The influence of α- and β-Si3N4 seeds on the properties of reaction bonded silicon nitride foams produced via wet processing

Abstract

This work presents the influence of α- and β-Si3N4 seeds on the properties of reaction bonded silicon nitride (RBSN) foams fabricated via wet processing of Si–PMMA–Si3N4 suspensions. Characterization of the seeded foams was performed in terms of porosity, density, and mechanical strength. Phase and microstructural analysis were also used to investigate the influence of α- and β-seeding on the α/β phase ratio and microstructure of the RBSN foams.It was observed that both α- and β-seeds can improve the foam strength by 30% and 85%, respectively, due to more uniform nitridation. A foam strength of 2.23MPa was obtained for RBSN foams seeded with 5wt% α-Si3N4 with a foam porosity of 72vol%. Higher contents of α-seeds create a loose structure of the Si3N4 seed particles within the nitrided matrix resulting in a considerable decrease in foam strength. The addition of 10wt% of β-Si3N4 resulted in the maximum strength of 2.31MPa at a porosity level of 72vol%. The addition of β-seeds also resulted in an increase in the vapor phase reactions during nitridation leading to a significant increase in the β-whisker content of the microstructure. It was observed that even 30wt% of β-seed addition increases the strength of the RBSN foams compared to the unseeded foams.