Semimetallic friction materials

Sintered Materials or Cermets. Heavy weights and high landing speeds of modem aircraft or high speed trains require friction materials that are extremely stable thermally. Organic or semimetallic friction materials are frequendy unsatisfactory for these applications. Cermet friction materials are metal-bonded ceramic compositions (see Composite materials) (12—14). The metal matrix may be copper or iron (15). 

Fig. 36 Performance levels of disk brake systems, whereby a = friction material containing asbestos b = asbestos-free semimetallic friction material c = asbestos-free ATE friction material (fiber substitute)...

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 (4) called semimetallics, also called semimets and carbon—metaUics. Because of the allegedly adverse health effects associated with asbestos [1332-21 -4] (qv) fibers, a second new class of friction materials called nonasbestos organics (NAOs) came about (5). Such materials are called either asbestos-free or nonasbestos friction materials (2). 

Property Modifiers. Property modifiers can, in general, be divided into two classes nonabrasive and abrasive, and the nonabrasive modifiers can be further classified as high friction or low friction. The most frequently used nonabrasive modifier is a cured resinous friction dust derived from cashew nutshell Hquid (see Nuts). Ground mbber is used in particle sizes similar to or slightly coarser than those of the cashew friction dusts for noise, wear, and abrasion control. Carbon black (qv), petroleum coke flour, natural and synthetic graphite, or other carbonaceous materials (see Carbon) are used to control the friction and improve wear, when abrasives are used, or to reduce noise. The above mentioned modifiers are primarily used in organic and semimetallic materials, except for graphite which is used in all friction materials. 

Abrasive modifiers are used in several types of friction materials. Very hard materials such as alumina, silicon carbide, and kyanite [1302-76-7] are used in fine particle sizes in organic, semimetallic, and cermet materials that are generally less than 74 im (200 mesh). Particle size is limited by the fact that large particles of such hard materials would groove cast-iron mating surfaces. Larger particle sizes are possible for harder mating surfaces in the special steel rotors used with sintered materials. 

Initially, the primary constituent of practically all semimetallics was iron powder in conjunction with a small amount of steel fiber (type I) (11). Later, large amounts of steel fiber were used along with small amounts of iron powder (type II). Various property modifiers, eg, ceramic powders, organic or mbber particles, and graphite powders, are added to enhance performance to desired levels, and a resin binder, which is necessary to hold the materials together in a mass, is also added (11). Compared to asbestos-based Class B organics that semimetallics originally replaced, semimetallics offered stable friction, improved fade resistance and durability, rotor compatibility, and quiet operation. 

The high frictional coefficient (0.4 to 0.5 compared with < 0.1 for glass fibers) of asbestos fibers is crucial to its utilization in the frictional lining sector. In the manufacture of brake and clutch linings 20 to 60% asbestos is incorporated together with fillers, metal chips and preferably phenol resins and rubber into a composite material, which has to satisfy many requirements. Currently there are asbestos-free so-called semimetallic brake linings, which consist of mixtures of metal fibers, metal powders, cellulose fibers, aluminum silicate fibers and mineral wool bonded with synthetic resins. 

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