Throat Liner Ablation-resistant Carbon-carbon Composites Products
In the field of weaponry, the throat liner is a critical high-temperature component in various types of rocket projectiles. During its service life, it must withstand temperatures of approximately 3,000°C, internal gas pressures exceeding 15 MPa, and high-speed, high-temperature gas erosion. Traditional high-silica resin and high-strength graphite throat liner products have exhibited issues such as prone to cracking and insufficient ablation resistance during service, necessitating urgent upgrading and replacement. By leveraging the advantages of carbon-carbon composite materials—low density, high high-temperature strength, ablation resistance, and excellent thermal shock resistance—they can replace traditional high-silica resin and high-strength graphite materials for application in throat liners of various rocket projectiles. For tactical weaponry, the products must also have a low manufacturing cost. Carbon-carbon composite throat liners manufactured using isothermal CVI and liquid-phase impregnation pyrolysis composite technology have successfully passed multiple test firings, but issues such as unstable performance and severe ablation damage have emerged. Carbon-carbon composites manufactured using thermal gradient CVI technology with needle-punched carbon felt as the reinforcing material not only have a short production cycle and relatively low cost but also exhibit excellent ablation resistance due to the pyrolytic carbon structure formed during deposition, fully meeting the requirements of various rocket types. Thermal gradient CVI-manufactured ablation-resistant carbon-carbon composites are also the preferred material for developing high-performance rocket nozzle liners in the future.
Compared to rocket motor throat liners, the operating environments for large rocket engine throat liners and advanced missile engine throat liners and ablation rings in the aerospace propulsion field are more severe, requiring higher internal pressure resistance, longer operating times, and lower ablation rates. Previously, high-density carbon-carbon composite throat liners and ablation rings made using asphalt liquid-phase impregnation and pyrolysis exhibited issues such as cracking and unstable performance during actual use, necessitating product upgrades. The primary causes of product failure are twofold: first, during the asphalt pyrolysis carbonization and graphitization process, the escape of a large number of carbon-hydrogen molecules results in the formation of numerous micro-cracks and pores; second, when the temperature rises to a certain range, the thermal expansion coefficient of asphalt carbon increases sharply, causing extreme thermal stress to develop instantaneously within the product during service, leading to explosions. Subsequently, anti-erosion carbon-carbon composite throat liners and erosion rings were developed using punctured carbon felt as the reinforcing material and a thermal gradient CVI process. The product’s matrix is composed of pyrolytic carbon fibers, featuring high structural density, low thermal expansion coefficient, and excellent mechanical and anti-erosion performance. It exhibits stable operational performance and fully meets the usage requirements.
Advanced missile gas rudder thermal protection systems, such as heat shield rings, fireproof baffles, fireproof pads, heat sink plates, and other thermal protection components for rocket engine expansion sections, must be able to withstand extremely high temperatures and low ablation during use. They also need to have thin walls and certain high-temperature mechanical properties. Similar products made of high-strength graphite or resin-based composite materials cannot meet the performance and weight reduction requirements. After upgrading to erosion-resistant carbon-carbon composite materials, the products demonstrate excellent performance.
Future updates to products such as throat liners, ablation rings, and thermal protection systems will primarily focus on improving manufacturing processes to achieve optimal mechanical, thermal-physical, and ablation-resistant properties of the carbon matrix structure, as well as modifying the carbon matrix of carbon-carbon composite materials to enhance ablation resistance. From the perspective of performance reliability, carbon-carbon composite materials produced using the rapid and efficient CVI process hold significant market application potential.
Carbon-carbon Brake Sssembly
The function of an aircraft brake assembly is to slow down and stop an aircraft moving rapidly on the ground. The brake assembly absorbs the aircraft’s enormous kinetic energy through relative sliding friction. During braking, the surface temperatures of the stationary and moving discs exceed 500°C, requiring the braking materials to possess excellent thermal shock resistance and superior friction and wear performance. Traditional steel brake assemblies suffer from issues such as high self-weight, high wear rates, and short service life. Especially during emergency braking scenarios like aborted takeoffs, localized melting deformation or welding may occur, resulting in the loss of braking effectiveness.
Carbon-carbon composite materials possess excellent thermal conductivity and a low thermal expansion coefficient, making them an ideal upgrade for traditional aircraft steel brake discs. The superiority of carbon-carbon composite brake discs is primarily manifested in three aspects: weight reduction, high-temperature resistance, and excellent friction properties.
Countries such as the UK, the US, and France began using carbon-carbon composite materials as aircraft brake materials almost simultaneously in the early 1970s. Since their first use as brake materials on the Concorde supersonic aircraft in 1973, carbon-carbon composite brake discs are now used in the vast majority of civil and military aircraft (such as the B737-777, A300-340, and other large commercial airliners, as well as advanced military aircraft like the F-16 and F-22), with their annual usage accounting for 90% of global carbon-carbon composite materials.
With the rapid development of China’s civil aviation industry, the number of various types of transport aircraft in operation has been increasing year by year. According to relevant data, China has imported over 500 A320 aircraft from Airbus, and annually requires over 10,000 carbon-carbon brake discs for replacement. Additionally, the demand for carbon-carbon brake discs for military aircraft is also increasing year by year. However, the international market for carbon-carbon brake discs is largely monopolized by five companies from the UK, the US, and France, resulting in high prices. Compared to Western countries, research into the manufacturing of carbon-carbon composite brake discs in China began relatively late. After decades of effort, carbon-carbon brake discs developed by institutions such as Central South University, the 43rd Research Institute of the Fourth Academy of Aerospace Science and Technology, and the 621st Aviation Research Institute have been applied domestically. However, they still fail to meet the demands of civil aircraft, resulting in China’s reliance on imports for most of its carbon-carbon brake discs, with annual foreign exchange expenditures reaching hundreds of millions of dollars. To swiftly meet the demand for brake assemblies in both civil and military aircraft, break through foreign technical barriers and market monopolies in aircraft brake assemblies, and capitalize on the high growth potential of the vast market, it is imperative to intensify R&D efforts for domestically produced brake assemblies and promote the industrialization of R&D outcomes.
The primary method currently used to manufacture carbon-carbon composite aircraft brake discs is the CVI process. The traditional isothermal CVI process tends to form a crust on the surface during densification, resulting in a lengthy manufacturing cycle (approaching 1,000 hours). However, by optimizing and improving the narrow-slot flow-restricted isothermal CVI process or adopting the thermal gradient CVI process, the densification cycle can be reduced to 300–400 hours. A composite process combining thermal gradient CVI with resin liquid-phase impregnation pyrolysis can regulate the friction and wear properties of the matrix carbon by adjusting the relative content of pyrolyzed carbon and resin carbon, thereby producing carbon-carbon composite materials that meet the requirements for aircraft brake systems. The CVI and liquid-phase impregnation pyrolysis composite process can also shorten the manufacturing cycle of carbon-carbon composite materials and reduce the production cost of brake discs.
In addition to being used in aircraft braking systems, carbon-carbon composite brake discs can also be used in Formula One racing cars, motorcycles, and high-speed trains. In the future, the upgrading of carbon-carbon composite brake discs will require further reductions in manufacturing costs, as well as improvements in the wear resistance and static friction coefficient of carbon-carbon composites, extending the service life of the product, and promoting its application in high-energy braking systems.
Carbon-carbon Crucibles, Heating Elements, and Other Civilian Products
Carbon-carbon composite materials can leverage their high-temperature resistance, low thermal expansion, and excellent high-temperature mechanical properties in fields such as mechanical manufacturing and chemical engineering, replacing traditional metal and graphite products and offering significant market application potential.
(1) Traditional high-temperature furnace graphite heating elements often require thicker walls due to insufficient mechanical properties and thermal shock resistance, resulting in limited heating element dimensions and a limited service life. By replacing graphite products with carbon-carbon composite heating elements, the mechanical strength of the heating elements is significantly enhanced due to the reinforcing effect of carbon fibers. Additionally, the high retention rate of high-temperature mechanical properties of carbon-carbon composites enables them to withstand thermal shock without damage. Since carbon-carbon composites have higher resistance than graphite, they can provide higher power, allowing for the production of large, thin-walled heating elements that more effectively utilize furnace chamber volume. For example, in high-temperature isostatic presses, carbon-carbon composite heating elements with a length of 2 meters are used, with wall thicknesses of only a few millimeters. Such heating elements can operate at temperatures exceeding 2500°C.
(2) Metal fasteners experience severe degradation of mechanical properties at high temperatures, and may even fail due to high-temperature welding, making replacement impossible. Fasteners such as screws, nuts, bolts, and washers made from carbon-carbon composite materials can fully utilize their excellent high-temperature mechanical properties at high temperatures. Their low thermal expansion coefficient and high temperature resistance ensure dimensional stability of the fasteners.
(3) In the production of single-crystal silicon, carbon-carbon composite materials, due to their high-temperature stability, can be used for crucible liners and seed crystal holders. Compared to traditional graphite crucibles, carbon-carbon composite crucibles have significantly extended service lives, and CC seed crystal holders can address the issue of traditional metal holders contaminating single-crystal silicon.
(4) Due to their lightweight and refractory properties, carbon-carbon composites can replace steel and graphite in the manufacture of superplastic forming blow molds and hot-pressing molds in powder metallurgy, reducing mold thickness, shortening heating cycles, saving energy, and increasing production. Carbon-carbon composite hot-pressing molds have been tested in cobalt-based powder metallurgy, showing more uses and a longer lifespan than graphite molds.
(5) Compared to metal internal combustion engine pistons, carbon-carbon composite materials have higher emissivity and lower thermal conductivity, allowing the removal of the piston outer ring and side edges. Additionally, carbon-carbon composite pistons can operate at higher temperatures and pressures, thereby improving the mechanical and thermal efficiency of internal combustion engines.
(6) In the chemical industry, carbon-carbon composites can replace traditional metal products, primarily used in corrosion-resistant equipment, pressure vessels, sealing materials, spiral tubes, etc.
Currently, the bottleneck constraining the widespread application of carbon-carbon composites in civilian fields is their high production costs. A key pathway for the future upgrading of civilian carbon-carbon composite materials is the engineering application of rapid and efficient densification processes such as thermal gradient CVI and liquid-phase vaporization CVI, which can significantly reduce manufacturing costs. Additionally, in high-temperature oxygen-rich environments, carbon-carbon composite material products require the preparation of anti-oxidation coatings on their surfaces to enhance service stability and extend their service life.
Aerospace High-Temperature Structural Carbon-Carbon Products
Research into the application of carbon-carbon composite materials in high-performance aircraft engine thermal structural components and advanced aerospace vehicle thermal protection systems has been a focus of attention. Carbon-carbon composite materials developed in the United States for sharp leading edges and wing leading edges have been successfully applied. China is still in the research and development phase in this area, and the manufacturing and service performance of carbon-carbon composite components still need to be improved.
Currently, China’s high-performance aerospace engine carbon-carbon composite thermal structural components and advanced aerospace vehicle carbon-carbon composite thermal protection systems are still in the research and development phase. To further enhance the service performance of such components, efforts should be focused on the following three areas:
Mixed pyrolytic carbon texture achieves comprehensive performance optimization.
Smooth-layer pyrolytic carbon has high hardness, while rough-layer pyrolytic carbon has high ablation resistance +3.
The thermal conductivity and thermal expansion coefficient of the two textures also differ. By designing and preparing mixed-textured pyrolytic carbon with a specific smooth layer/rough layer ratio tailored to the actual application environment of carbon-carbon composite components, the comprehensive performance of carbon-carbon composites can be optimized.
Synergistic strengthening and toughening of carbon fibers and nanotubes.
The carbon matrix in carbon-carbon composites has high brittleness and low toughness, making it prone to sudden and catastrophic failure when used as a structural material. By uniformly introducing carbon nanotubes into pyrolytic carbon, where carbon nanotubes serve as the reinforcing phase of pyrolytic carbon, and ensuring an appropriate bonding strength between pyrolytic carbon and carbon nanotubes, the interfacial bonding strength near the crack tip weakens under stress field conditions. This not only redirects the crack, thereby enhancing the fracture toughness of pyrolytic carbon, but also induces the pull-out phenomenon of carbon nanotubes. Carbon nanotubes can act as bridges between the upper and lower crack surfaces, thereby reducing stress concentration at the crack tip, increasing crack propagation resistance, and ultimately improving the mechanical properties and fracture toughness of carbon/carbon composite materials. The synergistic strengthening and toughening of carbon fibers and nanotubes represent an important direction for the upgrading of high-performance carbon-carbon composite materials.
Matrix modification combined with coating for oxidation resistance.
High-performance aerospace engine thermal structural materials must operate for extended periods under high-speed gas flow at temperatures above 1600°C. Advanced aircraft require prolonged resistance to oxidation erosion and severe erosion at temperatures exceeding 2000°C, imposing stringent demands on the oxidation resistance and erosion resistance of carbon/carbon composite materials. The combination of matrix modification and coating technology for oxidation resistance addresses the limitations of using matrix modification alone, which cannot completely prevent oxygen penetration, as well as the issues of interface incompatibility and unstable service performance associated with using coatings alone.