Effects of Impurity Oxygen Content in Raw Si Powder on Thermal and Mechanical Properties of Sintered Reaction-Bonded Silicon Nitrides

Sintered reaction-bonded silicon nitrides (SRBSNs) were fabricated from commercially available low-grade Si powder containing 1.6 mass% of impurity oxygen and 500 ppm of metallic impurities. Powder compacts of the raw Si powder doped with Y2O3 and MgO/MgSiN2 as sintering additives were nitrided at 1400 degrees C for 8 h under a N2 pressure of 0.1 MPa, followed by postsintering at 1900 degrees C for 6 h under a N2 pressure of 0.9 MPa. The SRBSN with Y2O3 and MgO as sintering additives had a four-point bending strength of about 700 MPa, but lower thermal conductivity of about 89 W/m/K. Thermal conductivity could be improved to over 100 W/m/K without degrading the bending strength by replacing MgO with MgSiN2. It is thought that MgSiN2 decreased the amount of oxygen in the liquid phase formed during sintering, which resulted in reduced lattice oxygen content in the developed beta-Si3N4 grains. In addition, the SRBSN with Y2O3 and MgSiN2 additive exhibited high fracture toughness of about 9 MPa m1/2. These good properties were equivalent to those of the SRBSN fabricated by using a reagent-grade high-purity Si powder containing only 0.3 mass% of impurity oxygen.