Brake pads are the most critical safety parts in the brake system, which plays a decisive role in the quality of the brake effect, and a good brake pad is the protector of people and vehicles (aircraft).
First, the origin of brake pads
In 1897, HerbertFrood invented the first brake pads (using cotton thread as reinforcing fiber) and used them in horse-drawn carriages and early cars, from which the world-famous Ferodo Company was founded. Then in 1909, the company invented the world's first solidified asbestos-based brake pad; In 1968, the world's first semi-metal-based brake pads were invented, and since then, friction materials have begun to develop toward asbestos-free. At home and abroad began to study a variety of asbestos replacement fibers such as steel fiber, glass fiber, aramid fiber, carbon fiber and other applications in friction materials.
Second, the classification of brake pads
There are two main ways to classify brake materials. One is divided by the use of institutions. Such as automobile brake materials, train brake materials and aviation brake materials. The classification method is simple and easy to understand. One is divided according to the material type. This classification method is more scientific. Modern brake materials mainly include the following three categories: resin-based brake materials (asbestos brake materials, non-asbestos brake materials, paper based brake materials), powder metallurgy brake materials, carbon/carbon composite brake materials and ceramic based brake materials.
Third, automobile brake materials
1, the type of automobile brake materials according to the manufacturing material is different. It can be divided into asbestos sheet, semi-metal sheet or low metal sheet, NAO(asbestos free organic matter) sheet, carbon carbon sheet and ceramic sheet.
1.1.Asbestos sheet
From the very beginning, asbestos has been used as a reinforcement material for brake pads, because asbestos fiber has high strength and high temperature resistance, so it can meet the requirements of brake pads and clutch discs and gaskets. This fiber has strong tensile capacity, can even match high-grade steel, and can withstand high temperatures of 316 ° C. What's more, asbestos is relatively cheap. It is extracted from amphibole ore, which is found in large quantities in many countries. Asbestos friction materials mainly use asbestos fiber, namely hydrated magnesium silicate (3MgO·2SiO2·2H2O) as reinforcement fiber. A filler for adjusting friction properties is added. An organic matrix composite material is obtained by pressing the adhesive in a hot press mold.
Before the 1970s. Asbestos type friction sheets are widely used in the world. And dominated for a long time. However, due to the poor heat transfer performance of asbestos. Friction heat cannot be dissipated rapidly. It will cause the thermal decay layer of the friction surface to thicken. Increase material wear. In the meantime. The crystal water of asbestos fiber is precipitated above 400℃. The friction property is significantly reduced and the wear is dramatically increased when it reaches 550℃ or more. The crystal water has been largely lost. The enhancement is completely lost. More importantly. It's medically proven. Asbestos is a substance that has serious damage to human respiratory organs. July 1989. The U.S. Environmental Protection Agency (EPA) announced that it would ban the import, manufacture, and processing of all asbestos products by 1997.
1.2, semi-metal sheet
It is a new type of friction material developed on the basis of organic friction material and traditional powder metallurgy friction material. It uses metal fibers instead of asbestos fibers. It is a non-asbestos friction material developed by the American Bendis Company in the early 1970s.
"Semi-metal" hybrid brake pads (Semi-met) are mainly made of rough steel wool as a reinforcing fiber and an important mixture. Asbestos and non-asbestos organic brake pads (NAO) can be easily distinguished from the appearance (fine fibers and particles), and they also have a certain magnetic properties.
Semi-metallic friction materials have the following main characteristics:
(l) Very stable below the coefficient of friction. Does not produce thermal decay. Good thermal stability;
(2) Good wear resistance. The service life is 3-5 times that of asbestos friction materials;
(3) Good friction performance under high load and stable friction coefficient;
(4) Good thermal conductivity. The temperature gradient is small. Especially suitable for smaller disc brake products;
(5) Small braking noise.
The United States, Europe, Japan and other countries began to promote the use of large areas in the 1960s. The wear resistance of semi-metal sheet is more than 25% higher than that of asbestos sheet. At present, it occupies a dominant position in the brake pad market in China. And most American cars. Especially cars and passenger and cargo vehicles. Semi-metal brake lining has accounted for more than 80%.
However, the product also has the following shortcomings:
(l) Steel fiber is easy to rust, easy to stick or damage the pair after rust, and the strength of the product is reduced after rust, and the wear is increased;
(2) High thermal conductivity, which is easy to cause the brake system to produce gas resistance at high temperature, resulting in the friction layer and the steel plate detachment:
(3) High hardness will damage the dual material, resulting in chatter and low-frequency braking noise;
(4) High density.
Although "semi-metal" has no small shortcomings, but because of its good production stability, low price, it is still the preferred material for automotive brake pads.
1.3. NAO film
In the early 1980s, there were a variety of hybrid fiber reinforced asbestos-free brake linings in the world, that is, the third generation of asbestos-free organic matter NAO type brake pads. Its purpose is to make up for the defects of steel fiber single reinforced semi-metallic brake materials, the fibers used are plant fiber, aramong fiber, glass fiber, ceramic fiber, carbon fiber, mineral fiber and so on. Because of the application of multiple fibers, the fibers in the brake lining complement each other in performance, and it is easy to design the brake lining formula with excellent comprehensive performance. The main advantage of NAO sheet is to maintain good braking effect at low or high temperature, reduce wear, reduce noise, and extend the service life of the brake disc, representing the current development direction of friction materials. The friction material used by all the world famous brands of Benz/Philodo brake pads is the third-generation NAO asbestos-free organic material, which can brake freely at any temperature, protect the life of the driver, and maximize the life of the brake disc.
1.4, carbon carbon sheet
Carbon carbon composite friction material is a kind of material with carbon fiber reinforced carbon matrix. Its frictional properties are excellent. Low density (steel only); High capacity level. It has a much higher heat capacity than powder metallurgy materials and steel; High heat intensity; No deformation, adhesion phenomenon. Operating temperature up to 200℃; Good friction and wear performance. Long service life. The friction coefficient is stable and moderate during braking. Carbon-carbon composite sheets were first used in military aircraft. It was later adopted by Formula 1 racing cars, which is the only application of carbon carbon materials in automotive brake pads.
Carbon carbon composite friction material is a special material with thermal stability, wear resistance, electrical conductivity, specific strength, specific elasticity and many other characteristics. However, carbon-carbon composite friction materials also have the following shortcomings: the friction coefficient is unstable. It is greatly affected by humidity;
Poor oxidation resistance (severe oxidation occurs above 50 ° C in the air). High requirements for the environment (dry, clean); It's very expensive. The use is limited to special fields. This is also the main reason why limiting carbon carbon materials are difficult to widely promote.
1.5, ceramic pieces
As a new product in friction materials. Ceramic brake pads have the advantages of no noise, no falling ash, no corrosion of wheel hub, long service life, environmental protection and so on. Ceramic brake pads were originally developed by Japanese brake pad companies in the 1990s. Gradually become the new darling of the brake pad market.
The typical representative of ceramic based friction materials is C/ C-sic composites, that is, carbon fiber reinforced silicon carbide matrix C/SiC composites. Researchers from the University of Stuttgart and the German Aerospace Research Institute have studied the application of C/ C-sic composites in the field of friction, and developed C/ C-SIC brake pads for use in Porsche cars. Oak Ridge National Laboratory with Honeywell Advnanced composites, HoneywellAireratf Lnading Systems, and Honeywell CommercialVehicle systems The company is working together to develop low-cost C/SiC composite brake pads to replace cast iron and cast steel brake pads used in heavy-duty vehicles.
2, carbon ceramic composite brake pad advantages:
1, compared with the traditional gray cast iron brake pads, the weight of carbon ceramic brake pads is reduced by about 60%, and the non-suspension mass is reduced by nearly 23 kilograms;
2, the brake friction coefficient has a very high increase, the brake reaction speed is increased and the brake attenuation is reduced;
3, the tensile elongation of carbon ceramic materials ranges from 0.1% to 0.3%, which is a very high value for ceramic materials;
4, the ceramic disc pedal feels extremely comfortable, can immediately produce the maximum braking force in the initial stage of braking, so there is even no need to increase the brake assist system, and the overall braking is faster and shorter than the traditional braking system;
5, in order to resist high heat, there is ceramic heat insulation between the brake piston and the brake liner;
6, ceramic brake disc has extraordinary durability, if normal use is lifetime free replacement, and ordinary cast iron brake disc is generally used for a few years to replace.
Post time: Sep-08-2023