Analysis of Stir Die Cast Al-SiC Composite Brake Drums Based On Coefficient of Friction [PDF]

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Tribology International 51 (2012) 36–41

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Analysis of stir die cast Al–SiC composite brake drums based on coefficient of friction A. Rehman a,n, S. Das b, G. Dixit a a b

Department of Mechanical Engineering, M.A.N.I.T. Bhopal, India A.M.P.R.I., Bhopal, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 July 2011 Received in revised form 23 October 2011 Accepted 15 February 2012 Available online 2 March 2012

The work reported here is to analyze the suitability of Aluminum alloy–Silicon Carbide MMC (Al–SiC MMC) in the automobile brake drum applications in comparison with cast iron (CI) brake drum. A brake drum dynamometer test rig was developed for the purpose. Al–SiC MMC was reinforced with 10% and 15% SiC particle by weight. The effect of heat treatment of the Al–SiC MMC brake drum was also studied. Performance was mainly evaluated on the basis of brake drum coefficient of friction (m). Scanning electron microscope was also used to study the effect of braking on the sliding surface of the brake drum. & 2012 Elsevier Ltd. All rights reserved.

Keywords: Coefficient of friction Metal–matrix composites (MMCs) Brakes Scanning electron microscope

1. Introduction The aluminum alloy composites are made by mixing the particulate in a molten alloy. The composites are stronger, more rigid, harder, and more wear- and abrasion-resistant. On the other hand, their ductility and fracture toughness are less, and machining typically requires the use of polycrystalline diamond cutting tools for efficient production. In general, aluminum decreases electrical and thermal conductivity and the coefficient of thermal expansion where as silicon carbide increases thermal conductivity while decreasing electrical conductivity and thermal expansivity [1]. The properties of MMC materials have been widely examined and would appear to offer several major advantages over cast iron like lower density, better resistance to corrosion, lower thermal expansion, higher thermal conductivity and higher thermal diffusivity [2–6]. When developing brakes and suitable friction partners, engineers have to consider a variety of parameters like temperature at braking interface, applied braking load, speed etc. which influence the friction process of the surfaces in abrasive contact [3,4]. The important parameters to evaluate the brake drum material are friction power, friction work, friction surface temperature, material of the friction partners, wear, geometry of the friction partners, environmental influences and local mechanism [3].

n

Corresponding author. Tel.: þ91 9826203022. E-mail address: [email protected] (A. Rehman).

0301-679X/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2012.02.007

A material for a tribo component must possess a precise balance of physical and mechanical properties such as thermal expansion, damping capacity, conformability, strength, stiffness and fatigue life [2]. Significant efforts to develop materials for brake drums are underway, however, available literature resources are still scarce. These efforts are spread in many directions, namely: development of Aluminum brake components, developing different testing apparatus, characterization of different materials, analysis of brake system and comparative study with cast iron brake components apart from optimization of brake material formulation [6–32]. Design optimization is essential as drums constitute major weight in brake assemblies.

2. Development of the Al–SiC metal matrix composite brake drums In the present work the emphasis has been on developing affordable Al–SiC MMC brake drum, reinforced with SiC. Fabrication of Al–SiC MMC brake drum was done by stir die casting technique as also been undertaken by Natarajan et al. [9]. Different MMC brake drums casted with aluminum alloy ADC12 had 10 wt% SiC, 15 wt% SiC and they have been referred to as ADC12-10SiC, ADC12-15SiC, respectively. ADC12-10SiC and ADC12-15SiC were also heat treated and were referred to as ADC12-10SiC (HT) and ADC12-15SiC (HT), respectively. Brake drum with cast iron ring, as braking surface, was used as baseline reference material and named as CI (Base). Brake drum with cast iron ring is normally used in scooter applications.

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2.1. Matrix alloy for composite brake drums Aluminum–silicon alloy (ADC-12) was used as the metal matrix alloy. ADC-12 alloy has 10.29% Si, 0.12% Mn, 0.47% Mg, 1.98% Cu, 0.75% Fe and 0.80% Ni in aluminum. Al–Si alloy has high resistance to corrosion, is easy to weld and has lower coefficient of thermal expansion.

2.2. Size distribution of silicon carbide particles The particles are sieved using standard sieving practice with an aim to get particles within the size range of 40–80 mm. Grading of the particles shows that 30–35% of the SiC particles are in the range of 50–55 mm sizes and 25% of the SiC particles are in the range of 45–60 mm sizes.

2.3. Microstructure of Al–SiC MMC materials Microstructure of cast aluminum silicon ADC-12 alloy and its composites is studied in scanning electron microscopes. Fig. 1 (a) to (c) show that the distribution of SiC particles throughout the matrix is fairly uniform for both ADC-12 SiC composites and heat treated ADC-12 SiC composites samples. Fig. 1(a) Shows a typical microstructure of ADC12 alloy which consists of aluminum dendrites and eutectic silicon in the inter-dendritic regions and around the dendrites some inter metallic phases were also observed. Fig. 1(b) shows a typical scanning electron micrograph of ADC-12 SiC composite sample which clearly depicts uniform distribution of SiC particles in aluminum matrix. The composites were heat treated using T6 heat treatment cycle. Fig. 1(c) shows a typical microstructure of heat treated alloy clearly depicts the alteration of silicon morphology from needle shape to more or less spherical in nature.

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A higher magnification micrograph Fig. 1(d) shows good interface bonding between aluminum and SiC particles and also depicts eutectic Silicon around the SiC particles.

2.4. Al–SiC MMC preparation and brake drum casting The composite melt was prepared by dispersing the second phase SiC particles on to the vortex of the molten alloy. The speed of rotation was maintained around 500 to 600 rpm. The second phase dispersoid particles (10 wt% and 15 wt%) were heated to 1000 1C in graphite crucibles in a muffle furnace. Magnesium metal ( 1.0%wt) of the melt weight was also added to the melt during dispersing the SiC particles to induce wettability between the SiC particles and the melt. After the complete addition of SiC particles, the speed of the stirrer was brought down to 200–400 rpm and the stirring continued for 3–5 min after which the stirrer was withdrawn from the melt. In continuation to the above sequence and with the mixing of SiC particles, the composite melt was poured and solidified in the hot cast iron die, which was designed and developed to cast the Al–SiC MMC brake drums. Fig. 2 shows the photograph of various machined MMC brake drums along with a CI brake drum. Comparative analysis of Al–SiC MMC brake drums with commercially used two wheeler CI braking ring brake drum was done. The CI brake drum was cut across to understand its constructional details and its casting technique. Sectional view of the CI drum is shown in Fig. 3. which clearly shows the CI braking ring.

2.5. Heat treatment of the Al–SiC metal matrix composite brake drum The alloy and composites were heat treated in a muffle electric furnace. Following three stages were involved during

Fig. 1. Typical scanning electron micrograph. (a) ADC12 Alloy, (b) ADC12 Composite, (c) Heat Treated Composite and (d) Aluminum and SiC Interphase Particles.

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heat treatment of the materials:

3. Test rig for brake drum performance evaluation

(i) Solution treatment: the alloy or composite are heated for 8 h at a temperature of 490 1C until the alloying solute elements are completely dissolved in Al solid solution, (ii) Quenching: the solution treated material is cooled rapidly in the water to prevent the precipitation of the solute elements and to obtain a super saturated solid solution and (iii) Artificial aging: hardening can be done by reheating the quenched alloy to a temperature of 180 1C for 8 h in order to get improved properties.

Friction material manufacturers and researchers around the world have explored various methods to predict the performance of a friction material in a specific vehicle braking system [29]. Important test results of different materials for brake applications and their analysis with their tribological behavior were reported by Sallit et al. [5], Mosleh et al. [7], Natarajan et al. [10], Uyyuru et al. [14], Shorowordi et al. [16], Cueva et al. [25], Daoud et al. [26], Kennedy et al. [27] and Gomes et al. [31] using a pin on disk machine. A sub-scale disk brake testing (SSBT) system was built to enable instrumented tests to be performed on candidate truck brake materials by Blau et al. [6]. A brake drum test rig was developed to test brake drums by Natarajan et al. [9]. Fash et al. developed a small specimen testing apparatus to characterize the friction behavior of the braking pairs [11]. Wear processes at the interfaces of the specific rotor-pad combinations have been studied through the analysis of friction and the use of electron microscopy by Howell et al. [13] on a test setup. Laden et al. [15] tested frictional characteristics of Al–SiC MMC brake disk on a specially designed braking stand. Shaoyang et al. [17] developed a chase machine to test and compare the performance of two Al–SiC MMC brake drums each reinforced with different SiC particle size. In order to simulate the heating produced during braking, an apparatus was developed to evaluate the effect of thermal stresses with SiC reinforced Al–SiC MMC brake drum by Marco et al. [18]. Straffelini et al. used block on disk machine to study the influence of load and temperature on the sliding behavior of Al–SiC MMC material against friction material [20]. Comparison of Chase and inertial brake Dynamometer testing of automotive friction materials was done by Tsang et al. [23]. Greening et al. [29] used dual dynamometer differential effectiveness to predict the whole vehicle braking performance. Kuroda et al. [30] developed a scale dynamometer testing facility for observing rapid performance characteristic. Ramachandra Rao et al. [32] used an inertia dynamometer with a data-logging system. In the light of the study of test setups used by various researchers and on the basis of their reported performance of various materials a brake drum test rig was designed and developed to compare the performance of brake drums casted with different Al–SiC MMC materials with that of CI brake drum. Fig. 4 shows the test rig developed for testing brake drums of two wheelers. All the parts of the setup are placed coaxial to each

Fig. 2. Brake drums made up of different materials.

Fig. 3. Sectioned view of CI brake drum.

Fig. 4. Brake drum test rig.

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other in such a fashion that the driving element of the test rig, an AC motor (A), is connected to a flywheel shaft (B) through an electromagnetic clutch (C). The flywheel shaft on the other end is overhung and is mounted with a chuck (D) to hold the brake drum to be tested. Chuck is placed coaxial to a tailstock (E) which is mounted so as to align the brake shoe plate (F) with the brake drum. Brake shoe plate also holds a high precision load cell such that it records the tangential load on the braking surface. This tangential load is used to calculate the braking torque and coefficient of friction of the braking surface. Speed is measured by an electromagnetic pickup. Gravity braking mechanism (G) has been designed to maintain a uniform braking load during all the tests. The brake drum is driven by an electric motor which is controlled by a variable frequency drive for speed variation. One thermocouple was embedded in the stationary friction liner to record the dynamic changes in liner temperature during braking. A data acquisition card on the computer acquires real time data and displays it on a computer display screen and also on the analog display panel (H). Performance is then calculated and then displayed on the result screen.

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the contact surface, rather, because of their angular shape, scratches the counter surface material which is oxidized during the sliding process because of higher degree of localized heating. SiC particles increase the surface to surface friction due to their inherent property of hardness. The brake drum test samples of ADC12-10SiC (HT) and ADC12-15SiC (HT) have further higher coefficient of friction at all applied brake force and speeds as compared to the values obtained with ADC12-10SiC, ADC1215SiC and CI. Heat treatment of the composite material further hardens the aluminum alloy and makes the SiC particles more firm in the matrix which results in increased coefficient of friction. Hence, in application heat treated composite material brake drum would need less effort to stop the vehicle in comparison to other test materials. The material pair used for brake drum applications should have higher and stable coefficient of friction [10]. However, very high coefficient of friction has its own disadvantages in automotive applications [26]. Results shown here can be used as a reference data to tailor a material for specific application other than the automotives. The wear and frictional behavior of the material pairs is complex due to additional factors like contact asperities, wear debris, surface contact percentage of drum and liner rubbing surface [22].

4. Results and discussion Each new pair of brake shoes and drum was operated with about 250 brakes to stop the drum at medium speeds and loads so as to wear off the mating surfaces to ensures more than 90% surface to surface contact. The burnishing process helps in repeatability of results. However, each test was repeated three times and average values of the performance parameters are reported here. Tests at three different operational speeds were conducted and on each speed six braking forces were applied. Testing sequences were intended to create actual operating conditions by implementing user defined braking forces, and speeds. The test speed has been shown on the graph in kilometer per hour (km/h) in place of revolutions per minute (rpm) so as to show the on road test speed for the brake drum. 4.1. Effect of braking load and speed on coefficient of friction Fig. 5 shows that the coefficient of friction of the brake drums varies in a narrow scatter band for brake drums of different materials which is according to the fundamentals of dry sliding solids under dry lubrication. The coefficient of friction for CI brake drum is minimum under all speeds and braking force and lower than ADC12-10SiC and ADC12-15SiC brake drums. The SiC particles are considerably harder and are not plastically deformed at

4.2. Coefficient of friction and influence of transfer layer The results shown in Fig. 5 for coefficient of friction need to be understood in the light of transfer of layer from one surface to the other surface during rubbing/sliding. The formation of third body or tribo induced films as a result of sliding between a pair of contacting surfaces has a role in reducing friction and wear. During sliding contact, transfer and back transfer of material between contacting sliding surfaces can occur [2]. The transfer layer is a body composed of various elements of different sizes. If the hardness of the transfer layer is greater than that of the matrix, meaning that it acts as a protection as well as a semilubricant. The transfer layer does not have a regular thickness and is composed of various elements whether it is stratified or fragmented [5]. The films tend to have complex, heterogeneous microstructures since the friction materials from which they form may contain in excess of 15 different additives [6]. Uyyuru et al. reported transfer of the pin material occurs because of ploughing action of the hard asperities of disk material against the relatively soft pin material [14]. Laden et al. has also reported that under certain operating conditions the transfer layer breaks which again affects coefficient of friction [15]. The fall in coefficient of friction for CI after 540 N and for heat treated composite after 720 N can be attributed to the phenomena of transfer layer making and

Fig. 5. Trend of coefficient of friction with increasing order of brake load at different speeds.

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breaking. Experimental results of Shorowordi et al. [16] also show that a mixed layer is formed on the worn surface of MMCs. The layer mostly contained the constituents of the brake pad which act as counter body. Straffelini et al. [20] reported presence of transfer layer on the basis of SEM micrograph and reported that the layer contained friction material on the basis of EDXS analysis. Straffelini et al. also reported that increase in contact temperature decreases coefficient of friction. The paper also states the fact that friction coefficient is in general determined by two phenomena, firstly due to the adhesive interaction between the contacting asperities and secondly due to the ploughing contribution due to abrasion. Further, it reports that one of these two dominates the other depending upon the SiC weight percentage in the aluminum composite material. Higher SiC in the aluminum composite results in adhesive interaction rather than abrasive one [20]. At low applied braking force and speed the friction coefficient is observed to be less due to less formation of transfer film at the interface [10]. Critically examining the trends of the coefficient of friction curves, in Fig. 5, against braking force it can be suggested that with increasing order of braking force from 180 N to 540 N the coefficient of friction increased marginally and then decreased for cast Iron at all speeds. Similar trends were observed with Al–SiC MMC brake drums with the coefficient of friction getting maximum at slightly higher braking force of 720 N. The coefficient of friction response with CI and Al–SiC MMC brake drums can also be understood in the light of the explanation presented by Howell et al. and Uyyuru et al. [13,14]. During light braking force, the product of specific heat and density significantly affects the peak disk temperature than does the thermal conductivity. Uyyuru et al. reported that during moderate to heavy braking, thermal conductivity plays a predominant role in determining peak disk temperatures [14]. Howell et al. reported that during light braking, cast iron rotors will run cooler than aluminum MMC rotors but as the applied pressure increases, aluminum MMC rotors with their high thermal conductivity will run cooler and should show superior frictional stability over cast iron rotors [13]. Also it has been reported that temperature under certain range slightly increases coefficient of friction while excessively high temperature has a fading effect and coefficient of friction falls [3,32]. The excessively high temperature were not encountered in the present work. However, making and breaking aspect of transfer layer may have played a very important role in the nominal fluctuations observed during drum testing. As reported by Osterle et al. [8] the coefficient of friction decreased with increasing braking stress. It was reported that during the run in period the coefficient of friction increases steadily from 0.4 to 0.7 at the low contact pressure and from 0.3 to 0.5 from the high contact pressure, respectively. Osterle et al. points out that, though this behavior can be explained by the formation and growth of contact areas, chemical and microstructural changes may play an important role as well. In fact, the material formed during the run in period at the surface, the so called friction layer, differs considerably from the original pad material. Straffelini et al. reported increase in the contact temperature produced a decrease in the coefficient of friction [20]. Rhee et al. [3] has investigated that the frictional force during sliding, is to be the power function of applied load and sliding velocity at a particular temperature. Under heavy braking conditions, the value of frictional force reduces due to rise in temperature and result in brake fade [10]. 4.3. Overall average coefficient of friction of all brake drums Fig. 6 shows overall average coefficient of friction of different Al–SiC MMC and CI brake drums for all speeds and braking force. Overall average coefficient of friction was calculated at all test

Fig. 6. Overall average coefficient of friction for different brake drums.

Fig. 7. Maximum temperature at the braking interface.

speeds and braking force. The figure has been included so that the values reported here can be used for brake drum design purposes. As shown in the figure overall average coefficient of friction for CI, ADC12-10SiC, ADC12-15SiC, ADC12-10SiC (HT) and ADC12-15SiC (HT) brake drums was 0.56, 0.78, 0.80, 0.90 and 0.93, respectively. Higher coefficient of friction with composite material can be attributed to the hard SiC particles which penetrate deep into the counter surface leading to formation of microchips from counter surface. As a result greater amount of frictional force is required for sliding of composite over the counter surface. As the SiC content increases number of SiC particles penetrating to the counter surface increases and thus the coefficient of friction in composites increases with increase in SiC content. 4.4. Braking surface interface temperature Fig. 7 shows the maximum interface temperature recorded at maximum braking force and speed for all brake drums. Braking interface temperature with CI brake drum was 205 1C followed by ADC12-15SiC (HT), ADC12-10SiC (HT), ADC12-15SiC and ADC1210SiC which was 184, 175, 125 and 120 1C, respectively, during braking. The results are in agreement with Howell et al. who suggested that as the applied braking force increases, aluminum

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MMC rotors with their high thermal conductivity will run cooler and should show superior frictional stability over cast iron rotors [13]. Aluminum has high thermal diffusivity property [2] which allows the heat to dissipate within short time. While applying the brake, the major share of thermal energy (about 95%) is dissipated through the drum while the rest of it goes into the brake shoe. A fast response data logging equipment is required [32] as rapid deceleration produces high heat generation in a single stop, the braking time may be less than the time required for the heat to penetrate through the drum and the shoe which will lead to temperature rise at the interface [9].

5. Conclusions The essence of this work is that Al–SiC MMC can be successfully used in brake drums in automobiles due to its comparative coefficient of friction. In general It was found that the coefficient of friction narrowly changed with applied braking load and speed for brake drums of any material. The coefficient of friction of the composites is found to be higher than cast iron brake drum. Higher coefficient of friction of heat treated Al–SiC MMC suggests a limited scope of its use in smaller load carrying automobiles. It is of interest to note that brake drum and brake shoe interface temperatures with all composite material brake drum are lower than that observed with CI brake drums. However, it was observed that heat treated composite material brake drums had higher interface temperatures as compared to that obtained with other Al–SiC MMC composite brake drums. Interface temperature has an important effect on the coefficient of friction of the braking surfaces which affects braking efficiency of the brake drums. Coefficient of friction with composite material is higher due to the presence of SiC particles in the aluminum alloy. In order to take the benefits of the composite materials, which has higher coefficient of friction as compared to cast iron, the brake shoe friction material can be tailored to bring down the level to suit the application. It is possible to investigate on a suitable friction material to match the composite material brake drums to get coefficient of friction within limits of the automobile applications. Sallit et al. [5] briefly discussed regarding important aspects of friction material requirements and formulation which indicate that the material for the brake shoes and pads can be designed to suit a particular Al–SiC MMC brake drum. Consequently, its properties have to be like low compressibility, good resistance to severe temperatures and good resistance to abrasion. The main components of a friction material are binder, reinforcing elements, fibers, abrasive elements (SiO2, Al2O3), filling charges (BaSO4, CaCO3, Al2O3), lubricants (MoS3, Sb2S3, as well as sulfides of Cu, Sn, Sb and brass), fire proofing substances, and aluminum hydroxides to protect the pad from fire. Brake drum material with higher coefficient of friction can be used for applications other than automobile brakes. Use of higher coefficient of friction material can be extended to clutches used in various machines. References [1] Brady George S, Clauser Henry R, Vaccari John A. Materials Handbook. 15th ed. McGraw-Hill Handbooks; 2002, pp. 49–61. [2] Prasad SV, Asthana R. Aluminum metal–matrix composites for automotive applications: tribological considerations. Tribology Letters 2004;17/3:445–53. [3] Rhee SK. Wear equation for polymers sliding against metal surfaces. Wear 1970;16/6:431–45. [4] Nakanishi Hiroaki, Kakihara Kenji, Nakayama Akinori, Murayama Tomiyuki. Development of aluminum metal matrix composites (Al-MMC) brake rotor and pad. JSAE Review 2002;23/3:365–70.

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