1. Research on adhesive materials
The main function of the binder is to tightly bond the components of the brake pad together and maintain the structural integrity of the brake pad under high temperature mechanical action. The most commonly used binder in brake pads is phenolic resin, which has excellent heat resistance and mechanical properties, good electrical insulation and molding performance, and is easy to obtain from raw materials, cheap in price, and simple in process and production equipment. However, the use of pure phenolic resin will cause the brake pads to be too hard and brittle, and the heat resistance limit temperature is only about 250 °C. When it exceeds 300°C, the thermal decomposition phenomenon is quite serious, which will lead to a significant decrease in the performance of the brake pads. Therefore, the phenolic resin must be toughened and heat-resistant modified. The modified resins used abroad mainly include COPNA resin (decomposition temperature is about 400~500℃), silicone modified phenolic resin, boric acid modified phenolic resin, cyanate modified phenolic resin (can withstand high temperature above 350℃), epoxy resin Modified phenolic resin (normal use at 400°C), thermoplastic polyimide resin (good heat resistance and wear resistance), suspension resin. Suspension resin is named after the suspension polymerization process of phenolic resin. It is a new type of phenolic resin developed abroad in the 1970s. It is also called Phenolic Thermosphere (PTS for short). The decomposition temperature reaches 490 ℃. Brake pads made of adhesive have the advantages of stable friction coefficient, good high temperature friction performance, low noise and small thermal recession. The study found that the friction properties of brake pads using modified phenolic resin (including friction coefficient before recession, friction coefficient after recession, wear rate, damage to counterparts, etc.) are better than those using traditional phenolic resin. ; There is no necessary connection between the strength and wear performance of resin and brake pads. Among them, the brake pads prepared with boric acid-modified phenolic resin still maintain a high friction coefficient (above 0.4) at 400 °C.
2. Friction performance modifier
Friction performance modifiers are substances added to friction materials to improve the friction coefficient and wear rate. They are mainly divided into two categories: lubricants and abrasives. The main purpose of the lubricant is to reduce the change in the coefficient of friction during braking. Commonly used lubricants include graphite and various types of metal sulfides. Metal sulfides are considered to be better lubricants than graphite, because the low bonding strength of phenolic resin binders and graphite cannot meet the requirements of efficient braking in the modern automobile industry, and will accelerate the wear of friction materials, while metal sulfides do not exist. question. But some compounds such as lead and antimony sulfides are toxic, so safer metal sulfides such as tin, copper, and molybdenum sulfides may be ideal lubricants. Abrasives can increase the friction coefficient of the friction material, but at the same time increase the wear of the counterpart. They remove iron oxides from the dual material and the detrimental surface film created during braking, but high levels of abrasives increase the variability of the coefficient of friction. Abrasives are mainly hard particles of metal oxides, quartz powder and silicate compounds. Its Mohs hardness value is generally 7~8; commonly used abrasives are zirconium oxide, zirconium silicate, alumina, silicon carbide, silicon dioxide and chromium oxide. The addition of alumina can improve the friction coefficient and reduce the wear rate; the addition of silicon carbide can greatly improve the friction coefficient, while the wear rate is only slightly increased; a certain amount of antimony trisulfide (Sb2S3) and zirconium silicate (ZrSiO4) The size and stability of the friction coefficient of the brake pads have a great impact. The friction performance modifier has a great influence on the friction characteristics of the friction material. Increasing the content of lubricant can improve the stability of the coefficient of friction, while increasing the content of abrasives will increase the coefficient of friction. Therefore, it is very important to coordinate the amount of lubricant and abrasive in the brake friction material.
3. Application of Reinforcing Fibers in Brake Pad Materials
In the 1970s, friction materials began to develop towards non-asbestos, and various substitutes for asbestos fibers appeared, mainly including ceramic fibers, aramid fibers, carbon fibers, steel fibers, copper fibers, aluminum fibers, glass fibers, mineral fibers, and fibers. fiber, potassium titanate whisker and sepiolite fiber, etc. With the deepening of research, the performance of single fiber reinforced friction material is not comprehensive, and there are various defects. However, when several fibers are mixed together, the performance can be complementary, and the hybrid effect can be exerted. The prepared friction material has excellent performance, so the hybrid fiber enhances friction Materials have become a research hotspot in recent years. Studies have shown that the adhesion of potassium titanate whiskers to aramid fibers can improve the heat resistance and strength of the friction surface film, but when the friction material contains only one of the two fibers, this favorable The synergistic effect is greatly reduced. Brake pads containing glass fiber and aluminum fiber can not provide the ideal friction coefficient and wear rate, and the use of aramid fiber instead of glass fiber and potassium titanate as a friction performance modifier,
It can improve the performance of brake pads in an all-round way; compared with the brake pads without potassium titanate, the stability of friction coefficient, thermal decay resistance and wear resistance of brake pad materials containing potassium phthalate are improved. It is reported that the friction coefficient of copper fiber and steel fiber brake pads decreases with the increase of sliding speed, and the friction coefficient of aluminum fiber brake pads does not change much; adding copper fiber can make the friction material have a high and stable friction coefficient and very low wear rate. Some studies have compared the effects of aramid fibers, serofibers, PAN fibers (polyacrylonitrile fibers), and carbon fibers on the friction coefficient and wear resistance of friction materials. The results show that aramid fiber can overcome the thermal sensitivity of resin, improve the stability of friction coefficient, and reduce the wear rate; saluluo fiber can significantly improve the friction coefficient, but the wear is the largest; carbon fiber reinforced friction material has the best resistance Thermal decay properties; polyacrylonitrile fibers are the least sensitive to braking load and sliding speed, and have little effect on friction coefficient and wear rate. Compared with resins, fillers, and friction modifiers, reinforcing fibers have attracted more attention from brake pad researchers. The influence of various reinforcing fibers on the friction performance of automobile brake pads has been studied a lot at home and abroad, but there are not many studies in the field of ceramic fiber reinforced brake pads, only limited to potassium titanate whiskers and silica alumina fibers. In the future, it is necessary to explore the application of other types of ceramic fibers in brake pads.
4. The influence of filler type on the performance of brake pad material
Fillers are divided into two categories: organic fillers and inorganic fillers. Inorganic fillers include barium sulfate, calcium carbonate, feldspar powder, mica, talc, vermiculite, kaolin, diatomaceous earth, and the like. Barium sulfate and calcium carbonate are both commonly used fillers, which can improve the thermal stability of friction materials and also improve the thermal decay performance of the material, but at higher temperatures, the former is not as stable as the latter. Mica and vermiculite are the other two commonly used fillers, which have a planar network structure and can suppress low-frequency braking noise, but vermiculite peels off rapidly in flakes at about 800 °C, and mica has poor wear resistance at high temperatures. Cashew nut shell oil friction powder and rubber powder are commonly used organic fillers with similar properties and excellent viscoelasticity, so they are often added to brake pads to reduce noise. Molybdenum trioxide is a new type of filler family with a higher melting point, about 800 °C. Molybdenum trioxide is reported to prevent thermal recession and cracking of friction materials at high temperatures. The study found that barium sulfate has a great influence on the friction properties of brake pads. Through the research on the influence of five commonly used fillers, vermiculite, barite, kyanite, potassium feldspar and foam iron powder on the properties of brake friction materials, it is found that the effect of filler content on the friction coefficient of the material is not obvious, and the effect on the wear rate is not obvious. The effect is also small, with the exception of potassium feldspar. With the increase of potassium feldspar content, the wear rate of the friction material also increases. In addition, there are reports using a new combined method to study the effect of various fillers on the wear performance of brake pads, and propose three mechanisms to reduce the wear rate. The filler has an important influence on the mechanical properties, physical properties and friction properties of the friction material. It can adjust the hardness of the friction material, improve the braking noise and the appearance of the product, and reduce the cost. However, in the development and application of brake pads, most of the researchers focus on the study of reinforcing fibers, while there is little research on fillers. In actual production, fillers are often selected only by experience and habits, and there is no systematic scientific theory. as a guide.