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

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 2.23 MPa was obtained for RBSN foams seeded with 5 wt% α-Si3N4 with a foam porosity of 72 vol%. 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 10 wt% of β-Si3N4 resulted in the maximum strength of 2.31 MPa at a porosity level of 72 vol%. 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 30 wt% of β-seed addition increases the strength of the RBSN foams compared to the unseeded foams.