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 °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.