Summary
While all the components of a brake system are important, it is imperative to acknowledge that some components play significant roles in the system as compared to others. One such component is the brake pad. In most cases, the brake pad is referred to as the friction material. It is responsible for generating the friction which opposed the movement of the wheel thus causing a stop to the movement of the automotive. This report is aimed at selecting a section or component of the brake system and presents its modification. As a consequence, the report highlights and discusses the operation and possible modification of the friction material used in a brake system. It presents different modifications through an understanding of the materials used in the production of brake friction material as well as the friction and wear of the brake friction material.
Introduction
It is imperative to note that the friction material used in the brake system is arguably at the core of any brake system. The performance characteristics of the friction material are highly defined by the property of the material. The braking ability presented by the brake pads are contributed and determined by physical and chemical properties of the material used in making and producing the braking pads. Apart from the physical and chemical properties of the frication material, there are several factors that affect the braking ability and performance characteristics of the brake pads (Chan & Stachowiak, 2004).
A considerable amount of stress is exerted on the friction materials by high-performance applications like race track able brake systems present in high powered automobiles. The high amount of stress exerted on the friction materials by the high-performance application is expressed in several different forms such as high brake and clamp torque loads, extremely high flux loads, and high operating temperatures. For these reasons, it is imperative for high-performance applications to pick and implement competent friction materials. Additionally, it is significant to comprehend extremely the operating circumstances the friction material will deal with in the selected application.
Figure 1: Illustration of brake friction material. Retrieved from http://www.johnstuartpowerbrake.com/Brake-Pads-Shoes.page
Automotive dynamic effects in the event of testing on the race track as well as the overall impact on the braking traction present at each wheel can greatly affect the distribution of braking energy within the brake system. In most cases, the dynamic effects can result into driving of the specific rotor temperature appreciably higher than mainly simplified types would foresee. It is worth noting that factors like tire traction, brake force distribution, chassis controls behavior, and front vs. rear brake fade behavior can highly affect the distribution of front to rear braking energy (Cho et al., 2006). In the same way, conflicting rotor cooling characteristics in effects like front wheel steer angle and vehicle slip angle will have an impact on the distribution of cooling aptitude in the automotive. There are several modifications and adjustments that can be implemented so as to improve the performance characteristics of the friction materials as well as the entire brake system (Jang et al., 2004). Within a specific brake section, deflection of the brake rotor, pads and calipers under braking torque and clamp loads, as well as the overall changes in the rotor to pad pressure distributions, can generate a considerable temperature difference over the surface of the brake rotor.
It is imperative to note that a heat-flux or temperature associated issue on any one of the brake corners can considerably reduce the resultant performance of the brake system. Any issue related to heat influx or temperature changes can negatively impact the overall performance of the brake system in several ways such as thermal roughness, pedal travel increase, rotor cracking and fade (Cho et al., 2006). It is imperative for the design of a successful brake system to address all the effects and avoid the issues associated with high temperature or heat influx on even the highly loaded brake corner. One way to achieve a successful brake design is by understanding the operation and performance characteristics of the friction materials. As a consequence, the selection and implementation of the friction material used for brake pads will be relevant and able to overcome possible operational limitations (Jang, 2013).
Theory
Even though there are different brake systems, they all exhibit the same principle of operation. The different brake systems are made up of several and different components depending on the type of brake system is used. In most cases, the operation of the brake system exhibits similar principle. The main principle of operation is based on the transfer of force from the brake pedal to the brake pads of friction material employed and finally to the wheel. When the operator steps on the brake pedals, the force used is transferred to the brake fluid in the brake system. The brake fluid then transfers the energy to the brake pistons of the brake caliper. This movement, in turn, forces the brake pads against the brake rotor. The clamping movement of the brake pads on the brake rotor causes friction which deters the rotational movement of the brake rotor as well as the axel that is mounted on it (Jang, 2013). As a consequence, the kinetic energy of the automotive is transformed into heat energy which is originally generated by the brake pads and the rotor.
Apart from public awareness, consumer demands have sparked the start of extreme research and development of various brake pads and friction materials used in brake systems. It is imperative to note that friction material used in brake systems have undergone substantial development and modifications over the past few decades. These developments and modifications are always aimed at improving the performance characteristics of the friction material as well the overall performance of the brake system (Chan & Stachowiak, 2004).
The manufacturers of brake friction materials have moved from the use of certain materials and have embraced the application of different materials with better and enhanced performance properties (Cho et al., 2006). It is imperative to highlight that numerous brake friction material manufacturers moved away from the use of asbestos in the production and manufacture of brake friction material so as to gain consumer acceptance. As a consequence, numerous brake pads have sprung composed of a different material with different performance properties.
Brake friction materials
The brake system of an automotive operates by transforming the vehicle’s kinetic energy into thermal energy. In the event of braking, the thermal energy is originally generated by the two contact areas of the brake; the brake pads and the brake disk, shoe or drum. The thermal energy is then related to the contacting constituents of the brake system like the calipers of the brake and the surroundings. The other end of the brake system is made up of such component as hydraulic line, caliper, pistons, brake pads and the brake rotor (Jang et al., 2004). The hydraulic line issued to transfer the pressure from the brake pedal to the pistons in the calipers. The hydraulic fluid in the caliper expands forcing the pistons to push the brake pad against the brake rotor.
It is imperative to highlight the basic requirements for brake pads or brake friction materials. It is important that the brake friction material selected for use and application in an automotive be able to sustain a adequately high friction coefficient with the brake disc; demonstrates a consistent and stable friction coefficient with the brake rotor; and not break down or decompose in such a manner that compromised the friction coefficient between it and the brake disc at elevated temperatures.
In most cases, the brake friction material is made up of several subcomponents such as the following; frictional additive, a binder, fillers and reinforcing fibers. The fractional additives establish the frictional characteristics of the brake friction material. The additives are made up of lubricants and abrasives (Cho et al., 2006). Apart from the frictional additives, brake pads are also made up of fillers. Fillers are used to improving the manufacturability of the brake friction materials through cost reduction. Additionally, binders are used to make up brake pads as well. They are used to hold the components of the brake friction material in place. Lastly, the brake pads are composed of reinforcing fibers which are used to provide mechanical strength to sustain the friction and heat generated during braking.
It is imperative to point out that brake friction materials are classified into three categories depending on the ingredients used in their manufacture. The general classification of brake pads includes metallic, semi-metallic and non-asbestos organic. The ingredients used in metallic brake pads are primarily metallic like copper fibers and steel fibers. On the other hand, the materials used in the production of semi-metallic brake pads are a mixture of organic ingredients and metallic ingredients (Jang et al., 2004). Lastly, non-asbestos organic brake pads are made from predominantly organic ingredients like rubber, mineral fibers, and graphite.
Reinforcing fibres
One of the basic components that have been used as reinforcing materials in brake friction material for a long time is asbestos fibers. Originally, friction lining was made up of a combination of brass wires, asbestos, and resins. It is imperative to note that some of the properties that propelled asbestos to be a popular and preferred material for brake pads include; its low cost, and it presented frictional lining with thermal resilience as well as excellent durability (Cho et al., 2006). The high-temperature resilience of the material used as a reinforcing fiber is imperative given that braking temperatures can attain hundreds of degrees Celcius. Reinforcing fibers used in brake pads are employed to present mechanical strength to the brake friction material.
Research and studies have discovered that the braking load is in a real sense carried by small plateaus that rise past the nearby lowlands on the brake friction material. The plateaus are generated by the reinforcing fiber circled by the compacted materials which are much softer. As a consequence, the importance of reinforcing fibers in brake friction materials cannot be ignored or underestimated. There are different materials used in the production of reinforcing fiber used in brake friction materials (Cho et al., 2006). Different material exhibit different mechanical properties thus present different braking properties to brake friction material. Some of the materials used in the production of reinforcing fibers include; Glass, Metallic, Sepiolite, Aramid and Potassium titanate (ceramic). Glass exhibit sufficient thermal resilience since it has a high melting point of 1430 degree Celcius. On the other hand, it is brittle. Similarly, metallic reinforcing fibers such as copper and steel fibers exhibit high heat resilience with a melting point above 1000 degrees Celcius. One major disadvantage of metallic reinforcing material is that large amounts of this material may result in extreme rotor wear.
Aramid presents sufficient stiffness to weight ratio, good wear resistance, and excellent thermal resistance. However, Aramid is soft and must be used in conjunction with other materials. Ceramic presents good stiffness to weight ratio and excellent thermal resilient. It has high melting point ranging from 1700 to 2040 degrees Celcius. The only disadvantage is that it is brittle. To avoid the negative impacts associated with elevated temperatures and wear of the brake disc, it is imperative to embrace the use of ceramic in the manufacture of reinforcing materials for brake friction materials (Chan & Stachowiak, 2004). Apart from the high thermal resilience, ceramic exhibit high strength as well as light weight. These properties make them very suitable for use as a reinforcing fiber in the production of brake friction material. Ceramic possess the high strength to weight ratio which means that they are favored over metallic fiber which exhibits low strength to weight ratio. Apart from their application in brake pads, ceramics are used in reinforcing brake discs.
Binders
Binders perform a very significant role in the braking system of any vehicle. They are tasked with the role of maintaining the structural integrity of the brake pad under thermal and mechanical stresses. It is mandatory for the binder to hold the constituents of a brake pad together as well as to prevent its components from falling apart. The selection and choice of the material used in brake pad binder are very imperative since the brake friction material do not remain structurally intact throughout the braking operation. The other components of the brake friction materials like the reinforcing fiber and the fillers will crumble during the braking operation (Jang et al., 2004). As a consequence, the material used in the production of brake binders should have a high heat resistance. While most breaking applications employ the use of various materials, the use and application of silicone and epoxy modified resins presents an ideal binder for most braking applications.
COPNA Resin, Phenolic resin, Cyanate ester resin, Thermoplastic polyimide resin, Epoxy-modified phenolic resin and Silicone modified phenolic resin are among the materials used in the production brake friction material binders. Phenolic resin is characterized by low cost and ease of production. On the other hand, it has low impact resistance; it is brittle, and it is extremely toxic (Cho et al., 2006). Also, Phenolic resin decomposes at a considerably low temperature of around 450 degrees Celcius. Similarly, COPNA resin is characterized by better wear resistance given that it has high bonding strength with graphite (Jang, 2013). However, the material decomposes at a relatively low temperature of between 450 to 500 degrees Celcius. As opposed to both Phenolic resin and COPNA resin, Silicone-modified phenolic resin exhibits enhanced impact resistance, chemical and heat resistance and improved water repellency. However, it still exhibits the toxic nature of the phenolic material (Chan & Stachowiak, 2004). Thermoplastic polyimide resin is regard as one of the best materials for binders given that it exhibits abrasion resistance and does not demonstrate thermal fade. The major disadvantage demonstrated by this material is the extremely low thermal conductivity. It exhibits a thermal conductivity that is three times below that of phenolic resin.
Experimental results
Research conducted to determine the brake friction material to be used in making the braking pads for improved performance was conducted on different materials under different conditions. The conditions which the materials were exposed to include: high level of friction coefficient, high efficiency, changes of the coefficient of friction with very low speed and maintaining the properties of elevated temperature. As a consequence, there are numerous composite materials which have been examined and complied with the imposed conditions at a rate of 75 percent.
Discussion
Research and studies have discovered that the braking load is in real sense carried by small plateaus that rise past the nearby lowlands on the brake friction material. The plateaus are generated by the reinforcing fiber circled by the compacted materials which are much softer. As a consequence, the importance of reinforcing fibers in brake friction materials cannot be ignored or underestimated. There are different materials used in the production of reinforcing fiber used in brake friction materials (Jang et al., 2004).
Ceramic possess the high strength to weight ratio which means that they are favored over metallic fiber which exhibits low strength to weight ratio. Apart from their application in brake pads, ceramics are used in reinforcing brake discs. Additionally, it is imperative to embrace the use of silicone-modified resin. They are reacted by combining phenolic resins with silicone rubber or silicon oil. Tougheners are employed to modify phenolic resins so as to reduce their brittleness.
Conclusion
It is very important to point out that brake friction materials are classified into three categories depending on the ingredients used in their manufacture. The general classification of brake pads includes metallic, semi-metalic and non-asbestos organic. The ingredients used in metallic brake pads are primarily metallic like copper fibres and steel fibres. On the other hand, the materials used in the production of semi-metallic brake pads are a mixture of organic ingredients and metallic ingredients. Lastly, non-asbestos organic brake pads are made from predominantly organic ingredients like rubber, mineral fibers and graphite.
References
Chan, D., & Stachowiak, G. W. (2004). Review of automotive brake friction
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Cho, M. H., Ju, J., Kim, S. J., & Jang, H. (2006). Tribological properties of solid lubricants
(graphite, Sb 2 S 3, MoS 2) for automotive brake friction materials. Wear, 260(7), 855-
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Jang, H. O. (2013). Brake friction materials. In Encyclopedia of Tribology(pp. 263-273).
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Jang, H., Ko, K., Kim, S. J., Basch, R. H., & Fash, J. W. (2004). The effect of metal fibers on the
friction performance of automotive brake friction materials. Wear, 256(3), 406-414.
