- Email: [email protected]
Scoping Review of Brake Friction Material for Automotive
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 16 (2019) 927–933
www.materialstoday.com/proceedings
ICAMMAS17
Scoping Review of Brake Friction Material for Automotive S. Venkatesh a*, K. Murugapoopathiraja b, ab
Department of Physical Chemistry Madurai Kamaraj University.
Abstract The past decade, the automotive brake friction material based on continuous research and developments phase out the asbestos and the friction product market increase the asbestos free friction material as a safer alternative. As a result, the friction industries was developing the different quality of brake lining and brake pads each with their own unique composition, yet performing the very same task and claiming to be better than others. The selection of brake friction materials is based more on tradition and experimental trial and error rather than fundamental understanding. This review was common ingredients used in brake lining, brake pad and its business trend. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials, Manufacturing and Applied Sciences. Keywords: automotive brake friction materials; brake pads; brake shoes; braking; brake lining;
Introduction Two main types of friction materials are disc brake pads and drum brake lining. Brake pads are a component of disc brakes used in automotive and other applications. Brake pads are steel backing plates with friction material bonded to the surface that faces the disc brake rotor. A drum brake is a brake that uses friction caused by a set of shoes that press outward against a rotating cylinder-shaped part called a brake drum. This paper was a review of the Brake lining raw materials and its current business trend. Figure 1 shows a typical drum brake commonly found in commercial vehicles. Driver the pressure on the brake pedal, brake fluid is effectively pushed against the pistons of the slag adjuster, which in turn forces the scam against the brake shoe. This de clamping action of the brake shoe retards the rotational movement of the brake drum and the axle that it is mounted on. The kinetic energy of the vehicle is converted into thermal energy which is primarily borne by the drum and brake linings. Brake pad operates based on similar principles. Normally the design of the brakes affects heat flow, reliability and noise characteristics. Brake Friction Material Herbert Frood is credited with inventing the first brake lining materials in 1897(Nicholson, 1995). Technological advances in the brake system are vital to public safety, as new vehicles capable of achieving high speed are being introduced in the Indian automotive market. Advances have been achieved in developing superior products to improve the effectiveness of the braking application in braking system. Brake friction parts have seen lot of improvements in braking technology and friction material used. The Indian aftermarket for brake friction products is well developed in terms of products and technology.
* Corresponding author. Tel.: +91-9488192881 E-mail address: [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials, Manufacturing and Applied Sciences.
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Figure: 1 Drum Brake lining & Brake assembly 1. Fiber 1.1 Glass fiber Glass fibers have been used as reinforcing Fibers from the mid 1970s. Being physically strong when bonded together with resin, the glass Fibers are suitable for use as reinforcing Fibers as they also exhibit thermal resilience [24]. Typical glass has a melting point of 1430 °C [1].Glass Fibers only have a conductivity of 0.04 W/m K, which is even lower than that of asbestos of 0.15 W/m K [11] and very much less than metallic Fibers. 1.2 Metallic fiber Metallic Fibers include steel, brass and copper. Metallic chips or granules are commonly used as reinforcing Fibers. The drawback of using steel Fibers is that they will rust, if the vehicle has an extended rest period or if the vehicle has been operating near a more moisture environment. Steel Fibers attacked by rust will be less to withstand, thereby compromising their functionality as reinforcing Fibers. Brake pads include metals such as zinc distributed over the cross-section of the friction lining, thereby forming a sacrificial anode for rusting to occur [12]. Steel Fibers is that they might cause excessive wear of the brake disc or brake drum if they are present in large proportions. Steel Fibers have also been shown to increase friction coefficient fluctuations [13], Some brake pads contain oxidized or phosphatized Fibers, resulting in improved fracture toughness and strength [29]. 1.3 Aramid fiber Aramid Fibers made from the condensation product of isophthalic or terephthalic acids and m- or pphenylenediamine [27], such as Kevlar Fibers are mostly used as reinforcing Fibers. They are light and exhibit high thermal stability, with a very good stiffness weight ratio. According to Smith and Boyd of R.K. Carbon Fibers, aramid Fibers have superior anti-fade properties compared to asbestos [30]. Aramid Fibers in pulp form have also been utilized in maintaining the uniformity of the brake pad material mixture during the processing of a moulded brake pad [7]. Aramid fiber have is that of superior wear resistance [4]. Due to their relative softness, however, they will be the only Fibers supporting the braking load; there would most probably be other harder Fibers same like metallic Fibers in the friction lining. 1.4 Ceramic Ceramic Fibers are a relatively new raw material in friction product compared to metallic Fibers like steel. They are typically made of various metal oxides such as alumina (aluminium oxide) carbides and silicon carbide. With a high thermal resistance, melting points ranging from 1850 to 3000 °C [35], light weight and high strength [5], they are very suitable as reinforcing Fibers. They are preferred over metallic Fibers because their high strength weight ratio. 2 Binders The purpose of a binder is to maintain the friction product structural integrity under mechanical and thermal stresses. It has to hold the components of a friction product together and to prevent its constituents from crumbling apart also. The figures listed are typical values for that particular binder. The selection of binders for friction product is an important issue, because if it does not remain structurally intact at all times during the braking application, the other constituents like reinforcing Fibers or lubricants will disintegrate. So it has to have a high heat resistance. For this reason, silicone and epoxy modified resins would generally be ideal as the binder for most of the friction product in braking applications. All other binders would have to be application specific such that their disadvantages would not be compromise their functionality [8].
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2.1 Phenolic resin Phenolic resin is the most common resin binder used in brake friction material. Phenol resin is a type of polymer formed by a condensation reaction between phenol and formaldehyde, and is able to act as a matrix for binding together different substrates [6]. This condensation reaction may be initiated by acidic or alkali catalysts, resulting in different classes of phenolic resins. When these phenolic resins are cured, they change from a thermoplastic state to a closely compacted, cross-linked thermoset matrix with relatively high heat resistance. In high-energy braking applications, the temperature induced can be high enough to decompose the phenolic resin via means of high-temperature oxidation. Phenolic resins carbonize at approximately 450 °C [36]; temperatures beyond this, it decomposes by partial burning and evaporation. This process decreases the density of the brake friction material at the wear surface and also increases its porosity, thereby losing its structural integrity [21], disadvantage of phenolic resins is that they are brittle and have a very low impact resistance. So they are usually modified with toughness material like epoxy resin or by incorporating wood flour to improve its flexibility [18]. Moreover, Jang et al.’s experiment [13] shows that the larger the quantity of phenolic resin used, the larger the friction coefficient fluctuations will be. 3. FILLERS The fillers in friction products are present for the purpose of improving its manufacturability as well as to reduce the overall cost of the brake pad [9]. It is a loose term which could also mean anything used in a large proportion in a brake friction material. Fillers was not as critical as other components such as reinforcing Fibers, its play an important role in modifying certain characteristics of friction product materials. The actual choice of fillers depends on the particular components in the friction material as well as the type of friction material. Example a metallic pad generates a lot of braking noise would require extra fillers such as cashew and mica (noise suppressors) than barium sulphate (heat stability). On the semi-metallic brake pads with a mixture of metallic and organic compounds having thermal expansion coefficients would require a large amount of molybdenum trioxide to prevent lining cracking. Alkali metal would not require as filler, when using friction product with large quantities of graphite or antimony sulphide as lubricants. Therefore the specific filler to be used depends on the constituents of the friction material [8]. 3.1 Inorganic fillers Barium sulphate, mica, vermiculite and calcium carbonate are included in inorganic fillers. The common property of these fillers is that they possess a relatively high melting point. For example, barium sulphate has a melting point of 1350 °C [22], while vermiculite exfoliates rapidly into flakes at approximately 800 °C [32]. One of the more commonly used fillers is barium sulphate. It imparts heat stability to the brake friction product material, at the same time aiding the friction characteristic of the friction material [19].Calcium carbonate is considered to be an alternative to barium sulphate, because it has a similar function properties, it imparts heat stability to the friction product material, thereby improving the friction material’s brake fade properties [28]. Calcium carbonate is the low cost of the two, but it is not as stable at higher temperatures as barium sulphate [2]. Mica is one of commonly used filler. It is able to suppress low-frequency brake noise [1] due to it having a plane netlike structure [15]. However, due to its stratified structure, it has a low interlayer strength. Mica causes interlayer splitting of the friction product, especially at high braking loads. Aluminium phosphate can be used as a coating on mica powder to prevent the interlayer splitting of mica [31].Like mica, vermiculite can also suppress noises generated during braking [25]. It also has a plane netlike structure and resembles mica in appearance. However, it is porous and its wear resistance at high temperatures is compromised [15].Vermiculate has a relatively high melting point of approximately 800 °C [23]. 3.2 Organic fillers Cashew dust and rubber in the form of dust are commonly used of organic fillers. Both have similar properties in that they are usually incorporated into friction products for the purpose of reducing brake noises due to their superior viscoelastic characteristics [17].Cashew particles, fall off the friction surface easily, leaving behind large pores that eventually crack [16]. Also, certain brake friction material manufacturers use cashew or rubber particles as under layer material because their low thermal conductivity prevents heat from transmitting to the backing plate of the brake
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friction material [26]. Cashew particles are help to reduce fluctuations in friction coefficients, especially at elevated temperatures [13]. 4. Frictional Additives Frictional additives are help to friction product materials in order to modify the friction coefficients and wear rates. Additives are divided into two main categories: lubricants, which decrease the friction coefficients and wear rates, and abrasives increase friction coefficients and wear rates. It is also important that certain frictional additives may be loosely regarded as fillers by certain manufacturers if they are present in more quantities [8]. 4.1 Lubricants The purpose of a lubricant in friction product is to stabilize the developed friction coefficient during braking, particularly at high temperatures. Commonly used lubricants include graphite and various metal sulphides [8].Graphite is widely used as it is able to form a lubricant layer on the opposing counter friction material rapidly [34]. The graphite used in brake friction materials can be of natural or synthetic graphite, and can exist in flake or powder form. Graphite in the flake form has improved lubrication properties [32], while graphite in the powder form is able to dissipate heat generated during braking more effectively [3].However, the bonding strength between graphite and phenolic resin is very weak, so graphite cannot be used too liberally in phenolic resins, leading to low shear strengths [14]. 4.2 Abrasives The abrasives material in a friction product material increase the friction coefficient and also increasing the rate of wear of the counter face material. They remove iron oxides from the counter friction material as well as other undesirable surface film layer formed during braking application. The friction materials with higher abrasive content exhibit a more variation of friction coefficient, resulting in instability of braking torque. Abrasives are hard particles of metal oxides and silicates. The abrasives have to be hard enough to at least abrade the counter friction material like cast iron. The abrasives typically have Mohs hardness values of around 7–8, and a few examples of the commonly used abrasives include zirconium oxide, zirconium silicate, aluminium oxide and chromium oxide [20]. 5. Friction Product Market Opportunities and Growth Factors 5.1 Production The Automotive industry produced a total 23,960,940 vehicles (Figure 2). This including passenger vehicles, commercial vehicles, three wheelers and two wheelers in April-March 2016 a marginal growth of 2.58 percent over the same period last year.
Ref: SIAMA India
Figure: 2 Production Trend 5.2 Domestic sales The Passenger Vehicles grow by 7.24 percent in April-March 2016 compared to last year. The overall Commercial Vehicles segment growth of 11.51 percent in April-March 2016 as compared to last year. Medium &
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Heavy Commercial Vehicles registered a growth at 29.91 percent and Light Commercial Vehicles grow marginally by 0.30 percent during April-March 2016 compared to last year. 5.3 Exports Overall automobile exports grew by 1.91 percent in April-March 2016. Passenger Vehicles, Commercial Vehicles, Three Wheelers and Two Wheelers registered a growth of 5.24 percent, 16.97 percent (-) 0.78 percent and 0.97 percent respectively in April-March 2016 compared to last year. (Figure.3) The aftermarket for brake friction parts was valued at US$46.7 million in the year 2001.The growth in the market for brake friction parts is on the backdrop of expected increase in new vehicle sales. Vehicle segments like the multi utility vehicle and passenger car are expected to lead the market in growth rates.
Ref: SIAMA India
Figure: 3 Export Trend 6. Future Trends The future developments in the field of friction product materials will closely walk through the current trends of the automotive industry. The future emphasis on cars will be on lower emissions and fuel efficiency as environmental regulations become a more stringent. This shift towards environmentally friendly cars has already seen the release of hybrid cars like Toyota Prius, Honda Insight and Ford Escape SUV.The focus on vehicle fuel efficiency and lower emissions that means brakes will have to be lighter and not release any toxic and carcinogenic substances into the atmosphere during use. This means that the choice of brake friction materials will need to be more environmentally friendly and not include toxic substances such as asbestos. The Santa Clara Valley Nonpoint Source Pollution Control Program in United States has identified automotive brake pads as a major contributor of copper in storm water, leading to the southern reach of San Francisco Bay to be labeled as an ‘impaired water body’ [11]. This is not perceived to be a major problem as the varieties of existing dry friction constituents mean that safer alternatives usually exist. For example, ceramic Fibers or glass Fibers can be used in place of asbestos Fibers. One more area needs further investigation and development would be that of friction material binders. There is a need to develop nonphenolic resin binders as current choices are limited. Conclusion In the last ten years, new generation vehicles have entered in the market. As a result, the aftermarket has also geared up with high quality parts and components to support the modern vehicle platforms. In this paper, the advantages of several alternative materials for use as brake friction product material were reviewed for the required function of a friction material. There is a growing focus on a more organized and structured aftermarket trade in the Friction product industry which is helping the segment evolve faster. The brake friction market is in a growth phase. The growth rates are across the various vehicle segments. The commercial vehicle segment is expected to reach market maturity in the next 7-8 years. The high growth segments of the market are multi utility vehicles and passenger cars. The technological changes are likely to tilt the market in favour of friction products Companies with superior product
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quality, technology, extensive distribution, and proactive market initiatives are expected to gain in this competitive market. Friction product Companies need to target high growth vehicle segments with the right product mix. Acknowledgements Author extends his thanks to MKU, Madurai for providing research fellowship. References [1] Avallone, E. A. and Baumeister III, T. Marks Handbook for Mechanical Engineers, 10th edition, 1997, pp. 6–142 (McGraw-Hill, New York). [2] Blau, P. Compositions, functions, and testing of friction brake materials and their additives. Technical Report ORNL/TM-2001/64, Oak Ridge National Laboratory, 2001, p. 29; available through NTIS, Springfield, Virginia.
[3] Booher, B. V. Pultrusion method of making brake linings. US Pat. 5156787, 1992 (United States Patent and Trademark Office). [4] Brinzey, A. E. Friction materials with universal core of non-asbestos Fibers. US Pat. 5041471, 1991 (United States Patent and Trademark Office). Ceramic Fibers (respirable size). Tenth Report on Carcinogens, US Department of Health and Human Services, 2002. Critchley, J. P., Knight, G. J. and Wright, W. W. Heat-Resistant Polymers, 1983, pp. 21–29 (Plenum Press, New York). Carlson, R. A. and Headley, J. L. Fiber mixtures for brake pads, US Pat. 5871159, 1999 (United States Patent and Trademark Office). D.Chan and G W Stachowiak Review of automotive brake friction materials Proc. Instn Mech. Engrs Vol. 218,p 953-965. Eriksson, M., Bergman, F. and Jacobson, S. On the nature of tribological contact in automotive brakes. Wear, 2002, 252, 26–36. Engberg, C. C. The regulation and manufacture of brake pads: the feasibility of reformulation to reduce the copper load to the San Francisco Bay. Palo Alto Regional Water Quality Control Plant, 1995. [11] Holman, J. P. Heat Transfer, 8th edition, pp. 641–642 (McGraw-Hill, Singapore). [12] Hell, M., Jaworek, W., Huppatz, W. and Wieser, D. Friction lining, especially for brakes and clutches, and a method for producing a friction lining. US Pat. 6481555, 2002 (United States Patent and Trademark Office).
[5] [6] [7] [8] [9] [10]
[13] Jang, H., Lee, J. S. and Fash, J. W. Compositional effects of the brake friction material on creep groan phenomena. Wear, 2001, 251, 1477– 1483.
[14] Komori, T., Miyake, S. and Senoo, Y. Brake friction material. US Pat. 4954536, 1990 (United States Patent and Trademark Office). [15] Kimura, K., Goto, Y., Torii, N., Katagiri, H., Miyazawa, H. and Motoyoshi, Y. Friction material for dampers and pro-cess for producing the same. US Pat. 5830566, 1998 (United States Patent and Trademark Office).
[16] Kinouchi, S., Hara, Y. and Yamaguchi, J. Friction material composition, production of the same and friction material. US Pat. 6372817, 2002 (United States Patent and Trademark Office).
[17] Kamioka, N., Tokumura, H. and Yoshino, T. Friction material containing BT resin dust. US Pat. 5384344, 1995 (United States Patent and Trademark Office).
[18] Kane, J. F. and Mowrer, N. R. Phenolic resin compositions with improved impact resistance. US Pat. 5736619, 1998 (United States Patent and Trademark Office).
[19] Komori, T., Miyake, S. and Senoo, Y. Brake friction material. US Pat. 4954536, 1990 (United States Patent and Trademark Office). [20] Kobayashi, M. Non-asbestos friction materials. US Pat. 6413622, 2002 (United States Patent and Trademark Office). [21] Lamport, R. A., Biermann-Weaver, J. M., Jain, V. K. and Shih, P. Resin mixture for friction materials. US Pat. 5753018, 1998 (United States Patent and Trademark Office).
[22] Lide, D. R. and Kehiaian, H. V. CRC Handbook of Thermophysical and Thermochemical Data, 1994, p. 26 (CRC Press, Boca Raton, Florida).
[23] Lide, D. R. and Kehiaian, H. V. CRC Handbook of Thermophysical and Thermochemical Data, 1994, p. 30 (CRC Press, Boca Raton, Florida).
[24] Marzocchi, A., Jannarelli, A. E. and Garrett, D. W. Friction materials for brake linings and the like. US Pat. 3967037, 1976 (United States Patent and Trademark Office).
[25] Nakagawa, M., Yamashita, Y., Ibuki, M. and Kishimoto, H. Friction material and method of manufacturing such material. US Pat. 5268398, 1993 (United States Patent and Trademark Office).
[26] Nakagawa, M. Disk-brake pad. US Pat. 6193025, 2001 (United States Patent and Trademark Office). [27] Okubo, H. S., Albertson, C. E. and Nibert, R. K. Asbestos-free friction materials. US Pat. 4446203, 1984 (United States Patent and Trademark Office).
[28] Ohya, K. and Kimbara, H. Disc brake pad. US Pat. 4944373, 1990 (United States Patent and Trademark Office). [29] Samuels, G. J. Friction composition and friction element fabricated thereform. US Pat. 5516816, 1996 (United States Patent and Trademark Office).
[30] Smith, W. N. and Boyd, P. Carbonaceous friction materials. US Pat. 5965658, 1999 (United States Patent and Trademark Office). [31] Seki, K. Non-asbestos friction material. US Pat. 5217528, 1993 (United States Patent and Trademark Office). [32] The exfoliation of vermiculite. Report 23, Western Australian Mining and Petroleum Research Institute, 1989.
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[33] Takahasi, K., Yoshida, M., Hagiwara, Y., Kondoh, K., Takano, Y. and Yamashita, Y. Titanium and/or titanium alloy sintered friction material. US Pat. 5922452, 1999 (United States Patent and Trademark Office).
[34] Taylor, A. J., Taylor, S. K., Hubbard, D. A. and Lotfipour, M. Friction pads for use in disc brakes. US Pat. 5725077, 1998 (United States Patent and Trademark Office).
[35] Warren, R. Ceramic-Matrix Composites, 1992, p. 2 (Blackie, New York). [36] Yesnik, M. A. Friction material comprising powdered phenolic resin and method of making same. US Pat. 5529666, 1996 (United States Patent and Trademark Office).
[37] Yamashita, Y., Nakagawa, M., Ibuki, M. and Kishimoto, H. Friction material for making brake pads. US Pat. 5266395, 1993 (United States Patent and Trademark Office).
ScienceDirect Materials Today: Proceedings 16 (2019) 927–933
www.materialstoday.com/proceedings
ICAMMAS17
Scoping Review of Brake Friction Material for Automotive S. Venkatesh a*, K. Murugapoopathiraja b, ab
Department of Physical Chemistry Madurai Kamaraj University.
Abstract The past decade, the automotive brake friction material based on continuous research and developments phase out the asbestos and the friction product market increase the asbestos free friction material as a safer alternative. As a result, the friction industries was developing the different quality of brake lining and brake pads each with their own unique composition, yet performing the very same task and claiming to be better than others. The selection of brake friction materials is based more on tradition and experimental trial and error rather than fundamental understanding. This review was common ingredients used in brake lining, brake pad and its business trend. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials, Manufacturing and Applied Sciences. Keywords: automotive brake friction materials; brake pads; brake shoes; braking; brake lining;
Introduction Two main types of friction materials are disc brake pads and drum brake lining. Brake pads are a component of disc brakes used in automotive and other applications. Brake pads are steel backing plates with friction material bonded to the surface that faces the disc brake rotor. A drum brake is a brake that uses friction caused by a set of shoes that press outward against a rotating cylinder-shaped part called a brake drum. This paper was a review of the Brake lining raw materials and its current business trend. Figure 1 shows a typical drum brake commonly found in commercial vehicles. Driver the pressure on the brake pedal, brake fluid is effectively pushed against the pistons of the slag adjuster, which in turn forces the scam against the brake shoe. This de clamping action of the brake shoe retards the rotational movement of the brake drum and the axle that it is mounted on. The kinetic energy of the vehicle is converted into thermal energy which is primarily borne by the drum and brake linings. Brake pad operates based on similar principles. Normally the design of the brakes affects heat flow, reliability and noise characteristics. Brake Friction Material Herbert Frood is credited with inventing the first brake lining materials in 1897(Nicholson, 1995). Technological advances in the brake system are vital to public safety, as new vehicles capable of achieving high speed are being introduced in the Indian automotive market. Advances have been achieved in developing superior products to improve the effectiveness of the braking application in braking system. Brake friction parts have seen lot of improvements in braking technology and friction material used. The Indian aftermarket for brake friction products is well developed in terms of products and technology.
* Corresponding author. Tel.: +91-9488192881 E-mail address: [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials, Manufacturing and Applied Sciences.
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Figure: 1 Drum Brake lining & Brake assembly 1. Fiber 1.1 Glass fiber Glass fibers have been used as reinforcing Fibers from the mid 1970s. Being physically strong when bonded together with resin, the glass Fibers are suitable for use as reinforcing Fibers as they also exhibit thermal resilience [24]. Typical glass has a melting point of 1430 °C [1].Glass Fibers only have a conductivity of 0.04 W/m K, which is even lower than that of asbestos of 0.15 W/m K [11] and very much less than metallic Fibers. 1.2 Metallic fiber Metallic Fibers include steel, brass and copper. Metallic chips or granules are commonly used as reinforcing Fibers. The drawback of using steel Fibers is that they will rust, if the vehicle has an extended rest period or if the vehicle has been operating near a more moisture environment. Steel Fibers attacked by rust will be less to withstand, thereby compromising their functionality as reinforcing Fibers. Brake pads include metals such as zinc distributed over the cross-section of the friction lining, thereby forming a sacrificial anode for rusting to occur [12]. Steel Fibers is that they might cause excessive wear of the brake disc or brake drum if they are present in large proportions. Steel Fibers have also been shown to increase friction coefficient fluctuations [13], Some brake pads contain oxidized or phosphatized Fibers, resulting in improved fracture toughness and strength [29]. 1.3 Aramid fiber Aramid Fibers made from the condensation product of isophthalic or terephthalic acids and m- or pphenylenediamine [27], such as Kevlar Fibers are mostly used as reinforcing Fibers. They are light and exhibit high thermal stability, with a very good stiffness weight ratio. According to Smith and Boyd of R.K. Carbon Fibers, aramid Fibers have superior anti-fade properties compared to asbestos [30]. Aramid Fibers in pulp form have also been utilized in maintaining the uniformity of the brake pad material mixture during the processing of a moulded brake pad [7]. Aramid fiber have is that of superior wear resistance [4]. Due to their relative softness, however, they will be the only Fibers supporting the braking load; there would most probably be other harder Fibers same like metallic Fibers in the friction lining. 1.4 Ceramic Ceramic Fibers are a relatively new raw material in friction product compared to metallic Fibers like steel. They are typically made of various metal oxides such as alumina (aluminium oxide) carbides and silicon carbide. With a high thermal resistance, melting points ranging from 1850 to 3000 °C [35], light weight and high strength [5], they are very suitable as reinforcing Fibers. They are preferred over metallic Fibers because their high strength weight ratio. 2 Binders The purpose of a binder is to maintain the friction product structural integrity under mechanical and thermal stresses. It has to hold the components of a friction product together and to prevent its constituents from crumbling apart also. The figures listed are typical values for that particular binder. The selection of binders for friction product is an important issue, because if it does not remain structurally intact at all times during the braking application, the other constituents like reinforcing Fibers or lubricants will disintegrate. So it has to have a high heat resistance. For this reason, silicone and epoxy modified resins would generally be ideal as the binder for most of the friction product in braking applications. All other binders would have to be application specific such that their disadvantages would not be compromise their functionality [8].
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2.1 Phenolic resin Phenolic resin is the most common resin binder used in brake friction material. Phenol resin is a type of polymer formed by a condensation reaction between phenol and formaldehyde, and is able to act as a matrix for binding together different substrates [6]. This condensation reaction may be initiated by acidic or alkali catalysts, resulting in different classes of phenolic resins. When these phenolic resins are cured, they change from a thermoplastic state to a closely compacted, cross-linked thermoset matrix with relatively high heat resistance. In high-energy braking applications, the temperature induced can be high enough to decompose the phenolic resin via means of high-temperature oxidation. Phenolic resins carbonize at approximately 450 °C [36]; temperatures beyond this, it decomposes by partial burning and evaporation. This process decreases the density of the brake friction material at the wear surface and also increases its porosity, thereby losing its structural integrity [21], disadvantage of phenolic resins is that they are brittle and have a very low impact resistance. So they are usually modified with toughness material like epoxy resin or by incorporating wood flour to improve its flexibility [18]. Moreover, Jang et al.’s experiment [13] shows that the larger the quantity of phenolic resin used, the larger the friction coefficient fluctuations will be. 3. FILLERS The fillers in friction products are present for the purpose of improving its manufacturability as well as to reduce the overall cost of the brake pad [9]. It is a loose term which could also mean anything used in a large proportion in a brake friction material. Fillers was not as critical as other components such as reinforcing Fibers, its play an important role in modifying certain characteristics of friction product materials. The actual choice of fillers depends on the particular components in the friction material as well as the type of friction material. Example a metallic pad generates a lot of braking noise would require extra fillers such as cashew and mica (noise suppressors) than barium sulphate (heat stability). On the semi-metallic brake pads with a mixture of metallic and organic compounds having thermal expansion coefficients would require a large amount of molybdenum trioxide to prevent lining cracking. Alkali metal would not require as filler, when using friction product with large quantities of graphite or antimony sulphide as lubricants. Therefore the specific filler to be used depends on the constituents of the friction material [8]. 3.1 Inorganic fillers Barium sulphate, mica, vermiculite and calcium carbonate are included in inorganic fillers. The common property of these fillers is that they possess a relatively high melting point. For example, barium sulphate has a melting point of 1350 °C [22], while vermiculite exfoliates rapidly into flakes at approximately 800 °C [32]. One of the more commonly used fillers is barium sulphate. It imparts heat stability to the brake friction product material, at the same time aiding the friction characteristic of the friction material [19].Calcium carbonate is considered to be an alternative to barium sulphate, because it has a similar function properties, it imparts heat stability to the friction product material, thereby improving the friction material’s brake fade properties [28]. Calcium carbonate is the low cost of the two, but it is not as stable at higher temperatures as barium sulphate [2]. Mica is one of commonly used filler. It is able to suppress low-frequency brake noise [1] due to it having a plane netlike structure [15]. However, due to its stratified structure, it has a low interlayer strength. Mica causes interlayer splitting of the friction product, especially at high braking loads. Aluminium phosphate can be used as a coating on mica powder to prevent the interlayer splitting of mica [31].Like mica, vermiculite can also suppress noises generated during braking [25]. It also has a plane netlike structure and resembles mica in appearance. However, it is porous and its wear resistance at high temperatures is compromised [15].Vermiculate has a relatively high melting point of approximately 800 °C [23]. 3.2 Organic fillers Cashew dust and rubber in the form of dust are commonly used of organic fillers. Both have similar properties in that they are usually incorporated into friction products for the purpose of reducing brake noises due to their superior viscoelastic characteristics [17].Cashew particles, fall off the friction surface easily, leaving behind large pores that eventually crack [16]. Also, certain brake friction material manufacturers use cashew or rubber particles as under layer material because their low thermal conductivity prevents heat from transmitting to the backing plate of the brake
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friction material [26]. Cashew particles are help to reduce fluctuations in friction coefficients, especially at elevated temperatures [13]. 4. Frictional Additives Frictional additives are help to friction product materials in order to modify the friction coefficients and wear rates. Additives are divided into two main categories: lubricants, which decrease the friction coefficients and wear rates, and abrasives increase friction coefficients and wear rates. It is also important that certain frictional additives may be loosely regarded as fillers by certain manufacturers if they are present in more quantities [8]. 4.1 Lubricants The purpose of a lubricant in friction product is to stabilize the developed friction coefficient during braking, particularly at high temperatures. Commonly used lubricants include graphite and various metal sulphides [8].Graphite is widely used as it is able to form a lubricant layer on the opposing counter friction material rapidly [34]. The graphite used in brake friction materials can be of natural or synthetic graphite, and can exist in flake or powder form. Graphite in the flake form has improved lubrication properties [32], while graphite in the powder form is able to dissipate heat generated during braking more effectively [3].However, the bonding strength between graphite and phenolic resin is very weak, so graphite cannot be used too liberally in phenolic resins, leading to low shear strengths [14]. 4.2 Abrasives The abrasives material in a friction product material increase the friction coefficient and also increasing the rate of wear of the counter face material. They remove iron oxides from the counter friction material as well as other undesirable surface film layer formed during braking application. The friction materials with higher abrasive content exhibit a more variation of friction coefficient, resulting in instability of braking torque. Abrasives are hard particles of metal oxides and silicates. The abrasives have to be hard enough to at least abrade the counter friction material like cast iron. The abrasives typically have Mohs hardness values of around 7–8, and a few examples of the commonly used abrasives include zirconium oxide, zirconium silicate, aluminium oxide and chromium oxide [20]. 5. Friction Product Market Opportunities and Growth Factors 5.1 Production The Automotive industry produced a total 23,960,940 vehicles (Figure 2). This including passenger vehicles, commercial vehicles, three wheelers and two wheelers in April-March 2016 a marginal growth of 2.58 percent over the same period last year.
Ref: SIAMA India
Figure: 2 Production Trend 5.2 Domestic sales The Passenger Vehicles grow by 7.24 percent in April-March 2016 compared to last year. The overall Commercial Vehicles segment growth of 11.51 percent in April-March 2016 as compared to last year. Medium &
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Heavy Commercial Vehicles registered a growth at 29.91 percent and Light Commercial Vehicles grow marginally by 0.30 percent during April-March 2016 compared to last year. 5.3 Exports Overall automobile exports grew by 1.91 percent in April-March 2016. Passenger Vehicles, Commercial Vehicles, Three Wheelers and Two Wheelers registered a growth of 5.24 percent, 16.97 percent (-) 0.78 percent and 0.97 percent respectively in April-March 2016 compared to last year. (Figure.3) The aftermarket for brake friction parts was valued at US$46.7 million in the year 2001.The growth in the market for brake friction parts is on the backdrop of expected increase in new vehicle sales. Vehicle segments like the multi utility vehicle and passenger car are expected to lead the market in growth rates.
Ref: SIAMA India
Figure: 3 Export Trend 6. Future Trends The future developments in the field of friction product materials will closely walk through the current trends of the automotive industry. The future emphasis on cars will be on lower emissions and fuel efficiency as environmental regulations become a more stringent. This shift towards environmentally friendly cars has already seen the release of hybrid cars like Toyota Prius, Honda Insight and Ford Escape SUV.The focus on vehicle fuel efficiency and lower emissions that means brakes will have to be lighter and not release any toxic and carcinogenic substances into the atmosphere during use. This means that the choice of brake friction materials will need to be more environmentally friendly and not include toxic substances such as asbestos. The Santa Clara Valley Nonpoint Source Pollution Control Program in United States has identified automotive brake pads as a major contributor of copper in storm water, leading to the southern reach of San Francisco Bay to be labeled as an ‘impaired water body’ [11]. This is not perceived to be a major problem as the varieties of existing dry friction constituents mean that safer alternatives usually exist. For example, ceramic Fibers or glass Fibers can be used in place of asbestos Fibers. One more area needs further investigation and development would be that of friction material binders. There is a need to develop nonphenolic resin binders as current choices are limited. Conclusion In the last ten years, new generation vehicles have entered in the market. As a result, the aftermarket has also geared up with high quality parts and components to support the modern vehicle platforms. In this paper, the advantages of several alternative materials for use as brake friction product material were reviewed for the required function of a friction material. There is a growing focus on a more organized and structured aftermarket trade in the Friction product industry which is helping the segment evolve faster. The brake friction market is in a growth phase. The growth rates are across the various vehicle segments. The commercial vehicle segment is expected to reach market maturity in the next 7-8 years. The high growth segments of the market are multi utility vehicles and passenger cars. The technological changes are likely to tilt the market in favour of friction products Companies with superior product
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quality, technology, extensive distribution, and proactive market initiatives are expected to gain in this competitive market. Friction product Companies need to target high growth vehicle segments with the right product mix. Acknowledgements Author extends his thanks to MKU, Madurai for providing research fellowship. References [1] Avallone, E. A. and Baumeister III, T. Marks Handbook for Mechanical Engineers, 10th edition, 1997, pp. 6–142 (McGraw-Hill, New York). [2] Blau, P. Compositions, functions, and testing of friction brake materials and their additives. Technical Report ORNL/TM-2001/64, Oak Ridge National Laboratory, 2001, p. 29; available through NTIS, Springfield, Virginia.
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