Fabrication and characterizations of high density Si3N4 - ZrO2 ceramics

Abstract

Silicon nitride and zirconia were mixed with SiO2, MgO, Y2O3 as sintering additives and pressureless sintered at 1650℃ in N2 atmosphere for 2 h. The XRD results showed a-Si3N4 was partially transformed to b-Si3N4 with 3:3:5 weight ratio of SiO2 : MgO : Y2O3. However, at 5 wt% of ZrO2 addition promoted phase transformation of a-Si3N4 to b-Si3N4 while 35 wt% of ZrO2 completely transformed to b-Si3N4 phase. Si3N4 has a lower density than ZrO2, bulk density of samples increases in correlation with the amount of ZrO2. Because there was no difference in hardness and flexural strength between sintered Si3N4 samples with hardness of 13.41 GPa and 648.13 MPa along with increasing ZrO2 variation up to 55 wt%. Furthermore, with 75 wt% ZrO2, the hardness was reduced to 10.57 GPa and the flexural strength decreased to 208.16 MPa. SEM images of Si3N4 samples demonstrated the dense microstructure and 5 wt% ZrO2 showed homogeneous ZrO2 distributed among the Si3N4 grains. As a result, the hexagonal rod-like form of b-Si3N4 is clearly visible in 75 wt% ZrO2. Therefore, Si3N4 with ZrO2 can be sintered with the homogeneous microstructure of the a-Si3N4 to b-Si3N4 transformation and tolerable mechanical properties vary with ZrO2 content.