Microstructure and mechanical properties of the in situ [beta]-Si[sub 3]N[sub 4]/[alpha][prime]-SiAlON composite

The in situ [beta]-Si[sub 3]N[sub 4]/[alpha][prime]-SiAlON composite was studied along the Si[sub 3]N[sub 4]-Y[sub 2]O[sub 3]:9AlN composition line. This two-phase composite was fully densified at 1,780 C by hot pressing. Densification curves and phase developments of the [beta]-Si[sub 3]N[sub 4]/[alpha][prime]-SiAlON composite were found to vary with composition. Because of the cooperative formation of [alpha][prime]-SiAlON and [beta]-Si[sub 3]N[sub 4] during its phase development, this composite had equiaxed [alpha][prime]-SiAlON ([approximately]0.2 [mu]m) and elongated [beta]-Si[sub 3]N[sub 4] fine grains. The optimum mechanical properties of this two-phase composite were in the sample with 30--40% [alpha][prime], which had a flexural strength of 1100 MPa at 25 C, 800 MPa at 1,400 C in air, and a fracture toughness 6 MPa[center dot]m[sup 1/2]. [alpha][prime]-SiAlON grains were equiaxed under a sintering condition at 1,780 C or lower temperatures. Morphologies of the [alpha][prime]-SiAlON grains were affected by the sintering conditions.