Abstract

Brake linings and clutch facings consist of friction materials. Prior to the mid-1970s, the most common friction materials in use in brakes and clutches for normal duty were termed organics. These materials usually contained about 30–40 wt % of organic components and were asbestos-based. After the mid-1970s, the downsizing of North American vehicles and the introduction of front wheel drive vehicles brought about the widespread usage of a new class of friction materials called semimetallics, also called semimets and carbon–metallics. Because of the allegedly adverse health effects associated with asbestos fibers, a second new class of friction materials called nonasbestos organics (NAOs) came about. Gray cast iron is of reasonably low cost, provides good wear resistance and damping characteristics, and has long been in use as a brake drum or disk material for passenger cars and trucks. Developments in metal-matrix composites technology has resulted in aluminum matrix materials filled with silicon carbide, SiC, that provide the possibility of weight reduction for brakes. These composite materials are being tested and evaluated. An analysis of friction mechanisms suggests that a frictional force is likely to consist of several components such as adhesion-tearing, ploughing (or abrasion), elastic and plastic deformation, fracture, shearing of a friction film (glaze), and asperity interlocking, all occurring at the sliding surface. In the case of automotive friction materials, the coefficient of friction is usually found to decrease with increasing unit pressure and sliding speed at a given temperature, contrary to Amontons' law. Effectiveness, essentially a measure of the stopping efficiency, can be expressed as the coefficient of friction, deceleration rate, hydraulic or air line pressure required, torque developed, or distance required to stop a vehicle. For a fixed amount of braking the amount of wear of automotive friction materials tends to remain fairly constant or increase slightly with respect to brake temperature, but once the brake rotor temperature reaches , the wear of resin-bonded materials increases exponentially with increasing temperature because of thermal degradation of organic components and other chemical changes. Synthetic resins, such as phenolic and cresylic resins, are the most commonly used friction material binders. The asbestos usually used in friction materials is chrysotile, , the principal mineral of the serpentine group. Glass fibers and glassy fibers such as SMF or ceramic fibers are generally more thermally resistant than asbestos. Several types of organic fibers are used: the cellulose-based include cotton (linters), solkafloc, paper, sisal, and other natural fibers; synthetics include acrylics and polyaromatics. Because of the higher costs associated with nonasbestos fibers and the performance requirements needed in replacing asbestos, platy minerals such as mica and talc, and metal powders such as iron and copper, are being used as a portion of the NAOs. Most linings are produced from resin wet mix by either an extrusion or a rolling process. Segments for heavy-duty use such as for medium-sized trucks are produced by a dry-mix process. Organic and semimetallic disks are produced by similar processes. The mix for organics is prepared in an intensive mixer, that for semimetallics generally in less intensive blender. The mix for organic blocks is prepared as for segments, and the mix for semimetallic blocks is prepared as for semimetallic disk pads. Grinding, drilling, and chamfering produce the final block. Methods for producing most manual clutch friction materials are concerned with the placement of the reinforcement strand or wire within the matrix using some sort of winding operation. Cermet materials are manufactured using the powder metallurgy technique. Manufactured friction materials are characterized by various chemical, physical, and mechanical tests in addition to friction and wear testing. The chemical tests include thermogravimetric analysis (tga), differential thermal analysis (dta), pyrolysis gas chromatography (pgc), etc. Physical and mechanical tests determine properties such as thermal conductivity, specific heat, tensile or flexural strength, and hardness. Much attention has been placed on noise vibration characterization. Friction materials are evaluated in the laboratory by a great variety of tests and equipment. The full brake dynamometer, when properly instrumented and controlled, reflects the actual brake performance in a vehicle with reasonable accuracy. Numerous vehicle test procedures are used by different organizations. Performance tests are essentially designed to appraise initial effectiveness, burnish and normal effectiveness, fade and recovery, and final effectiveness. Vehicle tests are considered the ultimate in friction material evaluation. Asbestos-based friction materials have been virtually phased out for new vehicle installations because OSHA regulations have limited the exposure of workers to airborne asbestos fibers. Organic friction materials continue to serve the drum brake industry, but are being replaced by a trend to 4-wheel disk brakes, which are also preferred for antiskid brake systems. As brakes become smaller, producing higher brake temperatures, the Class A NAOs are expected to become less suitable, requiring Class B NAOs. Future brakes must satisfy health standards, and most vehicle manufacturers have moved toward removing all asbestos from brakes. There is much interest and activity aimed toward friction couples having reduced noise/vibration properties.