Designing a Composite Material for Use in Brake Applications

Transcription

1 Materials Science Forum Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Designing a Composite Material for Use in Brake Applications Jason Lo CANMET, Natural Resources Canada, 568, Booth Street, Ottawa, Ontario K1A 0G1, CANADA jlo@nrcan.ga.ca Keywords: Composite brake rotors, metal matrix composites, hybrid composites. Abstract. Traditionally, automotive brake rotors are made with cast iron. Besides having economical advantage, cast iron rotor provides many disadvantages due to its weight, such as reduction in fuel efficiency, increase in green house gas emission, and increase in noise, vibration and hardness. With the development of commercial aluminum composites, composite brake rotors are now manufactured. However, the present commercial composite materials are not specifically made for brake application and there are drawbacks. A major drawback is their poor elevated temperature property. In this paper, the unique properties offered by an aluminum composite for brake application is addressed, and an approach to compensate its properties for brake application is highlighted. Introduction Traditionally, automotive brake rotors are made from cast iron which provides good wear resistance and excellent high temperature properties. With the environmental concerns along with government legislation pushing for the development of lighter vehicles, a general target to reduce the average weight of vehicles in excess of 5% by 2007[1] is required. Presently, there are a number of materials that can offer properties superior to the traditional rotor materials. Materials range from steels, titanium alloys, polymers, ceramics to ceramic matrix composites. While some of them offer better properties for brake application, their processing or raw material costs have deterred their use. Consequently, the use of metal matrix composites (MMCs) for brake rotors seems to be a reasonable solution. According to Stratecasts, Inc, aluminum MMC (Al MMC) usage in automobiles will increase significantly over the next decade due to the ever-tightening weight restrictions imposed on auto-makers. Based on conservative estimates, production of composite disc brakes will increase from 360,000 units in 1999 to 800,000 units in 2008 [2]. Unique Properties Offered by a Commercial Al MMC In the selection of brake rotor materials, weight is not the only concern. There are many physical, mechanical, thermal and corrosion property requirements need to take into consideration, as each of these properties will influence the performance of a brake rotor. In Table 1, some of the key material properties important to the selection of a material for brake rotor application are listed. In addition, the material and functional properties of cast iron and a commercial Al MMC (Duralcan SiC/Al with 30 vol.% of SiC), in the light of brake application, are compared. The main advantage of replacing cast iron with Al MMC is the weight saving, an extent of over 60% is achievable. A material with high coefficient of thermal expansion (CTE) is All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-10/05/16,21:35:04)

2 1110 PRICM-5 undesirable, since it may lead to distortion and thermal fatigue due to the development of thermal stresses at elevated temperatures. Al MMC can be made to have an equivalent CTE as cast iron. Thermal conductivity describes the property for heat dissipation in a material. Al MMC has a much higher thermal conductivity value (~150 W/m.K) than cast iron (58 W/m.K). Consequently, this will reduce thermal stress gradients on the brake rotor caused by localized hot spots on the wear surfaces[3]. It is desirable for a brake rotor material to have high specific heat. A high specific heat material doesn t heat up easily during braking, thus experiencing little thermal stresses. Unfortunately, Al MMC has lower specific heat than cast iron. Thermal shock resistance is the ability of a material to withstand thermal stresses brought about by sudden and severe changes in the temperature at the surface of a solid body. Values for cast iron and SiC/Al are 6,800 W/m and 40,000 W/m, respectively. It is apparent that the thermal shock resistance of SiC/Al is almost ten times that of cast iron. Thermal fatigue occurs when the heating and cooling cycles cause accumulation of residual stresses in a material. To minimize thermal fatigue, a material should have a low modulus and a low coefficient of thermal expansion. Both properties are significantly lower in Al MMC than in cast iron. In addition to the thermal properties mentioned above, it is important that a brake rotor material has stable properties at elevated temperatures [4,5] or more precisely at maximum service temperatures. Therefore, a parameter, namely, Maximum Operating Temperature (MOT), is introduced to indicate the maximum operable temperature for the material. It is important to use the MOT to judge if the selected material is suitable for use in the service temperature range. For Al MMC, the MOT is between o C, and it is o C for cast iron. This is the major drawback of Al MMC for brake application. For brake rotor application, a high strength material is required. The tensile strength of a cast iron is 200 MPa, and those of a typical Al MMC is 330 MPa. The high strength property in a brake rotor is required to carry clamping loads and braking torque and also to resist thermal stresses. Elastic modulus relates to the stiffness of the material. For cast iron, the modulus is 100 GPa; and for Al MMC, it is 110 GPa. A material with high modulus would lower the thermal shock resistance, while a low modulus material might be more prone to mechanical deformation. Under both sliding and abrasion wear, the wear rate of MMCs decreases with an increase in volume fraction. A wear test study indicated that Al MMC has much better wear resistance than cast iron [6]. This is a novel feature of Al MMC. Material damping is a manifestation of internal energy dissipation in a material during dynamic deformation. In other words, damping capacity is defined as the energy absorbed during a cycle of vibration. Damping is closely linked to the suppression of noise and vibration. High damping capacity materials help to meet requirements for increased quietness and durability of structural materials under vibratory loading. Al MMC provides high damping capacity.

3 Materials Science Forum Vols Corrosion is a major issue for brake rotors used in countries where there are severe winter condition and the use of salt in deicing roads. Corrosion in brake rotors leads to the loss of cooling efficiency, damage of the braking surface, and degradation in structural integrity. It is extremely desirable to have a brake rotor material which demonstrates higher corrosion resistance than cast iron. Owing to the abrasive nature of the ceramic reinforcements such as silicon carbide, the machinability of Al MMC is much poorer than cast iron. Unavoidably, this has increased the cost of machining. Despite the advantageous properties of commercial Al MMC over the cast iron, there are specific properties such as wear resistant, machining costs, elevated temperature properties, etc. that need to be improved. Remedies of the Commercial Al MMC Brake Rotor An effective way to make an Al MMC brake rotor is by the squeeze casting (SQ) route. That is, a ceramic particulate preform is first made, followed with the pressure infiltration of molten aluminum into the preform to form an Al MMC. The advantages here are five folds. First, the commercial Al MMC can only accommodate up to 35 vol% of reinforcement, whereas with the preform route, an reinforcement loading of over 50 vol% is possible. This undoubtedly will improve the wear resistance of Al MMC. Second, the commercial Al MMC is restricted with certain grades of aluminum alloy and types of reinforcement, but not as much in the SQ Al MMC systems. This obviously will improve the elevated temperature properties of Al MMC. Third, the commercial Al MMC requires the whole brake rotor to be reinforced, but a SQ rotor would have the reinforcement only selectively placed on the wear surfaces and not in the hub region. This will reduce the costs of reinforcement. Fourth, since the reinforcement is only selectively placed in the wear surfaces, for the major machining required at the hub region, no extra machining cost is required. Lastly and most important of all, the maximum operating temperature of Al MMC needs to be raised. The maximum operating temperatures can be raised by (i) using a high solidus temperature aluminum alloy, (ii) an increase in the volume fraction of reinforcement, or (iii) an addition of an alloying element such as magnesium. All of the above processes can be incorporated with the SQ route of making Al MMC. In a CANMET hybrid Al MMC made specifically for brake rotor application, in addition to SiC particulates, a proprietary ingredient was also added to the composite to encourage in-situ reaction to boost up the matrix alloy high temperature properties. The measured compressive strengths at various temperatures of the CANMET material are shown in Figure 1. A marked improvement in high temperature strength is shown to achieve. References 1. University of Michigan Office for the Study of Automotive Transportation, Delphi IX Forecast and Analysis of the North American Automotive Industry Materials (1998) 2. David Weiss, Barri Chamberlain, and Rick Bruski Justifying Aluminum Metal Matrix Composites in an Era of Cost Reduction, Modern Casting, Feb. 2000, p T. Zeuner, On Track with MMC Brake Discs, Materials World, January 1998, p E.A. Feest, J.J.R. Churchman-Davies, K. Ellis, Advances in Automnotive Braking Technology, Mechanical Engineering Publication, London and Bury St Edmunds (1996)

4 1112 PRICM-5 5. R. Fisher, Carbon-Carbon Composites in Aircraft Brakes, Institute of Mechanical Engineers, London 20 March N. Oda, Y. Sugimoto, T. Higuchi and K. Minesita, SAE Paper , Table 1. Comparing Al MMC with cast iron in brake rotor application Material Property Functional Property (in comparison with cast iron) Density Al MMC has a density of 2.7gm/cc; and cast iron is 7.3gm/cc Coefficient of Thermal Expansion (CTE) Al MMC has a bit higher CTE thus providing less dimensional stability and thermal fatigue resistance Thermal Conductivity (TC) Al MMC has 3 times higher TC, a much effective heat conductor Specific Heat (SH) Al MMC has lower SH Thermal Shock Resistance Thermal Shock Resistance of Al MMC is ten times higher Fatigue Resistance To minimize thermal fatigue, low modulus and CTE is desired. Al MMC has a lower modulus and CTE Elevated Temperature Properties (ETP) Al MMC has a lower Maximum Operating Temperature, thus contributing to lower Friction Surface Integrity and Maximum Service Temperatures Strength Al MMC has higher strength, thus can carry higher clamping loads, braking torque and resist thermal stresses Elastic Modulus Almost equivalent modulus is achievable. A high modulus would lower TSR, and too low would lead to mechanical deformation Wear & Friction Al MMC has higher Braking Response and Service Life Damping Capacity Al MMC offers lower Noise, Vibration and Harshness Corrosion Resistance Al MMC has better corrosion resistance Machinability Higher cost but can be avoided Figure 1. Compressive strengths of commercial and CANMET Al MMC Yield Strength (MPa) al CC C Temperat R.T Temperature ( o C) CC Commercial Composite; C2 CANMET Composite

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