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1 Research Article THE EFFECT OF FILLER ON THE FRICTION PERFORMANCE OF AUTOMOTIVE BRAKE FRICTION MATERIALS S. Manjunath Yadav 1, S. Basavarajappa 1, Chakrasali Chandrakumar 1, K.V.Arun 2 Address for Correspondence 1 Department of studies in Mechanical Engineering, Univ. B.D.T. College of Engineering, Davangere-5774, Karnataka 2 Department of studies in Industrial and Production Engineering, Univ. B.D.T. College of Engineering, Davangere-5774, Karnataka ABSTRACT The need for the use of newer materials to combat wear situations has resulted in the emergence of polymer based systems. The polymers and their composites are being increasingly employed in view of their possessing good strengths and low densities. Besides, wider choice of material and ease of manufacturing make them an ideal case for engineering applications. Despite the number of research studies completed on the mechanism of friction in automotive brake lining materials, the phenomenon is still not fully understood. Complex mechano-chemical processes occurring on the friction interface of a composite friction material make it difficult to understand the correlation between the formulation of brake lining and the frictional performance. This paper concentrates on the comparative frictional performance of glass-epoxy composite with influence of silicon carbide particles (SiC p ) and graphite (Gr) filler are experimentally investigated under varying applied load, sliding distance and sliding velocity using a pin-on-disc apparatus. For increased applied load situation, higher coefficient of friction was recorded. KEYWORDS Polymer Matrix material, fillers, friction, Brake lining INTRODUCTION In the past last Two decades, rapid developments in the automotive industry have been accompanied by increases of speed, loads, and engine power. The friction materials are required to provide a stable friction coefficient and a low wear rate at various operating speeds, pressures, temperatures, and environmental conditions. Friction material must also be compatible with the rotor material in order to reduce its extensive wear, vibration, and noise during braking. All of these requirements need to be achieved at a reasonable cost and minimum environmental load [1]. Polymer matrix composites are widely used in automotive, air, and railway transport systems as brake linings. They represent a replaceable (scarified) element in a friction couple and are typically rubbed against pearlitic gray cast iron, steel, or aluminum matrix composite material [2]. The reinforcing fibers commonly used for automotive brake friction materials are glass fiber, aramid pulp, metallic fiber, ceramic fiber, acrylic fiber, and others. In general, commercial friction materials contain 5 25 vol.% of fibrous ingredients and the types and the relative amounts of the fibers affect many aspects of brake performance and wear life. One possible way to widen the scope and usage of these materials is to resort to the introduction of fillers into the polymeric system having fibrous reinforcement [3-6]. This would enable the user to have optimum wear rate and coefficient of friction. However, the use of these filler based materials in actual service requires a careful cataloguing of the processing conditions employed and the attendant structure that follows. Significant changes in formulation of these materials in the recent decade reflect the requirements of safer and faster transportation as well as environmental needs [7 1]. However, the relationship between formulation and performance is not clear and complex problems involving instabilities in the coefficient of friction, excessive wear, vibration and noise accompany the friction processes of polymer matrix composite materials. In a review of some of the literature concerning frictional behavior of composites, Mohanty et al [11] have carried out experimental investigations on development of friction composites, using fly ash as a filler material. It can be noticed that, the developed brake lining composites have exhibited consistent coefficients of friction in the range of.35, and wear rates lower than 12 wt%.ray and Gnanamoorthy [12] reported that the friction and wear behavior of vinylester resin matrix composites filled with fly ash particles. The result shows that a better wear resistance was shown by the 4% filled composites with a lower wear loss, lesser linear wear, and a minimum value of the coefficient of friction. It was also observed that the difference between the coefficients of friction of the 4% and 5% filled composites decreased with the increase in the normal load. Jang et al [ 13 ] were experimentally investigate that, the effect of different metallic fibers upon friction and wear performance of various brake friction couples. The test results showed that the friction material with steel fibers was not compatible with Al-MMC disks due to severe material transfer and erratic friction behavior during sliding at elevated temperatures. On the other hand, elevated temperature tests showed that the friction material with Cu fibers exhibited better fade resistance than the others Unal et al [14 ] were studied on the sliding friction and wear behaviour of pure polytetrafluoroethylene (PTFE), glass fibre

2 reinforced (GFR) and bronze and carbon (C) filled PTFE polymers. The results showed that, the friction coefficient decrease with the increase in load and also found that, adding glass fiber, bronze and carbon fillers to PTFE were found effective in reducing the wear rate of the PTFE composite. Type of Composite Volume fraction in % Glass Epoxy Fillers fabric resin G-E-SiCp G-E-SiCp-Gr The tribological characteristics depend on the coefficient of friction. A friction process is always accompanied by the development of friction debris, which adheres to the rubbing couple. Due to the complexity of friction phenomena in polymer matrix composites, the mechanisms of friction have not been fully understood [15-17]. This paper presents results of the brake lining composite samples made of G-E composites filled with Silicon Carbide Particles (SiC p ) and Graphite (Gr). 2. MATERIALS AND EXPERIMENTATION 2.1 Materials The reinforcement material employed was 7-mil E- glass fiber. The silicon carbide (SiC p ) 22-mesh size and graphite (Gr) are used as filler materials. The matrix material used in the present work was a medium viscosity epoxy resin (LAPOX L-12) and a room temperature curing polyamine hardener (K-6), both supplied by ATUL India Ltd, Gujarat, India. This matrix was chosen since it provides good resistance to alkalis and has good adhesive properties. 2.2 Specimen preparation The composition of the friction materials used in this present work is listed in the Table 1. The glass-epoxy composites with Silicon carbide particles and solid lubricant (Graphite) as filler materials are used to prepare brake lining specimens by using Hand Lay- Up Technique. The laminate so prepared has a size 1 mm X 1 mm X 6 mm. To prepare the solid lubricant glass-epoxy composites, all the constituents like silicon carbide particles (SiC p ) and graphite (Gr) were weighed using an analytical balance and mixed with a known amount of epoxy resin for 1 min using a commercial blender. The laminate was cured at ambient conditions for a period of about 24 hours. The cured materials are cut using a diamond tipped cutter to yield brake lining test specimen of size 12.7 mm X 12.7 mm X 6 mm. Table 1 Composition of laminate composite 2.3 Experimental procedure The experimental set up used in this study was a pinon-disc type friction tester. The brake lining test specimen 12.7 mm X 12.7 mm X 6 mm is affixed to a pin of dimensions 6 mm diameter and 27 mm height with an adhesive of high bonding strength. The surface of the specimen comes in contact with the counter surface of the cast iron disc is used in the present work, which is having dimensions of 165 mm diameter, 8 mm thick. The experiment was conducted on a track of 13 mm diameter for a specified test duration, applied load and sliding velocity. First the brake lining specimen surface was prepared as per standard metallurgical procedure. The surfaces of both the sample and the disc were cleaned with acetone before starting the experiment. The initial weight of the pin assembly was recorded accurately using a digital electronic balance of least count.1 grams. After fixing both the disc and the specimen pin in their respective positions, the normal load to the pin was applied through a pivoted loading lever with a string and pan assembly as seen in Fig. 1. Fig.1. Schematic Diagram of Pin-on-Disc Apparatus The required loads were applied by placing known weights on the pan. The tests were conducted by selecting the test duration, applied load and sliding velocity. At the end of the test, the pin assembly was again weighed in the same balance and difference was taken as weight loss. 2.4 Test parameters Table 2.Process Parameters with their values no Applied load in N Sliding velocity in ms-1 Speed in rpm Sliding distance in m The test parameters used in the experiment are applied load, sliding velocity and sliding distance. The tests were conducted in two phases, in first phase applied load is varied, but sliding velocity and sliding distance are kept constant. During second phase

3 Co-efficent of friction (µ) sliding velocity is varied where as applied load and sliding distance is kept constant. The levels of the process variables used for testing are presented as in Table RESULTS AND DISCUSSION 3.1 EFFECT OF APPLIED LOAD ON COEFFICIENT OF FRICTION FOR G-E-SIC P COMPOSITE Fig. 2 refers to the variation of coefficient of friction on sliding distance over range of 5m for G-E-SiC p composites under 5 varied loads, at a constant sliding velocity of 4.8 ms -1. It is inferred from the Figure that the coefficient of friction (µ) increases with increase in load, because increase of applied load increases the area of contact which in turn coefficient of friction increases N 5N 6N 7N 8N Fig. 2. Variation of coefficient of friction on sliding distance for G-E-SiCp composites In addition, G-E-SiC p sample showed a continuous increase in the friction coefficient (µ) with increases in sliding distance. Such an increase is often associated with the adhesion of silicon carbide particles in the brake lining to the friction surface of the cast iron disc. It is observed from Figure that, there is some fluctuation in coefficient of friction (µ) at load of 6 N testing. This is because the surface of the sample revealed that soft glass fibers in the composites were plastically deformed and smeared over the contact surface by generating excessive adhesion on the counter surface, resulting in to a smooth and molten polymer film which leads to detach of soften stable friction layer was developed on the surface of specimen. The oscillation of the coefficient of friction (a sudden change within a short period of time) was higher in that time period. If applied load increases to maximum value the soften friction layer was removed from the surface, thereby friction increases between specimen surface and disc. The coefficient of friction is found to be high for a load of 8N and found to least for a load of 4N for 5m distance. 3.2 EFFECT OF APPLIED LOAD ON COEFFICIENT OF FRICTION FOR G-E-SIC P - Gr COMPOSITE The variation of coefficient of friction on sliding distance over range of 5m for G-E-SiC p composites under 5 varied loads, at a constant sliding velocity of 4.8 ms -1 is represented as shown in Fig. 3.It is observed from the Figure that the coefficient of friction (µ) increases with increase in load, because increase of applied load increases the area of contact which in turn co-efficient of friction increases. In addition, G-E-SiC p sample showed a continuous increase in the friction coefficient (µ) with increases in sliding distance. Co-efficent of friction (µ) N 5N 6N 7N 8N Fig. 3. Variation of coefficient of friction on sliding distance for G-E-SiCp-Gr composites The variation of increase in co-efficient of friction with increase in sliding distance is more in G-E- SiC p -Gr composites compared to G-E-SiC p composites. The variation of increase in co-efficient of friction with increase in sliding distance is more in G-E-SiC p -Gr composites compared to G-E-SiC p composites. The co-efficient of friction is found to be more at 8N load and least at 4N over run of 5m sliding distance. It is clearly seen from figure that, coefficient of friction of G-E-SiC P -Gr composite less compared to G-E-SiC P composites. The reason for this reduction in coefficient of friction is attributed to the presence of the smeared graphite layer at the sliding surface, which acts as a solid lubricant. This smeared layer becomes thicker with the increase in applied load and becoming high adherent and compacted. This graphite is change the roughness of the contact surface. 3.3 EFFECT OF SLIDING SPEED ON COEFFICIENT OF FRICTION Fig. 4 shows the effect of sliding speed on the coefficient of friction of GFRP composites with silicon carbide (SiCp) and graphite (Gr) as secondary fillers.sliding speed is varied form 2 to 1 rpm, whereas applied load and sliding distance are kept constant 6 N and 5 m respectively.

4 .5 G-E-SiCp G-E-SiCp-Gr Co-efficient of friction (µ) For 2 RPM For 4 RPM For 8 RPM For 1 RPM Fig. 4.variation of co-efficient of friction on sliding speed The experimental results shows that the coefficient of friction decreases with increase in sliding speed, but increases with increase in sliding distance for both types of composites. As the sliding distance increases the co-efficient friction also increases irrespective of the sliding speed. G-E-SiC p composites possess higher co-efficient of friction compared to G-E-SiC p - Gr composites for varied speed conditions. 4. CONCLUSIONS Based on the experimental study on the frictional behavior of G-E Composites with SiC p and SiC p + Gr as secondary fillers with varying sliding velocity, applied load and sliding distance under dry sliding wear conditions, the following conclusions drawn. As the applied load increases correspondingly the coefficient of friction also increases for both types of composites with keeping constant sliding velocity, but G-E-SiC p composite possess higher value of co-efficient of friction compared with G-E-SiC p -Gr composites for varied load conditions. The co-efficient of friction increases with increase in sliding distance for varied load conditions at a constant velocity, but the variation of increase is large in case of G-E- SiC p -Gr composites. The increase of sliding speed decreases the coefficient of friction for both types composites keeping the applied load constant, G-E-SiC p composites posses higher value of co-efficient of friction for varied sliding distance conditions. The co-efficient of friction increases with increase in sliding distance for varied sliding speed conditions at a constant applied load, but the variation of increase is large in case of G- E-SiC p composites. REFERENCES 1. P. Filip, L. Kovarik, M.A. Wright, Automotive Brake Lining Characterization.Proceedings of 15th Annual SAE Brake Colloquium 1997, SAE, Warrendale, PA, 1997, pp A.E. Anderson, Friction and wear of automotive brakes, ASM Handbook, Vol. 18, 1992, pp H. V. Ramakrishna, S. Padma Priya, and S. K. Rai, Tensile and Flexural Properties of Unsaturated Polyester/Granite Powder and Unsaturated Polyester/Fly Ash Composites, J. Reinf. Plast. & Comp.( 24 ) B. P. Kishore, and S. M Kulkarni, Compression Strength of Saline Water-ExposedEpoxy System Containing Fly Ash Particles, J. Reinf. Plast. & Comp.(24 ) H. V. Ramakrishna, S. Padma Priya, S. K. Rai, and A. Varadarajulu,Studies ontensile and Flexural Properties of Epoxy Toughened with PMMA/Granite Powder and Epoxy Toughened with PMMA/Fly Ash Composites, J. Reinf. Plast. & Comp.(24) S. S. Kishore, Impact Studies in Elastomer, Fly Ash and Hybrid Filled Epoxy Composites: Comparison of Data and Fracture Features of the Samples Cured via a Single Room Temperature and Multiple High Temperatures, J. Reinf. Plast. & Comp.(24) P. Filip, L. Kovarik, M.A. Wright, Automotive brake lining characterization, in: Proceedings of the 15th Annual SAE Brake Colloquium 1997, SAE, Warendale, PA, 1997, pp

5 8. J. Bijwe, Polym. Composites 18 (1997) H. Smales, Proc. Inst. Mech. Eng. Part D: J. Automobile Eng. 29 (1995) M. Abe, J. Jpn. Soc. Tribol. 41 (1996) (in Japanese). 11. Samrat Mohanty,Y.P. Chugh, Development of fly ash-based automotive brake lining, Tribology International 4 (27) Dipa ray and Gnanamoorthy, R., Friction and Wear Behavior of Vinylester Resin Matrix Composites Filled with Fly Ash Particles, Journal of Reinforced Plastics And Composites, 26 (1) (27), H. Jang, K. Ko, S.J. Kim, R.H. Basch, J.W. Fash, The effect of metal fibers on the friction performance of automotive brake friction materials, Wear 256 (24) H. Unal, A. Mimaroglu, U. Kadıoglu, H. Ekiz, Sliding friction and wear behaviour of polytetrafluoroethylene and its composites under dry conditions, Materials and Design 25 (24) Wirth, D. Eggleston, R. Whitaker, Wear 179 (1994) K.J. Wahl, I.L. Singer, Tribol. Lett. 1 (1995) P. Filip, M.A. Wright, Characterization of composite materials for automotive braking industry, Pract. Metallogr. (Special Issue) 3 (1999)