Friction, Lubrication, and Wear Technology (1997) Part 10 potx

Brake Rubbing Speed Effects. Organic brake linings show little variation in their specific wear rate with braking speed, when tested from low initial cast iron temperatures. There typically is a slight rise of wear rate below a rubbing speed of 2 m/s (6.5 ft/s). Brake asperity "flash" temperatures are known to vary primarily with speed, as has been described by Blok (Ref 2) and, more recently, by Lim and Ashby (Ref 3). Above 1 m/s (3.3 ft/s), asperity temperatures appear to range from 1000 to 1100 °C (1830 to 2010 °F). It is presumed that the variation of lining wear rate and the associated variation of friction level are related to this flash temperature transition. At very high rubbing speeds, the lining wear rate increases. This increase is greater when the initial brake drum or disk temperature is high. It is presumed that this speed effect is simply the result of higher interfacial temperatures. Model wear data, presented later, support this presumption. Semimet friction materials also exhibit unique behavior with speed. At rubbing speeds below 2 m/s (6.5 ft/s), semimet lining wear rates also increase, but to a significantly greater extent than do the organic linings. Thereafter, the semimet brake linings provide a nearly constant specific wear rate with rubbing speed, until a transition condition is reached. Higher speeds then generate much higher lining wear rates (to 100 times, or more). During full brake dynamometer testing of semimet brake linings, it was found that four brake stops from 160 km/h (100 mph) produced as much lining wear as over 500 brake stops from 50, 65, 100, and 130 km/h (30, 40, 60, and 80 mph). There are differences among commercial semimet brake lining formulations in terms of rubbing speed transition values. Higher transition speeds were found with a semimet lining that contained a small amount of para-aramid pulp. It is conjectured that this high-strength thermoplastic material provided enhanced near-surface brake lining strength and helped to prevent "friction welding" wear of the lining to the brake rotor. Brake Temperature Effects. The variation of brake lining specific wear rate with brake drum or disk cast iron temperature is shown in Fig. 1 for four representative lining classes. The friction materials are divided into semimet (SM), light-duty (LD) organic, heavy-duty (HD) organic, and original equipment (OE) organic classes. Because asbestos and NAO materials have overlapping wear properties, they are not separated. Fig. 1 Temperature effects on brake lining specific wear rates Semimet linings also exhibit a typical behavior that requires some explanation. These friction materials establish a transfer layer to the cast iron during break-in and initial service. Until established, the semimet wear rates are several times the normal rate. This cast iron conditioning process occurs faster with higher surface temperatures, higher rubbing speeds, and higher unit pressures. It also varies somewhat among the different semimet lining formulations, and even with differences of cast iron countersurface, such as texture, residual stress, and oxidation. Once the cast iron surface is conditioned, it is believed that a back-and-forth transfer of material takes place. Material exchange between the cast iron transfer layer and the semimet lining surface helps to keep the measured lining wear rate low. No direct cast iron contact or wear occurs, except for incidental scoring that is due to particulate contamination, after the transfer layer is established. This is further described in the section on brake drum and disk wear. Brake Usage Severity Effects. Figure 2 shows the wear life behavior for several classes of brake linings under different severities of usage. It can be seen that no friction material type is best for all usage conditions. Inexpensive aftermarket (AM) friction materials have acceptable wear lives under light usage conditions, but wear rapidly under more stringent conditions. Heavy-duty (HD) friction materials have a lower wear life variation with usage severity than most automotive materials, but are superior in wear behavior only for severe-duty usage. Sintered metallic friction materials also may have this wear characteristic, with low variations of wear rate at different usage conditions. Semimet (SM) brake linings are best in the middle range, but poor in very light duty conditions. Original equipment (OE) brake linings have a broad range of acceptable wear life. From Fig. 2, it should be clear that friction material wear life values are essentially meaningless unless usage conditions are specified. It is not possible to select the optimum friction material for wear without knowledge of the customer brake usage distribution. Fig. 2 Brake lining wear life versus usage severity Brake lining Wear Modeling Archard's equation is applicable to automotive brake systems only at very low rubbing speeds and component temperatures. Consequently, this linear relation between volume of wear and the product of sliding distance and applied load is not generally applicable for brake wear modeling. Forcing friction material wear data to fit polynomials of speed, rubbing distance, and load terms has been tried, but with very limited applicability. Rhee (Ref 4) and others have related measurements on friction material wear with the brake cast iron temperature. These data indicated an essentially constant wear rate for low temperatures and a nearly exponential one at elevated temperatures. Lining Cure Effect. Weintraub and Bernard (Ref 5) derived a model equation which showed that some pyrolytic gas chromatography (PGC) peak areas of a brake lining resin behaved in an orderly manner with curing time and temperatures. Simple laboratory-prepared formulations were used. They also showed that laboratory friction material wear rates for these simple brake linings were linearly related to these PGC peak areas. This work led to a simple, second- order Arrhenius-type model that was expressed as: m = W + Kte (R/T) (Eq 1) where m is the mass loss (on a test of fixed frictional work), W is a wear constant, K is related to the Arrhenius collision coefficient, R is related to the Arrhenius activation energy, t is the brake lining cure time, and T is the brake lining cure temperature. Additional studies by Weintraub, Anderson, and Gealer (Ref 6) showed that this wear relationship was applicable to additional friction material binder resin types in brake lining model formulations that approximated production linings. Interfacial Temperature Effects. A similar expression was shown by Anderson (Ref 7) to be applicable to most brake lining wear data, but now relating the in-use friction material wear rate with the mean interface temperature by V = A + Be (C/T) (Eq 2) where V is the specific wear rate (wear volume per unit frictional work), A is the low-temperature lining wear constant, B is a friction material wear constant, C is another friction material constant, and T is the average interface temperature during braking. This equation has been quite effective in relating brake lining wear data with testing conditions for a broad range of materials, over most of the brake operating temperature range. Model formulations and production friction materials had full-brake wear behavior that was found to be well-characterized by this relationship. The production linings had several organic components, making the applicability of this equation surprising. Experimental values found for the constant C appear to have functional significance. For most friction materials, this activation-energy-related term has been determined experimentally to vary from 65 to 85 kJ/mol (16 to 20 kcal/mol). Such values might well be expected from bimolecular reactions in the wear process. Because the constant term A dominates at low operating temperatures, it is easily determined experimentally. However, following some severe usage conditions, especially after high friction material soak temperatures, the value of A was found to increase and subsequently remain at this increased value. The increase appeared to be associated primarily with the friction material soak temperature, but also with some dependence on the time at temperature. It does not appear to be simply related to the mean interface temperature during braking. Thus, a better model expression would have the constant term replaced by one that includes prior thermal history. This new term also could include particulate contamination factors for an even more sophisticated model. As the temperature increases, the thermally sensitive Arrhenius-type term increases in influence. The temperature required in the above wear equation is the mean rubbing interfacial temperature. This is not easily measured, but it can be calculated. The readily calculable interfacial temperature rise for the brake application is added to the measurable bulk cast iron temperature of the brake drum or disk. Values for B and C can then be determined from experimental wear data for temperatures, usually in the 150 to 350 °C (300 to 660 °F) range. Whether this wear equation is technically correct or not is not at issue. It generally works well, and unique wear mechanisms appear to be involved when it does not. To a pragmatic experimentalist, this has been quite acceptable. Brake Drum and Disk Wear Gray cast iron is the dominant material for both brake drums and disk brake rotors. Brake cast iron typically has a type A graphite, with a pearlitic matrix and low ferrite and carbide content. Normal brake iron provides adequate damping and good resistance to thermal fracture in hard service. The wear of either the cast iron or other countersurface is not readily determined by the Arrhenius-type model relationship. It appears that several mechanisms affect the specific wear rate of the cast iron. These include abrasive, adhesive, and oxidative terms. The abrasive terms include components from the abrasive content of the friction material, external contaminants, and abrasive particles that are "manufactured" at the rubbing interface. The latter can result, for example, from vitrification (firing) of clays at the hot rubbing surface. Brake Lining Chemistry Effects. Some friction materials have constituents that are chemically active at the rubbing interface. Such brake linings have been found to be extremely responsive to changes in the cast iron chemical composition. Hatch (Ref 8) described the effect of titanium and rare earth oxide content on lining friction. Examples of large brake effectiveness differences with the same brake lining, but different cast irons, were shown. Regrettably, this article did not make clear that most friction materials do not provide any detectable difference in effectiveness, only the few that chemically interact with the cast iron. With one such disk brake lining, the in-service cast iron wear life was increased by a factor of 50 when the titanium content was increased from 0.02 to 0.04%. The presence of small, hard particles in the cast iron from the titanium was credited for the improved wear resistance on this countersurface. Titanium content control can be particularly effective in disk brakes, if parasitic drag of the brake linings causes local wearing of the rotor. Cast iron machinability considerations limit the useful titanium content of 0.05%. Graphite Morphology Effects. The graphite size and shape were found to affect the cast iron and lining wear rates for many brake linings. "Damped" gray cast iron, with its very large graphite flakes, has been used in some brake applications to decrease brake squeal. However, this iron is weaker, and tends to have poorer wear resistance than conventional brake iron. It also increases brake lining wear rates for some friction materials. This presumably results from the cutting action of sharp iron edges found around the large graphite flakes. Linings with hard resins and rigid matrixes were found to have lower wear rates with decreasing graphite size. Special cast iron brake test parts were made with very fine graphite structures and with about 0.04% titanium. Using a hard and abrasive nonasbestos truck brake block, this permanent mold cast iron provided a 30% reduction in the brake lining wear rate, and had a 240% improvement in cast iron life, compared with a conventional brake cast iron. For the NAO heavy truck brake blocks in particular, the cast iron chemistry and graphite morphology can exert a strong influence on the countersurface wear life. It appears that many nonasbestos materials cause the cast iron to become a more active member of the friction and wear couple. Consequently, closer control of the cast iron may be required. Each new friction material should be tested for cast iron sensitivity, to ensure acceptable service life. Normal Cast Iron Wear. Usually, a worn cast iron rubbing surface develops a satin gray appearance. Low- abrasiveness passenger car brake linings provide cast iron specific wear rates from about 0.2 to 1 mg/MJ (1.6 to 8 × 10 - 5 in. 3 /hp·h) in the absence of external contamination or rusting. Countersurface wear and scoring can result from abrasives that are an intended friction material constituent. Some abrasive content to the friction material is desirable, for example, to remove rubbing surface rust after periods of extended nonuse and to control brake lining transfer. Such abrasives also can be used to increase the lining friction level, or to control cast iron crack growth in severe-duty brake linings. Cast iron wear rates from 5 to 16 mg/MJ (4 to 13 × 10 -4 in. 3 /hp ·h) can result. At the higher values, the weight loss of cast iron can exceed that of the brake lining wear. The cast iron wear rate is sensitive to a number of factors. An abrasive will wear the cast iron if it has higher hardness, higher melting-softening temperature, and sufficient particle size. Litharge and barite are both softer than cast iron at room temperature. With normal brake usage, barite will increase the cast iron wear, whereas litharge will not, because barite has a higher softening temperature than cast iron. Even materials that meet the hardness and softening criteria may not produce severe cast iron wear, unless they are of a size that can cause abrasion. A brake lining matrix hardness varies considerably with formulation and with temperature for a given formulation. Some brake linings soften significantly, and can produce cast iron wear rates as shown in Fig. 3, trace A. Other linings have a nearly constant matrix hardness, and result in wear like that of trace B. The cast iron wear rate increases at elevated temperatures, mirroring the lining wear rate. When abrasives are evenly distributed throughout the brake lining, cast iron wear rates directly follow the lining wear rates. If external abrasives are present and dominant, wear behavior like that of trace C results. The decrease of cast iron wear at higher usage temperatures then appears to result from softening of the brake lining matrix and abrasive clearance from higher lining wear rates. Fig. 3 Cast iron specific wear rates versus brake temperature. See text for discussion. Local Cast Iron Wear. Brake rotors can generate local brake lining contact zones when there is a significant axial runout. Figure 4 shows the axial runout and thickness variation (TV) of a rotor that had minimal brake usage, but developed a thickness variation from parasitic wear (unintended brake dragging) during highway driving conditions. Road dust contamination aggravated the wear. Similar wear patterns have been seen with semimet brake linings, because of their magnesium oxide abrasive content. Fig. 4 Rotor axial runout and thickness variation from local drag wear Highly localized cast iron wear is found on the inboard face of the rotor, centered at a site slightly in advance of the maximum runout position. The resultant brake pedal vibrations from this wear were sufficient to require the remachining of the rotors after only 17,000 km (10,560 miles) of dusty highway service. Often, a small increase of cast iron matrix hardness and hard particle content will reduce the local cast iron wear rate greatly. Extreme scoring of cast iron can result from an inadvertent contamination of the lining by abrasives. For example, silicon carbide was a "tramp" impurity in the synthetic graphite used in a semimet brake lining formulation. After 2000 km (1240 miles) of customer service, one rotor face was deeply scored over the entire rubbed surface. The opposite rotor face showed no scoring, with the original grinder marks still evident on the surface. Both brake linings were of the same semimet formulation, but from different production batches, one with silicon carbide and the other without. Laboratory testing showed the contamination level at a few tenths of a percent. Hot spotting of the cast iron can produce local wear at the heated sites. If martensite is formed on the cast iron surface, local high spots will result. Martensite and hot spotting have been addressed by Anderson and Knapp (Ref 9). External Abrasive Effects. Dust or splash shields can be used to reduce disk lining and rotor cast iron wear from particulate contamination, but attendant restricted brake cooling may increase lining wear rates. External contaminants can be kept from drum brakes by labyrinth seals or shrouding with a small loss of cooling efficiency. However, retention of normal wear products and accumulated rust particles within the brake sometimes then contribute to increased wear rates. With riveted linings, the cast iron wear may be heavily biased along the path of the rivet holes, particularly those at the leading edge of the linings. Careful testing and intelligent compromises may be needed to balance cooling and contamination effects. Transfer coatings onto the cast iron can be generated by semimet brake linings, as described in the section on brake lining wear. These coatings can vary from a few tenths of a micron to 40 m (1.6 mils) or more. Once formed, the wear of the original cast iron becomes zero, except from local scoring that is due to abrasive action. However, the transfer coating is by no means static. With extended operation at low temperatures and especially for rubbing speeds below 2 m/s (6.5 ft/s), the coating will deplete. This appears to result from a preferential transfer to the friction material, because very low lining wear rates are measured during the period when the coating is depleting. Once depleted, the brake lining wear rate increases by a factor of about 4. The cast iron then wears at a substantial rate, and its appearance changes from a dark blue-black coloration, with a matte finish, to a shiny gray. With time, the cast iron usually develops uneven wear across the rubbing path. During near-transition usage, the cast iron coloration includes bands of brown, blue, and purple. With continued low-temperature, low-speed usage, the cast iron wear mass removal rate has been found to approach that of the friction material. If these conditions continue, the disk brake linings will wear out at about 20% of the expected mileage, and the rotors will have become unserviceable from the high and generally uneven wear. Normally, such high wear rates are associated with severe usage. In this case, a change of wear mechanism causes high wear under very light- duty, low-temperature usage. If the cast iron wears under light-duty conditions, but not under hard service, one might redesign the brake to ensure higher temperatures. This measure is acceptable, within limits. Continued high temperature and hard brake usage cause another unique condition. The transfer layer may become so thick that it locally delaminates from the cast iron rotor. This occurs when the transfer layer thickness is about 30 m (1.2 mils). Figure 5 shows a cracked and incipient failure state for the semimet transfer layer on a medium truck disk brake rotor that had seen severe service on a mountainous test route. Local high spots, around 1 mm (0.04 in.) in diameter, signal these delamination sites. The light bands near the center of the photograph are regions where the coating has broken away locally. Fig. 5 Semimet transfer layer cracks and blisters Figure 6 shows a more severe case, with the same semimet lining and brake usage. The cast iron is visible at the bottom of the recesses, where the transfer coating has flaked off. Machining marks could still be seen at these sites. Based on a simple scratch test, the coating was found to be quite adherent. Presumably, the delamination process resulted from high thermal strains at the coating-cast iron interface. Fig. 6 Semimet transfer layer delamination Design analysis of brakes for semimet linings thus appears to involve assurance that light-duty brake usage will not produce rotor wear and severe usage will not result in transfer layer delamination. Consequently, optimal design would prevent these usages in most operating situations, and would result in rare occurrences of either wear type. The greater the variation of usage severity in customer service, the greater the possibility of such cast iron wear issues. Toxicity of Brake Wear Debris When brake wear debris is considered, asbestos fiber is a common concern. However, several studies of brake lining wear emissions have shown only about 0.03% asbestos in the airborne wear debris. The low emission results from the high frictional contact asperity temperatures. Chrysotile asbestos converts to forsterite at about 800 °C (1470 °F), well below brake flash temperatures. Organic brake lining fiber emissions averaged 0.7 m (28 in.) in length. Therefore, the vast majority of the airborne asbestos did not meet the definition of a fiber by the Occupational Safety and Health Administration, the World Health Organization, and other authorities. Epidemiological data for automotive brake mechanics have shown no detectable excess cancer risk. The Environmental Protection Agency, using computer models for fiber exposure and cancer risk, predicted about 15 excess cancer cases a year from all U.S. brake wear. Semimet brake wear emissions appear to be of minimal toxicity. Fibers from the wear of other nonasbestos brake linings have not yet been collected and measured. These include mineral wool, glass fiber, slag wool, titanate fiber, phosphate fiber, carbon, and para-aramid fiber. Although lead was removed from most brake linings by 1980, barium, usually in the form of barium sulfate (barite) powder, is often found. Such heavy metal wear particles may affect chest x-rays of brake mechanics. Brake usage generates some fused-ring aromatic material, but in miniscule quantities, like those in lettuce. Until complete and definitive brake wear toxicity studies are completed, it would be prudent to minimize dust exposure from any brake wear debris. Automotive Brake Frictional Characteristics Brake linings inherently have some performance attributes, such as fade and fade recovery, but full evaluation of a friction material requires that it be installed into a full brake system. Vehicle brakes are required to operate under a wide range of conditions, from hard braking with a heavily loaded vehicle on a steep downhill slope to minimal brake usage on interstate highways. Vehicle brakes should be highly reliable and minimally affected by temperature, water, or other contaminants. Brake lining friction must be consistent throughout the life of the material. Brake Design Basis. Brakes are designed primarily on the basis of wear, pedal travel, brake system stability, and effectiveness properties. Brake effectiveness is defined as the ratio of the brake friction torque to the applied force. This term is used, rather than friction coefficient, because brake geometry factors often make the brake torque not linearly related to friction level. For example, effectiveness must be used to describe frictional performance of drum brakes, because of their large self-actuation characteristics. Self-Actuation. Frictional forces acting on the shoes of a simple drum brake may cause it to be further loaded against the drum, increasing its effectiveness. Such shoes are referred to as leading. Friction force on a trailing shoe causes it to oppose the application force, decreasing effectiveness. The increase in brake shoe loading, which is due to friction and geometry effects, is referred to as self-actuation. Although the geometric considerations of self-actuation influence brake effectiveness, it also is influenced by frictional properties of the brake lining. Figure 7 shows design effectiveness curves for different brakes as a function of friction coefficient. Assuming a nominal friction coefficient of 0.4 for each 1% change in friction coefficient, the brake effectiveness changes by approximately 3.5% with a duo-servo drum brake (two leading shoes, coupled), and by 2.6% on a leading-trailing drum brake, but the change is just 1% for a nonservo disk brake. Fig. 7 Effectiveness versus friction coefficient for several brake types Design Factors. In addition, the design of brakes can affect the degree to which friction material wear and thermomechanical properties influence brake effectiveness. A large change of brake effectiveness is possible, for the same value of friction coefficient, simply because of a change of lining pressure distribution. Figure 8 shows the change of effectiveness on a leading shoe that results from different pressure distributions. For a given value of friction coefficient, the effectiveness may vary by a factor of 4 as the pressure distribution varies from center-biased to end- biased. On a drum brake, the lining pressure distribution is affected by the stiffnesses of drum, shoes, and brake linings, in addition to thermal distortions, wear patterns, and prior usage history effects. [...]... per 10, 000 km of travel Tread wear index: (Wear rate of reference or control tire)/ (Wear rate of experimental tire) × 100 Standard wear rate: mils per 100 0 miles per 100 lbf cornering force at tire test temperature Camber angle: The vertical angle between the wheel plane and a line perpendicular to the pavement (Fig 1) Slip angle: The horizontal angle between the tire circumferential midpoint plane and. .. steel brakes, the wear that takes place at these low energies is not as significant as the main landing wear and is normally considered as being part of the main landing stop In contrast, the taxi wear in carbon brakes accounts for a large portion of the brake wear Aircraft brakes are designed to handle one rejected take-off (RTO) stop (the wear rate is normally one hundred to one thousand times greater... Walter, G.N Augeropoulos, M.L Janssen, and G.R Potts, Tire Sci Technol., Vol 1, 1973, p 210 33 A Grosch, The Speed and Temperature Dependence of Rubber Friction and Its Bearing on the Skid Resistance of Tires, The Physics of Tire Traction, D.F Hays and A.L Browne, Ed., Plenum Press, 1974, p 143 34 A.H Muhr and A.D Roberts, Friction and Wear, Natural Rubber Science and Technology, A.D Roberts, Ed., Oxford... temperature, the wear is characterized by abrasive wear Abrasive wear occurs when hard particles cut grooves across the opposing surface and displace material When brake energies are higher, the interface temperatures are also higher and an adhesive type of wear takes place Adhesive wear occurs when surface asperities, bonded together under localized high temperatures and pressures, shear apart during sliding... shear apart during sliding of the surfaces The wear debris may become trapped between the surface and contribute to abrasive wear Wear Rates Figure 2 shows wear rates based on normalized stack loading and average energy flux (AEF) For an aircraft to attain 100 0 stops (a stop is equivalent to a landing), assuming a five-pair brake with a 50 mm (2 in.) wear pin, wear rates of 0.0025 mm/face/stop (0.0001 in./face/stop)... Gealer, Wear of Resin Asbestos Friction Materials, Adv in Polymer Friction and Wear, Plenum Press, 1974 7 A.E Anderson, Wear of Brake Materials, ASME Wear Control Handbook, American Society of Mechanical Engineers, 1980 8 D Hatch, Cast Iron Brake Discs, J Automot Eng., Oct 1972, p 39 9 A.E Anderson and R.A Knapp, "Hot Spotting in Automotive Friction Systems," presented at 1989 International Wear of... treatments are employed for wear protection The different operating environments and types of materials in each section of the engine result in a variety of wear types, including fretting, impact, adhesive, high-speed and oscillatory sliding, oxidational, ingested particle erosion, and abrasive wear High-speed sliding wear occurs in rotating gas path seals Impact wear can occur in loose part assemblies or... can be related to a reference force value and corrected for surface temperature variation This corrected rate is known as the standard wear rate Incorporating Schallamach's temperature-correction relationship (Ref 8), Veith (Ref 12) developed the following equation for the standard wear rate: (Eq 5) where W°50 is the standard wear rate at 50 °C (120 °F) and 100 lb cornering force, F is the average cornering... understand wear in engineering terms, it is necessary to run special tests at single energies For example, 100 stops may be run at taxi energies, 100 more at service energies, 50 more at normal energies, and 20 at overload conditions After each condition, the brake is disassembled and wear measurements taken This approach enables the determination of which conditions result in the highest and the lowest wear. .. reflectance The dull bands have a high number of fissures or shallow grooves and scratches in the longitudinal fiber bundles, both of which are indicative of fiber removal caused by abrasion The debris material in the dull bands does not produce a high reflectance and appears less dense (more porous) than the bright bands The debris in the dull bands is particulate and is comprised of fibers and matrix This . 10, 000 km of travel • Tread wear index: (Wear rate of reference or control tire)/ (Wear rate of experimental tire) × 100 • Standard wear rate: mils per 100 0 miles per 100 lbf cornering force at. of brake linings and systems. These standards cover automobile, truck, and trailer brake system tests using both vehicles and dynamometers. Prior to the adoption of FMVSS 105 and 121, the SAE. Asbestos Friction Materials, Adv. in Polymer Friction and Wear, Plenum Press, 1974 7. A.E. Anderson, Wear of Brake Materials, ASME Wear Control Handbook, American Society of Mechanical Engineers,