Recent advances in forced-flow, thermal-gradient CVI for refractory composites

Chemical vapor infiltration (CVI) is simply chemical vapor deposition (CVD) on the internal surfaces of a porous preform and has been used to produce a variety of developmental and application materials. The greatest use of C VI is to infiltrate continuous-filament preforms taking advantage of the re latively low-stress CVD process. In CVI, reactants are introduced in the po rous preform via either diffusion or forced convection and the CVD precurso rs deposit the appropriate phase(s). As infiltration proceeds, the deposit on the internal surfaces becomes thicker. Thus, after some length of time, the growing surfaces meet bonding the preform and fill much of the free vol ume with deposited matrix. The forced-flow/thermal-gradient technique (FCVI ) developed at Oak Ridge National Laboratory overcomes the problems of slow diffusion and restricted permeability, and has demonstrated a capability t o produce thick-walled, simple-shaped, SiC-matrix components in times of th e order of hours. A model has been developed for the process that predicts flow, thermal and density profiles as a function of time. The results have been compared with an initial set of experiments and indicate qualitative a greement. It is expected that improved property relationships, such as perm eability and thermal conductivity as a function of density, will allow the model to closely represent the FCVI process and be useful in fabrication an d product optimization. (C) 1999 Published by Elsevier Science S.A. Ail rig hts reserved.