in contact are moving relative to each other. The static coefficient of friction, when the bodies in contact have no relative motion between them, is represented here by ?s.) The iron tyre on a wooden cart wheel worked well as a rotor surface against materials such as wood, leather and felt (Newcomb and Spurr, 1989), but these were sensitive to the environment (e.g. mud and rain), and as vehicle speed, size and weight increased, the amount of energy to be dissipated increased, and operating temperatures increased beyond the limits of these materials. Recognising the importance of temperature stability of the coefficient of friction, resin-bonded composite friction materials were invented to extend the operating range and durability of the stator material. Modern resin-bonded composite friction materials have developed so far that few car drivers nowadays give a moment’s consideration to the work that the friction material has to do when they apply the brakes, or the environment in which they have to function. Sliding friction between two dry contacting surfaces is often known as ‘Coulomb friction’ after Charles Coulomb (1736–1806), but despite its everyday nature the friction forces involved can usually only be estimated from previous experience and experimental evidence. In friction brakes the surfaces in sliding contact are often coated with ‘transfer films’ as a result of the sliding process so that the surfaces in contact are not just the bare metal or friction material. For example, a metal surface will have its surface disturbed by abrasion, adhesion or deformation during sliding to create metal fragments and other particles. Superimposed on these could be layers of oxide, which could in turn be covered by layers of organic material transferred from the friction material. A friction surface, even if it appears to be geometrically smooth, is very rough on the microscopic scale with a distribution of asperities across it. One explanation of the genesis of friction is the interaction of microscopic asperities on the two surfaces, but when two such surfaces are forced together it is very difficult to say where and how contact between them occurs. Model systems have been studied in which the materials and surfaces were scientifically controlled in the expectation that once the friction of such systems was understood, more and more complicated systems could be examined. Although the scientific understanding of friction has benefited from such research, the extremely complicated nature of braking friction, involving high energy, high temperature, high speed and high pressure, remains an inexact science that relies upon specialist knowledge and understanding. Whilst from an engineering point of view a constant coefficient of friction between two sliding bodies may seem a reasonable assumption, in working with friction brakes it is vital to understand that the coefficient of friction is most likely to be variable, and also it is helpful to understand why. The basic empirical laws of friction, which are known as Amontons’ laws after Guillaume Amontons (1736–1806), are stated below. Coulomb introduced a fourth law, which stated that the friction force is independent of sliding speed, but whereas Amontons’ laws of friction represent a good practical basis for brake friction pairs, the Coulomb law does not, for reasons explained later. 1. Friction force is independent of the nominal or apparent area of the surfaces in sliding contact. 2. Friction force F is proportional to the normal force N between two bodies in sliding contact, i.e. F = ?N, where ? is the coefficient of friction. 3. The friction force always opposes the direction of sliding (i.e. the relative motion). A simple physical explanation of the first two laws relates to the difference between the ‘real’ area of contact AR (based on the total surface areas of the microscopic asperities in contact) and the ‘apparent’ (or ‘nominal’) area of contact AN (indicated by the overall size of the contact interface) at any friction interface. The real area of contact AR is very much less than and independent of the apparent area AN, but is proportional to the normal force between the two bodies in sliding contact. A simplified model of the contacting surfaces based on the idea of contacting asperities is a series of elastic hemispheres (i = 1 ? n) on one surface pressing against another perfectly flat, rigid surface. Each hemisphere can be considered to adhere to the flat surface, generating a shear force that is proportional to the area of contact between it and the rigid surface, and the sum of all the areas of the hemispheres in contact with the rigid surface equates to the real area of contact:

R=?ARi

(2.1)

If the constant f (N/m2) denotes the specific friction force (i.e. the tangential friction force per unit real area of contact), then for any individual contact, i :

i=fARi

(2.2)

Making the assumption that the area of contact between each hemisphere and the rigid surface is proportional to the normal force between them:

Ri/Ni=constantandthereforeAR/N=constant

(2.3)

Thus, since ? = F/N = fAR/N, ? must also be constant. This simple theory assumes that the surfaces adhere to one another at the real areas of contact when pressed together by a normal force, and that no adhesion remains when the normal load is removed. Any variation in ? is attributed to variations in f arising from differing degrees of contamination of the surfaces. For a full understanding of the phenomenon of friction it would be necessary at the very least to be able to determine AR and f, but the more this is investigated the more complicated it appears and there is no easy way of doing this for the types of friction material pairs used in modern automotive braking systems. Hence it remains necessary to measure rather than calculate the frictional properties of any friction material, although skilled formulators are able to predict approximate friction (and wear) behaviour based on their knowledge, expertise and experience. ‘Tribology’ is the generic name for the science of friction, lubrication and wear. Braking friction constitutes ‘dry’ friction and so the science of friction and wear are mostly of interest. There are...