Mechanical and Dielectric Properties of Porous Slip Cast Silicon Nitride Using Ytterbium Oxide Sintering Aids

ABSTRACT Silicon nitride (Si ₃ N ₄ ) was slip cast with 5, 10, or 15 wt% ytterbium oxide (Yb ₂ O ₃ ). The 5 wt% Yb ₂ O ₃ samples were pressurelessly sintered at 1700°C–1850°C for 2 h. The porous β‐Si ₃ N ₄ microstructure varied from 65% to 81% relative density depending on the Yb ₂ O ₃ content and sintering temperature. The ∼81% dense 5 wt% Yb ₂ O ₃ samples sintered at 1850°C had a characteristic strength of 445 MPa and a Weibull modulus of 10.3. However, the high‐temperature strength of 5 wt% Yb ₂ O ₃ sintered at 1750°C ranged from 240 to 292 MPa at 1000°C. The room temperature flexural strength of 15 wt% Yb ₂ O ₃ with 32% porosity was 275 ± 5 MPa. Elastic modulus testing found the onset of intergranular glass softening occurred at decreasing temperatures between 770°C and 1070°C as Yb ₂ O ₃ content increased. Room temperature dielectric constants were measured by the focused beam method over 20–40 GHz between 5.75 and 4.17 for porosities ranging from 19% to 35%.