In the past, many different remedies have been tried to eliminate the sources of annoying brake noise, e.g., replacing brake hardware, adding shims, resurfacing the rotors, etc. The composition of the brake linings was seldom considered, since asbestos-filled materials possess many characteristics important to low noise operation.
With the disappearance of asbestos from brake linings, many other materials were evaluated in order to maintain performance while keeping brake noise to a minimum. Fillers like calcium silicate, glass fiber, mica, etc. offer some similar performance characteristics to asbestos. Friction modifiers, such as metal oxides, metal sulfides, rubber scrap, etc are also used to maintain a more constant coefficient of friction during braking.
Among the metal sulfides, molybdenum disulfide (MoS2) is often used since it possesses very good dry lubricating properties. Graphite has similar dry lubricating properties, but exhibits poorer adhesion to metal surfaces and a coefficient of friction highly affected by humidity levels.
New surface treatment technology for graphite has resulted in significant improvements to the tribological performance of this material. These “polarized” graphite particles show very good adhesion to metal surfaces and a much greater resistance to changes in humidity. Using polarized graphite in brake lining formulations can substantially reduce their potential for being a source of brake noise. Several European brake pad manufacturers are now using this new technology, and their findings are presented.