Material availability: carbon-carbon composite materials are readily available and are primarily used in the aerospace and defense industries for aircraft brakes, rocket nozzles, and heat shields. The use of C/C materials has been limited by its poor oxidation resistance at high temperatures. Aircraft braking materials capture the majority of the market and are the cheapest components available, whereas high-density, 3D composites are more expensive.
The price of carbon-carbon varies considerably depending on the end-use and method of production. Carbon-carbon raw material costs vary according to the type and geometries of the fibers and matrix precursor. Epoxy resins are less expensive than other high-carbon-yield polymers. Chopped, low-strength fibers are less expensive than continuous high-modulus fibers, and the cost of weaving varies according to the weave geometry, number of dimensions/directions, and type of fiber.
Joining issues: Integrating a carbon-carbon composite into an otherwise all-metallic energy conversion system raises joining concerns. The CTE mismatch at C/C to metal interfaces may create excessive thermal stresses causing failure at the joint and catastrophic gas leakage. The joining technology between C/C and metal is underdeveloped and unproven for high-temperature joint reliability over a long mission lifetime.
In order to join a C/C recuperator to metallic headers, Keneel and deutchman described a technique that used an lon beam to deposit a metallic interface material at shallow depths into the surface of each material to be bonded. With the surfaces treated, they could be joined together using a metal-to-metal bonding technique. Materials resources international advertised a braze method to join C/C with metal and withstand temperatures up to 2000C. The method uses liquid infiltration and liquid phase sintering of a powder based perform to produce wide gap joints able to accommodate expansion mismatch. These joining technologies are unproven in long-term, high-temperature service, and further research must be conducted.
Compatibility concerns: In addition to potential gas leakage, the degradation of plant or core materials due to mass transport of impurities via the gas stream is a serious concern. In the direct gas Brayton system, materials from the reactor core and ECS are coupled thermodynamically by circulating He-Xe gas, which serves as the core coolant and working fluid in a single, high-temperature, gas circuit. Although core and plant materials are compatible with inert He-Xe gas, there may be significant compatibility issues with impurities entering the circulating gas, specifically oxygen, nitrogen, and carbon. These impurities are of particular concern since they are reactive with, and may be detrimental to, plant and core materials at elevated temperatures. Degradation mechanisms include interstitial embrittlement of refractory metals, deposition of surface carbides, decarburization of superalloys, and diffusion of interstitials into the material matrix; all of which could be detrimental to important materials properties such as creep, fracture toughness, and fatigue.
Incorporating a carbon-carbon composite into the working fluid gas circuit would add to the compatibility concern. Although C/C composites are chemically compatible and corrosion resistant with He-Xe gas, they are sensitive to exposure to corrosive impurities in the gas stream. Logically, carbon-carbon composites must be considered a potential source of active interstitial contaminants. In the case where an oxygen potential is required for superalloy maintenance, carbon-carbon may produce CO by corrosion reactions. On the other hand, when a very low oxygen potential environment is necessary for refractory metal maintenance, there is still a concern over surface transport of free carbon “dust” into the working fluid. Both situations could yield concentrations of C or CO that are unacceptable to the contamination sensitivity of refractory metals, or potentially drive the coolant chemistry into a regime where superalloys carburize or decarburize.