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 (Si 3 N 4 /Si 2 N 2 O) 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 2 N 2 O was formed via the reaction between FAC and Si 3 N 4 . Microstructural analysis revealed that spherical‐shaped pores obtained from FAC were uniformly dispersed in the composite. Si 3 N 4 /Si 2 N 2 O composite ceramic foams with apparent porosities of 62.0%–67.4% and a bending strength of 21–39 MPa were obtained eventually.