Gradient Porous Silicon Nitride by Slip Casting and Vacuum Foaming

Gradient porous silicon nitride was fabricated by a combination method of slip casting and vacuum foaming. Firstly, foaming agent was added into Si3N4 aqueous slurry, then the uniformly stirred slurry was poured into the mold with plaster as base. Under low pressure and steady temperature, a green body with gradient pores was formed. Based on our research, the gradient structure and porosity were greatly influenced by the vacuum pressure. With the pressure decrease from 80 kPa to 9 kPa, the porosity increased from 59.01% to 80.85% and the gradient structure became more noticeable. When the pressure was 80 kPa, the sample obtained a uniform pore structure. After the pressure being decreased to 9 kPa, the sample obtained dispersed pore size distribution including values of 0.5, 20, 30, 40, 60, 100 and 200 μm measured through mercury intrusion method. Gradient porous ceramics with adjustable porosity and pore size distribution, as well as achievable complex shape and large scale, may fabricate through this simply and low cost method.