Brake pad friction coefficient is too high or too low will affect the braking performance.
Especially when the car needs emergency braking while driving at high speed, if the friction coefficient is too low, the brakes will not be sensitive, and if the friction coefficient is too high, the tires will lock up, which will cause the vehicle to flick and skid, which seriously poses a serious threat to driving safety. threaten.
According to national standards, the suitable working temperature of brake pads is 100-350 ℃. However, when the temperature of many inferior brake pads reaches 250°C, the friction coefficient will drop sharply, and the brake will completely fail at this time. Generally speaking, according to the SAE standard, manufacturers of brake friction pads will choose the FF rating coefficient, that is, the friction rating coefficient is 0.35~0.45.
Friction from adhesion is always present at the interface. In boundary lubrication friction or dry friction in a wet environment, the liquid forms a meniscus or adhesion bridge around the asperity, and the viscous effect of the meniscus plays an important role and in some cases constitutes the entire friction force. The adhesion coefficient μa of dry friction is:
μa=Ar•τa/N=τa/Pr= τa/σs
In the formula: Pr—is the actual average pressure, τa, σs—are the shear strength limit and yield limit of softer materials, respectively.
In-depth studies have shown that when the contact between asperities on the surface is elastic contact or plastic contact and the effect of surface film, it affects the joint strength of asperity contact, thus affecting the friction resistance.
It can be seen that the increase of the shear strength of the material or the decrease of the compressive yield limit will lead to the increase of the friction coefficient of the material. In the formulation design of friction materials, high hardness fillers are often used or the amount of high hardness fillers is increased to improve the friction coefficient based on this principle. High hardness filler particles will increase the embedding point and embedding depth of the friction surface between objects, and increase the shear force of the friction roughness item. The shear strength and coefficient of friction are correspondingly increased. On the other hand, when the friction material contains a considerable amount of soft components to form a soft friction material, such as rubber-based friction material, or when the temperature of the friction surface rises within a certain range, the compressive yield strength limit of the material decreases, resulting in friction The larger the real contact area of the surface, the higher the coefficient of friction.
