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

Abstract Silicon nitride (Si 3 N 4 ) is a promising candidate for ceramic radar frequency (RF) radomes. In this study, suspensions have been developed to slip cast Si 3 N 4 for RF windows utilizing 6 wt.% yttrium oxide (Y 2 O 3 )‐2 wt.% aluminum oxide (Al 2 O 3 ) 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 Si 3 N 4 components after sintering at 1850°C, where the Y 2 O 3 –Al 2 O 3 system had higher relative density compared with the Yb 2 O 3 systems. Extending this work, layered structures of alternating Si 3 N 4 compositions and hollow, curved Si 3 N 4 radome shapes were also fabricated through slip casting. The layered structures had a graded microstructure and smooth interfaces between Si 3 N 4 regions. The radomes were able to be cast and sintered crack‐free. Overall, slip casting was demonstrated as a processing pathway to produce porous Si 3 N 4 components desirable for RF and high‐temperature applications.