The effect of gelcasting parameters on microstructural optimization of porous Si3N4 ceramics

In this article we are reporting the optimization of gelcasting process which enabled us to manufacture silicon nitride bodies with around 30-37% porosity and 182-250 MPa flexural strength. Owing to their potential applications mainly in biomedical and aerospace, porous Si3N4 ceramics have gained ever increasing interests. However, the main challenge has been ensuring their high strength while maintaining high porosity. Si3N4 bodies were prepared via a controlled gelcasting followed by pressureless sintering in a coke bed. Monomers including acrylamide (AM) and N, N -methylenebisacrylamide (MBAM) were employed to formulate the primary slurry. Sub-micron Si3N4 powder of 35 vol% was used as solid loading where ammonium persulfate (APS) and N, N, N, N -tetramethylethylenediamine (TEMED) were added to initiate and catalyze polymerization reactions. Sintering was carried out at 1650 degrees C for 2-6 h and at 1750 degrees C for 2 h where 6 wt% mixture of Al2O3 - Y2O3 was included as a sintering aid. Phase characterization and microstructural evolution were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively; while mechanical evaluation was based on bending test. It was found that by optimizing the viscosity of slurry and the idle time of gelation, a well distribution of high porosity structure is developed. The high strength of the sintered bodies is related to a high volume and unique microstructure of fine and elongated beta- Si3N4 grains evolved during optimized sintering conditions. At temperatures above 1650 degrees C and sintering times of more than 4 h, the strength is decreased due to coarsening of beta- Si3N4 grains, however.