Friction is the force that acts to prevent two surfaces in contact from sliding against each other. When friction occurs, the kinetic energy (motion) of the sliding surfaces is converted into thermal energy (heat).
Some combinations of materials, such as ice on glass, have a very low coefficient of friction. That means that there is very little friction between them, and therefore almost no sliding resistance. Rubber tires against a hard road surface have a high coefficient of friction, which means they tend to grip and resist sliding against each other.
Disc brake pads and drum brake linings are made from materials which have a high coefficient of friction. They also need to have an ability to absorb and disperse large amounts of heat without their braking performance being adversely affected.
As the heat in brake pads and linings builds up, the friction capability of the material – and consequently its stopping power – reduces. This is called brake fade, and minimizing or overcoming fade is a major factor in the design of brakes and the development of brake friction materials.
Brake friction materials were commonly made from asbestos compounds, because of the excellent heat resistance of that material, but as that has now been proven to be highly toxic, asbestos is generally banned and no longer used.
Brakes are now manufactured from a variety of different materials that may be:
The choice of compound depends on the application. Lighter passenger vehicles generate less heat in the brakes than heavy or high performance vehicles. The optimum brake composition for any given vehicle or use will therefore be a combination of weighted attributes which include:
For instance, owners of small domestic vehicles will probably tend to value a longer pad life rather than higher performance in extreme conditions. In racing cars, however, fade resistance and stopping power at high speeds would be considered more important than noise levels or wear rate.
