The microstructure development and mechanical properties of sintered Y-Si-Al-O-N prepared from two different crystalline phases of $Si_3N_4$ powder have been investigated. α'-SiAlON, β'-SiAlON and α'-β' two phase composites were prepared by hot-pressing $Si_3N_4$ and $Y_2O_3$ㆍ9AlN powder mixture at 1700, 1840 or 1940℃ under 20 MPa in a nitrogen atmosphere. The microstructures of hot-pressed specimens were observed by optical and scanning electron microscopy and fracture toughnesses were measured by an indentation method.
The microstructure of hot-pressed β'-SiAlON was highly anisotropic and had broad distribution of grain size when using $α-Si_3N_4$, in contrast to relatively isotropic microstructure and narrow size distribution of grains when using $β-Si_3N_4$. This difference in microstructure seems to result from preferential nucleation of β' grains on preexisting $β-Si_3N_4$ particles in raw powder mixture. In case of α'-β' two phase composite, a similar dependence of microstructure development on starting $Si_3N_4$ powder was observed. The nucleation of SiAlON grain on preexisting grains of similar phase was supported by cored structures observed in hot-pressed specimens.
In case of α'-SiAlON, broad grain size distribution was obtained by using $α-Si_3N_4$ starting powder, in contrast to narrow size distribution when using $β-Si_3N_4$. This result cannot be explained by a preferential nucleation of SiALON on a preexisting similar phase. It seems that the miscibility gap between α and α' inhibited the nucleation of α' on preexisting $α-Si_3N_4$ and the nucleation of α'-SiAlON occurred only in a glass matrix.
The fracture toughness of α'-SiAlON by using $β-Si_3N_4$, and β'-SiAlON and α'-β' composite by using $α-Si_3N_4$ was enhanced. In β'-SiAlON and α'-β' composite, the crack propagation was inhibited by large grains with high aspect ratio, but in α'-SiAlON, it was not inhibited by large grains resulting in lower fracture toughness.
Y-SiAlON 계에서 출발상을 각각 α-, $β-Si_3N_4$로 달리하여 α'-, β'-SiAlON, α'-β' 복합체를 제조하였다. $β-Si_3N_4$를 출발상으로 한 경우에 비해 $α-Si_3N_4$를 출발상으로 사용하는 경우 β'-SiAlON, α'-β' 복합체는 입도 분포가 넓고 주상의 입자가 발달한 미세구조를 나타내었다. $α-Si_3N_4$를 출발상으로 사용한 α'-SiAlON은 입자 크기가 10 ㎛ 이상인 주상의 입자와 1-3 ㎛의 등축의 입자가 흔재해 있는, 입도 분포가 넓은 미세구조를 나타내었고 $β-Si_3N_4$를 출발상으로 한 α'-SiAlON은 비교적 좁은 입도분포를 가진 미세구조를 나타내었다. 파괴인성치는 입자길이가 5 ㎛ 내외이고 장경비가 10 이상으로 큰 입자를 많이 가진 $α-Si_3N_4$를 출발상으로 사용하는 α'-β'복합체일 때 가장 높았다.
