Mechanical and Dielectric Properties of Porous Slip Cast Silicon Nitride: Yttrium Oxide‐Aluminum Oxide Sintering Aids

ABSTRACT Porous silicon nitride (Si 3 N 4 ) is a candidate for high‐strength and low‐dielectric constant radar frequency radomes of high‐speed flight vehicles. In this investigation, Si 3 N 4 was fabricated via slip casting a suspension with 6 wt.% yttrium oxide and 2 wt.% aluminum oxide sintering aids, followed by pressureless sintering at 1850°C for 2 h. The density of the resulting porous b‐Si 3 N 4 microstructure varied between 71% to 93% depending on the pH of the slip. The 93% dense samples had a characteristic strength of 710 MPa and a Weibull modulus of 5.9. The high‐temperature strength ranged from 445 to 498 MPa at 1200°C, and the elastic modulus measured by impulse excitation dropped from 268 GPa at 25°C to 235 GPa at 1200°C. The focused beam method found that the room temperature dielectric constant measured between 20 and 40 GHz decreased from 7.6 at 4% porosity to 5.99 at 17% porosity.