JSAE Review 23 (2002) 365–370

Development of aluminum metal matrix composites (Al-MMC)brake rotor and pad

Hiroaki Nakanishia, Kenji Kakiharaa, Akinori Nakayamaa, Tomiyuki Murayamab

aFoundation Brake Engineering Department, ADVICS CO. LTD., 918-11, Mitukuri Aza Sakashita, Fujioka-cho, Nishikamo-gun,

Aichi 470-0424, JapanbLight Metals R&D Department, AISIM TAKAOKA CO. LTD., 1 Tennoh, Takaokasin-machi, Toyota, Aichi 473-8501, Japan

Received 18 January 2002; received in revised form 8 March 2002

Abstract

Improvement in fuel consumption rate requires a reduction in vehicle weight. Research and development for materials

substitution in the brake rotor, from the conventional cast iron to aluminum, has been undertaken. In this study, we developed

aluminum metal matrix composites brake rotor and pads, which have equivalent braking effects and wear resistance to those of the

conventional cast iron rotor, by optimization of the quantities and the particle diameter ratio of hard particles used for the rotor and

the pad. r 2002 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved.

1. Introduction

It is highly desired to improve the fuel consumptionrate by a reduction in total weight of the vehicle. Thebrake rotor accounts for a large part of the total chassisweight. In recent years, research and development hasbeen carried out to substitute the conventional cast ironbrake rotor with an aluminum brake rotor. For thedevelopment of aluminum brake rotors, however, thereare diverse technical problems to be solved, in additionto the manufacturing cost. In this study, we developed anew combination of an aluminum brake rotor with abrake pad and examined its application, in order tosolve the problems. Optimization of the quantity andthe particle diameter ratio of hard particles used for thebrake rotor and the brake pad resulted in developing animproved brake rotor of higher heat resistance thanthose of the previous aluminum metal matrix compo-sites (AL-MMC) rotors. The optimization also gave abrake pad having equivalent friction properties, includ-ing the braking effects and wear resistance, to those ofthe conventional cast iron rotors.

2. Problems

It has been thought that aluminum material is notsuitable for parts used under extremely severe frictionalconditions, such as a brake. The frictional heat of the

brake naturally raises the temperature, and the alumi-num material does not ensure sufficient thermalresistance. Application of hard particles like ceramicparticles for the reinforcement of the aluminum brakerotor has been examined to enhance the thermalresistance, although the results are not yet satisfactory[1,2].

Fig. 1 shows microscopic pictures of the structuresof the Al-MMC brake rotor and the conventional castiron (FC) brake rotor. While the FC brake rotor iscomposed of a material containing graphite, the Al-MMC brake rotor is composed of a composite materialin which ceramic particles are dispersed in aluminumalloy.

It is known that the combination of the Al-MMCbrake rotor with a conventional brake pad for the FCbrake rotor results in poor braking effects andinsufficient wear properties and is thus not suitable forpractical use.

Fig. 1. Micro structure of FC rotor and Al-MMC rotor.

0389-4304/02/$22.00 r 2002 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved.

PII: S 0 3 8 9 - 4 3 0 4 ( 0 2 ) 0 0 2 0 3 - 5 JSAE20024258

2.1. Heat resistance of rotor

The heat resistance of the Al-MMC brake rotor wasevaluated by measuring the temperature at whichscoring was observed. Here scoring represents thescratch-out phenomenon on the frictional surface dueto an increase in temperature of the brake rotor. Thescoring may be ascribed to the facts that the aluminummaterial has a significant decrease in strength with atemperature increase and that the aluminum basematerial on the frictional surface is softened. Theoccurrence of scoring causes abnormal wear of boththe brake pad and the brake rotor and abruptly lowersthe friction coefficient m: The brake pad used for the testwas the pad with less accumulation of film on itsfrictional surface among the pads previously testedagainst the FC brake rotor [3]. The test rubbed the brakepad against the rotor 200 times in conformity withJASO C406-82, and iteratively performed snap brakingat a deceleration of 4.4m/s2 to attain the speedreduction from the initial vehicle speed of 100 km/h tothe speed of 95 km/h. The temperature of the rotor wasraised under such conditions, and the temperature atwhich the friction coefficient m abruptly decreased wasmeasured. The graph of Fig. 2 shows the results of thetest. As seen from the plot of Fig. 2, the limittemperature of the previous Al-MMC rotor was 4101C.

In order to attain sufficient heat capacity of the brakerotor affected by this limit temperature, a relatively largeweight is required for the brake rotor mounted on thevehicle. This removes the merit of weight reduction. Afurther increase in limit temperature is accordinglydemanded.

2.2. Friction properties

The test repeated the braking operations with regardto the Al-MMC brake rotor, in combination with thebrake pad used for the test discussed in Section 2.1, at adeceleration of 3.4m/s2, a before-braking rotor tem-perature of 1201C and an initial vehicle speed of 65 km/h, and measured the friction coefficient m: The wear ofthe brake pad and the brake rotor was measured witha micrometer. The results of the test are shown in

Figs. 3 and 4, together with the results for theconventional FC brake rotor. The results of Figs. 3and 4 show that the repeated braking of the Al-MMCbrake rotor lowered the friction coefficient m and causedsignificant wear of the brake pad. The brake rotorthickness also increased with substance transferred fromthe brake pads. The friction properties of the Al-MMCbrake rotor are thus remarkably poorer than those ofthe conventional FC brake rotor. Further improvementis required with regard to the variation in frictioncoefficient and the wear of the brake pad due to therepeated braking.

2.3. Estimation of friction mechanism

The friction-related phenomena were estimated fromthe results of the test performed in Section 2.2. Fig. 5

Fig. 2. Scoring test result in conventional Al-MMC rotor.

Fig. 3. Variation in m with braking times.

Fig. 4. Wear of the rotor and the pad.

Fig. 5. Cross section of the frictional surface of FC rotor and Al-

MMC rotor.

H. Nakanishi et al. / JSAE Review 23 (2002) 365–370366

shows photographs of the sectional frictional surfaces ofthe respective rotors after the test. As shown in Fig. 5, asubstance transferred from the brake pad is accumulatedon the surface of the Al-MMC rotor to form a thickfilm. This is because hard particles included in the brakerotor protrude from the surface of the rotor and thesubstance transferred from the brake pad is accumulatedbetween the protrusions. Thus it is thought that the hardparticles function as a whetstone.

The estimated friction mechanism models with regardto the Al-MMC brake rotor and the conventional FCbrake rotor are shown in Fig. 6. These process modelsare explainable with a difference in hardness between thebase materials of the respective brake rotors and thehard particles included in them (see Table 1). In the case

of the FC brake rotor, the initial rough surface isscratched out by the hard particles in the brake padhaving the higher hardness than that of the cast ironbase material. The mixture of the wear powder of thebrake pad and the scratched-out material of the brakerotor fills the recesses on the surface of the brake rotorto form a film. The film accumulates over the wholefrictional surface. The hard particles included in thebrake pad then scratch off both the accumulated filmsand the base material of the brake rotor. As theaccumulation and scratch-off of the film are repeatedin this manner, the thickness of the film is keptsubstantially constant through the repeated brakingoperations. The brake pad is accordingly worn in asubstantially constant manner, which leads to arelatively stable friction coefficient m:

In the case of the Al-MMC brake rotor, on the otherhand, the hard particles on the surface of the rotor havethe higher hardness and the greater particle diameterthan those of the hard particles included in the brakepad, so that the brake rotor scratches out and wears thebrake pad. The wear powder fills the recesses on thesurface of the brake rotor to form a film.

The braking operation makes the hard particlesprotrude from the surface of the brake rotor and furtherwears the brake pad to thicken the film. Unlike the FCbrake rotor, the hard particles on the surface of theAl-MMC brake rotor prevent the brake pad fromscratching off the film covering over the whole frictionalsurface of the brake rotor. This leads to wear of thebrake pad. In the case of the Al-MMC brake rotor,there is accordingly no cycle of accumulation andscratch-off of the film. Thus the repeated brakingoperations thicken the accumulated film. This acceler-ates wear of the brake pad and lowers the frictioncoefficient m:

Based on the above discussion, it is very important toselect adequate hard particles included in the aluminumbrake rotor and the brake pad.

3. Development of new Al-MMC brake rotor and brake

pad

We increased the content of the hard particles in thebrake rotor for the purpose of improving the heatresistance and optimized the particle diameter ratio ofthe hard particles in the brake rotor to suit those in thebrake pad for the purpose of improving the frictionproperties.

3.1. Improvement in heat resistance of rotor

As is well known, the heat resistance of the Al-MMCbrake rotor is enhanced by increasing the quantity ofhard particles included in the brake rotor [1,2]. The high

Fig. 6. Friction process model with regard to Al-MMC and FC brake

rotor.

Table 1

Properties of hard particles in the rotor and the pad, and properties of

the rotor base material

Mohs

hardness

number

Particle

diameter

(mm)

Hard particles in the Al-MMC rotor 9.5 20

Base material of the Al-MMC rotor o3 —

Hard particles in the pad 4–7.5 1–20

Base material of the FC rotor 4 —

H. Nakanishi et al. / JSAE Review 23 (2002) 365–370 367

temperature lowers the strength of the aluminum basematerial and leads to easy scratch-out and destruction ofthe outer-most frictional surface. The high content ofthe hard particles, however, increases the ratio of thehard particles relative to the whole frictional surface andeffectively prevents scratch-off. In the conventionalparticle distributed-type cast material, the optimumcontent of the particles for good productivity isVf=20%. In this study, we adopted the preformtechnique and increased the content of the hard particlesfor enhanced heat resistance. We found that theoptimum content of the particles is Vf=30% for goodproductivity.

Evaluation of the heat resistance was performedunder the same test conditions as those discussedin Section 2.1. The results of the test are shown inFig. 7.

As seen in the graph of Fig. 7, the limit temperature ofthe preform-based brake rotor reached 4901C, while thelimit temperature of the conventional particle distrib-uted-type cast iron brake rotor was 4101C. Thisproves that the higher content of the hard particleseffectively enhances the heat resistance. We adoptedthe preform technique to arrange a higher content ofhard particles in the rotor than the limit in theconventional particle distributed-type cast technique,thereby favorably improving the heat resistance. Fig. 8shows the manufacturing process of the new Al-MMCbrake rotor developed in this study. Casting wasperformed under high pressure to shorten the formingtime. The preform was used only for the slidingparts. This decreased the number of parts havingdifficulties in processing, and thereby reduced themanufacturing cost.

3.2. Improvement in wear properties

The conventional brake pad for the FC brake rotorwas used as a base and was modified by mixing amaterial adequately selected corresponding to the hardparticles included in the Al-MMC brake rotor. It isnecessary to determine the particle diameter of thehard particles included in the brake pad by takinginto account the size and the quantity of the hard

particles included in the Al-MMC brake rotor. Thesame test as that of Section 2.2 was performedwith regard to the brake rotor developed in Section3.1 for evaluation of the wear of the brake pad andthe brake rotor. The results of the test are shown inFig. 9.

As seen in the graphs of Fig. 9, the desirable particlediameter ratio of the hard particles ranges from 2 to 20(rotor/pad) in order to ensure sufficient wear propertiesof both the brake rotor and the brake pad. Fig. 10 showsthe variation in friction coefficient m against thefrequency of braking at the particle diameter ratio of10. The results of Fig. 10 show that the repeated brakingoperations did not lower the friction coefficient m; whichwas kept substantially constant in the range of 0.39–0.4and was thus suitable for practical use.

Fig. 11 is a photograph showing the section of thebrake rotor after the test. The hard particles on the outermost frictional surface of the brake rotor were worndown to a rounded shape, the film covered oversubstantially the whole frictional surface, and had athickness equivalent to that of the film formed on thecast iron brake rotor (see Fig. 5).Fig. 7. Scoring test result in developed Al-MMC rotor.

Fig. 8. Developed rotor manufacturing process.

Fig. 9. Effects of the particle diameter ratio of hard particles on wear

of the Al-MMc rotor and the pad.

H. Nakanishi et al. / JSAE Review 23 (2002) 365–370368

4. Evaluation of new Al-MMC brake rotor and brake pad

The brake rotor developed in this study was subjectedto tests with a brake dynamo for evaluation of thebraking properties.

4.1. Evaluation of heat resistance of brake rotor

The heat resistance of the brake rotor was evaluatedby the same test as discussed in Section 2.1. The resultsof the test are shown in Fig. 12. The graph of Fig. 12shows that the brake rotor had a limit temperature of5001C while keeping the high friction coefficient m:

4.2. Evaluation of braking effects

The level and stability of the braking effects wereevaluated by a test in conformity with JASO C406-82.Fig. 13 shows observed variations in secondary brakingeffects at different initial vehicle speeds. The results ofthe test for the combination of the conventional FCbrake rotor with the conventional brake pad discussedin Section 2 are also shown for the purpose ofcomparison. The level of the braking effects of thedeveloped Al-MMC brake rotor was approximately10% higher than that of the conventional FC brakerotor at an initial vehicle speed of 50 km/h, while beingsubstantially equivalent to that of the conventional FCbrake rotor at an initial vehicle speed of 100 km/h. The

friction coefficient m of the developed Al-MMC brakerotor was substantially equivalent to or less than thatof the conventional FC brake rotor against thedeceleration, the vehicle speed, the temperature andthe heat history. Thus the Al-MMC brake rotordeveloped in this study fulfilled the requirements forpractical use.

4.3. Evaluation of wear of brake pad

The test repeated the braking operations 1000 times atdifferent temperatures of under the initial vehicle speedof 50 km/h and the deceleration of 1.5m/s2 andmeasured the variation in thickness of the brake pad,so as to evaluate the wear resistance of the brake pad.The results of the test are shown in Fig. 14. The resultsof the test with regard to the combination of theconventional FC brake rotor with the brake paddiscussed in Section 2 are also shown for the purposeof comparison. As shown in Fig. 14, the wear of thebrake pad developed in this study against the Al-MMCrotor was substantially equivalent to that of theconventional brake pad against the conventional FCbrake rotor. Thus the brake pad developed in this studyfulfilled the requirements for practical use.

4.4. Evaluation of brake squeal

Reduced brake squeal is one of the essential factorsrequired for the brake parts. The test repeated thebraking operations approximately 2000 times under aninitial vehicle speed of 10–30 km/h, a brake fluid

Fig. 11. Cross section of the frictional surface of developed Al-MMC

rotor.

Fig. 12. Scoring test result in developed Al-MMC rotor and pad.

Fig. 10. Variation in m with in braking times of developed Al-MMC

rotor and pad.

Fig. 13. JASO test results.

H. Nakanishi et al. / JSAE Review 23 (2002) 365–370 369

pressure of 0.2–4Mpa, and a before-braking padtemperature of 40–2001C. The frequency of occurrenceof brake squeal was 4 times and the maximum soundvolume was 78 dB. This is substantially equivalent to thebrake squeal level of the conventional FC rotorcombined with the conventional brake pad.

5. Conclusions

We have developed an improved Al-MMC disc brake,based on the findings discussed below:

(1) The higher content of the hard particles on thefrictional surface of the brake rotor enhances the heatresistance of the brake rotor.

(2) The film formed on the frictional surface of thebrake rotor is affected by the ratio of the quantity andthe size of the hard particles included in the brake rotorto those of the hard particles included in the brake pad.Regulation of the thickness of the film ensures stablefriction.

We will further examine the adaptability of thedeveloped Al-MMC disc brake to the vehicle, such asthe cooling properties. We will also try to furtherreduce the manufacturing cost and the total weight ofthe disc brake by using aluminum for calipers andjunctions.

References

[1] R. Dwivedi, Development of advanced reinforced aluminum brake

rotors, SAE950264.

[2] O. Osawa, Development of Al-MMC disc rotors, Proc. Technical

Presentation of JSAE Chubu Branch, 1998 (in Japanese with

English abstract).

[3] N. Odani, M. Kobayashi, K. Kakihara, Effects of transferred

surface film on disc brake pad in humid environment, SAE 1999-

01-3391.

Fig. 14. Pad wear per 1000 braking times.

H. Nakanishi et al. / JSAE Review 23 (2002) 365–370370