Preparation of Si3N4 ceramics by aqueous gelcasting using non-toxic agar powder as gelling agent without cooling crosslink process

Abstract

Abstract A non-toxic and low-cost gelcasting system was successfully developed using agar as gelling agent to fabricate silicon nitride ceramic green bodies without cooling crosslink process, and silicon nitride ceramic was obtained by pressureless sintering. The effects of agar contents on rheological behavior of silicon nitride suspension, microstructure and mechanical properties of silicon nitride ceramics were investigated. The silicon nitride ceramic suspension with 45 vol% solid loading using 1.2 wt% ammonium polyacrylate (PAA-NH4) as dispersant was successfully prepared for gelcasting. After removing binder at 800 °C for 2 h and pressureless sintering at 1800 °C for 2 h, the silicon nitride ceramics exhibited homogeneous and interlocking microstructure composed of elongated β-Si3N4 phase, which contributes to its mechanical properties. With the increase of agar powder content from 0.25 wt% to 1.5 wt%, the shrinkage and flexural strength of the silicon nitride ceramics increased firstly and then decreased. When the agar powder content was 0.5 wt%, the silicon nitride ceramics had the optimized flexural strength (218 ± 25 MPa) with relative density of 92.85 ± 2.19% and the shrinkage of 16.03 ± 0.15%. Using agar as gelling agent to prepare silicon nitride ceramics by gelcasting is believed to be a novel and low-cost method, which could be used to prepare other high performance ceramics in the future.