Friction Materials

materials having a high coefficient of friction, used to make components that operate under conditions of sliding friction. They are characterized by the ability to maintain their coefficient of friction and high resistance to wear over a wide temperature range, low adhesiveness (to prevent seizure), high thermal conductivity and heat capacity, and favorable resistance to the thermal shock that occurs as a result of the intense evolution of heat by friction. Friction materials must also satisfy requirements for corrosion resistance, run-in, suitability for industrial production, and economy.

Metallic friction materials include certain grades of pig iron and steel. For example, gray iron is commonly used for railroad brake shoes. Varieties of pig iron are not prone to warping, but at temperatures above 400–600°C their coefficient of friction is sharply reduced, which restricts the temperature conditions for their use. Friction pairs made of grade 40, 45, 65G, and other steels are used for friction clutches in tracked vehicles. A major disadvantage of steel friction pairs is the tendency to warp and seize when overheated. Metals are gradually being replaced by plastics as friction materials.

Nonmetallic friction materials are prepared primarily on an asbestos base; rubbers, resins, and other materials serve as binders. Plastic materials with a rubber binder have a relatively high and stable coefficient of friction up to 220–250°C; they are used for motor vehicle brake linings and clutch bands. Plastic materials with a resin binder have higher wear resistance but a somewhat lower coefficient of friction. One of the best materials in this group is Retinaks, which consists of a phenol-formaldehyde resin, barite, asbestos, and other components; it is designed for use in heavy-duty brake units where the temperature at the friction surface can reach 1000°C, as in aircraft brakes.

Sintered friction materials (seeSINTERED MATERIAL) are widely used in brake gear and friction clutches subject to heavy loads; they are well suited to such applications because of their high wear resistance, coefficient of friction, thermal stability, and thermal conductivity. The components used in sintered materials ensure good service characteristics under severe operating conditions; some contribute high wear resistance and high coefficient of friction (carbides and oxides of metals), and others impart stability to the frictional properties and prevent seizure (graphite, asbestos, barite, and molybdenum disulphide). Such materials are used to fabricate disks, pad segments, and shoes by sintering blanks precompacted from powdered mixtures. In order to increase strength, sintered friction materials may be formulated on a steel base, to which they are usually welded during sintering. The most widely used sintered materials have a copper or iron base. Copper-based materials contain tin, graphite, lead, and other components; they have a coefficient of friction between 0.08 and 0.12 when operating in oil and between 0.17 and 0.25 for dry friction, and the upper temperature limit for use is 300°C. In comparison, sintered materials using an iron base have greater strength, sustain higher unit loads, and withstand considerably higher temperatures. Depending on component composition, the coefficient of friction for brake materials ranges from 0.2 to 0.4. The materials usually contain copper, nickel, chromium, barite, asbestos, graphite, carbides of metals, and other components. They tolerate high temperatures on the frictional surface (up to 1200°C), which is particularly important in brake units.