Reactive spark plasma sintering of Si2N2O ceramic: Reaction process, microstructure, mechanical and dielectric properties

Dense Si2N2O ceramic was successfully prepared through the spark plasma sintering method at 1750 °C with nano-sized amorphous Si3N4 powder as raw material. The reaction sintering process, mechanical and dielectric properties of the Si2N2O ceramic were investigated in detail. The results revealed that the amorphous Si3N4 was initially crystallized to β-Si3N4, then oxidized to Si2N2O during the sintering process. With prolonging the dwell time, the grain orientation of Si2N2O gradually developed to the c-axis direction of the orthorhombic crystal. Meanwhile, the density of as-sintered ceramic was improved significantly, which greatly influenced the mechanical and dielectric properties of Si2N2O ceramic. In this work, the optimal dwell time of sintering process was 20 min, which is obviously shorter than the ordinary sintering methods. The Si2N2O ceramic fabricated at the above conditions exhibited the most favorable mechanical and dielectric properties with a bulk density of 2.80 ± 0.01 g cm−3, Vickers hardness of 13.1 ± 0.1 GPa, flexural strength of 214.1 ± 15.8 MPa, fracture toughness of 2.8 ± 0.3 MPa m1/2, and combined with low dielectric constant of around 3.5 to 4.5 at 8.2−12.4 GHz. Thus, the spark plasma sintering method is potentially regarded as a rapid way to prepare Si2N2O ceramic used as the structural and wave-transmitting functional material.