Use of Electrophoretic Impregnation and Vacuum Bagging to Impregnate SiC Powder into SiC Fiber Preforms

Abstract

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.

Microwave heated chemical vapour infiltration of SiC powder impregnated SiC fibre preforms

“…Consequently, if full advantage is to be taken of the inverse temperature profile that is achieved using microwave heating of preforms, it is essential to achieve homogeneous initial impregnation. There is some evidence that this can be achieved by combining the EPI and VB techniques 23…”

“…Not all the green samples prepared were subsequently processed by MCVI as some were sectioned after the powder impregnation to assess the microstructural powder distribution; additional details can be found elsewhere. 23 Table 3 lists the characteristics of the samples processed by MCVI. The system itself has previously been described elsewhere; 15 essentially, it consisted of a multimode, 2?45 GHz microwave cavity containing a quartz bell jar in which the sample was located sandwiched between two discs of high purity alumina foam (Fig.…”

“…where r f is the density of NL-202 Nicalon fibre (2?55 g cm 23 ), and r SiC is the density of stoichiometric SiC (3?21 g cm 23 ).…”

“…Not all the green samples prepared were subsequently processed by MCVI as some were sectioned after the powder impregnation to assess the microstructural powder distribution; additional details can be found elsewhere 23 Table 3. lists the characteristics of the samples processed by MCVI.…”