The influence of granulation on the gelcasting of pressureless-sintered silicon carbide ceramics

Gelcasting combined with pressureless sintering is a promising technological process to fabricate large scale or complex-shaped silicon carbide ceramics (SiC) products with excellent properties. To obtain high solid loading SiC slurry with low viscosity, two promising compact granulation and spray granulation methods were proposed to coarsen raw submicron SiC powders into a larger secondary granule. By compact and spray granulation processes, the average particle sizes were enlarged from 0.51 μm to 40.7 μm and 8.2 μm, respectively. The enlarged particles decreased the volume of structural water in the slurries remarkably, resulting in the decrease in the viscosity of slurry. The viscosities of slurry prepared from spray-granulated powders and compact-granulated powders were 1/2.5 and 1/3.5 to that from submicron powder at a shear rate of 100 s−1, respectively. The lower viscosities benefited to the escaped air bubble in slurries, and leaded to a dense microstructure in green and sintered bodies. The bending strengths of green bodies and sintered bodies prepared from granulated powders were about 3 times and 2.2 times to that from submicron powders. It is worth to notice that even though the dried spray-granulated powders with sphere-like appearance displayed higher tap density and well fluidence, the slurry viscosity was 58 Pa s higher than that of compact-granulation powders due to the hollow particle structure. As a result, compact granulation powders showed desired slurry fluidity and dense green and sintered bodies, thus high bending strength.