Carbon/carbon (C/C) composites consist of carbon fiber reinforcement embedded in a graphite matrix. These advanced materials have low density, high strength-to-stiffness ratio, high thermal and electrical conductivity, and are resistant to ablation, particle erosion, and thermal shock. In addition, C/C composites can be tailored to a specific target application by using the correct reinforcing fibers and controlling the micropores within the matrix through appropriate processing science. These lightweight structures are thus ideal for high-temperature structural applications like hypersonic aerospace vehicles, nose cones and wing leading edges of space shuttle orbiters, rocket nozzles, heat shields, aircraft disc brakes, and nuclear fusion reactors [1].
Our group is the first to manufacture a C/C composite with a (micro)vascular network (embedded micro-channels) [2], using a combination of the VaSC technique (vaporization of sacrificial components) [3] and traditional fabrication methods for C/C composites. Microchannels are incorporated during the prepreg fabrication procedure, allowing for the implementation of a complex channel architecture, which can be optimized for heat transfer where any fluid can flow. To improve the mechanical properties, one of the crucial fabrication steps is densification (cycles) through either a liquid route (using the same resin), or by using a gas carbon source (e.g. methane). In a first instance, in our previous work[2], to fill the pores of the composite, traditional isothermal CVI was used using methane at 1000 °C and 250 mbar.
It is well understood that most of the enhancement in C/C composites fabrication is based on improving the densification step through chemical vapor infiltration (CVI), pitch infiltration with intermediates or by impregnation/pyrolysis of the pre-densified preform with pitch. We propose a novel densification method by taking advantage of our unique embedded channels within the composite. In this new methodology, the gaseous carbon source flows through the channel and out to the interior of the specimen. Thus, deposition can occur from the hotter surface to the colder interior, allowing not only for a uniform densification to be obtained, but also ensuring that the surface of the channels are properly sealed. As a gas source for CVI, methane (CH4) is used, which breaks down above 900 0C, depositing carbon at the surface of the composite, hence slowly densifying the material. This procedure is dictated mainly by two factors: diffusion of the gas (methane) into the pores and C deposition reaction. If the reaction is too fast, deposition mostly occurs at the surface and entrance of the pores, clogging the access to its interior and limiting the densification capabilities. By favoring the diffusion of the gas, deposition can occur from the interiors of the pores. In this work, this is achieved by diluting CH4 with N2, which slows the rate of reaction, allowing for the gas to first diffuse into the composite. However, this may cause processing time to increase, which is a trade-off to having a greater deposition coverage throughout the whole of the composite. Furthermore, the channels will be used as an additional pathway to flow the gas from the interior of the composite, which can assist in a better densification coverage. For example, a carbonized sample was densified at 1000 0C for only 30 hours using our new method. Scanning Electron Microscopy (SEM) imaging of the sample’s cross-section shows a layer of CVI carbon deposited at the surface of the channel. The density increased by more than 8%. This sample was leak tested, confirming that this new CVI method seals off the pores at the surface, leaving the channel intact.
References
[1] Cheng, Ceram. Int. 46 (13), 21395 (2020).
[2] Cordeiro et al. Comp Part A: App Science & Man, 1;180:108069 (2024).
[3] Esser Kahn et al. Adv. Mater. 23, 3654–3658 (2011).
