There are various manufacturing processes for carbon-carbon composite materials, primarily including preforming of the blank, impregnation, carbonization, densification, graphitization, and oxidation resistance, among others. The production of carbon-carbon composite materials is a multi-step, high-precision process, with the core being the integration of carbon fibers and the carbon matrix into a unified whole through two critical stages: the forming of carbon fiber preforms and the densification of the carbon matrix. Based on the source and formation method of the carbon matrix, the primary manufacturing methods can be categorized as follows:
Fabrication of Carbon Fiber Preforms (Basic Steps)
The fabrication of all C/C composite materials begins with the construction of a carbon fiber skeleton (preform), whose structure directly influences the performance of the final material:
Weaving Process
Based on performance requirements, carbon fibers are woven into different structures, such as two-dimensional fabrics (plain weave, twill weave), three-dimensional knitting (integrated fabrics with better isotropy), and needle-punched felt (randomly interwoven chopped fibers with lower cost), etc.
Preform Processing
Surface treatment is performed on the woven preform (e.g., oxidation, coating) to enhance the bond strength between carbon fibers and the carbon matrix (interface bond strength must be precisely controlled; excessive strength may cause brittle fracture, while insufficient strength may lead to delamination).
Carbon Matrix Densification Methods (Core Process)
The formation of the carbon matrix is the core of C/C composite material preparation, with the aim of filling the gaps between carbon fibers to enhance material density and integrity. The main methods include:
Chemical Vapor Deposition (CVD, one of the most commonly used methods)
The principle of chemical vapor deposition is to place the preform in a high-temperature furnace (800-1200°C) and introduce carbon-containing gases (such as methane, propane, or propylene). The gases undergo cracking on the surface of the carbon fibers, depositing solid carbon (pyrolytic carbon), which gradually fills the pores. Its advantages include high purity of the carbon matrix, controllable structure (the crystal structure of carbon can be controlled by adjusting temperature and gas flow rate, such as producing isotropic carbon or anisotropic carbon); good interface bonding with carbon fibers, and excellent mechanical properties of the material.
This method also has certain drawbacks. For example, it has a long cycle time (densification requires tens to hundreds of hours), low efficiency; high cost, suitable for high-end fields (such as aerospace).
Chemical Vapor Infusion (CVI, an improved version of CVD)
The principle of chemical vapor infusion is similar to that of CVD, but it emphasizes the “infusion-deposition” process of gas in the pores of the preform. By controlling the temperature gradient and gas pressure, carbon is preferentially deposited in the internal pores, avoiding premature closure of the surface that could lead to internal porosity. CVI enables more uniform densification, particularly suitable for thick-walled or complex-structured preforms, but it still faces challenges such as long cycles and high costs.
Liquid-phase Impregnation – carbonization Method (liquid-phase method, commonly used for low-cost mass production)
The preform is impregnated with a liquid carbon-containing precursor (such as resin or asphalt), then carbonized at high temperature to form a carbon matrix. This process is divided into:
Resin impregnation carbonization:
- Steps: Vacuum impregnation of the preform with thermosetting resin (e.g., phenolic resin) → Curing (150–200°C) → Carbonization (800–1000°C in an inert atmosphere), where the resin decomposes into a carbon matrix;
- Advantages: Simple process, lower cost, suitable for complex-shaped products;
- Disadvantages: Significant volume shrinkage after carbonization (approximately 50%), requiring multiple impregnation-carbonization cycles (typically 3–5 times) to achieve high density.
Asphalt impregnation carbonization:
- Steps: Impregnate the preform with molten asphalt (petroleum asphalt or coal tar asphalt, with higher carbon content) → Carbonization (1000–1500°C) → Graphitization (optional, above 2000°C);
- Advantages: High carbon yield in the matrix (residual carbon content after asphalt carbonization reaches 50%-80%, higher than the 30%-50% of resins), enabling the production of high-density materials;
- Drawbacks: High viscosity of asphalt makes impregnation difficult, requiring high-pressure assistance, and it is prone to introducing impurities.
Process Optimization
High-temperature graphitization treatment
The densified C/C composite material is heated to 2000-3000°C in an inert atmosphere, causing the carbon matrix to convert to a graphite structure, significantly improving the material’s thermal conductivity, electrical conductivity, and high-temperature stability (but slightly reducing its strength).
Antioxidant treatment (coating protection, etc.)
Carbon is a chemically stable element in nature and exhibits chemical inertness at room temperature or under normal conditions. However, at higher temperatures, carbon readily reacts chemically with other oxidizing agents. The surface layer of carbon-carbon composite materials has fine cracks, allowing oxygen to penetrate through these cracks into the matrix interior. Once oxygen diffuses into the matrix, it can trigger oxidation reactions on the matrix surface or continue to diffuse within microscopic cracks between the matrix and carbon fiber. Both types of diffusion involve oxygen diffusing from the external environment into the matrix, as well as the outward diffusion of reaction products such as carbon monoxide and carbon dioxide resulting from the reaction between carbon and oxygen. Since the graphitization degree of the matrix carbon is far inferior to that of the carbon fibers, the oxidation process primarily occurs in the matrix. Therefore, carbon-carbon composite materials require anti-oxidation treatment. For example, methods such as SiC coating or plasma spraying can be used to form an anti-oxidation coating (with an applicable temperature range extending above 1600°C).
Typical Manufacturing Process (Integrated Process)
In actual production, multiple methods are often combined to balance performance and cost, for example:
- First, CVD/CVI is used to quickly form a preliminary substrate (to improve the density foundation);
- Then, liquid phase impregnation and carbonization are used to supplement the filling (to reduce cost and cycle time);
- Finally, high-temperature graphitization and anti-oxidation coating treatments are performed (to optimize performance).
The manufacturing process for carbon-carbon composite materials centers on CVD/CVI and liquid phase impregnation – carbonization. The former is suitable for high-end, high-performance applications (such as aerospace engine components), while the latter is more suitable for mass production (such as brake discs). The selection of processes must balance material properties (density, strength, high-temperature resistance), cost, and production efficiency. The current industry trend is to combine processes (such as “CVI + liquid-phase impregnation”) and implement automated control to shorten preparation cycles, reduce costs, and drive expansion into civilian applications.