Microstructure and Mechanical Properties of the in situ β‐Si 3 N 4 /α′‐SiAlON Composite

The in situ β‐Si 3 N 4 /α′‐SiAlON composite was studied along the Si 3 N 4 –Y 2 O 3 : 9 AlN composition line. This two phase composite was fully densified at 1780°C by hot pressing Densification curves and phase developments of the β‐Si 3 N 4 /α′‐SiAlON composite were found to vary with composition. Because of the cooperative formation of α′‐Si AlON and β‐Si 3 N 4 during its phase development, this composite had equiaxed α′‐SiAlON (∼0.2 μm) and elongated β‐Si 3 N 4 fine grains. The optimum mechanical properties of this two‐phase composite were in the sample with 30–40%α′, which had a flexural strength of 1100 MPa at 25°C 800 MPa at 1400°C in air, and a fracture toughness 6 Mpa·m 1/2 . α′‐SiAlON grains were equiaxed under a sintering condition at 1780°C or lower temperatures. Morphologies of the α°‐SiAlON grains were affected by the sintering conditions.