Preparation of porous SiC-Al2O3 ceramics via gelcasting utilising a shrinkable pore-forming agent and oxidised coarse-grained SiC

By utilising soaked millet as a shrinkable pore-forming agent, porous silicon carbide-alumina (SiC-Al2O3) ceramics were prepared via gelcasting. The fabrication of SiC-Al2O3 ceramics based on oxidised and unoxidised coarse-grained SiC was also studied. The water swelling, drying shrinkage, and low-temperature carbonisation of the millet were investigated. We found that the shrinkage of the soaked millet was greater than that of gel body during drying, which left large gaps that prevented shrinkage stresses from destroying the gel body. Low-temperature carbonisation of the millet should be performed slowly at 220–240 °C because its expansion rate increases to 45% at 250 °C, resulting in the cracking of samples. At a constant sintering temperature, the flexural strength of the SiC-Al2O3 ceramics prepared with SiC powders oxidised at 1000 °C was the highest, indicating that oxidised powders can successfully decrease the required sintering temperature and improve the flexural strength of composite ceramics. Based on our optimised process, porous SiC-Al2O3 ceramics were sintered at 1500 °C for 2 h. When their skeletons were fully developed, their pore sizes were in the range of 1.5–2 mm. Their porosity and flexural strength were 60.2–65.1% and 8.3–10.5 MPa, respectively.