Effect of SiO2 content on the microstructure, mechanical and dielectric properties of Si3N4 ceramics

This study investigated the effect of SiO2 content in the Y2O3–Al2O3 additive system on the microstructure, mechanical and dielectric properties of silicon nitride (Si3N4) ceramics. The total sintering additive content was fixed at 8 wt% and the amount of SiO2 was varied from 0 to 7 wt%. The crystalline phases of the samples were determined by X-ray diffraction analysis. Complete α-to-β transformation of the Si3N4 occurred during sintering of all of the samples, which indicated that the phase transformation was unaffected by the SiO2 content. However, the microstructures showed that the aspect ratio of the β-Si3N4 grains decreased and the residual porosity increased with increasing SiO2 content. Additionally, the flexural strength and the dielectric constant decreased with increasing SiO2 content because of the residual porosity and the formation of the Si2N2O phase via a reaction of SiO2 with Si3N4.