Size effect of raw Si powder on microstructures and mechanical properties of RBSN and GPSed-RBSN bodies

Size effect of raw Si powder on microstructures and mechanical properties of reaction-bonded Si3N4 (RBSN) and of GPSed-RBSN bodies was investigated using Si powder of 2 and 7 μm in average size. The percent nitridation of RBSN body at 1410 °C using 2 μm Si powder was 80.5%, which was lower than that of using 7 μm Si powder (94.2%). In the GPSed-RBSN body sintered at 1850 °C for 3.5 h using 7 μm Si powder, many large, rod-like Si3N4 grains were formed relative to the specimen using 2 μm Si powder. This was caused by the heterogeneous distribution of liquid phase as increasing the size of raw Si powder. The fracture strength and fracture toughness of GPSed-RBSN body using 7 μm Si powder were, respectively, 865 MPa and 6.5 MPa m1/2, which were higher than those of using 2 μm Si powder with 633 MPa and 4.8 MPa m1/2. The main reason for the increase in mechanical properties of GPSed-RBSN body using 7 μm Si powder was due to the formation of large, rod-like Si3N4 grains, which promoted the remarkable crack bridging and crack deflection on the fracture surfaces.