Effect of Pyrolysis Temperature on Properties of Porous Si3N4-BN Composites Fabricated Via PIP Route

The Si3N4-BN composites have been prepared via die pressing and the precursor infiltration and pyrolysis route using borazine as the precursor. The Si3N4-BN composites are composed of h-BN, alpha-Si3N4, and beta-Si3N4 produced at a pyrolysis temperature from 1200 to 1750 degrees C with only 0.17-3.9 wt.% phase transition of Si3N4. The effect of pyrolysis temperature on properties of the composites has been investigated. The density and mechanical properties of the composites, at both room temperature and 1000 degrees C, increase along with the elevating of the pyrolysis temperature. The density of the composites achieves 2.33 g/cm(3) at 1750 degrees C with the porosity of 14.1%. The flexural strength, elastic modulus, and fracture toughness at room temperature of the Si3N4-BN composites pyrolyzed at 1750 degrees C are 219.1 MPa, 75.5 GPa, and 2.62 MPa m(1/2), respectively. A desirable flexural strength of 184.9 MPa with a residual ratio of 84.4% has been obtained when the composites are exposed at 1000 degrees C in the air. Micrographs of SEM and TEM illustrate the bonding structure of the pyrolysis BN and Si3N4 grains.