1. Characteristics of Carbon-Based Composite Materials
Carbon fiber-reinforced carbon composites (carbon-based composites, C/C) are novel engineering materials with unique properties, composed primarily of carbon or graphite fibers as reinforcement and carbon or graphite as the matrix. Due to their almost entirely carbon composition, these materials can withstand extremely high temperatures and rapid heating rates. They possess high ablation heat, low ablation rates, excellent thermal shock resistance, and high strength in ultra-high temperature environments, making them ideal for high-performance ablative materials in reentry environments. Additionally, they exhibit strong thermal shock resistance and chemical inertness. When used as missile nose cones, carbon-based composites demonstrate low and uniform ablation rates, enhancing the missile's penetration and hit rate. Their exceptional wear resistance and high thermal conductivity also make them suitable for applications in aircraft, automotive brake pads, and bearings.
Carbon-based composites not only share the advantages of other composite materials but also have unique characteristics, including:
Entirely composed of carbon, they exhibit excellent stability at both low and high temperatures due to the strong affinity between carbon atoms. Their high melting point endows them with superior heat resistance, capable of withstanding temperatures around 2000°C, making them the best material for high-temperature mechanical performance in an inert atmosphere. Moreover, their strength increases with temperature, surpassing that at room temperature, which is unmatched by other materials.
Low density (less than 2.0g/cm³), only 1/4 of nickel-based superalloys and 1/2 of ceramic materials.
Excellent ablation resistance, capable of withstanding short-term ablation at temperatures above 3000°C, making them suitable for rocket engine nozzles and throat liners.
Outstanding friction and wear resistance, with a low friction coefficient and stable performance, making them ideal for various wear-resistant and friction components.
Good biocompatibility, with density and modulus comparable to human bones, showing promising applications in bone repair and replacement materials.
2. Manufacturing Process of Carbon-Based Composite Materials
The manufacturing process of carbon-based composites includes the selection of carbon fibers and their fabrics, choice of carbon precursors for the matrix, preform forming processes, densification of the carbon matrix, and final product processing and inspection. The chosen manufacturing process depends on the application requirements of the carbon-based composite materials, influencing the selection of fibers and matrix.
Preforms are created according to product shape and performance requirements, forming carbon fibers into the necessary structure for subsequent densification processes. Weaving techniques for preforms include machine weaving and hand weaving, with hand weaving being more widely used due to fewer issues with fuzzing or breakage. Advanced techniques include hand-wound weaving, cross weaving, and stitching weaving.
The densification process of carbon-based composites involves filling the voids around carbon fibers with high-quality carbon to obtain materials with excellent structure and properties. Common densification methods include Chemical Vapor Infiltration (CVI) and Liquid Phase Impregnation (LPI). Precursor materials for forming the carbon matrix include hydrocarbons (e.g., methane, propylene, natural gas) for CVI and thermosetting resins (e.g., phenolic resin, furan resin) and thermoplastic pitches (e.g., coal tar pitch, petroleum pitch) for LPI. CVI involves placing carbon fiber preforms in a CVI furnace, heating them to the required temperature, and introducing hydrocarbon gases, which decompose and deposit carbon around and within the fiber voids. Parameters for the CVI process are chosen based on product thickness, desired densification, and the structure of the pyrolytic carbon.
3. Properties of Carbon-Based Composite Materials
3.1 Mechanical Properties
Carbon-based composites are brittle materials with low fracture strain at failure. Their strength is closely related to the direction and content of reinforcing fibers. They have higher tensile strength and modulus along the fiber axis and lower values in directions deviating from the fiber axis. The strength is also influenced by the interfacial bonding between carbon fibers and the carbon matrix. High-temperature graphitization significantly improves the strength and modulus of carbon-based composites, increasing strength by 29.5% and modulus by 119.2%. Graphitization alters the material's interface properties, weakening the bonding between fibers and matrix, which improves tensile strength and fracture strain.
3.2 Thermal Physical Properties
Carbon-based composites possess thermal physical properties characteristic of carbon and graphite materials, with high thermal conductivity. Their thermal conductivity mechanism lies between that of metals and non-metals, involving both phonon and electron conduction. Conductivity increases with graphitization and density and varies with fiber orientation. They exhibit excellent thermal shock resistance due to the reinforcement of carbon fibers and the void network structure, which prevents catastrophic failure under thermal stress. The small coefficient of thermal expansion ensures dimensional stability under temperature changes, making them suitable for aerospace applications.
3.3 Ablation Resistance
Ablation refers to the material loss phenomenon due to thermal and mechanical processes when a missile or spacecraft re-enters the atmosphere. Among existing ablative materials, carbon-based composites are the best. They are sublimation-radiation type ablative materials with high ablation heat, large radiation coefficient, and high surface temperature, effectively absorbing and radiating heat, providing excellent ablation resistance.
3.4 Friction and Wear Properties
Carbon-based composites exhibit high specific strength, specific modulus, fracture toughness, low density, excellent thermal performance, friction and wear properties, and long service life, making them ideal for friction components in civilian and military aircraft brake materials. Their friction and wear performance are influenced by manufacturing processes, fiber volume fraction, structure, reinforcement forms, friction surface direction, and actual usage conditions.
3.5 Biocompatibility
Carbon is considered one of the best materials for biocompatibility. Carbon-based composites inherit carbon's biocompatibility and possess high toughness, strength, fatigue resistance, and excellent friction properties. Their mechanical properties can be tailored, making them suitable for bone repair and replacement materials due to their similarity to human bone modulus and favorable micro-porous structure for tissue growth.
4. Anti-Oxidation Technology of Carbon-Based Composite Materials
Carbon-based composites, with high strength and modulus, high thermal stability, high thermal conductivity and electrical conductivity, low density, low thermal expansion coefficient, ablation resistance, corrosion resistance, and stable friction coefficient, maintain these properties at temperatures above 2000°C. However, their excellent performance is limited to inert environments. Carbon-based composites begin to oxidize at 400°C in an oxygen environment, with oxidation rates increasing rapidly with temperature, leading to catastrophic consequences in high-temperature oxidation environments. Thus, anti-oxidation technology is crucial for their application as high-temperature structural materials.
Currently, anti-oxidation strategies include matrix modification and anti-oxidation coating technologies. Matrix modification provides effective low-temperature oxidation resistance, while surface coating technologies have made significant progress, producing multi-layer gradient coatings for long-term service at 1600°C.
4.1 Matrix Modification Technology
Matrix modification involves adding oxygen barrier components to the carbon source precursor during the synthesis of carbon-based composites, forming composites with inherent anti-oxidation capabilities. The oxygen barrier components must have good chemical compatibility with the matrix carbon, low oxygen and moisture permeability, no catalytic effect on oxidation reactions, and should not affect the original mechanical properties of the composites. Despite challenges in matching thermal expansion coefficients between coatings and the composite matrix, matrix modification remains a promising approach, especially for low-temperature oxidation protection or combined with coating technology for high-temperature protection.
4.2 Anti-Oxidation Coating Technology
Anti-oxidation coatings on the surface of C/C composites aim to prevent oxygen contact and diffusion, achieving high-temperature oxidation resistance. Effective coatings must provide an oxygen barrier, have low volatility to prevent loss under high-speed airflow or high-temperature conditions, adhere well to the substrate, and be chemically and mechanically compatible. Coating forms include single-layer, double-layer composite, functional gradient, and multi-layer coatings, each designed to meet specific performance requirements.
5. Applications of Carbon-Based Composite Materials
5.1 High-Performance Brake Materials
Carbon-based composite brake discs are widely used in high-speed military aircraft and large supersonic civilian aircraft. They require high specific heat capacity, high melting point, and high-temperature strength. Carbon-based composites meet these requirements with their lightweight, high-temperature resistance, high energy absorption, and excellent friction performance, making them ideal for aircraft brake systems. Carbon brake discs offer long service life, stable braking torque, and resistance to high temperatures, making them a significant advancement in brake material technology.
5.2 Ablative Materials
As ablative materials, carbon-based composites are used in intercontinental missile nose cones, solid rocket nozzles, and space shuttle nose cones and wing leading edges. They provide high-temperature strength, ablation resistance, erosion resistance, and thermal shock resistance, protecting missile warheads during atmospheric reentry. In solid rocket engines, they serve as throat liners and nozzles, and in liquid rocket engines, they are used in thrust chamber liners, movable thrust chambers, nozzle extension cones, and thermal shields.
5.3 Biomedical Applications
Carbon-based composites offer biocompatibility, stability within the body, and mechanical compatibility with bone elasticity, making them suitable for bone repair and replacement materials. They maintain high strength, fatigue resistance, toughness, and can be tailored for specific mechanical properties. Although clinical applications are currently limited, their potential advantages predict promising future applications in biomedical materials.
Additionally, carbon-based composites are used in self-lubricating bearings, mechanical fasteners, hot-press molds, helium-cooled nuclear reactor heat exchange pipes, chemical pipelines and container linings, high-temperature seals, bearings, and crystal pulling machine components.
6. Development Trends of Carbon-Based Composite Materials
Technological advancements are driving carbon-based composites from ablative carbon-based composites to multifunctional and thermal structural carbon-based composites. The evolution has progressed from 2D-carbon-based to 3D-carbon-based and multi-dimensional carbon-based composites, incorporating advanced hybrid (weaving and matrix) carbon-based structures. These materials are becoming increasingly mature in aerospace applications and are finding new uses in civilian industries. Current research focuses on three main areas: improving matrix performance (especially shear along the plane and tensile perpendicular to the plane), enhancing anti-oxidation coatings (for higher usage temperatures and longer service lives), and seeking cost-effective production methods.
6.1 Multifunctional Carbon-Based Composites
The trend is moving towards carbon-based composites that serve multiple functions, combining thermal structural capabilities with other desirable properties. This includes enhancing their usability in extreme conditions while maintaining or improving mechanical and thermal properties.
6.2 From 2D to 3D and Multi-Axial Weaving
There has been a significant shift from 2D to 3D carbon-based composites, providing greater structural integrity and performance. Multi-axial weaving techniques, including automated and programmed hybrid weaving, are being developed to create more complex and resilient composite structures.
6.3 Advanced Matrix and Hybridization
Improving the matrix composition and integrating hybrid materials have become key focus areas. This involves using different types of fibers and matrices to achieve desired properties and performance metrics, tailored for specific applications.
6.4 Enhanced Anti-Oxidation Coatings
Research is heavily invested in developing advanced anti-oxidation coatings that can withstand higher temperatures and longer durations. This includes multi-layer and gradient coatings that provide robust protection against oxidation, extending the service life of the composites in harsh environments.
6.5 Cost-Effective Manufacturing Techniques
Efforts are ongoing to discover and implement cost-effective manufacturing methods. This includes optimizing existing processes like CVI and LPI, as well as developing new techniques such as rapid densification processes to reduce production costs and improve efficiency.
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