Preparation and process optimization of C/C composites based on the multi-channel flow field and thermal field control

Fiber-reinforced composite materials are widely used in aerospace due to high-temperature resistance and high specific strength. The popular chemical vapor infiltration (CVI) method slowly penetrates the porous preform through a gas source and decomposes the carbon source gas into pyrolytic carbon for deposition. This method offers the advantages of less fiber damage and designability of special-shaped components; however, the CVI preparation cycle is long, and the pore blockage on the fiber preform surface makes the densification of the porous preform uneven. To effectively improve the controllability and uniformity of the deposition of porous composite materials, a dual-process chemical vapor infiltration/deposition (CVI/CVD) system bases on a dual-temperature zone-dual-channel structure is proposed in this study, with strong designability and manufacturability and an improved CVI.

Bases on the influence of gas flow direction and temperature on the deposition process, CVD is introduced to combine the two parameters, thus improving the densification uniformity while shortening the fabrication cycle. Since the complex transport and reaction of carbon source and carrier gases is a multivariate multi-coupling process with a great impact on the deposition efficiency and uniformity, numerous modeling and experimental studies are required. In this paper, the process design and system optimization are conducted to generate carbon/carbon composites (C/C composite) bases on the integrated research idea of equipment design-build-theory-preparation-optimization. By establishing flow, heat transfer, and mass transfer reaction models, the effects of temperature, velocity, and concentration on the densification process are analyzed. The two-step deposition simulation of the porous composites is conducted via the dual-process CVI/CVD system.

The results showed that reducing the deposition temperature could reduce the deposition rate, avoid saturated adsorption on the preform surface before complete deposition, and improve the deposition uniformity in the thickness direction. Increasing the temperature to increase the reaction rate could easily lead to surface deposition, causing the surface pores to close and block, thus hindering further deposition. The deposition position could be controlled by changing the spatial temperature gradient. For the same temperature gradient, the higher the control temperature, the larger the main deposition area; for a higher control temperature, the main deposition area decreased with the increasing temperature gradient. When the time for the reaction carbon source gas and the carrier gas to be transported to the deposition area was equal to that for the deposition reaction to consume the atmosphere source, the initial velocity matched the concentration, improving the densification efficiency. The two-step deposition simulation results showed that the porous preforms achieved a relatively uniform density in horizontal and vertical directions after the two-step CVI/CVD.

By combining the two processes, a dual-temperature zone-dual-channel CVI/CVD system is constructed. A mathematical and a physical model of densification are established, based on the basic theories of chemical reaction kinetics and heat and mass transfer to perform the variable parameter simulation experiment. The flow and thermal field control process optimization design is performed for the prepared C/C composite material, verifying the feasibility of the uniformity control and the process optimization of the C/C composite material deposition sample and providing significant guidance to conduct the future experiments.