Densification of C/C composites using film boiling chemical vapor infiltration with two heaters

Two-dimensional needle-punched carbon fiber felts were densified using film boiling chemical vapor infiltration with heaters above and below the felt and using xylene pyrolysis at 900-1200°C. The pyrocarbon deposition rate was calculated from the mass gain of the composites. The density of the composites and the thickness of the pyrocarbon were measured by the Archimedes method and polarized light microscopy, respectively. The effects of deposition temperature and heater configuration on the densification behavior of the composites were investigated. Results showed that C/C composites with a density of 1.70-1.73 g/cm³ were produced after 30-35 h densification. The deposition front thickness and initial deposition rate of the pyrocarbon increased with deposition temperature from 900-1000°C to 1100-1200°C. However, the difficulty of precursor transfer increased at higher deposition temperatures due to a more rapid densification at the preform edge, leading to a density decrease of the composites from 1.72-1.73 g/cm³ at 900-1000°C and 1000-1100°C to 1.70 g/cm³ at 1100-1200°C if upper heater was not used. Moreover, the densification uniformity of the composites decreased, and their density gradients along both the axial and radial directions were larger than 0.04 g/cm³. When the upper heater with many holes along its axial direction was used, the thickness of the deposition front decreased whereas the mass transfer efficiency of precursor into the preform increased, which had the advantage of improving the density and uniformity of the composites for the higher deposition temperature of 1100-1200°C.