Fabrication of pressureless sintered Si3N4 ceramic balls by powder injection molding

Abstract

Pressureless sintered Si3N4 ceramic balls were fabricated by powder injection molding technology. Yttria and alumina as a sintering aid were added to improve the sintered density. The mixing was performed with twin-screw extrude mixer to fabricate the homogeneous feedstock. The rheological behavior of Si3N4 feedstock was analyzed with Power Law-Arrhenius model and moldability index. With the optimized injection molding parameters, green parts without defects were obtained. A two-step debinding process was applied to remove the binders in the green parts. Solvent debinding was firstly conducted in n-hexane at 50 °C for 5 h. The optimized thermal debinding was secondly conducted in air atmosphere based on TGA experiment results. The sintering was conducted in nitrogen atmosphere at 1750 and 1800 °C for 4 h. Embedding powder (Si3N4:BN = 1:1) was used to avoid dissociation of silicon nitride. The balls with 5 mm diameter were finally fabricated with no defects. The relative density and mechanical properties including 3-point bending strength, Vickers hardness and fracture toughness had a higher value at the sintering temperature of 1800 °C than at 1750 °C. It indicates that the Si3N4 ceramic balls with high physical and mechanical properties can be successfully fabricated by optimized PIM process.