Electrophoretic deposition [EPD] applied to reaction joining of silicon carbide and silicon nitride ceramics

Electrophoretic Deposition (EPD) was used to deposit a mixture of SiC or Si 3N4 "filler" and reactive carbon (graphite and carbon black) particles onto various SiC or Si3N4 parts in preparation for reaction bonding. The partic les had gained a surface charge when mixed into an organic liquid consistin g of 90 w % acetone + 10 w % n-butyl amine to form a slurry. The charged pa rticles then moved when placed under the influence of an electric field to form a "green" deposit on the ceramic parts. The green parts were then drie d and subsequently joined using a reaction bonding method. In this reaction bonding, molten Si moves into the joint via capillary action and then diss olves carbon and precipitates additional SiC. An optimum mixture of SiC "fi ller" to C powder ratio of 0.64 was identified. Residual un-reacted or "fre e" Si was minimized as a result of selecting powders with well-characterize d particle size distributions and mixing them in batch formulas generated a s part of the research. Image analysis of resulting microstructures indicat ed residual "free" Si content as low as 7.0 vol % could be realized. Seven volume percent compares favorably with the lowest "free" Si levels availabl e in experimental samples of bulk siliconized (reaction-bonded) SiC manufac tured using conventional reaction-bonding techniques. The joints retained t he residual silicon over a large number of high-temperature thermal cycles (cycling from below to above the melting point of silicon). Comparisons to commercial reaction-bonded SiC indicated the majority of residual silicon o f the joint was retained in closed porosity. This infers that parts made wi th these joints might be successfully utilized at very high temperatures. I t was demonstrated that the EPD technique could be applied to butt, lap, an d scarf type joints, including the capability to fill large gaps or undercu t sections between parts to be joined. The overall results indicate that EP D, combined with reaction bonding, should allow for the fabrication of larg e complex structures manufactured from smaller components consisting of sil icon carbide or silicon nitride. (C) 2000 Kluwer Academic Publishers.