Effect of Sintering Aid Composition and Porosity on the Oxyacetylene Torch Response of Silicon Nitride for RF Windows

While silicon nitride (Si 3 N 4 ) is a promising candidate for radar frequency (RF) radomes, there are limited data available for performance under high‐temperature, high heat flux conditions relevant to hypersonic flight. In this study, flat plates of Si 3 N 4 were slip cast using 6 wt.% yttrium oxide (Y 2 O 3 )/2 wt.% aluminum oxide (Al 2 O 3 ) or 5–15 wt.% ytterbium oxide (Yb 2 O 3 ) sintering aids. Sintered billets ranged from 10.5% to 40% porosity. These were compared to a fully dense Si 3 N 4 commercial standard hot‐pressed with Y 2 O 3 –Al 2 O 3 . Torch testing was performed by an oxyacetylene flame for 60 and 240 s to evaluate a generic boost‐glide trajectory with a heat flux of 200 W/cm 2 . The physical, thermal, and microstructural responses of Si 3 N 4 were investigated. Front‐face temperature increased with porosity, from 1464°C when fully dense to a maximum of 2457°C with 40% porosity and 15 wt.% Yb 2 O 3 sintering aids. Surface roughening, spectral emissivity, secondary phase crystallization at the surface, and microstructural features of the surface oxide scale were assessed. As Yb 2 O 3 content increased, a transition to a liquid surface resulted in greater recession, spallation, and volatilization compared to the solid‐state surface of the Y 2 O 3 –Al 2 O 3 samples. These results inform design considerations for porous Si 3 N 4 components operating in high‐temperature applications.