MECHANICAL AND DIELECTRIC PROPERTIES OF Si3N4-BASED CERAMICS

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 degrees C/min and 35 MPa, respectively. The holding time at a sintering temperature of 1800 degrees 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 degrees 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 degrees 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% (epsilon similar to 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).