Secondary crystalline phases and mechanical properties of heat-treated Si3N4

The silicon nitrides containing various amount of Yb2O3 were sintered at 1800 °C and a pressure of 25 MPa for 1 h. The fracture toughness of silicon nitride improved with the content of Yb2O3. XRD and transmission electron microscopy (TEM) showed that a Yb4Si2O7N2 phase appeared when 10 wt.% of Yb2O3 was added. Then, a heat-treatment of 6 h was made on the ceramics. Compared with the as-sintered Si3N4, the fracture toughness of the post-heat-treated Si3N4 increased with addition of 6 or 8 wt.% Yb2O3, but the fracture toughness did not increase with addition of 4 or 10 wt.% Yb2O3. Based on SEM analysis, the increase in fracture toughness was due to the formation of a microstructure which contained small matrix grains with some exaggerated grains. In the silicon nitride containing 10 wt.% Yb2O3, the grain boundary crystalline phase was changed from Yb4Si2O7N2 to Yb2SiO5, which resulted in an adverse influence in the fracture toughness.