The brake materials for modern transportation means such as airplanes, automobiles, and high - speed trains have evolved from cast iron, synthetic materials, powder metallurgy materials to carbon/carbon composites and carbon/ceramic composites. The carbon/ceramic composite brake material, developed in the 1990s, is a multi - phase composite brake material with high - strength carbon fibers as the reinforcement and pyrolytic carbon, SiC, etc. as the matrix. Based on the carbon/carbon composite brake material, it introduces SiC ceramic hard material with excellent anti - oxidation performance as the matrix. It not only retains the advantages of low density and high - temperature resistance of the carbon/carbon composite brake material but also overcomes the shortcomings of the carbon/carbon brake material such as low static friction coefficient, large wet - state attenuation, insufficient friction life, and poor environmental adaptability, thus becoming the new - generation brake material. In 2008, the carbon/ceramic aircraft brake disc jointly developed by Xi'an Aerospace Brake Technology Co., Ltd. and Northwestern Polytechnical University was first installed and applied on domestic aircraft, and has been promoted on multiple aircraft models such as carrier - based aircraft, fighter jets, and early warning aircraft. There are no relevant application reports abroad yet.

This paper introduces the main preparation methods of carbon/ceramic composite brake materials and elaborates on three key issues affecting the performance of carbon/ceramic composite brake materials: component and structure design, residual Si content control, and anti - oxidation technology.

Carbon/ceramic composite; Brake material; Preparation process; Residual silicon; Anti - oxidation

The development of modern transportation means such as airplanes, automobiles, and high - speed trains has put forward increasingly higher requirements for the performance of brake materials. The performance of brake materials is directly related to the safe operation of transportation means. As a new type of brake material, carbon/ceramic composite brake materials show broad application prospects in fields such as aerospace and automobiles due to their unique performance advantages. This paper will introduce in detail the preparation processes, key preparation issues, and future research directions of carbon/ceramic composite brake materials.

The key to the preparation of carbon/ceramic composite brake materials lies in minimizing fiber damage as much as possible, forming an appropriate bonding strength at the fiber/matrix interface, overcoming the “bottleneck effect” of matrix densification, and reducing the preparation cost. Currently, the main preparation processes for carbon/ceramic composite brake materials include Chemical Vapor Infiltration (CVI), Polymer Impregnation Pyrolysis (PIP), and Reactive Melt Impregnation (RMI).

The CVI method refers to the process in which a gas - phase precursor undergoes a chemical reaction at high temperature to deposit pyrolytic carbon and SiC inside the pores of the preform. The CVI method mainly includes two process routes: one is to use propylene or natural gas/propane as the carbon - source gas to first deposit a layer of pyrolytic carbon on the surface of the carbon fibers in the preform, and then use CH₃SiCl₃ as the SiC gas source, and H₂ or N₂ as the dilution gas and carrier gas for pyrolytic deposition of SiC to obtain the carbon/ceramic composite material; the other is to use a mixed gas of SiCl₄, CCl₄, and H₂ as the gas source to co - deposit pyrolytic carbon and the SiC matrix to obtain the carbon/ceramic composite material. The Institute of Metal Research, Chinese Academy of Sciences, first prepared a C/C - SiC composite material with a gradient - changing matrix composition using the CVI process. The CVI process can achieve component design at the micro - scale and causes little damage to fibers, but it has a long preparation cycle, high cost, low material density, a large density gradient, and a high porosity.

The PIP method is to impregnate a C/C pre - form with a certain density with a solution of an organic polymer precursor (such as polycarbosilane) under pressure, dry and solidify it, and then subject it to heat treatment to pyrolyze the organic polymer into SiC ceramics. The Oak Ridge National Laboratory in the United States adopted the PIP method to theoretically solve the problem of high - temperature adhesion of brake discs caused by excessive residual Si. When using this method to prepare carbon/ceramic composite brake materials, due to the low ceramic yield during the polymer transformation process and matrix shrinkage, a completely dense material cannot be obtained. The advantages of this method are strong designability, good processability, and a low sintering temperature. However, it has a long process cycle, high cost, a large matrix shrinkage rate, and cracks are generated, resulting in an un - dense structure.

The RMI method refers to infiltrating a porous carbon/carbon material with molten Si, causing the molten Si to react with part of the matrix carbon to in - situ generate a SiC ceramic matrix. The RMI method has a simple process flow and low production cost, and is currently the most widely used preparation process for carbon/ceramic composites. However, the RMI method has the defect that during the reaction, the molten Si inevitably reacts with the carbon fibers, reducing the fiber toughening effect. At the same time, a certain amount of Si element will inevitably remain in the material, resulting in a decrease in the working temperature of the material and a reduction in creep resistance.

The carbon/ceramic brake discs prepared by the German Aerospace Center using the RMI process have been successfully applied in high - end cars such as Porsche and Audi. However, it is mainly formed by short - fiber compression molding, resulting in an isotropic structure of the material, enhanced brittleness, and easy failure of the material. At the same time, the high residual Si content is likely to cause disc - sticking accidents, affecting braking safety. Wang Linshan et al. prepared a C/C - SiC composite material with excellent friction and wear performance by precisely controlling the process parameters. Dong Benxing et al. used graphite powder as a filler, prepared a porous C/C composite material by unidirectional pressure impregnation - pyrolysis, and then successfully prepared a low - cost needle - punched felt C/C - SiC brake material using RMI. Currently, there are few research reports on the influence of residual Si on the friction and wear performance of carbon/ceramic composite materials. The controllable technology of the residual Si content will be an urgent problem to be solved in the preparation of carbon/ceramic brake materials by the RMI method.

The above - mentioned single carbon/ceramic preparation processes have certain limitations. Therefore, multiple processes can be combined to prepare C/C - SiC brake materials by integrating various methods to improve their performance. For example, the 43rd Institute of the Fourth Academy of Aerospace uses the CVI method combined with the PIP method to prepare C/C - SiC with good uniformity and high mechanical properties, but its preparation cycle is long and the cost is high. Xu Yongdong combined the CVI method with the RMI method to develop a preparation method for low - cost, high - temperature - resistant, and anti - oxidation C/C - SiC composite materials.

Brake materials are required to have an appropriate friction coefficient and a low wear rate. The friction and wear performance, as the main performance index of braking materials, determines the quality and service safety of braking materials. Although carbon/ceramic composite materials have great advantages as brake materials, their friction and wear performance are affected by multiple factors such as material composition, phase distribution, and microstructure. In particular, a large amount of residual Si is likely to melt during the braking process, triggering an adhesion effect, affecting braking stability and braking safety. In addition, the high - temperature environment during the braking process can easily cause the oxidation of local C/C structural units, greatly reducing the friction and wear performance of the material. Therefore, when designing and preparing carbon/ceramic composite materials, three key issues should be fully considered: component and structure design, residual Si content control, and high - temperature oxidation resistance.

The microstructure of carbon/ceramic composite brake materials includes carbon fiber reinforcement, pyrolytic carbon matrix, SiC ceramic matrix, and a small amount of residual Si. The three - dimensional needled carbon fiber preform, as the skeleton of the carbon/ceramic brake disc, the needled fibers perpendicular to the friction surface can improve the thermal conductivity and inter - laminar shear strength of the material, which is beneficial to improving the braking stability and wear resistance. Pyrolytic carbon, as the matrix phase, its content has an important impact on the friction and wear performance of carbon/ceramic materials: as the content of pyrolytic carbon increases, the hardness of the material gradually decreases, and the friction coefficient gradually increases.

The SiC ceramic phase is a hard phase, and its function is to increase the friction coefficient. However, when the SiC content on the friction surface is too high, a “plowing groove” effect occurs, accelerating wear. The Si phase on and near the friction surface will melt due to excessive temperature during the braking process, resulting in adhesive wear, seriously affecting braking safety. Liu Ying et al. studied the influence of Si content on the friction performance of carbon/ceramic composite materials. The results showed that as the Si content increased, the open - pore rate of the material gradually decreased, the hardness gradually increased, and the wear gradually changed from single abrasive wear to a mixed wear mechanism of abrasive wear and adhesive wear. When the Si content (mass fraction) was 28.42%, the material had better mechanical and physical properties, a higher friction coefficient, and the lowest wear rate, corresponding to the most excellent friction and wear performance.

Li Jinwei et al. prepared a short - carbon - fiber - reinforced C/C - SiC composite material using the Warm Pressing - Liquid Silicon Infiltration (WPLSI) process and studied the influence of carbon fiber content on the mechanical properties of the material. The results showed that the mechanical properties of the material increased with the increase of fiber content in the range of 20% - 30%. When the fiber content was 30%, its flexural strength and vertical compressive strength reached 104.63 MPa and 167.99 MPa, respectively.

Adding fillers to modify the matrix is an effective way to improve the friction performance of C/C - SiC composite materials. Xiao Peng et al. added metal Fe to the C/C - SiC composite material to obtain a C/C - SiC - Fe brake material. During the braking process, non - oxidizable FeSi and FeSi₂ formed on the material surface filled the pores, playing a buffering role, effectively preventing the braking instability caused by high - frequency vibration of the brake material during high - speed braking. However, at too high a temperature, Fe will erode the carbon fibers, causing damage to the material.

Cu is incompatible with carbon fibers even at high temperatures, so it will not erode the carbon fibers. After modifying the C/C - SiC composite material by infiltrating C/C pre - forms with Cu and Si, due to the addition of the copper alloy, the heat - conduction and heat - dissipation capabilities of the friction surface are improved, which is beneficial to stabilizing the friction coefficient, effectively preventing the oxidation wear of the material, playing a role in reducing friction and wear, and improving the service life of the material. Liu Lei et al. prepared C/C - SiC composite materials with different Al contents using the reactive melt infiltration method and found that the mass wear rate was the lowest when the Al content was 40%. The corrosion morphology showed that increasing the density of the composite material can effectively improve the erosion resistance, and increasing the Al content can further improve the erosion resistance of the material.

Tülbez et al. impregnated carbon nanotubes (CNTs) into the C/C pre - form before infiltrating liquid Si. The results showed that adding excessive carbon in the form of CNTs to the C/C pre - form can significantly improve the infiltration efficiency of Si, thereby improving the density and microstructure uniformity of the C/C - SiC composite material. In addition, the unreacted CNTs and the lower residual porosity increased the fracture strength of the composite material by 40% compared with that without CNTs.

As the reinforcement of carbon/ceramic composite brake materials, the structure and uniformity of the pre - form directly affect the friction and wear performance of the material. Yang Shangjie adopted a new “sandwich” - structured pre - form with pure net tires on both sides and three - dimensional needling in the middle to avoid the problem of unstable friction and wear performance caused by the non - uniform structure of the pre - form. Liu Rongjun studied the influence of different structures of carbon fiber pre - forms on the performance of C/C - SiC composite materials and found that the C/C green body prepared from the carbon - cloth laminated - structure pre - form was beneficial to the penetration reaction of Si, and the obtained composite material had a high density and good mechanical properties.

The residual Si in carbon/ceramic composite brake materials significantly affects the friction and wear performance of the material: on the one hand, it increases the non - uniformity of the microstructure, affecting the stability of the friction coefficient; on the other hand, the existence of residual Si will cause a large adhesive force on the friction surface, resulting in adhesive wear during the friction process, affecting braking safety. Therefore, the residual Si content in carbon/ceramic brake materials needs to be strictly controlled.

In research, fillers are generally introduced to react with the matrix to form compounds to reduce the residual Si content. Fan et al. prepared a three - dimensional needled carbon/carbon pre - form with a fiber content of 32% (volume fraction) using the CVI method. Subsequently, the C/C porous body was directly infiltrated with liquid Si. The Si melt spontaneously infiltrated into the pre - form and reacted with the carbon matrix to form a SiC matrix, successfully preparing a C/C - SiC composite material. Its structural composition mass fraction was 70% C, 22% SiC, and 8% Si. Subsequently, the C/C porous body was infiltrated with a water slurry of TiC powder to obtain a C/C - TiC composite material, and then a C/C - SiC - Ti₃SiC₂ composite material was obtained through a liquid Si infiltration reaction. No residual Si was found, indicating that the formed Ti₃SiC₂ replaced the residual Si. In addition, by adding Ti powder to the Si powder and using the liquid Si infiltration method to prepare the C/C - SiC composite material, Ti reacts with SiC to form Ti₃SiC₂. The hardness of Ti₃SiC₂ is lower than that of SiC. Its introduction can effectively reduce the “plowing groove” effect on the material surface, facilitate the formation of a friction film on the material surface, and effectively reduce the content of the residual Si phase and the wear rate in the material.

The high - temperature environment during the braking process can easily cause the oxidation of local C/C structural units, greatly reducing the friction and wear performance of the material. Compared with carbon/carbon composite brake materials, most of the carbon phases in carbon/ceramic brake discs are coated with SiC, giving the carbon/ceramic brake discs a certain degree of anti - oxidation. However, due to the thermal expansion mismatch of the multi - phase material, matrix cracks are generated, which become the diffusion channels for oxygen. Under normal braking conditions, the temperature of the non - friction surface of the aircraft brake disc is 600 - 900 °C. Therefore, developing an anti - oxidation technology for carbon/ceramic composite brake materials in a high - temperature environment is of great significance.

As the bridge connecting the fibers and the matrix, the interface can not only improve the toughness of carbon/ceramic materials but also enhance the anti - oxidation performance of the fibers. Pyrolytic carbon (PyC) has a wide range of sources and good compatibility with fibers and the SiC matrix. As an interface phase, it can significantly improve the toughness of carbon/ceramic materials. However, its disadvantage of being easily oxidized at high temperatures has gradually led to its replacement by complex - structured (PyC - SiC)n and (BN - SiC)n composite interfaces. These complex - structured composite interfaces can not only increase the diffusion path of the oxidation medium, improve the anti - oxidation performance of the material, but also regulate the matching of thermal expansion and modulus between the fibers and the matrix, comprehensively improving the mechanical properties of the material. However, disadvantages such as high cost and complex processes restrict its further widespread application.

In addition to interface anti - oxidation, matrix modification is also an effective method to improve anti - oxidation. By introducing components in the matrix that can react with the oxidation medium at high temperatures to form a glassy phase (such as B, BxC), carbides or borides of refractory metals (such as ZrC, ZrB₂, HfC, TaC), substances such as B₂O₃ and SiO₂ are generated. At high temperatures, these substances form a glass system to seal the diffusion channels of the oxidation medium, thereby inhibiting the oxidation of the material. Matrix modification is divided into two types: multi - component dispersed matrix modification and multi - component multi - layer matrix modification. Multi - component dispersed matrix modification is to disperse the components in the matrix in the form of particles with a size of 1 - 100 μm, while multi - component multi - layer matrix modification is to achieve the controllable distribution of the modified components in the matrix through the process.

Liu Jiangong et al. infiltrated a C/C porous body with a B₄C slurry, dried it, and then embedded it with an appropriate amount of Si powder. A modified C/C - SiC composite material was obtained through molten Si infiltration, which not only eliminated the residual Si but also improved the mechanical properties and anti - oxidation of the material. Other studies have shown that a CMC - SiC composite material modified with ZrB₂ and ZrC simultaneously by the RMI and PIP combined processes has excellent high - temperature resistance, anti - oxidation, and anti - ablation properties. The difficulty of multi - component dispersed matrix modification is that it is difficult to control the distribution of its components, and it is difficult to achieve synergistic optimization. Li Siwei et al. deposited a PyC interface layer in the pre - form, then continuously deposited two layers of SiC, and then two layers of B - C. After repeating the process, a B