Materials
Brake pad materials range from asbestos to organic or semi-metallic formulations. Each of these materials has proven to have advantages and disadvantages regarding environmental friendliness, wear, noise, and stopping capability. Semi-metallic pads provide strength and conduct heat away from rotors but also generate noise and are abrasive enough to increase rotor wear.
Ceramic compounds and copper fibers in place of the semi-metallic pad's steel fibers accommodate higher temperatures with less heat fade and generate less dust and wear on both the pads and rotors. They also provide much quieter operation due to the ceramic compound that helps dampen noise by shifting its resonant frequency beyond the human hearing range and reduced metal use (approximately 15% metal content by weight).
There are environmental factors that govern the selection of brake pad materials. For example, recent legislation in Washington State (SSB 6557) and other states will limit the amount of copper that is allowed to be used in friction materials, to be eventually phased out to trace amounts. Other materials like antimony compounds will be monitored as well.
Asbestos was widely used in pads for its heat resistance but, due to health risks, has been replaced with alternative materials, such as mineral fibers, cellulose, aramid, PAN, chopped glass, steel, and copper fibers. Depending on material properties, disc wear rates vary. The properties that determine material wear involve trade-offs between performance and longevity. Newer pads can be made of exotic materials like ceramics, aramid fibres, and other plastics.
Vehicles have different braking requirements. Friction materials offer application-specific formulas and designs. Brake pads with a higher coefficient of friction provide good braking with less brake pedal pressure requirement, but tend to lose efficiency at higher temperatures, increasing stopping distance. Brake pads with a smaller and constant coefficient of friction don’t lose efficiency at higher temperatures and are stable, but require higher brake pedal pressure.
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“In how few words, for instance, the Greeks would have told the story of Abelard and Heloise, making but a sentence of our classical dictionary.... We moderns, on the other hand, collect only the raw materials of biography and history, “memoirs to serve for a history,” which is but materials to serve for a mythology.”
—Henry David Thoreau (1817–1862)
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—Lyndon Baines Johnson (1908–1973)