Research On Carbon-Carbon Composite Brake Pads

 

Characteristics of Carbon Carbon Composite Brake Pads

Carbon-carbon composite materials have become a new generation of brake materials to replace metal matrix composite materials due to their unique mechanical, thermal and friction and wear properties. Its main features are as follows:

 

Low Density.

The density of this material is about 1.8g/cm3, which is 1/3 to 1/4 of metal matrix composite materials. Compared with brake components made of this material, the weight is reduced by about 40%. For military aircraft, this can improve the payload and combat technical indicators of the aircraft; for commercial aircraft, a weight reduction of 1Kg is equivalent to saving 3,000 liters of fuel per year. Therefore, its cost-saving effect is significant.

 

Good Thermal Stability.

When the aircraft aborts takeoff, the surface temperature of the brake pads exceeds 2000°C. The carbon/carbon composite material will neither melt, bond, or warp and deform, and can continue to be used after cooling; while the temperature of the metal matrix composite material exceeds 2000°C. Above 660°C, warping deformation will occur, leading to melt bonding and requiring overhaul of the brake components. Therefore, brake pads made of carbon/carbon composite materials not only improve the design margin of the brake components, but also improve the safety of the brake components.

 

Large Specific Heat Capacity.

The specific heat capacity of this material is 2.5 times that of metal matrix composite materials. It has good heat absorption function, improves the heat storage capacity of the thermal bank, and reduces the working temperature of the thermal bank.

 

The Friction Coefficient is Stable.

The material has a stable friction coefficient in a wide temperature range, and the aircraft braking process is gentle, improving the aircraft’s braking comfort.

 

Low Wear Rate and Long Service Life.

More than 2 times that of metal matrix composite materials, reducing the number of brake component repairs.

High Specific Strength.

In particular, the high-temperature strength is more than twice that of steel. Compared with metal matrix composite materials, the material itself can be used as a structural element and does not require other materials to make a skeleton support structure, simplifying the structure of the brake components and improving the reliability of the brake components. performance and maintainability.

 

Because this material has the above characteristics, it is particularly suitable for use as aircraft brake material. Therefore, since the 1990s, carbon brakes have become a standard configuration of new aircraft and are widely used.

 

Basic production process of carbon carbon composite brake pads

Carbon-carbon composite materials are composed of carbon fiber reinforcement and matrix carbon.

 

According to the form of carbon fiber existing in the matrix carbon, it can be divided into the following three types:

 

The first type of carbon fiber is randomly distributed in the matrix carbon in the form of short fiber bundles. (Currently represented by European and American brands such as brembo)

 

The second type of carbon fiber is woven into a cloth with continuous filament bundles and then laminated into a laminate in the matrix carbon.

 

The third type of carbon fiber exists in the matrix carbon in the form of needle-punched felt, a transitional structure between the two structures of carbon cloth laminate and three-dimensional braid. (Lema carbon ceramic and carbon brake discs both use this technology)

 

 

According to its source and form, matrix carbon can be divided into two categories: liquid phase method and gas phase method:

 

Liquid phase method refers to the use of liquid phase such as resin or asphalt. Carbon-containing organic matter is impregnated with carbon fiber and solidified and carbonized to remove the volatile components in the organic matter, leaving the carbonaceous components to form the matrix carbon in the composite material.

 

Gas phase method refers to the introduction of low molecular organic hydrocarbons into the preheated carbon fiber reinforcement in the form of gas phase, and cracking and polymerization at high temperature to form matrix carbon.

 

Combining the distribution shape of carbon fiber reinforcement with the formation of matrix carbon, a variety of carbon/carbon composite brake pad preparation processes are formed. The actual processes used are divided into three categories:

 

The first category: short fiber molding impregnation carbonization process.

 

This process first impregnates carbon fiber filaments with resin to form a prepreg, then cuts them into short fiber bundles of a certain length, hot presses them in a mold, and then impregnates and carbonizes them with resin or asphalt after curing and carbonization, and cycles them many times until the required material density is reached. Finally, after certain processing, a carbon/carbon composite brake pad is formed.

 

This process can be divided into normal pressure, medium pressure, high pressure and other types according to parameters such as carbonization pressure. Honeywell and ABSC in the United States mainly use this process to produce carbon brake pads, and their products are mainly used in aircraft such as B767 and MD11.

 

The advantages of this process are high carbon fiber utilization, simple process and low production cost. The produced carbon/carbon composite material is in an isotropic state. The disadvantage is low mechanical properties (see Table 1), and the friction surface of the brake pad is prone to breakage during use. The reason is that the carbon fiber reinforcement is a short-cut fiber bundle that does not form a complete structure. Another disadvantage is the high wear rate (see Table 1), which is because the matrix carbon is spherical and is prone to abrasive wear. (Currently mainly represented by European and American brands such as brembo)

 

The second category: carbon cloth laminate or needle felt preform and chemical vapor deposition (CVD) process

 

This process first weaves carbon fiber into carbon cloth, then cuts and stacks it into a preform or weaves and needles the carbon fiber into a needle-punched felt preform, deposits it with low molecular hydrocarbons in a chemical vapor deposition furnace until the required density is reached, and finally forms a carbon-carbon composite brake pad after a certain processing.

 

This process can be divided into isothermal and isobaric, pressure difference method, temperature difference method, etc. according to the heating method of the preform and the direction of airflow.

 

Goodrich Company of the United States, Dunlop Company of the United States, and Messier Company of France use this process to produce carbon brake pads, and the products are mainly used in B757, B747-400, Airbus series, etc.

 

The advantage of this process is that the matrix carbon is gradually grown along the surface of the carbon fiber, so the structure between the matrix carbon and the carbon fiber is tight. Therefore, the material produced by this process has high mechanical properties (see Table 1). In addition, CVD carbon is a graphite-like layer structure, and a layer of graphite microcrystalline film is easily formed on the friction surface during friction, reducing the material wear rate (see Table 1). The disadvantage is that the deposition efficiency of this process is low and the deposition time is long.

 

Table 1 Performance comparison of major foreign carbon brake disc materials

 

 

 

The Third Category: Composite Process.

 

The so-called composite process is to combine the above two types of processes to prepare carbon-carbon composite brake pads. For example, the CVD process is formed by the gradual growth of matrix carbon on the surface of carbon fiber. It is suitable for filling the smaller gaps between carbon fibers. The large voids are blocked by the surrounding small pores to block the source of deposition gas, forming large voids in the material. In the impregnation carbonization process, the impregnated resin or asphalt carbonization is internally contracted to form spherical matrix carbon, which often breaks away from contact with the surrounding carbon fibers to form uniform small gaps.

 

Therefore, the large voids left by the CVD process are easily filled by the small gaps formed between the impregnated carbonized matrix carbon and the fibers, which are suitable for CVD carbon. The combination of the two can reduce the densification process time and improve production efficiency. The disadvantage is that the process equipment is complex and the investment is large.

 

Development of carbon-carbon composite brake pad production process

Since the composite process is a combination of the first two, its development is based on the development of the two, so the development of the first two types of processes will be mainly introduced in the afternoon.

 

1. Development Of Resin Compression Molding Impregnation Carbonization Process

 

Since the resin contains non-carbonaceous components, it is easy to produce some low-molecular volatiles during carbonization, which reduces the residual carbon rate and forms pores in the material. Therefore, this process generally requires up to seven times of repeated impregnation and carbonization to achieve the required density of the material. In order to reduce the number of impregnations and increase the residual carbon rate, high residual carbon rate resins and increased carbonization pressure were developed. The hot isostatic pressing process that continuously performs impregnation and carbonization was further developed to reduce the number of impregnation and carbonization times to 2-3 times.

 

Because the new resin is expensive and difficult to promote and apply in production, the hot isostatic pressing equipment is complex, the investment is large, and the cost of use is high, so the new process reduces the material production cost, but the reduction is limited.

 

2. Development Of CVD Process

 

The CVD process developed in the early stage is the isothermal pressing method, which is to heat the carbon fiber preform evenly, and the deposition gas randomly flows through the preform, freely diffuses and deposits in the preform. The deposition rate of this process is controlled by the gas diffusion rate and can only be carried out at a very low pressure to balance the deposition rate and the gas diffusion rate. Even so, the deposition rate on the surface of the preform is greater than the internal deposition rate, and a crust is easily formed on the surface of the preform, which blocks the free diffusion and deposition of the gas.

 

Therefore, shelling treatment is required, so the CVD process of this process is discontinuous and is carried out in five or six stages to achieve the required density of the material. The cumulative CVD time is as long as thousands of hours. Although the CVD time of this process is long, the process is highly adaptable and can densify preforms of different sizes and shapes. The microstructure of the matrix carbon is easy to control, the material quality is stable and reliable, and single-furnace multi-column large-capacity production can be achieved, and the product cost is low. At present, the French messier and the British dunlop companies both use this process to produce carbon brake pads.

 

In order to shorten the CVD time and number of times, a differential pressure CVD process has been developed. This method uses a certain tooling to guide the deposition gas to flow through the preform in a directional manner and form a pressure difference on the preform. This method can increase the deposition temperature and gas volume. Therefore, the CVD time can be shortened to 500-600 hours, and the number of CVD times can be reduced to 2-3 times. At present, Goodrich Company in the United States adopts this process.

 

The temperature difference CVD process is to form a certain temperature gradient on the carbon fiber preform to solve the influence of surface crusting on the airflow in the isothermal method, which can make the CVD process continuous. Once completed, the CVD temperature and gas volume can be further increased, and the CVD time can be shortened to 200-400 hours.

 

The disadvantage of this process is that the equipment and fixtures are matched with the formation and size of the preform, that is, the equipment and fixtures of this process are dedicated, and cannot be used for densification of different preforms, and multi-material column CVD cannot be achieved.

 

The temperature difference and pressure difference CVD process combines the temperature difference method and the pressure difference method to further reduce the CVD time, which can be reduced to less than 100 hours. The disadvantage is that this process cannot increase the density of the preform to the required density of the material alone.

 

Technology Development Trend Of Carbon-Carbon Composite Brake Pads

From the perspective of the structure of carbon fiber reinforcement, the mechanical properties of the material are low due to the failure of short fibers to form a complete fiber reinforcement, so the carbon brake pads prepared by this structure are mainly used for early models such as B767 and MD11 that use carbon brakes.

 

The material made of the second carbon cloth laminate structure is affected by factors such as low interlayer shear strength and low vertical thermal conductivity (see Table 1), so the carbon brakes made by this structure are only used for early models such as B757.

 

The third structure is a needle felt structure, which is very suitable for CVD process densification, and the material has good mechanical, thermal and friction and wear properties (see Table 1), becoming the basic structure of carbon-carbon composite reinforcement at present, and has been continuously improved: on the one hand, the molding technology realizes automated assembly line production. It improves the designability, controllability and consistency of the prefabricated structure, and also improves production efficiency. On the other hand, the scrap recycling technology improves the utilization rate of carbon fiber.

 

From the perspective of densification process and matrix carbon formation process: the matrix carbon formed by the impregnation carbonization process is spherical in structure. Abrasive wear is formed during the friction process, and the wear rate is high, which makes it difficult to improve the service life of the brake pad. Therefore, the carbon brake pad prepared by this process is only used for early models such as the B757 MD11 mentioned above. In recent years, new models developed all use CVD process such as B777 A330 A340 A318 B717, and the CVD process is constantly improving, and the following development trends have emerged:

 

1) Reduce the heat treatment temperature of the material to below 1600° or even no heat treatment. The graphitization degree of the material is only about 10%, such as A340–600, to reduce the wear rate of the material and extend the service life of the brake pad.

 

2) Quality anti-oxidation technology, such as the sepcarb carbon-carbon composite material used in B777-200/300, which adds antioxidant components during the deposition process to reduce the wear rate of the material caused by oxidation.

 

3) Brake pads with unequal thickness design, such as A330/A340 brake pads, dynamic discs and static discs are designed with unequal thickness structures, so that thin discs can be processed into two-in-one discs after one use cycle and thick discs can be processed in small amounts after two use cycles, and continue to be used. Such a structure reduces the amount of cutting during renovation compared to the equal thickness structure, and fully improves the utilization rate of brake materials.

 

In summary, the development trend of carbon-carbon composite brake pads is as follows:

 

1) Needle felt structure has become the basic structure of carbon-carbon composite carbon fiber reinforcement. Its automatic forming technology not only ensures the designability of the reinforcement structure, but also improves production efficiency: its recycling and reuse technology improves the utilization rate of carbon fiber.

 

2) CVD process has become the standard densification process of carbon-carbon composite materials. Its material anti-oxidation technology and low-temperature treatment technology reduce the material wear rate and extend the service life of brake pads.

 

3) The unequal thickness design of brake pads improves the utilization rate of carbon-carbon composite materials.