Generally speaking, the properties of carbon-carbon composites are mainly divided into two categories: mechanical properties and thermophysical properties. Mechanical properties mainly include tensile properties, compression properties, bending properties, shear properties and fatigue properties, and thermophysical properties mainly include thermal expansion properties and thermal conductivity.
Density is an important index of carbon-carbon materials, only to achieve a certain density of carbon-carbon composites can have good mechanical properties, and the density of the composite material is mainly contributed by the carbon fibers and matrix carbon, so the mechanical properties of carbon-carbon composites are affected by the type of carbon fibers and their volume fractions, the structure of the pre-fabricated body, the structure of the matrix carbon, the interface between carbon fibers/matrix, and the subsequent heat treatment and other factors. Different matrix carbon types correspond to different mechanical properties of the materials. The type of pyrolytic carbon weave and the structure of the precast body are closely related to the preparation process, and therefore the effects on the mechanical properties need to be considered.
In general, the mechanical properties of carbon-carbon composites are greatly influenced by the structure of the precast body and the type of fiber. The main materials used as precast materials for carbon-carbon composites are carbon cloth, carbon mats, and woven carbon fiber bundles. Due to the different fiber content and the different direction of fiber distribution and the different external load sharing when loaded, the prefabricated body structure has a greater impact on the mechanics of carbon-carbon composites.
Interface is an extremely important microstructure in composites, its structure and morphology directly affect the performance of composites, and play a role in transferring, blocking, absorbing, scattering and inducing to play the function of materials, such as electric, optical, thermal, acoustic, magnetic and other properties. Therefore, the chemical composition, atomic or molecular arrangement, physicochemical properties of the interface of the composite material can show gradient gradient or mutation characteristics, the same material elements can be different due to the interface state and make the material has different macroscopic properties.
Heat treatment is a very important process in the preparation of carbon-carbon composites, on the one hand, it leads to the release of thermal stresses in the material, which can reduce the stress damage of the material; on the other hand, heat treatment can change the type of weaving of the graphitized matrix carbon, and the change of the type of matrix carbon will further affect the mechanical properties of the material as a whole, so the actual production will be based on the actual situation of heat treatment. Because according to the existing research results, too high heat treatment temperature will seriously reduce the mechanical properties of composites, the degree of reduction is closely related to the material itself and the type of organization and heat treatment conditions. And moderate heat treatment is conducive to the improvement of fracture toughness of composite materials.
Generally speaking, the thermal properties of carbon-carbon composites include specific heat capacity, thermal expansion, thermal conductivity, thermal diffusion and thermal radiation. Among them, thermal expansion and thermal conductivity are not only the basic performance data for evaluating and measuring the service condition of composites in high-temperature environments, but also an important means to study the microstructural changes of composites, such as interfacial thermal stresses, phase transitions, microcracks and defects.
The coefficient of thermal expansion of carbon fibers is less than that of the carbon matrix, and the thermal conductivity of carbon fibers is greater than that of the carbon matrix, so the type of carbon fibers in the carbon-carbon composites as well as the structure of the preforms have different effects on the thermal properties of the carbon-carbon composites. The anisotropy of carbon fibers affects the axial and radial thermal properties of the fibers, which further affects the properties of the prepared composites. Differences in the structure of the precast body affect the overall thermal properties of the material, mainly related to the arrangement of the carbon fibers in the precast body. For thermal conductivity, the anisotropy of the carbon fibers and the difference in thermal conductivity between the carbon fibers and the matrix carbon, the carbon fibers and the structure of the precast body also affect the overall thermal conductivity of the prepared material. Overall, the amount and orientation of fibers in the precast body play a dominant role in the thermal properties of the material, but the different sources of carbon fibers and the indexing process also result in differences in the weave, and thus the material thermal properties have variability. The type of matrix carbon also has an effect on the thermophysical properties of carbon-carbon composites. Since the coefficient of thermal expansion of matrix carbon in carbon-carbon composites is greater than that of carbon fibers, the type of matrix carbon will definitely affect the overall thermal properties of the material.
For thermal conductivity, the thermal conductivity of pyrolytic carbons with different weave configurations varies. For resin carbon and pitch carbon, the incorporation of modifiers also affects their thermal properties. Heat treatment also affects the thermal properties of carbon-carbon composites since the heat treatment temperature affects the graphitization degree of matrix carbon and carbon fibers. In general, the increase in heat treatment temperature reduces the coefficient of thermal expansion of the material to a certain extent, with the rate of reduction in the low-temperature section (1300-1500°C) being higher than that in the high-temperature section (2100-2500°C). The low-temperature section is dominated by the change of fibers, and the high-temperature section is dominated by the change rule of the matrix; and the thermal conductivity is shown to increase with the increase of heat treatment temperature in the whole process.
At present, many types of carbon-carbon composite materials have been used in weapons and equipment. They will be subjected to more high and low temperature environments. Therefore, it is of great significance to carry out research on their thermal properties. However, due to the types of carbon-carbon composite preform fibers, preform structures, matrix carbon with various textures, diversity of matrix interfaces and different preparation processes, the rules that affect thermal properties are not universal. Therefore, specific problems still need to be studied in combination with specific materials.
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