MEASUREMENT OF GAS-TRANSPORT THROUGH FIBER PREFORMS AND DENSIFIED COMPOSITES FOR CHEMICAL-VAPOR INFILTRATION

Gas transport via pressure-driven permeation or via concentration-driv en diffusion is a key step in the chemical vapor infiltration (CVI) pr ocess. This paper describes methods for the measurement of these prope rties for CVI preforms and partially infiltrated composites. Results a re presented for Nicalon(TM)-fiber cloth layup preforms and composites , Nextel(TM)-fiber braid preforms and composites, and a Nicalon-fiber three-dimensional (3-D) weave composite. The permeability of Nicalon c loth layup preforms is strongly dependent on the packing density over the range of 29-40 vol% but is only weakly dependent on the orientatio n of the alternating cloth layers. The specimen-to-specimen variation is high (standard deviation of 20%), which reflects the statistical na ture of the interlayer separation and alignment. This variability also affects the results for the partially densified specimens, where the permeability values for specimens of similar density can be very diffe rent, Trends in the diffusion factor results are similar to those for permeability, which reflects the fact that both transport parameters a re dependent on the structure of the same pore network, although in so mewhat different ways, The permeability of Nextel braid preforms is de pendent on the thread count and the weight for cloths with similar con struction and packing density. The gas permeability of the finer wave (6.3 tows/cm (16 tows/in.)) is approximately one-half that of the coar ser weave (3.5 tows/cm (9 tows/in.)). Results are reported for a small number of infiltrated composites with Nextel fiber reinforcement, Att empts to mount a Nicalon-fiber 3-D weave preform specimen have been un successful. Results for a small number of composite specimens with 3-D weave reinforcement are reported.