Flexural strength and dielectric properties of in-situ Si3N4-SiO2-BN composite ceramics

Abstract

Silicon nitride (Si3N4) is regarded as one of the most promising materials for high temperature structural applications due to its excellent mechanical properties at both room and elevated temperatures. However, one high-temperature Si3N4material intended for use in radomes has a relatively high dielectric constant of 7.9-8.2 at 8-10 GHz. In order to reduce the dielectric constant of the Si3N4, an in-situ reaction process was used to fabricate Si3N4-SiO2-BN composites. In the present study, an in-situ reaction between B2O3and Si3N4, with or without addition of BN in the starting powder mixture, was used to form the composite. The in-situ reaction process resulted in the uniform distribution of the constituents making up the composite ceramic, and resulted in good flexural strength and dielectric constant. The composite was produced by pressure-less sintering and hot-pressing at 1650°C in a nitrogen atmosphere. Microstructure, flexural strength, and dielectric properties of the composites were evaluated with respect to their compositions and sintering processes. The highest flexural strength (193 MPa) and lowest dielectric constant (5.4) was obtained for the hot-pressed composites. The strength of these Si3N4-SiO2-BN composites decreased with increasing BN content.