Slip casting porous silicon nitride for high-temperature radar frequency radomes

Silicon nitride (Si₃N₄) is a promising candidate for ceramic radar frequency (RF) radomes. In this study, suspensions have been developed to slip cast Si₃N₄ for RF windows utilizing 6 wt.% yttrium oxide (Y₂O₃)-2 wt.% aluminum oxide (Al₂O₃) and 5–15 wt.% ytterbium oxide sintering aids. Suspensions were evaluated via rheology, zeta potential, and casting rate to determine the optimal ball milling time, concentration of polyethylenimine dispersant, and suspension pH for a low viscosity, high stability slip. Choice of sintering aids tailored porosity from 7% to 38% in slip-cast Si₃N₄ components after sintering at 1850°C, where the Y₂O₃–Al₂O₃ system had higher relative density compared with the Yb₂O₃ systems. Extending this work, layered structures of alternating Si₃N₄ compositions and hollow, curved Si₃N₄ radome shapes were also fabricated through slip casting. The layered structures had a graded microstructure and smooth interfaces between Si₃N₄ regions. The radomes were able to be cast and sintered crack-free. Overall, slip casting was demonstrated as a processing pathway to produce porous Si₃N₄ components desirable for RF and high-temperature applications.