Role of Interface Structure on Mechanical Properties of Fluorine-Doped Si<sub>3</sub>N<sub>4</sub>-SiC Ceramics

Abstract

Changes in interface chemistry and, accordingly, in interface structure of a Si3N4-SiC ceramic, due to the presence of fluorine, was found to markedly affect the mechanical response of the material. Owing to a lowered cohesive interface strength, as a consequence of fluorine segregation at grain and phase boundaries, an increased fraction of intergranular fracture was monitored upon crack propagation. Moreover, in comparison to undoped specimens, fluorine-doped samples revealed markedly higher creep rates, which are related to a decrease in the apparent grain-boundary viscosity. The experimental results allow a correlation between the macroscopic mechanical properties and the atomic interfacial glass structure. A glass structure model is presented, which emphasizes the influence of broken (non-bridging) bonds on the bulk mechanical performance of Si3N4-SiC-based composites.