The brake pads of automobiles and construction machinery are a kind of friction material. The main function is to absorb kinetic energy through friction with the metal to make the vehicle work safely and reliably. It should have good friction performance and wear resistance, and at the same time Has good mechanical strength and heat resistance. According to the composition of the brake pad material, it is divided into asbestos (asbestos) brake pads, semi-metal brake pads, NAO (non-asbestos organic friction material) brake pads, and ceramic-based brake pads. The production and use of asbestos brake pads is now banned; the dust and metal powders that escape into the air after the friction of the semi-metal brake pads are harmful to the human body and the environment; NAO brake pads do not have good heat resistance and friction coefficient stability at high temperatures ; Ceramic-based brake pads have stable friction coefficient, good thermal stability and good wear resistance, do not contain metal components, and have a service life more than double that of other brake pads. They have superior overall performance and are clean and environmentally friendly products. Will gradually be widely used in the market.
Components of ceramic based brake pads
Ceramic brake pads are mainly composed of reinforced fiber, mineral filler, friction performance regulator and binder, and are made of products after production and processing.
Its composition (mass %) is: reinforced fiber 25 ~ 40, filler 10 ~ 30, adhesive matrix 15 ~ 30, friction property regulator 10 ~ 20, anti-wear lubricant 15 ~ 30, elastic toughening agent 5 ~ 10.
1.1 Reinforcement fibers in ceramic brake pads and their roles
Table 1 Ceramic fiber performance parameters
Fiber is used as a reinforcing frame material in ceramic-based brake pads and plays an important role in the strength, friction performance and wear resistance of the brake pads. The commonly used reinforcing fibers for ceramic-based brake pads are aluminum silicate fiber, alumina fiber, carbon fiber, and zirconia fiber. Initially, aluminum silicate fiber and alumina fiber are mostly used. Later studies have found that zirconia fiber is better than alumina fiber and aluminum silicate. Fiber has more superior performance, its oxidation resistance, corrosion resistance, low thermal conductivity, better high temperature performance, is a better reinforcing fiber material. The reinforcing fiber used in the ceramic-based brake pad is a hybrid fiber with the above-mentioned fibers in a good proportion, which has a better reinforcing effect than a single fiber. Several ceramic fiber performance parameters are shown in Table 1.
1.2 Mineral fillers and functions in ceramic-based brake pads
The mineral filler in ceramic-based brake pads can improve the physical and mechanical properties of the brake pads (such as thermal conductivity, thermal expansion, density, strength, stiffness, and hardness, etc.), and can also adjust the friction performance and product cost. Packing is divided into anti-friction packing, friction-increasing or friction packing. Anti-friction fillers can improve the wear resistance of brake pads, reduce friction coefficient, and reduce braking noise; friction-increasing or friction fillers can improve the physical and mechanical properties of brake pads, increase friction resistance, stabilize friction coefficient and improve wear resistance.
In order to improve the overall performance of the brake pads, friction reducing fillers and friction enhancing fillers will be mixed and used in a certain proportion according to the requirements of use. The main mineral fillers used in ceramic-based brake pads are: barite, expanded vermiculite, sepiolite, corundum, zircon, graphite, talc, wollastonite, zeolite, diatomaceous earth, brucite, hydrotalcite, etc.
Barite can increase and stabilize the friction coefficient, reduce wear and braking noise, and form a relatively stable friction interface at high temperatures, which can prevent the friction pair surface from scratching and make the surface of the friction pair smoother. It is a friction-increasing control material.
Expanded vermiculite has excellent physical and chemical properties, which can reduce the electrostatic adhesion on the surface of the brake pad, reduce the product density and braking noise, and increase its elasticity. It is a filler for regulating noise reduction and shock absorption.
Sepiolite is a fibrous silicate mineral. Sepiolite colloid is added to the brake pad as a reinforcing base material. It has good toughness, high tensile and flexural strength, high impact strength, high temperature resistance and low wear resistance, and good high temperature decay. .
Corundum and zircon have high hardness and are hard fillers. Adding zircon or corundum can exert a good friction-increasing effect and lower braking noise.
Graphite is an anti-friction control material and friction performance regulator. It has good thermal conductivity, can reduce the wear rate, can stabilize the friction coefficient, and can also accelerate the dispersion of friction heat on the surface of the brake pad and improve the thermal stability of the brake pad.
Talc is a silicate mineral with a layered structure. It plays a role of reducing friction and regulating fillers. It has good adhesion with binders and can improve the strength of brake pads.
Wollastonite has needle-like characteristics, low thermal expansion, and excellent thermal shock resistance, which can increase the friction coefficient and reduce the wear rate, help the brake pad to form a smooth contact surface and reduce wear.
Zeolite and diatomaceous earth have excellent adsorption properties, which can fully absorb the small molecular substances decomposed by the brake pad at high temperature, the heat generated on the surface of the friction pair, and the braking noise. It is a friction performance control filler.
Brucite contains a certain amount of crystal water, and the plate-like crystal structure becomes fibrous brucite fiber when it is deformed. It has flexibility and flexibility, high temperature resistance, and excellent alkali resistance.
Hydrotalcite-like has excellent thermal weight loss and heat absorption, can absorb frictional heat, undergo phase change at high temperature, and gradually transform into spinel minerals, which has an enhanced effect on the physical and mechanical properties of brake pads, and is lubricating, reducing friction, and repairing Regulated materials.
1.3 Binders and functions in ceramic-based brake pads
The binder has good wetting properties for reinforcing fibers, fillers and other components. It binds these materials together and forms a stable chemical bond with them. It is the matrix of the brake pad. The binder used in ceramic-based brake pads is phenolic resin (modified resin) and synthetic rubber powder, mainly phenolic resin. Under a certain heating temperature, it will soften first and then enter a viscous fluid state to generate flow and distribute it evenly. In the ceramic-based brake pad forming material, the reinforcing fiber and the filler are bonded together through resin curing and vulcanization to form a product with a dense texture, a certain mechanical strength and meeting the friction performance requirements of the brake pad; rubber powder can be improved The softness of the product reduces its hardness.
Ceramic-based brake pads work under high temperature conditions of 200-500 ℃ during braking. In this temperature range, inorganic reinforced ceramic fibers and fillers have stable performance and will not be thermally decomposed. Organic binders, phenolic resin and The rubber enters the thermal decomposition zone, so the choice of an adhesive with good heat resistance has a very important effect on the performance of the brake pad.
2 Test of ceramic-based brake pads
2.1 Raw material formulation test
Table 2 Basic parameters of raw material formula
The friction performance of ceramic-based brake pads mainly depends on the selection of raw materials, formula ratio and production process. During the test, it is necessary to solve the problems of uniform fiber dispersion, accurate material measurement, uniform mixing, control of the press forming process, and stable friction coefficient. . A lot of test data were obtained through experiments and analysis, and the basic parameters of a better formula were determined. The results are shown in Table 2.
2.2 Preparation process test
2.2.1 Mixing
The feeding amount is 5 kg, the mixer spindle speed is 180 r/min, the reamer speed is 2 200 r/min, and the mixing time is 210 s.
2.2.2 Press forming
The injection volume in the mold cavity is 200 g±2 g, the pressure is 3.0 MPa, 4.0 MPa, and 5.0 MPa respectively. The exhaust time is 3.0 s, the mold exhaust lift distance is 3.0 mm, and the exhaust residence time is 3.5 s. The holding pressure is 5.0 MPa, the holding time is 180 s, the vacuum degree is 0.4 MPa, and the sample thickness is 11.5 mm ± 0.2 mm.
2.2.3 Heat treatment
The holding time of the sample oven is 140 ℃/1 h, 160 ℃/1 h, 180 ℃/1 h, 200 ℃/2 h, 210 ℃/3 h, and the heating rate is 1 ℃/10 min.
2.3 Determination of friction coefficient
Use the XD-MSM constant-speed friction tester to test the friction coefficient of the sample according to the GB/T 5763-2008 inspection method. The friction coefficient parameters are shown in Table 3.
Table 3 Test friction coefficient of samples
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3 Production process and characteristics of ceramic-based brake pads
3.1 Production process of ceramic-based brake pads
Ceramic-based brake pads are mainly composed of reinforced ceramic fibers, mineral fillers, and binders. The production process is shown in Figure 1.
Raw material → weighing ingredients → mixing → pre-forming (mold) → sintering → grinding → grooving (drilling) → spraying → modification → inspection → coding → packaging
Figure 1 Production process of ceramic-based brake pads
3.2 Characteristics of ceramic-based brake pads
Ceramic-based brake pads have a stable coefficient of friction, good thermal stability, low thermal conductivity, and good wear resistance. In the process of braking, friction generates heat and the working temperature increases. As the working temperature increases, the friction coefficient of ordinary brake pads begins to decrease, and the friction force decreases, thereby reducing the braking effect. The friction coefficient of ceramic-based brake pads is still stable at 0.45～0.50 when the working temperature is as high as 650 ℃. It can withstand greater pressure and shear force, has good mechanical strength and physical properties, and is suitable for various high-performance brake materials. Requirements to meet the high speed, safety, and high wear resistance of brake pads.
Semi-metallic brake pads and NAO (None Asbestos Organic) brake pads have no good solutions due to material reasons; ceramic-based brake pads have low content of reinforced fiber slag balls, good reinforcement, and contain no metal, which can greatly reduce the brake pads Dual wear and braking noise. The coefficient of friction of ceramic-based brake pads is higher than that of semi-metal brake pads and NAO (None Asbestos Organic) brake pads. The thermal attenuation is low, which can improve braking performance and safety. It is caused by abnormal friction between the brake pads and the mating during braking. For noise, the pictures of the three types of brake pads are shown in Figure 2.
The service life of semi-metal brake pads and NAO (None Asbestos Organic) brake pads is less than 60,000 km, while the service life of ceramic-based brake pads is more than 100,000 km, and the service life is increased by more than 50%. The brake pad overcomes the defects of traditional brake pad materials and technology, and is the most advanced brake pad product.
Ceramic-based brake pads are made of new ceramic fiber materials, with stable ingredient quality and quality, no metal components, stable friction performance, and long service life. They are clean and environmentally friendly products with stable performance. Widely promoted and applied to improve corporate image and brand Value plays an important role in product upgrading and transformation and high-quality development.