HIGH-TEMPERATURE STRENGTH OF FLUORINE-DOPED SILICON-NITRIDE

High-purity Si3N4 (with 2.5 wt% glassy SiO2) doped with F was prepared by immersion of the starting powder into dilute HF and hot isostatic pressing without sintering additives, using a glass encapsulation method. Oxygen content and cation impurity content were almost the same for the F-doped and undoped materials. However, X-ray fluorescence analysis revealed the order of 100 ppm of F in the doped material, and a considerable amount of F was detected from the amorphous SiO2 phase at grain-boundary triple points by analytical transmission electron microscopy. High-resolution electron microscopy found that an amorphous intergranular film was omnipresent in both of the materials, with an equilibrium thickness of 10 +/- 1 angstrom. Subcritical crack-growth resistance and creep resistance at 1400-degrees-C were degraded significantly by the presence of F. Internal friction of doped materials exhibited a clear grain-boundary relaxation peak, which suggested that F was present in the intergranular film at the two-grain junctions; this decreased the grain-boundary viscosity considerably. The film thickness of the doped material showed no apparent chemical effects and was explained by taking into account competing repulsive forces acting normal to the film.