Research and Development of Ceramic Brake Pads
In order to meet the increasingly demanding requirements of high-speed, safety, and comfort in modern cars, it is necessary to continuously strive to develop brake systems and brake materials. Especially during high-temperature braking, the stability and safety of the material are crucial. This article provides a brief overview of the research and development of ceramic brake pads, explores the preparation process and key technical issues of automotive ceramic brake pads, as well as their huge market potential. It proposes that ceramic brake pads will be the development trend of automotive brake friction materials, which is of great significance for the development of ceramic brake materials.
Due to the problems of noise, vibration, high wear rate, and short service life of the semi metal friction materials and asbestos free friction materials mainly used in automotive braking systems, carbon/carbon composite friction materials that can be replaced may have excellent performance, but their costs are relatively high and are usually only used on aircraft. Ceramic materials have the advantages of small specific gravity, high melting point, high hardness, good chemical stability, and corrosion resistance, and have been widely used in friction materials. Therefore, developing new ceramic friction materials with stable friction performance, low wear rate, long service life, no noise and vibration has become a hot field of friction material research.
1.History of brake pad material usage
The traditional brake pad material is made of asbestos fiber filled phenolic resin, where asbestos is made from silicate minerals and contains a certain amount of crystalline water in its composition. The increase in speed of modern cars has resulted in surface temperatures of brake components reaching as high as 300-500 ℃. Asbestos friction materials have poor thermal conductivity and heat resistance, and will lose crystalline water at around 400 ℃. At 550 ℃, the crystalline water will be completely lost, and the strengthening effect has basically lost. After dehydration of asbestos, it can cause unstable friction, deterioration of the working layer material, intensified wear, and obvious "heat recession" phenomenon. In addition, asbestos has a carcinogenic effect on its dust during processing and use, so asbestos based brake pads have been banned in Western industrial countries, and China has also restricted the use of this type of material. It is obvious that asbestos organic friction materials are no longer suitable for the development needs of the automotive industry and modern society, and will gradually be replaced by new materials.
The second generation brake pad material is a semi metallic graphite composite material. Its main components are steel fiber, graphite, metal powder and their auxiliary materials, which are bonded and formed with modified phenolic resin. It is named after the metal content in the brake pad accounts for half of the total weight. Countries such as the United States, Europe, and Japan began to promote their use on a large scale in the 1960s. The wear resistance of semi metal sheets is more than 25% higher than that of asbestos sheets, and they also have advantages such as high friction coefficient, good thermal conductivity, and easy processing and forming. Therefore, this type of material currently dominates the brake pad market in China.
However, the product still has the following drawbacks: ① steel fibers are prone to rust, which can easily adhere or damage the product after rust, and the strength of the product decreases and wear increases after rust; ② High thermal conductivity can easily cause air resistance in the braking system at high temperatures, resulting in the detachment of the friction plate layer from the steel plate; ③ High hardness can damage the dual material, resulting in vibration and low-frequency braking noise; ④ High density.
The third generation brake pad material is asbestos free organic friction material (NAO). Glass fibers, aromatic polyimide fibers, or other fibers (such as carbon and ceramics) are mainly used as reinforcement materials. Its main advantage is that it maintains good braking performance both at low and high temperatures, reduces wear, reduces noise, and extends the service life of brake discs. The material of NAO type brake pads has undergone several changes, and now NAO materials have effectively surpassed the performance of asbestos brake pads in many aspects, mainly in terms of anti friction performance and noise.
Ceramic brake pads are a new variety of friction materials. Ceramic brake pads are composed of ceramic fibers, iron free fillers, adhesives, and a small amount of metal. They have advantages such as no noise, no dust, no corrosion of wheel hubs, long service life, and environmental protection.
2.Physical and mechanical properties of 2 ceramic brake pads
The test results of the physical properties and impact strength comparison between the new ceramic based brake pads and resin based brake pads are shown in Table 1
The new ceramic formula uses a variety of lightweight porous fillers, and does not contain any metal components (steel fiber, foam, iron powder, heavy metals, etc.). Compared with the semi metal formula, the ceramic brake pads have large open porosity and low density. Meanwhile, compared to resin based friction materials, ceramic based friction materials also have lower hardness. This is mainly due to the following reasons: firstly, there are many pores on the surface of ceramic based brake pads, which reduces the effective area of the material itself to withstand external loads, resulting in smaller values during hardness testing; Secondly, phenolic resin is tightly stacked with rigid aromatic rings connected by methylene, and after high-temperature curing, its hardness is relatively high; In the resin based formula (ordinary semi metallic formula), there are more metal components (steel fibers), which also makes the overall hardness of the resin based friction material higher. In terms of impact strength, compared to ceramic based brake pads, resin based brake pads have a relatively higher content of organic components, especially the addition of nitrile rubber powder, which disperses an appropriate amount of flexible rubber particles on a continuous rigid and hard resin matrix, forming the so-called "high molecular alloy". When the resin rubber matrix is composed of rubber particles, due to the elasticity of the rubber particles, the cracks generated by stress concentration are absorbed when the material is impacted, hindering the further expansion of the cracks and slowing down the degree of material damage. At the macro level, it is manifested as an improvement in the material's impact strength; Although inorganic binders in ceramic formulas have good high-temperature bonding effects, their flexibility is not good. However, this disadvantage can be achieved by modifying the material of ceramic based brake pads, attempting to change the elastic modulus of ceramic based brake pads. Griffith microcrack theory can be used to add plastic particles or fibers to the ceramic material matrix to make cermet or composite materials, and can also artificially cause a large number of extremely fine cracks in the material, Use it to absorb energy and prevent crack propagation. The recent emergence of ductile ceramics is the addition of zirconia to alumina, which utilizes the phase transformation of zirconia to generate volume changes and form a large number of microcracks or significant internal stresses in the matrix, thereby improving the toughness of the material. The above methods can be applied to the research and improvement of ceramic based brake pads, so that ceramic based brake pads not only have extremely high hardness, but also have toughness comparable to resin based brake pads.
3.Key technical issues of ceramic brake pads
At present, there are several key technical issues that need to be improved for automotive ceramic friction materials:
(1) Improve the performance and quality of adhesives (resin and rubber) for friction materials (such as high thermal decomposition temperature, low heat loss, low free phenol content, high softening point, fine particle size, good toughness, etc.), and actively develop new types of resins for friction materials;
(2) Improve the thermal conductivity of friction materials to accelerate the transfer of friction heat and prevent performance degradation caused by the decomposition of friction materials;
(3) Actively developing lightweight and high-performance porous fillers to reduce product density and reduce braking noise;
(4) Improve the level of key process equipment such as crushing, mixing, hot pressing, and heat treatment (solidification), improve process operations, quality testing and control methods, and ensure the stability of product mass production quality;
(5) Reduce brake noise induced by friction pads;
(6) Improve the shear strength and compressive strength of brake friction pads for safe braking.
Through the research and analysis of the physical and mechanical properties of new ceramic based automotive brake pads, it can be seen that ceramic brake pads have characteristics such as stable friction coefficient, good temperature resistance, long service life, comfortable braking, no damage to the dual disc, and no braking noise. Moreover, they have high cost-effectiveness and broad application prospects.
Although ceramic brake pads are unlikely to replace traditional brake pads in the short term, modern cars are moving towards high-performance, high-speed, safety and comfort. This requires that the brake system, as an important component of automobiles, must be safe and reliable. At the same time, new brake materials must be continuously developed to meet stricter requirements. Ceramic brake pads will inevitably become a development trend in the future.
The research and further development of new types of automotive ceramic brake pads, especially high-temperature resistant, low noise, metal free, and fiber free brake pads, is expected to change the application status of existing brake pads in China, and will have very important social and economic significance.
