Abstract: In order to deeply understand the forced-flow-thermal-gradient chemical vapor infiltration (FCVI) process for fabricating carbon-carbon composites, the model concerning pores in
preforms was established in this paper, the pores in preforms were composed of opened pores and closed pores, the CVI process is the closing process
of opened pores. On the basis of the chemical reaction dynamics and the heat and mass transmission theory, a new concept, the residual time of
densification (RTD), was put forward, from which a theoretical method was deduced to predict the densification efficiency of the process and control
the technique qualifications, based on this theory, the highest densification efficiency of FCVI process can be acquired when the density distribution of
performs accord with a certain mathematical relationship during CVI process. As a result, the relationship between the density distribution of preforms
and the process efficiency was analyzed comprehensively, and a variable factor, ψ, was defined to evaluate the efficiency of FCVI process.
The practical experiments showed that, if the ψ valued around 0 in any stage of the process, the highest densification efficiency of FCVI
process can be acquired. Guided by the presented method, the densification efficiency was improved to a satisfactory level in practical FCVI processes.

Key words: density gradient C/C, FCVI, residual time of densification （RTD）, efficiency of the FCVI