Effects of Impurity Iron Content on Characteristics of Sintered Reaction‐Bonded Silicon Nitride

The effects of impurity iron content on characteristics of sintered reaction‐bonded silicon nitrides were examined by adding iron powder to a high purity raw Si powder. Powder compacts of the raw Si powder doped with 2 mol% Y 2 O 3 and 5 mol% MgSiN 2 as sintering additives and Fe as impurity (0 mass%, 0.1 mass%, 1.0 mass% and 5.0 mass%) were nitrided at 1400°C for 8 h under a N 2 pressure of 0.1 MPa, followed by post‐sintering at 1900°C for 6 h under a N 2 pressure of 0.9 MPa. All the SRBSN (Sintered Reaction‐Bonded Silicon Nitride) specimens had about the same 4‐point bending strength of 730–770 MPa. The fracture toughness of the specimens was gradually decreased with increasing Fe additive amount due to the inhibition of development of rodlike β‐ Si 3 N 4 grains by SiFe x particles formed during nitridation process. The thermal conductivity was also decreased with an increase in Fe amount. It seems that the increasing oxygen in grain‐boundary phase caused by the oxidation of Fe during milling resulted in the increase in lattice oxygen of β‐ Si 3 N 4 grains, which caused phonon scattering and thereby decreased thermal conductivity of β‐ Si 3 N 4 . There was little change in the dielectric breakdown strength of the specimens: 24, 22, 22, and 21 kV/mm for the specimens without Fe , and with 0.1 mass%, 1.0 mass% and 5.0 mass% Fe , respectively. The surface resistivity of the specimens with 0 mass%, 0.1 mass% and 1.0 mass% Fe was in the range of 10 13 Ω, but the specimen with 5 mass% Fe was about one order lower than the others.