Antifriction Materials

materials employed for machine parts (bearings, bushings, and others) which operate with sliding friction and which under certain conditions have a low coefficient of friction. They are notable for low adhesion, good wear-in, thermal conductivity, and stable characteristics. Under hydrodynamic lubrication conditions when parts (which are not distorted by the pressure in the lubricating layer) are completely separated by a relatively thick layer of lubricant, the properties of the material from which the parts are made has no effect on the friction. The antifriction capability of materials becomes apparent under imperfect lubrication conditions (or for friction without lubrication) and is a function of the physical and chemical properties of the material, including high thermal conductivity and heat capacity, the ability to form strong boundary layers that reduce friction, and the capability of being easily (elastically or plastically) deformed or worn away so that the load can be uniformly distributed over the contacting surface (the wear-in feature). Also related to the antifriction capability are the microstructure of the surface (the specific degree of roughness or porosity in whose recesses the lubricating oil is retained) and the ability of the material to “absorb” solid abrasion particles that fall on the friction surface, thereby protecting the associated part from wear. Antifriction capability is displayed under dry friction conditions by a material containing components that have a lubricating action and make for low friction by being present on the friction surface (for example, graphite, molybdenum disulphide, and others). One of the important properties of antifriction materials as a result of their antifriction capability under all friction conditions is their inability or poor ability to seize (adhere) with the material of the associated part. The greatest tendency to seizure in friction is with ductile metals of the same composition in a pair having face-centered and body-centered cubical lattices. For steel the greatest tendency to seize in friction is with silver, tin, lead, copper, cadmium, antimony, bismuth, and alloys based on them.

The most common antifriction materials are the bearing materials used for sliding bearings. Besides antifriction properties they must possess the requisite strength, resistance to corrosion in the lubricating medium, suitability for production, and economy. Because of the difference in requirements between a bearing material forming a friction surface (antifriction capability) and a bearing part providing support (adequate strength), the bearing material is distributed on a bearing having a strong structural material as a base (steel, for example) and a friction surface consisting of a layer of antifriction material, such as babbitt metal. The antifriction materials are applied to a semifinished bearing or to a continuously moving steel strip by casting; from the bimetallic thicknessgauged strip, bearings (inserts and bushings) are fabricated by stamping.

Bearing materials are divided into metallic and nonmetallic types. The metallic bearing materials include alloys based on tin, lead, copper, zinc, aluminum, and also some cast irons; the nonmetallic bearing materials include certain kinds of plastics, materials based on wood, graphitic carbon materials, and rubber. Some bearing materials are a combination of metals and plastics—for example, a porous layer formed by sintered bronze beads impregnated with tetrafluoroethylene (Teflon) or tetrafluoroethylene with fillers.

Bearing materials based on tin or lead (babbitt metal) are employed in bearings in the form of a layer cast on steel (or sometimes on bronze). A strong adhesion is achieved by special cleaning of the steel; it is also possible to melt the babbitt metal (for large bearings) and line the bearing surface with cavities or grooves for better adhesion. Automobile bearings are fabricated from bimetallic steel-babbitt strip.

The bearing materials having a copper base are the tin bronzes, the tin-lead bronzes, the leaded bronzes, some tin-less bronzes, and also some brasses. For the very high stress bearings of internal combustion engines, leaded laminated bronzes (25 percent or more lead) are used in the form of a thin layer poured on the steel.

Bearing materials having a zinc base serve as substitutes for bronzes—for instance the type TsAm9–1.5 alloy used in steam locomotive bearings both for the fabrication of entire inserts and for lining the steel. There is also the well-known method of cladding steel with this alloy during the production of bimetallic strips by rolling.

Bearing materials having an aluminum base, used extensively for internal combustion engine bearings, can be subdivided into two groups according to the degree of ductility (hardness rating). Compared with the babbitts, the ductile aluminum alloys have greater heat conductivity and better mechanical properties at elevated temperatures; they are far cheaper but worse on wear-in and less capable of “absorbing” hard particles, and they wear the associated steel shaft somewhat more. Their properties are improved by depositing a thin (25 micron) layer of a tin-lead alloy on the working surface. The best antifriction properties are found in an aluminum alloy with 20 percent of tin having a microstructure obtained by means of plastic deformation and annealing. Alloys having a Brinell hardness (HB) of HB < 350 mega-newtons per sq m (35 kilograms-force per sq mm) are used in producing by rolling jointly with steel a bimetallic strip or stock from which bearing inserts are subsequently stamped. Alloys having a higher hardness (HB=450 meganewtons per sq m, or 45 kilograms-force per sq mm) are used to make diesel bearings.

Gray pearlitic cast iron having a specific microstructure (average to large laminar pearlite, graphite of average coarseness, phosphide eutectic in the form of isolated inclusions) has antifriction properties and is employed for bearings operating under high loads at low speeds.

Bearing materials based on a plastic with fillers of fabric (textolite) and wood chips are used successfully in bearings—liberally lubricated with water—for low shaft rotation rates. Even more widely used as bearing materials are plastics (polyamides, tetrafluoroethylene, and others) operating with lubricating oil or water. Polyamides are also used in the form of a thin coating (for instance, 0.3 mm) on the metallic base of a bearing, thus increasing the permissible load. The operating condition of bearings made of plastic is limited by the temperature at the friction surface (for example, no more than 80° to 100°C for polyamides). A feature of certain bearings made of polyamides is the nearly complete absence of wear on an associated steel shaft. The best antifriction capability at low sliding velocities without lubrication is that of tetrafluoroethylene, which retains its low friction over a temperature operating range from -200°C up to 260°C.

Bearing materials based on wood generally include natural wood, compressed woods, and laminated woods. An example of a natural bearing material is guaiacum or lignum vitae, which is used with a water lubricant. Bearing materials based on wood are used with liberal water lubrication in bearings for rolling mills, water turbines, and ships’ propellers.

The graphitic carbon bearing materials are produced by pressing and heat-treating mixtures of petroleum coke and coal tar with a small amount of natural graphite. They are used as bearing materials to operate without lubrication under low specific loadings, at temperatures up to 480°C, and in air. Graphitic carbon bearing materials impregnated with metals or resin are also prepared.

Rubber is used as a bearing material when well lubricated with water at low specific loadings and low sliding velocities. The operating condition is limited to a temperature of 50° to 70°C at the friction surface.

Metal-ceramic self-lubricating bearing materials are employed in the form of porous bushings (mainly of small size, operating at low speeds, and with no external supply of lubricant). They are fabricated by sintering previously compressed blanks made of tin bronze powders (10 percent Sn) with a graphite admixture or of iron with graphite. The degree of porosity is about 25 percent. The bushings are impregnated with oil.