MICROSTRUCTURAL EVALUATION OF DEFORMATION MECHANISMS IN SILICON NITRIDE CERAMICS

摘要

Changes of phase composition and morphology were investigated in silicon nitride (Si_3N_4) before and after compressive deformation testing. Using different viscosity additives, rapid consolidation techniques were used to achieve specimens with different α-phase content and grain morphology. Low viscosity additive systems in which the phase change occurred before densification produced elongated microstructure with preferred orientation noted in the prismatic [210] direction. Limited deformation was accommodated with this grain morphology which indicated a flow stress dependency of n=1. High viscosity additive systems in which the densification occurred before the phase change produced equiaxed microstructures with virtually no preferred orientation. Enhanced deformation was accommodated in this grain morphology with an indicated flow stress dependency of n=2. Transmission electron microscope (TEM) photographs corresponded with x-ray diffraction (XRD) analysis for the grain morphology. Selected area diffraction (SAD) was used to verify phases.