The influence of the nitriding parameters on the microstructure and strength of the open-cell reaction bonded silicon nitride foams fabricated via wet processing

Abstract

In this study, the parameters which influence strength of the open-cell reaction bonded silicon nitride foams were investigated. These parameters include the monomer content in the suspension, the porosity level of the foam, the nitriding atmosphere including N2 and N2–4 %H2, and the nitriding temperature ranging from 1350 to 1425 °C. The nitriding mechanisms dominating under different nitriding conditions were also studied based on the phase and microstructural analysis. It was observed that there is a minimum monomer concentration of 25 wt% required in the premix solution to obtain a defect-free and homogeneous RBSN foam. Increasing the monomer content only from 15 to 20 wt% resulted in a threefold increase in the foam strength. The high porosity level of the foam which is above 70 vol% significantly affects the nitriding mechanisms and microstructures compared to those of dense RBSN ceramics. The maximum strength was obtained for the foams nitrided under N2–H2 atmospheres, and the nitriding temperature had a negligible effect on the foam strength when H2 is present in the atmosphere. α-Si3N4 is also the dominant phase in the microstructure in the presence of H2 regardless of the nitriding temperature. It was observed that β-Si3N4 can also be present in high quantities when N2 atmospheres are used. β-Si3N4 is present in the microstructures in two different morphologies including interlocking rods and angular grains. Each morphology forms based on a specific nitriding mechanism.