Do You Know About Carbon-Ceramic Brake Discs on Aircraft? HUSCH Popular Science
Don't underestimate an aircraft's brake discs; they are one of the aircraft's critical components, as vital as the aircraft's "heart"—the engines—and its "brain"—the flight control system. The principle is actually similar to that of cars: during landing, the braking system on the wheels (commonly a multi-disc system) provides the vast majority of the deceleration, converting the aircraft's enormous kinetic energy into the brake discs' internal energy (heat). When an aircraft encounters an emergency during high-speed rollout requiring an aborted takeoff, emergency braking subjects the brake discs to an even more severe test, causing their temperature to rise sharply to an incandescent state.
Furthermore, aircraft brake system design must also consider many potential operational anomalies, placing even higher demands on the brake discs. For instance, when an aircraft is rolling at high speed on the runway preparing for takeoff and encounters an unexpected situation requiring a rejected takeoff. Or, if a system fault is discovered shortly after takeoff necessitating a return, and at that moment the flaps and slats cannot be fully extended. Should any of these emergencies occur, the brake discs need to absorb significantly more energy than during a normal landing.
Due to aircraft's higher speeds and weights, the challenge for brake discs is most severe. Therefore, a "Most Severe Landing Stop Test" must be conducted to verify if the brake discs meet design requirements. Under this test condition, the aircraft is heavier than during a normal landing (fuel tanks are nearly full), faster, with less aerodynamic drag (flaps/slats not fully deployed), and with thrust reversers deactivated—relying almost entirely on the brake discs to absorb the aircraft's kinetic energy. So, how much energy must the brake discs absorb in this scenario? For an aircraft weighing 78 tons landing at a speed of 200 knots, this energy is approximately 360 megajoules. Based on high school physics knowledge, 1 joule of energy can lift a 1 Newton (N) object by 1 meter. Assuming this 1N object is a small apple, then 360 megajoules of energy is enough to lift 36,000 tons of apples by 1 meter, or to lift that single small apple by 360,000 kilometers—which is roughly the distance from Earth to the Moon.
At this point, having absorbed this immense energy, the brake disc's temperature rises sharply, reaching over a thousand degrees Celsius. The high temperature causes it to glow a bright orange-red, looking like a blazing fireball.
The material used to make aircraft brake discs must withstand both friction and high temperatures. What material can meet such conditions? The answer is: carbon/carbon composite material.
A Surprise from an Accident: Carbon/Carbon Composites
Early aircraft used powder metallurgy steel brake discs, which suffered from high weight, poor high-temperature performance, and short service life. In comparison, carbon/carbon composite brake discs offer superior performance and are about 40% lighter than steel brake discs (for large aircraft with multiple wheels, this translates to weight savings of several hundred kilograms up to a ton), leading to their widespread adoption.
So-called carbon/carbon composite material is a composite material
consisting of carbon fibers as the reinforcing skeleton and carbon as the
matrix. The form of the carbon fiber can be a three-dimensional framework made
of continuous fibers, or randomly distributed short-cut fibers; the carbon
matrix is obtained either by carbonizing impregnated resin or pitch, or by
pyrolytic deposition from hydrocarbon gases (e.g., natural gas, propane).
How Are Aircraft Carbon Brake Discs Made?
After decades of research, modern production processes for carbon/carbon composites have achieved characteristics such as high specific strength, high specific modulus, high-temperature resistance, and excellent friction and wear performance, meeting the comprehensive performance requirements for aerospace materials under high-temperature and high-speed conditions. Consequently, carbon/carbon composites have become a key development direction for the next generation of aerospace materials.
Brake discs are critical to aviation safety, with very high industry barriers. Giants like France's Safran and the US's Honeywell monopolize over 80% of the global aircraft brake disc market.
Taking the production process of French aviation giant Safran as an example: First, layered carbon fibers are needled, causing a portion of the fibers to turn in the vertical direction, thereby connecting the layers to form carbon fiber felt. Multiple layers of carbon fiber felt are stacked and needled to obtain a porous carbon fiber preform. The preform is densified by impregnation with resin followed by carbonization, and by depositing pyrolytic carbon using the Chemical Vapor Infiltration (CVI) method to fill the pores between the carbon fibers. Rapid carbon deposition on the surface forms a hard crust, which must be removed by machining to facilitate subsequent infiltration and to shape the brake disc. The densification and machining processes are repeated multiple times until the required density and dimensions are achieved, followed by the application of anti-oxidation coatings such as phosphate. Finally, a high-temperature heat treatment at two to three thousand degrees Celsius is performed to graphitize the disordered pyrolytic carbon, improving the brake disc's thermal conductivity and friction performance.
From Sky to Road: The Evolution and Advantages of Carbon Ceramic (C/SiC) Brakes
Carbon ceramic (C/SiC) brake material, as a new type of brake material developed rapidly in recent years, offers advantages like low density, high-temperature resistance, and high, stable friction performance. The core of a carbon ceramic brake system lies in the brake disc. Its material is an enhanced composite ceramic synthesized from carbon fiber and silicon carbide at high temperatures around 1700°C. It is not only exceptionally outstanding in withstanding high temperatures but also weighs over half as much as traditional brake discs of the same size.
Currently, the brake materials widely used in high-speed trains, automobiles, and aircraft are mainly powder metallurgy and C/C composites. However, powder metallurgy brake materials have shortcomings such as a tendency to bond at high temperatures, susceptibility to friction performance degradation, significant decline in high-temperature strength, poor thermal shock resistance, and short service life. C/C brake materials have issues like low static and wet friction coefficients (wet state friction can be about 50% lower than dry state), large heat sink volume, long production cycles (about 1200 hours), and high production costs, limiting their further development and application.
Real-World Application Examples of Carbon Ceramic Brake Material:
Researchers like Walter Krenkel at the German Aerospace Center (DLR) studied C/SiC brake material targeting high-end car brake discs. It is now used in high-end automobiles like the Porsche 911, Audi A8L, and Ferrari.
The US company Starfire researched the preparation of C/SiC brake material via the precursor conversion method and applied it to motorcycle brake pads.
The research team led by Academician Zhang Litong at Northwestern Polytechnical University in China pioneered research on carbon ceramic brake material in 2001. By 2014, carbon ceramic brake discs had been finalized for two key fighter jet models, entering batch production, and have undergone flight testing on multiple aircraft models.
Carbon ceramic brake material is a high-performance brake material developed in recent years, following powder metallurgy and C/C composite materials. Compared with traditional metal and semi-metallic brake materials, it offers advantages such as low density, high strength, stable friction performance, low wear, high specific braking capacity, high-temperature resistance, and long service life. Compared with C/C composites, the introduction of an appropriate amount of SiC ceramic hard material as the matrix enhances the material's oxidation resistance and friction coefficient. Moreover, its friction performance is less sensitive to external environmental media (mold, oil contamination, humidity, etc.). Therefore, carbon ceramic C/SiC brake material holds broad application prospects in fields like high-speed trains, automobiles, and aircraft.
The reason carbon ceramic brake discs can improve the thermal fade phenomenon during automotive braking is that, as a ceramic-modified brake lining, it alters the sliding friction mechanism, changing the traditional shear friction mode, and does not score the brake disc during braking. A carbon film forms during use, leaving the brake disc surface mirror-smooth. Friction grooves do not appear, making the surface flatter, thereby increasing the friction contact area, making the friction force more stable and braking performance stronger. Beyond the weight advantage, another major benefit of wear-resistant carbon ceramic brake systems lies in their surface containing a significant amount of silicon carbide. As one of the hardest substances known to humankind, silicon carbide contributes to a potential service life of carbon ceramic brakes of up to 30 years.
Disadvantage: Despite its numerous advantages, carbon ceramic brake production is extremely complex. Raw materials must be subjected to high temperature and high pressure for extended periods during die-casting. The stringent manufacturing process makes it difficult to significantly increase output, leading to high prices, which also limits the widespread application of carbon ceramics.