Mechanical and Dielectric Properties of Si3N4-Based Ceramics

Abstract

The effect of sintering-activating Y₂O₃ and SiO₂–Y₂O₃ additives on the mechanical and dielectric properties of Si₃N₄ and Si₃N₄–BN ceramics consolidated by spark plasma sintering was examined. The heating rate and applied pressure were maintained at 50°C/min and 35 MPa, respectively. The holding time at a sintering temperature of 1800°C varied depending on the composition of the oxide additives. The Si₃N₄–BN ceramics with Y₂O₃–SiO₂ additives exhibited a 30% reduction in mechanical properties (hardness and fracture toughness) compared to Si₃N₄–Y₂O₃ or Si₃N₄–Y₂O₃–SiO₂ ceramics. The Si₃N₄ ceramics demonstrated resistance to deformation at temperatures ranging from 20 to 900°C. Specifically, Si₃N₄ ceramics with Y₂O₃ or Y₂O₃–SiO₂ additives showed average strengths of approximately 950 and 820 MPa, whereas Si₃N₄–BN ceramics demonstrated a strength of 490 MPa. An increase in temperature from 1000 to 1400°C for all ceramics studied resulted in a gradual decrease in bending strength to approximately 200 MPa. The strength at room and elevated temperatures, Vickers hardness of approximately 4 GPa and 15.5 GPa, and fracture toughness of about 7.7 MPa · m^1/2 meet the current requirements for this type of ceramics. Radiofrequency measurements showed that dense Si₃N₄-based ceramics had a dielectric constant of 8. When 10 wt.% BN was added, the dielectric constant of the composite decreased by approximately 8%. Additionally, residual porosity of about 10% further decreased the dielectric constant of the Si₃N₄–BN composite by around 13% (ε ~ 6.3). This reduction in the dielectric constant had a positive effect on radio transparency. The dielectric loss tangent of the test ceramics did not exceed 2 · 10^–3.