The excellent performance of carbon-carbon composites mainly depends on three major parts: carbon fiber, matrix carbon and interface. The high strength and high modulus of carbon-carbon composites mainly come from carbon fibers, of which the utilization of carbon fiber strength can generally reach 25%-50%. Carbon carbon composites at high temperatures can still maintain the mechanical properties of the indoor temperature, and even the temperature rises, carbon carbon composite tensile strength increases, the modulus of elasticity decreases, which is inherited from the carbon fiber of this characteristic. Because a part of the crack is closed at high temperature, which is caused by the mismatch between the expansion coefficient of matrix carbon and orientation consistent carbon fiber at room temperature. Although carbon fiber is a brittle material, but the carbon carbon composites are relatively high fracture toughness, which is mainly the matrix carbon fracture stress is much lower than the carbon fiber, when the matrix carbon and carbon fiber interface bonding is relatively weak, the stress makes the matrix cracking and cracks do not go through the carbon fiber, the carbon fiber can still continue to withstand the load, which presents a non-brittle fracture mode, usually referred to as the pseudo-plasticity of the fracture. However, if the interfacial bond is too strong or too weak, it can cause catastrophic damage.
Therefore, the matrix and carbon fibers can be improved to obtain carbon-carbon composites to meet the performance requirements of specific conditions, such as in the aerospace field to choose the overall performance of good three-dimensional woven structure, deposited carbon or asphalt-carbon based hybrid matrix to produce high-performance solid rocket engine ablative materials; in the aerospace field, in order to improve the friction resistance of the disc, wear and con-thermal vibration properties of the brake disc, the use of non-continuous short fiber. In order to improve the brake disc friction, abrasion and thermal shock performance in the field of aviation, the use of discontinuous short fibers, carbon cloth pavement or carbon felt as a reinforcing item prepared brake disc materials; in addition, there is the use of asphalt carbon carbon composite materials as electrode materials, the use of continuous fibers to make the force components, and so on.
1 Improvement of Matrix
Carbon matrix has a large proportion in the composition of carbon-carbon composites, so the type and structural state of the carbon matrix often have a significant impact on the various properties of carbon-carbon composites, and the designability of the carbon matrix can further provide a variety of possibilities for the improvement of different properties of the material.In general, the mechanical properties of pyrolytic carbon-carbon composites are better than those of resin-based composites, while asphalt-based carbon-carbon composites are more conducive to fiber-reinforcement effects and improved interlaminar shear strengths due to microcracks in the asphalt-carbon lamellae.
There are two main types of matrix improvement methods at present, one is to diffuse the additives into the matrix carbon precursor by supplying ball milling and supplying precipitation during the material synthesis, and jointly molding them into carbon-carbon composites, so as to enhance the mechanical properties of the composites, oxidation resistance as well as friction properties. These additives on the one hand can be an internal protective layer, so that the performance of the matrix can be improved, on the other hand, blocking the pores in the carbon-carbon composites, reducing the effective contact surface with the air, which can extend the service life of carbon-carbon composites. The other type is matrix replacement, that is, with a certain role of the material added to the carbon-carbon composite materials, the use of a variety of methods (chemical gas phase reaction, impregnation, reaction melt infiltration method, etc.) to make it replace part or all of the matrix carbon, the formation of a multi-matrix materials, and thus improve the performance of the material itself.
2 Improvement of Carbon Fiber and Precast Body Structure
Different carbon fiber types and prefabricated body structures have a great influence on the mechanical and thermophysical properties of the final carbon-carbon composites. It is well known that carbon fiber types have a great influence on the properties of prepared carbon-carbon composites. There are various types of carbon fibers due to different precursors, such as PAN-based carbon fibers, viscose-based carbon fibers, asphalt-based carbon fibers, and intermediate-phase asphalt-based carbon fibers, and the differences in the structure of these precursors lead to the differences in the properties of the prepared fibers, which further affect the properties of the prepared carbon-carbon composites. According to the characteristics of carbon fibers, carbon fibers can be divided into ordinary carbon fibers, high-strength carbon fibers, high-modulus carbon fibers and activated carbon fibers and so on, of which high-strength, high-modulus carbon fibers are mostly used in the preparation of carbon-carbon composites.
The improvement of the prefabricated body structure is mainly related to the manufacturing technology. In terms of the type of carbon fiber tow used, there are different sizes of carbon fiber tows such as 1K, 3K, 6K, 12K, etc.; in terms of the length of the carbon fibers, there are short-cut carbon fibers, short-fiber mats, long-fiber mats, and weftless cloths; in terms of the fabrication process of the carbon fiber tows, there are carbon fiber cloth layups, 2D carbon fiber piercing carbon fiber cloth overlays, and needle-punched carbon fiber precast bodies, and the structures change with the different angles of the needling. In terms of the dimensions of the preforms manufactured, there are various structures such as 1D, 2D, 2.5D and 3D, which change with the felting technology of the textile industry. Different types of carbon fibers and preformed body structures also affect the ablative properties of the material. The treatment of the carbon fiber surface is an improvement of the carbon fiber preform in some extent , and carbon carbon composites with improved properties can also be obtained using this method. In addition, the addition of some substances to the carbon fiber precursor can also improve the antioxidant properties of carbon-carbon composites.
In summary, it can be seen that the improvement of the structure of the prefabricated body is mainly from the carbon fiber type, length, arrangement, etc. The current development trend is to combine the modern textile industry felt process to prepare a multi-dimensional, multi-directional multi-dimensional needled prefabricated body, so as to improve the performance of the carbon carbon composite material, so that carbon carbon composite material as a structural component becomes possible. In addition, the surface treatment of carbon fiber and the additives in the carbon fiber application process that improve the performance of carbon fiber can improve the material properties to a certain extent by exerting the performance of carbon fiber.
3 Interface Improvement
The interface in carbon carbon composites has a very important influence on the overall performance of the material, and the interface type in carbon carbon composites is diversified, so the interface improvement work mainly starts from the following aspects. First, the carbon fiber surface treatment to improve the interface state. The main effect of carbon fiber surface treatment can be divided into improving carbon fiber or precursor interface connection and weakening carbon fiber and precursor interface connection. Secondly, a variety of mixed carbon as the matrix carbon can obtain multi-level interfacial effects. Third, the interfacial layer is prepared on the surface of carbon fiber to change the interfacial structure by introducing interfacial layer substances. In addition, there are some other ways of interfacial improvement. For example, brushing organic solvent on the surface of carbon fiber surface to increase the active sites, and growing carbon nanotubes on the surface of carbon fiber to increase the deposited active sites can provide some degree of improvement.
The development of advanced equipment has put forward higher requirements on the performance of carbon-carbon composite materials, which not only require them to have certain mechanical properties, but also require them to have other properties such as thermal conductivity, stealth, and lightweight. Realizing structural and functional integration is one of the development trends of carbon-carbon composite materials in the future. This requires designers to use a variety of means to improve carbon fiber, carbon matrix and interface. In some ultra-high temperature and harsh environments, where carbon-carbon composite materials were not used at all before, structural and functional integrated composite materials can now be prepared by connecting carbon-carbon composite materials with metal materials or ceramic materials. On the one hand, the advantages of high strength retention rate of carbon-carbon composite materials at high temperatures are utilized, and on the other hand, the purpose of weight reduction is achieved. The connection and use of carbon-carbon composite materials with other materials will greatly expand the application of carbon-carbon composite materials in the field of weapons and equipment. The excellent high temperature resistance of carbon-carbon composite materials will also enable them to be better applied in some cutting-edge scientific and technological fields such as aerospace after modification of high temperature resistant coatings and heat-resistant and ablation-resistant matrices.
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