Silicon oxinitride Si2N2O has many interesting properties such as good thermal shock and oxidation resistance, low thermal expansion coefficient, low dielectric constant and losses and thermal stability which leads to a wide range of potential applications ranging from wave-transparent components and optical wave-guide materials to high-temperature structural materials. However, due to its covalent bonds, low diffusion coefficient and decomposition temperature around 1700 °C, it is difficult to elaborate dense and pure Si2N2O at low temperature without the use of oxides sintering aids.
This study focused on the elaboration of dense Si2N2O ceramics using an equimolar mixture of Si3N4 and SiO2 powders precursors and Al2O3, Y2O3 and Sm2O3 sintering aids, according to four different compositions. Dense samples with a 40 mm diameter containing at least 95 vol % of Si2N2O phase were obtained by a reactive liquid phase Spark Plasma Sintering. The influence of the sintering aids on the microstructure, the dielectric properties and the oxidation resistance of the ceramics was investigated. The results show that the obtained nano-microstructures are homogeneous in all samples in terms of grain size and intergranular phase content, with a slight impact of the liquid phase viscosity during the sintering on the Si2N2O grains aspect ratio. The dielectric constant values at ambient temperature are not influenced by the sintering aids whereas the dielectric losses seem to increase with the presence of elongated grains in the microstructure. After an oxidation test at 1600 °C under air, the formation of a rich containing Al viscous glass is observed on the top of the Al2O3 samples whereas the use of Y2O3 and Sm2O3 sintering aids leads to the formation of a protective SiO2 / disilicate phase layer.