The influence of thermal gradient on pyrocarbon deposition in carbon/carbon composites during the CVI process

A thermal gradient CVI process was investigated. A graphite heater in the center of a carbon felt disk preform was heated by Joule heating to a temperature of 900 °C, the carbon felt had a low thermal conductivity, and the rapid natural gas flow cooled the exterior surface of the preform. The rate constant of the chemical vapor deposition reaction increased exponentially with increasing temperatures with pyrocarbon being formed only in the designated deposition zone. Plugging of surface pores in the preforms, which often occurs in the isothermal CVI technology was unusual in this thermal gradient CVI process. As the deposition process went on, the deposited zone moved progressively towards the outside surface of the preform. The electrical resistance between two electrodes decreased gradually while the power of the thermal gradient CVI furnace increased non-linearly with the progressive densification. The temperature distribution in the thermal gradient furnace changed non-linearly with time and position. The relationship between temperature and position in the deposition zone followed the classical Fourier law. The microstructure of pyrocarbon at different positions was discussed.