Use of electrophoretic impregnation and vacuum bagging to impregnate SiC powder into SiC fiber preforms

Techniques based on vacuum bagging (VB) and electrophoretic impregnation (EPI) have been investigated for the impregnation of SiC powder into layered Nicalon SiC fabric preforms. The aim was to produce preimpregnated samples for subsequent chemical vapor infiltration (CVI) with reduced intertow porosity that arises from the construction of the fabric layers while leaving unblocked the intratow porosity that is so indispensable for a successful infiltration. Because the goal was simply to learn about the ability to impregnate the samples, no interphase coating was applied to the fibers as would normally be used when producing SiC_f/SiC composites. While the VB process generally yielded much stronger preforms, depending on the pressure used and the powder particle size, it resulted in powder becoming located in the intratow rather than the intertow porosity. In contrast, provided an appropriate electrode arrangement was used, EPI offered the potential for a more controlled impregnation process with the powder primarily found in the intertow porosity; however, the preforms were very weak and delaminated easily. The combination of the two processes resulted in a very successful approach, with greater uniformity of particle infiltration and higher green strengths, while largely avoiding impregnating the intratow porosity.