M e t a l l i c Fo Fo a m s J. Banhart, J. Baumeister, and M. Weber, Fraunhofer-Institute for Applied Materials Research, Bremen
References 1. J.A. Ridgeway, "Cellarized Metal and Method of Producing the Same," U.S. Patent 3,297,431, 1967 2. S. Akiyama et al., "Foamed Metal and Method for Producing the Same," European Patent Application EP 0 210 803 Al, 1986 3. J. Baumeister, "Method for Producing Porous Metal Bodies," German Patent DE 40 18 360, 1990 4. J. Baumeister and H. Schrader, "Methods for Manufacturing Formable Metal Bodies," German Patent DE 41 01 630, 1991 5. J. Baumeister, J. Banhart, and M. Weber, Int. Conf. on Materials by Powder Technology, F. Aldinger, Ed., DGM Informationsgesellschaft Oberursel (Germany), 1993, p 501 6. M. Weber, Ph.D. thesis, Technical University Clausthal, 1995 7. M. Weber, J. Baumeister, J. Banhart, and H.D. Kunze, Proc. Powder Metallurgy World Congress PM94 (Paris), Les Editions de Physique, 1994, p 585 8. L.J. Gibson and M. Ashby, Cellular Solids, Oxford, 1988 M e t a l l i c Fo Fo a m s J. Banhart, J. Baumeister, and M. Weber, Fraunhofer-Institute for Applied Materials Research, Bremen
Se l e c t e d R e f e r e n c e s •
Baumeister et al., U.S. Patent 5,151,246, 29 Sept 1992
F r i c t i o n P o w d e r M e t a l l u r g y M at at e r i al s Introduction FRICTION MATERIALS are the components of a mechanism that converts mechanical energy into heat upon sliding contact. The conversion product, heat, is absorbed or dissipated by the friction material. The coefficient of friction, an index of shearing force of the contacting parts, determines the degree of performance of the friction material. The required level of the coefficient of friction depends on the operating conditions and the end use of the product. Sintered metal friction materials have been used as brake disks, especially for heavy-duty application. Because of their good breaking performance and low wear rate under high temperatures and heavy-duty conditions, sintered friction materials have become more important. Sintered friction materials typically comprise: • •
Sinterable metal powders (e.g., copper and iron) ir on) Friction modifiers, such as abrasive particles (e.g., alumina, silica, and mullite) and lubricants (e.g., graphite and molybdenum disulfide)
Metallic friction materials are used in heavy-duty applications, such as in aircraft brake linings and as clutch facings on tractors, heavy trucks, earth-moving equipment, and heavy presses. There are two principal types of applications or operating conditions for metallic friction materials: "wet" and "dry." Under wet conditions, the friction components, such as clutches in powershift and automatic transmissions, are immersed in oil. Dry operating conditions involve direct contact of friction components without oil, such as in aircraft brakes and standard clutches. A low apparent density (AD) is a key characteristic of metal powders for friction applications. For example, powders with high surface area and internal porosity enhance brake formulations by: • •
•
Lowering costs: Lowest apparent density reduces brake weight and lowers total material usage. Improving brake surface: High internal porosity ensures a more homogeneous brake surface, and high surface area improves particle bonding with the matrix. Increasing molded strength: Higher green strength is achieved when compacting powders with lower apparent densities (Fig. 1).
Highly porous and irregular iron powders with very low apparent densities can be produced by the hydrogen reduction (Pyron) process. A comparison of iron powder apparent density follows: • • •
Hydrogen-reduced iron powder, 1.20 to 2.5 g/cm 3 CO-reduced powder, 2.30 to 2.60 g/cm 3 Atomized powder, 2.95 to 3.25 g/cm
3
Fig. 1 Green strength of compacts pressed from hydrogen-reduced powders with various apparent densities (AD). Source: Pyron Corporation
Acknowledgement This article was adapted from an article by S. Ozsever of the Raymark Corporation titled "P/M Friction Materials" in Powder Metallurgy, Volume 7 of the ASM Handbook.
Fr i c t i o n P o w d e r M e t a l l u r g y M a t e r i a ls
Manufacturing Metallic friction materials are produced by compacting and sintering mixes of metal powders and friction-producing ceramic materials such as silicon dioxide or aluminum oxide. Friction materials consist of a dispersion of a frictionproducing ingredient in a metallic matrix. Originally, a copper-tin alloy was used as the metallic matrix, in which copper powder, tin powder, the friction-producing ingredient in powder form, and other ingredients that modify the frictional behavior were mixed, compacted, and sintered. This matrix material is still used, but other compositions such as copperzinc matrix materials have been developed, mainly for oil-immersed (wet) applications. The copper-zinc materials have the ability to maintain a strong, yet porous, matrix that retains oil. The more porous copper-zinc materials have higher friction coefficients and higher energy absorption capacity than copper-tin materials. R a w M a t e r i a l B l e n d i n g . Sintered metal friction materials involve a wide variety of compositions. The choice of the
composition depends on the nature of the application, such as the use and energy-power requirements. In general, metallic friction materials can be classified as either copper or iron base. Table 1 provides the range of compositions of metallic friction materials. Proportions of the components greatly affect physical properties of the materials. By varying the percentages of the individual components, different coefficients of friction may be achieved. Before mixing, powders generally are brush screened to break up agglomerated particles. Mixing usually is done in cone-type blenders. During mixing, small amounts of additives are introduced, mainly to prevent possible segregation of components by specific gravity. These additions are light-fraction oils that are volatilized easily during sintering.
Table 1 Nominal compositions of copper-base and iron-base friction materials Premix Copper base
Compositions, % Copper Iron Lead 65-75 ... 2-5
Tin 2-5
Zinc 5-8
Silicon dioxide 2-5
Graphite 10-20
C o m p a c t i n g . Most compacting of powders is done in hydraulic presses. Compacting pressure varies from 165 to 276
MPa (12 to 20 tsi) and is determined by the type of powder and the compacted density required. Before starting a production run, a series of density checks should be made. Low densities often cause handling problems, especially with thin cross sections. This can be overcome by increasing pressure. Surface parallelism is a major consideration and, in general, parts are kept in the 0.05 to 0. 10 mm (0.002 to 0.004 in.) tolerance range. S i n t e r i n g o f C l u t c h P l a t e F a c i n g s . Green compacts, placed on supporting steel backing plates, are stacked in
sintering furnaces. Backing plates or cores generally are cleaned and copper plated to achieve good bonding with the friction material. The type of sintering furnace used depends on the shape of the parts being produced. Bell-type furnaces are generally used for clutch discs because of the compression requirement. However, for disc brake pads and other odd shapes, the use of bell-type furnaces is not essential because compression is not required during sintering. In bell furnaces, pressure is applied on the vertical stack of discs to reach the desired sintered density level and to prevent warpage or distortion. However, excess pressure causes high sintered densities and loss of low-melting-point metals from the friction material. Sintering is carried out at temperatures of 550 to 1000 °C (1020 to 1830 °F) in a protective atmosphere to prevent oxidation. Figure 2 shows the structure of a copper-base friction material, copper-plated layer, and steel backing plate after sintering at 650 °C (1200 °F) for 2 h.
Fig. 2 Structure of sintered copper-base friction material
Fi n a l O p e r a t i o n s . Dimensional accuracy and additional design considerations are of major importance for applications.
Parts are machined after sintering to meet dimensional specifications. Grooving is required in the production of friction facing. Various types of grooves may be used, and each performs in a different manner. Two of the most frequently used types of grooves are combined in one friction facing (Ref 1). The purpose for grooving varies from wet friction to dry friction materials, but grooves are very important in wet clutch systems. In wet applications, the heat energy is partly removed by the oil. Grooves allow cooling oil to flow across the surface of the friction faces, yet allow fast oil runoff during engagement to minimize oil thickness on the surfaces and maximize the coefficient of friction. In dry systems, grooves serve somewhat different purposes than in wet systems. These include (a) prevention of the crushing effect of thermal expansion during high application temperatures, as in the disc brake pads of heavy-duty vehicles; (b) removal of operation debris; and (c) transfer of water and other liquids if the surface becomes wet.
R ef e r e n c e c i t e d i n t h i s s e ct i o n
1. R.L. Fish, "Wet Friction Applications: Some Design Considerations," technical report, Raymark Corp., Stratford, CT Fr i c t i o n P o w d e r M e t a l l u r g y M a t e r i a ls
Friction Applications The coefficient of friction ( ) is the most important property in selection of a facing material. Friction may be expressed as static or dynamic values. The static coefficient of friction is the friction value of two surfaces at zero speed, while the dynamic coefficient of friction is measured at speeds greater than zero. The coefficient of friction is a function of conditions such as rubbing speed, pressure, and temperature. The relationship between rubbing speed and the coefficient of friction is shown in Fig. 3. With increasing rubbing speeds of the two surfaces, the coefficient of friction tends to drop. Applied pressure has a similar effect on the stability of the coefficient of friction. In wet friction applications, any increase in the temperature of the two rubbing surfaces above 150 °C (300 °F) will cause a substantial drop in the coefficient of friction. This temperature is commonly referred to as "breakdown temperature."
Fig. 3 Effect of unit pressure and rubbing speed on the coefficient of friction in a wet system. Solid line indicates dynamic coefficient of friction. Dashed line indicates static coefficient of friction.
In wet applications, wear rates are usually low; therefore, thinner facings may be produced. Wear rate depends on factors such as temperature, number of engagements, mating or coupling plate surface finish, coupling plate, and facing material. Dry friction applications are simple and lightweight compared to wet applications. Their major disadvantage is their erratic behavior in wet environments (Ref 2). For instance, on passenger cars, considerable fade is encountered when water enters the brake assembly. Figure 4 shows the effect of pressure and speed on the dynamic coefficient of friction of an automobile disc brake. Similar to wet friction materials, the drop in the dynamic coefficient of friction with pressure is noticeable.
Unit pressure Coefficient of friction kPa psi 32 km/h (20 mph) line 0.336 2760 400 4825 700 0.336 6900 1000 0.317 8965 1300 0.291 64 km/h (40 mph) line 2760 400 0.336 0.306 4825 700 6900 1000 0.298 8965 1300 0.277
Fig. 4 Effect of pressure and speed on the coefficient of friction in an iron-base by braking surface
Dry friction materials vary in composition from wet friction materials. In wet friction compounds, the matrix of the sintered material is copper, but in dry friction compounds, the percentage of copper is reduced and iron is increased. The effect of temperature on dry friction material is shown in Fig. 5(a) and fig 5(b). Between 95 and 315 °C (200 and 600 °F), the drop in the dynamic coefficient of friction is noticeable. After a certain level, the rate of decrease slows and eventually stabilizes.
Temperature °F °C Fig. 5(a) 93 200 120 250 150 300 180 350 205 400 230 450 260 500 290 550 315 600 Fig. 5(b) 315 600 260 500 205 400 150 300 93 200
Coefficient of friction
0.425 0.460 0.415 0.405 0.395 0.380 0.360 0.350 0.345 0.330 0.340 0.360 0.370 0.400
Source: Raymark Corporation
Fig. 5 Effect of temperature on the coefficient of friction for a proprietary iron-base compound for brake usage
In some materials, a slight increase in the coefficient of friction with increasing temperature is found after stabilization. This is mainly because of a change in surface morphology and the high-temperature effect of graphite in the facing material. In many cases, the drop in the friction level is recoverable, as shown in Fig. 5(b). As the temperature drops, the coefficient of friction increases. At 95 °C (200 °F), frictional recovery is almost complete. However, the level of recovery depends on the material used. Clutch and brake applications include:
Dry friction materials • • • • • •
Earth moving equipment Agricultural equipment Cranes and hoists Lift trucks Highway trucks (clutches) Aircraft (brakes)
Wet friction materials • • • •
Earth moving equipment Agricultural equipment Military Lift trucks (clutches)
R ef e r e n c e c i t e d i n t h i s s e ct i o n
2. W. Jenson, Friction Materials, Machine Design, Jan 1972, p 108-113
Fr i c t i o n P o w d e r M e t a l l u r g y M a t e r i a ls
References 1. R.L. Fish, "Wet Friction Applications: Some Design Considerations," technical report, Raymark Corp., Stratford, CT 2. W. Jenson, Friction Materials, Machine Design, Jan 1972, p 108-113 Fr i c t i o n P o w d e r M e t a l l u r g y M a t e r i a ls
Se l e c t e d R e f e r e n c e s •
Friction and Antifriction Materials, Perspectives in Powder Metallurgy, Vol 4, Plenum Press, New York, 1970
•
R.H.T. Dixon and A. Clayton, Powder Metallurgy for Engineers, Machinery Publishing Co., Ltd., London, 1971
•
J.R. Zimmerman, Clutches and Brakes, Standard Handbook of Machine Design, McGraw-Hill, 1996
Po w d e r M et a ll u r g y B ea r i n g s Norbert A. Arnold, Keystone Carbon Co., Victor C. Straub, Keystone Powdered Metal Company; Michael Schloder, Specialty Pressed Components, Inc.
Introduction SELF-LUBRICATING BEARINGS are one of the oldest industrial applications of porous P/M parts, dating back to the mid-1920s (Ref 1, 2, 3). They remain the highest volume part produced by the P/M industry. The major advantage of porous bearings is that porosity in the bearing acts as an oil reservoir. The pores are filled with a lubricant that comprises about 25 vol% of the material. When the journal in an oil-impregnated self-lubricating bearing starts to turn, friction develops, the temperature rises, and oil is drawn out of the press because of the greater coefficient o expansion of the oil compared with the metal and because of the hydrodynamic pressure differential in the oil film between the journal and the bearing. When rotation stops and the bearing cools, the oil is reabsorbed by capillary action. For many self-lubricating bearings, lubricant contained in the pores of the bearing remains for the entire service life of the bearing. On some heavy-duty bearing applications, an oil reservoir that feeds additional oil through the bearing wall may be provided on the outside diameter of the bearing. Figure 1 shows typical examples of arrangements for supplementary lubrication of porous bearings. The most common shapes of self-lubricating bearings are shown in Fig. 2.
