Experimental Evaluation of a New Flocculation-Filtration Model for Ceramic Shape Forming Processes

The effect of suspension rheology on the casting mechanics of 37 vol% aqueous silicon carbide slips was examined. Slip viscosities of 180, 120, 90, and 58 mPa.s were evaluated. The 58 mPa.s slip was considered to be fully deflocculated; the others were underdeflocculated to varying degrees. The fully deflocculated slip cast according to parabolic rate behavior, while the others cast at a faster than parabolic rate. Deviations from parabolic rate behavior were correlated with a nonuniform cake structure observed through ultrasound and mercury porosimetry measurements. The observed casting behavior was explained on the basis of a suspension flocculation-filtration model. From rheological measurements, it was found that flocculation of the SiC particles during casting in the underdeflocculated suspensions caused the local void volume within the forming cake to increase as a function of cake thickness. The fully deflocculated slip was rheologically stable, and therefore cast with a uniform microstructure.