Friction coefficient is one of the most important performance indexes to evaluate any kind of friction material, which is related to the quality of the transmission and braking functions of the friction plate. It is not a constant, but a number that changes under the influence of temperature, pressure, friction speed or surface state and surrounding medium factors. The ideal friction coefficient should have ideal cold friction coefficient and controllable temperature decay. Because of the heat generated by friction and the increase of working temperature, the friction coefficient of friction material changes.
Temperature is an important factor affecting the friction coefficient. In the process of friction, due to the rapid rise of temperature, the friction coefficient of friction materials begins to decline when the temperature is above 200 ℃. When the temperature reaches the decomposition temperature range of resin and rubber, the friction coefficient suddenly decreases. This phenomenon is called "heat decay". Serious "heat recession" will lead to the deterioration of braking efficiency. In practical application, friction will be reduced, thus reducing the braking effect, which is very dangerous and must be avoided. It is an effective way to reduce and overcome the "heat recession" by adding high temperature friction regulator filler into friction materials. The friction coefficient of the friction plate after the "heat recession" will gradually return to the original normal condition when the temperature gradually decreases, but sometimes the friction coefficient will return to be higher than the original normal friction coefficient and recover too much, which we call "over recovery".
Friction coefficient usually decreases with the increase of temperature, but too much reduction can not be ignored. There are requirements for braking torque and speed stability in the bench test standard of automobile brake linings in China. (brake performance requirements for freight cars and passenger cars; brake performance requirements for passenger cars; brake bench test methods for passenger cars; brake bench test methods for freight cars and passenger cars). Therefore, when the vehicle speed is increased, the factors that reduce the brake efficiency should be prevented.
When the surface of friction material is stained with water, the friction coefficient will also decrease. When the water film on the surface is removed and restored to dry state, the friction coefficient will return to normal, which is called "wading recovery". When the surface of the friction material is stained with oil, the friction coefficient decreases significantly, but a certain amount of friction should be maintained so that it still has a certain braking efficiency.
The wear resistance of friction material is not only the reflection of its service life, but also an important technical and economic index to measure the durability of friction material. The better the wear resistance is, the longer its service life is. But the wear of friction materials in the working process is mainly caused by the shear force produced by the friction contact surface. The working temperature is an important factor affecting the amount of wear. When the surface temperature of the material reaches the thermal decomposition temperature range of the organic binder, the organic binder such as rubber and resin will decompose, carbonize and lose weight. With the increase of temperature, the phenomenon is intensified, the bonding effect is decreased, and the amount of wear is increased rapidly, which is called "hot wear". Choosing suitable antifriction packing and resin and rubber with good heat resistance can effectively reduce working wear of materials, especially thermal wear, and prolong their service life.
There are many ways to express the wear resistance index of friction materials. According to the national standard of GB5763-98 "automobile brake lining", the wear index (constant speed friction tester) is the wear rate at the temperature range of 100 ℃ ~ 350 ℃ (50 ℃ is the first gear). The wear rate is the amount of volume wear per unit of friction work in the process of relative sliding between the sample and the dual surface, which can be calculated by measuring the sliding distance of the friction force and the reduction of the thickness of the sample due to wear. However, in the process of friction test, affected by high temperature, the tested sample will produce different degree of thermal expansion, which covers the thickness wear of the sample, sometimes even a negative value, that is, the thickness of the sample increases after high temperature wear. This can't really reflect the actual wear. Therefore, some manufacturers not only determine the volume wear of samples, but also determine the weight wear rate of samples.
Before the friction material products are assembled and used, they need to be drilled, riveted, assembled and other mechanical processing before they can be made into brake pad assembly or clutch assembly. In the process of friction work, friction materials not only have to bear high temperature, but also bear large pressure and shear force. Therefore, the friction material must have enough mechanical strength to ensure that there is no damage and fragmentation in the process of processing or use. For example: riveting brake pad: it is required to have certain impact strength, riveting stress, compressive strength, etc. Bonded brake pad: the disc pad shall have enough room temperature bonding strength and high temperature (300 ℃) bonding strength to ensure that the friction material is firmly bonded to the steel back, which can withstand the high shear force of the disc pad in the braking process, without separation from each other, resulting in serious consequences of braking failure. The clutch plate is required to have sufficient impact strength, static bending strength, maximum strain value and rotation failure strength. In order to ensure that the clutch plate will not be damaged in the process of transportation, riveting and processing, and also to ensure that the clutch plate will not break under the working conditions of high-speed rotation.