Dielectric constant and flexural strength of micro porous slip cast partially sintered silicon nitride

Slip casting of commercial Si3N4 powder dispersed in de-ionized water with Y2O3 and Al2O3 as sintering aids was carried out using optimum slurry conditions arrived through zeta potential and viscosity measurements. The cast specimens were sintered in graphite furnace in the temperature range of 1600 °C to 1800 °C under 0.15 MPa static N2 pressure. The sintered Si3N4 show density variation from 59% to 82% of theoretical value in the sintering temperature range studied. The 3-point flexural strength varied between 80 MPa and 320 MPa and dielectric constant 3.9 to 5.7 as a function of decreasing porosity from 39% to 17%. The samples exhibit uniform distribution of micro porosity in the range 0.5 μm–2.0 μm and grain size of 0.5 μm–3.0 μm. The microstructure changed from equiaxed and platelet shaped α-Si3N4 grains to needle shaped β- Si3N4 with increase in sintering temperature from 1600 °C to 1800 °C. The dielectric and flexural strength properties found promising for high temperature radome applications.