The mixture of α-$Si_3N_4$ and Si powder was used to investigate effects of unreacted Si on α/β transformation densification process, and strength of Sintered Reaction Bonded Silicon Nitride.
In order to avoid nitridation of Si, α-$Si_3N_4$ and Si mixture compacts were heat-treated in Ar. β-fraction increased with Si additions and heat-treating time sigmoidally at 1700℃ in Ar. As β-fraction increased, morphology of equiaxed α$-Si_3N_4$ changed elongated. Also, large β phase grown into Si was observed. In case of no Si addition to $Si_3N_4$, α/β transformation was not occurred and morphology had not changed.
During heat-treating of RBSN, the same results were obtained, but significant decomposition of $Si_3N_4$ was observed because of high surface energy of α-needles and α-matte.
During sintering of RBSN, amount of Si increased with sintering time and sintering temperature. But this phenomenon was not occurred significantly in Sintered Silicon Nitride.
The Sintered specimens with and without 5wt% Si addition had 53kg/㎟ and 73kg/㎟ in three point bending strength, respectively.