Evolution of automotive brake material
Until the late 196Os, the U.S. passenger car and light truck automotive market used drum brakes on all four wheels and organic brake linings. In the late 1960s and early 1970s there was a transition to disc front-drum rear brake systems. Following these changes, which were still not fully settled, the past decade produced changes that are even more drastic. Beginning in the mid- 197Os, three important new requirements were imposed on the automotive industry: the Federal braking regulations, the need to eliminate asbestos from friction materials, and the conversion of the heavy vehicles to more energy-efficient lighter and smaller front wheel drive vehicles. These changes necessitated development of a new generation of friction materials
Before the 1970s, all brake pads had asbestos in them. The substance was proven to be dangerous to human health, so it was banned in this product, as well as other areas of industry. With the disappearance of asbestos pads came the Organic Brake Pad. These pads do contain between 10 to 30% metal, but the rest of the friction material comes from plant-derived fibers, high-temperature resins, and other materials. Depending on the proportion of metal in them, they might also have Kevlar, carbon, rubber, and even glass in them.There is also a kind of organic brake pad called a Low-metallic NAO, which refers to Non-Asbestos Organic, which comes with more metal than the “fully organic” ones, and also bring an enhanced capability of heat transfer. Most volume manufacturers and premium manufacturers fit their cars from the factory with Organic Brake Pads, but this only refers to over 50% of new vehicles. Others employ different types of brake pads from the production line.
The ceramic friction material for brake pads was developed as a replacement for both organic and semi-metallic parts. This has not happened yet, but there is a good reason for that. Ceramic brake pads are the most expensive you can buy, and their abilities are not suited for all the consumers targeted by suppliers and automakers. Organic material found the first type of brake pad described above, these components have a dense ceramic material. Do not think of glass, but something similar to pottery that is made in a kiln, which is mixed with copper (or other metal) fibers. Together, the combination of materials provides better performance, and they are more silent than the other types. Ceramic brake pads are appreciated for their long lifespan, as well as stable and consistent performance throughout their operating life. However, these pads are sometimes criticized for the “feel” they provide in operation, but also for reduced effectiveness in cold climates when compared to semi-metallic pads. This type of brake pad must not be confused with carbon-ceramic braking systems, which are found in supercars. Some high-end sports cars offer them as optional equipment. They come with ceramic pads, but the discs are made with composite materials instead of cast iron. They provide the highest level of performance available in cars, but also come at a hefty cost, and need to be warmed up for optimal performance.
The second type of brake pad friction material is named “semi-metallic”,the reason for this is because they feature from 30 to 65% metal by weight. Multiple types of metal are used, from copper and iron to steel. The rest of the friction surface is made of fillers, modifiers, and other substances required enhancing performance and maintaining reliability. This type of brake pad friction material is extremely popular among automakers, and they are considered the most versatile type of brake pad on the market. They have their disadvantages, evidently, but some believe that getting semi-metallic brake pads is the best option. It all depends on the application. Before the appearance of ceramic brake pads, semi-metallic pads used to be the best performing pads available on the market. Evidently, some of that advantage has disappeared with new technology, but they still can keep up with their superior competitors from many points of view.
Asbestos brake pads ruled for many years but with the health concern it got replaced with other materials. Now there was a concern to find asbestos free materials. Replacement of asbestos are suggested as carbon, Kevlar, glass fibre, steel wool, wollastonite, graphite fibres and a number of other mineral fibre types. Then the organic brake pads came into existence containing some amount of metal, but the rest of the friction material comes from plant-derived fibers, high-temperature resins, and other materials. Depending on the proportion of metal in them, they might also have Kevlar, carbon, rubber, and even glass in them..There is also a kind of organic brake pad called a Low-metallic NAO, which refers to Non-Asbestos Organic, which comes with more metal than the “fully organic” ones, and also bring an enhanced capability of heat transfer. Ceramic brake pads are appreciated for their long lifespan, as well as stable and consistent performance throughout their operating life. Further experiments were done with coconut shell, palm kernel shell (PKS), Phenolic resin (phenol formaldehyde), Bagasse etc. Ingredients such as phenolic resin, aramid pulp, glass fiber, potassium titanate, graphite, aluminum fiber and copper powder were used in the composite development phase, in addition to the fly ash. Still researchers are studying the properties of the constituent brake pads and experimented with samples in practical conditions to bring out the best. The future materials for brake pads should concentrate on the part that it should not cause any health hazard, better heat resistance, higher coefficient of friction, extended durability, dissipate the heat to the atmosphere easily and should be cost effective for the purpose of use.
Prof. Sasthi Charan Hens
Department of Mechanical Engineering,
University of Engineering & Management (UEM), Jaipur