Degradation behavior of amorphous silicon nitride fibers in low atmospheric pressure

We investigated the degradation behavior of amorphous silicon nitride (Si3N4) fibers in low air pressure and presumed the evolution mechanism. The obtained Si3N4 fibers were characterized by tensile strength, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and elemental analysis after being annealed (air pressure: 1 Pa-0.1 MPa, temperature: 1000-1600 degrees C, dwell time: 0-4 h). When air pressure was lower than 100 Pa or higher than 1000 Pa the strength of fibers dropped sharply. Due to the moderate partial pressure of oxygen in 100-1000 Pa, both active oxidation and passive oxidation were restrained resulting in the best mechanical property of fibers in 100-1000 Pa. Besides air pressure, annealing time also affected the thermal behavior of fibers. Firstly Si3N4 fibers were passive oxidized to form SiO2 layer on the surface, and then Si3N4 decomposed into free Si and SiO2 released gaseous SiO. Finally crystallization inside of fibers and formation of nanowires on the outer surface played the leading role in the progress of degradation.

Automated summary

The study found that annealing amorphous silicon nitride fibers at air pressure between 100-1000 Pa resulted in optimal mechanical properties, as the moderate oxygen partial pressure could simultaneously inhibit both active and passive oxidation, maintaining high strength of the fibers. Additionally, annealing time also influenced the thermal behavior of the fibers, ultimately leading to degradation through internal crystallization and formation of nanowires on the outer surface.