Fabrication and Properties of Porous Si₃N₄ Ceramic with High Porosity

A novel process combining oxidation-bonding and sintering was developed to fabricate porous Si₃N₄ ceramic with high porosity. After sintering at 1800 °C, the SiO₂ in porous Si₃N₄ ceramic transforms into Si₂N₂O because of the reaction of SiO₂ and Si₃N₄. Due to the reaction of SiO₂ and carbon, the porosity of porous Si₃N₄ ceramic increases obviously from 40.2% to 76.8% with the weight decreasing by 35.6%. As the porosity increases, the dielectric constant and loss of the porous Si₃N₄ ceramic decrease obviously from 3.08 to 1.61 and from 3.70×10^-3 to 0.74×10^-3, but due to the production of Si₂N₂O whose mechanical properties are much higher than SiO₂ and the increase of the bonding strength among Si₃N₄ particles, the flexural strength and the fracture toughness of the porous Si₃N₄ ceramic decrease from 55 to 39 MPa and from 0.7 to 0.5 MPa·m^1/2, respectively, but its Vickers hardness increases from 1.2 to 1.3 GPa.