To produce C/C composite parts, Spirit weaves tows of carbon fiber, each consisting of bundles of thousands of individual carbon fibers, into three-directional arrangements of straight or uncrimped fibers.
“After quality assurance inspections ensure the fiber architecture is correct, then heated, pressurized pitch is forced into the spaces between the fibers,” Boshers explains. “Following this infusion process, the billet is heated to char the pitch, leaving only carbon behind. The infusion process is repeated multiple times to fill up any remaining voids, resulting in a very dense, very uniform block, or billet, of carbon-carbon.” The company can mill multiple aircraft components from a single billet.
Spirit is working to automate the C/C manufacturing process to reduce labor costs and improve product quality, to accelerate the densification process and to produce near-net shape components. Boshers says those changes could improve the buy-to-fly characteristics of the C/C material, making it more affordable.
Spirit is also developing ways to make production-scale quantities of ceramic matrix composite (CMC) materials for hypersonic vehicles. CMCs maintain strength and stiffness like C/C composites, but operate at a lower temperature range of 1,500 F to 2,200 F. CMCs do have better environmental resistance, especially to oxidation, than C/Cs. They can be used for components in non-peak-heat areas of aircraft, for some thermal shielding and for scramjet inlet ducts.
Spirit’s CMC materials include either carbon, silicon carbide (SiC) or aluminum oxide (Ox) fibers. With the carbon and silicon carbide fibers, the company uses silicon carbide as the matrix material. Aluminum oxide fibers are used with an aluminum oxide matrix.
To perform the specialized, high-temperature testing (up to 5,000 F) of both C/Cs and CMCs, Spirit has built its own energy materials testing laboratory. It is also partnering with the National Institute for Aviation Research (NIAR) on construction of a high-temperature materials test facility in Wichita. “This facility will provide the capability to characterize high-temperature C/C and CMC materials, and provide certified B-basis statistical allowables for the design of hypersonic vehicle structures,” says Boshers. (B-basis is the strength value at which only 10 in 100 specimens will fail with a 95% confidence level.)
Boshers sees many additional opportunities for composite materials in the hypersonic market. “We are exploring new fiber architectures and materials that offer cost/performance benefits in hypersonic applications,” he explains. “Near-net shape materials, larger size components to reduce or eliminate joints, and materials that can more effectively resist ablation or erosion are currently being developed. Longer term, the use of high-temperature-resistant, durable materials could be used in jet engines to reduce weight and improve efficiency, resulting in reduced fuel use and lower CO2 emissions.”
Although the defense industry has been the biggest customer for hypersonic aircraft materials, there is growing interest from the commercial sector as well. Companies like SpaceX and Virgin Galactic are developing technologies that will slash the time it currently takes to transport both passengers and cargo around the world. Composite materials will play a key role not only in the development of these hypersonic aircraft, but also in the production of many other innovative future aircraft.
3D-Printed Aircraft Components
Additive manufacturing (AM) of composites enables quick production of components of almost unlimited geometries with very little waste. But most AM work today is done with thermoplastic materials, which don’t have the properties necessary for aircrafts’ structural components. Thermoset composites made with carbon fiber can provide the required strength and durability, but incorporating carbon fibers into the printing mixture has been problematic. When the carbon fiber count gets too high, the fibers clump together and clog the nozzle of the 3D printer so that it can’t print.
Emrah Celik, assistant professor of mechanical and aerospace engineering at the University of Miami, has developed a process that solves this problem. He found that vibrating the carbon fibers at the printer’s nozzle prevents them from clumping and allows the printing process to proceed.
In the course of working on AM projects for NASA and the Air Force, Celik was able to customize off-the-shelf, 3D extrusion printers to produce carbon fiber thermoset parts. In 2019, Celik and his Air Force research partners published a paper describing how they had been able to manufacture a thermoset part with about 6% carbon fiber by volume. “That was great at the time, and we exceeded what was state-of-the-art practice,” he says.
But Celik wanted to add more fiber to produce even stronger composite components. With his graduate assistant, Nashat Nawafleh, he explored several variables, including the optimal length of the carbon fibers, the surface treatment of those fibers and the impact of different levels of vibration on the thermoset material.
With vibration, for example, “You don’t want to vibrate too much, because you don’t want to reduce your resolution or change the dimensional accuracy,” Celik says. “But the vibration has to be the right amount to separate the fibers from each other so they don’t clog the nozzle.”
After experimenting with Kevlar® and glass fibers, Celik focused on carbon fibers because of the strength they provide to the composite material. He initially worked with chopped carbon fibers about 6 millimeters in length, but eventually chose milled carbon fibers approximately 50 microns long. After testing a variety of surface chemistries for these shorter fibers, Celik found that sized materials provide the best linkage between the resin and the carbon fibers.
In thermoplastic AM, the bonding between layers of a composite may not be strong because the material is heated up for printing and quickly cools down. Celik’s thermoset AM process delivers a more isotropic result; the printing “ink” is composed of nanoclays or silica, mixed with a thermoset resin and then with the carbon fibers right at the nozzle. “It becomes a gel type of material, and when you extrude it on a surface it has enough strength to keep its shape,” he explains. The gel in each layer fuses well to the layer underneath, and when the part is completed, it is put in an oven to cure overnight.
The isotropic properties are also improved because of the use of the shorter carbon fibers. During the extrusion process, the fibers in the printing ink tend to align in the direction of the printing, reducing the strength in other directions. Since the shorter fibers don’t align as much as longer ones, the thermoset composites in Celik’s process show 80% strength in the transverse direction.
With all of these changes, Celik realized an impressive improvement in the carbon fiber content of the printed thermoset composite. “Instead of 5% to 6% fiber, we could now fabricate materials with 46% carbon fiber by volume,” he explains. The material is as strong as metal, but 80% lighter than steel and 50% lighter than aluminum.
Celik believes the lightweight thermoset composite parts produced through this AM process will be able to replace heavier metal components. In addition, with 3D-printed composites, aircraft manufacturers could re-engineer components. Only the parts that require structural integrity have to be solid; in other areas, honeycomb configurations could be used to further reduce an aircraft’s weight.
Several Department of Defense agencies have already expressed an interest in Celik’s work. While aerospace may be the first industry to use the technology, he sees additional applications in transportation and other industries, especially if the cost of producing carbon fiber can be reduced.
From 3D printing to advances in materials, the variety of R&D going on today reflects the many possibilities that composites offer for aircraft and spacecraft of the future.
