ROLE OF INTERFACE STRUCTURE ON MECHANICAL-PROPERTIES OF FLUORINE-DOPED SI3N4-SIC CERAMICS

Changes in interface chemistry and, accordingly, in interface structur e of a Si3N4-SiC ceramic, due to the presence of fluorine, was found t o markedly affect the mechanical response of the material. Owing to a lowered cohesive interface strength, as a consequence of fluorine segr egation at grain and phase boundaries, an increased fraction of interg ranular fracture was monitored upon crack propagation. Moreover, in co mparison to undoped specimens, fluorine-doped samples revealed markedl y 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 interfac ial glass structure. A glass structure model is presented, which empha sizes the influence of broken (non-bridging) bonds on the bulk mechani cal performance of Si3N4-SiC-based composites.