Design and fabrication of porous Si3N4-Si2N2O in situ composite ceramics with improved toughness

In order to improve the fracture toughness and ease the machining process of porous silicon nitride (Si3N4), pre oxidation combining with gel-casting near-net forming method was adopted to prepare a kind of porous silicon nitride-silicon oxynitride (Si3N4-Si2N2O) in situ composite ceramics. The oxidation of Si3N4 powders and its influence on the rheological properties of Si3N4 slurries were carefully studied, and the microstructure, composition, mechanical and dielectric properties of the sintered composite ceramics were also investigated. It is interesting that the existence of the SiO2 coating improved the maximal absolute Zeta potential of Si3N4 powders, which could in turn increase the stability and decrease the viscosity of ceramic slurries. Si2N2O phase was synthesized by the reaction between SiO2 and Si3N4 at 1750 degrees C, and it was unstable at temperatures over 1850 degrees C. Appropriate introduction of Si2N2O could optimize the toughness of porous Si3N4, but it was always at the expense of the flexural strength. In addition, the dielectric constant of the composite ceramics increased with the decrease of theoretical porosities, while the loss tangent reversed and remained at a relatively low level. (C) 2016 Elsevier Ltd. All rights reserved.