The Chemical Vapor Deposition (CVD) process for producing carbon-carbon composites involves high-temperature chemical reactions where carbon source gases decompose and deposit carbon inside and on the surface of a carbon fiber substrate. This method is commonly used for manufacturing high-performance carbon-carbon composites, such as those used in aerospace applications, high-temperature structural components, thermal field components, and high-performance heat-resistant parts.

1. Basic Principle of CVD Process

CVD involves the following steps:

1.1 Carbon Source Gas Injection: Introduce carbon source gases such as methane (CH₄), acetylene (C₂H₂), ethylene (C₂H₄), or benzene (C₆H₆) into a heated reactor.

1.2 Decomposition and Carbon Deposition: At high temperatures (typically 800-1200℃), the carbon source gases decompose and release carbon atoms, which deposit on the carbon fiber substrate, filling the pores and gradually forming a dense carbon structure.

1.3 Multiple Deposition Cycles: To improve the material’s density, strength, and high-temperature performance, carry out multiple deposition cycles typically. Each cycle of gas injection and deposition further enhances the carbon content of the material, reducing its porosity.

1.4 Final Graphitization in a graphitization furnace: After several cycles, the material is exposed to even higher temperatures (typically around 2000℃) in a final graphitization step to enhance its thermal conductivity and mechanical properties.

2. Detailed Steps of the CVD Process

2.1 Substrate Preparation

• Substrate Selection: Typically use carbon fibers (such as PAN-based or pitch-based carbon fibers) as the substrate. Prepare the carbon fiber substrate by weaving, winding, or layering to achieve the desired shape.

2.2 Gas Injection and Deposition

• Gas Source Selection: Common gases include methane (CH₄), acetylene (C₂H₂), ethylene (C₂H₄), or benzene (C₆H₆), chosen based on the desired deposition rate and material properties.
• Deposition Conditions: Introduce carbon source gases into the reaction chamber under low pressure (1-100 Torr) and heated to high temperatures (typically 800℃ to 1200℃). The carbon atoms released from the gas react and deposit on the fiber surface.

2.3 CVD Deposition Process

• Pre-heating Stage: Heat the carbon fiber substrate to around 800-1200℃.
• Gas Injection: Introduce Carbon source gases and carrier gases (such as hydrogen or argon) into the reactor, with careful control over the flow rate, gas concentration, and pressure.
• Carbon Deposition: The gases decompose at the high temperature, and carbon atoms deposit on the fiber surface and within the pores, gradually filling the structure.
• Deposition Control: Temperature, gas concentration, and pressure are adjusted to optimize deposition rate and uniformity.
• Cooling and Post-processing: After deposition, the material is cooled, and post-treatment may be performed, such as surface treatments to enhance hardness.

3. Key Factors Affecting CVD Deposition Quality

3.1 Temperature

• 800-1200℃: Ideal for carbon deposition.
• Below 800℃: Deposition rate is too slow and ineffective.
• Above 1200℃: Carbon structure may change, affecting material performance.

3.2 Gas Flow and Pressure

• Low pressure (1-100 Torr) supports uniform deposition.
• High pressure may lead to uneven deposition and porosity.
• Low pressure can reduce the deposition rate, affecting production efficiency.

3.3 Gas Source Selection

• Methane (CH₄): High purity, suitable for uniform deposition.
• Acetylene (C₂H₂): Faster deposition rate, but can lead to uneven structures.
• Benzene (C₆H₆): Used for specific high-density C/C composites.

4. Advantages and Disadvantages of the Chemical Vapor Deposition Process

4.1 Advantages

• High purity carbon deposition: CVD produces carbon layers with no impurities, leading to dense structures.
• Excellent high-temperature performance: Ideal for aerospace, semiconductor, and thermal field applications.
• Uniform microstructure: Produces dense, uniform carbon layers that enhance mechanical strength and thermal shock resistance.
• Controllable porosity: Allows for control of porosity, tailoring the material’s strength and thermal conductivity properties.

4.2 Disadvantages

• Slow deposition rate: The CVD process is slow, with complete deposition potentially taking several days or weeks.
• High production cost: The complex equipment and high consumption of carbon source gases make it a costly process.
• Requires precise control: Meticulously controls process parameters such as gas flow, temperature, and pressure to ensure uniform deposition.

5. Applications of Chemical Vapor Deposition-Produced Carbon-Carbon Composites

Due to their excellent high-temperature strength, low density, and outstanding thermal conductivity, CVD-produced carbon-carbon composites are used in:

• Aerospace: Thermal protection systems for spacecraft, rocket nozzles, and aircraft brake discs.
• Semiconductor industry: High-temperature parts for silicon crystal growth furnaces, graphite heaters.
• Nuclear and defense: Nuclear reactor components and high-temperature fuel containers.
• High-temperature furnaces: Components for vacuum furnaces, graphite heating elements.