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 Y 2 O 3 and MgO/MgSiN 2 as sintering additives were nitrided at 1400°C for 8 h under a N 2 pressure of 0.1 MPa, followed by postsintering at 1900°C for 6 h under a N 2 pressure of 0.9 MPa. The SRBSN with Y 2 O 3 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 MgSiN 2 . It is thought that MgSiN 2 decreased the amount of oxygen in the liquid phase formed during sintering, which resulted in reduced lattice oxygen content in the developed β‐Si 3 N 4 grains. In addition, the SRBSN with Y 2 O 3 and MgSiN 2 additive exhibited high fracture toughness of about 9 MPa m 1/2 . 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.