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.”