Handbook of Friction Materials and their Applications

Roberto C Dante

• Publisher: Woodhead Publishing
• Year: 2015
• Print ISBN: 9780081006191
• eBook ISBN: 9780081006207
• Topic: Technology & Engineering
• Subtopic: Mechanical Engineering
• Index: Technology & Engineering

In the past few decades, friction material engineering has become more sophisticated with many tests and techniques to investigate the properties of the materials and their counterparts before, during and after friction occurred. There has not been too much information available on the different raw materials used for friction materials. This book is more focused towards the raw materials that formulate the different friction materials. It explains about their main friction effects and material structure. Handbook of Friction Materials and Their Applications begins by explaining about different friction materials and how they can be used for brakes. It then goes onto explain the tribology of friction materials. Further out it discusses how different friction materials are formulated and produced. Noise and vibration are explained in a further chapter. The later part talks about how different raw materials can be used for friction materials, such as metals, carbon, organic and inorganic materials. - Explains how different friction materials can be used for brakes - Discusses the noise and vibration effects in friction materials - Covers the raw materials that are used in friction materials

1

Friction materials

Friction for brakes

Abstract

Friction and tribology make many things possible in our everyday lives, such as ground transportation. The major types of brakes for ground vehicles are presented here, including drum and disk brakes. Sketches of these brakes, as well as aircraft brakes, are provided in this chapter. The friction coefficient can be seen as proportional to the dissipated power and, depending on the dissipated power, different types of friction materials may be required. The binding component's thermal stability is the fundamental issue in this case. Phenolic resins may achieve 400 °C, sintered metals 1000–1500 °C depending on the alloy, while ceramic, carbon–carbon, and carbon–ceramic may achieve temperatures above 2000 °C.

Keywords

Drum brake

Disk brakes

Coefficient of friction

Dissipated power

Friction is one of the most common phenomena in nature; it is everywhere in our daily lives without our even noticing it, since it is intimately part of almost everything we do that involves movement or change. It is so common that many times we are not aware that, without it, many aspects of our life would be very difficult, if not impossible. The purpose of this short chapter is not to introduce the reader to the principles of friction, but only to provide a different insight into friction, in order to look at friction, friction materials, and brakes as part of a larger context. A frictionless world would be that ideally depicted in the physics created by Galileo and Newton, characterized by a movement without energy losses. However, the simple acts of holding a water glass with our fingers or walking would be impossible without a friction force that balances and opposes the force of gravity or the reaction force, respectively. Friction makes many things both difficult and possible at the same time: for example, car movement, which consumes energy to feed engines to overcome the friction of wheels against the ground, but also needs the wheels' friction against the ground to make movement possible.

Another important aspect to be mentioned is that friction is characterized by systems composed of many fundamental particles (atoms, molecules, crystals, etc.), such as surfaces in sliding contact, gases, and liquids. Systems composed of many particles imply interactions among them, and therefore imply irreversible processes—that is, a net entropy increment in our world. These interactions can be attractive or repulsive, mechanical, and ultimately they generate forces. This kinetic energy dissipation causes heat and produces wear, as well as many other forms of energy. The science that studies friction between surfaces is called tribology and was founded as a new discipline in the 1960s. The major concern at that time was to decrease the coefficient of friction between sliding surfaces and consequently reduce wear in machines, especially those dedicated to manufacturing, so that energy consumption and parts replacement could be improved. Another important topic is tribochemistry, which is a new scientific branch dealing with chemical reactions activated by the work of friction. Triboreactions exhibit rates much higher than those of thermally activated chemical reactions.

There is only one technical device in which a high coefficient of friction is desired: the brake. Since machine motion exists, the manner to decrease velocity and stop the motion is a fundamental issue. The need for powerful brakes began when vehicle speeds increased and became necessary to control them. Brakes fulfill this role by transforming kinetic energy into other forms of energy, with heat being the most relevant. This, then, also implies the task of dissipating that heat.

The basic concept of any brake is to dissipate kinetic energy, whether in a horse-drawn carriage, a car, a train or an aircraft; this energy is dissipated through friction due to viscosity, such as in magnetorheological brakes, or friction between sliding surfaces. The following equation of power dissipation can be considered the basic equation for each type of brake, because it introduces the concept that an external force is necessary and that is the origin of the force opposing motion:

$\frac{\delta W_{\text{d}}}{\text{d}t}=w\left(\overrightarrow{X}\right)F_{\text{N}}v$

(1.1)

where W_d is work dissipated, t is time, $w\left(\overrightarrow{X}\right)$

is a coefficient of proportionality and $\overrightarrow{X}$

the set of variables that affect $w\left(\overrightarrow{X}\right)$

, v is the speed, F_N is the force component normal to and between the interacting surfaces, which can be either electric, or magnetic, incident pressure, or a combination of several forces. F_N can be formally considered to be an attractive force between the two opposing surfaces. Controlling F_N means to control the power dissipation and therefore the vehicle and machine speed. This equation highlights the importance of instant dissipation since this determines many surface effects, which depend upon the energy shots rather than on the total amount of energy dissipated.

A more general equation than Equation (1.1) is the following, which relates the friction force to the dissipated power:

$\frac{\delta W_{\text{d}}}{\text{d}t}=F_{\text{T}}v$

(1.2)

In the case of dry friction $w\left(\overrightarrow{X}\right)$

is the coefficient of friction μ, which is affected by temperature, speed, and other variables (for an outlook on the friction law, see the book Tribology by I.M. Hutchings [1]), so that μ can be expressed in the following way:

$\mu \left(\overrightarrow{X}\right)=\left(\frac{\delta W_{\text{d}}}{\text{d}t}\right)/F_{\text{N}}v$

(1.3)

This equation expresses the dependence of the coefficient of friction on some of the main variables that may affect it in vehicle brakes.

For example, consider an 80 kW car moving at maximum power to be stopped within 10 m and...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Dedication
• Biography
• Preface
• 1: Friction materials: Friction for brakes
• 2: Tribology of friction materials
• 3: Types of friction material formulas
• 4: Production processes for organic brake pads
• 5: Noise and vibration
• 6: Metal sulfides
• 7: Carbon materials
• 8: Abrasives, ceramic, and inorganic materials
• 9: Metals
• 10: Binders and organic materials
• Index