Microwave Sintering of Silicon Nitride with Zirconia as a Secondary Additive

Abstract

The effect of 3 mol % Y2O3 stabilized ZrO2 in microwave sintered Si3N4 was investigated in the present study. A Finite Difference Time Domain (FDTD) numerical technique was used to estimate the microwave power absorption and temperature distribution within Si3N4 samples with MgO and Y2O3 sintering aids and different levels of ZrO2 during microwave sintering. Higher temperatures were developed in the sample containing the highest dielectric loss additive (4 wt% MgO, 6 wt% Y2O3 and 2.5 wt% ZrO2) when compared to the lower loss additive (4 wt% MgO and 6 wt% Y2O3) or pure Si3N4. The numerically calculated temperatures within the samples were used to correct the values experimentally measured by an optical pyrometer, which were 75-125 ºC lower. Isothermal densification and α → β phase transformation rates of Si3N4 containing 4 wt% MgO, 6 wt% Y2O3 and between 0 and 5 wt% ZrO2 were examined. An addition of 2.5 wt% ZrO2 enhanced the densification rate during microwave sintering. Complete α→β phase transformation occurred at a lower temperature during microwave sintering than conventional sintering. Grain boundary phases of microwave sintered samples were characterized by transmission electron microscopy (TEM). Small ZrO2 grains (d<200 nm) were remained dispersed in the matrix. Both tetragonal and cubic Zr-rich phases were identified by selected-area electron diffraction. Additions of ZrO2 improved the hardness of Si3N4. However, no variation in fracture toughness was observed despite differences in microstructures. The strong bonding between the grains and grain boundary phase resulted in predominantly transgranular crack propagation paths.