DEVELOPMENT OF MATERIALS FOR AUTOMOTIVE DISC BRAKES

O. Maluf a; M. Angeloni a; M.T. Milana; D. Spinelli a; W.W. Bose Filho a

a Automobilistic, Aeronautics and Materials Engineering Department, São Carlos Engineering

School, University of São Paulo, São Carlos, SP, Brazil

Abstract

At the end of the 19th century the development of a brake system for the newly

invented automobile vehicles was needed. From that moment on, this equipment, which

makes use of several components (the brake disc among them), was developed. It was after

the beginning of the Second World War, in 1938, that the brake system technological advance

got great impulse due to the aeronautics industry necessity. Historically, the first material used

to make brake discs was the gray cast iron, which is a material that fits the requirements it is

intended for, such as: good thermal conductivity, good corrosion strength, low noise, low

weight, long durability, steady friction, low wear rate, and a good price/benefit ratio.

Therefore, for more than one hundred years, a great number of materials were developed with

this intention, but the most used until today is the cheap and easily produced gray cast iron.

Nowadays, a lot of emphasis has been given to the study of fatigue strength of gray cast iron

alloys through modeling to improve the service life of the component. Although this kind of

analysis presents meaningful results, experimental works are necessary to validate them, i.e.,

the component must be studied under real rather than only virtual conditions.

Keywords: Brake discs; gray cast iron; fatigue; graphite flakes.

Introduction

It was at the end of the 19th century, with the development of trains and the beginning

of car production, that the necessity of developing a brake mechanism for them emerged. Its

purpose is to decelerate a vehicle transforming kinetic energy into thermal energy using

friction and dissipating the heat to the atmosphere.

The history of automobiles began in 1700 when Nicholas Cugnot convinced the king

of France to finance his design of an almost 10-ton vehicle to drag the cannons used then, as

see in “Fig. 1”.

Figure 1. First known automobile.

The vehicle, powered by a steam boiler, reached the speed of 10 km/h. However, the

inventor found out, in his first voyage, that he had not thought about how to stop his

invention. Thus, the first automobilistic accident we have ever heard of happened, when the

heavy car brought a wall down. For this reason, he was parted from the court and died in

misery, but his merit cannot be denied. Cugnot's vehicle is preserved until today in Turin

Automobile Museum, in Italy.

Around 1886, in Germany, Gotlieb Daimler and Carl Benz would change the history

of the world forever, because they created, independently, the first prototypes of internal

combustion automobiles. They started to be effectively produced ten years later. This

invention gave rise to the development of several automobile components, and among them

was the brake system.

In the United States, in 1890, according to “Hughes, (1971)”, the American Elmer

Ambrose Sperry invented a brake similar to the present electromagnetic disc “Sperry, (1894-

6)”. The disc, known as magnetic brake, was placed in contact with another disc (brake disc)

to apply a braking torque. The interesting thing is that Sperry noticed that braking torque was

partially due to the Foucault streams and partially to the friction between the discs.

According to “Nicholson, (1995)”, the inventor of the first material used in brake

lining was Herbert Frood, in 1897. The material was made of cotton impregnated with a

bitumen solution and was used in both railway car wheels and the first automobiles. His

invention led to the foundation of Ferodo Company, a firm that produces brake materials until

today.

The history records the use of many types of different material for brakes (friction

materials). For instance, wood and leather were used in the brakes of railway cars. Until

today, many brake materials have organic based elements such as polymers and vegetable

fibers.

As a result of railroad transportation development, the brake material technology for

high loads and speeds was also developed and the first braking experiences began to be

carried out around the end of the 19th century “Fig 2”.

0 10 20 30 40 50 60

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Fric

tion

coef

ficie

nt(0

-3 s

econ

ds o

f bra

ckin

g )

Speed (mph)

Cast iron brake blocks Wrought iron brake blocks

Figure 2. Test results from the work of Captain D. Galton (1878) using a special railroad

wheel brake simulator and steel test wheels.

One of the first known equipments for braking with disc brakes is credited to the

English engineer Frederick Willian Lanchester, who patented the product “Clark, (1995) and

Harper, (1998)”. In his patent, he describes the disc brake as a metal disc, leaf shaped rigidly

connected to each of the back wheels of the vehicle. It has its borders pressed by a pair of

claws to brake.

This period also evidenced the development of new braking technology. For instance,

according to “Newcomb and Spurr, (1989)”, Mercedes and Renault companies introduced

variations in their prototypes of what would be the present drum brakes in 1903.

Below, in “Tab. 1”, there's a chronological sequence of the most common materials used for

brakes appearance, according to “Nicholson, (1995)”:

Table 1- Historical Compositions of Automotive Friction Brake Materials

Material Description

Application(s)

Approximate Year

Cast iron on steel used in blocks and other

components of railroad car brakes

1870

Cotton or hair (straps) cart wheel and the first automobiles 1897

Asbestos tissue and other wires to increase strength and

performance trucks and automobiles 1908

Bronze particles with low ash content bituminous coal, produced with internal

reinforcement of short fiber

trucks and automobiles 1926

Material molded from a dry mixture of post-metallic to replace the fragile cast iron brake pad on electric trains

London subway 1930

Flexible resin developed with agglutinant elements more

intricately formulated drum brake reinforcement 1930

Metal alloyed with resin for aeronautic industry use 1950 Glass, mineral, metallic,

carbon, and synthetic fibers to give semi-metals better

performance than amianthus (beginning of the discussion

about amianthus safety)

automobiles and trucks 1960

Compounds without amianthus drum brakes and car equipment 1980

Suggestion of carbon fiber use automotive brakes 1991

It is not necessary to say that “Lanchester, (1902) and Sperry, (1895)” first brake

designs were substantially modified along the 20th century, especially as to the materials used

and performance methods that were improved. Until the beginning of the Second World War,

in 1938, the braking system development took place slowly and gradually. Later, according to

“Harper, (1998)”, there was a fast and consistent advance impelled by aviation industry

demand. Clutch-type disc brakes were developed for aircrafts. The contact between the pads

and the disc takes place in an annular area, which extends itself over most part of the disc

angular area and it is this contact that generats friction. The concept is different from the one

used today on spot-type disc brakes of passenger cars, where the friction area is the upper

medium of the disc.

Ideally, the materials used to make brake systems must meet the following criteria:

good thermal conductivity, good corrosion strength, low noise, low weight, long durability,

steady friction, low wear rate and good cost/benefit ratio.

To attend the required properties, it is often to combine many materials with different

shapes and sizes. Nowadays there are more than 2000 types of different materials used in

commercial brake components “Weintraub, (1998)”.

The spot-type disc brake evolution for automobiles can be traced by the development

of Dunlop, Girdlingand Lockheed Corp. “Newcomb and Spurr, (1969)”, whose discs were

really similar to those used on automobiles today.

The geometry, the dimension and the chemical composition of brakes affect the heat

flow, the durability, the noise generation, the maintenance ease and consequently the

performance of the two main brake systems existing, the drum and the disc brakes.

Compared to the drum brake system, the disc brake system has higher wear resistance

and easier maintenance, especially in terms of its geometry “Halderman and Mitchell, (2000).

Among the several brake system components, this brief review will mainly focus on the disc

materials and their evolution during the last decades. The reader who is interested in a deeper

knowledge of automobile brake systems should refer to Halderman & Mitchell's book.

Brake discs

1. Chemical Composition

As it was seen before, the two main functions of a brake disc are the transmission of

considerable mechanical force and dissipation of the heat produced in the operation. The

temperature of work for passenger cars, depending on the severity and the number of

breakings, can be placed between room temperature and, in some cases, until 700º C. This

great range creates high temperature gradients in the disc, which causes thermal fatigue

problems. However, the component is also exposed to the action of a cyclic mechanical load

applied by the pads during a stop, what contributes to the component's wear. Therefore, in real

conditions of work, the disc is exposed to a thermomechanical fatigue loading condition

instead of only thermal “Mackin, (2002)”.

From a theoretical point of view, several materials would be able to attend the

requirements for a good performance. However, due to the better metallurgical stability

behavior, lower cost and comparatively ease of production, cast iron is often more used

“Brembo, (1997)”. Other types of material are used for high performance braking conditions,

in which the component is submitted to very high temperatures. For instance, composite

materials of carbon matrix are used to produce brake discs for competition cars and aircrafts.

They present excellent heat performance, but the manufacturing cost and the way of using it

make it not practical for common automobiles. Another example is the brake disc made of

cast iron with titanium, element that increases the component strength, but reduces the friction

coefficient, which can be a major problem when there is little braking distance. “Fig. 3”

shows the reduction of the friction coefficient with the increase of titanium percentage in the

disc “Brembo, (1997)”.

0.00 0.02 0.04 0.06 0.08 0.10

0.340.360.380.400.420.440.460.480.500.520.54

Fric

tion

coef

ficie

nt

% of Titanium

Figura 3. Friction coefficient against the percentage of titanium for a brake disc cast

iron“Brembo,(1997)”.

Aluminum alloys containing silicon carbide can also be used because they have low

density. However, in spite of lower weight, these alloys do not dissipate the heat as well as,

for instance, the gray cast iron. To use it, the exit strategy would be to increase the component

superficial area which would at least reduce the advantage of its lower weight “Mackin,

(2002)”.

Thus, comparatively, cast irons are the preferred materials to produce braking system

components and are usually classified in the following way: gray, nodular, vermicular,

malleable and white. Nowadays, gray cast iron alloys present the best price/benefit ratio for

automotive brake discs use, especially in those used for more popular automobiles. For the

production of the several types of cast iron, the metallic load to be molten is usually

composed of pig iron (reduced iron ore), scrap iron and alloy elements such as silicon iron,

manganese iron, chrome iron, nickel iron, and others.

All the alloy elements tend to increase mechanical strength, and the most efficient are

vanadium, molybdenum and chrome. For example, the most used alloys to produce brake

discs by General Motors in Brazil are the gray cast irons, with or without other chemical

elements, as seen in “Tab. 2”.

Table 2 – Chemical composition (Weight %) of some grey cast irom employed by GM

Brazil, “GME-05002, (1999)”.

As a result, gray cast iron alloys, depending on the chemical composition and cooling

speed, present microstructures with the carbon shaped as graphite flakes over matrices that

can be ferritic, ferritic-pearlitic, and pearlitic.

2. Physical and Mechanical Properties

2.1 Diffusivity

Cast irons are indicated to produce brake discs because besides the low costs of the

production, they have excellent thermal conductivity, which eases the dissipation of the heat

generated by the friction of the pads during a stop, and the capacity of damping vibrations,

which are prime characteristics of this kind of component.

One of the ways to improve the heat transfer ability is to increase its thermal

diffusivity. It describes the heat diffusivity during the transient process, and is also a

fundamental parameter to the brake design. Thermal diffusivity may be used to calculate the

heat conductivity, which is an inherent measure of how well the material transmits heat.

Brake discs with improved thermal conductivity present increase in thermal fatigue strength

“Jimbo et al. (1990)”, making it possible to increase its service life, that is, if the gray cast

iron characteristic of conducting heat can be increased without sacrificing other objective

criteria, the disc performance can be improved. The main factors that influence the heat

transfer in cast irons are distribution, size and morphology of the graphite phase, because

graphite flakes have much higher heat conductivity than the matrices. The cast iron heat

conductivity is proportional to the ratio between its surface area and its respective volume.

Thus, cast irons with nodular graphite “Fig. 4(a)” are worse conductors than the vermicular

ones “Fig. 4(b)”, which in turn are worse than the ones with graphite flakes “Fig. 4(c)”.

(a) (b) (c)

Figure 4. (a) Pearlitic ductile cast iron showing the “bull’s eye” structure, which consists of

graphite nodules surrounded by free ferrite (white). Etching: Nital 2% “Milan et al. (2004)”.

(b) Vermicular cast iron. Etching: Nital 2% “Milan et al. (2004)”. (c) Pearlitic matrix gray cast

iron depicting several graphite flakes. Ataque: Nital 2% “Milan et al. (2004)”.

In 1983, “Okamoto et al. (1983)” reported that the cast iron thermal conductivity is

proportional to the graphite flakes shape (or the graphite nodules). In 1990, “Jimbo et al.

(1990)” described the benefits of using cast iron with high thermal conductivity in brake

discs. However, the change in friction characteristics such as the friction coefficient value and

the amount of particles generated by friction, analyzed as a gray iron microstructure function,

has not been much discussed in literature. Little attention has been given to the correlation

between the relative quantity of phases in gray iron and the performance under friction. In

1999, “Hecht et al.(1999)” observed that gray cast iron thermal diffusivity is influenced by a

subtle change in chemical composition and presented a linear relation between thermal

diffusivity and equivalent carbon (or maximum length of graphite flakes), as observed in “Fig.

5”. Linear dependence was expected, because increasing C or CE amounts is the fastest way

of improving gray cast iron graphitization. There was a 25% increase of diffusivity in brake

discs with a increase of 0.40% of EC “Yamada and Kurikuma, (1998)”.

“Hecht et al.(1999)” also noticed that in gray cast irons, diffusivity is reduced with the

increase of the temperature until 500º C. Above this temperature, the difference noticed in

diffusivity of alloys with similar values in 500º C is reduced. These data suggest that high

diffusivity and thermal benefits can be reached by controlling the casting process of brake

discs in gray cast iron with longer graphite flakes.

4,10 4,15 4,20 4,25 4,30 4,35 4,40 4,45 4,50 4,550,15

0,16

0,17

0,18

0,19

0,20

Them

al D

iffus

ivity

cm

3 /sec

Carbon Equivalent %

Brake rotor samples Cast iron slug samples

Figure 5. Room temperature thermal diffusivity increases as a function of carbon

equivalent (CE), where CE = %C + 1/3(%Si + %P) “Hecht et al. (1999)”.

The solidification rate influences the morphology of graphite flakes, so different

configuration in casting molds, such as the number of cores, type of molding sand and process

or new component design can modify, even if just a little, graphite flakes morphology.

Knowing how graphite morphology influences the heat transfer, it is possible to analyze how

a process variation during casting the discs can change their performances, because as it was

seen before, an important three-dimensional property for heat transfer is the graphite

superficial area/volume ratio. Nowadays there are computer programs capable of measuring

morphological parameters of graphite flakes analyzing two dimensional images, dividing the

perimeter (P) by the area (A) enclosed by it, as seen in “Fig. 6”. It is also known that the

longer the length of the flakes, the higher the diffusivity and conductivity “Hecht et al.

(1999)”.

Figure 6. Schematic representation showing the parameters defined to characterize graphite

flake morfology. A commercial image analysis software program was used to measure the

dimensions defined here: P perimeter length of graphite shape, A area of graphite shape, L

actual length of graphite shape, X Major axis of graphite flake shape, Y Minor axis of

graphite flake shape“Hecht et al. (1999)”.

2.2 Abrasion strength

The diffusivity value on ferrite phase is higher than the one presented by the pearlitic

microstructure. Thus, it could be concluded that best iron alloy to brake discs production

would be a gray iron presenting longer graphite flakes on a ferritic matrix. This is not true

because the matrix type influences diffusivity just a little “Fitzgeorge and Pope, (1959), and

Omerod, (1978)” and the disc must present good mechanical strength, especially regarding

abrasion. This is why most brake discs used in passenger cars are produced with cast irons

whose microstructure presents graphite flakes on pearlitic matrix. Pearlite, consisting of

intercalated lamellae of ferrite phase and cementite phase (Fe3C), results in a matrix with

higher mechanical strength “Omerod, (1978)”.

To avoid producing the components from gray cast iron alloys with identical chemical

composition and different mechanical properties, it is necessary that the solidification rate is

the same, so that the microstructure is not modified. As a consequence, molding, fusion and

casting parameters have to be strictly controlled at every new production.

If the chemical composition is modified by means of adding alloy elements, the

mechanical properties will be changed. In other words, to modify mechanical properties, two

factors are really important: cooling speed and chemical composition.

Despite being possible to produce good quality cast irons without adding alloy

elements, just controlling the carbon, silicon and manganese contents and the production

technique, it is usual to add alloy elements to obtain not only higher strengths with higher

equivalent carbon, but also because microstructures are less susceptible to thickness variation

of the parts. Practically all alloy elements tend to increase the tensile and toughness strength,

and the most efficient to gray cast iron are manganese, vanadium, molybdenum, chrome and

copper. Manganese promotes the pearlitic microstructure formation, increasing mechanical

strength and toughness; vanadium is a strong pearlite former and graphite refiner;

molibdenum increases mechanical strength to heat, creep and corrosion, Cr increases

corrosion and abrasion strength and Cu is a strong pearlite stabilizer.

The hardening effect produced by alloy elements in solid solution in ferrite can be

seen schematically in “Tab. 3” “Milan et al. (2004)”. Notice that despite all elements increase

the alloy strength, the ones that form interstitial solid solution have a stronger effect than the

substitutional ones.

Besides the direct way, by forming a solid solution and second-phase precipitates,

alloy elements can indirectly act upon the increase in strength through grain refining,

dessulfuring or globulizing sulfides, stabilizing carbides, degasifying and also increasing the

material hardenability

Table 3 – Solid solution hardening effect of alloying elements in ferrite phase. The

arrow points to an increasing effect “Milan et al. (2004)”.

“Rhee et al. (1972)” pointed that a change in the disc chemical composition affected

very strongly both mechanical properties and the lining/disc wear rates. “Libsch and Rhee,

(1979)” related the effects of variation in chemical composition upon the disc. They showed

that small changes on alloy elements level changed the lining and disc wear level. “Anderson,

(1992)” stated that thin graphite flakes reduce lining and disc wear. “Zhang et al. (1993)”

studied cast iron triblological properties in terms of phosphorus concentration and graphite

morphology and observed that cast iron with phosphorus and compact (or vermicular)

graphite morphology presented high friction coefficient and low mass loss due to wear. “Cho

et al. (2003)” investigated gray cast iron features regarding phase quantities, graphite and

ferrite, and the microconstituent pearlite, controlling the microstructure by the carbon quantity

variation (equivalent carbon) and cooling speed. They noticed the free ferrite quantity and the

pearlite quantity in the gray iron disc do not affect the friction coefficient. Additionally, they

observed a friction coefficient increase with a higher graphite area percentual.

2.3 Fatigue

It is known that the brake disc form and physical properties can affect the braking

performance and cause problems such as vibration, material loss during the stop and noise

“Metzler, (1990) and Rhee, (1972)”. This happens because gray cast iron physical properties,

such as thermal conductivity, vibration weakening, thermal expansion coefficient and the

specific heat, change according to the phases present in gray cast iron Chapman and Mannion,

(1982)”.

In the last decades, great attention has been given to improve brake discs performance

concerning its behavior when there is friction with the brake pads. This great effort led to

materials development, such as: non-ferrous copper alloys, aluminum matrix composites and,

nowadays, carbon composites “Wycliffe, (1993) and Rhee, (1970)”.

However, gray cast iron is still very used in automobile disc brakes because, as

mentioned earlier, besides having good physical and mechanical properties, it is cheap and

easily produced. Nevertheless, a more detailed characterization of these alloys is necessary,

especially concerning the wear strength to determine which one of them is more appropriate

to braking because, with trucks and passenger cars performance improvement, there is a

growing increase of the demand for brake discs technological improvement.

Because it is, perhaps, the most important piece of the disc brake system, the brake

disc is submitted to three types of fatigue: thermal, where the component is only submitted to

abrupt temperature variations with no load; isothermal, where the temperature is stable and

the load, variable; and thermomechanical (in or out-of-phase), where temperature and load are

changeable. The most important, for representing better the conditions the disc is submitted to

during the braking process, is the out-of-phase thermomechanical fatigue, where the

maximum load is applied when the temperature is the lowest possible and vice-versa.

The discs exposed to high thermal stresses require materials that offer better strength

to thermal fatigue. In these cases, gray cast irons with nickel, chrome and molybdenum have

been used “Yamabe et al. (2002)”, as the ones used by General Motors, for example, which

posses 12% nickel, more chrome and molybdenum “Sakamoto et al., (1993)”. However, the

disadvantage of these alloys is, without a doubt, the high cost.

“Yamabe et al. (2002)” studied the possibility of producing truck discs with a cheaper

cast iron alloy with high thermal fatigue strength. They used prototypes of different alloys

with variable quantities of elements and some of them had cerium inoculated. They verified

the connection between the material thermal fatigue strength, its physical and mechanical

properties, and its corresponding microstructure. They noticed that the higher graphite

quantity is distributed on a matrix with refined grains, the lower the crack propagation rate is,

and that the number of graphite flakes increases with nickel addition and some cerium

inoculation. They also observed that the thermal fatigue limit is not severely affected by the

number of cycles before the beginning of the crack, but after it. From this survey, a low cost

brake discs alloy was developed, equivalent to the one used nowadays.

3. Conclusion

It is noticed that, historically, despite all the evolution that took place during the 20th

Century, brake discs of automobile vehicles are still produced with the centennial and very

cheap gray cast iron, which still offers the best price/benefit ratio, even after receiving some

alloy elements addition.

From the technological point of view, along the time a great number of experiments

were accomplished, especially after 1938, with the beginning of the Second World War.

However, as experimental projects like the ones described above are really expensive and

slowly executed, nowadays the advance of the development of materials and brake discs

project has been almost exclusively based on modeling in terms of low cost and speed. In

modeling, one tries to predict the component behavior (life), through finite elements,

simulating several parameters under thermal, isothermal, and especially thermomechanical

fatigue, which is the one that represents best the real braking. Although this type of analysis

presents meaningful results, there is the need of experimental works to validate them, i.e., it is

necessary to study the component under real rather than only virtual conditions.

The literature about experimental thermal and isothermal fatigue is scarce, and even

less about thermomechanical fatigue can be found. This is preoccupying, because only

experimental fatigue tests can definitely validate the results obtained through modeling and

elucidate the active mechanisms in the component.

With a view to cooperating for the enlightening of the issue, a techno-scientific

project, with an experimental nature, is being developed in the Automotive, Aeronautics and

Materials Department at the University of São Paulo - USP in association with General

Motors of Brazil, in which it is intended to determine isothermal, thermal and

thermomechanical fatigue properties in several temperatures by low cycle fatigue testing of

seven different gray cast iron alloys used to produce brake discs. Moreover, traditional tensile,

impact, toughness and diffusivity tests will help to distinguish them better. From then, it will

be tried to establish life prediction models to disc brakes, trying to reduce the number of

possible alloys and creating a database concerning the alloys mostly used in the component

production. The bigger range of knowledge will allow a faster development of more trustful

products which will bring important commercial benefits by the gain in scale economy and

the decrease of item variety in stock.

Acknowledgements

This work is a part of an applied research program in the behavior of the automotive

brake discs and the financial support of General Motors of Brazil through the FIPAI is

gratefully acknowledged.

References Anderson, A.E., 1992, Friction and wear of automotive brakes, Materials Park, OH, ASM Handbook, vol. 18. Brembo, 1997, Il Manual del Disco Freno, capitulo 2. Chapman, B.J., Mannion, G., 1982, Titanium-bearing cast irons for automotive braking applications, Foundry Trade Journal, vol. 23, pp.230–246. Cho, M.H., Kim, S.J., Baschk R.H., Fashk J.W., Jang, H., 2003, Tribological study of gray cast iron with automotive brake linings: The effect of rotor microstructure, Tribology International, vol. 36, pp. 537-545. Clark, C.S., 1995, The Lanchester Legacy, a Trilogy of Lanchester Works, Vol. 1, Butler &

Tanner, Frome and London, England, pp. 1895–1931. Fitzgeorge, D. and Pope, J.A., 1959, Transactions of the North East Coast Institution of Engineers and Shipbuilders, vol.75, pp. 284. GME-05002, 1999, Engineering Standards Europe, General Specification to Brake Disc., pp.1-7. Halderman, J.D., Mitchell Jr., C.D., 2000, Automotive Brake Systems, 2nd Edition, Prentice-Hall, Upper Saddle River, NJ. Harper, G.A., 1998, Brakes and Friction Materials: The History and Development of the Technologies, Mechanical Engineering Publications Limited, London, England. Hecht, R.L., Dinwiddie, R.B., Porter, W. and Wang, H., 1996, SAE Technical Paper Series 962126. Hughes, T.P., Sperry, E., 1971, Inventor and Engineer, John Hopkins Press, Baltimore, MD. Jimbo, Y., Mibe, T., Akiyama, K, Matsui, H., Yoshida, M. and Ozawa, A., 1990, SAE International SAE 900002 pp. 22. Lanchester, F.W., 1902, Improvements in the brake mechanism of power-propelled road vehicles, G.B. Patent No.26407. Libsch, T.A., Rhee, S.K., 1979, The effects of compositional variations in heavy truck rotors upon rotor and pad wear, Wear of Materials vol.34, pp. 350. Mackin, T.J., 2002, Thermal cracking in disc brakes, Engineering Failure Analysis, February, vol 9, no. 1, pp. 63-76(14). Metzler, H., 1990, The brake rotor-friction partner of brake linings, SAE Technical Papers, no. 900847. Milan, M., Maluf, O., Spinelli, D., Bose Filho, W.W., 2004, Metais - Uma visão objetiva, Ed. Suprema, São Carlos, Brasil. Newcomb, T.P., Spurr, R.T., 1969, Automobile Brakes and Braking Systems, R. Bentley Inc., Cambridge, MA. Newcomb, T.P., Spurr, R.T., 1989, A Technical History of the Motor Car, Adam Hilger, New York. Nicholson, G., 1995, Facts About Friction, P&W Price Enterprises, Inc., Croydon, PA. Okamoto, T, Kagawa, A, Kamei, K, Matsumoto, H., 1983, Effects of graphite shape on thermal conductivity, electrical resistivity, damping capacity and Young’s Modulus of cast iron below 500 deg. C, Journal of the Japanese Foundrymen’s Society, vol. 55, pp.32-36. Omerod, J., Taylor, R.E. and Edwards, R.J., 1978, Met. Tech., Abril ,pp.109. Palmer, K.B., 1960, J. Bcira, vol.8, pp. 266. Rhee, K., Turak, J.L., Spurgeon, W.M., 1970, An inertial dynamometer evaluation of three alloys for automotive brake drums, SAE Technical Papers, no. 700138. Rhee, S.K., DuCharme, R.T., Spurgeon, W.M., 1972, Characterization of cast iron friction surfaces, SAE Technical Papers, 720056. Rhee, S.K., Thesie,r P.A., 1972, Effects of surface roughness of brake drum on coefficient of friction and lining wear, SAE Technical Papers, no.720449. Sakamoto, H, et al., 1993, Heavy-duty brake disk for high speed trains(in Japanese with English summary), Technical Report of Sumitomo Metal Industries, Ltd., Vol. 45, No. 6, pp. 11–29. Sperry, E.A., 1894, The electric brake in practice, Transactions AIEE 11, pp. 682–728. Sperry, E.A., 1895, Electric brake, US Patent Nos. 534974, 534977. Sperry, E.A., 1896, Electric brake, US Patent No. 565937. Weintraub, M., 1998, Brake additives consultant. Private communication. Wycliffe, P., 1993, Friction and wear of Duralcan reinforced aluminum composites in automotive braking systems, SAE Technical Papers, no. 930187. Yamabe, J., Takagi, M., Matsui, T., Kimura, T., Sasaki, M., 2002, Development for disc

brake rotors for trucks with high thermal fatigue strength, JSAE Review, vol. 23, pp. 105-112. Yamada, S. and Kurikuma, T., 1998, Paper F98T683, Proceedings form 1998 FISITA World Automotive Congress, Paris, September-October. Zhang, Y., Chen, Y., He, R., Shen, B., 1993, Investigation of tribological properties of brake shoe materials-phosphorous cast irons with different graphite morphologies, Wear, vol. 166, pp.179–186.

DESENVOLVIMENTO DE MATEIRIAS PARA DISCOS DE FREIO AUTOMOTIVOS

O. Maluf a; M. Angeloni a; M.T. Milana; D. Spinelli a; W.W. Bose Filho a

a Depto de Engª de Materiais, Aeronáutica e Automobilística, Escola de Engenharia de São Carlos, Universidade de São Paulo, São Carlos, SP, Brasil

Resumo No final do século XIX surgiu a necessidade da criação de sistemas de frenagem para

os recém inventados veículos automotivos. A partir de então, e durante o século XX, deu-se o desenvolvimento destes aparatos que utilizam diversos componentes, dentre eles, o disco de freio. Foi após o início da segunda guerra mundial, em 1938, que o avanço tecnológico dos sistemas de freio, em termos de projetos e materiais, ganhou grande impulso devido à demanda da industria aeronáutica. Historicamente,o primeiro material usado na fabricação dos discos de freio foi o ferro fundido cinzento que é um material que se adequa as características que se exige para o fim a que se destina, ou seja: boa condutividade térmica; boa resistência à corrosão; baixo ruído; baixo peso; longa durabilidade; atrito estável; baixa taxa de desgaste e boa relação custo/benefício. A partir daí e ao longo de mais de cem anos foram desenvolvidos um grande número de materiais para este fim, mas o que ainda continua sendo o mais usado é o ferro fundido cinzento que é barato e fácil de produzir. Atualmente muita ênfase tem sido dada ao estudo da resistência à fadiga das ligas de ferro fundido cinzento através de modelagem computacional, com o intuito aumentar a vida útil do componente. Embora este tipo de análise apresente resultados significativos, existe a necessidade de trabalhos experimentais que os validem, ou seja, há necessidade do componente ser estudado sob condições reais e não apenas virtuais. Palavras-chave: Disco de freio; Ferro fundido cinzento; Fadiga; Veios de grafita.