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.