Novel method for fabrication of silicon nitride/silicon oxynitride composite ceramic foams using fly ash cenosphere as a pore-forming agent

Silicon nitride/silicon oxynitride (Si₃N₄/Si ₂N₂O) ceramic foams with tailored microstructures were prepared by a gelcasting process using fly ash cenosphere (FAC) as a pore-forming agent. The slurry has a suitable viscosity so that the floating velocity of FAC in the slurry is slow enough to avoid delamination between the FAC and the matrix during consolidation. Experimental observation showed that a uniform distribution of FAC was achieved in the matrix. Sintering was carried out at 1500°, 1600°, and 1700°C, respectively, for 1 h under a nitrogen atmosphere. X-ray diffractometry analysis indicated that Si ₂N₂O was formed via the reaction between FAC and Si ₃N₄. Microstructural analysis revealed that spherical-shaped pores obtained from FAC were uniformly dispersed in the composite. Si₃N₄/Si₂N₂O composite ceramic foams with apparent porosities of 62.0%-67.4% and a bending strength of 21-39 MPa were obtained eventually.